Structure of the mammalian TRPM7, a magnesium channel required during embryonic development.

The transient receptor potential ion channel subfamily M, member 7 (TRPM7), is a ubiquitously expressed protein that is required for mouse embryonic development. TRPM7 contains both an ion channel and an α-kinase. The channel domain comprises a nonselective cation channel with notable permeability to Mg2+ and Zn2+ Here, we report the closed state structures of the mouse TRPM7 channel domain in three different ionic conditions to overall resolutions of 3.3, 3.7, and 4.1 Å. The structures reveal key residues for an ion binding site in the selectivity filter, with proposed partially hydrated Mg2+ ions occupying the center of the conduction pore. In high [Mg2+], a prominent external disulfide bond is found in the pore helix, which is essential for ion channel function. Our results provide a structural framework for understanding the TRPM1/3/6/7 subfamily and extend the knowledge base upon which to study the diversity and evolution of TRP channels.

TRPM7 senses oxidative stress to release Zn2+ from unique intracellular vesicles.

TRPM7 (transient receptor potential cation channel subfamily M member 7) regulates gene expression and stress-induced cytotoxicity and is required in early embryogenesis through organ development. Here, we show that the majority of TRPM7 is localized in abundant intracellular vesicles. These vesicles (M7Vs) are distinct from endosomes, lysosomes, and other familiar vesicles or organelles. M7Vs accumulate Zn2+ in a glutathione-enriched, reduced lumen when cytosolic Zn2+ concentrations are elevated. Treatments that increase reactive oxygen species (ROS) trigger TRPM7-dependent Zn2+ release from the vesicles, whereas reduced glutathione prevents TRPM7-dependent cytosolic Zn2+ influx. These observations strongly support the notion that ROS-mediated TRPM7 activation releases Zn2+ from intracellular vesicles after Zn2+ overload. Like the endoplasmic reticulum, these vesicles are a distributed system for divalent cation uptake and release, but in this case the primary divalent ion is Zn2+ rather than Ca2.

Mitochondrial calcium uniporter regulator 1 (MCUR1) regulates the calcium threshold for the mitochondrial permeability transition.

During the mitochondrial permeability transition, a large channel in the inner mitochondrial membrane opens, leading to the loss of multiple mitochondrial solutes and cell death. Key triggers include excessive reactive oxygen species and mitochondrial calcium overload, factors implicated in neuronal and cardiac pathophysiology. Examining the differential behavior of mitochondrial Ca(2+) overload in Drosophila versus human cells allowed us to identify a gene, MCUR1, which, when expressed in Drosophila cells, conferred permeability transition sensitive to electrophoretic Ca(2+) uptake. Conversely, inhibiting MCUR1 in mammalian cells increased the Ca(2+) threshold for inducing permeability transition. The effect was specific to the permeability transition induced by Ca(2+), and such resistance to overload translated into improved cell survival. Thus, MCUR1 expression regulates the Ca(2+) threshold required for permeability transition.

Expression of Hedgehog ligand and signal transduction components in mutually distinct isocitrate dehydrogenase mutant glioma cells supports a role for paracrine signaling.

Hedgehog (Hh) signaling regulates the growth of malignant gliomas by a ligand-dependent mechanism. The cellular source of Sonic Hh ligand and mode of signaling have not been clearly defined due to the lack of methods to definitively identify neoplastic cells in glioma specimens. Using an antibody specific for mutant isocitrate dehydrogenase protein expression to identify glioma cells, we demonstrate that Sonic Hh ligand and the pathway components Patched1 (PTCH1) and GLI1 are expressed in neoplastic cells. Further, Sonic Hh ligand and its transcriptional targets, PTCH1 and GLI1, are expressed in mutually distinct populations of neoplastic cells. These findings support a paracrine mode of intratumoral Hh signaling in malignant gliomas.

Ca2+-dependent facilitation of Cav1.3 Ca2+ channels by densin and Ca2+/calmodulin-dependent protein kinase II.

