The PDZ scaffold protein INAD abolishes apparent store-dependent regulation of the light-activated cation channel TRP

TRPM7 and Ischemic CNS Injury

Ischemic brain damage represents a major source of morbidity and mortality in westernized society and poses a significant financial burden on the health care system. To date, few effective therapies have been realized to treat stroke and once promising avenues such as antiexcitotoxic therapy with NMDA receptor antagonists have not proven clinically useful. Thus, we need to identify new targets for research and therapeutic intervention of the neurodegeneration caused by stroke. Transient receptor potential (TRP) channels are an exciting new family of cation channels that respond to intracellular and extracellular stimuli. Indeed, several members can be induced by oxidative stress and oxygen free radicals. We have recently demonstrated that one member, TRPM7, is an essential mediator of anoxic neuronal death that is activated by oxidative stress, in parallel to excitotoxic signal pathways. Thus, future treatment of ischemic brain injury may need to include strategies that inhibit or modulate TRPM7 activity. Further investigation of the physiology and pathophysiology of TRPM7 and other TRP family members is needed to provide both pharmacological targets and a better understanding of ischemic brain disorders.

Drosophila visual transduction

Visual transduction in the Drosophila compound eye functions through a pathway that couples rhodopsin to phospholipase C (PLC) and the opening of transient receptor potential (TRP) channels. This cascade differs from phototransduction in mammalian rods and cones, but is remarkably similar to signaling in mammalian intrinsically photosensitive retinal ganglion cells (ipRGCs). In this review, I focus on recent advances in the fly visual system, including the discovery of a visual cycle and insights into the machinery and mechanisms involved in generating a light response in photoreceptor cells.

TRPMs and neuronal cell death

Death of CNS neurons during acute injury occurs as a result of a complex combination of excitotoxicity, necrosis, apoptosis, oedema and inflammatory reactions. Neuroprotection via glutamate receptor blockade or antioxidant or anti-inflammatory therapy have not proven effective in the clinical treatment of brain damage due to narrow therapeutic windows, poor pharmacokinetics or blockade of the signalling essential for normal excitatory neurotransmission and neuronal survival. Recent work in neuronal biochemistry, genomics and proteomics has increased understanding of the molecular organization of the excitatory synapse and the neuronal postsynaptic density. Transient receptor potential (TRP) channels are an exciting new family of cation channels that are highly expressed in the brain. Several members can be induced by oxidative stress and oxygen free radicals, both of which play important roles in neurodegeneration. Recent work has indicated that members of the melastatin subfamily (TRPM) of TRP proteins, particularly TRPM7 and TRPM2, may play key roles in neuronal death that is activated by oxidative stress and downstream from excitotoxic signal pathways. This discovery provides an exiting new avenue for research into the pathophysiology and treatment of acute neurodegeneration.

TRPC5 activation kinetics are modulated by the scaffolding protein ezrin/radixin/moesin-binding phosphoprotein-50 (EBP50)

TRPC1-7 proteins are members of a family of mammalian non-specific cation channels that mediate receptor-operated, phospholipase Cbeta/Cgamma dependent Ca(2+) influx in various cell types. TRPC4 and TRPC5 form a subfamily within TRPCs. Uniquely in the TRPC family, these channels possess a C-terminal "VTTRL" motif that binds to PDZ-domains of the scaffolding protein, EBP50 (NHERF1; Tang et al., J Biol Chem 275:37559-37564). The functional effects of EBP50 on TRPC4/5 activity have not been investigated. We have cloned rat TRPC5 (rTRPC5), functionally expressed it in HEK293 cell, and studied channel regulation with patch-clamp techniques. Both rTRPC5 and its VTTRL deletion mutant (r5dV) were localized to the plasma membrane. rTRPC5 did not display any significant basal activity in unstimulated HEK293 cells. In cells co-expressing rTRPC5 and H1 histamine receptor, rTRPC5 current evoked by GTPgammaS or histamine developed in two phases: a slowly developing, small inward current was followed by a rapidly developing, transient, large inward current. Each phase had a characteristic non-linear current-voltage (I-V) relationship. Deletion of the VTTRL motif had no detectable effect on the biophysical properties of the channel. Co-expression of EBP50 with rTRPC5 caused a significant delay in the time-to-peak of the histamine-evoked, transient large inward current. EBP50 did not modify the activation kinetics of the VTTRL-deletion mutant. We conclude that the VTTRL motif is not necessary for activation of TRPC5, but may mediate the modulatory effect of EBP50 on TRPC5 activation kinetics.

