Intestinal cell phosphate uptake and the targeted knockout of the 1,25D3-MARRS receptor/PDIA3/ERp57

Vitamin D-Mediated Regulation of Intestinal Calcium Absorption

Vitamin D is a critical regulator of calcium and bone homeostasis. While vitamin D has multiple effects on bone and calcium metabolism, the regulation of intestinal calcium (Ca) absorption efficiency is a critical function for vitamin D. This is necessary for optimal bone mineralization during growth, the protection of bone in adults, and the prevention of osteoporosis. Intestinal Ca absorption is regulated by 1,25 dihydroxyvitamin D (1,25(OH)2 D), a hormone that activates gene transcription following binding to the intestinal vitamin D receptor (VDR). When dietary Ca intake is low, Ca absorption follows a vitamin-D-regulated, saturable pathway, but when dietary Ca intake is high, Ca absorption is predominately through a paracellular diffusion pathway. Deletion of genes that mediate vitamin D action (i.e., VDR) or production (CYP27B1) eliminates basal Ca absorption and prevents the adaptation of mice to low-Ca diets. Various physiologic or disease states modify vitamin-D-regulated intestinal absorption of Ca (enhanced during late pregnancy, reduced due to menopause and aging).

Vitamin D and Gut Health

Vitamin D is a conditionally required nutrient that can either be obtained from skin synthesis following UVB exposure from the diet. Once in the body, it is metabolized to produce the endocrine hormone, 1,25 dihydroxyvitamin D (1,25(OH)2D), that regulates gene expression in target tissues by interacting with a ligand-activated transcription factor, the vitamin D receptor (VDR). The first, and most responsive, vitamin D target tissue is the intestine. The classical intestinal role for vitamin D is the control of calcium metabolism through the regulation of intestinal calcium absorption. However, studies clearly show that other functions of the intestine are regulated by the molecular actions of 1,25(OH)2 D that are mediated through the VDR. This includes enhancing gut barrier function, regulation of intestinal stem cells, suppression of colon carcinogenesis, and inhibiting intestinal inflammation. While research demonstrates that there are both classical, calcium-regulating and non-calcium regulating roles for vitamin D in the intestine, the challenge facing biomedical researchers is how to translate these findings in ways that optimize human intestinal health.

Intestinal Phosphorus Absorption in Chronic Kidney Disease

Chronic kidney disease (CKD) affects approximately 10% of adults worldwide. Dysregulation of phosphorus homeostasis which occurs in CKD leads to development of CKD-Mineral Bone Disorder (CKD-MBD) and contributes to increased morbidity and mortality in these patients. Phosphorus is regulated by multiple hormones (parathyroid hormone (PTH), 1,25-dihyxdroxyvitamin D (1,25D), and fibroblast growth factor 23 (FGF23)) and tissues (kidney, intestine, parathyroid glands, and bone) to maintain homeostasis. In health, the kidneys are the major site of regulation for phosphorus homeostasis. However, as kidney function declines, the ability of the kidneys to adequately excrete phosphorus is reduced. The hormonal changes that occur with CKD would suggest that the intestine should compensate for impaired renal phosphorus excretion by reducing fractional intestinal phosphorus absorption. However, limited studies in CKD animal models and patients with CKD suggest that there may be a break in this homeostatic response where the intestine fails to compensate. As many existing therapies for phosphate management in CKD are aimed at reducing absolute intestinal phosphorus absorption, better understanding of the factors that influence fractional and absolute absorption, the mechanism by which intestinal phosphate absorption occurs, and how CKD modifies these is a much-needed area of study.

Vitamin D receptor is present on the neuronal plasma membrane and is co-localized with amyloid precursor protein, ADAM10 or Nicastrin

Our recent study indicated that vitamin D and its receptors are important parts of the amyloid processing pathway in neurons. Yet the role of vitamin D receptor (VDR) in amyloid pathogenesis is complex and all regulations over the production of amyloid beta cannot be explained solely with the transcriptional regulatory properties of VDR. Given that we hypothesized that VDR might exist on the neuronal plasma membrane in close proximity with amyloid precursor protein (APP) and secretase complexes. The present study primarily focused on the localization of VDR in neurons and its interaction with amyloid pathology-related proteins. The localization of VDR on neuronal membranes and its co-localization with target proteins were investigated with cell surface staining followed by immunofluorescence labelling. The FpClass was used for protein-protein interaction prediction. Our results demonstrated the localization of VDR on the neuronal plasma membrane and the co-localization of VDR and APP or ADAM10 or Nicastrin and limited co-localization of VDR and PS1. E-cadherin interaction with APP or the γ-secretase complex may involve NOTCH1, NUMB, or FHL2, according to FpClass. This suggested complex might also include VDR, which greatly contributes to Ca⁺² hemostasis with its ligand vitamin D. Consequently, we suggested that VDR might be a member of this complex also with its own non-genomic action and that it can regulate the APP processing pathway in this way in neurons.

