Modulation Of NGF Signaling By Caveolin-1 And Caveolin-2

Nerve Growth Factor (NGF) was the first discovered and the best characterized neurotrophic polypeptide. It has been shown to exert a variety of functions in the nervous system, which includes regulating neuronal specification, differentiation, survival, axon growth guidance, and neuronal plasticity. In addition to the above, NGF has also been shown to participate in other physiological processes, for example, in breast and prostate cancers, it is highly expressed and to be mitogenic for the tumor cells. Studies on NGF trafficking and signaling have continued to flourish underlining the importance and pertinence of further dissecting and understanding pathways for NGF signalling.NGF binds to and signals via two receptors:a receptor common to all neurotrophins, p75 Neurotrophin Receptor (p75NTR), member of the tumor necrosis factor receptor superfamily, and a receptor more specific to NGF, with a tyrosine kinase activity, Tropomyosine receptor kinase A (TrkA). At the plasma membrane, both TrkA and p75 receptors have been shown to localized to caveolae, specific subdomains that are enriched in cholesterol, sphingolipids and the presence of caveolin proteins. Caveolae has also been documented in the receptor endocytosis, cell signaling and breast cancer tumorigenesis. Cav-1 is the main component of caveolae, it can interact with numerous proteins involved in signal transduction, which results in compartmentalization of signaling molecules and their maintenance in an inactive conformation. Cav-2 is the other major component of caveolae, while being much less studied.The focus of this work is on this membrane microenvironment mediated modulation of NGF signaling. The signaling endosome hypothesis proposes that TrkA internalized into endosomal compartments would promote a sustained signalling from the growth cone along the axon, all the way to the cell body. In the present work we found that overexpression of Cav-1 in mouse dorsal root ganglia neurons significantly impacted neurite extension. Similarly, overexpression of Cav-1 in PC12 cells strongly inhibits their ability to grow neurites in response to NGF. It inhibits NGF signaling without, impairing transient MAPK pathway activation. Rather, it does so by sequestering NGF receptors in lipid rafts, which correlates with the cell surface localization of downstream effectors, and phosphorylated-Rsk2, resulting in the prevention of the phosphorylation of CREB. By contrast, overexpression of Cav-2 potentiates NGF induced differentiation, which is accompanied by sustained activation of downstream effectors, and standard internalization of the receptors. This differential effect could be due to the different localization of Caveolins, that results perturbing microenvironment, thereby affects the NGF signaling. Furthermore, PC12 cells expressing the non-phosphorylatable Cav-1 mutant (S80V), no longer present inhibited TrkA trafficking or CREB phosphorylation, but, on the contrary, behave much like Cav-2 PC12 cells.These results offer novel insight into the relationship between membrane microdomains, receptors, and caveolins, specific scaffolding proteins important for regulating the NGF signaling pathway. Finally, our results open the way for investigation of the impact of TrkA and Cav-1 interplay in other models, such as tumorigenesis.