Neurobiology: channels for pathfinding

How filopodia respond to calcium in the absence of a calcium-binding structural protein: non-channel functions of TRP

Background

For many cell types, directional locomotion depends on their maintaining filopodia at the leading edge. Filopodia lack any Ca²⁺-binding structural protein but respond to store-operated Ca²⁺ entry (SOCE).

Methods

SOCE was induced by first replacing the medium with Ca²⁺-free salt solution with cyclopiazonic acid (CPA). This lowers Ca²⁺ in the ER and causes stromal interacting molecule (STIM) to be translocated to the cell surface. After this priming step, CPA was washed out, and Ca²⁺ influx restored by addition of extracellular Ca²⁺. Intracellular Ca²⁺ levels were measured by calcium orange fluorescence. Regulatory mechanisms were identified by pharmacological treatments. Proteins mediating SOCE were localized by immunofluorescence and analyzed after image processing.

Results

Depletion of the ER Ca²⁺ increased filopodia prevalence briefly, followed by a spontaneous decline that was blocked by inhibitors of endocytosis. Intracellular Ca²⁺ increased continuously for ~ 50 min. STIM and a transient receptor potential canonical (TRPC) protein were found in separate compartments, but an aquaporin unrelated to SOCE was present in both. STIM1- and TRPC1-bearing vesicles were trafficked on microtubules. During depletion, STIM1 migrated to the surface where it coincided with Orai in punctae, as expected. TRPC1 was partially colocalized with Vamp2, a rapidly releasable pool marker, and with phospholipases (PLCs). TRPC1 retreated to internal compartments during ER depletion. Replenishment of extracellular Ca²⁺ altered the STIM1 distribution, which came to resemble that of untreated cells. Vamp2 and TRPC1 underwent exocytosis and became homogeneously distributed on the cell surface. This was accompanied by an increased prevalence of filopodia, which was blocked by inhibitors of TRPC1/4/5 and endocytosis.

Conclusions

Because the media were devoid of ligands that activate receptors during depletion and Ca²⁺ replenishment, we could attribute filopodia extension to SOCE. We propose that the Orai current stimulates exocytosis of TRPC-bearing vesicles, and that Ca²⁺ influx through TRPC inhibits PLC activity. This allows regeneration of the substrate, phosphatidylinositol 4,5 bisphosphate (PIP2), a platform for assembling proteins, e. g. Enabled and IRSp53. TRPC contact with PLC is required but is broken by TRPC dissemination. This explains how STIM1 regulates the cell’s ability to orient itself in response to attractive or repulsive cues.

TRPC1

The TRPC1 ion channel was the first mammalian TRP channel to be cloned. In humans, it is encoded by the TRPC1 gene located in chromosome 3. The protein is predicted to consist of six transmembrane segments with the N- and C-termini located in the cytoplasm. The extracellular loop connecting transmembrane segments 5 and 6 participates in the formation of the ionic pore region. Inside the cell, TRPC1 is present in the endoplasmic reticulum, plasma membrane, intracellular vesicles, and primary cilium, an antenna-like sensory organelle functioning as a signaling platform. In human and rodent tissues, it shows an almost ubiquitous expression. TRPC1 interacts with a diverse group of proteins including ion channel subunits, receptors, and cytosolic proteins to mediate its effect on Ca(2+) signaling. It primarily functions as a cation nonselective channel within pathways controlling Ca(2+) entry in response to cell surface receptor activation. Through these pathways, it affects basic cell functions, such as proliferation and survival, differentiation, secretion, and cell migration, as well as cell type-specific functions such as chemotropic turning of neuronal growth cones and myoblast fusion. The biological role of TRPC1 has been studied in genetically engineered mice where the Trpc1 gene has been experimentally ablated. Although these mice live to adulthood, they show defects in several organs and tissues, such as the cardiovascular, central nervous, skeletal and muscular, and immune systems. Genetic and functional studies have implicated TRPC1 in diabetic nephropathy, Parkinson's disease, Huntington's disease, Duchenne muscular dystrophy, cancer, seizures, and Darier-White skin disease.

