A TRP that makes us feel hyper

In a recent issue of The Journal of Physiology, the combined powers of the Wehner and Okada groups outline a new chapter in molecular cell physiology with implications for the fields of cell growth and various other arenas (Numata et al. 2012), discussed more below. They report that a C-terminal splice variant of TRPM2 in concerted interaction with CD38 is necessary for HeLa cells to respond to hypertonicity with an inward cationic current, and necessary furthermore for their proliferation. This discovery represents a novel characterization of a specific TRP channel that is activated by hypertonicity. Moreover, the authors demonstrate an intriguing interplay between TRPM2ΔC and CD38 that relies on a gradient of products of CD38 enzymatic activity, rather than on the particular product itself. With respect to the latter result, the authors overcame a prima-vista paradox with remarkable persistence, namely that presence of products of CD38 enzymatic activity in the patch-pipette did not rescue efficient siRNA-mediated CD38 knock-down to activate TRPM2ΔC.

As with any significant discovery, these novel findings reveal a new set of pertinent questions. In particular, based on physiological function now assigned to these molecules, the following rational aims can be addressed in keeping with the new molecular logic. What genetic programmes related to TRPM2ΔC–CD38 underlie growth arrest in HeLa cells ? What domains of TRPM2ΔC and CD38 are critical for their concerted interaction? What role does post-translational modification play, e.g. via phosphorylation, so that hypertonicity-activated kinases could be functioning up-stream? The other way round, could TRMP2ΔC–CD38 function up-stream of known hypertonicity-regulated kinases such as SGK1 (Vallon & Lang, 2005)? In case hypertonicity-dependent signalling is known to rely on Ca²⁺, could this be a two-step process whereby TRMP2ΔC activation leads to depolarization, followed by Ca²⁺ influx through voltage-gated channels, such as observed in β-cells (Lange et al. 2009)?

Interestingly, in regard to Ca²⁺, the authors conclude that TRPM2ΔC is much less Ca²⁺ permeable than TRPM2_WT. Although this interpretation is consistent with the apparent permeability of native hypertonicity induced cationic conductances (HICCs) (as referenced in the new study), it is at odds with a previous study (Wehage et al. 2002), which suggests that both TRPM2_WT and TRPM2ΔC display similar levels of Ca²⁺ permeability. Moreover from a structural–functional standpoint, the loss of Ca²⁺ permeability in TRPM2ΔC is somewhat surprising since the putative pore region is identical to the Ca²⁺-permeable TRPM2_WT (Sumoza-Toledo & Penner, 2010). Ca²⁺ permeability of native HICCs and TRPM2ΔC therefore awaits future in-depth quantitative analysis and molecular deconstruction (as e.g. in Voets et al. 2002; Chung et al. 2008).

Beyond the cell-physiological perspective, the Wehner–Okada discovery, based on their previous discoveries (Wehner et al. 2003; Numata et al. 2008), now allows a rational search for a role of TRPM2ΔC in organs/cells that are known to respond to hypertonicity provided they express the channel and CD38. These studies will be based on efficient siRNA-mediated knockdown in primary cells, plus on Trpm2^−/− cells/tissue that can be specifically transfected with TRPM2ΔC, in the absence of specific chemical antagonists. In regard to chemical modulators of TRPM2ΔC function, these can now be rationally devised as well, especially in view of a cancer cell's growth depending on TRPM2ΔC–CD38.

Of a number of possible locations and physiological systems in vertebrate organisms, we specifically comment on the following.

