InteractiveFly: GeneBrief

Transient receptor potential cation channel, subfamily M: Biological Overview | References


Gene name - Transient receptor potential cation channel, subfamily M

Synonyms -

Cytological map position - 2R

Function - cation channel

Keywords - Trp family channel - mediates the calcium influx and initiates the calcium wave during Drosophila egg activation/a> - senses oxidative stress to release Zn(2+) from unique intracellular vesicles - mediates aversion to cold

Symbol - Trpm

FlyBase ID: FBgn0265194

Genetic map position - chr2R:15,262,808-15,291,302

NCBI classification - LSDAT_euk: SLOG in TRPM

Cellular location - surface transmembrane, surface of intracellular vesicles



NCBI links: EntrezGene, Nucleotide, Protein
BIOLOGICAL OVERVIEW

The Drosophila Trpm channel mediates calcium influx during egg activation

Egg activation is the process in which mature oocytes are released from developmental arrest and gain competency for embryonic development. In Drosophila and other arthropods, eggs are activated by mechanical pressure in the female reproductive tract, whereas in most other species, eggs are activated by fertilization. Despite the difference in the trigger, Drosophila shares many conserved features with higher vertebrates in egg activation, including a rise of intracellular calcium in response to the trigger. In Drosophila, this calcium rise is initiated by entry of extracellular calcium due to opening of mechanosensitive ion channels and initiates a wave that passes across the egg prior to initiation of downstream activation events. This study combined inhibitor tests, germ-line-specific RNAi knockdown, and germ-line-specific CRISPR/Cas9 knockout to identify the Transient Receptor Potential (TRP) channel subfamily M (Trpm) as a critical channel that mediates the calcium influx and initiates the calcium wave during Drosophila egg activation. A reduction was observed in the proportion of eggs that hatched from trpm germ-line knockout mutant females, although eggs were able to complete some egg activation events including cell cycle resumption. Since a mouse ortholog of Trpm was recently reported also to be involved in calcium influx during egg activation and in further embryonic development, these results suggest that calcium uptake from the environment via TRPM channels is a deeply conserved aspect of egg activation (Hu, 2019).

In almost all animals, mature oocytes are arrested in meiosis at the end of oogenesis and require an external trigger to be activated and transition to start embryogenesis. This 'egg activation' involves multiple events that transition eggs to embryogenesis, including meiosis resumption and completion, maternal protein modification and/or degradation, maternal mRNA degradation or translation, and egg envelope changes (Hu, 2019).

Triggers of egg activation vary across species. In vertebrate and some invertebrate species, fertilization triggers egg activation. However, changes in pH, ionic environment, or mechanical pressure can also trigger egg activation in other invertebrate species. A conserved response to these triggers is a rise of intracellular free Ca2+ levels in the oocyte. This calcium rise is due to influx of external calcium and/or release from internal storage, depending on the organism. The elevated Ca2+ concentration is thought to activate Ca2+- dependent kinases and/or phosphatases, which in turn change the phospho-proteome of the activated egg, initiating egg activation events (Hu, 2019).

Drosophila eggs activate independent of fertilization and the trigger is mechanical pressure. When mature oocytes exit the ovary and enter the lateral oviduct, they experience mechanical pressure from reproductive tract tract. As the oocytes swell due to the influx of oviductal fluid, their envelopes become taut. Drosophila oocytes can be activated in vitro by incubation in a hypotonic buffer, although some egg activation events do not proceed completely normally in vitro. Intracellular calcium levels rise in oocytes occur egg activation, as observed with the calcium sensor GCaMP. This calcium rise takes the form of a wave that starts at the oocyte pole(s) and traverses the entire oocyte. Initiation of this calcium wave requires influx of external Ca2+, as chelating external Ca2+ in in vitro egg activation assays blocks the calcium wave and egg activation. Propagation of the calcium wave relies on the release of internal Ca2+ stores, likely through an Inositol 1,4,5-trisphosphate (IP3) mediated pathway, as knocking down the endoplasmic reticulum (ER) calcium channel IP3 receptor (IP3R) prevents propagation of the calcium wave (Hu, 2019).

