“TRP inflammation” relationship in cardiovascular system

Viral and bacterial infections are logical candidates for environmental triggers of immune reactions associated with TRP channel-dependent signaling and inflammasome activation. Upon recognition of microbial pathogens, TLRs serve as germline-encoded PRRs that play a central role in host cell recognition and responses. However, how TLR-dependent signaling links to TRP channel was unclear until very recently.

Several studies in the past few years revealed intriguing connections of TLRs to TRP channels. One study reported that hemolytic streptococcal infection affects the expression levels of at least seven TRP members, i.e., TRPC4, TRPM6, TRPM7, TRPM8, TRPV1, TRPV4, and TRPA1 [30, 31]. Another study showed that TRPC1 plays a functional role in host defense against gram-negative bacteria. Upon infection, TRPC1 (−/−) mice exhibited decreased survival, severe lung injury, and systemic bacterial dissemination. Furthermore, siRNA silencing of TRPC1 resulted in decreased Ca²⁺ entry, reduced proinflammatory cytokine production, and lowered bacterial clearance. Importantly, bacterium-mediated activation of TRPC1 was coupled with a cascade of TLR4 signaling; TLR4-dependent, TRPC1-mediated Ca²⁺ entry triggers PKCα activity to facilitate NF-κB/c-Jun N-terminal kinase (JNK) activation and augment the proinflammatory response, leading to tissue damage and eventually mortality. These findings favor the view that activation of TRPC1 is required for the host defense against bacterial infections through the TLR4-TRPC1-PKCα signaling pathway, but its excessive activity may lead to exacerbation of inflammation [32]. A similar but in-opposite-direction involvement of TRPC1-mediated Ca²⁺ entry in TLR-mediated inflammation has been demonstrated in microglia and macrophages from mice intracranially inoculated with a helminth Mesocestoides corti [33, 34]; it has been known that humans infected with a related helminth cestode Taenia solium have immunosuppressive rather than inflammatory responses in the asymptomatic phase after the infection. Experiments using soluble parasite factors from Mesocestoides-infected mice showed that suppression of TRPC1-mediated store-operated Ca²⁺ entry by these factors and consequent inhibition of NF-κB, JNK, and MAPK pathways are likely responsible for the immunosuppression. This novel immunosuppressive mechanism appears therapeutically useful to prevent the initiation of TLR-dependent inflammatory response via suppression of TRPC1 activity. In cultured macrophages, however, degradation of TRPC1 by caspase-11, an inducible caspase which is activated by NLRP3 inflammasome activator lipopolysaccharide (LPS), was found to increase the secretion of IL-1β. This negative regulation by TRPC1 occurred independently of caspase-1 cleavage or cell death [35] and thus likely reflects a distinctive mechanism from those described above. Consistently, a higher IL-1β secretion was observed in the sepsis model of TRPC1-deficient mice made by intraperitoneal LPS injection [35]. Although there is always a caveat to the relevance of knockout studies such as compensatory expression of homologous or other types of molecules which might affect downstream signaling (similar arguments may also hold for other TRP knockout models; see below), these results collectively imply the presence of multiple signaling pathways involving TRPC1 that regulate TLR-mediated inflammation. Further detailed analyses will be necessary to understand how manipulation of TRPC1 activity could be utilized for immune-modulatory interventions of inflammation.

In addition, there is evidence linking other TRP members to TLR-mediated signaling. In airway smooth muscle (ASM) cells, exposure to a proinflammatory cytokine TNF alpha (TNFα) or a mixture of allergens (ovalbumin, house dust mite, Alternaria, and Aspergillus extracts) causes both acute and chronic inflammations. These inflammatory responses involve at least in part increased secretion of brain-derived neurotrophic factor (BDNF) in a manner dependent on TRPC3-mediated Ca²⁺ entry [36]. In endothelial cells (ECs), endotoxin (LPS) induces pathological vascular leakage. This occurs through the interaction between TLR4 signaling and TRPC6-mediated Ca²⁺ entry, which causes increased endothelial permeability via activation of non-muscle myosin light chain kinase (MYLK) and NF-κB. Genetic deletion of TRPC6 rendered mice resistant to endotoxin-induced barrier dysfunction and inflammation and protected against sepsis-induced lethality [21].

TRPM4 is causally related to LPS-induced endothelial cell death via intracellular Na⁺ overloading. Pharmacological inhibition of TRPM4 activity with 9-phenathrol or glibenclamide was found to attenuate this consequence, suggesting a therapeutic potential of TRPM4 for endotoxin shock [22]. TRPM7-mediated intracellular concentration of Ca²⁺ ([Ca²⁺]_i) elevation serves as a key regulator for endotoxin-induced endothelial fibrosis through endothelial to mesenchymal transition [23]. This channel is also implicated in LPS-induced endothelial cell migration via TLR4/NF-κB pathway [37]. TRPM2-deficient mice shows compromised innate immunity against Listeria monocytogenes infection which allows uncontrolled replication of the bacteria with significantly reduced production of IL-12 and interferon-γ [38]. Consistent with this finding, in a cecal ligation and puncture (CLP)-induced mouse sepsis model, genetic disruption of TRPM2 was found to cause impaired host defense, leading to increased mortality associated with increased bacterial burden, organ injury, and systemic inflammation. Interestingly, this finding appears to reflect failed upregulation of heme oxgenase (HO)-1 in macrophages which is normally induced by TRPM2-mediated Ca²⁺ influx and essential for bacterial clearance [39].

