Abstract

The generation of all neurons and glial cells that populate the central nervous system (CNS) depends on the complex biology of neural stem cells (NSCs). Defects in the proliferative capacity or in the balance between self-renewal and differentiation of NSCs can lead to neurodevelopmental disorders such as tuberous sclerosis, hydrocephalus, autism, epilepsy and schizophrenia. Thus, investigation of the genes and mechanisms governing NSCs biology is relevant to understand the pathogenesis of several disorders affecting brain development. Soluble N-ethylmaleimide-sensitive fusion (NSF) attachment protein-alpha (α-SNAP) has traditionally been implicated in intracellular SNARE (SNAP receptor)-mediated membrane fusion by recruiting/activating the ATPase NSF and disassembling SNARE complexes. Deletion of the gene encoding for α-SNAP (Napa) in mice is lethal at early embryonic stages, but the naturally occurring hyh (hydrocephalus with hop gait) missense mutation of Napa causes an autosomal recessive disorder characterized by (i) disruption of the neuroepithelial organization of NSCs, (ii) cortical, hippocampal and cerebellar defects, and (iii) severe hydrocephalus. What is the link between the loss of function of an ubiquitous protein such as α-SNAP and the neuropathology of hyh mouse? Some recent evidence obtained in our lab demonstrates that a disruption in N-cadherin-based adherens junctions between NSCs and abnormal cell polarity during the neurogenic period precedes the pathological hyh phenotype. In this context, it could be assumed that disruption of adherens junctions and cell polarity in hyh mice is a consequence of defects in membrane trafficking. However, studies published in the last two years suggest that, in addition to the canonical (NSFdependent) role on SNARE-mediated membrane fusion, α-SNAP has a broader repertoire of ‘novel’ noncanonical (NSF-independent) biological functions. Indeed, it has been shown that α-SNAP modulates apoptosis, AMPK activity and autophagy in different cell types. Are defects in non-canonical functions of α-SNAP involved in the pathogenesis of hyh phenotype? Our hypothesis is that “canonical (NSF-dependent) and non-canonical (NSF-independent) roles of α-SNAP are key determinants of NSCs biology during the neurogenic period”. Thus, in hyh mice (α-SNAP mutants), defects in both canonical (i.e. membrane trafficking/fusion) and noncanonical (i.e. modulation of autophagy and AMPK activity) roles of α-SNAP would be the pathogenic mechanism leading to an aberrant neurogenic process and abnormal brain development. To test this hypothesis, the goals of the present proposal are: (1) to characterize biochemical properties of α-SNAP WT and hyh (M105I) mutant, focusing in protein-lipid and protein-protein interactions relevant for the canonical and non-canonical roles of α-SNAP, (2) to study the (canonical) role of α-SNAP (WT and M105I) on N-cadherin trafficking dynamics, (3) to evaluate the (non-canonical) role of α-SNAP on AMPK activity and autophagy in WT and hyh mice, and (4) to analyze the consequences (rescue?) of modulating AMPK activity and/or autophagy in the hyh phenotype (cellular and tissue level). We plan to identify biochemical defects in α-SNAP M105I by GST-pulldown, co-immunoprecipitation, and liposome binding assays. We will study the canonical role of α-SNAP WT and M105I by analyzing the trafficking dynamics ( Golgi-to-surface and recycling) of N-cadherin. Further, the non-canonical role of α- SNAP on AMPK activity (at cell population and single cell level) and autophagy (autophagosome formation, autophagic flux, translational regulation, role of AMPK/mTOR) will be investigated in WT and hyh mice (brain tissue samples, MEFs and cultured NSCs). Finally, using AMPK and autophagy modulators, we will test the role of these mechanisms in the pathogenesis of hyh phenotype. In summary, this project aims (i) to build a bridge between α-SNAP dysfunction and hyh pathology, and (ii) to increase our understanding of the molecular basis of neurodevelopmental disorders.