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

Abstract

Calbindin (CaBP)-D28k and CaBP-D9k are cytosolic vitamin D-dependent calcium-binding proteins long thought to play an important role in transepithelial calcium transport. However, recent genetic studies suggest that CaBP-D28k is not essential for calcium metabolism. Genetic ablation of this gene in mice leads to no calcemic abnormalities. Genetic inactivation of the vitamin D receptor (VDR) gene leads to hypocalcemia, secondary hyperparathyroidism, rickets, and osteomalacia, accompanied by 90% reduction in renal CaBP-D9k expression but little change in CaBP-D28k. To address whether the role of CaBP-D28k in calcium homeostasis is compensated by CaBP-D9k, we generated VDR/CaBP-D28k double knockout (KO) mice, which expressed no CaBP-D28k and only 10% of CaBP-D9k in the kidney. On a regular diet, the double KO mice were more growth-retarded and 42% smaller in body weight than VDRKO mice and died prematurely at 2.5-3 months of age. Compared with VDRKO mice, the double KO mice had higher urinary calcium excretion and developed more severe secondary hyperparathyroidism and rachitic skeletal phenotype, which were manifested by larger parathyroid glands, higher serum parathyroid hormone levels, much lower bone mineral density, and more distorted growth plate with more osteoid formation in the trabecular region. On high calcium, high lactose diet, blood-ionized calcium levels were normalized in both VDRKO and the double KO mice; however, in contrast to VDRKO mice, the skeletal abnormalities were not completely corrected in the double KO mice. These results directly demonstrate that CaBP-D28k plays a critical role in maintaining calcium homeostasis and skeletal mineralization and suggest that its calcemic role can be mostly compensated by CaBP-D9k.It is well established that calcium homeostasis is primarily maintained by the vitamin D and parathyroid hormone (PTH) 1 endocrine systems. In this regard, the principal role of vitamin D is to regulate calcium transport in the intestine and kidney. Intestinal and renal distal tubular calcium transfer consists of a passive, concentration gradient-dependent paracellular transport and an active, ATP-dependent transcellular transport (1). The latter is largely regulated by vitamin D and is crucial when calcium supply is low. It is believed that the vitamin D-mediated transcellular calcium transport in the duodenum and renal distal tubules involves three sequential steps: entry, intracellular movement, and extrusion. 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 (2-4). 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 (5-7). It is believed that the apical to b...

Gene Expression Changes in the Jejunum of Rats during the Transient Suckling-Weaning Period

“…In addition, we found that vitamin D receptor (Vdr) was induced during this period. Previous studies have shown that VDR regulates calcium absorption by regulating related genes such as calcium transporter 1 and calbindin-D9K in the small intestine (38)(39)(40). Our present results support the notion proposed in a previous study using Vdr null mice (41) that vitamin D is important for development after weaning.…”

Cytosolic Ca2+ Buffers

“…The same holds true for CR-immunoreactive or CB-D28k-immunoreactive neurons in the respective knockout strains, CBD28k 2/2 and CR 2/2 (Airaksinen et al 1997;Schiffmann et al 1999). The most notable exception is the up-regulation of CB-D9k in epithelial kidney cells in CB-D28k 2/2 mice (Zheng et al 2004). Two plausible explanations for the absence of compensation/homeostatic mechanisms at the level of other Ca 2þ buffers are presented: (I) Neurons once committed to express a certain Ca 2þ buffer have permanently inactivated/repressed the promoter for other Ca 2þ buffers; (II) The specific properties (affinities, kinetics, cooperativity, mobility) of any other Ca 2þ buffer would not be adequate to restore "normal" Ca 2þ signaling (Schwaller 2009 Purkinje cells are characterized by extensive Ca 2þ signaling and high expression levels of PV and CB-D28k and, thus, are well-suited to address these questions.…”

Proteomic Identification and Immunolocalization of Increased Renal Calbindin-D28k Expression in OVE26 Diabetic Mice

“…Renal calbindin-D28k appears to enhance tubular Ca 2+ reabsorption, and a reduction of renal calbindin-D28k is associated with hypercalciuria [28][29][30][31][32]. Findings from a recent in vivo study using a gene-knockout model confirm the critical role of calbindin-D28k in maintaining renal calcium homeostasis [33]. Hypercalciuria is commonly associated with diabetes [4][5][6][7][8][9].…”