CaMKIIβ-mediated Phosphorylation Enhances Protein Stability of Spastin to Promote Neurite Outgrowth

Neurite outgrowth is critically controlled by calcium influx-mediated cytoskeleton dynamics. Spastin, a AAA ATPase microtubule-severing protein, also plays an important role in neurite outgrowth. However, the detailed mechanisms underlying posttranscriptional fine-tuning spastin activity, particularly in the context of calcium signaling, remain elusive. Here, we identified that the Ca²⁺/calmodulin-dependent protein kinase II beta (CaMKIIβ) isoform acted as an upstream kinase to mediate the phosphorylation of spastin. CaMKIIβ interacted with and phosphorylated spastin on Ser233 and Ser562 amino acids. Moreover, CaMKIIβ-mediated phosphorylation reduced the polyubiquitination level of spastin and suppressed its proteasomal degradation. This enhanced protein stability by CaMKIIβ increased the microtubule-severing activity of spastin and coordinately promoted the neurite outgrowth in hippocampal neurons. Inhibition of spastin or CaMKIIβ impaired synaptic activity, as evidenced by reduced frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs). Behaviorally, treatment with spastin or CaMKIIβ inhibitors led to deficits in short-term working memory and spatial learning, as assessed by Y-maze and Morris water maze tests in male mice, respectively. In general, this study unveils a novel mechanism whereby CaMKIIβ-mediated phosphorylation of spastin connects extracellular calcium signaling to the regulation of cytoskeleton dynamics and neurite outgrowth.