Timothy Gomez

Position title: Principal Investigator | Professor

Email: tmgomez@wisc.edu

Phone: (608) 263-4554

RESEARCH INTERESTS - The long-term objective of our research is to better understand the intracellular signaling cascades and downstream molecular mechanisms responsible for the assembly of neural circuits during development. For this we must understand how developing axons and dendrites are guided to their proper targets where they assemble specific synaptic connections. Mutations in genes involved in axon guidance, circuit assembly and synaptic transmission are responsible for many deficits in cognitive function, including autisms, dyslexia and numerous other learning disabilities.

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.

Research in my laboratory combines a variety of fluorescent probe technologies with confocal and total internal reflection fluorescence (TIRF) microscopy to visualize the dynamic behavior of growth cones and assess their physiological responses during axon extension in vitro and guided outgrowth in the intact spinal cord. We use two model systems for our studies. First, we study spinal cord and retinal ganglion cell (RGC) neuron development using the African Clawed frog Xenopus Laevis due to the large size, rapid development, and ease of molecular and surgical manipulation of its embryos. Second, we are studying the development of human forebrain, motoneuron and RGCs using neurons derived from human induced pluripotent stem cells (iPSCs). Various gain and loss of function techniques are used to alter the physiology of growth cones both in vitro and in vivo. In addition, we are using iPSCs derived from human patients with various autism spectrum disorders. By combining the latest advances in imaging technologies with improved optical probes including fluorescent fusion proteins and FRET-based reporter molecules we hope to answer the following questions:

1. How does tyrosine kinase signaling by Src and FAK non-receptor tyrosine kinases regulate filopodial protrusion and adhesion dynamics downstream of axon guidance cues?

2. How does calcium influx and release through specific channels exert differential affects on neurite outgrowth.

3. What types of mechanosensitive Trp channels are expressed by developing neurons and what roles do they play in axon guidance and regeneration.

4. How does local protein synthesis control axon guidance in human neurons and what roles do autism related genes such as FMRP and TSC have in the control of local protein synthesis downstream of axon guidance cues.

Figure 1.(below – click to enlargeGrowth cones assemble macromolecular adhesion complexes (point contacts) that link ECM proteins to actin filaments. A. A Xenopus spinal neuron growth cone immuno-labelled for phospho-Tyr118-paxillin (green) and F-actin (red). Note multiple point contact adhesions along actin filaments (arrows). B. A Xenopus spinal neuron growth cone immuno-labelled for b-tubulin (green) and F-actin (red). Note that microtubules may regulate the trafficking of vesicles containing integrin and guidance cue receptors. Scale, 5 mm. C. Schematic representation of a growth cone (adapted from (Kamiguchi and Lemmon, 2000)) on the ECM with several integrin receptors (blue) linked to actin filaments through adhesion complexes (green). Integrin receptor trafficking within recycling endosomes (blue vesicles) along microtubules (dark green) may regulate axon guidance (see text for details) A guidance cue/growth factor receptor is illustrated on the apical surface (orange). D. Schematic representation of key molecular components of growth cone point contact adhesions. Integrin ab heterodimeric receptors (dark blue lines) bind to proteins within the ECM, such as Col, LN and FN. Integrin activation leads to the assembly of multiple scaffolding proteins, such as talin, paxillin and vinculin to the cytoplasmic tail of integrins. In addition, FAK and Src are activated by clustering of integrin receptors, and they modulate the composition of adhesions through phosphorylation of key residues that allow for binding of many additional proteins (not shown). Several scaffolding proteins bind directly to actin filaments (red), which is believed to restrain retrograde flow and allow the force of actin polymerization to generate membrane protrusion. Guidance cue receptors (orange) can also regulate adhesion-associated proteins through binding and activation of FAK and Src. Cross-talk through FAK/Src signaling modulates adhesion assembly and turnover, as well as regulation of the actin cytoskeleton.

Fig 1

Figure 2.(below) Time-lapse movie of a Fluo-4 loaded Xenopus spinal neuron growth cone exhibiting frequent filopodial calcium transients and less frequent global growth cone transients. Note that the frequency of calcium transients is controlled by substratum-dependent mechanical signaling through TRP channels. Images were collected every 0.5 sec and playback is at 14X. See Gomez et al. (2001) Science; Jacques-Fricke et al. (2006) J Neurosci.

Selected Publications

  • Santiago-Medina, M., Gregus, K. A., Nichol, R.H. and Gomez, T. M. (2015). Regulation of ECM degradation and axon guidance by growth cone invadosomes. Development, Feb 1, 142(3):486-496.
  • Doers ME, Musser MT, Nichol R, Baker MW, Berndt ER, Gomez TM, Zhang SC, Abbeduto L, Bhattacharyya A. (2014). Induced pluripotent stem cell derived forebrain neurons from FXS individuals show defects in initial neurite outgrowth. Stem Cells Dev. March 24. Epub ahead of print.
  • Gomez, T. M. and Letourneau, PC. (2014). Actin dynamics in growth cone motility and navigation. J Neurochem, Oct 24. Epub ahead of print.
  • Santiago-Medina, M., Gregus, K. A. and Gomez, T. M. (2013) PAK-PIX interactions regulate adhesion dynamics and membrane protrusion to control neurite outgrowth. J Cell Science, Jan 15, Epub ahead of print.
  • Kerstein, P., Jacques-Fricke, B., Rengifo, J., Mogen, B., Williams, J., Gottlieb, P., Sachs, F. and Gomez, T. M. (2013) Mechanosensitive TRPC1 channels promote calpain proteolysis of talin to regulate spinal axon outgrowth J Neurosci. Jan 2, 33(1): 273-285. PMID: 23283340
  • Myers, J. P., Robles, E., Ducharme-Smith, A. and Gomez, T. M. (2012) Focal adhesion kinase modulates Cdc42 activity downstream of positive and negative axon guidance cues J of Cell Science. Jun, 5;125(Pt 12):2918-2929. PMID: 22393238
  • Santiago-Medina, M., Myers, J. P. and Gomez, T. M. (2011) Imaging adhesion and signaling dynamics in Xenopus laevis growth cones. Dev. Neurobio. Apr 4, Epub ahead of print. PMCID: 3158960
  • Myers, J. P. and Gomez, T. M. (2011) Focal adhesion kinase promotes integrin adhesion dynamics necessary for chemotropic turning of nerve growth cones. J Neurosci. 21 September; 31(38):13585-13595. PMCID: 3193056
  • Myers, J. P., Santiago-Medina, M. and Gomez, T. M. (2011) Regulation of axonal outgrowth and pathfinding by integrin-ECM interactions. Dev. Neurobio. 71 (11): 901-923. PMCID: 3192254
  • Moon, M-s and Gomez, T. M. (2010) Balanced Vav2 GEF activity regulates neurite outgrowth and branching in vitro and in vivoMol. Cell. Neurosci. Jun;44(2):118-128. PMCID: 2862809