TPC2 polymorphisms associated with a hair pigmentation phenotype in humans result in gain of channel function by independent mechanisms

Abstract

Two-pore channels (TPCs) are endolysosomal cation channels. Two members exist in humans, TPC1 and TPC2. Functional roles associated with the ubiquitously expressed TPCs include VEGF-induced neoangiogenesis, LDL-cholesterol trafficking and degradation, physical endurance under fasting conditions, autophagy regulation, the acrosome reaction in sperm, cancer cell migration, and intracellular trafficking of pathogens such as Ebola virus or bacterial toxins (e.g., cholera toxin). In a genome-wide association study for variants associated with human pigmentation characteristics two coding variants of TPC2, rs35264875 (encoding M484L) and rs3829241 (encoding G734E), have been found to be associated with a shift from brown to blond hair color. In two recent follow-up studies a role for TPC2 in pigmentation has been further confirmed. However, these human polymorphic variants have not been functionally characterized until now. The development of endolysosomal patch-clamp techniques has made it possible to investigate directly ion channel activities and characteristics in isolated endolysosomal organelles. We applied this technique here to scrutinize channel characteristics of the polymorphic TPC2 variants in direct comparison with WT. We found that both polymorphisms lead to a gain of channel function by independent mechanisms. We next conducted a clinical study with more than 100 blond- and brown/black-haired individuals. We performed a genotype/phenotype analysis and subsequently isolated fibroblasts from WT and polymorphic variant carriers for endolysosomal patch-clamp experimentation to confirm key in vitro findings.

Targeting Two-Pore Channels: Current Progress and Future Challenges

“…Advances in molecular biology have enabled cell type-specific conditional TPC1 and TPC2 KO cells and animals, allowing the study of the distinctive roles of TPC1 and TPC2. In addition, a recent report revealed that two variants of point mutations on human TPC2 are linked with alteration of hair pigmentation [ 93 ]. Therefore, our understanding of TPCs would also be broadened by studying data from national and international biobanks and investigating clinical symptoms or tissue samples of individuals identified with naturally occurring mutations on human TPC1 and TPC2 (see Outstanding Questions).…”

The protein interaction networks of mucolipins and two-pore channels

“…While TPC2 sensitivity to ATP requires mTORC1 kinase activity, the mTOR target site on TPC2 remains uncharacterised. Recently however, a widely distributed human TPC2 gain-of-function polymorphism (G734E) was linked to decreased ATP sensitivity [34]. This could provide a stepping stone to further elucidate the exact mechanism of mTORC1-dependent TPC regulation.…”

Membrane transport proteins in melanosomes: Regulation of ions for pigmentation

“…Symptoms of the Menkes patients vary but include abnormalities in hair and skin [62]. TPC2 defect leads to abnormalities of melanin production and pigmentation [63]; however, their corresponding disease was not reported (Table 1). Here, we summarize the current knowledge of the ion transport proteins-their physical properties, biochemical functions, physiological roles and pathological effects in melanosomes.…”

“…Meanwhile, Bellono et al demonstrated the PI(3,5)P 2 activates TPC2-mediated Na + -selective conductance [126]. The response to PI(3,5)P 2 is further observed in a TPC2-expressing endolysosome models [63]. Nevertheless, loss of TPC2 clearly affects melanosomal pH and size, and melanin production.…”