Loren W. Runnels

The trp ion channel family

Mammalian homologues of the Drosophila transient receptor potential (TRP) channel gene encode a family of at least 20 ion channel proteins. They are widely distributed in mammalian tissues, but their specific physiological functions are largely unknown. A common theme that links the TRP channels is their activation or modulation by phosphatidylinositol signal transduction pathways. The channel subunits have six transmembrane domains that most probably assemble into tetramers to form non-selective cationic channels, which allow for the influx of calcium ions into cells. Three subgroups comprise the TRP channel family; the best understood of these mediates responses to painful stimuli. Other proposed functions include repletion of intracellular calcium stores, receptor-mediated excitation and modulation of the cell cycle.

The TRPM7 channel is inactivated by PIP(2) hydrolysis.

TRPM7 (ChaK1, TRP-PLIK, LTRPC7) is a ubiquitous, calcium-permeant ion channel that is unique in being both an ion channel and a serine/threonine kinase. The kinase domain of TRPM7 directly associates with the C2 domain of phospholipase C (PLC). Here, we show that in native cardiac cells and heterologous expression systems, Gαq-linked receptors or tyrosine kinase receptors that activate PLC potently inhibit channel activity. Numerous experimental approaches demonstrated that phosphatidylinositol 4,5-bisphosphate (PIP2), the substrate of PLC, is a key regulator of TRPM7. We conclude that receptor-mediated activation of PLC results in the hydrolysis of localized PIP2, leading to inactivation of the TRPM7 channel.

TRPM7 Regulates Cell Adhesion by Controlling the Calcium-dependent Protease Calpain

m-Calpain is a protease implicated in the control of cell adhesion through focal adhesion disassembly. The mechanism by which the enzyme is spatially and temporally controlled is not well understood, particularly because the dependence of calpain on calcium exceeds the submicromolar concentrations normally observed in cells. Here we show that the channel kinase TRPM7 localizes to peripheral adhesion complexes with m-calpain, where it regulates cell adhesion by controlling the activity of the protease. Our research revealed that overexpression of TRPM7 in cells caused cell rounding with a concomitant loss of cell adhesion that is dependent upon the channel of the protein but not its kinase activities. Knockdown of m-calpain blocked TRPM7-induced cell rounding and cell detachment. Silencing of TRPM7 by RNA interference, however, strengthened cell adhesion and increased the number of peripheral adhesion complexes in the cells. Together, our results suggest that the ion channel TRPM7 regulates cell adhesion through m-calpain by mediating the local influx of calcium into peripheral adhesion complexes.

Theory and application of fluorescence homotransfer to melittin oligomerization.

Fluorescence homotransfer (electronic energy transfer between identical fluorophores) has the potential to quantitate the number of subunits in membrane protein oligomers. Homotransfer strongly depolarizes fluorescence emission as a result of intermolecular excitation energy exchange between an initially excited, oriented molecule and a randomly oriented neighbor. We have theoretically treated fluorescein labeled subunits in an oligomer as a cluster of molecules that can exchange excitation energy back and forth among the subunits within that group. We find that the larger the number of subunits, the more depolarized is the emission. The general equations to calculate the expected anisotropy for complexes composed of varying numbers of labeled subunits are presented. Self-quenching of fluorophores, orientation, and changes in lifetime are also discussed and/or considered. To test this theory, we have specifically labeled melittin on its N-terminal with fluorescein and monitored its monomer to tetramer equilibrium both in solution and in lipid bilayers. The calculated anisotropies are close to the experimental values when non-fluorescent fluorescein dimers are taken into account. Our results show that homotransfer may be a promising method to study membrane-protein oligomerization.

Mechanism of activation of the Formin protein Daam1

The Formin proteins are central players in mediating cytoskeletal reorganization and are epistatically positioned in a pathway downstream of Rho activation. These proteins exist in the cytoplasm in an autoinhibited state, which is mediated by intramolecular interactions between the amino-terminal GTPase binding domain (GBD) that encompasses the diaphanous inhibitory domain (DID) and the carboxyl-terminal diaphanous autoregulatory domain (DAD). It has been proposed that the binding of Rho within the GBD releases this molecule from autoinhibition by disrupting the DID/DAD interactions. Here we report that Daam1 is not significantly activated by Rho binding but rather by its interaction with Dishevelled (Dvl). Removal of the DAD domain disrupts interactions between Dvl and Daam1, and the binding of Dvl to Daam1 disrupts the interaction between the GBD and DAD that mediates Daam1 autoinhibition. Mutations within or removal of the DAD converts Daam1 into an active protein that can induce Rho activation. We further demonstrate that Dvl synergizes with Daam1 to regulate gastrulation during Xenopus embryogenesis and that expression of activated Daam1 can rescue impaired convergent extension movements resulting from deregulated noncanonical Wnt signaling. Our studies together define the importance of a carboxyl-terminal binding partner, Dvl, that leads to the activation of Daam1.

Profilin is an effector for Daam1 in non-canonical Wnt signaling and is required for vertebrate gastrulation.

Non-canonical Wnt signaling plays important roles during vertebrate embryogenesis and is required for cell motility during gastrulation. However, the molecular mechanisms of how Wnt signaling regulates modification of the actin cytoskeleton remain incompletely understood. We had previously identified the Formin homology protein Daam1 as an important link between Dishevelled and the Rho GTPase for cytoskeletal modulation. Here, we report that Profilin1 is an effector downstream of Daam1 required for cytoskeletal changes. Profilin1 interacted with the FH1 domain of Daam1 and was localized with Daam1 to actin stress fibers in response to Wnt signaling in mammalian cells. In addition, depletion of Profilin1 inhibited stress fiber formation induced by non-canonical Wnt signaling. Inhibition or depletion of Profilin1 in vivo specifically inhibited blastopore closure in Xenopus but did not affect convergent extension movements, tissue separation or neural fold closure. Our studies define a molecular pathway downstream of Daam1 that controls Wnt-mediated cytoskeletal reorganization for a specific morphogenetic process during vertebrate gastrulation.

