Magnesium excretion in C. elegans requires the activity of the GTL-2 TRPM channel

Intestinal Development and Gut Disease: Contributions From the Caenorhabditis elegans Model

The mammalian intestine is a highly organized and complex system essential for nutrient absorption, immune response, and homeostasis. Disruptions in its development can lead to various gut diseases, ranging from congenital anomalies to inflammatory and neoplastic disorders. Caenorhabditis elegans (C elegans) has emerged as a valuable model organism for studying intestinal development and gut diseases due to its genetic tractability and transparent body. This review explores the significant contributions of C elegans research to our understanding of intestinal biology, examining historical milestones, anatomical and physiological insights, and its utility in modeling gut diseases and drug discovery. We also draw comparative insights into mammalian systems and propose future research directions. The findings highlight the potential of C elegans as an essential model system for advancing our knowledge of intestinal development and its implications for human health.

Differential regulation of degradation and immune pathways underlies adaptation of the ectosymbiotic nematode Laxus oneistus to oxic-anoxic interfaces

Eukaryotes may experience oxygen deprivation under both physiological and pathological conditions. Because oxygen shortage leads to a reduction in cellular energy production, all eukaryotes studied so far conserve energy by suppressing their metabolism. However, the molecular physiology of animals that naturally and repeatedly experience anoxia is underexplored. One such animal is the marine nematode Laxus oneistus. It thrives, invariably coated by its sulfur-oxidizing symbiont Candidatus Thiosymbion oneisti, in anoxic sulfidic or hypoxic sand. Here, transcriptomics and proteomics showed that, whether in anoxia or not, L. oneistus mostly expressed genes involved in ubiquitination, energy generation, oxidative stress response, immune response, development, and translation. Importantly, ubiquitination genes were also highly expressed when the nematode was subjected to anoxic sulfidic conditions, together with genes involved in autophagy, detoxification and ribosome biogenesis. We hypothesize that these degradation pathways were induced to recycle damaged cellular components (mitochondria) and misfolded proteins into nutrients. Remarkably, when L. oneistus was subjected to anoxic sulfidic conditions, lectin and mucin genes were also upregulated, potentially to promote the attachment of its thiotrophic symbiont. Furthermore, the nematode appeared to survive oxygen deprivation by using an alternative electron carrier (rhodoquinone) and acceptor (fumarate), to rewire the electron transfer chain. On the other hand, under hypoxia, genes involved in costly processes (e.g., amino acid biosynthesis, development, feeding, mating) were upregulated, together with the worm's Toll-like innate immunity pathway and several immune effectors (e.g., bactericidal/permeability-increasing proteins, fungicides). In conclusion, we hypothesize that, in anoxic sulfidic sand, L. oneistus upregulates degradation processes, rewires the oxidative phosphorylation and reinforces its coat of bacterial sulfur-oxidizers. In upper sand layers, instead, it appears to produce broad-range antimicrobials and to exploit oxygen for biosynthesis and development.

TRPM channels mediate learned pathogen avoidance following intestinal distention

Upon exposure to harmful microorganisms, hosts engage in protective molecular and behavioral immune responses, both of which are ultimately regulated by the nervous system. Using the nematode Caenorhabditis elegans, we show that ingestion of Enterococcus faecalis leads to a fast pathogen avoidance behavior that results in aversive learning. We have identified multiple sensory mechanisms involved in the regulation of avoidance of E. faecalis. The G-protein coupled receptor NPR-1-dependent oxygen-sensing pathway opposes this avoidance behavior, while an ASE neuron-dependent pathway and an AWB and AWC neuron-dependent pathway are directly required for avoidance. Colonization of the anterior part of the intestine by E. faecalis leads to AWB and AWC mediated olfactory aversive learning. Finally, two transient receptor potential melastatin (TRPM) channels, GON-2 and GTL-2, mediate this newly described rapid pathogen avoidance. These results suggest a mechanism by which TRPM channels may sense the intestinal distension caused by bacterial colonization to elicit pathogen avoidance and aversive learning by detecting changes in host physiology.

