Kindlin-1 enhances axon growth on inhibitory chondroitin sulfate proteoglycans and promotes sensory axon regeneration

Growing and regenerating axons need to interact with the molecules in the extracellular matrix as they traverse through their environment. An important group of receptors that serve this function is the integrin superfamily of cell surface receptors, which are evolutionarily conserved αβ heterodimeric transmembrane proteins. The function of integrins is controlled by regulating the affinity for ligands (also called "integrin activation"). Previous results have shown that CNS inhibitory molecules inactivate axonal integrins, while enhancing integrin activation can promote axon growth from neurons cultured on inhibitory substrates. We tested two related molecules, kindlin-1 and kindlin-2 (Fermitin family members 1 and 2), that can activate β1, β2, and β3 integrins, for their effects on integrin signaling and integrin-mediated axon growth in rat sensory neurons. We determined that kindlin-2, but not kindlin-1, is endogenously expressed in the nervous system. Knocking down kindlin-2 levels in cultured sensory neurons impaired their ability to extend axons, but this was partially rescued by kindlin-1 expression. Overexpression of kindlin-1, but not kindlin-2, in cultured neurons increased axon growth on an inhibitory aggrecan substrate. This was found to be associated with enhanced integrin activation and signaling within the axons. Additionally, in an in vivo rat dorsal root injury model, transduction of dorsal root ganglion neurons to express kindlin-1 promoted axon regeneration across the dorsal root entry zone and into the spinal cord. These animals demonstrated improved recovery of thermal sensation following injury. Our results therefore suggest that kindlin-1 is a potential tool for improving axon regeneration after nervous system lesions.