How mechanical forces trigger calcium entry during Drosophila egg activation was unknown. However, the lack of initiation of a calcium wave in the presence of Gd3+, an inhibitor of mechanosensitive ion channels, and N-(p-Amylcinnamoyl) anthranilic acid (ACA), an inhibitor of TRP-family ion channels, suggested that TRP family ion channels are likely involved. Further supporting this idea is the recent discovery that a TRP family channel, TRPM7, is needed for calcium influx that leads to calcium oscillations in activating mouse eggs. The Drosophila genome encodes 13 TRP family channels, but according to RNAseq data, only 3 (Painless, Trpm, and Trpml) are expressed in the ovary. This study used specific inhibitors, existing mutants, germline- specific RNAi knockdown, and new knockouts that were created with CRISPR/Cas9 to screen these 3 candidates for their roles in the initiation of the calcium wave. Trpm, the single Drosophila ortholog of mouse TRPM7, was shown to mediate the calcium wave initiation, whereas the other two TRP channels are not necessary to initiate the calcium wave (Hu, 2019).

Calcium wave phenotypes are normal in oocytes from pain or trpml null mutants. However, the frequency of the calcium wave is diminished in wildtype oocytes in the presence of Trpm inhibitors and in oocytes from trpm germline knockdown or knockout mutants. These results consistently indicated that Trpm mediates the calcium influx that initiates the calcium wave during Drosophila egg activation. trpm germline knockout females also displayed significantly decreased egg hatchability, due to defects after cell cycle resumption. The reduced hatchability suggested that maternal trpm function or the calcium wave is required for further embryogenesis after egg activation (Hu, 2019).

TRP family ion channels are non-selective and respond to a wide array of environmental stimuli. Drosophila Trpm has been reported to play multiple roles throughout larval development, including maintaining Mg2+ and Zn2+ homeostasis (Georgiev, 2010; Hofmann, 2010), and sensing noxious cold in larval Class III md neurons (Turner, 2016). However, the role of Trpm in reproduction had not been investigated because of the pupal lethality of trpm null mutants. In this study germline specific RNAi knockdown and CRISPR/Cas9 mediated knockout revealed three novel functions of Drosophila Trpm: supporting early oogenesis, mediating influx of environmental calcium to initiate the calcium wave during egg activation, and maternally supporting embryonic development after egg activation (Hu, 2019).

A previous study suggested that calcium influx during Drosophila egg activation is mediated through mechanosensitive ion channels (Homer, 2008). Both Drosophila Trpm and its mouse ortholog TRPM7 are reported to be constitutively active and permeable to a wide range of divalent cations (Georgiev, 2010). Mouse TRPM7 is known to respond to mechanical pressure, but further study will be needed to determine whether Drosophila Trpm is similarly responsive to mechanical triggers, such as those that occur during ovulation (Hu, 2019).

Germline knockout of trpm significantly reduced the frequency of observing calcium waves in in vitro egg activation assays and egg hatchability. However, this reduced egg hatchability was not due to failure of cell cycle resumption during egg activation. There are two possible explanations for the reduced egg hatchability of trpm germline knockout females (Hu, 2019).

First, it is possible that trpm plays a maternal role, independent of its role in initiating the calcium wave, such that lack of maternally deposited Trpm proteins leads to defects during embryogenesis. In mouse, TRPM7 is also required for normal early embryonic development, apart from its role in calcium oscillations. Inhibition of TRPM7 function impairs pre-implantation embryo development and slows progression to the blastocyst stage. Drosophila trpm mutant lethality had been reported to occur during the pupal stage. However, those homozygous mutants were offspring of heterozygous mothers, and thus did not lack maternal Trpm function. Germline specific depletion of trpm reveals a maternal role for Trpm in embryogenesis (Hu, 2019).

Alternatively, or in addition, it is possible that oocytes lacking Trpm do not take up sufficient Ca2+ from the environment to form a calcium wave, but that at least some events of egg activation can occur despite this. In mouse, an initial calcium rise is induced by sperm-delivered PLCζ via the IP3 pathway. Yet although sperm from PLCζ null males fails to trigger normal calcium oscillations, some eggs fertilized by those sperm develop. Multiple oscillations following fertilization require influx of external calcium, mediated by TRPM7 and CaV3.2 (Bernhardt, 2018). Even though these oscillations were reported to be needed for multiple post-fertilization events, some TRPM7 and CaV3.2 double-knockout embryos still develop, albeit not completely normally. Together, these data suggest that egg activation can still occur in mouse with diminished intracellular calcium rises, analogous to what is seen in Drosophila in the absence of maternal Trpm function (Hu, 2019).

