Timothy Gomez, Ph.D. Associate Professor Department of Anatomy tmgomez@wisc.edu Gomez Lab Web Site

Research Description:

Work in my laboratory focuses on the intracellular signaling mechanisms that regulate growth cone motility and guidance. Growth cones are sensory-motor specializations at the tips of developing axons and dendrites, which detect and transduce extracellular cues into guided extension. Guidance of growing axons to their proper synaptic targets sites serves a crucial early step in the development of specific synaptic connectivity. Great advances have been made in recent years in our understanding of the factors that contribute to guided axon extension. Many new classes of ligands and their receptors have been discovered and we are beginning to appreciate how growth cones integrate multiple extracellular stimuli and convert those signals into stereotyped behaviors (Figure 1). However, it is clear that the number of specific synaptic connections far exceeds the number of guidance cues and receptors that are expressed by neurons. Therefore, epigenetic mechanisms, such as biochemical signal cascades, must provide additional information that is required to organize the highly complex interconnections of the adult nervous system.

Research in my laboratory combines a variety of fluorescent probe technologies with confocal microscopy to visualize the dynamic behavior of living growth cones and assess their physiological responses during axon extension in vitro and guided outgrowth in the intact spinal cord or retinotectal pathway. We use the African Clawed frog Xenopus Laevis as our model system due to the large size, rapid development, and ease of molecular and surgical manipulation of its embryos. Manipulation of protein expression, as well as photolytic uncaging techniques are used to alter the physiology of growth cones both in vitro and in vivo. By combining the latest advances in imaging technologies with improved optical probes including fluorescent fusion proteins and FRET-based reporter molecules, we are addressing questions regarding the molecular basis of axon outgrowth and guidance.

Figure 1: Soluble and bound extracellular factors guide axon outgrowth

Figure 2: Sites of calcium transient activity (red) colocalize with integrin receptor clusters (blue) in growth cone filopodia (for details see Gomez et al. (2001) Science 291, 1983).	Figure 3: An embryonic Xenopus spinal cord and notochord (below) double-labeled for ß tubulin (red) and actin (green). Individual image slices were captured on a laser scanning confocal microscope and projected into a single image as a maximum projection.

Current Projects:

Several ongoing research projects in my laboratory are focused on the intracellular signaling cascades activated by diffusible, cell surface and extracellular matrix associated guidance cues. One intracellular signal that has particularly diverse effects on axonal and dendritic growth is cytosolic calcium (Ca2+) ions (Figure 2). Although Ca2+ signaling clearly has profound influences on the motility of a variety of cell types from many species, we still have little mechanistic understanding of how Ca2+ exerts such diverse affects. One current focus of my laboratory is in the identification of novel plasma membrane Ca2+ channels that are activated by mechanical stretch. We have evidence that Ca2+ influx through stretch-activated channels (SACs) slows outgrowth both in vitro and in vivo. We are also interested in the downstream targets of Ca2+ signals that both positively and negatively influence axon outgrowth. We have evidence that the Ca2+-dependent protease calpain negatively influences outgrowth in opposition to tyrosine kinase signaling that positively influence outgrowth. Other positive and negative influences being studied in my laboratory are: Rho family GTPases, tyrosine phosphorylation by FAK and Src, lipid signaling, regulation by protein synthesis, regulation of point contact dynamics, and endocytic cycling. With our studies, we hope to provide a more complete understanding of how growth cones integrate diverse extracellular influences into the stereotyped motile behavior required to build a functional nervous system (Figure 3).

Calcium transients in growth cone filopodia

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Selected Publications: Articles on PubMed

• Pal S, Wu J, Murray JK, Gellman SH, Wozniak MA, Keely PJ, Boyer ME, Gomez TM, Hasso SM, Fallon JF, Bresnick EH. (2006). An antiangiogenic neurokinin-B/thromboxane A2 regulatory axis. J Cell Biol. 174:1047-1058. Erratum in: J Cell Biol. 175:671. PMID 17000881
  
  
• Robles E and Gomez TM. (2006). Focal adhesion kinase signaling at sites of integrin-mediated adhesion controls axon pathfinding. Nat Neurosci. 9:1274-1283. PMID 16964253
  
  
• Jacques-Fricke BT, Seow Y, Gottlieb PA, Sachs F, and Gomez TM. (2006). Opposing regulation of neurite growth by Ca2+ influx through a mechanosensitive channel and release from intracellular stores. J Neurosci. 26:5656-5664. PDF PMID 16723522
  
  
• Woo SW and Gomez TM. (2006). Rac1 and RhoA promote neurite outgrowth through formation and stabilization of growth cone point contacts. J Neurosci. 26:1418-1428. PDF PMID 16452665

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