ER and SOCE Ca²⁺ signals are not required for directed cell migration in human iPSC-derived microglia

The central nervous system (CNS) is constantly surveilled by microglia, highly motile and dynamic cells deputed to act as the first line of immune defense in the brain and spinal cord. Alterations in the homeostasis of the CNS are detected by microglia that respond by extending their processes or - following major injuries - by migrating toward the affected area. Understanding the mechanisms controlling directed cell migration of microglia is crucial to dissect their responses to neuroinflammation and injury. We used a combination of pharmacological and genetic approaches to explore the involvement of calcium (Ca²⁺) signaling in the directed migration of human induced pluripotent stem cell (iPSC)-derived microglia challenged with a purinergic stimulus. This approach mimics cues originating from injury of the CNS. Unexpectedly, simultaneous imaging of microglia migration and intracellular Ca²⁺ changes revealed that this phenomenon does not require Ca²⁺ signals generated from the endoplasmic reticulum (ER) and store-operated Ca²⁺ entry (SOCE) pathways. Instead, we find evidence that human microglial chemotaxis to purinergic signals is mediated by cyclic AMP in a Ca²⁺-independent manner. These results challenge prevailing notions, with important implications in neurological conditions characterized by perturbation in Ca²⁺ homeostasis.