Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 323
10
Advances in Medicine, the Biochemical
Process Industry, and Animal Agriculture
;..
The impact of biological research on health care has long been recognized by
the medical and pharmaceutical industries. The rapidly expanding fields of
cellular and molecular biology continue to generate possibilities for new pharma-
ceutical products and medical practices that will have major impacts on the pre-
vention, diagnosis, and treatment of human disease.
The application of biochemical and molecular understanding of cellular pro-
cesses to product development in plant and animal agriculture lags behind that for
medicine. Where such applications have been made in animal agriculture, they
have generally resulted from the transfer of biomedical breakthroughs made origi-
nally in biomedical research. Currently, however, the biological knowledge is
being exploited with increasing effectiveness in the development of new plant and
animal agricultural products.
ADVANCES IN MEDICINE
The long and successful overlap of all areas of biology makes it impossible to
separate their relative contributions to the development of medical practices and
pharmaceutical products. Instead it is more fruitful to discuss some key examples
in which the fundamental knowledge of biology is likely to lead to advances in
. .
medicine.
323
OCR for page 324
324
OPPORTUNITIES IN BIOLOGY
Molecular Pharmacology and Human Disease
A Primary Objective of Biomedical Research Is the Development of Chemical
Agents That Can Selectively Relieve and Abolish Pathological Processes
Traditionally, the discovery of new drugs has been based on the chemical
synthesis of large numbers of analogs, which are then analyzed empirically in
screening programs. The success rate has been low relative to the effort and
expense involved. In contrast to these traditional methods, most drugs can now be
discovered by a concerted effort with a limited number of compounds because the
investigators proceed with a rational approach based on a clear understanding of
the fundamental properties of the systems~hat is, the receptor-recognition prop-
erties or the identity of key regulating enzymes. Some notable examples have
been the development of propranolol, an agent used worldwide in the treatment of
cardiovascular diseases; cimetidine, a histamine-2 agonist, which has dramati-
cally altered ulcer therapy; and lovostatin (Mevacor3, a selective inhibitor of
cholesterol production a powerful new tool in the attack against atherosclerosis.
Scientific and analytical developments in the past decade have provided
powerful tools for the dissection and understanding of many critical biological
processes. For perspective, it took 40 years to elucidate the structure of the
neurotransmitter norepinephrine after the demonstration in 1921 that adrenergic
neurons released an agent that increased the rate and force of cardiac contraction.
It took an additional 20 years to understand the sites and mechanism of synthesis,
storage, release, and response to norepinephrine. These inquiries are valuable
because the chemical manipulation of intrinsic norepinephrine production, re-
lease, and action is the primary basis for the treatment of hypertension in millions
of patients.
In contrast to the time required to develop norepinephrine, the application of
modem analytical chemical techniques (especially thin-layer chromatography,
high-pressure liquid chromatography, and gas-chromatography-mass-spectrome-
try) have resulted in the rapid discovery and identification of trace amounts of
extremely labile but potent substances produced in the body from arachidonic
acid that are involved in the regulation of cardiac, pulmonary, allergic, inflamma-
tory, and blood disorders. In 1973, the prostaglandin endoperoxides were de-
scribed; in 1975, thromboxane A2 (vasoconstrictor and platelet aggregator); in
1976-1977, prcstacyclin (vasodilator and inhibitor of platelet aggregation); and in
197S, leukotrienes (chemotactic and anaphylactic substances). Chemical agents
that modify the synthesis of such arachidonate metabolites have already been
discovered and proven effective in experimental animals and in clinical trials.
The recognition Hat normal bodily function, cell-cell communication, and
disease processes (such as inflammation, myocardial infarction, immune response,
and allergy) are mediated by minute amounts of biologically active substances
OCR for page 325
MEDICINE, THE BIOCHEMICAL PROCESS INDUSTRY, ED ANIMAL AGRICULTURE 325
produced in the body and intricately regulated has provided targets vulnerable to
chemical manipulation for therapeutic benefit. Such studies require the develop-
ment of interdisciplinary programs that bring together cell and molecular biolo-
gists, analytical and synthetic chemists, whole-animal physiologists and pharma-
cologists, and clinical scientists, who collectively focus their research on under-
standing the molecular bases of disease processes and on their therapeutic modifi
cation. The efforts are directed at elucidating (1) the characteristics of the
intrinsic biochemical pathways that mediate and modulate normal and pathologi-
cal function, (2) the determinants and mechanisms of receptor recognition and
coupling to intracellular pathways, (3) the design, synthesis, testing, and targeting
of new chemical entities, and (4) the evaluation of human disease through the
development of new diagnostic strategies, and (5) the application of newly dis-
covered potential therapeutic agents. Thus, opportunities presented by modem
analytical, cell, and molecular biological techniques will not only elucidate struc-
ture, but when coupled to synthetic efforts, can provide large amounts of material
for whole-organ and whole-animal pharmacology studies and ultimately to thera-
peutic applications. Two specific examples of such efforts are illustrated below.
Prostaglandins, Thrornboxane, and LeuLotrienes Have Great Potential
for Therapeutics
For the past decade, arachidonic acid research has emphasized the discovery
and elucidation of locally synthesized potent metabolites that influence regional
tissue function. Two primary metabolic routes have been characterized exten-
sively. The first of these is the cyclooxygenase pathway, which produces
prostaglandins and thromboxane; the second is the lipooxygenase pathway, which
produces the leukotrienes. The cyclooxygenase products have been characterized
for their role in renal, cardiovascular, and platelet function. The leukotrienes
subdivide into two types: (1) those, such as leukotriene (LT)C/D, that are slow-
reacting substances of anaphylaxis and that seem to be intimately involved in pul-
monary smooth muscle during anaphylaxis and allergy; and (2) LTB4, which is an
extremely potent chemotactic substance most likely involved in inflammation and
. . .
tissue injury.
The manipulation of arachidonic acid metabolism provides an ideal approach
for the development of new therapeutic modalities. The progression of these
investigations has created a situation that dictates the metabolic targets that could
be usefully modified by pharmacological agents.
A fascinating discovery has arisen from the dietary manipulation of fatty
acids in experimental animals. The depletion of arachidonic acid or its precursor
from the diet produces essential fatty acid deficiency, which is life-saving in the
autoimmune destruction of the kidney in mice genetically affected with glomeru-
lonephritis (lupus). The lethal consequence of this disease arises from macro
OCR for page 326
326
OPPORTUNITIES IN BIOLOGY
phage invasion and destruction of the glomerulus. Essential fatty acid deficiency
blocks the leukocyte attack on the glomerulus, apparently by blocking local LTB
synthesis. Although essential fatty acid deficiency is not a practical treatment for
lupus, it provides a special insight on what might be a useful therapeutic ap-
proach. Alternatively, substitution of fish oil for arachidonic acid or its precursors
in the daily diet could achieve similar results if they were adequate to interfere
with macrophage function.
