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| Modulation of TRP channel activity by calmodulin and calcium |
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| In a wide variety of nonexcitable and in many excitable cells, activation of G-protein-coupled receptors initiates a linear sequence of events leading to depletion of intracellular calcium storage compartments (the endoplasmic reticulum, ER)1 via the production of inositol 1,4,5-trisphosphate (IP3) by phospholipase C, and the subsequent induction of calcium influx from
the extracellular space. Depletion of the ER appears to be a prerequisite for the activation of calcium influx, because several experimental maneuvers that induce depletion of the ER, such as the introduction of IP3 into the cell or blockade of the microsomal calcium ATPase with thapsigargin (TG) and other selective blockers, are equally effective activators of calcium influx. The activation of calcium influx after store depletion has been termed store-operated calcium entry (SOCE) and appears to be a well preserved mechanism from insects to humans. Our initial finding that the transient receptor potential protein (TRP) from the Drosophila photoreceptor encodes a calcium-permeable channel activated after depletion of intracellular calcium stores, provided the first evidence for the identification of the molecular entity responsible for SOCE. An intense search in mammalian tissues led to the isolation of several mammalian TRP homologues, some of which are activated after depletion of the ER.
The diverse TRP superfamily of channels has been divided in three subfamilies (TRPC, TRPV, and TRPM) based on structural motifs. Members from the TRP superfamily are found in a wide range of organisms, from yeast to humans. One of the most prominent features in this superfamily is the wealth of regulatory mechanisms responsible for channel activation, including changes in osmolarity (TRPV4), ligands such as vanilloid (TRPV1), cold and menthol (TRPM8), store depletion and diacylglycerol (TRPC subfamily). Although the search for new TRPC homologues in a wide variety of organisms has proven very fruitful, the identification of the mechanism communicating the depleted state of the ER to the plasmalemmal channel remains elusive. Several models have been proposed to explain the communication between the ER and store operated channels (SOC), which can be divided into two main classes: one involving a physical coupling between the ER and the plasmalemmal channel and another suggesting the presence of a diffusible messenger responsible for communicating the depleted state of the ER to the plasma membrane.
Our group showed for the first time that calmodulin (CaM) is an essential element in the modulation of TRPC channel activity. Other groups have found that CaM is involved also in the regulation of members from different TRP subfamilies.
We are currently involved in revealing the mechanisms of TRPC channel modulation by CaM and intracellular calcium. For this purpose we use different techniques including molecular biology (point mutations, production of chimeras, RNAi, etc), confocal microscopy, intracellular calcium measurements with fluorescent dyes and electrophysiology. |
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