Increased levels of phosphoinositides cause neurodegeneration in a drosophila model of amyotrophic lateral sclerosis

The human VAMP-associated protein B (hVAPB) has been shown to cause a range of motor neurodegenerative diseases, including amyotrophic lateral sclerosis 8 (ALS8) and spinal muscular atrophy (SMA). However, the molecular mechanisms underlying VAPB-induced neurodegeneration remain elusive. We sought to address this question by identifying VAPB interacting proteins, which may be affected by the disease causative mutations. Using a combination of biochemical and genetic approaches in Drosophila, we confirmed the evolutionarily conserved phosphoinositide phosphatase Sac1 (Suppressor of Actin 1), as a DVAP binding partner and showed that the two proteins colocalise in the endoplasmic reticulum. We also show that DVAP function is required to maintain normal levels of phosphoinositides (PIs) and that downregulation of either Sac1 or DVAP at the larval neuromuscular junction (NMJ) affects a number of synaptic processes, including axonal transport, synaptic growth, microtubule integrity and localisation of several postsynaptic components. We found that double knock down of DVAP and Sac1 induces no further increase in the severity of the mutant phenotypes when compared to either single mutant alone. This, together with the similarity in mutant phenotypes, indicates that the two genes function in a common pathway. In flies carrying the ALS8 mutation (DVAP-P58S), we observed reduced viability, locomotion defects and early death in surviving adults, closely matching the phenotypes of both DVAP and Sac1 downregulation. Additionally, transgenic expression of DVAP-P58S in the motor system elicits synaptic defects similar to those of either Sac1 or DVAP loss-of-function, including an increase in the levels of PtdIns-4-Phosphate (PI4P), the substrate of Sac1. Consistent with these observations, we found that Sac1 is sequestered into DVAP-P58S mediated aggregates and that downregulation of PI4P in neurons rescues the neurodegenerative and the synaptic phenotypes associated with DVAP-P58S transgenic expression. Together our data unveil a previously unknown function for Sac1 in neurodegeneration and synaptic function, as well as provide evidence for a dominant negative mechanism for phosphoinositide-mediated ALS8 pathogenesis. We also highlight a causative role for increased PI4P levels in VAPB-P56S induced neurodegeneration