Insufficient influx of calcium in the absence of Trpm function could disrupt later (but maternally- dependent) embryogenesis. The oocyte-to-embryo transition involves multiple events. In mouse egg activation, these events take place sequentially as calcium oscillations progress, with developmental progression associated with more oscillations and more total calcium signal. Some of the events start after a certain number of oscillations but require additional oscillations to complete. It is possible that mechanisms critical for Drosophila embryo development also depend on reaching a precise level of calcium. A low-level calcium rise might be sufficient to trigger some egg activation mechanisms such as vitelline membrane crosslinking and cell cycle resumption, but high-levels of calcium may be required for further progression (Hu, 2019).

Given the importance of calcium in egg activation, it was surprising that although trpm knockout eggs lacked a calcium wave in vitro, in vivo such eggs could progress in cell cycles and even, sometimes, hatch. There may be insufficient calcium influx in the absence of Trpm for full and efficient development, but some egg activation events may still occur. Alternatively, it is possible that redundant mechanisms permit a sufficient calcium-level increase without producing a detectable wave form. Despite being able to trigger a series of egg activation events including meiosis resumption and protein translation, osmotic pressure during in vitro activation may have different properties from mechanical pressure exerted on mature oocytes during ovulation. The latter might allow opening of other calcium channels to initiate a normal calcium rise and complete egg activation. Two channels, TRPM7 and Cav3.2, are needed for the calcium oscillations following mouse fertilization, but the Drosophila ortholog of mouse Cav3.2, Ca-α1T, is not detectably expressed in fly ovaries. Other unknown channels might play this redundant role in vivo. In this light it is noted that levels of basal GCaMP fluorescence varied among oocytes incubated in IB that were inhibited from forming a wave, suggesting the possibility of a calcium increase by a redundant mechanism (Hu, 2019).

It was intriguing that Drosophila Trpm is essential for the calcium rise at egg activation, and that its mouse ortholog, TRPM7, was recently reported to be required (along with CaV3.2) for the calcium influx needed for post-fertilization calcium oscillations that are in turn required for egg activation events. This apparent conservation in mechanisms in egg activation involving orthologous Trpm channels in a protostome (Drosophila) and a deuterostome (mouse) prompts asking whether Trpm-mediated calcium influx is a very ancient and basal aspect of egg activation, with other more variable aspects such as sperm-triggered calcium rises being more derived, if better known, features. It is interesting in this light that a sperm-delivered TRP channel (TRP-3) has also been reported to mediate calcium influx and a calcium rise in another protostome, C. elegans (Hu, 2019).

The TRP channels Pkd2, NompC, and Trpm act in cold-sensing neurons to mediate unique aversive behaviors to noxious cold in Drosophila

The basic mechanisms underlying noxious cold perception are not well understood. This study developed Drosophila assays for noxious cold responses. Larvae respond to near-freezing temperatures via a mutually exclusive set of singular behaviors-in particular, a full-body contraction (CT). Class III (CIII) multidendritic sensory neurons are specifically activated by cold and optogenetic activation of these neurons elicits CT. Blocking synaptic transmission in CIII neurons inhibits CT. Genetically, the transient receptor potential (TRP) channels Trpm NompC, and Polycystic kidney disease 2 (Pkd2) are expressed in CIII neurons, where each is required for CT. Misexpression of Pkd2 is sufficient to confer cold responsiveness. The optogenetic activation level of multimodal CIII neurons determines behavioral output, and visualization of neuronal activity supports this conclusion. Coactivation of cold- and heat-responsive sensory neurons suggests that the cold-evoked response circuitry is dominant. This Drosophila model will enable a sophisticated molecular genetic dissection of cold nociceptive genes and circuits (Turner, 2016).