Thus, on the basis of a sound foundation of studies of arachidonic acid
metabolism, specific therapeutic target areas can now be approached on a rational
basis. Manipulation of the cyclooxygenase pathway could yield beneficial effects
in immune response regulation and thrombosis, while manipulation of He lipoxy-
genase pathway, especially leukotriene synthesis, could provide a major new
class of agents for the treatment of asthma, allergy, anaphylaxis, and inflamma-
tion (including myocardial infarction and renal disease).
Atriopeptin Is a Cardiac Hormone Intimately Involved in Fluid, Electrolyte,
awl Blood-Pressure Homeostasis
Atriopeptin, a peptide hormone, also termed atrial natriuretic factor (ANF) is
intimately involved in the regulation of renal and cardiovascular homeostasis.
Figure 10-1 illustrates the way in which atriopeptin links the heart, kidneys,
adrenals, blood vessels, and brain in a complex hormonal system that regulates
volume and pressure homeostasis. Basal levels of circulating atriopeptin can be
increased by atrial stretch caused by volume expansion, constrictor agents that
elevate atrial pressure, immersion in water, atrial tachycardia, and high-salt diets.
Once in the circulation, atriopeptin exerts a number of effects related to renal
and cardiovascular functions. Because it suppresses elevated renin in the plasma
and relaxes blood vessels, atriopeptin reduces vascular resistance, thereby lower-
ing blood pressure.
Atriopeptin-immunoreactive neurons have been observed in rat brains, espe-
cially in a hypothalamic region that has extensive connections with structures that
regulate cardiovascular functions. Animals with lesions that include this region
show profound alterations in the regulation of fluid balance, and neither renal
hypertension nor salt-induced hypertension develops. Thus, atriopeptin may
serve as both a central neuromodulator and a peripheral hormone in the regulation
of cardiovascular and renal function.
Current evidence suggests that the major actions of the hormone are to
promote loss of fluid and electrolytes and to reduce vascular tone. These com-
bined actions would be expected to reduce atrial stretch and suppress the release
and processing of atriopeptigen, the 12~amino acid prohormone that is the
primary form of atnal peptide stored in atrial myocytes.
Effects on Renal Function. Atriopeptin also alters salt and water metabolism.
The infusion of atriopeptin into laboratory animals produces rapid and transient
OCR for page 327
MEDICINE, THE BlOCH~IC~ PROCESS INDUSTRY, ED DIME AG~CULTU~ 327
Brain
~~~~ TV_
/
Vasopr~ssin , ~
FIRM, electrolyte, Elevation Atrial
arm vascular ~ stretch
,
.-
__-
/ hi; Decreased blood pressure
Decreased aldosterone
\ Increased urine
volume, sodium
excretion, etc.
Heart
~3~-
Blood vessels
~\
Adrenal\
it\ \1
L
Circulating form
an I
FIGURE 10-l Schematic diagram of the atnopepiin hormonal system. [Adapted from
P. Needleman and J.E. Greenwald, New Engl. J. Med. 314:828 (1986)]
Atriopeptigen
126-amino-acid prohormone
I Selectrve cleavage
excretion of water, sodium, and potassium. The glomerular filtration rate and the
filtration fraction remain elevated throughout the peptide infusion and diminish
abruptly when the infusion is stopped. At present, there is no strong evidence of a
direct inhibition of sodium transport in the renal tubules.
The atriopeptin-induced increase in glomerular filtration rate is striking,
especially since it occurs at doses of peptide that decrease blood pressure and total
renal blood flow. Autoradiography techniques have revealed high-affinity recep-
tor sites for atriopeptin localized on the glomeruli and in the papillae of the
kidney. Those data, together with the finding that cyclic guanosine mono
sphosphate (cGMP) increases markedly in isolated glomeruli and medullary
collecting-duct suspensions incubated with atriopeptin, suggest that the glomeruli
and the medullary collecting duct are the sites that regulate volume and electrolyte
excretion. Such action at these sites would be useful in situations in which
conventional diuretics, which act through tubular sites, are ineffective.
A potent natriuretic and diuretic agent that is a selective renal vasodilator
would have therapeutic potential in pathophysiological states characterized by
fluid and electrolyte imbalances. Some promising clinical targets for research on
atriopeptin therapy include renal failure, hepatorenal syndrome, and congestive
heart failure.
OCR for page 328
328
OPPORTUNITIES INBIOWGY
Human Physiology and Pathophysiology. With the development of radioim-
munoassays, it has become possible to study the release of human atriopeptin. As
in other mammalian species, the concentration of atriopeptin in humans is ele-
vated when intravascular volume and right atrial pressure are increased. Plasma
atriopeptin is elevated in adults with chronic renal failure or with congestive heart
failure, and a worsening of the condition is directly correlated with the concentra-
tion of circulating atriopeptin.
Thus, atriopeptin may have a unique role as a therapeutic agent, especially in
critical-care situations in which patients are undergoing intravenous therapy. The
demonstration that atriopeptin can increase the glomerular filtration rate in ani-
mals with severe renal insufficiency suggests that it has the potential to reduce the
frequency of dialysis. The doses of a number of currently used drugs, such as
certain antibiotics and chemotherapeutic agents, are limited by their toxicity to
kidney tissue. Concurrent administration of atriopeptin with such agents may
increase their therapeutic efficacy by inhibiting renal deterioration.
The maintenance of circulatory volume and salt homeostasis, despite changes
in diet, fluid intake, posture, physical exertion, and stress, requires the integrated
action of several endocrine systems that have contrasting functions. Imbalances
of these complex compensatory systems become unmasked in certain disease
states. One approach to the therapeutic manipulation of endocrine systems is to
administer agents such as atriopeptin to mimic, release, enhance, or antagonize
the intrinsic hormone. The discovery of nonpeptide mimics of the atrial peptides
or of agents that could stimulate the synthesis and release of endogenous atrio-
peptin would allow the testing of long-term atriopeptin therapy for hypertension
as well as for numerous renal and hepatic diseases.
New Approaches to Understanding Health and Disease: The Lipoproteins
Atherosclerosis Is the Principal Cause of Death in the United States, and
Heart Attack (Myocardial Infarction) and Stroke (Cerebral Infarction)
Are Its Overt Manifestations
Atherosclerosis, the disease process leading to the life-threatening events of
heart attack and stroke, causes the progressive narrowing and eventual blocking
of critical regions of the arterial bed. A major feature of the atherosclerotic lesion
is the deposition of cholesterol in association with cell proliferation and connec-
tive tissue elaboration. Extensive epidemiological and pathological studies have
pointed to an important association of lipids and lipoproteins in this disease.
Certain species of lipoproteins that accumulate in plasma as a result of aberrant
production and catabolism are the source of the cholesterol that accumulates in
the artery wall.
