The vertebrate peripheral nervous system (PNS) has significant capacity for axon regeneration. Despite this capacity, regenerating axons often fail to reach their target tissues, leaving patients with persistent pain, loss of sensation, or paralysis. Thus, understanding the underlying molecular-genetic mechanisms that promote sustained and directed growth of regenerating axons will generate a strong foundation for therapeutic applications aimed to promote functional PNS recovery. The Granato laboratory has established a system to laser transect larval zebrafish peripheral nerves and monitor regeneration in a live vertebrate, in real time. The lab recently made a surprising discovery and identified the low-density lipoprotein receptor (LDLR)-related protein 4 (Lrp4) receptor to be required for motor axon regeneration in vivo. In lrp4 mutant zebrafish, nerves and associate Schwann cells develop normally but transected motor axons fail to regrow and Schwann cells adopt an abnormal, granular morphology that persists after injury. These observations are consistent with the idea that Lrp4 plays a critical role to sustain robust axon regeneration, possibly though interactions between axons and Schwann cells. In development, lrp4 is expressed on skeletal muscle and stimulates MuSK receptor signaling to initiate synapse formation. Surprisingly, we find that the MuSK receptor is dispensable for axon regeneration, indicating that Lrp4 functions independently of the canonical MuSK pathway to promote axon regeneration. Furthermore, we find that an lrp4 allele lacking the transmembrane region disrupts synapse development but not axon regeneration, strongly arguing that secreted Lrp4 promotes axon regeneration. Finally, transgenic, muscle specific expression of lrp4 in lrp4 mutants restores synaptic development but not regeneration. Taken together, these findings provide very strong evidence that Lrp4 promotes axon regeneration through mechanisms largely independent of those critical for synapse development. The goal of this proposal is to 1) determine the biologically relevant cell-type for Lrp4 to promote axon regeneration and 2) to identify the ligand and binding partners that mediate Lrp4-dependent signaling in regeneration. Combined, the proposed studies will make significant contributions to the fundamental science of how injured PNS axons robustly regrowth towards their original targets. This will result in a better understanding of peripheral nerve regeneration across the board and will help to address the urgent therapeutic needs for patients suffering from peripheral neuropathies caused by diabetes, injury, and autoimmune disorders. Finally, the proposed project will greatly expand my research experience at the ?hands on level? as I will learn to use cutting edge methods to manipulate the zebrafish genome, utilize genetic mutants and visualize axon regeneration in a live intact vertebrate animal, and equally important will enable me to build a solid foundation for my long-term goals to elucidate the molecular mechanisms that promote axon regeneration.
Incomplete regeneration of peripheral nerves, in cases of spinal cord injury, diabetic neuropathy, or other peripheral neuropathies, often results in persistent pain, loss of sensation, or paralysis. This work will address the cellular and molecular mechanisms that promote axon regeneration in vertebrates. Insights from this study will inform clinical strategies to promote functional nerve regeneration in cases of injury or disease.