Marie B. Demay

Vitamin D and Human Health: Lessons from Vitamin D Receptor Null Mice

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

TBX1 is responsible for cardiovascular defects in velo-cardio-facial/DiGeorge syndrome

Velo-cardio-facial syndrome (VCFS)/DiGeorge syndrome (DGS) is a human disorder characterized by a number of phenotypic features including cardiovascular defects. Most VCFS/DGS patients are hemizygous for a 1.5-3.0 Mb region of 22q11. To investigate the etiology of this disorder, we used a cre-loxP strategy to generate mice that are hemizygous for a 1.5 Mb deletion corresponding to that on 22q11. These mice exhibit significant perinatal lethality and have conotruncal and parathyroid defects. The conotruncal defects can be partially rescued by a human BAC containing the TBX1 gene. Mice heterozygous for a null mutation in Tbx1 develop conotruncal defects. These results together with the expression patterns of Tbx1 suggest a major role for this gene in the molecular etiology of VCFS/DGS.

Rescue of the Skeletal Phenotype of Vitamin D Receptor- Ablated Mice in the Setting of Normal Mineral Ion Homeostasis: Formal Histomorphometric and Biomechanical Analyses*

1,25-Dihydroxyvitamin D3 has been shown to play an important role in vitro in regulating osteoblast gene transcription and promoting osteoclast differentiation. To address the role of the vitamin D receptor (VDR) in skeletal homeostasis, formal histomorphometric analyses were performed in VDR null mice in the setting of impaired mineral ion homeostasis as well as in VDR null mice in whom normal mineral ion homeostasis had been preserved. In hypocalcemic VDR null mice, there was an increase in bone volume as a result of a dramatic increase in osteoid. There was also an increase in the number of osteoblasts without a significant change in the number of osteoclasts. Examination of the growth plate revealed marked disorganization, with an increase in vascularity and matrix. Biomechanical parameters demonstrated increased bone fragility in the hypocalcemic VDR null mice. In the VDR ablated mice in whom normal mineral ion homeostasis had been preserved, none of these measurements was significantly different fro...

The Nonskeletal Effects of Vitamin D: An Endocrine Society Scientific Statement

Significant controversy has emerged over the last decade concerning the effects of vitamin D on skeletal and nonskeletal tissues. The demonstration that the vitamin D receptor is expressed in virtually all cells of the body and the growing body of observational data supporting a relationship of serum 25-hydroxyvitamin D to chronic metabolic, cardiovascular, and neoplastic diseases have led to widespread utilization of vitamin D supplementation for the prevention and treatment of numerous disorders. In this paper, we review both the basic and clinical aspects of vitamin D in relation to nonskeletal organ systems. We begin by focusing on the molecular aspects of vitamin D, primarily by examining the structure and function of the vitamin D receptor. This is followed by a systematic review according to tissue type of the inherent biological plausibility, the strength of the observational data, and the levels of evidence that support or refute an association between vitamin D levels or supplementation and maternal/child health as well as various disease states. Although observational studies support a strong case for an association between vitamin D and musculoskeletal, cardiovascular, neoplastic, and metabolic disorders, there remains a paucity of large-scale and long-term randomized clinical trials. Thus, at this time, more studies are needed to definitively conclude that vitamin D can offer preventive and therapeutic benefits across a wide range of physiological states and chronic nonskeletal disorders.

Vitamin D receptor is essential for normal keratinocyte stem cell function

The major physiological role of the vitamin D receptor (VDR) is the maintenance of mineral ion homeostasis. Mutation of the VDR, in humans and mice, results in alopecia. Unlike the effects of the VDR on mineral ion homeostasis, the actions of the VDR that prevent alopecia are ligand-independent. Although absence of the VDR does not prevent the development of a keratinocyte stem cell niche in the bulge region of the hair follicle, it results in an inability of these stem cells to regenerate the lower portion of the hair follicle in vivo and impairs keratinocyte stem cell colony formation in vitro. VDR ablation is associated with a gradual decrease in keratinocyte stem cells, accompanied by an increase in sebaceous activity, a phenotype analogous to that seen with impaired canonical Wnt signaling. Transient gene expression assays demonstrate that the cooperative transcriptional effects of β-catenin and Lef1 are abolished in keratinocytes isolated from VDR-null mice, revealing a role for the unliganded VDR in canonical Wnt signaling. Thus, absence of the VDR impairs canonical Wnt signaling in keratinocytes and leads to the development of alopecia due to a defect in keratinocyte stem cells.

Metabolic and cellular analysis of alopecia in vitamin D receptor knockout mice

Targeted ablation of the vitamin D receptor (VDR) results in hypocalcemia, hypophosphatemia, hyperparathyroidism, rickets, osteomalacia, and alopecia--the last a consequence of defective anagen initiation. To investigate whether the markedly elevated levels of 1,25-dihydroxyvitamin D led to the alopecia, we raised VDR-null mice in a ultraviolet light-free environment and fed them chow lacking vitamin D for five generations. Despite undetectable circulating levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, alopecia persisted in the VDR-null mice, demonstrating that the alopecia was not secondary to toxic levels of 1,25-dihydroxyvitamin D interacting with an alternative receptor. Furthermore, alopecia was not seen in control littermates, suggesting that absence of ligand and absence of receptor cause different phenotypes. To identify the cell population responsible for the alopecia, we performed hair-reconstitution assays in nude mice and observed normal hair follicle morphogenesis, regardless of the VDR status of the keratinocytes and dermal papilla cells. However, follicles reconstituted with VDR-null keratinocytes demonstrated a defective response to anagen initiation. Hence, alopecia in the VDR-null mice is due to a defect in epithelial-mesenchymal communication that is required for normal hair cycling. Our results also identify the keratinocyte as the cell of origin of the defect and suggest that this form of alopecia is due to absence of ligand-independent receptor function.