OCR for page 329
MEDICINE, THE BlOCH~IC~ PROCESS INDUSTRY, ED DIME AG~CULTU~ 329
Several Major Classes of Lipoproteins mist
Plasma lipoproteins normally serve important homeostadc functions in bio-
logical structure and hormone action as well as in energy production. They are
spheroidal particles constituted of a central core of hydrophobic lipids (cholesteryl
esters and triglycerides) surrounded by a surface layer of hydrophilic lipid
(phospholipids) containing cholesterol and unique proteins called apolipoproteins
(Figure 10-2~. The major classes of lipoproteins in plasma are the chylomicrons,
very-low-density lipoproteins (VLDL3, low-density lipoproteins (LDL), interme-
diate-density lipoproteins (IDL3, and high-density lipoproteins (HDL). Each
major class contains particles that are structurally and compositionally heteroge-
neous, are routed into different metabolic channels and may or may not give rise
to potentially atherogenic particles. A detailed understanding of this heterogene-
ity, therefore, is crucial to an understanding of the basic biology of lipoproteins
UNFSTERIFIED
PHOSPHOLI PI
P~PR~TFIN ~-100
CHOLESTERYL
FIGURE 1~2 Low-density lipoprotein. [Reproduced, with pennission, from M. S. Brown and J. L.
Goldstein, Scientific American 251(5):58 66 (1984)]
OCR for page 330
330
OPPORTUNITIES IN BIOLOGY
and to the establishment of a rational basis for the treatment of lipoprotein
abnormalities associated with atherosclerosis.
Cigarette smoking, hypertension, and diabetes increase the chance of devel-
oping artherosclerosis. Most individuals who are predisposed to the disease do
not show clear-cut defects in lipoprotein metabolism. They may, however, show
characteristic features in their plasma lipoprotein profiles, such as high blood
concentrations of LDL and low concentrations of HDL. The data suggest that
both genetic control and environmental modulation of lipoprotein production,
structural and compositional properties, metabolic channeling, and catabolism
may produce a net accumulation of lipid, including cholesterol, in arterial cells.
The detailed study of these features provides an exciting opportunity for acceler-
ated progress in understanding the function of normal and abnormal lipoproteins.
Research on the Structure and Synthesis of Apolipoproteins Will Lead to
Insight into Cardiovascular Disease
Apolipoproteins are determinants of major processes in lipoprotein assem-
bly, secretion, extracellular processing, and catabolic removal, particularly via
receptor-mediated pathways. Sequencing of many of the apolipoproteins has
been accomplished, DNA clones for some of these have been obtained, and the
mapping of apolipoprotein genes has proceeded. Altered forms of some of these
genes have been associated with hypertriglyceridemia and with an HDL defi-
ciency state with premature atherosclerosis. The incisive techniques of cell and
molecular biology offer exciting prospects for increasing our understanding of
apoliprotein gene regulation and expression. It is important to elucidate how
dietary, hormonal, and environmental factors regulate apolipoprotein gene ex-
pression. In addition, a host of other questions remain on topics such as the
assembly of lipoproteins, the role of mutations, and developmental changes in
lipoproteins. Studies like these should be linked to studies on heritability. In this
connection, research on genetic polymorphism is very important.
Information on Critical Aspects of Lipoprotein Assembly and Secretion
Is Lacking
Some of the general features of lipoprotein assembly and secretion are
known, but many important components of these processes are poorly under-
stood. For example, signal peptides on apolipoproteins are probably involved in
intracellular targeting and secretion. The secreted nascent lipoproteins are ex-
ceedingly heterogeneous in particle size and apolipoprotein and lipid composi-
tion, and therefore in their metabolic fates. Analysis of the steps in assembly and
secretion of the different species of lipoproteins can now be approached by the
techniques of molecular and cellular biology.
While the major emphasis in understanding lipoprotein assembly and secre-
tion has been on the VLDL particles, basic information on the determinants of
OCR for page 331
MEDICINE, THE BlOCHEMIC~ PROCESS INDUSTRY, ED DIME AG~CULTU~ 331
HDL and LDL assembly and secretion by the liver is equally important but
lacking. The processes involved in the assembly and secretion of nascent HDL
are still little understood, and they gain importance in view of the role played by
HDL in the control of cellular cholesterol. Knowledge of the effects of dietary,
environmental, and pharmacological factors on lipoprotein assembly and secre-
tion is rudimentary and in need of detailed molecular characterization.
Insight into What Determines Which Lipoproteins Are Directed into One
Metabolic Channel Versus Another One Is Crucial to Our Understanding of the
Origins of Atherosclerosis
Once they have been secreted, lipoproteins circulate in the bloodstream,
where they interact with enzymes, receptors, and other factors that modulate their
catabolic clearance from the blood. As these interactions occur, atherogenic
products are sometimes produced. Organisms deficient in components of the
extracellular channeling system (lipases, esterifying enzymes, or cofactor lipo-
proteins) demonstrate extremes of abnormal lipoprotein distributions. For ex-
ample, the investigation of the cellular biology of the LDL receptor has delineated
a finely tuned system for regulating cellular cholesterol that affects plasma LDL
levels. Receptor-mediated internalization of LDL and its lysosomal degradation
produces free cholesterol within the liver cell which, in excess, can inhibit
cholesterol synthesis and the synthesis of the LDL receptor. Intracellular storage
of excess cholesterol is facilitated by activation of an esterifying enzyme. Some
cholesterol is also secreted from the liver cells in the form of bile acids that enter
the intestine and are partially recycled back to the liver.
These control mechanisms have been exploited in pharmacological approaches
to the control of plasma LDL levels in humans with hypercholesterolemia The
oral administration of a resin that binds bile salts secreted from the liver into the
intestine can remove some cholesterol from the body. This treatment, while
increasing the number of LDL receptors, also steps up cholesterol synthesis,
thereby reducing the the treatment's effectiveness. Administration of inhibitors
of hepatic cholesterol synthesis also increases receptor synthesis, but to a greater
degree than the resin. A promising approach for control of LDL levels in
hypercholesterolemia is the use of combined drug regimes, which control choles-
terol synthesis and reduce LDL in the plasma. The further molecular analysis of
the regulation of LDL receptor action and cholesterol metabolism is of high
priority. Such studies need to be linked with others at the level of the whole
organism, so that dietary and environmental factors in heart disease can be
evaluated.
Mechanisms of Atherogenesis Involving Lipoproteins Are Being Determined
In the bloodstream, the diverse classes of lipoproteins and their catabolic
intermediates and products make contact with platelets, endothelial cells, macro
OCR for page 332
332
OPPORTUNITIES INBIOLOCY
phages, and smooth muscle cells. Mounting evidence indicates that the interac-
tion of these cells with elevated levels of specific lipoprotein species, such as
beta-VLDL (a cholesterol-enriched VLDL remnant) and LDL, directly or indi-
rectly promote changes similar to those observed at various stages in the develop-
ment of the atherosclerotic lesion.