Cholinergic transmission in C. elegans: Functions, diversity, and maturation of ACh-activated ion channels

Acetylcholine is an abundant neurotransmitter in all animals. Effects of acetylcholine are excitatory, inhibitory, or modulatory depending on the receptor and cell type. Research using the nematode C. elegans has made ground-breaking contributions to the mechanistic understanding of cholinergic transmission. Powerful genetic screens for behavioral mutants or for responses to pharmacological reagents identified the core cellular machinery for synaptic transmission. Pharmacological reagents that perturb acetylcholine-mediated processes led to the discovery and also uncovered the composition and regulators of acetylcholine-activated channels and receptors. From a combination of electrophysiological and molecular cellular studies, we have gained a profound understanding of cholinergic signaling at the levels of synapses, neural circuits, and animal behaviors. This review will begin with a historical overview, then cover in-depth current knowledge on acetylcholine-activated ionotropic receptors, mechanisms regulating their functional expression and their functions in regulating locomotion.

Excessive Mg2+ Impairs Intestinal Homeostasis by Enhanced Production of Adenosine Triphosphate and Reactive Oxygen Species

Aims: Mg²⁺ is fundamental for life, and its shortage severely impairs vital functions. However, whether excessive Mg²⁺ has beneficial or adverse effects has remained unknown. To clarify this issue, we analyzed the effect of suppressing the functions of Cyclin M (CNNM) Mg²⁺ efflux transporters in various experimental systems. Results: Investigation of short-lived Caenorhabditis elegans worms mutated for CNNM genes revealed reactive oxygen species (ROS) augmentation in intestinal cells, coincidently with high levels of Mg²⁺. Knockdown of gtl-1, encoding Mg²⁺-incorporating channel into intestinal cells, reduced ROS levels and restored life span, confirming the causative role of excessive Mg²⁺. Also, inactivation of orthologous CNNM in human cultured cells and mice by RNA interference, expression of CNNM-inhibiting protein, phosphatase of regenerating liver 3, or gene knockout resulted in ROS overproduction. Moreover, biochemical analyses revealed that excessive Mg²⁺ stimulates adenosine triphosphate overproduction and accelerates mitochondrial electron transport, whose suppression shut down ROS generation. Innovation and Conclusion: These results provide definitive evidence that excessive Mg²⁺ drives overproduction of ROS by affecting energy metabolism, implying the crucial importance of the tight regulation of intracellular Mg²⁺ levels.

The Caenorhabditis elegans Excretory System: A Model for Tubulogenesis, Cell Fate Specification, and Plasticity

The excretory system of the nematode Caenorhabditis elegans is a superb model of tubular organogenesis involving a minimum of cells. The system consists of just three unicellular tubes (canal, duct, and pore), a secretory gland, and two associated neurons. Just as in more complex organs, cells of the excretory system must first adopt specific identities and then coordinate diverse processes to form tubes of appropriate topology, shape, connectivity, and physiological function. The unicellular topology of excretory tubes, their varied and sometimes complex shapes, and the dynamic reprogramming of cell identity and remodeling of tube connectivity that occur during larval development are particularly fascinating features of this organ. The physiological roles of the excretory system in osmoregulation and other aspects of the animal's life cycle are only beginning to be explored. The cellular mechanisms and molecular pathways used to build and shape excretory tubes appear similar to those used in both unicellular and multicellular tubes in more complex organs, such as the vertebrate vascular system and kidney, making this simple organ system a useful model for understanding disease processes.

Mg2+ Extrusion from Intestinal Epithelia by CNNM Proteins Is Essential for Gonadogenesis via AMPK-TORC1 Signaling in Caenorhabditis elegans

Mg²⁺ serves as an essential cofactor for numerous enzymes and its levels are tightly regulated by various Mg²⁺ transporters. Here, we analyzed Caenorhabditis elegans strains carrying mutations in genes encoding cyclin M (CNNM) Mg²⁺ transporters. We isolated inactivating mutants for each of the five Caenorhabditis elegans cnnm family genes, cnnm-1 through cnnm-5. cnnm-1; cnnm-3 double mutant worms showed various phenotypes, among which the sterile phenotype was rescued by supplementing the media with Mg²⁺. This sterility was caused by a gonadogenesis defect with severely attenuated proliferation of germ cells. Using this gonadogenesis defect as an indicator, we performed genome-wide RNAi screening, to search for genes associated with this phenotype. The results revealed that RNAi-mediated inactivation of several genes restores gonad elongation, including aak-2, which encodes the catalytic subunit of AMP-activated protein kinase (AMPK). We then generated triple mutant worms for cnnm-1; cnnm-3; aak-2 and confirmed that the aak-2 mutation also suppressed the defective gonadal elongation in cnnm-1; cnnm-3 mutant worms. AMPK is activated under low-energy conditions and plays a central role in regulating cellular metabolism to adapt to the energy status of cells. Thus, we provide genetic evidence linking Mg²⁺ homeostasis to energy metabolism via AMPK.

