Regulation of 3D Axon Guidance By Growth Cone Invadosomes
While extensive studies have investigated the molecular mechanisms that regulate axon guidance over two-dimensional substrata in vitro or along axonal tracks in vivo, little is known about the signals that control axon guidance across three dimensional tissues. Our preliminary and recently published data suggest that along with planar filopodia and lamellipodia, growth cones generate orthogonal protrusions in vitro and in vivo that resemble podosomes or invadopodia (Figure 1). Podosomes and invadopodia, collectively referred to as invadosomes, are actin-based cellular protrusions associated with extracellular matrix (ECM) degradation and tissue invasion. We hypothesize that growth cone invadosomes function to actively detect ligands through receptor interactions that regulate actin polymerization and participate in ligand and receptor degradation to modulate ligand-mediated guidance.
Current Open Questions:
1.How do grow factors and axon guidance cues influence invadosome formation?
2.What roles do scaffolding and signaling proteins such as Tks5, Cortactin and Src play in growth cone invadopodia formation and function?
3.What matrix metalloproteases and ADAM proteins are targeted to growth cone invadosomes and what roles do they play in basal lamina degradation and ligand activation?
4.How and where do invadosomes function in 3D axon guidance in vivo?
A growth cone labeled for F-actin (green) and microtubules (purple) has F-actin foci within its central domain (box). B) F-actin foci are believed to be precursors to invadosomes, which are basal protrusions that penetrate into the underlying substratum. C-D) 3D rendering view of Structured Illumination Microscope (SIM) images of a human forebrain neuron growth cone triple-labeled for F-actin (red), ßIII tubulin (blue) and ADAM17 (green). A view of the apical surface C) shows an F-actin foci in the central domain (arrowhead), which is revealed to be an apically directed protrusion in a lateral view D) with ADAM17 at the protrusion tip (arrowhead).