Phosphatidylinositol 4,5-bisphosphate (PIP2) controls magnesium gatekeeper TRPM6 activity

TRPM6 is crucial for human Mg2+ homeostasis as patients carrying TRPM6 mutations develop hypomagnesemia and secondary hypocalcemia (HSH). However, the activation mechanism of TRPM6 has remained unknown. Here we demonstrate that phosphatidylinositol-4,5-bisphophate (PIP2) controls TRPM6 activation and Mg2+ influx. Stimulation of PLC-coupled M1-receptors to deplete PIP2 potently inactivates TRPM6. Translocation of over-expressed 5-phosphatase to cell membrane to specifically hydrolyze PIP2 also completely inhibits TRPM6. Moreover, depolarization-induced-activation of the voltage-sensitive-phosphatase (Ci-VSP) simultaneously depletes PIP2 and inhibits TRPM6. PLC-activation induced PIP2-depletion not only inhibits TRPM6, but also abolishes TRPM6-mediated Mg2+ influx. Furthermore, neutralization of basic residues in the TRP domain leads to nonfunctional or dysfunctional mutants with reduced activity by PIP2, suggesting that they are likely to participate in interactions with PIP2. Our data indicate that PIP2 is required for TRPM6 channel function; hydrolysis of PIP2 by PLC-coupled hormones/agonists may constitute an important pathway for TRPM6 gating, and perhaps Mg2+ homeostasis.

TRPM6 and TRPM7: A Mul-TRP-PLIK-cation of channel functions.

Unique among ion channels, TRPM6 and TRPM7 garnered much interest upon their discovery as the first ion channels to possess their own kinase domain. Soon after their identification, the two proteins were quickly linked to the regulation of magnesium homeostasis. However, study of their physiological functions in mouse and zebrafish have revealed expanding roles for these channel-kinases that include skeletogenesis and melanopore formation, thymopoiesis, cell adhesion, and neural fold closure during early development. In addition, mutations in the TRPM6 gene constitute the underlying genetic defect in hypomagnesemia with secondary hypocalcemia, a rare autosomal-recessive disease characterized by low serum magnesium accompanied by hypocalcemia. Depletion of TRPM7 expression in brain, on the other hand, proved successful in mitigating much of the cellular devastation that accompanies oxygen-glucose deprivation during ischemia. The aim of this review is to summarize the data emerging from molecular genetic, biochemical, electrophysiological, and pharmacological studies of these unique channel-kinases.

TRPM7 activates m-calpain by stress-dependent stimulation of p38 MAPK and c-Jun N-terminal kinase.

TRPM7 is a Ca(2)(+)-permeant and Mg(2)(+)-permeant ion channel in possession of its own kinase domain. In a previous study, we showed that overexpression of the channel-kinase in HEK-293 cells produced cell rounding and loss of adhesion, which was dependent on the Ca(2+)-dependent protease m-calpain. The TRPM7-elicited change in cell morphology was channel-dependent and occurred without any significant increase in cytosolic Ca(2+). Here we demonstrate that overexpression of TRPM7 increased levels of cellular reactive oxygen species (ROS) and nitric oxide, causing the activation of p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK). Application of inhibitors of p38 MAPK and JNK blocked TRPM7-induced cell rounding and activation of m-calpain, without affecting the phosphorylation state of the protease. Overexpression of TRPM7 increased intracellular Mg(2+); however, when the concentration of either external Ca(2+) or Mg(2+) was increased to favor the permeation of one divalent cation over the other, a similar increase in cell rounding and calpain activity was detected, indicating that TRPM7-mediated activation of m-calpain is not dependent on the nature of the divalent conducted by the channel. Application of inhibitors of nitric oxide synthase and mitochondrial-derived ROS reduced TRPM7-induced increases in nitric oxide and ROS production, blocked the change in cell morphology, and reduced cellular calpain activity. Collectively, our data reveal that excessive TRPM7 channel activity causes oxidative and nitrosative stresses, producing cell rounding mediated by p38 MAPK/JNK-dependent activation of m-calpain.

TRPM7 Regulates Polarized Cell Movements

TRPM7 (transient receptor potential melastatin 7) is a Ca²+- and Mg²+-permeant ion channel in possession of its own kinase domain. As a kinase, the protein has been linked to the control of actomyosin contractility, whereas the channel has been found to regulate cell adhesion as well as cellular Mg²+ homoeostasis. In the present study we show that depletion of TRPM7 by RNA interference in fibroblasts alters cell morphology, the cytoskeleton, and the ability of cells to form lamellipodia and to execute polarized cell movements. A pulldown-purification assay revealed that knockdown of TRPM7 prevents cells from activating Rac and Cdc42 (cell division cycle 42) when stimulated to migrate into a cellular wound. Re-expression of TRPM7 reverses these phenotypic changes, as does, unexpectedly, expression of a kinase-inactive mutant of TRPM7. Surprisingly, expression of the Mg²+ transporter SLC41A2 (solute carrier family 41 member 2) is also effective in restoring the change in cell morphology, disruption of the cytoskeleton and directional cell motility caused by depletion of the channel-kinase. The results of the present study uncover an essential role for Mg²+ in the control of TRPM7 over the cytoskeleton and its ability to regulate polarized cell movements.