The antagonistic modulation of Arp2/3 activity by N-WASP/WAVE2 and PICK1 defines dynamic changes in astrocyte morphology

Astrocytes exhibit a complex, branched morphology, allowing them to functionally interact with numerous blood vessels, neighboring glial processes and neuronal elements, including synapses. They also respond to CNS injury by a process known as astrogliosis, which involves morphological changes including cell body hypertrophy and thickening of major processes. Following severe injury, astrocytes exhibit drastically reduced morphological complexity, and collectively form a glial scar. The mechanistic details behind these morphological changes are unknown.Here, we investigate the regulation of the actin-nucleating Arp2/3 complex in controlling dynamic changes in astrocyte morphology. In contrast to other cell types, Arp2/3 inhibition drives the rapid expansion of astrocyte cell bodies and major processes. This intervention results in reduced morphological complexity of astrocytes in both dissociated culture and in brain slices. We show that this expansion requires functional myosin II downstream of ROCK/RhoA. Knockdown of the Arp2/3 subunit Arp3 or the Arp2/3 activator N-WASP by siRNA also results in cell body expansion and reduced morphological complexity, whereas depleting WAVE2 specifically reduces the branching complexity of astrocyte processes. On the other hand, knockdown of the Arp2/3 inhibitor PICK1 increases astrocyte branching complexity. Furthermore, astrocyte expansion induced by ischemic conditions is delayed by PICK1 knockdown or N-WASP overexpression.Our findings identify a novel morphological outcome for Arp2/3 activation in restricting rather than promoting outward movement of the plasma membrane in astrocytes. Arp2/3 regulators PICK1 and N-WASP/WAVE2 function antagonistically to control the complexity of astrocyte branched morphology, and this mechanism underlies the morphological changes seen in astrocytes during their response to pathological insult.