The frequent observation of lipid-laden macrophage cells in lesions has
prompted intensive research on macrophage interaction with lipoproteins. These
studies have contributed valuable information on the major features of the macro-
phage receptors and their function. In addition, a role for the macrophage in
converting smooth muscle cells to foam cells (lipid-laden cells) has been sug-
gested by in vitro studies describing incorporation by smooth muscle cells of lipid
inclusions derived from the lysis of macrophage foam cells. Detailed information
on such systems will greatly enhance our ability to deal effectively with athero-
sclerosis.
New Opportunities Exist to Prevent and Treat Diseases of Cell Proliferation
Atherosclerosis can serve as an excellent example of a repair process in the
artery wall, which is modified in different ways by chronic hypercholesterolemia
and by factors generated from cigarette smoking, hypertension, diabetes, or other
factors associated with an increased incidence of this disease process. In the
United States and western Europe, chronic hypercholesterolemia is the risk factor
associated with the highest incidence of atherosclerosis. The study of chronic,
dietarily induced hypercholesterolemia in monkeys and pigs has permitted a
chronological assessment of the cells involved in the development of the lesions
of atherosclerosis. Within a few days after cholesterol concentrations equal to
those seen in humans afflicted by the disease have been induced, white blood cells
become increasingly sticky and adhere to the surfaces of the endothelial cells
lining the artery. The high cholesterol concentrations induce changes both in the
circulating white blood cells and in the endothelium. These white blood cells are
scavenger cells that take up foreign substances, including some forms of lipid.
White blood cells take up lipid in the innermost layer of the artery wall and
become foam cells. In so doing, they accumulate and create the first and most
common lesion of atherosclerosis, the "fatty streak." In the United States and
western Europe, such fatty streaks are found in individuals of all ages, including
infants, and represent the precursor lesion, which in many instances goes on to
become the advanced, proliferative, occlusive lesion of atherosclerosis, the fi-
brous plaque that causes myocardial and cerebral infarctions. Thus, experiments
with pigs and monkeys were of critical importance in establishing the connection
between hyperocholesterolemia and atherosclerotic lesions.
The next change that occurs in chronic hypercholesterolemic monkeys may
represent further"injury" to the endothelial cells. These cells seem to separate
from one another at particular sites in the artery. After the endothelial cells
OCR for page 333
MEDICINE, THE BIOCHEMICAL PROCESS INDU=RY, AND ANIM~ AGRICULTURE 333
~ ~:~ ~PORRI~:REE~:~NK:BE~E~EN ~POP~E~INS~AND~ ~ ~
:~ :; i:
~ :: ~::~:~:~:~:::: ~ ~ ~ ~;:~ ~:~TH~E~:~C~LorrlNG~SYsTEM~:~:~:~ ~:~:~:~ ~:~ ~ ~:~:~:~
::: ~ ~: ~: :: ~::~:~:~An ~ :LDL~ variant ~:n~am~sd~::~ ~:~Lp~a)~ ~ ~h~as~bben~shown~to:~poss~ess ~ An Amen ah:::: ~ :::
: ~ :: : : :: :: :: :: : : :: ~ ~ : :: ~: ~ : : :: ~ ~ :: ~ : : : : :
~: ~:~ ~applipoprdtein ~called :~ apo ~A. ~:~:lThis ~apolip~protain~ is ~bttached ~ td:~the norma
~:~LDL:apolipoprotei~a~apQ~ B-1:~0~0~ b~r~a~:~sU~:llide~li~nkag~e.~:~::~:Hig~h~pl~asma~=nce~n~-~ :~::~
trab~ons:~(a3~ ~are~stron9ly~as~soci~d~with~accelerdi:~ d - elopment~of
~:::~dtheroscle~rosis~ ~:~:~e :~n~an~ner i:n~:wh~ich~::~Lp(~a)~:infl~ue~n~cas:::~:ath~eroselerosIs de-~:: ~::~
~ ~.~ ~ ~ ~ ~ ~ ~ ~ ~ ~ .~ ~,~ ~ ~ ~ ~ ~ ~ . ~ .~ ~ ~ ~ ~ ~ ~ ~ ~:~ ~ ~ ~: ~ ~ ~: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~:~ ~ ~ :~ ~ ~ ~: ~ ~: ~ ~ ~ :~ ~ ~ ~ :
~ ~ ~ : ~ ~ ~ : ~ v Q I o : p m ~ e n t ~ ~ ~ ~ : i s : : : ~ ~ ~ ~ s t ~ l I : ~ ~ : a : ~ : ~ ~ m ~ y s t e ~ r y . ~ ~ ~ : ~ ~ : : ~ : ~ : H o w e v ~ r ~ ~ ~ ~ : ~ o : n : ~ : ~ ~ : ~ t ~ h e ~ ~ ~ ~ ~ ~ ~ ~ : b a s ~ l s ~ : : : ~ ~ ~ ~ o f ~ ~ ~ ~ t h ~ e ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ a r n l n o ~ ~ a c l d : : : :
~ ~:~ ~ ~ ~ ~ ~ ~:~ ~ ~ ~ ~ ~ ~ :~ ~ ~ ~ ~ ~ ~,~ C~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~,
:~ ~ :~s~eq~u~en~ce::~ :af ~apo A: ~sCfe~nbsts~h:ave ~ :recently~:~:~shown~:~ that~ ~tl1:~'s apolipoprote':n: ~:~::~::; ~
~:~:~:~ ~ ~:~ ~ ~ ~ ~ ~:~ ~:~ ~,~ ~ ~ ~: ~ ~ ~ ~:~ ~ ~ ~:~ ~: ~ :~ ~:~:~: ~ ~ :~: ~
~: ~s:hares cons'~d:e:rable~:s~eq~u~Q~nce ~ho~m:ol~ogy~w~th:~:plas~m~noge:~n.~ ~P~I~asm'~n~gen~
~is~a~ ks~f~ comp~nen~t~ ~i~n~ihe~ ~cluttin9:~sysiem,~whi~binds~to~fibrin~and~:when
~: :adtivatdd~ by: tissue~ plasminogen~=ivatbr~can enzy:rr~bt:~ally~dissolves~fibrin
~:~c~s~.~ ~: ~:lt~is~ ~interssting~to~:~s~c~ul~to o~n~the ~ ppssibld ~ ˘onn~ection~ between~ th~e~ ~
:plasminoge~n-likb :strutture~of ;~apo :~;~:an~d::~:~4s~:role: i:n::~bth~:rosclerosis:.~ ~Thers~is~:::: ~:~:
~avid~e~nce~ ~th~at~lip(~) is~ ~associdted~ w~th~ ~init~ial bthero~lerot:iŁ~plaques :~w~hich~ ~ ~
:;~i i :~:~ cobtai~n:~fib~ri~nog~e~n~(fib~rin ~precU~rsor)~and~fibri~n~i~n~ro~gh~r~the~sam~e~oonca~ntra~:~:
~:~tion~as;c~holesterol~.~ ~ It~h~s~been ~propo0~thdt~(a)~.~mediatted~by~th~e~apD~A:~:~:~
~:mino ~ac~d~ seq~uences~th~are~homologous~plas~minoge~n s~fibrtn~ ~binding~
~:~s~eq~uences ~:~::ca~n~:~he~Q:~ ~to:~the: :hth~eroscle~rot~;c~:pl~aqu:e~.~i~: :~EJucid:dt~io:n: ::6f~:~pos-~
~ ~ ~ , ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
: ~ ~:~s:ible~:apo~:~A~:~wit:h: co~m~pon~e~nts~of:~e clotting~sy~tem :awa:~:~:add~itio~nal r~search~ :~:~:~
~to~ dete~rmine~: the ~ex~= role~ ~ ~:Lpfa3 ~and ~:aDo~A ~in~ ~e~ accele~ted~develop-:~:~
~:::~::~:~: mQ:nt~nf ~h~arosŁ1erosis:.: ~:~
separate from one another, they then retract and expose the underlying white
blood cells that have become foam cells. In many instances, another circulating
blood cell, the platelet, attaches at these sites of exposure. Platelets seem to
release substances that stimulate the localized proliferation of smooth muscle.