Can systemic tonicity regulation by the brain involve CD38–TRPM2ΔC? Many types of neurons might also express a functional CD38–TRPM2ΔC complex, notably osmoreceptor neurons located in brain areas responsible for the control of systemic osmoregulation, such as those in the organum vasculosum lamina terminalis (Ciura et al. 2011), subfornical organ (Johnson, 1985; Bourque et al. 1994; McKinley et al. 2001; De Luca et al. 2010) and supraoptic nucleus (Sharif Naeini et al. 2006; Bourque, 2008). These neurons typically respond to extracellular fluid hypertonicity by a depolarization of their membrane potential and an accompanying increase in action potential firing rate. Osmotically induced changes in the electrical activity of these neurons modulate behavioural and physiological responses that participate in the maintenance of hydromineral homeostasis (Bourque, 2008; Sharif-Naeini et al. 2008). Inappropriate changes in the osmotic control of osmoregulatory responses could underlie some forms of hypertension (Toney et al. 2003) and have been linked to pathological states associated with many important clinical conditions such as myocardial infarction (De Smet et al. 2003), drug abuse (Kalantar-Zadeh et al. 2006) and septic shock (Sonneville et al. 2009). A better understanding of the cellular and molecular mechanism that mediate neuronal osmosensory responses may therefore lead to better therapeutic options under clinical conditions. Existing data suggest that various types of ion channels are involved in the responsiveness of central osmosensitive neurons, including TRPV1 (Sharif Naeini et al. 2006; Ciura et al. 2011), TRPV4 (Liedtke & Friedman, 2003; Carreno et al. 2009; Nedungadi et al. 2012), and Na_x (Hiyama et al. 2010). The study by Numata and colleagues indicates that it is now important to examine the possible involvement of the CD38–TRPM2ΔC complex in osmosensory neurons.

In regard to pain, aggravation of nociceptive signalling in trigeminal sensory neurons by hypotonicity has been linked to TRPV4, but hypertonicity-dependent mechanisms have remained elusive for lack of known hypertonicity-responsive signalling mechanisms (Liu et al. 2007; Li et al. 2011). The possible involvement of the TRPM2ΔC–CD38 complex in nociceptor neurons can now be addressed directly. Along the same lines, hypotonicity aggravates nocifensive behaviour, potentiated by prostaglandin-E2 (Alessandri-Haber et al. 2005). This depends on Trpv4 in the whole-animal setting. Interestingly, this was also true for hypertonicity-aggravated mouse pain behaviour, but it could not readily be explained at the level of cellular transduction – this question can now be addressed. Hypertonicity does increase the hurt in humans, and therefore a focus on CD38–TRPM2ΔC will be a welcome new starting point towards increased understanding of pathological pain mechanisms.

Finally, what role could TRPM2ΔC–CD38 hypertonicity signalling play in cartilage function? The novel osmosensory role of the TRPM2ΔC–CD38 complex may also have significant relevance to other cells in the body that are exposed to time-varying changes in their osmotic environment. For example, chondrocytes in articular cartilage, the connective tissue that serves as the load-bearing surface of synovial joints, experience relatively large diurnal changes in the osmolarity of their local tissue environment. These changes arise secondary to normal physiologic loading of the joint, which leads to the loss (and recovery) of interstitial fluid in the cartilage that alters the concentration of large negatively charged proteoglycans in the tissue. Increasing evidence indicates that these physical phenomena serve as critical signals for regulating chondrocyte function, both in health and disease (Guilak & Hung, 2005). While the response of the cells to hypertonic stimuli has long been recognized (Chao et al. 2006), a plausible mechanism for hypertonic osmosensation has remained elusive in these cells as well. On the other hand, recent studies have shown that the response of chondrocytes to hypotonic stimuli, as occurs with recovery from compression, is regulated by TRPV4 (Phan et al. 2009) and is necessary for chondroprotection in the joint (Clark et al. 2010). Furthermore, TRPM2ΔC is also activated by reactive oxygen species, which have been implicated in the disease pathogenesis of osteoarthritis, particularly with respect to tissue and cell injury and senescence (Henrotin & Kurz, 2007). While the expression and function of TRPMΔC in chondrocytes remains to be identified, it provides a rather attractive candidate mechano-osmosensor that could integrate physical and inflammatory signals within the joint.