Drosophila menthol sensitivity and the precambrian origins of transient receptor potential-dependent chemosensation

Transient receptor potential (TRP) cation channels are highly conserved, polymodal sensors which respond to a wide variety of stimuli. Perhaps most notably, TRP channels serve critical functions in nociception and pain. A growing body of evidence suggests that transient receptor potential melastatin (TRPM) and transient receptor potential ankyrin (TRPA) thermal and electrophile sensitivities predate the protostome-deuterostome split (greater than 550 Ma). However, TRPM and TRPA channels are also thought to detect modified terpenes (e.g. menthol). Although terpenoids like menthol are thought to be aversive and/or harmful to insects, mechanistic sensitivity studies have been largely restricted to chordates. Furthermore, it is unknown if TRP-menthol sensing is as ancient as thermal and/or electrophile sensitivity. Combining genetic, optical, electrophysiological, behavioural and phylogenetic approaches, this study tested the hypothesis that insect TRP channels play a conserved role in menthol sensing. Topical application of menthol to Drosophila melanogaster larvae was found to elicit a Trpm- and TrpA1-dependent nocifensive rolling behaviour, which requires activation of Class IV nociceptor neurons. Further, in characterizing the evolution of TRP channels, the hypotheses were put forward that three previously undescribed TRPM channel clades (basal, alphaTRPM and betaTRPM), as well as TRPs with residues critical for menthol sensing, were present in ancestral bilaterians (Himmel, 2019).

TRPM channels mediate zinc homeostasis and cellular growth during Drosophila larval development

TRPM channels have emerged as key mediators of diverse physiological functions. However, the ionic permeability relevant to physiological function in vivo remains unclear for most members. This study reports that the single Drosophila TRPM gene (dTRPM) generates a conductance permeable to divalent cations, especially Zn(2+) and in vivo a loss-of-function mutation in dTRPM disrupts intracellular Zn(2+) homeostasis. TRPM deficiency leads to profound reduction in larval growth resulting from a decrease in cell size and associated defects in mitochondrial structure and function. These phenotypes are cell-autonomous and can be recapitulated in wild-type animals by Zn(2+) depletion. Both the cell size and mitochondrial defect can be rescued by extracellular Zn(2+) supplementation. Thus these results implicate TRPM channels in the regulation of cellular Zn(2+) in vivo. It is proposed that regulation of Zn(2+) homeostasis through dTRPM channels is required to support molecular processes that mediate class I PI3K-regulated cell growth (Georgiev, 2010).

Transient receptor potential (TRP) proteins are a large family of cation channels with a range of ionic permeabilities and functions. Although many TRP channels appear to mediate sensory transduction in eukaryotes, it is increasingly clear that some members of this family are also involved in regulating fundamental cellular processes unrelated to sensory transduction (Georgiev, 2010).

TRP channels have been classified into several subfamilies. Of these, the TRPM subfamily is distinguished by the presence of a unique TRPM domain at the N terminus. Genes encoding TRPM-like channels are not found in the yeast genome but are found in a range of metazoan species including C. elegans, Drosophila, zebrafish, and mammals. Four TRPM genes have been described in C. elegans. Of these, gon-2 and gtl-1 have been implicated in intestinal electrolyte homeostasis, postembryonic mitosis in the gonad, and body wall contraction. The zebrafish genome encodes four TRPM genes; a mutant in trpm7 shows defects in skeletogenesis, embryonic melanophore formation, growth, and touch responses. Mammalian TRPM channels have been linked to key physiological processes such as pancreatic β cell function, magnesium homeostasis, cell viability, anoxic cell death, cold sensation, and taste transduction (Georgiev, 2010).

A range of ionic permeabilities has been reported for TRPM channels. Four members, TRPM2, TRPM3α2, TRPM6, and TRPM7, are reported to be permeable to Mg2+. Loss-of-function phenotypes in TRPM7 are reported to be rescued by Mg2+ supplementation, and supplementation with Ca2+ and Mg2+ has been reported to rescue distinct aspects of the zebrafish trpm7 mutant phenotype. However, a recent study reported no defects in acute Mg2+ uptake or total cellular Mg2+ in lymphoctyes from murine TRPM7 KO animals. Thus, presently the ionic permeability relevant to the cellular function of TRPM7 in vivo remains unresolved (Georgiev, 2010).

This study found that functional expression of the single dTRPM gene generates a conductance that is suppressed by high intracellular Mg2+ and is permeable to a range of divalent cations. This divalent permeability was abolished in a pore mutant (E1007Q), strongly suggesting that dTRPM is part of an ion-conducting pore. While the dTRPM conductance permeates a range of divalent cations, it is especially conductive to Zn2+, and its permeability profile is reminiscent of that reported for mammalian TRPM6 and TRPM7 and TRPM3α2 (Georgiev, 2010).