We now know that the proliferation of smooth muscle cells at sites where
platelets interact can be partly due to the powerful growth factors produced by
platelets, endothelial cells, macrophages, and the smooth muscle cells themselves.
One of these, the platelet-derived growth factor, is a potent mitogen that stimu-
lates the proliferation of cells that form connective tissue, such as smooth muscle
cells in the artery wall. Perhaps of equal importance is the secretion of other
growth factors into the local environment by activated macrophages or platelets.
Like platelet-derived growth factor, these factors could have a major impact on
the local multiplication of smooth muscle cells and macrophages and set the stage
for the development of advanced occlusive lesions of atherosclerosis. These
lesions could enlarge over a period of months or years and eventually sufficiently
OCR for page 354
354
OPPORTUN171ES IN BIOLOGY
Acquisition and Interpretation of Analytical Information Is a Major Problem
A major problem that has plagued efficient process development has been the
scarcity of analytical techniques for measuring the product of interest. The
biological activity of many new products is difficult to measure, and their struc-
ture is often poorly defined; thus, routine measurement of these molecules is
difficult. Assays for their biological activities are often cumbersome and impre-
cise. A powerful analytical technique that has recently evolved is He use of
monoclonal antibodies as the basis for molecular recognition and analysis. New
sensor technology is need, including biosensors, instrumentation, and software,
that will provide the power needed to support process development more effec-
tively.
Process Synthesis, or Manufacturing, Requires Multiple Steps from Compound
Production to Purification
Manufacturing technology for the production of biochemical products re-
quires a multiplicity of unit operations. A typical process for manufacturing a
highly pure therapeutic protein is diagnosed in Figure 1~3. The primary unit
operation in which the raw materials or nutrient medium is converted to a desired
product is carried out in a bioreactor. This reactor may be a fermenter for growing
microorganisms or a device for propagation of animal cells; in either case, the
cells may have been genetically engineered to overproduce the desired producL
Once formed, the product must be separated, concentrated, and purified to its
active form and formulated to stabilize, and perhaps enhance, its activity for
eventual use as a drug.
A process for manufacturing therapeutic proteins derived from recombinant
microorganisms is expensive and complex, not only because of the multiple unit
operations that must be carried out to recover highly pure materials, but also
because the final product must be made in its native form that is, with a single
three-dimensional structure. Thus, in the case of a protein that has a large number
of possible folding configurations, only one is permitted in the final product. This
will ensure its efficient use as a therapeutic agent and help prevent the body's
immune response from rejecting the molecule as a foreign protein. Both biologi-
cal and structural integrity must be retained. The ability to achieve this integrity
depends on developing a better understanding of how proteins behave during their
production and purification and as well as on developing high-resolution analyti-
cal techniques that are able to measure structural properties of biological mole-
cules.
Other problems that hinder successful process development include the pres-
ence of proteases that modify the product, insufficient reduction in nucleic acid
content, and the presence of isomeric forms of a protein product. Glycosylation
of proteins is a useful and important posttranslational modification implemented
OCR for page 355
MEDICINE, THE BIOCHEMICAL PROCESS INDUSTRY, AND ANIMAL AGRICULTURE 355
Medium preparation
Bioreactor
(Fermentation or
cell culture)
Cell harvesting
Cell disruption
Separation of
cell debris
Concentration
of product
Multistep purification
(Chromatography,
extraction, and
precipitation
Product concentration
(Interaction with
quality control)
Formulation
FIGURE 1~3 Bioprocessing. [Charles Cooney, Massachusetts ~sutute of Technology]
by cells, the function of which we need to better understand; we also need to
improve the techniques for characterizing, modifying, and recovering properly
glycosylated proteins. Additionally, a substantial number of proteins of interest
form insoluble aggregates or "inclusion bodies" inside genetically engineered
cells, and we need to better understand procedures for recovering these proteins.
Such recovery of many proteins could be enhanced by high levels of secretion;
however, the molecular basis of secretion is unlmown in many microorganisms.
OCR for page 356
356
OPPO~UNITIES IN BIOLOGY
Further advances in understanding the biology and biochemistry of protein secre-
tion will have a major impact on process development.
In recent years, many of the improvements in recovery of new biochemical
products have come about through scale-up of known techniques and the use of a
wider variety of materials that enhance purification. Improved scale-up has
occurred as a consequence of automation of many procedures, especially chroma-
tography, the availability of new chromatographic resins and other absorbents, the
wide use of membrane processes for concentration and desalting, and improve-
ments in mechanical devices such as centrifuges, cell homogenizers, and mem-
brane filters. Improvements are still needed. New techniques, such as affinity
chromatography and electrokinetic separations, are still not applicable to large-
scale purification because of scale-up cost. There thus remains a need for
innovation in both the development and implementation of new recovery proce-
dures, the development of improved materials to enhance unit operations such as
extraction and chromatography, and the development of techniques for achieving
process integration. As part of this process, computer-aided design software and
flow-sheet simulation techniques are likely to become major tools in the hands of
the process biochemist and the engineer.
In response to these needs, substantial research has been done to delineate the
necessary components of the complex and ill-de6'ned nutrient media that are used
for the growth of mammalian cells. From this effort has developed a set of better
defined and less expensive cell-culture media that avoid the use of animal sera.
