TRPM7: Channeling the Future of Cellular Magnesium Homeostasis?

Biological Pathway Specificity in the Cell-Does Molecular Diversity Matter?

Biology arises from the crowded molecular environment of the cell, rendering it a challenge to understand biological pathways based on the reductionist, low-concentration in vitro conditions generally employed for mechanistic studies. Recent evidence suggests that low-affinity interactions between cellular biopolymers abound, with still poorly defined effects on the complex interaction networks that lead to the emergent properties and plasticity of life. Mass-action considerations are used here to underscore that the sheer number of weak interactions expected from the complex mixture of cellular components significantly shapes biological pathway specificity. In particular, on-pathway-i.e., "functional"-become those interactions thermodynamically and kinetically stable enough to survive the incessant onslaught of the many off-pathway ("nonfunctional") interactions. Consequently, to better understand the molecular biology of the cell a further paradigm shift is needed toward mechanistic experimental and computational approaches that probe intracellular diversity and complexity more directly. Also see the video abstract here https://youtu.be/T19X_zYaBzg.

Magnesium and embryonic development

Important for energy metabolism, neurotransmission, bone stability, and other cellular functions, Mg(2+) has well-established and undisputedly critical roles in adult tissues. Its contributions to early embryonic development are less clearly understood. For decades it has been known that gestational Mg(2+) deficiency in rodents produces teratogenic effects. More recent studies have linked deficiency in this vital cation to birth defects in humans, including spina bifida, a neural fold closure defect in humans that occurs at an average rate of 1 per 1000 pregnancies. The first suggestion that Mg(2+) may be playing a more specific role in early development arose from studies of the TRPM7 and TRPM6 ion channels. TRPM7 and TRPM6 are divalent-selective ion channels in possession of their own kinase domains that have been implicated in the control of Mg(2+) homeostasis in vertebrates. Disruption of the functions of these ion channels in mice as well as in frogs interferes with gastrulation, a pivotal process during early embryonic development that executes the emergence of the body plan and closure of the neural tube. Surprisingly, gastrulation defects produced by depletion of TRPM7 can be prevented by Mg(2+) supplementation, indicating an essential role for Mg(2+) in gastrulation and neural fold closure. The aim of this review is to summarize the data emerging from molecular genetic, biochemical and electrophysiological studies of TRPM6 and TRPM7 and provide a model of how Mg(2+), through these unique channel-kinases, may be impacting early embryonic development.

Detailed examination of Mg2+ and pH sensitivity of human TRPM7 channels

TRPM7 channel kinase is a protein highly expressed in cells of hematopoietic lineage, such as lymphocytes. Studies performed in native and heterologous expression systems have shown that TRPM7 forms nonselective cation channels functional in the plasma membrane and activated on depletion of cellular Mg(2+). In addition to internal Mg(2+), cytosolic pH and the phospholipid phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)] are potent physiological regulators of this channel: protons inhibit, while PI(4,5)P(2) is required for TRPM7 channel activity. These channels are also inhibited from inside by other metal cations and polyamines. While the regulation of TRPM7 channels by internal metal ions, acidic pH, and PI(4,5)P(2) is voltage independent, extracellular metal cations and polyamines block voltage dependently at micromolar concentrations and appear to occupy a distinct blocking site. In the present study we investigated intracellular Mg(2+) and pH dependence of native TRPM7 currents using whole cell patch-clamp electrophysiology in human Jurkat T lymphocytes and HEK293 cells. Our main findings are 1) Mg(2+) inhibition involves not one but two separate sites of high (∼10 μM) and low (∼165 μM) affinity; and 2) while sharing certain characteristics with Mg(2+) inhibition, protons most likely inhibit through one inhibitory site, corresponding to the low-affinity Mg(2+) site, with an estimated IC(50) of pH 6.3. Additionally, we present data on amplitude distribution of preactivated TRPM7 currents in Jurkat T lymphocytes in the absence of prior Mg(2+) or proton depletion.

Functional TRPV4 channels are expressed in mouse skeletal muscle and can modulate resting Ca2+ influx and muscle fatigue

Skeletal muscle contraction is basically controlled by Ca(2+) release and its reuptake into the sarcoplasmic reticulum. However, the long-term maintenance of muscle function requires an additional Ca(2+) influx from extracellular. Several mechanisms seem to contribute to the latter process, such as store-operated Ca(2+) entry, stretch-activated Ca(2+) influx and resting Ca(2+) influx. Candidate channels that may control Ca(2+) influx into muscle fibers are the STIM proteins, Orai, and the members of the transient receptor potential (TRP) family of cation channels. Here we show that TRPV4, an osmo-sensitive cation channel of the vanilloid subfamily of TRP channels is functionally expressed in mouse skeletal muscle. Western blot analysis showed the presence of TRPV4-specific bands at about 85 and 100 kDa in all tested muscles. The bands were absent when muscle proteins from TRPV4 deficient mice were analyzed. Using the manganese quench technique, we studied the resting influx of divalent cations into isolated wild-type muscle fibers. The specific TRPV4-channel activator 4α-phorbol-12,13-didecanoate (4α-PDD) stimulated resting influx by about 60% only in wild-type fibers. Electrical stimulation of soleus muscles did not reveal changes in isometric twitch contractions upon application of 4α-PDD, but tetanic contractions (at 120 Hz) were slightly increased by about 15%. When soleus muscles were stimulated with a fatigue protocol, muscle fatigue was significantly attenuated in the presence of 4α-PDD. The latter effect was not observed with muscles from TRPV4(-/-) mice. We conclude that TRPV4 is functionally expressed in mouse skeletal muscle and that TRPV4 activation modulates resting Ca(2+) influx and muscle fatigue.

