TRPC3: a multifunctional, pore-forming signalling molecule.

TRPC3 represents one of the first identified mammalian relatives of the Drosophila trp gene product. Despite intensive biochemical and biophysical characterization as well as numerous attempts to uncover its physiological role in native cell systems, this channel protein still represents one of the most enigmatic members of the transient receptor potential (TRP) superfamily. TRPC3 is significantly expressed in brain and heart and likely to play a role in both non-excitable as well as excitable cells, being potentially involved in a wide spectrum of Ca2+ signalling mechanisms. Its ability to associate with a variety of partner proteins apparently enables TRPC3 to form different cation channels in native cells. TRPC3 cation channels display unique gating and regulatory properties that allow for recognition and integration of multiple input stimuli including lipid mediators and cellular Ca2+ gradients as well as redox signals. The physiological/pathophysiological functions of this highly versatile cation channel protein are as yet barely delineated. Here we summarize current knowledge on properties and possible signalling functions of TRPC3 and discuss the potential biological relevance of this signalling molecule.

Phospholipase C-dependent control of cardiac calcium homeostasis involves a TRPC3-NCX1 signaling complex.

OBJECTIVE: Members of the classical transient receptor potential protein (TRPC) family are considered as key components of phospholipase C (PLC)-dependent Ca2+ signaling. Previous results obtained in the HEK 293 expression system suggested a physical and functional coupling of TRPC3 to the cardiac-type Na+/Ca2+ exchanger, NCX1 (sodium calcium exchanger 1). This study was designed to test for expression of TRPC3 (transient receptor potential channel 3) and for the existence of a native TRPC3/NCX1 signaling complex in rat cardiac myocytes. METHODS: Protein expression and cellular distribution were determined by Western blot and immunocytochemistry. Protein-protein interactions were investigated by reciprocal co-immunoprecipitation and glutathione S-transferase (GST)-pulldown experiments. Recruitment of protein complexes into the plasma membrane was assayed by surface biotinylation. The functional role of TRPC3 was investigated by fluorimetric recording of angiotensin II-induced calcium signals employing a dominant negative knockdown strategy. RESULTS: TRPC3 immunoreactivity was observed in surface plasma membrane regions and in an intracellular membrane system. Co-immunolabeling of TRPC3 and NCX1 indicated significant co-localization of the two proteins. Both co-immunoprecipitation and GST-pulldown experiments demonstrated association of TRPC3 with NCX1. PLC stimulation was found to trigger NCX-mediated Ca2+ entry, which was dependent on TRPC3-mediated Na+ loading of myocytes. This NCX-mediated Ca2+ signaling was significantly suppressed by expression of a dominant negative fragment of TRPC3. PLC stimulation was associated with increased membrane presentation of both TRPC3 and NCX1. CONCLUSION: These results suggest a PLC-dependent recruitment of a TRPC3-NCX1 complex into the plasma membrane as a pivotal mechanism for the control of cardiac Ca2+ homeostasis.

Expression of Trp3 determines sensitivity of capacitative Ca2+ entry to nitric oxide and mitochondrial Ca2+ handling: evidence for a role of Trp3 as a subunit of capacitative Ca2+ entry channels.

The role of Trp3 in cellular regulation of Ca(2+) entry by NO was studied in human embryonic kidney (HEK) 293 cells. In vector-transfected HEK293 cells (controls), thapsigargin (TG)-induced (capacitative Ca(2+) entry (CCE)-mediated) intracellular Ca(2+) signals and Mn(2+) entry were markedly suppressed by the NO donor 2-(N,N-diethylamino)diazenolate-2-oxide sodium salt (3 microm) or by authentic NO (100 microm). In cells overexpressing Trp3 (T3-9), TG-induced intracellular Ca(2+) signals exhibited an amplitude similar to that of controls but lacked sensitivity to inhibition by NO. Consistently, NO inhibited TG-induced Mn(2+) entry in controls but not in T3-9 cells. Moreover, CCE-mediated Mn(2+) entry into T3-9 cells exhibited a striking sensitivity to inhibition by extracellular Ca(2+), which was not detectable in controls. Suppression of mitochondrial Ca(2+) handling with the uncouplers carbonyl cyanide m-chlorophenyl hydrazone (300 nm) or antimycin A(1) (-AA(1)) mimicked the inhibitory effect of NO on CCE in controls but barely affected CCE in T3-9 cells. T3-9 cells exhibited enhanced carbachol-stimulated Ca(2+) entry and clearly detectable cation currents through Trp3 cation channels. NO as well as carbonyl cyanide m-chlorophenyl hydrazone slightly promoted carbachol-induced Ca(2+) entry into T3-9 cells. Simultaneous measurement of cytoplasmic Ca(2+) and membrane currents revealed that Trp3 cation currents are inhibited during Ca(2+) entry-induced elevation of cytoplasmic Ca(2+), and that this negative feedback regulation is blunted by NO. Our results demonstrate that overexpression of Trp3 generates phospholipase C-regulated cation channels, which exhibit regulatory properties different from those of endogenous CCE channels. Moreover, we show for the first time that Trp3 expression determines biophysical properties as well as regulation of CCE channels by NO and mitochondrial Ca(2+) handling. Thus, we propose Trp3 as a subunit of CCE channels.

S-nitrosation controls gating and conductance of the alpha 1 subunit of class C L-type Ca(2+) channels.