Ca(v)1 (L-type) channels and calmodulin-dependent protein kinase II (CaMKII) are key regulators of Ca(2+) signaling in neurons. CaMKII directly potentiates the activity of Ca(v)1.2 and Ca(v)1.3 channels, but the underlying molecular mechanisms are incompletely understood. Here, we report that the CaMKII-associated protein densin is required for Ca(2+)-dependent facilitation of Ca(v)1.3 channels. While neither CaMKII nor densin independently affects Ca(v)1.3 properties in transfected HEK293T cells, the two together augment Ca(v)1.3 Ca(2+) currents during repetitive, but not sustained, depolarizing stimuli. Facilitation requires Ca(2+), CaMKII activation, and its association with densin, as well as densin binding to the Ca(v)1.3 alpha(1) subunit C-terminal domain. Ca(v)1.3 channels and densin are targeted to dendritic spines in neurons and form a complex with CaMKII in the brain. Our results demonstrate a novel mechanism for Ca(2+)-dependent facilitation that may intensify postsynaptic Ca(2+) signals during high-frequency stimulation.

CaMKII associates with CaV1.2 L-type calcium channels via selected beta subunits to enhance regulatory phosphorylation.

Calcium/calmodulin-dependent kinase II (CaMKII) facilitates L-type calcium channel (LTCC) activity physiologically, but may exacerbate LTCC-dependent pathophysiology. We previously showed that CaMKII forms stable complexes with voltage-gated calcium channel (VGCC) beta(1b) or beta(2a) subunits, but not with the beta(3) or beta(4) subunits (Grueter et al. 2008). CaMKII-dependent facilitation of Ca(V)1.2 LTCCs requires Thr498 phosphorylation in the beta(2a) subunit (Grueter et al. 2006), but the relationship of this modulation to CaMKII interactions with LTCC subunits is unknown. Here we show that CaMKII co-immunoprecipitates with forebrain LTCCs that contain Ca(V)1.2alpha(1) and beta(1) or beta(2) subunits, but is not detected in LTCC complexes containing beta(4) subunits. CaMKIIalpha can be specifically tethered to the I/II linker of Ca(V)1.2 alpha(1) subunits in vitro by the beta(1b) or beta(2a) subunits. Efficient targeting of CaMKIIalpha to the full-length Ca(V)1.2alpha(1) subunit in transfected HEK293 cells requires CaMKII binding to the beta(2a) subunit. Moreover, disruption of CaMKII binding substantially reduced phosphorylation of beta(2a) at Thr498 within the LTCC complex, without altering overall phosphorylation of Ca(V)1.2alpha(1) and beta subunits. These findings demonstrate a biochemical mechanism underlying LTCC facilitation by CaMKII.

Atypical calcium regulation of the PKD2-L1 polycystin ion channel.

Native PKD2-L1 channel subunits are present in primary cilia and other restricted cellular spaces. Here we investigate the mechanism for the channel's unusual regulation by external calcium, and rationalize this behavior to its specialized function. We report that the human PKD2-L1 selectivity filter is partially selective to calcium ions (Ca(2+)) moving into the cell, but blocked by high internal Ca(2+)concentrations, a unique feature of this transient receptor potential (TRP) channel family member. Surprisingly, we find that the C-terminal EF-hands and coiled-coil domains do not contribute to PKD2-L1 Ca(2+)-induced potentiation and inactivation. We propose a model in which prolonged channel activity results in calcium accumulation, triggering outward-moving Ca(2+) ions to block PKD2-L1 in a high-affinity interaction with the innermost acidic residue (D523) of the selectivity filter and subsequent long-term channel inactivation. This response rectifies Ca(2+) flow, enabling Ca(2+) to enter but not leave small compartments such as the cilium.

Identification of Hedgehog pathway responsive glioblastomas by isocitrate dehydrogenase mutation.