Vitamin D Receptor Regulates Amyloid Beta 1-42 Production with Protein Disulfide Isomerase A3

The challenge of understanding the biology of neuronal amyloid processing could provide a basis for understanding the amyloid pathology in Alzheimer's disease (AD). Based on our previous studies, we have suggested that AD might be the consequence of a hormonal imbalance in which the critical hormone is vitamin D. The present study primarily focused on the creation of a condition that prevents the genomic or nongenomic action of vitamin D by disrupting vitamin D receptors (VDR or PDIA3/1,25MARRS); the effects of these disruptions on the series of proteins involved in secretases that play a crucial role in amyloid pathology and on amyloid beta (Aβ) production in primary cortical neurons were observed. VDR and PDIA3/1,25MARRS genes were silenced separately or simultaneously in E16 primary rat cortical neurons. The expression of target genes involved in APP processing, including Presenilin1, Presenilin2, Nicastrin, BACE1, ADAM10, and APP, was investigated with qRT-PCR and Western blot in this model. 1,25-Dihydroxyvitamin D₃ treatments were used to verify any transcriptional regulation data gathered from siRNA treatments by determining the mRNA expression of the target genes. Immunofluorescence labeling was used for the verification of silencing experiments and intracellular Aβ1-42 production. Extracellular Aβ1-42 level was assessed with ELISA. mRNA and protein expression results showed that 1,25-dihydroxyvitamin D₃ might affect the transcriptional regulation of the genes involved in APP processing. The intracellular and extracellular Aβ1-42 measurements in our study support this suggestion. Consequently, we suggest that 1,25-dihydroxyvitamin D₃ and its receptors are important parts of the amyloid processing pathway in neurons.

The role of vitamin D in the endocrinology controlling calcium homeostasis

Vitamin D and its' metabolites are a crucial part of the endocrine system that controls whole body calcium homeostasis. The goal of this hormonal control is to regulate serum calcium levels so that they are maintained within a very narrow range. To achieve this goal, regulatory events occur in coordination at multiple tissues, e.g. the intestine, kidney, bone, and parathyroid gland. Production of the vitamin D endocrine hormone, 1,25 dihydroxyvitamin D (1,25(OH)2 D) is regulated by habitual dietary calcium intake and physiologic states like growth, aging, and the menopause. The molecular actions of 1,25(OH)2 D on calcium regulating target tissues are mediated predominantly by transcription controlled by the vitamin D receptor. A primary role for 1,25(OH)2 D during growth is to increase intestinal calcium absorption so that sufficient calcium is available for bone mineralization. However, vitamin D also has specific actions on kidney and bone.

Multifunctional molecule ERp57: From cancer to neurodegenerative diseases

The protein disulfide isomerase (PDI) gene family is a protein family classically characterized by endoplasmic reticulum (ER) localization and isomerase and redox activity. ERp57, a prominent multifunctional member of the PDI family, is detected at various levels in multiple cellular localizations outside of the ER. ERp57 has been functionally linked to a host of physiological processes and numerous studies have demonstrated altered expression and aberrant functionality of ERp57 in association with diverse pathological states. Here, we summarize available knowledge of ERp57's functions in subcellular compartments and the roles of dysregulated ERp57 in various diseases toward an emphasis on the potential utility of therapeutic development of ERp57.