Modulation of smooth muscle, L-type Ca(2+) channels (class C, Ca(V)1.2b) by thionitrite S-nitrosoglutathione (GSNO) was investigated in the human embryonic kidney 293 expression system at the level of whole-cell and single-channel currents. Extracellular administration of GSNO (2 mM) rapidly reduced whole-cell Ba(2+) currents through channels derived either by expression of alpha1C-b or by coexpression of alpha1C-b plus beta2a and alpha2-delta. The non-thiol nitric oxide (NO) donors 2,2-diethyl-1-nitroso-oxhydrazin (2 mM) and 3-morpholinosydnonimine-hydrochloride (2 mM), which elevated cellular cGMP levels to a similar extent as GSNO, failed to affect Ba(2+) currents significantly. Intracellular administration of copper ions, which promote decomposition of the thionitrite, antagonized its inhibitory effect, and loading of cells with high concentrations of dithiothreitol (2 mM) prevented the effect of GSNO on alpha1C-b channels. Intracellular loading of cells with oxidized glutathione (2 mM) affected neither alpha1C-b channel function nor their modulation by GSNO. Analysis of single-channel behavior revealed that GSNO inhibited Ca(2+) channels mainly by reducing open probability. The development of GSNO-induced inhibition was associated with the transient occurrence of a reduced conductance state of the channel. Our results demonstrate that GSNO modulates the alpha1 subunit of smooth muscle L-type Ca(2+) channels by an intracellular mechanism that is independent of NO release and stimulation of guanylyl cyclase. We suggest S-nitrosation of intracellularly located sulfhydryl groups as an important determinant of Ca(2+) channel gating and conductance.

Modulation of the smooth-muscle L-type Ca2+ channel alpha1 subunit (alpha1C-b) by the beta2a subunit: a peptide which inhibits binding of beta to the I-II linker of alpha1 induces functional uncoupling.

Modulation of the smooth-muscle Ca(2+) channel alpha1C-b subunit by the auxiliary beta2a subunit was studied in the HEK 293 (cell line from human embryonic kidney cells) expression system. In addition, we tested whether the alpha1-beta interaction in functional channels is sensitive to an 18-amino-acid synthetic peptide that corresponds to the sequence of the defined major interaction domain in the cytoplasmic I-II linker of alpha1C (AID-peptide). Ca(2+) channels derived by co-expression of alpha1C-b and beta2a subunits exhibited an about 3-fold higher open probability (P(o)) than alpha1C-b channels. High-P(o) gating of alpha1C-b.beta2a channels was associated with the occurrence of long-lasting channel openings [mean open time (tau)>10 ms] which were rarely observed in alpha1C-b channels. Modulation of fast gating by the beta2a subunit persisted in the cell-free, inside-out recording configuration. Biochemical experiments showed that the AID-peptide binds with appreciable affinity to beta2 subunits of native Ca(2+) channels. Binding of the beta2 protein to immobilized AID-peptide was specifically inhibited (K(i) of 100 nM) by preincubation with free (uncoupled) AID-peptide, but not by a corresponding scrambled peptide. Administration of the AID-peptide (10 microM) to the cytoplasmic side of inside-out patches induced a substantial reduction of P(o) of alpha1C-b.beta2a channels. The scrambled control peptide failed to affect alpha1C-b. beta2a channels, and the AID-peptide (10 microM) did not modify alpha1C-b channel function in the absence of expressed beta2a subunit. Our results demonstrate that the beta2a subunit controls fast gating of alpha1C-b channels, and suggest the alpha1-beta interaction domain in the cytoplasmic I-II linker of alpha1C (AID) as a possible target of modulation of the channel. Moreover, our data are consistent with a model of alpha1-beta interaction that is based on multiple interaction sites, including AID as a determinant of the affinity of the alpha1-beta interaction.

Motion detection in insect orientation and navigation.

The visual systems of insects are exquisitely sensitive to motion. Over the past 40 years or so, motion processing in insects has been studied and characterised primarily through the optomotor response. This response, which is a turning response evoked by the apparent movement of the visual environment, serves to stabilise the insect's orientation with respect to the environment. Research over the past decade, however, is beginning to reveal the existence of a variety of other behavioural responses in insects, that use motion information in different ways. Here we review some of the recently characterised behaviours, describe the inferred properties of the underlying movement-detecting processes, and propose modified or new models to account for them.

The significance of head movements in distance discrimination in praying mantis larvae

1. When larvae of the praying mantis Polyspilota sp. and Tenodera sinensis want to leave an exposed position and can choose to move between stationary objects at different distances, they usually choose the nearest. Their ability to select the nearest object is greatest when the background has horizontal stripes and is least when it has vertical stripes. Object preference is based on a successive distance comparison, which may involve content-related memory processes. 2. Mantid larvae can determine the absolute distance to a stationary object. Vertical contrasting borders play an important role in this process. 3. Side-to-side head movements (peering) are directly involved in the distance measurement, as shown (i) by the peering behaviour itself and (ii) by the fact that mantids can be deceived in distance measurement by arbitrary movements of target objects during the peering movement. It is supposed that the distance measurement involves the larger and faster retinal image shifts that near, as opposed to more distant, objects evoke. 4. Mantid larvae can distinguish a black-and-white rectangle in the foreground from a black-and-white striped background, even when both are similar with respect to luminance, contrast and texture. The ability to distinguish between figures and background could be explained by motion parallaxes, i.e. by the fact that during peering movements the nearer object moves faster and by a larger angle than the background structure. 5. From birth onwards, even when the eyes have yet to develop foveal specialization, mantids are capable of this visually controlled behaviour.