Requirement for the N-terminal coiled-coil domain for expression and function, but not subunit interaction of, the ADPR-activated TRPM2 channel

The coiled-coil domain of zebrafish TRPM7 regulates Mg·nucleotide sensitivity

TRPM7 is a member of the Transient-Receptor-Potential Melastatin ion channel family. TRPM7 is a unique fusion protein of an ion channel and an α-kinase. Although mammalian TRPM7 is well characterized biophysically and its pivotal role in cancer, ischemia and cardiovascular disease is becoming increasingly evident, the study of TRPM7 in mouse models has been hampered by embryonic lethality of transgenic ablations. In zebrafish, functional loss of TRPM7 (drTRPM7) manifests itself in an array of non-lethal physiological malfunctions. Here, we investigate the regulation of wild type drTRPM7 and multiple C-terminal truncation mutants. We find that the biophysical properties of drTRPM7 are very similar to mammalian TRPM7. However, pharmacological profiling reveals that drTRPM7 is facilitated rather than inhibited by 2-APB, and that the TRPM7 inhibitor waixenicin A has no effect. This is reminiscent of the pharmacological profile of human TRPM6, the sister channel kinase of TRPM7. Furthermore, using truncation mutations, we show that the coiled-coil domain of drTRPM7 is involved in the channel's regulation by magnesium (Mg) and Mg·adenosine triphosphate (Mg·ATP). We propose that drTRPM7 has two protein domains that regulate inhibition by intracellular magnesium and nucleotides, and one domain that is concerned with sensing magnesium only.

Novel Alleles of gon-2, a C. elegans Ortholog of Mammalian TRPM6 and TRPM7, Obtained by Genetic Reversion Screens

TRP (Transient Receptor Potential) cation channels of the TRPM subfamily have been found to be critically important for the regulation of Mg²⁺ homeostasis in both protostomes (e.g., the nematode, C. elegans, and the insect, D. melanogaster) and deuterostomes (e.g., humans). Although significant progress has been made toward understanding how the activities of these channels are regulated, there are still major gaps in our understanding of the potential regulatory roles of extensive, evolutionarily conserved, regions of these proteins. The C. elegans genes, gon-2, gtl-1 and gtl-2, encode paralogous TRP cation channel proteins that are similar in sequence and function to human TRPM6 and TRPM7. We isolated fourteen revertants of the missense mutant, gon-2(q338), and these mutations affect nine different residues within GON-2. Since eight of the nine affected residues are situated within regions that have high similarity to human TRPM1,3,6 and 7, these mutations identify sections of these channels that are potentially critical for channel regulation. We also isolated a single mutant allele of gon-2 during a screen for revertants of the Mg²⁺-hypersensitive phenotype of gtl-2(-) mutants. This allele of gon-2 converts a serine to phenylalanine within the highly conserved TRP domain, and is antimorphic against both gon-2(+) and gtl-1(+). Interestingly, others have reported that mutation of the corresponding residue in TRPM7 to glutamate results in deregulated channel activity.

Comparative genomic analyses reveal a vast, novel network of nucleotide-centric systems in biological conflicts, immunity and signaling

Cyclic di- and linear oligo-nucleotide signals activate defenses against invasive nucleic acids in animal immunity; however, their evolutionary antecedents are poorly understood. Using comparative genomics, sequence and structure analysis, we uncovered a vast network of systems defined by conserved prokaryotic gene-neighborhoods, which encode enzymes generating such nucleotides or alternatively processing them to yield potential signaling molecules. The nucleotide-generating enzymes include several clades of the DNA-polymerase β-like superfamily (including Vibrio cholerae DncV), a minimal version of the CRISPR polymerase and DisA-like cyclic-di-AMP synthetases. Nucleotide-binding/processing domains include TIR domains and members of a superfamily prototyped by Smf/DprA proteins and base (cytokinin)-releasing LOG enzymes. They are combined in conserved gene-neighborhoods with genes for a plethora of protein superfamilies, which we predict to function as nucleotide-sensors and effectors targeting nucleic acids, proteins or membranes (pore-forming agents). These systems are sometimes combined with other biological conflict-systems such as restriction-modification and CRISPR/Cas. Interestingly, several are coupled in mutually exclusive neighborhoods with either a prokaryotic ubiquitin-system or a HORMA domain-PCH2-like AAA+ ATPase dyad. The latter are potential precursors of equivalent proteins in eukaryotic chromosome dynamics. Further, components from these nucleotide-centric systems have been utilized in several other systems including a novel diversity-generating system with a reverse transcriptase. We also found the Smf/DprA/LOG domain from these systems to be recruited as a predicted nucleotide-binding domain in eukaryotic TRPM channels. These findings point to evolutionary and mechanistic links, which bring together CRISPR/Cas, animal interferon-induced immunity, and several other systems that combine nucleic-acid-sensing and nucleotide-dependent signaling.