During this study, four key observations were made: (1) Zn2+ depletion in wild-type larvae can recapitulate the cell size and mitochondrial defects seen in dTRPM-deficient salivary glands; (2) the effects of Zn2+ depletion on salivary gland cell size was smaller in dTRPM-deficient cells compared to wild-type; (3) the defects in cell size and mitochondrial morphology of dTRPM-deficient cells could be rescued by supplementation with extracellular Zn2+; (4) finally, dTRPM28 larvae show defects in Zn2+ homeostasis. These observations strongly suggest a causal relationship between altered Zn2+ homeostasis and the cellular phenotypes of dTRPM deficiency in vivo. Taken together, they provide compelling evidence of a role for dTRPM-dependent Zn2+ homeostasis in supporting cell growth during Drosophila larval development. Consistent with these findings of altered Zn2+ homeostasis and associated larval growth defect in dTRPM28, embryonic growth defects have been reported in a mouse KO of the SLC30A family Zn2+ transporter ZnT1, and RNAi knockdown of the Drosophila ortholog of ZnT1, required for Zn2+ uptake in intestinal cells, results in growth defects during larval development. Interestingly, although a number of previous studies have implicated Mg2+ permeation through mammalian TRPM7 as relevant for its physiological function, a recent study has reported that Mg2+ levels and homeostasis are unaffected in TRPM7 K/O mice. Like dTRPM, TRPM7 has also been reported to be particularly permeable to Zn2+; thus it will be interesting to study if Zn2+ homeostasis in mammalian systems is regulated by TRPM7 (Georgiev, 2010).

What is the role of dTRPM in regulating cellular Zn2+? The results demonstrate Zn2+ induced MtnC transcription following extracellular Zn2+ supplementation in dTRPM28 cells, implying Zn2+ entry into the cytoplasm. This result suggests that dTRPM is unlikely to be the exclusive or major route of Zn2+ influx across the plasma membrane; rather, the major route of Zn2+ entry across the plasma membrane is dependent on Zn2+ transporters. It is more likely that dTRPM regulates Zn2+ homeostasis in a specific subcellular compartment that plays a critical, nonredundant role in growth. Presently the subcellular localization of endogenous dTRPM channels is unknown; this information will be critical to defining the subcellular compartment in which dTRPM regulates Zn2+ homeostasis. Whatever the nature of this compartment, it is likely that there will be complex but important crosstalk between dTRPM and Zn2+ transporters. The observation of altered CG3994 transcription in dTRPM28 is an indication of this. Further analysis of the interactions between dTRPM with Zn2+ transporters with respect to cell size regulation will be required to unravel the nature of this crosstalk (Georgiev, 2010).

The rapid and dramatic increase in body mass during Drosophila larval development is almost entirely underpinned by increases in cell size in larval-specific tissues. Although a complex neuroendocrine axis coordinates this growth, in this study it was found that the requirement of dTRPM to support salivary gland cell growth was cell autonomous. This requirement for dTRPM function in cell growth appears to be subsequent to normal activation of class I PI3K and its key downstream growth regulator S6K and independent of altered dFOXO function. These data offer compelling evidence of a requirement for dTRPM function for molecular processes that operate subsequent to activation of dS6K. Interestingly, DT-40 chicken B-lymphoma cell lines with TRPM7 deleted have been reported to have a small reduction in cell size, although the proposed function of TRPM7 in this setting is mechanistically different (Georgiev, 2010).

Since the data indicate a strong causal relationship between altered Zn2+ homeostasis and cell growth in dTRPM28 that occurs downstream of S6K activation, what molecular processes might be affected? The Drosophila genome contains ca. 500 proteins with obvious Zn2+-binding motifs (such as Zn2+ fingers), and it is very likely that multiple Zn2+-dependent molecular processes are affected by the loss of dTRPM. However, given that the incorporation of new biomass is a key element of increase in cell size, it is likely that Zn2+-dependent processes that affect macromolecular biosynthesis are impaired. In most cells, protein synthesis consumes more energy than any other biosynthetic process; at least four high-energy phosphate bonds are split to make each new peptide bond. This study found defects in mitochondrial structure and function in dTRPM-deficient cells that could be recapitulated in wild-type larvae by Zn2+ depletion and suppressed in dTRPM-deficient salivary gland cells by extracellular Zn2+ supplementation. Taken together, these observations imply an important role for dTRPM-dependent Zn2+ homeostasis in regulating mitochondrial structure in Drosophila salivary gland cells. Along with similar larval growth phenotypes reported for mutants in genes encoding mitochondrial ribosomes, the altered mitochondrial structure and the reduced ATP levels in dTRPM28 that this study reports strongly suggest that the cellular growth defect in dTRPM28 larvae is underpinned by a major deficit in cellular energy supply required to undertake protein biosynthesis. However, this does not preclude additional roles for other Zn2+-dependent cellular processes (such as the function of transcription factors, chromatin remodeling enzymes, and other proteins) in the cell growth defect seen in dTRPM mutants (Georgiev, 2010).