These defined low-protein media not only enable efficient and controlled growth
of mammalian cells, but also facilitate the recovery of products excreted by these
cells. Because these products are often secreted into the medium in very low
concentrations (<10 milligrams per liter), the use of media containing high con-
centrations of protein makes subsequent downstream processing for product re-
covery difficult. Thus, the use of well-defined, low-protein media.areatly en-
hances efficient product recovery.
c ~_ ~
Modern Biology Coupled to the Biochemical Processing Industry Has
Enormous Potential
Human Growth Hormone. Human growth hormone is a protein produced by
the pituitary gland that stimulates the liver to produce somatomedins, which
stimulate bones to grow. Children produce large quantities of growth hormone,
whereas adults normally produce very little. Pituitary dwarfs represent a popula-
tion with a defect in the production of the pituitary growth hormone. Children
judged deficient in growth hormone used to be injected with the hormone ob-
tained directly from the pituitary glands of cadavers, which they received three
times per week. The National Hormone and Pituitary Program processed as many
as 50,000 pituitaries per year, supplying the growth hormone derived from them
OCR for page 357
MEDICINE, THE BlOCH~IC~ PROCESS INDUSTRY, ED DIME AG~CUL=M 357
to treat 3,500 individuals. Since the hormone was difficult to obtain, it was
always in short supply; the criteria for deciding who to treat were stringent. In
1985 three people who had been treated with the cadaver-derived human growth
hormone in the 1960s and 1970s died of Creutzfeldt-Jakob disease, a rare disease
that results in dementia and death. The proposed cause of the disease was
infectious virus derived from the pituitaries of victims of Creutzfeldt-Jakob dis-
ease. At the time of these deaths, biotechnology companies had been working on
the application of recombinant DNA technology to produce synthetic human
growth hormone.
In this effort, the gene coding for growth hormone was isolated from pituitar-
ies, and genetic expression vectors were developed to produce the hormone by
fermentation. This was followed by several years of careful research on the
isolation and purification of this rare protein. The successful completion of this
task in the fall of 1985 resulted in Food and Drug Administration approval for a
genetically engineered version of human growth hormone. Since the growth
hormone is manufactured in a bacterium, no human pituitaries are needed and the
threat of Creutzfeld-Jakob infections has been eliminated completely.
In addition to solving a major health and social problem, the development of
a commercial process for the manufacture of recombinant human growth hormone
has made major contributions to the identification of fundamental problems in
protein structure and function. Challenging problems such as determining the
factors that influence proper folding of proteins derived from the linear assembly
of amino acids, determining the crystalline structure of growth hormone, and
understanding its mechanism for effecting the observed physiological responses
are receiving widespread attention. The study of growth hormone will lead to a
greater understanding of the biology of other regulatory and mediating proteins,
as well as to the design of smaller and more effective molecules. We can antici-
pate improved treatments for hormone deficiencies, wound healing, osteoporosis,
bone fractures, and obesity.
Tissue Plasrrunogen Activator. Both strokes and heart attacks can occur
when a blood clot lodges in a blood vessel, blocking the flow of bloom Tissue
plasminogen activator is a naturally occurring enzyme that binds to fibrin clots
and is activated to convert plasminogen to plasmin, the latter being capable of
dissolving the blood clot. The primary initial applications for this protein are in
the treatment of heart attacks, with minor applications for other conditions such as
unstable angina, pulmonary embolism, and arterio-venous occlusions. More than
a score of companies are currently involved in the research and development of
tissue plasminogen activator.
The story differs considerably from that for human growth hormone produc-
tion. Tissue plasminogen activator is a glycosylated protein; that is, it contains
oligosaccharides (sugar complexes) attached to the backbone of the protein mole-
cule. Since bacteria cannot glycosylate proteins, the manufacture of tissue plas
OCR for page 358
358
OPPORTUNITIES IN BIOLOGY
minogen activator, or its variants, must be in higher organisms or cells representa-
tive of those organisms. At present, major production is achieved through
mammalian cell culture technology. This approach has initiated a new cycle of
chemical and biological research to develop analytical methods for characterizing
and analyzing the sugar attachments to the protein as well as basic biological
studies to develop an understanding of the role of these oligosaccharides in the
structure and function of tissue plasminogen activator.
Other thrombolytic agents such as urokinase and streptokinase are also being
marketed and intensively investigated. Second-generation clot-dissolving mole-
cules are being designed. It is hoped that new products with enhanced specific
activities, greater specificity, and fewer side effects (such as nonspecific bleed-
ing) will be generated. The challenge to increase the survival rates of the 900,000
heart attack victims in the United Stems who reach hospitals each year is great,
and the tools available today provide scientists with the means for solving these
problems.
Industrial Growth in Biotechnology Is Predicted, but International
Competition Will Be Great
Over the past 5 years, biotechnology, defined as the application of biological
organisms, systems, and processes to the manufacturing and service industries,
has developed dramatically, and commercial products have been made. Although
the number of products that have entered the market at this time is limited (for
example, insulin, human growth hormone, interferons, tissue plasminogen activa-
tor, and hepatitis B vaccine), the momentum to introduce more products contin-
ues. Within the next decade we should see the introduction of many new products
for medicine, agriculture, and environmental management.
Economic considerations and analyses have replaced some of the euphoric
hopes for products derived from biotechnology. This change has resulted in a
significant focus toward high-value specialty products and away from large-scale
production of general-use products. Combinatorial evaluations of gene engineer-
ing, mutagenesis, fermentation optimization, and downstream processing have
intensified as economic dictates have become more obvious.
Regulatory issues remain a major question worldwide, particularly with
regard to the deliberate environmental release of plants and microbes developed
through the use of recombinant DNA technology. As a result, there now exists a
regulatory patchwork with little uniformity from country to country. In the
United States, regulatory issues are currently under study, and debate continues
with regard to what risk-benefit information will be required and who will
regulate what.
Worldwide, there is little doubt that biotechnology is seeing major develop-
ments in Europe, Japan, and the United States. Other complementary and syner-
gistic technologies that will influence the economic trends and impact of biotech
OCR for page 359
MEDICINE, THE BIOCHEMICAL PROCESS INDUSTRY, AND ANIMAL AGRICULTURE 359
nology must be considered. Examples are more traditional plant breeding, so-
matic cell culture techniques, and computer science. The Japanese have organ-
ized to become a major force in biotechnology. This movement has involved
government, industry, and universities and has focused on key target areas such as
plant cell culture, hybridomas, bioreactors, and large-scale mammalian culture.
New opportunities to service the growing activities in biotechnology have led
to new businesses such as those that provide computer support for molecular
biology, build equipment for nucleic acid and pervade synthesis and analysis, and
produce mammalian cell cultures. Various government initiatives have led ~
broader joint ventures and venture capital operations on an international scale,
which eventually should have a significant impact on intemabonal trade.