Calcium signals and FGF-2 induced neurite growth in cultured parasympathetic neurons: spatial localization and mechanisms of activation

The growth of neuritic processes in developing neurons is tightly controlled by a wide set of extracellular cues that act by initiating downstream signaling cascades, where calcium signals play a major role. Here we analyze the calcium dependence of the neurite growth promoted by basic fibroblast growth factor (bFGF or FGF-2) in chick embryonic ciliary ganglion neurons, taking advantage of dissociated, organotypic, and compartmentalized cultures. We report that signals at both the growth cone and the soma are involved in the promotion of neurite growth by the factor. Blocking calcium influx through L- and N-type voltage-dependent calcium channels and transient receptor potential canonical (TRPC) channels reduces, while release from intracellular stores does not significantly affect, the growth of neuritic processes. Simultaneous recordings of calcium signals elicited by FGF-2 at the soma and at the growth cone show that the factor activates different patterns of responses in the two compartments: steady and sustained responses at the former, oscillations at the latter. At the soma, both voltage-dependent channel and TRPC blockers strongly affect steady-state levels. At the growth cone, the changes in the oscillatory pattern are more complex; therefore, we used a tool based on wavelet analysis to obtain a quantitative evaluation of the effects of the two classes of blockers. We report that the oscillatory behavior at the growth cone is dramatically affected by all the blockers, pointing to a role for calcium influx through the two classes of channels in the generation of signals at the leading edge of the elongating neurites.

Transient receptor potential canonical channels in angiogenesis and axon guidance

Wiring of vascular and neural networks requires precise guidance of growing blood vessels and axons, respectively, to reach their targets during development. Both of the processes share common molecular signaling pathways. Transient receptor potential canonical (TRPC) channels are calcium-permeable cation channels and gated via receptor- or store-operated mechanisms. Recent studies have revealed the requirement of TRPC channels in mediating guidance cue-induced calcium influx and their essential roles in regulating axon navigation and angiogenesis. Dissecting TRPC functions in these physiological processes may provide therapeutic implications for suppressing pathological angiogenesis and improving nerve regeneration.

Emerging roles of canonical TRP channels in neuronal function

Ca(2+) signaling in neurons is intimately associated with the regulation of vital physiological processes including growth, survival and differentiation. In neurons, Ca(2+) elicits two major functions. First as a charge carrier, Ca(2+) reveals an indispensable role in information relay via membrane depolarization, exocytosis, and the release of neurotransmitters. Second on a global basis, Ca(2+) acts as a ubiquitous intracellular messenger to modulate neuronal function. Thus, to mediate Ca(2+)-dependent physiological events, neurons engage multiple mode of Ca(2+) entry through a variety of Ca(2+) permeable plasma membrane channels. Here we discuss a subset of specialized Ca(2+)-permeable non-selective TRPC channels and summarize their physiological and pathological role in the context of excitable cells. TRPC channels are predominately expressed in neuronal cells and are activated through complex mechanisms, including second messengers and store depletion. A growing body of evidence suggests a prime contribution of TRPC channels in regulating fundamental neuronal functions. TRPC channels have been shown to be associated with neuronal development, proliferation and differentiation. In addition, TRPC channels have also been suggested to have a potential role in regulating neurosecretion, long term potentiation, and synaptic plasticity. During the past years, numerous seminal discoveries relating TRPC channels to neurons have constantly emphasized on the significant contribution of this group of ion channels in regulating neuronal function. Here we review the major groundbreaking work that has uniquely placed TRPC channels in a pivotal position for governing neuronal Ca(2+) signaling and associated physiological responses.

