Biomedical subjects

Carsten Schmitz

Publications and source records attributed to Carsten Schmitz.

6 recordsLinked to original sources

PubMed2005

The channel kinases TRPM6 and TRPM7 are functionally nonredundant.

TRPM7 and its closest homologue, TRPM6, are the only known fusions of an ion channel pore with a kinase domain. Deletion of TRPM7 in DT40 B-lymphocytes causes growth arrest, Mg(2+) deficiency, and cell death within 24-48 h. Amazingly, in analogy to TRPM6-deficient patients who can live a normal life if provided with a Mg(2+)-rich diet, TRPM7-deficient DT40 B-lymphocytes show wild type cell growth if supplied with 5-10 mm Mg(2+) concentrations in their extracellular medium. Here we have investigated the functional relationship between TRPM6 and TRPM7. We show that TRPM7 deficiency in DT40 cells cannot be complemented by heterologously expressed TRPM6. Nevertheless, both channels can influence each other's biological activity. Our data demonstrate that TRPM6 requires TRPM7 for surface expression in HEK-293 cells and also that TRPM6 is capable of cross-phosphorylating TRPM7 as assessed using a phosphothreonine-specific antibody but not vice versa. TRPM6 and TRPM7 coexpression studies in DT40 B-cells indicate that TRPM6 can modulate TRPM7 function. In conclusion, although TRPM6 and TRPM7 are closely related and deficiency in either one of these molecules severely affects Mg(2+) homeostasis regulation, TRPM6 and TRPM7 do not appear to be functionally redundant but rather two unique and essential components of vertebrate ion homeostasis regulation.

Cell Division↗

PubMed2004

Accumulation of free ADP-ribose from mitochondria mediates oxidative stress-induced gating of TRPM2 cation channels.

TRPM2 is a member of the transient receptor potential melastatin-related (TRPM) family of cation channels, which possesses both ion channel and ADP-ribose hydrolase functions. TRPM2 has been shown to gate in response to oxidative and nitrosative stresses, but the mechanism through which TRPM2 gating is induced by these types of stimuli is not clear. Here we show through structure-guided mutagenesis that TRPM2 gating by ADP-ribose and both oxidative and nitrosative stresses requires an intact ADP-ribose binding cleft in the C-terminal nudix domain. We also show that oxidative/nitrosative stress-induced gating can be inhibited by pharmacological reagents predicted to inhibit NAD hydrolysis to ADP-ribose and by suppression of ADP-ribose accumulation by cytosolic or mitochondrial overexpression of an enzyme that specifically hydrolyzes ADP-ribose. Overall, our data are most consistent with a model of oxidative and nitrosative stress-induced TRPM2 activation in which mitochondria are induced to produce free ADP-ribose and release it to the cytosol, where its subsequent accumulation induces TRPM2 gating via interaction within a binding cleft in the C-terminal NUDT9-H domain of TRPM2.

Adenosine Diphosphate Ribose↗

PubMed2004

Receptor-mediated regulation of the TRPM7 channel through its endogenous protein kinase domain.

TRPM7 is a ubiquitously expressed and constitutively active divalent cation-selective ion channel, whose basal activity is regulated by intracellular levels of Mg(2+) and Mg.ATP. We have investigated receptor-mediated mechanisms that may actively regulate TRPM7 activity. We here report that TRPM7 currents are suppressed by intracellular GTPgammaS, suggesting the involvement of heterotrimeric G proteins. TRPM7 currents are also inhibited by stimulating endogenous muscarinic receptors, which is mediated by G(i) because the inhibitory effect is blunted by pertussis toxin. Conversely, stimulation of endogenous G(s)-coupled beta-adrenergic receptors potentiates TRPM7 currents, whereas G(q)-coupled thrombin receptors have little effect. Consistent with the involvement of G(s)/G(i) in controlling adenylyl cyclase activity, elevations of intracellular cAMP levels enhance TRPM7 activity and prevent receptor-mediated modulation of TRPM7 activity by muscarinic and adrenergic agonists. This cAMP-dependent effect requires the functional integrity of both protein kinase A (PKA) and the endogenous kinase domain of TRPM7 because cAMP-mediated effects are abolished when treating cells with the PKA inhibitors H89 or KT5720 as well as in cells expressing phosphotransferase-deficient TRPM7 constructs. These mutant channels are also much less susceptible to GTPgammaS-mediated inhibition, suggesting that the main regulatory effect occurs through G(i)- and G(s)-mediated changes in cAMP. Taken together, our results demonstrate that TRPM7 activity is up- and down-regulated through its endogenous kinase in a cAMP- and PKA-dependent manner.

Calcium↗

PubMed2004

Dual-function ion channel/protein kinases: novel components of vertebrate magnesium regulatory mechanisms.

Although magnesium is the dominant divalent intracellular cation and is required for the function of diverse types of enzymes that participate in virtually every cellular process, the molecular mechanisms that regulate its homeostasis are poorly understood. Electrophysiologic and biochemical investigations of a novel dual-function ion channel/kinase protein have recently converged with the identification of the gene locus for an autosomal recessive form of inherited hypomagnesemia to provide new insight into vertebrate magnesium regulatory mechanisms.

Animals↗

PubMed2003

Regulation of vertebrate cellular Mg2+ homeostasis by TRPM7.

TRPM7 is a polypeptide with intrinsic ion channel and protein kinase domains whose targeted deletion causes cells to experience growth arrest within 24 hr and eventually die. Here, we show that while TRPM7's kinase domain is not essential for activation of its channel, a functional coupling exists such that structural alterations of the kinase domain alter the sensitivity of channel activation to Mg(2+). Investigation of the relationship between Mg(2+) and the cell biological role of TRPM7 revealed that TRPM7-deficient cells become Mg(2+) deficient, that both the viability and proliferation of TRPM7-deficient cells are rescued by supplementation of extracellular Mg(2+), and that the capacity of heterologously expressed TRPM7 mutants to complement TRPM7 deficiency correlates with their sensitivity to Mg(2+). Overall, our results indicate that TRPM7 has a central role in Mg(2+) homeostasis as a Mg(2+) uptake pathway regulated through a functional coupling between its channel and kinase domains.

Adenosine Triphosphate↗

PubMedDate not supplied

TRPM2 Ca2+ permeable cation channels: from gene to biological function.

TRPM2 is a recently identified TRPM family cation channel which is unique among known ion channels in that it contains a C-terminal domain which is homologous to the NUDT9 ADP-ribose hydrolase and possesses intrinsic ADP-ribose hydrolase activity. Here, available information on the TRPM2 gene, transcripts, predicted protein products, and assembled multimeric channels is comprehensively reviewed and synthesized to highlight important areas for future work and provide insight into potential biological function(s) of TRPM2 channels.

Adenosine Diphosphate Ribose↗