LAR (leukocyte common-antigen-related) family receptors, members of the receptor protein tyrosine phosphatase (RPTP) superfamily, are important in the formation, plasticity, and regeneration of neuronal circuits. Considerable progress has been made in identifying LAR-interacting proteins, which form the basis for the molecular relays that underly LAR function. However, our understanding of intracellular LAR signaling in axon guidance remains fragmentary. In my thesis work, I identified a novel LAR-interacting protein, Caskin, and elucidated its role in motor axon guidance in the developing Drosophila embryo.
Drosophila caskin encodes a neuronal scaffolding protein containing two SAM domains and a conserved C-terminal region. Pan-neuronal caskin overexpression leads to vast axon guidance defects in both the CNS and in motor axons, implicating Caskin as a regulator of growth cone behavior. To define Caskin’s function in axon guidance, ethyl methanesulfonate (EMS)-induced caskin loss-of-function (LOF) mutants were generated and characterized. caskin LOF mutants exhibit motor axon guidance defects identical to Dlar LOF mutants. Further biochemical and genetic examination demonstrated Caskin interacts with Dlar via its first N-terminal SAM domain and relays LAR signaling via its conserved C-terminal region. I examined downstream signaling of Caskin in order to further understand how LAR signaling regulates the cytoskeleton. I conducted a yeast two-hybrid screen to search for potential binding partners that act through interaction with Caskin’s C-terminus. Through this screen I found several novel Caskin-interacting proteins, some of which are known to regulate the cytoskeleton.
Protein phosphorylation is important in motor axon guidance. An attractive model was previously proposed in which a common substrate, regulated by Abl kinase and LAR phosphatase, can provoke different axonal responses. Interestingly, I found that Caskin can become tyrosine phosphorylated in an Abl kinase-dependent manner and that Caskin and Abl interact genetically in the regulation of motor axon guidance. Further studies will seek to determine whether tyrosine phosphorylation is important in Caskin’s function.
Recently, PTPsigma, a LAR family member in vertebrates, was identified as a receptor for CSPGs, which are molecules that inhibit axonal regeneration. I found that interactions between Caskin and LAR are conserved in vertebrates; mouse Caskin 2 is capable of binding to LAR and PTPsigma. Taken together, my thesis work reveals a function for Caskin in LAR-mediated motor axon guidance and identifies a potential molecular target for intervention following nervous system injury.