According to the nomenclature of transient receptor (TRP) channels TRPV6 represents the last member of the vanilloid-like TRP (TRPV) subfamily which were named after the first member TRPV1. TRPV1 can be activated by vanilloid-like compounds (Caterina et al. 1999; Montell et al. 2002). TRPV6 was first isolated from rat small intestine using a functional cloning approach (Peng et al. 1999). The human orthologue which shows only 90% identical amino acids to the rat sequence was cloned from a human placenta cDNA library (Wissenbach et al. 2001), and the mouse sequence was cloned by Hirnet and coworkers (Hirnet et al. 2003).
Properties of the TRPV6 Channel
TRPV6 currents can be blocked by the trivalent ions La3+ and Gd3+ in the micromolar range (Peng et al. 1999), by Xestospongin a natural isolate of a sponge (Vassilev et al. 2001) and by the antimycotic drugs econazole and miconazole as well as by ruthenium red (Hoenderop et al. 2001; Nilius et al. 2001a; Schwarz et al. 2006). Also a synthetic compound TH-1177 is supposed to inhibit TRPV6 currents with an IC50 of 0.44 μM (Haverstick et al. 2000; Landowski et al. 2011). 2-APB, a rather nonspecific modulator of TRP channels, inhibits TRPV6 (Kovacs et al. 2012). Soricidin a toxic peptide derived from saliva of short-tailed shrew inhibits currents although inhibition is not complete (Bowen et al. 2013). No specific activator of TRPV6 currents is known.
The expression of the TRPV6 gene seems to be not completely identical within different species. The murine TRPV6 is expressed in placenta, pancreas, salivary gland (Hirnet et al. 2003), epididymis (Weissgerber et al. 2011), prostate, and small intestine (Muller et al. 2000). In addition TRPV6 is expressed in bone marrow cells (Nijenhuis et al. 2003). In contrast, in humans TRPV6 expression in small intestine has never been shown convincingly and human prostate does not express TRPV6 transcripts, but TRPV6 is upregulated in prostate cancer (Fixemer et al. 2003; Wissenbach et al. 2004; Wissenbach et al. 2001); for details see the section “disease relation.”
Genomic Inactivation and Physiological Consequence
Furthermore, a knock-in mouse line was generated, in which the amino acid within the pore region responsible for Ca2+ selectivity, (D582A), was replaced by an alanine resulting in a dead channel (Weissgerber et al. 2011). These mice show an identical phenotype like the mice carrying the deletion of exons 13–15 (Weissgerber et al. 2012). This knock-in mouse was also used to generate a TRPV6 KO mouse by deleting exon 13–15, and it showed an almost identical phenotype.
The reason that independent groups found different phenotypes is not clear but one might speculate that the inactivation strategy as well as strain-based differences might have an influence on the result. Inactivation of the EphB6 receptor did not lead to one of the phenotypes described above (Luo et al. 2004).
Taking together, male hypofertility is the most robust phenotype found in TRPV6 KO mice so far.
TRPV6 and its next homolog TRPV5 which shows ∼75% identical amino acids are located in tandem on the human chromosome 7q33–35 which corresponds to the murine chromosome 6 and rat chromosome 4 (Hirnet et al. 2003; Hoenderop et al. 1999; Muller et al. 2000; Peng et al. 1999; Wissenbach et al. 2001). Nonmammalian animals contain only one TRPV5-/6-like gene which is slightly more similar to TRPV6. This indicates that TRPV5 and TRPV6 arose from gene duplication. TRPV5, which is mainly expressed in kidney, might be an adaptation to specific requirements of the secondary kidneys of mammalian animals (Peng 2011).
The TRPV6-related sequence of the green algae Chlamydomonas reinhardtii shows a clearly higher degree of homology to mammalian TRPV6 proteins as one would expect. From this finding one would speculate that the Chlamydomonas TRPV6 is the result of horizontal gene transfer at a late time point during evolution (Merchant et al. 2007).
One has to mention that in humans two alleles of TRPV6 are known; the ancestral form, TRPV6a which differs to TRPV6b by 3 amino acids, namely, R197V, V318M, and T721M (Wissenbach et al. 2001). The mammalian TRPV6 sequences of animals reflect the TRPV6a variant whereas the TRPV6b variant occurs exclusively in humans. With increasing distance to the African continent the allele frequency of TRPV6b increases with Asian populations being up to 97% homozygous for TRPV6b whereas homozygous TRPV6a individuals are hardly detectable. In contrast the TRPV6a allele frequency increases in African population from north to south resulting in up to 85% TRPV6a alleles (Akey et al. 2006). The reason for this unequal distribution is not clear since TRPV6a and TRPV6b show very similar electrophysiological properties (Hughes et al. 2008). The TRPV6b variant is not present in apes and as calculated from the molecular clock one would estimate that the TRPV6b variant came up ∼2.7 million years ago. This is clearly after the split of human and apes around 4 million years ago (Pennisi 2006) and before the modern humans left Africa around 150.000 years ago and replacing the Neanderthals in Europe (Caramelli et al. 2003; Stringer and Andrews 1988).
The cDNA of the human TRPV6 was cloned from human placenta, and the translated cDNA sequence showed no in frame stop codon upstream of the first in frame AUG triplet (Wissenbach et al. 2001). Translation of the whole cDNA sequence revealed that the amino acid sequences upstream of the first AUG codon is conserved among several mammalian species which is unusual. Fecher-Trost and coworkers studied if the translational start of TRPV6 is upstream of the first AUG triplet (Fecher-Trost et al. 2013). Indeed the TRPV6 translation starts exclusively at an ACG triplet 120 bp upstream of the first AUG triplet. MS data obtained by mass spectrometry show that this ACG triplet which would code threonine (T) is instead translated into methionine (M). The full length TRPV6 protein is slightly better transferred to the plasma membrane as the TRPV6 initiated at the first AUG codon. In fact this N-terminal-truncated TRPV6 does not occur in vivo. Nevertheless the electrophysiological properties of the full length TRPV6 channels formed from the full-length 765 aa protein is similar to channels formed by the truncated 725 aa variant.
The family of transient receptor potential, TRP, channels consists mostly of nonselective cation channels with the exception of TRPV6 and TRPV5 which are highly Ca2+ selective channels showing an ion conductance of PCa/PNa > 100. TRPV6 (and TRPV5) belong together with L-type calcium channels and CRAC channels (Orai/Stim) to the most calcium selective ion channels which are known. Genetic inactivation of the TRPV6 gene leads to hypofertility of male mice as the consequence of inadequate sperm maturation within the epididymis. In the human population two TRPV6 alleles are present which differ in three amino acids, TRPV6a and TRPV6b. The TRPV6 protein of apes reflects the human TRPV6a variant; thus human TRPV6b is one of the very rare proteins which is significantly different compared to higher primates. The translational start triplet of TRPV6 is an ACG codon which usually would be translated into threonine but instead is translated into methionine (Fecher-Trost et al. 2013). TRPV6 has a restricted expression pattern in humans and is most dominantly expressed in the human placenta, exocrine pancreas, and some exocrine gland tissues, but in addition, overexpressed in frequent malignancies as prostate and breast cancer (Bolanz et al. 2008; Fixemer et al. 2003; Wissenbach et al. 2004; Wissenbach et al., 2001). From these findings TRPV6 is a promising drug target for the treatment of several malignancies.
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