Drosophila TRPM channel is essential for the control of extracellular magnesium levels

The TRPM group of cation channels plays diverse roles ranging from sensory signaling to Mg2+ homeostasis. In most metazoan organisms the TRPM subfamily is comprised of multiple members, including eight in humans. However, the Drosophila TRPM subfamily is unusual in that it consists of a single member. Currently, the functional requirements for this channel have not been reported. This study found that the Drosophila TRPM protein was expressed in the fly counterpart of mammalian kidneys, the Malpighian tubules, which function in the removal of electrolytes and toxic components from the hemolymph. Mutations were generated in trpm; this resulted in shortening of the Malpighian tubules. In contrast to all other Drosophila trp mutations, loss of trpm was essential for viability, as trpm mutations resulted in pupal lethality. Supplementation of the diet with a high concentration of Mg2+ exacerbated the phenotype, resulting in growth arrest during the larval period. Feeding high Mg2+ also resulted in elevated Mg2+ in the hemolymph, but had relatively little effect on cellular Mg2+. It is concluded that loss of Drosophila trpm leads to hypermagnesemia due to a defect in removal of Mg2+ from the hemolymph. These data provide the first evidence for a role for a Drosophila TRP channel in Mg2+ homeostasis, and underscore a broad and evolutionarily conserved role for TRPM channels in Mg2+ homeostasis (Hofmann, 2010).


Functions of Trpm orthologs in other species

TRPM7 and CaV3.2 channels mediate Ca(2+) influx required for egg activation at fertilization

The success of mammalian development following fertilization depends on a series of transient increases in egg cytoplasmic Ca(2+), referred to as Ca(2+) oscillations. Maintenance of these oscillations requires Ca(2+) influx across the plasma membrane, which is mediated in part by T-type, CaV3.2 channels. This study shows using genetic mouse models that TRPM7 channels are required to support this Ca(2+) influx. Eggs lacking both TRPM7 and CaV3.2 stop oscillating prematurely, indicating that together they are responsible for the majority of Ca(2+) influx immediately following fertilization. Fertilized eggs lacking both channels also frequently display delayed resumption of Ca(2+) oscillations, which appears to require sperm-egg fusion. TRPM7 and CaV3.2 channels almost completely account for Ca(2+) influx observed following store depletion, a process previously attributed to canonical store-operated Ca(2+) entry mediated by STIM/ORAI interactions. TRPM7 serves as a membrane sensor of extracellular Mg(2+) and Ca(2+) concentrations and mediates the effects of these ions on Ca(2+) oscillation frequency. When bred to wild-type males, female mice carrying eggs lacking TRPM7 and CaV3.2 are subfertile, and their offspring have increased variance in postnatal weight. These in vivo findings confirm previous observations linking in vitro experimental alterations in Ca(2+) oscillatory patterns with developmental potential and offspring growth. The identification of TRPM7 and CaV3.2 as key mediators of Ca(2+) influx following fertilization provides a mechanistic basis for the rational design of culture media that optimize developmental potential in research animals, domestic animals, and humans (Bernhardt, 2018).

Signalling mechanisms mediating Zn(2+)-induced TRPM2 channel activation and cell death in microglial cells

