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

Abstract

The Vesicle-associated membrane protein (VAMP)-Associated Protein B (VAPB) is the causative gene of amyotrophic lateral sclerosis 8 (ALS8) in humans. Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by selective death of motor neurons leading to spasticity, muscle atrophy and paralysis. VAP proteins have been implicated in various cellular processes, including intercellular signalling, synaptic remodelling, lipid transport and membrane trafficking and yet, the molecular mechanisms underlying ALS8 pathogenesis remain poorly understood. We identified the conserved phosphoinositide phosphatase Sac1 as a Drosophila VAP (DVAP)-binding partner and showed that DVAP is required to maintain normal levels of phosphoinositides. Downregulating either Sac1 or DVAP disrupts axonal transport, synaptic growth, synaptic microtubule integrity and the localization of several postsynaptic components. Expression of the disease-causing allele (DVAP-P58S) in a fly model for ALS8 induces neurodegeneration, elicits synaptic defects similar to those of DVAP or Sac1 downregulation and increases phosphoinositide levels. Consistent with a role for Sac1-mediated increase of phosphoinositide levels in ALS8 pathogenesis, we found that Sac1 downregulation induces neurodegeneration in a dosage-dependent manner. In addition, we report that Sac1 is sequestered into the DVAP-P58S-induced aggregates and that reducing phosphoinositide levels rescues the neurodegeneration and suppresses the synaptic phenotypes associated with DVAP-P58S transgenic expression. These data underscore the importance of DVAP–Sac1 interaction in controlling phosphoinositide metabolism and provide mechanistic evidence for a crucial role of phosphoinositide levels in VAP-induced ALS.

Emerging mechanisms of molecular pathology in ALS

“…Secreted VAPB is a ligand for Eph receptors; its secretion is blocked by introduction of ALS-associated mutations (120). In drosophila, expression of mutant VAPB induced deficits in synaptic morphology and neurodegeneration (121).…”

Mitochondria and endoplasmic reticulum crosstalk in amyotrophic lateral sclerosis

“…Phosphoinositide phosphatase Sac1 is another fly VAP binding partner. In a fly model of ALS8, mutant VAP caused neurodegeneration associated with increased phosphoinositide levels, which could be prevented by reducing phosphoinositide levels (Forrest et al, 2013). …”

VAMP-associated Proteins (VAP) as Receptors That Couple Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Proteostasis with Lipid Homeostasis

“…However, the loss of dVAP can be partially compensated by the expression of ORP8, an FFAT-independent ER-anchored ORP (36). Thus VAP-FFAT binding and lipid transfer activities are likely involved in regulating cellular proteostasis and might contribute to the development of ALS (38). We therefore hypothesize that VAPs function as ER-localized receptors, which couple proteostasis with lipid homeostasis.…”

“…A P56S mutation in the MSP domain of VAPB destabilizes the domain and supports the aggregation of VAPB with VAPA leading to a general loss of function. Deletion of Drosophila dVAP is manifested as a proteostasis disease, leading to the accumulation of protein aggregates and the build up of ER quality control compartments characteristic of inhibited protein degradation (37)(38)(39). The cytosolic MSP domain of dVAP is cleaved and secreted (using an unconventional secretion pathway) serving as an activating ligand for Eph receptors (40,41).…”