A single amino acid mutation results in a rapid inactivation of epithelial calcium channels

Evolutionary analyses reveal independent origins of gene repertoires and structural motifs associated to fast inactivation in calcium-selective TRPV channels

Essential for calcium homeostasis, TRPV5 and TRPV6 are calcium-selective channels belonging to the transient receptor potential (TRP) gene family. In this study, we investigated the evolutionary history of these channels to add an evolutionary context to the already available physiological information. Phylogenetic analyses revealed that paralogs found in mammals, sauropsids, amphibians, and chondrichthyes, are the product of independent duplication events in the ancestor of each group. Within amniotes, we identified a traceable signature of three amino acids located at the amino-terminal intracellular region. The signature correlates with both the duplication events and the phenotype of fast inactivation observed in mammalian TRPV6 channels. Electrophysiological recordings and mutagenesis revealed that the signature sequence modulates the phenotype of fast inactivation in all clades of vertebrates but reptiles. A transcriptome analysis showed a change in tissue expression from gills, in marine vertebrates, to kidneys in terrestrial vertebrates. Our results highlight a cytoplasmatic structural triad composed by the Helix-Loop-Helix domain, the S2-S3 linker, and the TRP domain helix that is important on modulating the activity of calcium-selective TRPV channels.

TRPV5: a Ca(2+) channel for the fine-tuning of Ca(2+) reabsorption

TRPV5 is one of the two channels in the TRPV family that exhibit high selectivity to Ca(2+) ions. TRPV5 mediates Ca(2+) influx into cells as the first step to transport Ca(2+) across epithelia. The specialized distribution in the distal tubule of the kidney positions TRPV5 as a key player in Ca(2+) reabsorption. The responsiveness in expression and/or activity of TRPV5 to hormones such as 1,25-dihydroxyvitamin D3, parathyroid hormone, estrogen, and testosterone makes TRPV5 suitable for its role in the fine-tuning of Ca(2+) reabsorption. This role is further optimized by the modulation of TRPV5 trafficking and activity via its binding partners; co-expressed proteins; tubular factors such as calbindin-D28k, calmodulin, klotho, uromodulin, and plasmin; extracellular and intracellular factors such as proton, Mg(2+), Ca(2+), and phosphatidylinositol-4,5-bisphosphate; and fluid flow. These regulations allow TRPV5 to adjust its overall activity in response to the body's demand for Ca(2+) and to prevent kidney stone formation. A point mutation in mouse Trpv5 gene leads to hypercalciuria similar to Trpv5 knockout mice, suggesting a possible role of TRPV5 in hypercalciuric disorders in humans. In addition, the single nucleotide polymorphisms in Trpv5 gene prevalently present in African descents may contribute to the efficient renal Ca(2+) reabsorption among African descendants. TRPV5 represents a potential therapeutic target for disorders with altered Ca(2+) homeostasis.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Epithelial Ca2+ entry channels: transcellular Ca2+ transport and beyond

The recently discovered apical calcium channels CaT1 (TRPV6) and ECaC (TRPV5) belong to a family of six members called the 'TRPV family'. Unlike the other four members which are nonselective cation channels functioning as heat or osmolarity sensors in the body, CaT1 and ECaC are remarkably calcium-selective channels which serve as apical calcium entry mechanisms in absorptive and secretory tissues. CaT1 is highly expressed in the proximal intestine, placenta and exocrine tissues, whereas ECaC expression is most prominent in the distal convoluted and connecting tubules of the kidney. CaT1 in the intestine is highly responsive to 1,25-dihydroxyvitamin D3 and shows both fast and slow calcium-dependent feedback inhibition to prevent calcium overload. In contrast, ECaC only shows slow inactivation kinetics and appears to be mostly regulated by the calcium load in the kidney. Outside the calcium-transporting epithelia, CaT1 is highly expressed in exocrine tissues such as pancreas, prostate and salivary gland. In these tissues it probably mediates re-uptake of calcium following its release by secretory vesicles. CaT1 also contributes to store-operated calcium entry in Jurkat T-lymphocytes and prostate cancer LNCaP cells, possibly in conjunction with other cellular components which link CaT1 activity to the filling state of the calcium stores. Finally, CaT1 expression is upregulated in prostate cancer and other cancers of epithelial origin, highlighting its potential as a target for cancer therapy.