Mg²⁺ is the second most abundant cation in cells and serves as an essential cofactor for numerous enzymes. To avoid its shortage, cellular and organismal levels of Mg²⁺ are tightly regulated by the concerted actions of various Mg²⁺ transporters and channels. In this study, we analyzed Caenorhabditis elegans strains carrying mutations in genes encoding Mg²⁺ transporters and found that the mutations abrogated Mg²⁺ homeostasis. Additionally, these worms were sterile because of a developmental defect in the gonads with severely attenuated proliferation of germ cells. These abnormalities were rescued by additional Mg²⁺ supplementation to the medium, and thus were considered to be due to Mg²⁺ shortage. We investigated the mechanism of this Mg²⁺-associated attenuation of gonadal development, and found that disrupting of the function of AMP-activated protein kinase (AMPK) restored gonad elongation. It is well-known that AMPK is activated under low-energy conditions and plays a central role in regulating cellular metabolism to adapt to the energy status of cells. Thus, we demonstrated that Mg²⁺ homeostasis is intimately connected to energy metabolism via AMPK.

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.

A novel nondevelopmental role of the sax-7/L1CAM cell adhesion molecule in synaptic regulation in Caenorhabditis elegans

The L1CAM family of cell adhesion molecules is a conserved set of single-pass transmembrane proteins that play diverse roles required for proper nervous system development and function. Mutations in L1CAMs can cause the neurological L1 syndrome and are associated with autism and neuropsychiatric disorders. L1CAM expression in the mature nervous system suggests additional functions besides the well-characterized developmental roles. In this study, we demonstrate that the gene encoding the Caenorhabditis elegans L1CAM, sax-7, genetically interacts with gtl-2, as well as with unc-13 and rab-3, genes that function in neurotransmission. These sax-7 genetic interactions result in synthetic phenotypes that are consistent with abnormal synaptic function. Using an inducible sax-7 expression system and pharmacological reagents that interfere with cholinergic transmission, we uncovered a previously uncharacterized nondevelopmental role for sax-7 that impinges on synaptic function.

Size- and composition-dependent toxicity of synthetic and soil-derived Fe oxide colloids for the nematode Caenorhabditis elegans

Colloidal iron oxides (FeOx) are increasingly released to the environment due to their use in environmental remediation and biomedical applications, potentially harming living organisms. Size and composition could affect the bioavailability and toxicity of such colloids. Therefore, we investigated the toxicity of selected FeOx with variable aggregate size and variably composed FeOx-associated organic matter (OM) toward the nematode Caenorhabditis elegans. Ferrihydrite colloids containing citrate were taken up by C. elegans with the food and accumulated inside their body. The toxicity of ferrihydrite, goethite, and akaganeite was dependent on aggregate size and specific surface area, with EC50 values for reproduction ranging from 4 to 29 mg Fe L(-1). Experiments with mutant strains lacking mitochondrial superoxide dismutase (sod-2) showed oxidative stress for two FeOx and Fe(3+)-ions, however, revealed that it was not the predominant mechanism of toxicity. The OM composition determined the toxicity of mixed OM-FeOx phases on C. elegans. FeOx associated with humic acids or citrate were less toxic than OM-free FeOx. In contrast, soil-derived ferrihydrite, containing proteins and polysaccharides from mobile OM, was even more toxic than OM-free Fh of similar aggregate size. Consequently, the careful choice of the type of FeOx and the type of associated OM may help in reducing the ecological risks if actively applied to the subsurface.