Excessive Zn(2+) causes brain damage via promoting ROS generation. This study investigated the role of ROS-sensitive TRPM2 channel in H2O2/Zn(2+)-induced Ca(2+) signalling and cell death in microglial cells. H2O2/Zn(2+) induced concentration-dependent increases in cytosolic Ca(2+) concentration ([Ca(2+)]c), which was inhibited by PJ34, a PARP inhibitor, and abolished by TRPM2 knockout (TRPM2-KO). Pathological concentrations of H2O2/Zn(2+) induced substantial cell death that was inhibited by PJ34 and DPQ, PARP inhibitors, 2-APB, a TRPM2 channel inhibitor, and prevented by TRPM2-KO. Further analysis indicate that Zn(2+) induced ROS production, PARP-1 stimulation, increase in the [Ca(2+)]c and cell death, all of which were suppressed by chelerythrine, a protein kinase C inhibitor, DPI, a NADPH-dependent oxidase (NOX) inhibitor, GKT137831, a NOX1/4 inhibitor, and Phox-I2, a NOX2 inhibitor. Furthermore, Zn(2+)-induced PARP-1 stimulation, increase in the [Ca(2+)]c and cell death were inhibited by PF431396, a Ca(2+)-sensitive PYK2 inhibitor, and U0126, a MEK/ERK inhibitor. Taken together, this study shows PKC/NOX-mediated ROS generation and PARP-1 activation as an important mechanism in Zn(2+)-induced TRPM2 channel activation and, TRPM2-mediated increase in the [Ca(2+)]c to trigger the PYK2/MEK/ERK signalling pathway as a positive feedback mechanism that amplifies the TRPM2 channel activation. Activation of these TRPM2-depenent signalling mechanisms ultimately drives Zn(2+)-induced Ca(2+) overloading and cell death (Mortadza, 2017).

Reciprocal regulation of actin cytoskeleton remodelling and cell migration by Ca2+ and Zn2+: role of TRPM2 channels

Cell migration is a fundamental feature of tumour metastasis and angiogenesis. It is regulated by a variety of signalling molecules including H2O2 and Ca(2+) This study asked whether the H2O2-sensitive transient receptor potential melastatin 2 (TRPM2) Ca(2+) channel serves as a molecular link between H2O2 and Ca(2+) H2O2-mediated activation of TRPM2 channels induced filopodia formation, loss of actin stress fibres and disassembly of focal adhesions, leading to increased migration of HeLa and prostate cancer (PC)-3 cells. Activation of TRPM2 channels, however, caused intracellular release of not only Ca(2+) but also of Zn(2+) Intriguingly, elevation of intracellular Zn(2+) faithfully reproduced all of the effects of H2O2, whereas Ca(2+) showed opposite effects. Interestingly, H2O2 caused increased trafficking of Zn(2+)-enriched lysosomes to the leading edge of migrating cells, presumably to impart polarisation of Zn(2+) location. Thus, these results indicate that a reciprocal interplay between Ca(2+) and Zn(2+) regulates actin remodelling and cell migration; they call for a revision of the current notion that implicates an exclusive role for Ca(2+) in cell migration (Li, 2016).

TRPM6 and TRPM7 differentially contribute to the relief of heteromeric TRPM6/7 channels from inhibition by cytosolic Mg(2+) and Mg.ATP

TRPM6 and its homologue TRPM7 are alpha-kinase-coupled divalent cation-selective channels activated upon reduction of cytosolic levels of Mg(2+) and Mg.ATP. TRPM6 is vital for organismal Mg(2+) balance. However, mechanistically the cellular role and functional nonredundancy of TRPM6 remain incompletely understood. Comparative analysis of native currents in primary cells from TRPM6- versus TRPM7-deficient mice supported the concept that native TRPM6 primarily functions as a constituent of heteromeric TRPM6/7 channels. However, heterologous expression of the human TRPM6 protein engendered controversial results with respect to channel characteristics including its regulation by Mg(2+) and Mg.ATP. To resolve this issue, the mouse TRPM6 (mTRPM6) cDNA was cloned, and its functional characteristics were compared to mouse TRPM7 (mTRPM7) after heterologous expression. Notably, it was observed that mTRPM6 and mTRPM7 differentially regulate properties of heteromeric mTRPM6/7 channels: In the presence of mTRPM7, the extreme sensitivity of functionally expressed homomeric mTRPM6 to Mg(2+) is tuned to higher concentrations, whereas mTRPM6 relieves mTRPM7 from the tight inhibition by Mg.ATP. Consequently, the association of mTRPM6 with mTRPM7 allows for high constitutive activity of mTRPM6/7 in the presence of physiological levels of Mg(2+) and Mg.ATP, thus laying the mechanistic foundation for constant vectorial Mg(2+) transport specifically into epithelial cells (Ferioli, 2017).

