(NAS Colloquium) Molecular Kinesis in Cellular Function and Plasticity (2002)

Henri Tiedge^*†, Floyd E.Bloom^‡, and Dietmar Richter^§

^*Department of Physiology and Pharmacology, and Department of Neurology, State University of New York, Health Science Center, Brooklyn, NY 11203;

^‡Department of Neuropharmacology, Scripps Research Institute, La Jolla, CA 92037; and ^§Institut für Zellbiochemie und klinische Neurobiologie, Universität Hamburg, D-20246 Hamburg, Germany

Intracellular transport and localization of cellular components are essential for the functional organization and plasticity of eukaryotic cells. Although the elucidation of protein transport mechanisms has made impressive progress in recent years, intracellular transport of RNA remains less well understood. The National Academy of Sciences Colloquium on Molecular Kinesis in Cellular Function and Plasticity therefore was devised as an interdisciplinary platform for participants to discuss intracellular molecular transport from a variety of different perspectives. Topics covered at the meeting included RNA metabolism and transport, mechanisms of protein synthesis and localization, the formation of complex interactive protein ensembles, and the relevance of such mechanisms for activity-dependent regulation and synaptic plasticity in neurons. It was the overall objective of the colloquium to generate momentum and cohesion for the emerging research field of molecular kinesis.

The meeting-bound researcher, approaching one of our cities by air, cannot help but muse about the similarities in the way cities and cells appear to be organized. As the urban arteries come into view—streets, highways, railroad tracks—one can observe traffic in diverse forms, cars, trucks and buses traveling to their various destinations, trains proceeding along their tracks. The underlying rationale for every single movement may not be apparent to our airborne observer, but it is obvious that for the city to operate urban transportation is a prerequisite. Conversely, occasional congestion or traffic jams would indicate a breakdown of local traffic flows even if the cause of any such breakdown may not be immediately obvious from a bird’s-eye perspective.

Upon such reflections on urban traffic, and on the parallels with cellular transportation, our biologist may further ponder on purpose, underlying principles and mechanisms of the latter. Like cities, cells have developed diverse transport systems to ensure that the right components are delivered to, or manufactured at, the right location at the right time. What are these transport systems? What are the intracellular roads or tracks, what are the engines and motors, and how do they operate? How are the various types of cargo shipped, and how is such transport tailored to demand? What determines whether it is the finished product that is shipped, or rather smaller parts or subunits for local on-site assembly? How are such mechanisms regulated to maintain cellular function, react to physiological stimuli, and ensure flexible adaptation to changing environments?

These were some of the more basic questions that were addressed at the National Academy of Sciences Colloquium on Molecular Kinesis in Cellular Function and Plasticity, held at the Arnold and Mabel Beckman Center in Irvine, California, December 7–9, 2000. This colloquium was conceived as interdisciplinary in nature, bringing together researchers who examine principles of intracellular molecular motion from a diverse range of viewpoints. It has become apparent over the last few years that intracellular transport and localization of both proteins and RNAs play important roles in the development and function of eukaryotic cells as diverse as yeast and neurons.

However, although both mechanisms have been implicated in the establishment and maintenance of cellular polarity and plasticity, the two fields have developed essentially in parallel, with little interdisciplinary contact. Mechanisms of intracellular organelle transport have by now been sufficiently well established, as have the modes of action of various motor proteins that are underlying such mechanisms. In contrast, proteins responsible for RNA localization are only now being identified, and RNA-transporting molecular motors have remained elusive. Cross-disciplinary interactions between the areas of protein kinesis and RNA kinesis have been informal and sporadic. It was therefore the explicit intent of the National Academy Colloquium to overcome this fragmentation by providing a formal joint forum for scientific exchange between these disciplines.

Motor proteins such as myosins, dyneins, and kinesins are the engines of intracellular molecular transport. Kinesins in particular are seen as major movers in neurons as they have been implicated in microtubule-based transport in both axons and dendrites (1, 2). Kinesins form a rather large super family, and the individual superfamily proteins operate as motor molecules in various cell types with diverse cargoes. Given that transportation requirements are particularly demanding and complex in neurons, it does not come as a surprise that the highest diversity of kinesins is found in brain. In neurons, kinesin and dynein motor molecules have not only been implicated in intracellular axonal and dendritic transport, but also in neuronal pathfinding and migration (1). Given the various fundamental cellular functions they subserve in neurons, such mechanisms, should they become defective, also can be expected to contribute to onset or progression of neurological disorders.

