Investigating the role of Vitamin D in axon development and regeneration

The regulation of axonal regeneration in the central nervous system (CNS) remains an unresolved challenge in neurobiology. Primary neuron culture is considered the gold standard model for investigating various aspects of CNS development, including neuronal polarization, neurite outgrowth, axon guidance (pathfinding), synaptogenesis, neuronal network formation, maturation, and functional activity. Although a few studies have reported the potential role of vitamin D in promoting axonal regeneration within the CNS, the existing evidence remains limited. It has been demonstrated that regeneration of CNS axons can be partially induced through stimulation with appropriate neurotrophic factors. In the present study, we evaluated the effects of vitamin D on neuronal development between days 1 and 7 in primary neuron cultures, corresponding to the neuronal maturation phase. Furthermore, we examined the impact of vitamin D on regenerative processes in neurons that had acquired a mature phenotype by day 7, exhibiting characteristics similar to those of adult neurons. Our findings showed that vitamin D administration during the developmental period enhanced neurite length, neurite outgrowth rate, and dynamic microtubule density. In parallel, vitamin D increased the expression level of DLK, a key component of the DLK-MAPK signaling cascade, while reducing the levels of DCX and its phosphorylated form (pDCX). Additionally, vitamin D treatment led to upregulation of GAP43, STMN2, and 14-3-3 zeta/delta proteins, which are involved in neuronal development, axonal navigation, and synaptic plasticity. Co-localization analysis further revealed that DLK and STMN2 proteins associate with the vitamin D receptor (VDR) following vitamin D treatment. In axonal injury-induced regeneration models, vitamin D enhanced the expression of GAP43 and STMN2, both of which are linked to regenerative responses. Collectively, these results suggest that vitamin D may contribute not only to neuronal development but also to the modulation of regenerative mechanisms within the CNS.