Structural and functional regulation of growth cone, filopodia and synaptic sites by TRPV1

Specialized neuronal structures namely growth cones, filopodia and spines are important entities by which neurons communicate with each other, integrate multiple signaling events, consolidate interacting structures and exchange synaptic information. Recent studies confirmed that Transient Receptor Potential Vanilloid sub type 1 (TRPV1), alternatively known as capsaicin receptor, forms a signaling complex at the plasma membrane and integrate multiple exogenous and endogenous signaling cues there. This receptor localizes in the neuronal growth cones and also in filopodial tips. In addition, TRPV1 is endogenously present in synaptic structures and located both in pre- and post-synaptic spines of cortical neurons. Being nonselective Ca(2+)-channel, TRPV1 regulates the morphology and the functions of these structures by various mechanisms. Our studies indicated that physical interaction with signaling and structural molecules, modulation of different cytoskeleton, synaptic scaffolding structures and vesicle recycling by Ca(2+)-dependent and -independent events are the key mechanisms by which TRPV1 regulates growth cone, filopodia and spines in a coordinated manner. TRPV1 not only regulates the morphology, but also regulates the functions of these entities. Thus TRPV1 is important not only for the detection of noxious stimuli and transmission of pain signaling, but also are for the neuronal communications and network formation.

TRPV1 acts as a synaptic protein and regulates vesicle recycling

Electrophysiological studies demonstrate that transient receptor potential vanilloid subtype 1 (TRPV1) is involved in neuronal transmission. Although it is expressed in the peripheral as well as the central nervous system, the questions remain whether TRPV1 is present in synaptic structures and whether it is involved in synaptic processes. In the present study we gathered evidence that TRPV1 can be detected in spines of cortical neurons, that it colocalizes with both pre- and postsynaptic proteins, and that it regulates spine morphology. Moreover, TRPV1 is also present in biochemically prepared synaptosomes endogenously. In F11 cells, a cell line derived from dorsal-root-ganglion neurons, TRPV1 is enriched in the tips of elongated filopodia and also at sites of cell-cell contact. In addition, we also detected TRPV1 in synaptic transport vesicles, and in transport packets within filopodia and neurites. Using FM4-64 dye, we demonstrate that recycling and/or fusion of these vesicles can be rapidly modulated by TRPV1 activation, leading to rapid reorganization of filopodial structure. These data suggest that TRPV1 is involved in processes such as neuronal network formation, synapse modulation and release of synaptic transmitters.

Transient receptor potential canonical 5 channels activate Ca2+/calmodulin kinase Igamma to promote axon formation in hippocampal neurons

Functionality of neurons is dependent on their compartmentalized polarization of dendrites and an axon. The rapid and selective outgrowth of one neurite, relative to the others, to form the axon is critical in initiating neuronal polarity. Axonogenesis is regulated in part by an optimal intracellular calcium concentration. Our investigation of Ca(2+)-signaling pathways involved in axon formation using cultured hippocampal neurons demonstrates a role for Ca(2+)/calmodulin kinase kinase (CaMKK) and its downstream target Ca(2+)/calmodulin kinase I (CaMKI). Expression of constitutively active CaMKI induced formation of multiple axons, whereas blocking CaMKK or CaMKI activity with pharmacological, dominant-negative, or short hairpin RNA (shRNA) methods significantly inhibited axon formation. CaMKK signals via the gamma-isoform of CaMKI as shRNA to CaMKIgamma, but not the other CaMKI isoforms, inhibited axon formation. Furthermore, overexpression of wild-type CaMKIgamma, but not a mutant incapable of membrane association, accelerated the rate of axon formation. Pharmacological or small interfering RNA inhibition of transient receptor potential canonical 5 (TRPC5) channels, which are present in developing axonal growth cones, suppressed CaMKK-mediated activation of CaMKIgamma as well as axon formation. We demonstrate using biochemical fractionation and immunocytochemistry that CaMKIgamma and TRPC5 colocalize to lipid rafts. These results are consistent with a model in which highly localized calcium influx through the TRPC5 channels activates CaMKK and CaMKIgamma, which subsequently promote axon formation.

Submembraneous microtubule cytoskeleton: biochemical and functional interplay of TRP channels with the cytoskeleton

Much work has focused on the electrophysiological properties of transient receptor potential channels. Recently, a novel aspect of importance emerged: the interplay of transient receptor potential channels with the cytoskeleton. Recent data suggest a direct interaction and functional repercussion for both binding partners. The bi-directionality of physical and functional interaction renders therefore, the cytoskeleton a potent integration point of complex biological signalling events, from both the cytoplasm and the extracellular space. In this minireview, we focus mostly on the interaction of the cytoskeleton with transient receptor potential vanilloid channels. Thereby, we point out the functional importance of cytoskeleton components both as modulator and as modulated downstream effector. The resulting implications for patho-biological situations are discussed.