The 1,25D3 -MARRS receptor/PDIA3/ERp57 and lifespan

Using MRI on mice bearing a targeted knockout (KO) of the 1,25D3 -MARRS receptor/PDIA3/ERp57 we found that they had decreased body fat relative to their littermate (LM) controls, a condition associated with increased lifespan. Others have found that lower body fat is correlated with decreased lipid droplets in intestinal cells that may be mediated by a factor secreted by germ cells (possibly estradiol). In a reducing environment estradiol competed for binding to the 1,25D3-MARRS receptor/PDIA3/ERp57. A consequence of this was that estradiol stimulated calcium uptake in enterocytes isolated from LM mice. In time course studies, lipid droplets increased in response to 1 nM estradiol from 1-5 D of culture, relative to corresponding controls, while at 6 and 7 D this steroid decreased lipid droplets. Enterocytes from LM or KOs incubated with estradiol for 1-4 D showed the hormone increased lipid droplets. Using the 4 D culture period, 1 and 10 nM estradiol significantly increased the number of lipid droplets in cells from LM mice by 40-60%, compared to equivalent conditions in KO mice. In assessing signal transduction pathways, the hormone increased phospho-Akt levels, but no differences were observed in phospho-mTORC1, or phospho-S6K (although cells from chicks did exhibit a hormone-mediated difference). Finally, the remaining mice (which had stopped reproducing) were allowed to die naturally and lifespan recorded. LM mice lived 687 ± 77 D (without an outlying value) while KO mice lived 740 D ± 80 D. These data suggest the 25D3 -MARRS receptor/PDIA3/ERp57 may contribute to the length of lifespan in mammals.

Actin and Keratin are Binding Partners of the 1,25D3-MARRS Receptor/PDIA3/ERp57

We have shown that the 1,25D₃-MARRS receptor is necessary for the rapid, pre-genomic effects of 1,25(OH)₂D₃ on phosphate and/or calcium absorption in chick intestines. However, a clear understanding of the proteins involved in the signaling mechanisms by which the 1,25D₃-MARRS receptor facilitates 1,25(OH)₂D₃-mediated phosphate or calcium uptake, as well as other cellular effects, is still under investigation. We used co-immunoprecipitation studies and mass spectroscopy to identify actin and keratin as proteins that interact with the 1,25D₃-MARRS receptor. Using confocal microscopy, we visualized 1,25(OH)₂D₃- MARRS receptor localizations relative to actin and/or keratin distribution in chick enterocytes. Cells cultured in media containing phenol red had the 1,25D₃-MARRS receptor and actin localized largely in the nucleus, which was dispersed upon addition of (OH)₂ 1,25(OH)₂D₃. In the absence of phenol red, staining was cytoplasmic. Addition of steroid caused diminished staining at 10 s and 30 s, with a return of intensity between 1 and 5 min. Nuclear staining was observed after 1 min. We found that F-actin concentrations are maximal when 1,25D₃-MARRS receptor localizations within enterocytes are low suggesting that cyclical conversions of F-actin to G-actin are involved in the 1,25(OH)₂D₃-mediated redistribution of the 1,25D₃-MARRS receptor within the cell. We also found that keratin distribution remains constant with 1,25(OH)₂D₃ exposure when Factin depolymerizes into G-actin, which suggests that actin and keratin work in concert to facilitate hormonemediated redistribution of the 1,25D₃-MARRS receptor. We subsequently investigated whether the cyclical redistribution was related to either 1,25(OH)₂D₃-stimulated phosphate or calcium uptake, but no congruent pattern was found.

Role of the 1,25D3-MARRS receptor in the 1,25(OH)2D3-stimulated uptake of calcium and phosphate in intestinal cells

We have used mice with a targeted knockout (KO) of the 1,25D(3)-MARRS receptor (ERp57/PDIA3) in intestine to study rapid responses to 1,25-dihydroxyvitamin D(3) [1,25D(3)] with regards to calcium or phosphate uptake. Western analyses indicated the presence of the 1,25D(3)-MARRS receptor in littermate (LM) mice, but not KO mice. Saturation analyses for [(3)H]1,25D(3) binding revealed comparable affinities for the hormone in lysates from female and male LM, but a reduced B(max) in females. Binding in lysates from KO mice was absent or severely reduced. Enterocytes from KO mice failed to respond to hormone with regard to either ion uptake, while cells from LM mice exhibited an increase in uptake. For calcium uptake, the protein kinase (PK) A pathway mediated the response to 1,25D(3). Enterocytes from LM mice responded to 1,25D(3) with enhanced PKA activity, while cells from KO mice did not, although both cell types responded to forskolin. Calcium transport in LM mice in vivo was greater than in KO mice. Cells from LM and KO mice had cell surface VDR; however, anti-VDR antibodies had no effect on ion uptake. Unlike chicks, the PKC pathway was not involved in phosphate uptake. As in chicks and rats, intestinal cells from adult male mice lost the ability to respond to 1,25D(3) with enhanced phosphate uptake, whereas in female mice, uptake in cells from adults was greater than that observed in young mice. Finally, when we tested phosphate uptake in vivo, we found that young female mice had a much greater rate of transport than young male mice.