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 post-transcriptional fine-tuning spastin activity, particularly in the context of calcium signaling, remain elusive. Here, we identified Ca/calmodulin-dependent protein kinase II beta isoform (CaMKIIβ) 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 poly-ubiquitination 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. This work uncovers a novel molecular mechanism that links calcium signaling to cytoskeletal remodeling and neural function. We demonstrate that CaMKIIβ phosphorylates spastin, enhancing its stability by reducing polyubiquitination and proteasomal degradation. This post-transcriptional regulation increases spastin's microtubule-severing activity, thereby promoting neurite outgrowth in hippocampal neurons. Furthermore, inhibition of CaMKIIβ or spastin impairs synaptic transmission and cognitive performance, highlighting their critical roles in neuronal development and function. Overall, the study identifies CaMKIIβ as a key upstream regulator of spastin, offering new insights into how calcium influx governs neurite extension and memory-related behavior, with potential implications for neurological disease mechanisms and therapeutic strategies.