The TRPM7 chanzyme is cleaved to release a chromatin-modifying kinase

TRPM7 is a ubiquitous ion channel and kinase, a unique 'chanzyme,' required for proper early embryonic development. It conducts Zn(2+), Mg(2+), and Ca(2+) as well as monovalent cations and contains a functional serine/threonine kinase at its carboxyl terminus. This study shows that in normal tissues and cell lines, the kinase is proteolytically cleaved from the channel domain in a cell-type-specific manner. These TRPM7 cleaved kinase fragments (M7CKs) translocate to the nucleus and bind multiple components of chromatin-remodeling complexes, including Polycomb group proteins. In the nucleus, the kinase phosphorylates specific serines/threonines of histones. M7CK-dependent phosphorylation of H3Ser10 at promoters of TRPM7-dependent genes correlates with their activity. This study also demonstrated that cytosolic free [Zn(2+)] is TRPM7 dependent and regulates M7CK binding to transcription factors containing zinc-finger domains. These findings suggest that TRPM7-mediated modulation of intracellular Zn(2+) concentration couples ion-channel signaling to epigenetic chromatin covalent modifications that affect gene expression patterns (Krapivinsky, 2014).


REFERENCES

Search PubMed for articles about Drosophila Trpm

Bernhardt, M. L., Stein, P., Carvacho, I., Krapp, C., Ardestani, G., Mehregan, A., Umbach, D. M., Bartolomei, M. S., Fissore, R. A. and Williams, C. J. (2018). TRPM7 and CaV3.2 channels mediate Ca(2+) influx required for egg activation at fertilization. Proc Natl Acad Sci U S A 115(44): E10370-E10378. PubMed ID: 30322909

Ferioli, S., Zierler, S., Zaisserer, J., Schredelseker, J., Gudermann, T. and Chubanov, V. (2017). TRPM6 and TRPM7 differentially contribute to the relief of heteromeric TRPM6/7 channels from inhibition by cytosolic Mg(2+) and Mg.ATP. Sci Rep 7(1): 8806. PubMed ID: 28821869

Georgiev, P., Okkenhaug, H., Drews, A., Wright, D., Lambert, S., Flick, M., Carta, V., Martel, C., Oberwinkler, J. and Raghu, P. (2010). TRPM channels mediate zinc homeostasis and cellular growth during Drosophila larval development. Cell Metab 12(4): 386-397. PubMed ID: 20889130

Himmel, N. J., Letcher, J. M., Sakurai, A., Gray, T. R., Benson, M. N. and Cox, D. N. (2019). Drosophila menthol sensitivity and the precambrian origins of transient receptor potential-dependent chemosensation. Philos Trans R Soc Lond B Biol Sci 374(1785): 20190369. PubMed ID: 31544603

Hofmann, T., Chubanov, V., Chen, X., Dietz, A. S., Gudermann, T. and Montell, C. (2010). Drosophila TRPM channel is essential for the control of extracellular magnesium levels. PLoS One 5(5): e10519. PubMed ID: 20463899

Hu, Q. and Wolfner, M. F. (2019). The Drosophila Trpm channel mediates calcium influx during egg activation. Proc Natl Acad Sci U S A. PubMed ID: 31427540

Krapivinsky, G., Krapivinsky, L., Manasian, Y. and Clapham, D. E. (2014). The TRPM7 chanzyme is cleaved to release a chromatin-modifying kinase. Cell 157(5): 1061-1072. PubMed ID: 24855944

Li, F., Abuarab, N. and Sivaprasadarao, A. (2016). Reciprocal regulation of actin cytoskeleton remodelling and cell migration by Ca2+ and Zn2+: role of TRPM2 channels. J Cell Sci 129(10): 2016-2029. PubMed ID: 27068538

Mortadza, S. S., Sim, J. A., Stacey, M. and Jiang, L. H. (2017). Signalling mechanisms mediating Zn(2+)-induced TRPM2 channel activation and cell death in microglial cells. Sci Rep 7: 45032. PubMed ID: 28322340

Turner, H. N., Armengol, K., Patel, A. A., Himmel, N. J., Sullivan, L., Iyer, S. C., Bhattacharya, S., Iyer, E. P. R., Landry, C., Galko, M. J. and Cox, D. N. (2016). The TRP channels Pkd2, NompC, and Trpm act in cold-sensing neurons to mediate unique aversive behaviors to noxious cold in Drosophila. Curr Biol 26(23): 3116-3128. PubMed ID: 27818173


Biological Overview

date revised: 20 April, 2020

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