ADVANCES IN ANIMAL AGRICULTURE
Challenges Lie Ahead to Increase the Efficiency and Productivity of Domestic
Animals Under Widely Disparate Environmental, Economic, awl
Political Constraints
Of the thousands of animal species that might have been domesticated,
human beings have utilized only a handful. These have been selected over the last
10 millennia to occupy increasingly narrow ecological niches. These niches are
highly structured management systems designed to increase the efficiency of our
domestic species by removing negative environmental effects and maximizing the
nutritional status of these animals as they grow to maturity. This intensified
approach to domestic animal agriculture has reached its peak in the temperate
regions of the world, where the "developed" economies and, generally, a surplus
of agricultural products exist. Great discrepancies are found between He agricul-
tural productivity of the temperate regions and those of the tropics and subtropics,
where 60 percent of the world's population and half of its domestic animal species
reside. For instance, it requires from two to four times as many animals to
produce equivalent amounts of meat and milk in the tropics as can be produced by
domestic animals in the temperate regions.
Molecular biology has helped provide the ability to address these production
problems by unlocking the vast information found within the genome of each
animal. Using this information, we can increase productivity of specific tissues,
provide new weapons against disease, reduce inefficiencies caused by overpro-
duction of fat in growing animals, increase the efficiency of rumen fermentation,
and increase the genetic diversity of the domestic animal gene pool. Since the
amount of available information present in the gene pool of the domestic animal
population far exceeds our ability to interpret it, it is critical to define those areas
most likely to produce real gains in the productivity of domestic animal agricul-
ture.
OCR for page 360
360
OPPORTUNITIES IN BIOLOGY
Growth Regulation
Growth Occurs As a Result of an Increase in Cell Number, Cell Size, or Both
Increasing growth rate has a number of potential benefits to agricultural
productivity. Among these are decreased interval to reproductive maturity or
market weight, increased feed efficiency, and reduction of maintenance require-
ments. In addition to increasing the rate at which animals grow it is also possible
to consider increasing the growth rates of specific tissues (such as muscle) and
decreasing the growth of others (such as adipose tissues.
Regulation of the Somatotropin-Somatomedin System. The growth of whole
animals can be accelerated by growth hormone (somatotropin) and insulinlike
growth factors (somatomedin). The release of somatotropin into the circulatory
system is correlated with a concomitant increase in the circulating concentrations
of somatomedin. The release of somatotropin can be effected by the use of a
growth-hormone releasing factor, inhibitors of somatostatin, release, or antibod-
ies against somatostatin. Exogenous supplementation with the homologous so-
matotropin or somatomedins is also an available approach. Largely unexplored is
the regulation of the receptors or binding proteins that influence the rate of the
degradation and the degree of responsiveness of the target tissues for soma-
totropin or somatostatin. The approach of designing drugs by studying the
receptor-ligand interaction has been successful in the human pharmaceutical area,
but it is only recently that a similar approach has been attempted in the domestic
animal industry. A recent example is the p-agonists, which stimulate p-adrener-
gic receptors to partition energy away from adipose tissue and toward muscle. An
approach that has been envisioned for the future is to produce peptide mimics of
steroids by studying the steroid hormone receptor and building peptides that will
activate it. This would be an important breakthrough, since it would alleviate the
problem of steroid residues in meat.
To date the use of recombinant homologous somatotropins and chemically
and or recombinantly produced growth-hormone releasing factors has been most
carefully studied. Since growth hormone does more than produce somatomedins,
it is likely that this approach or the use of releasing factors will be the method of
choice for the immediate future. However, the use of somatomedins and epider-
mal growth factors has been proposed for wound healing in valuable animals.
Regulation of Growth of Specific Cell Types. The ability to favor or retard the
growth or development of a single cell Me has obvious advantages in increasing
the productivity of domestic animals. The specificity required to achieve either of
these results dictates a greater understanding of He biology of these cell types
than may be currently available. However, progress is being made with two cell
types: the adipocyte of adipose tissue and the myoblast and myotube of muscle.
OCR for page 361
MEDICINE, THE BlOCH~IC~ PROCESS INDUSTRY, ED DIME AG~CULTU~ 361
Excess storage of lipid is a major cause of reduced feed efficiency in domes-
tic animals and a major health problem in the human population. Reducing excess
fat storage in animals addresses both problems simultaneously, since the meat
marketed will be of higher quality. Present approaches include the exogenous use
of somatotropin and p-agonists during the finishing period to partition nutrients
away from adipose tissue and toward muscle synthesis. A more direct approach
being tested is the immunization of animals against membrane proteins of adi-
pocytes. This approach has had dramatic effects in laboratory animals, but is
largely untested in animal production systems. Much additional work is needed
to learn to regulate specific metabolic pathways in adipocytes by regulation of the
genome.
Increasing muscle mass in animals may be accomplished by increasing the
number of muscle cells or the amount of muscle protein present in cells. Two
basic in vitro cellular models are the fetal myoblast cells and myotubes. Two-
dimensional mapping of proteins during proliferation and myotube formation has
identified specific proteins produced during these processes. However, we do not
yet understand the regulation of the synthesis and degradation of these molecules
or their specific functions in proliferation and maturation of these muscle cells.
Additional basic research is warranted in these areas to provide tools and more
sophisticated approaches to increasing the availability of high-quality protein for
human consumption.
Reproduction
It Has Been Difficult to Increase the Reproductive Performance of
Domestic Animals
A great deal of basic research has been carried out in the past three decades
on the regulation of the reproductive process in an attempt to increase reproduc-
tion performance. Widespread use of artificial insemination and embryo transfer
has increased the rate of genetic gain in certain specifically desired traits, such as
milk yield. However, a reliable pregnancy diagnosis is not yet available. In
addition, the average litter size in the swine population has not increased in the
U.S. herd, although certain breeds in other countries have much higher litter
numbers. For example the average litter size of the Chinese hog is 24, versus 10
for the United States.
A major constraint in regulating the reproductive process is the large number
of complex cell types that influence reproductive success or failure. In addition,
the management systems in place for most domestic animals stress the reproduc-
tive process. For instance, increasing mink yield in cattle through genetic selec-
tion has had detrimental effects on reproductive performance because of the large
metabolic demands to increase mink production. Immediate gains in production
could be realized if sexing of semen were accomplished and if pregnancy-specific
peptides were identified that could be used to diagnose pregnancy on farms.
OCR for page 362
362
OPPORTUNITIES IN BIOLOGY
In the domestic fowl population, some potential exists for manipulating egg
formation to alter the lipid composition (reduce cholesterol), to alter the protein
content of eggs, or to express novel proteins that could be harvested. It is also
possible to harvest antibodies from eggs of chickens that have been immunized
against a foreign protein.
