Loss of INPP5K attenuates IP-induced Ca responses in the glioblastoma cell line U-251 MG cells.

INPP5K (inositol polyphosphate 5-phosphatase K) is an endoplasmic reticulum (ER)-resident enzyme that acts as a phosphoinositide (PI) 5-phosphatase, capable of dephosphorylating various PIs including PI 4,5-bisphosphate (PI(4,5)P), a key phosphoinositide found in the plasma membrane. Given its ER localization and substrate specificity, INPP5K may play a role in ER-plasma membrane contact sites. Furthermore, PI(4,5)P serves as a substrate for phospholipase C, an enzyme activated downstream of extracellular agonists acting on Gq-coupled receptors or tyrosine-kinase receptors, leading to IP production and subsequent release of Ca from the ER, the primary intracellular Ca storage organelle. In this study, we investigated the impact of INPP5K on ER Ca dynamics using a previously established INPP5K-knockdown U-251 MG glioblastoma cell model. We here describe that loss of INPP5K impairs agonist-induced, IP receptor (IPR)-mediated Ca mobilization in intact cells, while the ER Ca content and store-operated Ca influx remain unaffected. To further elucidate the underlying mechanisms, we examined Ca release in permeabilized cells stimulated with exogenous IP. Interestingly, the absence of INPP5K also disrupted IP-induced Ca release events. These results suggest that INPP5K may directly influence IPR activity through mechanisms yet to be resolved. The findings from this study point towards role of INPP5K in modulating ER calcium dynamics, particularly in relation to IP-mediated signaling pathways. However, further work is needed to establish the general nature of our findings and to unravel the exact molecular mechanisms underlying the interplay between INNP5K function and Ca signaling.