Localization of K⁺, H⁺, Na⁺ and Ca²⁺ fluxes to the excretory pore in Caenorhabditis elegans: application of scanning ion-selective microelectrodes

Although Caenorhabditis elegans is commonly used as a model organism for studies of cell biology, development and physiology, the small size of the worm has impeded measurements of ion transport by the excretory cell and hypodermis. Here, we use the scanning ion-selective microelectrode technique to measure efflux and influx of K(+), H(+), Na(+) and Ca(2+) in intact worms. Transport of ions into, or out of, immobilized worms produces small gradients in ion concentration in the unstirred layer near the surface of the worm. These gradients are readily detectable with ion-selective microelectrodes and the corresponding ion fluxes can be estimated using the Fick equation. Our data show that effluxes of K(+), H(+), Na(+) and Ca(2+) are localized to the region of the excretory pore, consistent with release of these ions from the excretory cell, and that effluxes increase after experimental preloading with Na(+), K(+) or Ca(2+). In addition, the hypodermis is a site of Na(+) influx.

Permeation, regulation and control of expression of TRP channels by trace metal ions

Transient receptor potential (TRP) channels form a diverse family of cation channels comprising 28 members in mammals. Although some TRP proteins can only be found on intracellular membranes, most of the TRP protein isoforms reach the plasma membrane where they form ion channels and control a wide number of biological processes. There, their involvement in the transport of cations such as calcium and sodium has been well documented. However, a growing number of studies have started to expand our understanding of these proteins by showing that they also transport other biologically relevant metal ions like zinc, magnesium, manganese and cobalt. In addition to this newly recognized property, the activity and expression of TRP channels can be regulated by metal ions like magnesium, gadolinium, lanthanum or cisplatin. The aim of this review is to highlight the complex relationship between metal ions and TRP channels.

Transient receptor potential melastatin (TRPM) channels mediate clozapine-induced phenotypes in Caenorhabditis elegans

The molecular mechanisms of action of antipsychotic drugs (APDs) are not fully understood. Here, we characterize phenotypes of missense and knockout mutations in the Caenorhabditis elegans transient receptor potential melastatin (TRPM) channel ortholog gtl-2, a candidate APD target identified in a genome-wide RNAi (RNA interference) screen for Suppressors of Clozapine-induced Larval Arrest (scla genes). We then employ the developmental phenotypes of gtl-2(lf) mutants to validate our previous gtl-2(RNAi) result. GTL-2 acts in the excretory canal cell to regulate Mg(2+) homeostasis. Using exc (excretory canal abnormal) gene mutants, we demonstrate that excretory canal cell function is necessary for clozapine-induced developmental delay and lethality. Moreover, cell-specific promoter-driven expression studies reveal that GTL-2 function in the excretory canal cell is important for its role in the SCLA phenotype. We then investigate the mechanism by which GTL-2 function in the excretory canal cell impacts clozapine-induced phenotypes. gtl-2(lf) mutations cause hypermagnesemia, and we show that exposure of the wild-type strain to high Mg(2+) phenocopies gtl-2(lf) with respect to suppression of clozapine-induced developmental delay and lethality. Our results suggest that GTL-2 TRPM channel function in the excretory canal cell is important for clozapine's developmental effects. TRP channels are expressed in mammalian brain and are implicated in the pathogenesis of mental illnesses but have not been previously implicated in APD action.

Evolutionarily conserved, multitasking TRP channels: lessons from worms and flies

The Transient Receptor Potential (TRP) channel family is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling. TRPs allow animals to detect chemicals, mechanical force, light, and changes in temperature. Consequently, these channels control a plethora of animal behaviors. Moreover, their functions are not limited to the classical senses, as they are cellular sensors, which are critical for ionic homeostasis and metabolism. Two genetically tractable invertebrate model organisms, Caenorhabditis elegans and Drosophila melanogaster, have led the way in revealing a wide array of sensory roles and behaviors that depend on TRP channels. Two overriding themes have emerged from these studies. First, TRPs are multitasking proteins, and second, many functions and modes of activation of these channels are evolutionarily conserved, including some that were formerly thought to be unique to invertebrates, such as phototransduction. Thus, worms and flies offer the potential to decipher roles for mammalian TRPs, which would otherwise not be suspected.

TRPM7

The channel kinases TRPM6 and TRPM7 are fusion proteins with an ion transport domain and an enzymatically active kinase domain. TRPM7 has been found in every mammalian tissue investigated to date. The two-in-one protein structure, the ubiquitous expression profile, and the protein's unique biophysical characteristics that enable divalent ion transport involve TRPM7 in a plethora of (patho)physiological processes. With its prominent role in cellular and systemic magnesium homeostasis, TRPM7 emerges as a key player in embryonic development, global ischemia, cardiovascular disease, and cancer.

