Investigating the Role of the ALS Causative Gene, VAPB, in Organelle Dynamics and Neuronal Differentiation

The organization and communication between cellular organelles are critical for maintaining homeostasis and supporting specialized functions, such as neuronal differentiation. Vesicle-associated membrane protein-associated B (VAPB) is a resident endoplasmic reticulum (ER)-tether protein for many organelle interactions and has been implicated in familial amyotrophic lateral sclerosis (ALS). Previous literature has established the role of VAPB in mediating ER-organelle membrane contact sites (MCS) but has yet to characterize how disruption in VAPB function impacts organelle dynamics. We used multispectral imaging in VAPB knockout human induced pluripotent stem cells (hiPSCs) to systematically assess how the disruption of VAPB-mediated contacts affect organelle morphology and interactions. Later we investigated the effect of the deletion of VAPB on the morphology and viability of developing hiPSC-derived neurons (iNeurons). We find differences in organelle number, volume, and interactions in VAPB DEL-DEL hiPSCs, that affect the viability of mature iNeurons. We observed that individual contact volumes decreased in many ER-organelle contacts. This finding suggests that the disruption of VAPB function may affect the structural integrity of the MCS formed. However, many contact frequencies and volumes remain conserved or upregulated in the VAPB DEL-DEL iPSCs, suggesting the presence of a compensatory response mediated by other tethering proteins, perhaps a paralog of VAPB, like VAPA. Moreover, iNeurons differentiated from VAPB DEL-DEL hiPSCs showed increased cell death at the stage of neuronal maturation, indicating defective differentiation and maturation when VAPB is absent. Together, these findings provide insight into how disruption in VAPB function perturbs organelle structure and neuronal differentiation.