Modulation of renal Ca²⁺transport protein genes by dietary Ca²⁺and 1,25‐dihydroxyvitamin D₃in 25hydroxyvitamin D₃‐1α‐hydroxylase knockout mice

Abstract

Pseudovitamin D-deficiency rickets (PDDR) is an autosomal disease characterized by hyperparathyroidism, rickets, and undetectable levels of 1,25-dihydroxyvitaminD3 (1,25(OH)2D3). Mice in which the 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-OHase) gene was inactivated presented the same clinical phenotype as patients with PDDR and were used to study renal expression of the epithelial Ca2+ channel (ECaC1), the calbindins, Na+/Ca2+ exchanger (NCX1), and Ca2+-ATPase (PMCA1b). Serum Ca2+ (1.20+/-0.05 mM) and mRNA/protein expression of ECaC1 (41+/-3%), calbindin-D28K (31+/-2%), calbindin-D9K (58+/-7%), NCX1 (10+/-2%), PMCA1b (96+/-4%) were decreased in 1alpha-OHase-/- mice compared with 1alpha-OHase+/- littermates. Feeding these mice a Ca2+-enriched diet normalized serum Ca2+ levels and expression of Ca2+ proteins except for calbindin-D9K expression. 1,25(OH)2D3 repletion resulted in increased expression of Ca2+ transport proteins and normalization of serum Ca2+ levels. Localization of Ca2+ transport proteins was clearly polarized in which ECaC1 was localized along the apical membrane, calbindin-D28K in the cytoplasm, and calbindin-D9K along the apical and basolateral membranes, resulting in a comprehensive mechanism facilitating renal transcellular Ca2+ transport. This study demonstrated that high dietary Ca2+ intake is an important regulator of the renal Ca2+ transport proteins in 1,25(OH)2D3-deficient status and thus contributes to the normalization of blood Ca2+ levels.

Critical Role of Calbindin-D28k in Calcium Homeostasis Revealed by Mice Lacking Both Vitamin D Receptor and Calbindin-D28k

“…Therefore, both CaBP-D9k and CaBP-28k are important for renal calcium handling. Other data from recent studies of 25-hydroxyvitamin D 3 1␣-hydroxylase knockout mice are consistent with this conclusion (5).…”

“…Influx of luminal calcium into the epithelial cells is propelled by a steep electrochemical gradient across the apical membrane, saturable at high levels of luminal calcium, and mediated by apical calcium channels (2)(3)(4). Once in the cytosol, calcium binds to the vitamin D-dependent calcium-binding proteins, calbindins (CaBPs), which ferry calcium from the apical to the basolateral portion of the cell, where the Na ϩ /Ca 2ϩ exchanger and Ca 2ϩ -dependent ATPase on the basolateral membrane extrude calcium into extracellular fluids (5)(6)(7). It is believed that the apical to basolateral intracellular movement of calcium ions is the rate-limiting step and is highly dependent upon the cellular concentration of CaBPs, which are regulated by 1,25-dihydroxyvitamin D 3 , the hormonal form of vitamin D.…”

The epithelial calcium channels TRPV5 and TRPV6: regulation and implications for disease

“…The reduced renal TRPV5 expression and accompanied hypocalcemia were completely restored after 1,25(OH) 2 D 3 supplementation ). Similar results were obtained using different animal models and cell lines, including VDR and 1α-OHase knockout mice and the human intestinal cell line Caco-2, confirming the vitamin D-dependent regulation of TRPV5 and TRPV6 (Fleet et al 2002;Hoenderop et al 2002a;van Abel et al 2003;van Cromphaut et al 2001;Weber et al 2001;Wood et al 2001). Together these findings indicate that the expression of TRPV5 and TRPV6 is indeed controlled by 1,25(OH) 2 D 3 .…”

“…In clinical practice, patients are treated with vitamin D analogues and/ or an enriched Ca 2+ diet to prevent rickets, which correct the major perturbations in Ca 2+ homeostasis. As previously mentioned, in 1α-OHase knockout mice, a downregulation of renal TRPV5 as well as intestinal TRPV6 expression was demonstrated, whereas supplementation with 1,25(OH) 2 D 3 or an enriched Ca 2+ diet restored expression levels of these Ca 2+ transporters and serum Ca 2+ concentrations (Hoenderop et al 2002a;van Abel et al 2003). These data suggest that decreased channel abundance contributes to defective renal and intestinal Ca 2+ absorption and, thereby, the sustained hypocalcemia in VDDR-I.…”

“…Interestingly, dietary Ca 2+ enrichment resulted in the normalization of the reduced renal TRPV5 and duodenal TRPV6 expression levels in 1α-OHase knockout mice, as well as for the other Ca 2+ transport proteins participating in active Ca 2+ (re)absorption (Hoenderop et al 2002a;van Abel et al 2003). In mice lacking a functional vitamin D receptor, dietary Ca 2+ levels regulated the expression of TRPV5 and TRPV6 in kidney and duodenum, respectively (Panda et al 2004;van Cromphaut et al 2001;Weber et al 2001).…”