Basolateral Mg2+ extrusion via CNNM4 mediates transcellular Mg2+ transport across epithelia: a mouse model

Transcellular Mg²⁺ transport across epithelia, involving both apical entry and basolateral extrusion, is essential for magnesium homeostasis, but molecules involved in basolateral extrusion have not yet been identified. Here, we show that CNNM4 is the basolaterally located Mg²⁺ extrusion molecule. CNNM4 is strongly expressed in intestinal epithelia and localizes to their basolateral membrane. CNNM4-knockout mice showed hypomagnesemia due to the intestinal malabsorption of magnesium, suggesting its role in Mg²⁺ extrusion to the inner parts of body. Imaging analyses revealed that CNNM4 can extrude Mg²⁺ by exchanging intracellular Mg²⁺ with extracellular Na⁺. Furthermore, CNNM4 mutations cause Jalili syndrome, characterized by recessive amelogenesis imperfecta with cone-rod dystrophy. CNNM4-knockout mice showed defective amelogenesis, and CNNM4 again localizes to the basolateral membrane of ameloblasts, the enamel-forming epithelial cells. Missense point mutations associated with the disease abolish the Mg²⁺ extrusion activity. These results demonstrate the crucial importance of Mg²⁺ extrusion by CNNM4 in organismal and topical regulation of magnesium.

Magnesium is an essential element for living organisms. Its absorption occurs at the intestine through the barrier comprised of epithelial cells. In this process, transcellular Mg²⁺ transport across epithelia, involving both entry from one side and extrusion from the other side, is important. Previous studies have revealed the role of Mg²⁺-permeable channel protein in Mg²⁺ entry into the epithelial cells. However, the identity of proteins involved in Mg²⁺ extrusion to the inner parts of body has remained unknown. Mice genetically engineered not to express CNNM4, which localizes to the epithelial membrane facing to the inner parts of body, show hypomagnesemia due to the defect in magnesium absorption. Functional analyses using culture cells directly reveal that CNNM4 can extrude intracellular Mg²⁺ to the outside of cells. These results indicate that CNNM4 mediates transcellular Mg²⁺ transport across the intestinal epithelia. Furthermore, we also show that these CNNM4-lacking mice also have a defect in amelogenesis, which is consistent with the disease symptoms of Jalili syndrome that is known to be caused by mutations in the CNNM4 gene.

A Gαq-Ca²⁺ signaling pathway promotes actin-mediated epidermal wound closure in C. elegans

BACKGROUND

Repair of skin wounds is essential for animals to survive in a harsh environment, yet the signaling pathways initiating wound repair in vivo remain little understood. In Caenorhabditis elegans, a p38 mitogen-activated protein kinase (MAPK) cascade promotes innate immune responses to wounding but is not required for other aspects of wound healing. We therefore set out to identify additional wound response pathways in C. elegans epidermis.

RESULTS

We show here that wounding the adult C. elegans skin triggers a rapid and sustained rise in epidermal Ca(2+) that is critical for survival after wounding. The wound-triggered rise in Ca(2+) requires the epidermal transient receptor potential channel, melastatin family (TRPM) channel GTL-2 and IP(3)R-stimulated release from internal stores. We identify an epidermal signal transduction pathway that includes the Gα(q) EGL-30 and its effector PLCβ EGL-8. Loss of function in this pathway impairs survival after wounding. The Gα(q)-Ca(2+) pathway is not required for known innate immune responses to wounding but instead promotes actin-dependent wound closure. Wound closure requires the Cdc42 small GTPase and Arp2/3-dependent actin polymerization and is negatively regulated by Rho and nonmuscle myosin. Finally, we show that the death-associated protein kinase DAPK-1 acts as a negative regulator of wound closure.

CONCLUSIONS

Skin wounding in C. elegans triggers a Ca(2+)-dependent signaling cascade that promotes wound closure, in parallel to the innate immune response to damage. Wound closure requires actin polymerization and is negatively regulated by nonmuscle myosin.

