Polymodal TRPC signaling

Abstract

TRPC proteins have been implicated in a large array of Ca²⁺ signaling processes and are considered as pore-forming subunits of unique polymodal channel sensors. The mechanisms of TRPC activation are so far incompletely understood but appear to involve a concert of signals that are generated typically downstream of receptor-mediated activation of phospholipase C. Specifically for the TRPC1/4/5 subfamily the activating scenario is ill-defined and appears enigmatic due to the observation of multiple modes of activation. TRPC4 was initially described as a store-operated cation channel and was repeatedly proposed as a pivotal element of the store-operated signaling pathways of various tissues. However, classical reconstitution of TRPC4 complexes in expression systems as well as recent knock-down strategies provided evidence against store-dependent regulation of this channel and raised considerable doubt in its proposed prominent role agonist-induced Ca²⁺ signaling. Recent analysis of the function of TRPC4 in vascular endothelial cells of divergent phenotype revealed a novel aspect of TRPC signaling, extending the current concept of TRPC regulation by a phenotype-dependent switch between Ca²⁺ transport and a potential intracellular scaffold function of the TRPC protein.

The Enigmatic Gating Control of TRPC Channels

All seven members of the “classical” family of transient receptor potential (TRP) proteins are considered as pore forming subunits of non-selective cation channels, which are controlled by cellular phospholipase C (PLC) activity (reviewed in refs. 1–4). Although the key role of PLC in channel activation is without controversy, the molecular mechanisms that govern TRPC channel gating downstream of PLC are in large part elusive. Historically, the TRPC subfamily received substantial interest immediately after identification of the mammalian trp genes, based on their potential function as sensors for depletion of intracellular Ca²⁺ stores,5 and most members have been implicated in store-operated Ca²⁺ entry phenomena in various tissues.6–10 However, an initial, detailed analysis of the channel properties in heterologous expression systems strongly suggested that upon heterologous overexpression in classical host cells such as HEK293 or CHO, most TRPC species form receptor-regulated cation channels that are activated independently of the filling state of the endoplasmic reticulum (ER).11–14 For a subset of TRPC proteins (TRPC3/6 and 7), diacylglycerol was identified as an activating mediator that links these channels to phospholipase C activity.11 Nonetheless, regulatory communication and even physical interaction with components of the ER has repeatedly been demonstrated for TRPC proteins including also TRPC3/6/7.15–22 Moreover, the potential importance of subunit heteromerization and the formation of signalcomplexes of certain stoichiometry was found to determine biophysical as well as regulatory properties of TRPC channels.12,23–25 A particular tight relation between channel activity and the Ca²⁺ content of the ER appear to exist for the TRPC1/4/5 subset of closer relatives. The more recently identified ER Ca²⁺ sensor Stim1, which is a key component of store-depletion activated Ca²⁺ entry via Orai channels, has been demonstrated to interact also with TRPC1/4 and 5 proteins and may confer store-dependent regulation of certain TRPC channel multimers.26–28 Although several gating determinants, key factors and potential mechanisms of channel activation have been reported for TRPC1/4/5 channels, the molecular chain of events leading to activation is inclompletely understood. Processes potentially involved in TRPC4/5 channel activation include the targeting of channels into specific membrane domains and interaction with the actin cytoskeleton mediated by ERM-domain proteins,29,30 changes in local PIP₂ content of the channels membrane environment and recruitment and activation of pertussis toxin sensitive G proteins.31 Moreover local changes in free Ca²⁺ and/or divalent concentrations at the cytoplasic face of the channel complex are a likely determinant of channel gating.32 Besides integration of lipid, Ca²⁺ and G-protein signals by the TRPC4 gating machinery, additional impact of physical coupling of TRPC4 channels to ER Ca²⁺ stores via electrostatic interaction with Stim1 needs consideration.27 Similar to other TRP channels it appears feasible to speculate that multiple modes of TRPC4 activation might exist, depending on the composition, stoichiometry and cellular localization of the signalplex.

Plasticity of Signalplexes as the Basis of Polymodal TRPC Function

Variations in TRPC signaling complex composition and stoichiometry have been suspected to underly the divergent regulatory properties of TRPC channels observed along with variation of expression levels and cell types.23,24,26,33,34 TRPC signalplex composition may change with expression and availablability of complex partners within a certain cellular background. We and others speculated that TRPC proteins may serve different cellular functions by recruitment into multiple distinct channel complexes.35–38 Consequently, the regulatory and (patho) physiological impact of a TRPC protein is expected to vary significantly with changes in the level of expression and cellular localization of signaling partners. This concept may explain differences in the function of a particular TRPC protein in divergent host cell environments. Nonetheless, gating modality of a TRPC protein are likely to change also within a given cell type along with changes cellular remodeling processes and phenotype switching.