The Hedgehog (Hh) pathway regulates the growth of a subset of adult gliomas and better definition of Hh-responsive subtypes could enhance the clinical utility of monitoring and targeting this pathway in patients. Somatic mutations of the isocitrate dehydrogenase (IDH) genes occur frequently in WHO grades II and III gliomas and WHO grade IV secondary glioblastomas. Hh pathway activation in WHO grades II and III gliomas suggests that it might also be operational in glioblastomas that developed from lower-grade lesions. To evaluate this possibility and to better define the molecular and histopathological glioma subtypes that are Hh-responsive, IDH genes were sequenced in adult glioma specimens assayed for an operant Hh pathway. The proportions of grades II-IV specimens with IDH mutations correlated with the proportions that expressed elevated levels of the Hh gene target PTCH1. Indices of an operational Hh pathway were measured in all primary cultures and xenografts derived from IDH-mutant glioma specimens, including IDH-mutant glioblastomas. In contrast, the Hh pathway was not operational in glioblastomas that lacked IDH mutation or history of antecedent lower-grade disease. IDH mutation is not required for an operant pathway however, as significant Hh pathway modulation was also measured in grade III gliomas with wild-type IDH sequences. These results indicate that the Hh pathway is operational in grades II and III gliomas and glioblastomas with molecular or histopathological evidence for evolvement from lower-grade gliomas. Lastly, these findings suggest that gliomas sharing this molecularly defined route of progression arise in Hh-responsive cell types.

Differential regulated interactions of calcium/calmodulin-dependent protein kinase II with isoforms of voltage-gated calcium channel beta subunits.

Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylates the beta2a subunit of voltage-gated Ca2+ channels at Thr498 to facilitate cardiac L-type Ca2+ channels. CaMKII colocalizes with beta2a in cardiomyocytes and also binds to a domain in beta2a that contains Thr498 and exhibits an amino acid sequence similarity to the CaMKII autoinhibitory domain and to a CaMKII binding domain in the NMDA receptor NR2B subunit (Grueter, C. E. et al. (2006) Mol. Cell 23, 641). Here, we explore the selectivity of the actions of CaMKII among Ca2+ channel beta subunit isoforms. CaMKII phosphorylates the beta1b, beta2a, beta3, and beta4 isoforms with similar initial rates and final stoichiometries of 6-12 mol of phosphate per mol of protein. However, activated/autophosphorylated CaMKII binds to beta1b and beta2a with a similar apparent affinity but does not bind to beta3 or beta4. Prephosphorylation of beta1b and beta2a by CaMKII substantially reduces the binding of autophosphorylated CaMKII. Residues surrounding Thr498 in beta2a are highly conserved in beta1b but are different in beta3 and beta4. Site-directed mutagenesis of this domain in beta2a showed that Thr498 phosphorylation promotes dissociation of CaMKII-beta2a complexes in vitro and reduces interactions of CaMKII with beta2a in cells. Mutagenesis of Leu493 to Ala substantially reduces CaMKII binding in vitro and in intact cells but does not interfere with beta2a phosphorylation at Thr498. In combination, these data show that phosphorylation dynamically regulates the interactions of specific isoforms of the Ca2+ channel beta subunits with CaMKII.

L-type Ca2+ channel facilitation mediated by phosphorylation of the beta subunit by CaMKII.

L-type Ca(2+) channels (LTCCs) are major entry points for Ca(2+) in many cells. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is associated with cardiac LTCC complexes and increases channel open probability (P(O)) to dynamically increase Ca(2+) current (I(Ca)) and augment cellular Ca(2+) signaling by a process called facilitation. However, the critical molecular mechanisms for CaMKII localization to LTCCs and I(Ca) facilitation in cardiomyocytes have not been defined. We show CaMKII binds to the LTCC beta(2a) subunit and preferentially phosphorylates Thr498 in beta(2a). Mutation of Thr498 to Ala (T498A) in beta(2a) prevents CaMKII-mediated increases in the P(O) of recombinant LTCCs. Moreover, expression of beta(2a)(T498A) in adult cardiomyocytes ablates CaMKII-mediated I(Ca) facilitation, demonstrating that phosphorylation of beta(2a) at Thr498 modulates native calcium channels. These findings reveal a molecular mechanism for targeting CaMKII to LTCCs and facilitating I(Ca) that may modulate Ca(2+) entry in diverse cell types coexpressing CaMKII and the beta(2a) subunit.