Cellular cation transport studied by 6/7Li and 23Na NMR in a porous Mo132 Keplerate type nano-capsule as model system

Li+ ions can interplay with other cations intrinsically present in the intra- and extra-cellular space (i.e. Na+, K+, Mg2+ and Ca2+) have therapeutic effects (e.g. in the treatment of bipolar disorder) or toxic effects (at higher doses), likely because Li+ interferes with the intra-/extra-cellular concentration gradients of the mentioned physiologically relevant cations. The cellular transmembrane transport can be modelled by molybdenum-oxide-based Keplerates, i.e. nano-sized porous capsules containing 132 Mo centres, monitored through 6/7Li as well as 23Na NMR spectroscopy. The effects on the transport of Li+ cations through the 'ion channels' of these model cells, caused by variations in water amount, temperature, and by the addition of organic cationic 'plugs' and the shift reagent [Dy(PPP)2](7-) are reported. In the investigated solvent systems, water acts as a transport mediator for Li+. Likewise, the counter-transport (Li+/Na+, Li+/K+, Li+/Cs+ and Li+/Ca2+) has been investigated by 7Li NMR and, in the case of Li+/Na+ exchange, by 23Na NMR, and it has been shown that most (in the case of Na+ and K+, all (Ca2+) or almost none (Cs+) of the Li cations is extruded from the internal sites of the artificial cell to the extra-cellular medium, while Na+, K+ and Ca2+ are partially incorporated.

Immunohistochemical localization of TRPC6 in the rat substantia nigra

Transient receptor potential channels (TRPC) are plasma membrane, nonselective cationic channels and have been proposed as candidates involved in the regulation of cellular Ca2+ influx [D.E. Clapham, L.W. Runnels, C., Strubing, The TRP ion channel family, Nat. Rev. Neurosci. 2 (2001) 387-396; A. Martorana, C. Giampa, Z. DeMarch, M.T. Viscomi, S. Patassini, G. Sancesario, G. Bernardi, F.R. Fusco, Distribution of TRPC1 receptors in dendrites of rat substantia nigra: a confocal and electron microscopy study, Eur. J. Neurosci. 24 (2006) 732-738]. Studies on regional localization patterns of TRPCs are necessary to provide helpful guidelines for correlating current types with particular channels. In this study, we examined the distribution of one particular member of TRPC superfamily, namely, TRPC6, in the substantia nigra of normal rat brain. Single and double label immunohistochemistry were employed to perform both light and confocal microscopy observations. Our single label studies showed that, in the substantia nigra, TRPC6 labeled the perikarya with a diffuse and intense immunoreaction product distributed throughout cell cytoplasm whereas only a light immunostaining was observed in the cell nuclei. No labeling of axon or terminals was observed, although TRPC6 was evenly distributed in the neuropil. Our dual label studies showed a TRPC6 immunoreactivity pattern that was localized into the proximal dendrites and axon hillock of the large dopaminergic neurons identified by TH immunoreaction. Furthermore, our double label immunofluorescence study for TRPC6 and mGluR1 showed a complete co-localization of the two markers in the substantia nigra. Moreover, TRPC6 did not co-localize with synaptophysin. Thus, our study shows the postsynaptic localization of TRPC6 and its association with mGluR1 in the midbrain dopamine neurons.

