These findings highlighted the possibility of paracrine production of 1,25-dihydroxyvitamin D3 production in the CNS. The glial cell expression of the 25(OH)D3 24-hydroxylase gene, CYP24A1, producing the enzyme needed to inactivate calcitriol, suggested further control of 1,25-dihydroxyvitamin D3 levels in the CNS [10]. In a rodent model, Napabucasin nmr Spach and Hayes varied the plasma 25-OHD level by varying dietary vitamin D3 and reported that CNS calcitriol correlated with plasma 25-OHD but not with plasma calcitriol [11]. These data provided evidence for calcitriol synthesis in situ in the CNS. Therefore, the presence of 25-hydroxylase and 1-α-hydroxylase
required to synthesize 1,25-dihydroxyvitamin D3 and 24-hydroxylase needed
to degrade 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 in the brain, along with evidence of in situ CNS calcitriol synthesis, consolidated the idea that the CNS is poised to locally metabolize (and regulate) the active form of vitamin D implicating the importance of this active hormone in brain health and disease. Calcitriol exerts its nuclear effect via the vitamin D receptor (VDR). The discovery of the VDR (mRNA and protein) throughout the brain and spinal cord consolidated the importance of this hormone in modulating nervous system function. Studies from adult rats and hamsters provided a detailed topography of the distribution of VDR in the CNS [2, 3], later shown to AZD4547 concentration be similar in humans [8, 12] (see Figure 2). VDR expression was noted in both neurones and PAK6 glial cells (microglia, astrocytes, oligodendrocytes) in different CNS regions,
including: (i) cortex [temporal (that is, auditory, olfactory, entorhinal), frontal (that is, prefrontal, orbitofrontal, primary motor), parietal, cingulated]; (ii) deep grey matter (thalamus, basal ganglia, hypothalamus, hippocampus, amygdala); (iii) cerebellum (granular and Purkinje cell layers); (iv) brainstem nuclei; (v) spinal cord (anterior horn cells); and (vi) ventricular system (that is, choroid plexus ependymal cells) [13, 14]. VDRs have also been reported in the nuclei of Schwann cells and in peripheral neurones [15, 16]. The VDR is a member of the steroid/thyroid hormone superfamily of transcription regulation factors. On binding of calicitriol, VDR heterodimerizes with the retinoid X receptor (RXR), and subsequently binds specific genomic sequences known as vitamin D response elements (VDREs) to influence gene transcription [17]. Recent construction of a genome-wide map of VDR binding provided evidence of enrichment of VDR-binding sites near autoimmune and cancer-associated genes identified from genome-wide association studies [17] (Figure 3).