In eukaryotic cells, the flow of information originates in the nucleus. Subsequent to its export into the cytoplasm, an mRNA may be translated in the perikaryal somatic region, or it may continue its travel to distant extrasomatic destinations for local on-site translation. These mechanisms may not be mutually exclusive for any given mRNA, but it is assumed that while en route, mRNAs are not actively translated. Many mRNAs, including those that are transported to and translated at extrasomatic target sites, are likely to be subject to specific translational control. Significant progress has been made in recent years in the functional dissection of translation initiation complexes and pathways (3–5), and it appears plausible, in view of such work, that translation initiation mechanisms play important roles in the

regulation of protein synthesis both at perykaryal somatic and at distant extrasomatic sites.

The analysis of RNA transport and localization has in recent years matured into a novel discipline in cell biology and neuroscience. In traditional cell biology, proteins are manufactured in the perikaryal soma and subsequently delivered to their respective sites of function. Although this may often be so, it is now accepted that this scenario does not necessarily represent the whole story. In diverse cell types, RNAs have been identified that are targeted to specific subcellular locations for on-site translation (6). In 1982, the first such localized mRNA, encoding myelin basic protein, was identified in oligodendrocytes (7). Subsequently, RNA localization was documented also in Xenopus oocytes, Drosophila embryos, and a variety of somatic eukaryotic cell types ranging from fibroblasts to neurons (8–12).

In neurons, localized RNAs were discovered rather late, and only after the presence of polyribosomes in postsynaptic dendritic microdomains (13, 14) had already been documented for a while. The first three RNAs identified in dendrites were the mRNAs encoding MAP2 (15) and CaMKIIa (16) as well as BC1 RNA, a noncoding RNA polymerase III transcript (17). These were joined by neuropeptide-encoding transcripts in the axonal domain (18). Today, research is focused on the mechanism of RNA transport in neuronal processes and on the elucidation of the signals involved—both at the level of RNA (cis-acting elements) and proteins (trans-acting factors). This work eventually will shed light on how a neuron administers translation of a distinct mRNA at or near a synapse in an input-specific and activity-dependent manner (11, 19).

In terms of subcellular location, the ultimate and critical determinant of cellular function is of course a correct spatio-temporal expression pattern of the protein repertoire, regardless of whether any given protein is delivered from the perikaryon or synthesized locally on site. Consequently, given the paramount importance of subcellular “location” in particular in neurons, protein targeting and anchoring mechanisms will directly impact long-term neuronal plasticity and are likely to figure prominently in the development of neurological disorders. In this respect, the discovery of novel scaffolding multidomain proteins that are involved in the functional organization of the postsynaptic density has significantly furthered our understanding of how signal transduction pathways might be regulated at the synapse. Activity-dependent modification of protein structure, location, and/or interaction may be essential for the molecular reorganization of postsynaptic functional architecture (20, 21). In addition, local translation of mRNA(s) encoding one or several of the scaffolding proteins also may contribute to the dynamic plasticity at a postsynaptic specialization after stimulation (21).

It then appears that neurons, being among the spatially most extended and functionally most complex of all eukaryotic cells, have to cope with organizational tasks that are indeed reminiscent of those associated with the maintenance and development of large metropolitan areas. And it thus holds true for cities and cells alike that the larger and more complex they are, the more relevant becomes an old New Yorker real estate adage that of all determinants of functional value, none are more important than the following three: location, location, location.

We thank the National Academy of Sciences for encouragement in planning this colloquium and for generous financial and administrative support. We also thank Mr. E.Patte of the National Academy of Sciences Executive Office and Ms. M.Gray-Kadar of the Beckman Center for their help in organizing the meeting and the National Academy for providing the excellent resources and facilities of the Arnold and Mabel Beckman Center in Irvine.

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