Axon guidance deregulation in ALS and frontotemporal dementia

About This Grant

Project Summary/Abstract The objective of this R21 application is understand how axon guidance genes contribute to the neuropathogenesis of amyotrophic sclerosis (ALS) and frontotemporal dementia (FTD), two devastating neurodegenerative disorders that lie on either end of a clinical disease spectrum. Among more than 40 different genes implicated in ALS/FTD, X-linked mutations in the ubiquitin (Ub) chaperone UBQLN2 cause diverse neurologic phenotypes, ranging from pure ALS with or without FTD, to primary lateral sclerosis and spastic paraplegia. ALS/FTD-associated mutations in UBQLN2 promote its aggregation; however, the mechanisms whereby UBQLN2 mutations instigate neurodegeneration in ALS/FTD are unclear. Our group employs a “fly to human” approach in which genetic modifier screens in Drosophila yield candidate UBQLN2 toxicity pathways that are then tested for phenotypic modulation in inducible pluripotent stem cells (iPSC)-derived motor neurons (iMNs) containing ALS-associated mutations in UBQLN2. One key to these studies has been the development of a combinatorial UBQLN2 mutant (UBQLN24XALS) harboring four different clinical mutations that further reduce its solubility, providing a sensitized system for the identification of UBQLN2 modifier pathways in flies and iMNs. Unbiased genetic screens identified the Unc-5 and Frazzled (Fra) axon guidance genes as determinants of UBQLN24XALS toxicity in flies. Fra is an attractive netrin receptor that promotes the movement of neuronal axons toward netrin ligands while Unc- 5 binding to netrin promotes axonal repulsion. Unc-5 and Fra also function as neuronal dependence receptors that promote apoptotic cell death in the absence of ligand binding. Suppression of human Unc- 5 orthologs UNC5B and UNC5C, or the Fra ortholog DCC (deleted in colon carcinoma) suppressed toxicity phenotypes in human inducible motor neurons (iMNs) harboring a UBQLN24XALS knockin mutation, suggesting that UNC5 and DCC are conserved mediators of UBQLN2 neurotoxicity. Based on these findings we propose that UNC5 and DCC signaling plays a deleterious role in ALS/FTD-UBQLN2 and further hypothesize that aberrant UNC5-DCC signaling may be a disease driver in both genetic and sporadic forms of ALS/FTD. These ideas will be tested using iMN models and a novel humanized UBQLN24XALS knockin mouse model that recapitulates histopathologic and behavioral features seen in UBQLN2-ALS/FTD patients. The objectives of the proposal are to: (i) Define the mechanism of UBQLN2 toxicity suppression by UNC5 and DCC inhibition; and (ii) Test whether UNC5 signaling contributes to neurodegenerative phenotypes in 4XALS mice. Successful completion of these aims will provide mechanistic insights into ALS/FTD-UBQLN2 and prompt further study of UNC5, DCC, and their netrin ligands as therapeutic targets for the treatment of ALS/FTD.