Transient receptor potential melastatin 6 and 7 channels, magnesium transport, and vascular biology: implications in hypertension

Magnesium, an essential intracellular cation, is critically involved in many biochemical reactions involved in the regulation of vascular tone and integrity. Decreased magnesium concentration has been implicated in altered vascular reactivity, endothelial dysfunction, vascular inflammation, and structural remodeling, processes important in vascular changes and target organ damage associated with hypertension. Until recently, very little was known about mechanisms regulating cellular magnesium homeostasis, and processes controlling transmembrane magnesium transport had been demonstrated only at the functional level. Two cation channels of the transient receptor potential melastatin (TRPM) cation channel family have now been identified as magnesium transporters, TRPM6 and TRPM7. These unique proteins, termed chanzymes because they possess a channel and a kinase domain, are differentially expressed, with TRPM6 being found primarily in epithelial cells and TRPM7 occurring ubiquitously. Vascular TRPM7 is modulated by vasoactive agents, pressure, stretch, and osmotic changes and may be a novel mechanotransducer. In addition to its magnesium transporter function, TRPM7 has been implicated as a signaling kinase involved in vascular smooth muscle cell growth, apoptosis, adhesion, contraction, cytoskeletal organization, and migration, important processes involved in vascular remodeling associated with hypertension and other vascular diseases. Emerging evidence suggests that vascular TRPM7 function may be altered in hypertension. This review discusses the importance of magnesium in vascular biology and implications in hypertension and highlights the transport systems, particularly TRPM6 and TRPM7, which may play a role in the control of vascular magnesium homeostasis. Since the recent identification and characterization of Mg2+-selective transporters, there has been enormous interest in the field. However, there is still a paucity of information, and much research is needed to clarify the exact mechanisms of magnesium regulation in the cardiovascular system and the implications of aberrant transmembrane magnesium transport in the pathogenesis of hypertension and other vascular diseases.

Cell (patho)physiology of magnesium

There is an unsettled debate about the role of magnesium as a ‘chronic regulator’ of biological functions, as opposed to the well-known role for calcium as an ‘acute regulator’. New and old findings appear to delineate an increasingly complex and important role for magnesium in many cellular functions. This review summarizes the available evidence for a link between the regulation of intracellular magnesium availability and the control of cell growth, energy metabolism and death, both in healthy and diseased conditions. A comprehensive view is precluded by technical difficulties in tracing magnesium within a multicompartment and dynamic environment like the cell; nevertheless, the last few years has witnessed encouraging progress towards a better characterization of magnesium transport and its storage or mobilization inside the cell. The latest findings pave the road towards a new and deeper appreciation of magnesium homoeostasis and its role in the regulation of essential cell functions.

Tissue-specific expression of TRP channel genes in the mouse and its variation in three different mouse strains

Background

The purpose of this work was to study the gene expression of transient receptor potential (TRP) channels in the mouse. The application of a standardized and quantitative technique, TaqMan RT-PCR, should give information about the pattern and relative importance of TRP channels for murine tissues and cell types. To verify data sets with an independent method, we studied the occurrence of some of the transcripts by in situ hybridization.

Results

We have characterized the mRNA expression of 22 TRP channels in the mouse with a focus on nerve and muscle tissues. This is the first study to describe the expression profiles of all channel isoforms of the four related Group 1 subfamilies (TRPC, TRPV, TRPM and TRPA) with a standardized and quantitative technique. Comparisons of transcript abundance showed a consistent dominance of TRPM7 and TRPC3 in most tissues. We further observed characteristic patterns and differences in gene expression of individual channels ranging over three orders of magnitude. The overall level of TRP channel mRNAs was highest in brain areas followed by kidney, lung, reproductive organs and muscle. In brain TRPM3 and TRPM7 dominated and 19 other isoforms were detected. In lung and kidney TRPV4, TRPV5 and TRPM7 were found in highest levels. TRPM7, TRPC3, TRPC6 and TRPM3 mRNAs were characteristically present in all tested muscle tissues. Most data obtained with the C57Bl/10 mouse strain were confirmed with Balb/c and NOD mice. However, TRPC3, C6, TRPM7, M3, TRPV2 and V4 expression showed marked differences in the three tested mouse strains. In situ hybridization revealed co-expression of transcripts on the cellular level and widely confirmed the data obtained with RT-PCR.

Conclusion

Transcripts coding for members of the TRPC, TRPV, TRPM and TRPA subfamilies of TRP cation channels are present in a broad spectrum of murine tissues. Several channel isoforms often coexist in a specific tissue or cell type. TRP channel expression does not show typical tissue specific dominance of individual members as is known from other ion channel families. Mouse strain specific variations of TRP channel expression indicate that genetic background or physiological requirements considerably influence expression levels.

Differential regulation of TRPM channels governs electrolyte homeostasis in the C. elegans intestine

The transient receptor potential (TRP) channels are implicated in various cellular processes, including sensory signal transduction and electrolyte homeostasis. We show here that the GTL-1 and GON-2 TRPM channels regulate electrolyte homeostasis in the C. elegans intestine. GON-2 is responsible for a large outwardly rectifying current of intestinal cells, and its activity is tightly regulated by intracellular Mg(2+) levels, while GTL-1 mainly contributes to appropriate Mg(2+) responsiveness of the outwardly rectifying current. We also used nickel cytotoxicity to study the function of these channels. Both GON-2 and GTL-1 are necessary for intestinal uptake of nickel, but GTL-1 is continuously active while GON-2 is inactivated at higher Mg(2+) levels. This type of differential regulation of intestinal electrolyte absorption ensures a constant supply of electrolytes through GTL-1, while occasional bursts of GON-2 activity allow rapid return to normal electrolyte concentrations following physiological perturbations.