TRPV1 at nerve endings regulates growth cone morphology and movement through cytoskeleton reorganization

While the importance of Ca(2+) channel activity in axonal path finding is established, the underlying mechanisms are not clear. Here, we show that transient receptor potential vanilloid receptor 1 (TRPV1), a member of the TRP superfamily of nonspecific ion channels, is physically and functionally present at dynamic neuronal extensions, including growth cones. These nonselective cation channels sense exogenous ligands, such as resenifera toxin, and endogenous ligands, such as N-arachidonoyl-dopamine (NADA), and affect the integrity of microtubule cytoskeleton. Using TRPV1-transiently transfected F11 cells and embryonic dorsal root ganglia explants, we show that activation of TRPV1 results in growth cone retraction, and collapse and formation of varicosities along neurites. These changes were due to TRPV1-activation-mediated disassembly of microtubules and are partly Ca(2+)-independent. Prolonged activation with very low doses (1 nM) of NADA results in shortening of neurites in the majority of isolectin B4-positive dorsal root ganglia neurones. We postulate that TRPV1 activation plays an inhibitory role in sensory neuronal extension and motility by regulating the disassembly of microtubules. This might have a role in the chronification of pain.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

Regulation of the mechanosensitive cation channels by ATP and cAMP in leech neurons

Single-channel recordings were used to study the modulation of stretch-activated channels (SACs) by intracellular adenosine nucleotides in identified leech neurons. These channels exhibited two activity modes, spike-like (SL) and multiconductance (MC), displaying different polymodal activation. In the absence of mechanical stimulation, internal perfusion of excised patches with ATP induced robust and reversible activation of the MC but not of the SL mode. The ATP effect on channel activity was dose-dependent within a range of 1 microM-1 mM and was induced at different values of intracellular pH and Ca2+. The non-hydrolyzable ATP analog AMP-PNP, ATP without Mg2+ or ADP also effectively enhanced MC activity. Adenosine mimicked the effect of its nucleotides. At negative membrane potentials, both ATP and adenosine activated the channel. Moreover, ATP but not adenosine induced a flickering block. Addition of cAMP during maximal ATP activation completely and reversibly inhibited the channel, with activation and deactivation times of minutes. However, cAMP alone only induced a weak and rapid channel activation, without inhibitory effects. The expression of these channels in the growth cones of leech neurons, their permeability to Ca2+ and their sensitivity to intracellular cAMP are consistent with a role in the Ca2+ oscillations associated with cell growth.

Effects of intracellular pH and Ca2+ on the activity of stretch-sensitive cation channels in leech neurons

The effects of intracellular pH and calcium on the activity of the leech mechanosensitive cation channels have been studied. These channels exhibited two activity modes denoted as spike-like (SL) and multiconductance (MC). In the absence of mechanical stimulation, acidification of the intracellular side of membrane patches from 7.2 to 6.2 reversibly increased the mean channel open time as well as the opening frequency in the SL mode. Channels in MC mode were activated by a pH(i) reduction from 7.2 to 6.2, but were inhibited at pH(i) 5.5. Unlike MC mode, SL mode was strongly activated by intracellular Ca(2+). Fura-2 imaging experiments showed that intracellular calcium was induced to increase by hypotonic cell swelling. The major component of this response did not require extracellular calcium. A component of the swelling-induced calcium response was sensitive to blockers of stretch-sensitive cation channels. The results indicate that the two activity modes of mechanosensitive channels of leech neurons respond differently to changes of intracellular pH and calcium. The sensitivity of the channel to micromolar concentrations of internal free calcium, along with its permeability to this ion, is consistent with a role in the amplification of mechanically induced Ca(2+) signals in leech neurons.

Calcium-sensing mechanism in TRPC5 channels contributing to retardation of neurite outgrowth

The calcium- and sodium-permeable transient receptor potential channel TRPC5 has an inhibitory role in neuronal outgrowth but the mechanisms governing its activity are poorly understood. Here we propose a mechanism involving the neuronal calcium sensor-1 (NCS-1) protein. Inhibitory mutants of TRPC5 and NCS-1 enhance neurite outgrowth similarly. Mutant NCS-1 does not inhibit surface-expression of TRPC5 but generally suppresses channel activity, irrespective of whether it is evoked by carbachol, store depletion, lanthanides or elevated intracellular calcium. NCS-1 and TRPC5 are in the same protein complex in rat brain and NCS-1 directly binds to the TRPC5 C-terminus. The data suggest protein-protein interaction between NCS-1 and TRPC5, and involvement of this protein complex in retardation of neurite outgrowth.