TRPC4, a Contested Player in Vascular Endothelial (Patho)physiology

Evidence from a murine TRPC4 knock-down model suggested that this channel protein is of particular importance in the cardiovascular system, representing a prominent endothelial TRPC species.9,39 Consequently, the role of TRPC4 in endothelial physiology and pathophysiology has been analyzed extensively using a variety of approaches including siRNA as well as dominant negative knock-down strategies. These investigations yielded highly controversial results ranging from evidence for a key role of TRPC4 in the endothelial store-operated pathway, which controls essential functions such as barrier stability, gene expression and mediator production,9,39–42 to a complete disqualification of TRPC4 as an endothelial Ca²⁺ transport system.43 Interestingly, despite the latter report caused series doubt in a prominent Ca²⁺ signaling function of TRPC4 in the endothelium, the relevance of this protein as a determinant of endothelial proliferation was further confirmed.

A Phenotype Window Discloses TRPC4 Channel Function in Vascular Endothelium

Recent mistrust in the involvement of TRPC4 in endothelial Ca²⁺ signaling was based on a convincing demonstration that store-operating Ca²⁺ entry into endothelial cells is mediated by the Stim1-Orai pathway.43 Hence, the cellular role of vascular TRPCs became again misty with the glimpse of a possible ion transport independent function of these proteins. At the same time, we investigated agonist-induced changes in plasma membrane expression of endothelial TRPC4 proteins and observed a striking phenotype dependence of surface expression of the channel protein.44 Most importantly, we failed to detect significant plasma membrane targeting of TRPC4 in single migrating endothelial cells. As this cellular state is typically the substrate for investigation of whole cell membrane conductances and also Ca²⁺ entry, our observation of unavailable TRPC4 membrane targeting was perfectly in line with the reported lack of Ca²⁺ signaling function.43 Nonetheless, proliferating, sub-confluent populations displayed a marked recruitment of TRPC4 into the plasma membrane upon stimulation with either EGF or thrombin, and we identified a TRPC4-mediated Ca²⁺ signaling pathway specifically in proliferating clusters of endothelial cells that formed immature cell-cell contacts (Fig. 1). TRPC4 expression was without significant impact on Ca²⁺ signaling in single cells as well as in mature endothelial barriers. In conclusion, TRPC4 appears to function as a Ca²⁺ entry channel exclusively at a certain cellular state during endothelial development and/or barrier repair/regeneration. Close inspection of phenotype transition and detailed analysis of a rather small phenotype window enabled us to verify and characterize Ca²⁺ transport function of TRPC4 in native endothelial cells. The transient function of TRPC4 as part of a plasma membrane Ca²⁺ channel complex raises the question if TRPC4 and other TRPC proteins are able to adopt specific cellular functions when targeted to intracellular membranes and protein complexes. A respective hypothesis was put forward by our finding that TRPC4 associates with the junctional protein β-catenin.44 This newly identified interaction partner represents another multifunctional signaling molecule that binds to TRPC4 also within the cell in nucleus associated compartments. Thus, TRPC4 may serve endothelial phenotype switching in a highly polymodal manner including intracellular crosstalk with other signaling pathways.

Perspectives and Outlook

The TRPC channel family is considered as a paradigm of multifunctional signaling molecules, based on their polymodal gating behavior and their typical promiscuity with respect to gating stimuli. The recently uncovered highly phenotype-dependent cellular localization and function of TRPC4 adds a new dimension to the concept of polymodal TRPC signaling. The observed intracellular targeting and integration into specific endothelial signalplexes including association and cotranslocation with β-catenin, opens the view on Ca²⁺ transport independent functions of TRPC proteins and crosstalk with important processes controlling gene expression such as the Wnt pathway. In aggregate, polymodal TRPC signaling appears to play a crucial role in phenotype transition within the vascular system. The potential role of TRPCs in tissue remodeling and repair needs particular attention and warrant further investigations.

Figures and Tables

Figure 1 Proposed model of phenotype-dependent TRPC4 function in growth factor-stimulated endothelial cells via interaction of the channel with β-catenin and targeting into cell-cell contacts. TRPC4 is for a large part sequestered in intracellular compartments and unavailable for Ca²⁺ signalling in single cells (contact deficient; upper left). By contrast, formation of immature cell adhesions promotes surface targeting of β-catenin-TRPC4 complexes and enables further recruitment of channels into the plasma membrane and Ca²⁺ entry function (immature contact; upper right). Once mature barriers are formed (mature contact; lower), TRPC4 resides for a large part in junctional complexes that are rapidly retrieved from the cell surface during growth factor stimulation and are barely available for contribution to global Ca²⁺ signaling.

Acknowledgements

I wish to thanks Dr. Michael Poteser for helpful discussions and FWF (P19820) as well as ÖNB (8216) for financial support.