VITAMIN D DEFICIENCY AND DISORDERS OF VITAMIN D METABOLISM

The disorders of vitamin D metabolism are inherited metabolic abnormalities involving mutations of the vitamin D receptor or enzymes involved in the metabolism of vitamin D to its biologically active form 1,25-dihydroxyvitamin D. Although these mutations are rare, studies in affected patients and animal models have helped to identify critical actions of vitamin D and the mechanism by which it exerts its effects. Vitamin D deficiency, however, is an increasingly recognized problem among the elderly and in the general population. Screening for vitamin D deficiency only in those patients with known risk factors will result in a large proportion of unrecognized affected patients.

Mechanism of vitamin D receptor action.

Abstract:  Studies in humans and in animal models have demonstrated that the receptor‐dependent actions of 1,25‐dihydroxyvitamin D are required for normal skeletal growth and maturation. Investigations were undertaken to address which consequences of vitamin D receptor deficiency are a direct result of impaired receptor‐dependent hormone actions versus being due to metabolic changes. Vitamin D receptor (VDR) knockout mice were therefore generated. Investigations were performed in mice with abnormal mineral ion homeostasis, as well as in mice in which the development of abnormal mineral ion homeostasis was prevented by dietary means. VDR null mice had hypocalcemia, hyperparathyroidism, and hypophosphatemia in the first month of life. Rickets and osteomalacia are observed as well. Institution of a high‐calcium, high‐phosphorus, lactose‐supplemented diet by the third week of life prevents abnormalities in mineral ion homeostasis. The bones of the VDR null mice with normal mineral ion homeostasis are indistinguishable from those of their wild‐type littermates. The rachitic changes in the growth plates are also prevented by maintenance of normal mineral ion homeostasis. Investigations into the pathophysiological basis for the growth plate abnormalities in the VDR null mice with abnormal mineral ion homeostasis demonstrated that impaired apoptosis of hypertrophic chondrocytes due to hypophosphatemia was the cause of rachitic changes. Studies investigating the cause of the alopecia demonstrate novel ligand‐independent VDR actions in the keratinocyte. The skeletal effects of VDR ablation are therefore indirect and reflect absence of ligand‐dependent receptor actions in the intestine. In contrast, the cutaneous phenotype of VDR ablation is a direct consequence of absence of ligand‐independent VDR actions in epidermal keratinocytes.

Osteoblasts lacking the vitamin D receptor display enhanced osteogenic potential in vitro.

1,25‐Dihydroxyvitamin D plays an important role in the regulation of osteoblast gene expression, regulating the expression of bone matrix proteins as well as that of Runx2, a key regulator of osteoblast differentiation. Studies in mice lacking the vitamin D receptor (VDR) have revealed that the actions of the VDR on the skeleton are not required in the setting of normal mineral ion homeostasis. Since paracrine and endocrine factors can compensate for gene defects in vivo, studies were performed to determine whether ablation of the VDR alters the program of osteoblast differentiation in vitro. Studies in primary calvarial cultures revealed that ablation of the VDR enhanced osteoblast differentiation. The cells from the VDR null mice exhibited an earlier onset and increased magnitude of alkaline phosphatase activity, as well as an earlier and sustained increase in mineralized matrix formation, demonstrating that this enhancement persists throughout the program of osteoblast differentiation. The expression of bone sialoprotein, which enhances mineralization, was also increased in the VDR null cultures. To determine whether the increase in osteoblast differentiation was associated with an increase in the number of osteogenic progenitors, the number of osteoblastic colony forming units (CFU‐OB) was evaluated. There was a twofold increase in the number of CFU‐OB in the cultures isolated from the VDR null mice. Furthermore, the VDR null CFU‐OB demonstrated an earlier onset and higher magnitude of expression of alkaline phosphatase activity when compared to the CFU‐OB from their wild‐type control littermates. These studies demonstrate that the VDR attenuates osteoblast differentiation in vitro and suggest that other endocrine and paracrine factors modulate the effect of the VDR on osteoblast differentiation in vivo.

Cloning and characterization of the vitamin D receptor from Xenopus laevis.

The Vitamin D receptor (VDR), a member of the nuclear receptor superfamily, mediates the effects of 1,25-dihydroxyvitamin D3 on mineral ion homeostasis. Although the mammalian and avian VDRs have been extensively studied, little is known about the VDR in lower vertebrate species. To address this, we have isolated the Xenopus laevis VDR (xVDR) complementary DNA. Overall, the xVDR shares 79%, 73%, 73%, and 75% identity at the amino acid level with the chicken, mouse, rat, and human VDRs, respectively. The amino acid residues and subdomains important for DNA binding, hormone binding, dimerization, and transactivation are mostly conserved among all VDR species. The xVDR polypeptide can heterodimerize with the mouse retinoid X receptor α, bind to the rat osteocalcin vitamin D response element (VDRE), and induce vitamin D-dependent transactivation in transfected mammalian cells. Northern analysis reveals two xVDR messenger RNA species of 2.2 kb and 1.8 kb in stage 60 Xenopus tissues. In the adult, xVDR expressi...