Lactation
Increases in Milk Production Will Occur with the Use of Bovine Somatotropin
and Increased Mammary Growth
It is generally believed that use of bovine somatotropin in lactating cattle will
increase average mink yields 10 to 15 percent across a 305-day lactation. Bovine
somatotropin increases the rate of milk synthesis, but does not increase the
amount of secretory tissue present. Increases in the growth of mammary tissue
would also increase milk yield since each gram of mammary tissue produces
approximately 1.6 grams of milk per day. Although several potential mammary
growth factors have been identified, the specific roles these various factors play in
regulating tissue growth are not well understood. If specific mammary growth
factors could be identified and put to use, it would benefit not only the dairy
industry, but also other animal industries as well, since milk production is a major
[imitating factor on the growth rate of neonatal swine and beef cattle.
Alteration of gene expression in mammary tissue to produce a milk with a
higher solid content is a possibility. Milk is 87 percent water, and it is therefore
expensive to transport. The major osmotic determinant of milk is lactose, the
mink sugar. In marine mammals, the production of lactose is greatly limited,
producing a milk that is extremely high in protein and fat. It would be possible to
achieve the same effect in cattle by inhibiting the expression of alpha lactalbumin
production. This protein, produced only in mammary tissue, is the rate-limiting
protein for lactose synthesis. Another possible approach to manipulation of milk
synthesis is the production of novel proteins that either improve the quality of
milk or are of economic significance.
Infectious Disease
Molecular Biology Will Provide Major Advances in the Diagnosis, Treatment,
and Prevention of Some of the Major Diseases That Threaten the
Livestock Industry
Disease is a major cause of reduced performance in domestic animals. Some
diseases that cause major losses in the livestock industry are foot-and-mouth
disease, neonatal enterotoxicosis, ~ypanosomiasis, mastitis, and respiratory dis-
eases. At present, molecular biology approaches are being used to develop
methods of diagnosis and treatment for these diseases.
OCR for page 363
MEDICINE, THE BlOCH~IC~ PROCESS INDUSTRY, ED DIME AG~CULTU~ 363
Some general approaches are the production of animal interferons for treat-
ment of respiratory diseases and the use of monoclonal antibodies for use in
diagnostic aids or as treatments against certain bacteria or viruses. Recently, a
vaccine against foot-and-mouth disease was developed; it uses a protein-surface
antigen encoded by a synthetic gene, which was cloned and expressed at high
levels in Escherichia colt. Similar approaches might be productive in the search
for a weapon against ~ypanosomes, which, by causing sleeping sickness in cattle,
are a major deterrent to increased productivity in the African cattle industry. Use
of antibodies against adhesion factors prevents the bacteria that cause neonatal
enterotoxicosis from attaching to gut epithelial cells. A similar approach has been
successful in preventing the attachment to mammary epithelial cells of certain
bacteria that cause mastitis.
Production of Feedstocks
Some Feed Additives, Such As Amino Acids and Vitamins, May Be Efficiently
Produced by Recombinant Techniques
The two major amino acids now used as feed additives are methionine and ~-
lysine. It is doubtful that methionine made by recombinant techniques would cost
less than that made by chemical synthesis. However, lysine, which can be
absorbed across the gut only in its ~-form, can be produced by fermentation that
uses coryneform bacteria. It is believed that recombinant techniques will make it
possible to produce bacteria that can make 130 g of ~-lysine per liter. At these
levels of production, the price of lysine will be lower than the equivalent soybean
meal source.
Lowering the cost of lysine production may permit the addition to feed of the
next two limiting amino acids, ~-'ryptophan and ~-threonine. Success in making
recombinant organisms that produce these amino acids in large quantity has been
reported.
The production of most vitamins will not be affected greatly by recombinant
technology since chemical synthesis will remain financially competitive. How-
ever, it has been proposed that vitamin Bit, riboflavin, and niacin might be
produced competitively by these techniques.
Livestock Improvement
Genetic Engineering and Expanded Breeding Programs Should Result in More
Efficient Production of Livestock
The insertion of foreign DNA into animals has received much publicity
recently. The challenge is to obtain tissue specificity of gene expression. Most
current work in domestic species involves manipulation of the somatotropin
OCR for page 364
364
OPPORTUNITIES IN BIOLOGY
system by inserting extra copies of the somatotropin gene into embryos. This
approach, which has as its goal improving feed efficiency and meat quality, has its
highest chance for success in litter-bearing species such as the pig, where the
number of embryos is high. Studies are also being canted out in sheep by using
embryo transfer from superovulated animals to increase the odds of success.
Although it has been possible to get expression of these extra copies of soma-
totropin genes in animals, the results to date have been disappointing. For
example, the health and reproduction capabilities of these animals have been
impaired, suggesting the need for a tissue-specific approach to gene expression.
Ideally, one would like to insert the extra copies of the desired gene into the target
tissue to avoid side effects in other tissues. It is likely that this will be a major
barrier to the practical use of this approach.
One novel approach to this problem has been to use lactating mammary
tissue to produce a foreign protein. This was accomplished by joining the desired
gene to the control region of the gene coding for lactoglobulin. This recombinant
gene is expressed only in lactating mammary tissue. Therefore, by using an
appropriate tissue-specif~c promoter, investigators can obtain tissue-specif~c ex-
pression.
Another possible example of the same approach would be to insert the gene-
regulating expression of the mediator of somatotropin action on mammary tissue.
This would permit autostimulation of secreting mammary tissue to increase yield.
As mentioned earlier, the productivity of animals in the temperate regions of
the globe far exceeds that of the same species in the tropics and subtropics.
Moreover, when the temperate breeds are exported to tropical regions, their
productivity falls rapidly, and they often die of diseases to which they have no
resistance. An obvious solution is to use local breeds in these regions, provide
exogenous hormone treatments to increase their productivity, or to insert genes
coding for metabolic enhancers. For example, adaptation to thermal stress is
associated with a reduction in circulating somatotropin concentrations. Growth
rates and mink yields of cattle in these regions might be enhanced markedly with
exogenous supplementation of somatotropin or insertion of extra copies of the
growth hormone gene.
The diversity of the domestic animal gene pool is larger than most scientists
realize because the predominant breeds of livestock represent only a small portion
of the total number of breeds. For example, 90 percent of the dairy cow popula-
tion in the United States consists of Holsteins, even though more than 40 different
dairy breeds exist throughout the world. No doubt many of these breeds have
genetic information of great value to the world cattle industry. In Asia alone,
there are five species of nondomesticated ruminants, which are close relatives to
cattle although poorly known. Another example is in the swine industry. What
genes make it possible for the Chinese sow to produce twice as many piglets in a
litter as the average U.S. sows? Southeast Asia is the home of a ruminant pig
(Babirusa) that might be used to produce a new pig breed, one more efficient at
meat production. We should consider not only the relatively few highly utilized
domestic animals in our search to increase domestic animal productivity, but also
their underutilized relatives.
Representative terms from entire chapter:
growth hormone
