HspB5/αB‐crystallin phosphorylation at S45 and S59 is essential for protection of the dendritic tree of rat hippocampal neurons

Rarefaction of the dendritic tree leading to neuronal dysfunction is a hallmark of many neurodegenerative diseases and we have shown previously that heat shock protein B5 (HspB5)/αB‐crystallin is able to increase dendritic complexity in vitro. The aim of this study was to investigate if this effect is also present in vivo, if HspB5 can counteract dendritic rarefaction under pathophysiological conditions and the impact of phosphorylation of HspB5 in this process. HspB5 and eight mutants inhibiting or mimicking phosphorylation at the three phosphorylation sites serine (S)19, S45, and S59 were over‐expressed in cultured rat hippocampal neurons with subsequent investigation of the complexity of the dendritic tree. Sholl analysis revealed significant higher complexity of the dendritic tree after over‐expression of wild‐type HspB5 and the mutant HspB5‐AEE. All other mutants showed no or minor effects. For in vivo investigation in utero electroporation of mouse embryos was applied. At embryonal day E15.5 the respective plasmids were injected, cornu ammonis 1 (CA1) pyramidal cells transfected by electroporation and their basal dendritic trees were analyzed at post‐natal day P15. In vivo, HspB5 and HspB5‐AEE led to an increase of total dendritic length as well as a higher complexity. Finally, the dendritic effect of HspB5 was investigated under a pathophysiological condition, that is, iron deficiency which reportedly results in dendritic rarefaction. HspB5 and HspB5‐AEE but not the non‐phosphorylatable mutant HspB5‐AAA significantly counteracted the dendritic rarefaction. Thus, our data suggest that up‐regulation and selective phosphorylation of HspB5 in neurodegenerative diseases may preserve dendritic morphology and counteract neuronal dysfunction. Heat shock protein HspB5 increases dendritic complexity of cultured rat hippocampal neurons in vitro and in pyramidal neurons of the hippocampus in vivo, which is dependent on phosphorylation at S45 and S59 (overexpression of mutant HspB5‐AEE). Furthermore, HspB5 and HspB5‐AEE but not the non‐phosphorylatable mutant HspB5‐AAA significantly counteracted the rarefaction of the dendritic tree in an in vitro model of deferoxamine (DFO) induced iron deficiency. Our data suggest that upregulation and selective phosphorylation of HspB5 in neurodegenerative diseases may preserve dendritic morphology and counteract neuronal dysfunction.