Inhibitor of myogenic family, a novel suppressor of store-operated currents through an interaction with TRPC1

Depletion of intracellular Ca2+ stores leads to the activation of Ca2+ inflow through store-operated Ca2+ channels. Although the identity of these channels is unknown, there is considerable evidence that the transient receptor potential channel 1 (TRPC1) participates in the formation of these channels. We show that TRPC1 physically interacts with the a-isoform of the inhibitor of the myogenic family (I-mfa), a known inhibitor of basic helix-loop-helix transcription factors, in vitro and in vivo. The interaction is mediated by the C-terminal cytoplasmic tail of TRPC1 and the C-terminal cysteine-rich domain of I-mfa. Using the whole cell configuration of the patch clamp technique, we show that ectopic expression of I-mfa in CHO-K1 cells reduces native store-activated Ca2+ currents, whereas knock-down of endogenous I-mfa in A431 cells by RNA interference enhances these currents. Pipette perfusion of purified recombinant I-mfa rescues the effect of I-mfa knock-down on store-operated conductance. Finally, cell dialysis with a monoclonal antibody specific to TRPC1 results in the suppression of store-activated conductance in cells lacking I-mfa, but not in I-mfa expressing cells. We propose that I-mfa functions as a molecular switch to suppress the store dependence of TRPC1.

Caveolin-1 contributes to assembly of store-operated Ca2+ influx channels by regulating plasma membrane localization of TRPC1

TRPC1, a component of store-operated Ca2+ entry (SOCE) channels, is assembled in a complex with caveolin-1 (Cav1) and key Ca2+ signaling proteins. This study examines the role of Cav1 in the function of TRPC1. TRPC1 and Cav1 were colocalized in the plasma membrane region of human submandibular gland and Madin-Darby canine kidney cells. Full-length Cav1 bound to both the N and C termini of TRPC1. Amino acids 271-349, which includes a Cav1 binding motif (amino acids 322-349), was identified as the Cav1 binding domain in the TRPC1 N terminus. Deletion of amino acids 271-349 or 322-349 prevented plasma membrane localization of TRPC1. Importantly, TRPC1Delta271-349 induced a dominant suppression of SOCE and was associated with wild-type TRPC1. Although the role of the C-terminal Cav1 binding domain is not known, its deletion did not affect localization of TRPC1 (Singh, B. B., Liu, X., and Ambudkar, I. S. (2000) J. Biol. Chem. 275, 36483-36486). Further, expression of a truncated Cav1 (Cav1Delta51-169), but not full-length Cav1, similarly disrupted plasma membrane localization of endogenously and exogenously expressed TRPC1 in human submandibular gland and Madin-Darby canine kidney cells. Cav1Delta51-169 also suppressed thapsigarginand carbachol-stimulated Ca2+ influx and increased the detergent solubility of TRPC1, although plasma membrane lipid raft domains were not disrupted. These data demonstrate that plasma membrane localization of TRPC1 depends on an interaction between its N terminus and Cav1. Thus, our data suggest that Cav1 has an important role in the assembly of SOCE channel(s).

Proteins modulating TRP channel function

TRP channels are involved in different signaling cascades; TRP channels can be activated via hormones and neurotransmitter in a receptor/G-protein-mediated manner or by osmotic, thermic or mechanic stimuli. The overall functional role of TRP channels within these processes of hormonal cellular control, nociception or cellular calcium homeostasis is still unclear, as these complex processes often involve macromolecular structures. Whereas the integration of Drosophila TRP in the phototransduction process is becoming clear, the understanding of the participation of mammalian TRP channels in signal transduction complexes is only beginning. TRP channels have been demonstrated to interact with PDZ domain proteins, and both scaffold and regulatory function have been shown for INAD, the PDZ domain protein of the Drosophila phototransduction complex. In mammalian cells, the interaction of NHERF and TRPC4 has been shown and it is anticipated that NHERF may abolish the apparent store-dependent regulation of TRPC4 and TRPC5. Whereas TRP channels and PDZ domain proteins form permanent heterodimeric proteins, the interaction of calcium-binding proteins is dependent on the calcium concentration and is, therefore, dynamic. The prototype of calcium-binding protein used for experiments is calmodulin; whether or not calmodulin is also the natural interaction partner of TRP channels is an open question.

Regulation of Ca2+ release-activated Ca2+ channels by INAD and Ca2+ influx factor

The coupling mechanism between depletion of Ca(2+) stores in the endoplasmic reticulum and plasma membrane store-operated ion channels is fundamental to Ca(2+) signaling in many cell types and has yet to be completely elucidated. Using Ca(2+) release-activated Ca(2+) (CRAC) channels in RBL-2H3 cells as a model system, we have shown that CRAC channels are maintained in the closed state by an inhibitory factor rather than being opened by the inositol 1,4,5-trisphosphate receptor. This inhibitory role can be fulfilled by the Drosophila protein INAD (inactivation-no after potential D). The action of INAD requires Ca(2+) and can be reversed by a diffusible Ca(2+) influx factor. Thus the coupling between the depletion of Ca(2+) stores and the activation of CRAC channels may involve a mammalian homologue of INAD and a low-molecular-weight, diffusible store-depletion signal.