The FGD homologue EXC-5 regulates apical trafficking in C. elegans tubules

Maintenance of the shape of biological tubules is critical for development and physiology of metazoan organisms. Loss of function of the Caenorhabditis elegans FGD protein EXC-5 allows large fluid-filled cysts to form in the lumen of the single-cell excretory canal tubules, while overexpression of exc-5 causes defects at the tubule's basolateral surface. We have examined the effects of altering expression levels of exc-5 on the distribution of fluorescently-marked subcellular organelles. In exc-5 mutants, early endosomes build up in the cell, especially in areas close to cysts, while recycling endosomes are depleted. Endosome morphology changes prior to cyst formation. Conversely, when exc-5 is overexpressed, recycling endosomes are enriched. Since FGD proteins activate the small GTPases CDC42 and Rac, these results support the hypothesis that EXC-5 acts through small GTPases to move material from apical early endosomes to recycling endosomes, and that loss of such movement is likely the cause of tubule deformation both in nematodes and in tissues affected by FGD dysfunction such as Charcot-Marie-Tooth Syndrome type 4H.

The Mg2+ transporter MagT1 partially rescues cell growth and Mg2+ uptake in cells lacking the channel-kinase TRPM7

Magnesium (Mg(2+)) transport across membranes plays an essential role in cellular growth and survival. TRPM7 is the unique fusion of a Mg(2+) permeable pore with an active cytosolic kinase domain, and is considered a master regulator of cellular Mg(2+) homeostasis. We previously found that the genetic deletion of TRPM7 in DT40 B cells results in Mg(2+) deficiency and severe growth impairment, which can be rescued by supplementation with excess extracellular Mg(2+). Here, we show that gene expression of the Mg(2+) selective transporter MagT1 is upregulated in TRPM7(-/-) cells. Furthermore, overexpression of MagT1 in TRPM7(-/-) cells augments their capacity to uptake Mg(2+), and improves their growth behavior in the absence of excess Mg(2+).

TRPM channels modulate epileptic-like convulsions via systemic ion homeostasis

Neuronal networks operate over a wide range of activity levels, with both neuronal and nonneuronal cells contributing to the balance of excitation and inhibition. Activity imbalance within neuronal networks underlies many neurological diseases, such as epilepsy. The Caenorhabditis elegans locomotor circuit operates via coordinated activity of cholinergic excitatory and GABAergic inhibitory transmission. We have previously shown that a gain-of-function mutation in a neuronal acetylcholine receptor, acr-2(gf), causes an epileptic-like convulsion behavior. Here we report that the behavioral and physiological effects of acr-2(gf) require the activity of the TRPM channel GTL-2 in nonneuronal tissues. Loss of gtl-2 function does not affect baseline synaptic transmission but can compensate for the excitation-inhibition imbalance caused by acr-2(gf). The compensatory effects of removing gtl-2 are counterbalanced by another TRPM channel, GTL-1, and can be recapitulated by acute treatment with divalent cation chelators, including those specific for Zn(2+). Together, these data reveal an important role for ion homeostasis in the balance of neuronal network activity and a novel function of nonneuronal TRPM channels in the fine-tuning of this network activity.

The channel-kinase TRPM7 regulates phosphorylation of the translational factor eEF2 via eEF2-k

Protein translation is an essential but energetically expensive process, which is carefully regulated in accordance to the cellular nutritional and energy status. Eukaryotic elongation factor 2 (eEF2) is a central regulation point since it mediates ribosomal translocation and can be inhibited by phosphorylation at Thr56. TRPM7 is the unique fusion of an ion channel with a functional Ser/Thr-kinase. While TRPM7's channel function has been implicated in regulating vertebrate Mg(2+) uptake required for cell growth, the function of its kinase domain remains unclear. Here, we show that under conditions where cell growth is limited by Mg(2+) availability, TRPM7 via its kinase mediates enhanced Thr56 phosphorylation of eEF2. TRPM7-kinase does not appear to directly phosphorylate eEF2, but rather to influence the amount of eEF2's cognate kinase eEF2-k, involving its phosphorylation at Ser77. These findings suggest that TRPM7's structural duality ensures ideal positioning of its kinase in close proximity to channel-mediated Mg(2+) uptake, allowing for the adjustment of protein translational rates to the availability of Mg(2+).

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. Here, we 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. We generated mutations in trpm and found that 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⁺. We conclude 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.