Lys1110 of TRPM2 is critical for channel activation

TRPM2 (transient receptor potential melastatin 2) is a non-selective Ca2+-permeable cation channel activated by ADPR (adenosine diphosphoribose) and H2O2. It is widely expressed in mammalian cells and plays an important role in the regulation of various cell functions. However, the mechanisms of TRPM2 channel activation are not fully understood. Previously, we reported that TRPM2 channel activation is induced by high intracellular Cl- concentration. In the present study, we investigated the functional role of Lys1110 in the membrane-proximal C-terminal region by site-directed mutagenesis. Replacement of the positively charged amino acid lysine (Lys1110) with the neutrally charged amino acid asparagine (K1110N) or the negatively charged amino acid glutamic acid (K1110E) generated mutants that failed to induce an increase in free cytosolic calcium concentration ([Ca2+]i) not only by intracellular injection of Cl-, but also by H2O2 or ADPR. However, a mutant generated by replacing the lysine residue with a positively charged amino acid arginine (K1110R) displayed channel activity similar to wild-type TRPM2. Interestingly, in the K1107N/K1110N double-point mutant, the impaired function of the K1110N mutant in response to ADPR and H2O2, but not to Cl-, was recovered. There were no changes in protein expression, membrane trafficking and oligomerization of the mutant channels. The extent of [Ca2+]i increase by H2O2 in HEK (human embryonic kidney)-293 cells expressing TRPM2 mutants was well correlated with the degree of susceptibility to H2O2-induced cell death. These results display the crucial role of a positively charged amino acid residue at position 1110 for TRPM2 channel activity.

Canonical transient receptor potential (TRPC) 1 acts as a negative regulator for vanilloid TRPV6-mediated Ca2+ influx

TRP proteins mostly assemble to homomeric channels but can also heteromerize, preferentially within their subfamilies. The TRPC1 protein is the most versatile member and forms various TRPC channel combinations but also unique channels with the distantly related TRPP2 and TRPV4. We show here a novel cross-family interaction between TRPC1 and TRPV6, a Ca²⁺ selective member of the vanilloid TRP subfamily. TRPV6 exhibited substantial co-localization and in vivo interaction with TRPC1 in HEK293 cells, however, no interaction was observed with TRPC3, TRPC4, or TRPC5. Ca²⁺ and Na⁺ currents of TRPV6-overexpressing HEK293 cells are significantly reduced by co-expression of TRPC1, correlating with a dramatically suppressed plasma membrane targeting of TRPV6. In line with their intracellular retention, remaining currents of TRPC1 and TRPV6 co-expression resemble in current-voltage relationship that of TRPV6. Studying the N-terminal ankyrin like repeat domain, structurally similar in the two proteins, we have found that these cytosolic segments were sufficient to mediate a direct heteromeric interaction. Moreover, the inhibitory role of TRPC1 on TRPV6 influx was also maintained by expression of only its N-terminal ankyrin-like repeat domain. Our experiments provide evidence for a functional interaction of TRPC1 with TRPV6 that negatively regulates Ca²⁺ influx in HEK293 cells.

Heteromerization of TRP channel subunits: extending functional diversity

Transient receptor potential (TRP) channels are widely found throughout the animal kingdom. By serving as cellular sensors for a wide spectrum of physical and chemical stimuli, they play crucial physiological roles ranging from sensory transduction to cell cycle modulation. TRP channels are tetrameric protein complexes. While most TRP subunits can form functional homomeric channels, heteromerization of TRP channel subunits of either the same subfamily or different subfamilies has been widely observed. Heteromeric TRP channels exhibit many novel properties compared to their homomeric counterparts, indicating that co-assembly of TRP channel subunits has an important contribution to the diversity of TRP channel functions.

State-dependent inhibition of TRPM2 channel by acidic pH

Transient receptor potential melastatin 2 (TRPM2) channel fulfills an important role in oxidative stress signaling in immune and other cells, to which local extracellular acidosis is known to occur under physiological or pathological conditions and impose significant effects on their functions. Here, we investigated whether the ADP-ribose-activated TRPM2 channel is a target for modulation by extracellular acidic pH by patch clamp recording of HEK293 cells expressing hTRPM2 channel. Induced whole cell or single channel currents were rapidly inhibited upon subsequent exposure to acidic pH. The inhibition in the steady state was complete, voltage-independent, and pH-independent in the range of pH 4.0–6.0. The inhibition was irreversible upon returning to pH 7.3, suggesting channel inactivation. In contrast, exposure of closed channels to acidic pH reduced the subsequent channel activation in a pH-dependent manner with an IC₅₀ for H⁺ of 20 μm (pH 4.7) and rendered subsequent current inhibition largely reversible, indicating differential or state-dependent inhibition and inactivation. Alanine substitution of residues in the outer vestibule of the pore including Lys⁹⁵² and Asp¹⁰⁰² significantly slowed down or reduced acidic pH-induced inhibition and prevented inactivation. The results suggest that acidic pH acts as a negative feedback mechanism where protons bind to the outer vestibule of the TRPM2 channel pore and inhibit the TRPM2 channels in a state-dependent manner.