In recent years, the potential benefits of TRPV1 activation have been recognized for the abatement of inflammatory response. For example, in Helicobacter pylori-positive patients, the genetic polymorphism of TRPV1 945G>C has been suggested to be one of the pathophysiological factors of functional dyspepsia [40]. In murine sepsis models, genetic or pharmacologic disruption of TRPV1 can affect mortality, blood bacteria clearance, and cytokine response, in such a pattern that may vary according to the sepsis-inducing events and the methods of TRPV1 disruption [41, 42]. In salivary glands, polyinosinic-polycytidylic acid or LPS activates, via TLR4 activation, NF-κB by IκB-α degradation and phosphorylation to release highly proinflammatory cytokines TNFα and IL-6. Capsaicin inhibits this process by interacting with the NF-κB pathway whereby to potentially alleviate inflammation of salivary glands [43]. Indeed, in healthy human subjects as well as patients, capsaicin has been suggested to have a therapeutic potential alone or in combination with other non-steroidal anti-inflammatory drugs [44], and in a mouse CLP model, capsaicin is shown to relieve the damaging impact of sepsis through TRPV1 activation [44, 45]

Autophagy is a highly evolutionarily conserved catabolic process to degrade and recycle cytoplasmic contents via a lysosomal route for reuse in downstream metabolism. It becomes increasingly clear that insufficiency of autophagy is an important pathogenic mechanism for inflammatory diseases [76]. Recently, a possible link between autophagy deficiency and increased inflammasome activation was suggested. The mechanism proposed includes the following causal sequence of cellular events: inefficient mitophagy, accumulation of damaged mitochondria, increased ROS production, and ROS-mediated inflammasome activation which occurs either directly or indirectly via DNA damage and secondary inflammatory signaling(s). Autophagy deficiency also reduces the efficiency of lysosomal degradation and thereby facilitates the accumulation of intra-lysosomal lipids and cholesterol crystals. This then leads to lysosomal membrane destabilization, lysosomal leakage, and inflammasome activation [77]. A similar consequence of autophagy deficiency can be expected for inefficient lysosomal degradation of damaged organelles and proteins. Therefore, normal function of the autophagy system is indispensable for keeping the cell healthy.

TRPML1 is an important player in endosomal sorting and transporting processes at the late endocytotic phase, specifically the formation of late endosome-lysosome hybrid vesicles [78‐81]. In other words, the role of this channel is to control the delivery of cellular materials to lysosomes, an essential process of autophagy [82‐84]. Altered activity of TRPML1 has been implicated in lysosomal dysfunction and impaired autophagy associated with AD-linked presenilin-1 mutations. In this pathological state, disrupted lysosomal acidification due to defective vesicular ATPase activity are thought to be primarily responsible for lysosomal and autophagy deficits, but concurrently, abnormal cytosolic Ca²⁺ elevation occurs via facilitated Ca²⁺ efflux through TRPML1 channel. However, correcting this abnormal Ca²⁺ homeostasis alone is not sufficient to restore normal lysosomal proteolytic and autophagic activities, thus suggesting that TRPML1 may play a permissive role in this process [85]. In this regard, it may deserve to mention that in humans, mutations in the gene encoding TRPML1 channel (MCOLN1) are the cause of the neurodegenerative disorder mucolipidosis type IV (MLIV) [86].

TRPML3 is a novel Ca²⁺ channel that plays a crucial role in the regulation of cargo trafficking along the endosomal [87, 88] and autophagosomal maturation [89] pathways. In infected bladder epithelial cells (BECs), TRPML3 triggers a non-lytic expulsion of bacteria (which is a powerful cell-autonomous defense strategy) to rapidly reduce infectious burden. This lysosomal channel is capable of sensing uropathogenic Escherichia coli-mediated lysosome neutralization and, in turn, releasing Ca²⁺, thereby triggering lysosomal exocytosis to expel the bacteria.[90]

The full-length form of TRPM2 channel (TRPM2-L) has a short splice variant consisting of only the N terminus and the first two transmembrane segments and lacking a pore domain (TRPM2-S). In expression system, coexpression of TRPM2-S suppressed oxidant-induced Ca²⁺ entry through TRPM2-L and subsequent cell death, presumably through a negative physical interaction [91]. Although its pathophysiological significance had been unclear, a recent study has revealed an interesting connection of this short variant (TRPM2-S) to autophagy. Mitochondrial homeostasis is dynamically regulated by the processes of autophagy/mitophagy and mitochondrial biogenesis. As compared with tumor cells expressing TRPM2-L isoform abundantly, those expressing TRPM2-S showed the accumulation of damaged mitochondrial DNAs with increased levels of unremoved heat shock protein 60 (Hsp60) and a mitochondrial protein translocase of outer membrane 20 (Tom20) in mitochondria. These results are interpreted to suggest that oxidant-induced Ca²⁺ entry mediated by TRPM2 may be crucial to maintain normal autophagy/mitophagy activity [92].

Oxidative stress induces pleiotropic responses ranging from cell survival to death. A recent study has given an interesting explanation about these differential cell fates, i.e., involvement of distinct poly(ADP-ribose) polymerase (PARP) isoforms (PARP1, PARP2) and distinctive cellular localization of TRPM2 channel. PARPs are enzymes producing poly(ADP-ribose) and, in conjunction with poly(ADP-ribose) glyocohydorase, capable of activating TRPM2 channel via immediate conversion of poly(ADP-ribose) into monomeric ADP-ribose. Under moderate oxidative stress conditions (5 mM H₂O₂), plasma membrane TRPM2 is under the control of PARP1, activation of which leads to the phosphorylation of p38, SAPK/JNK, and cAMP response element-binding protein (CREB)/ATF-1. This ultimately induces autophagy, thereby allowing cell survival. In contrast, high oxidative stress (10 mM H₂O₂) triggers late autophagy steps and PARP2 activation, leading to cell death with the activation of lysosomal TRPM2 channel [93].

NF-κB consists of a family of transcription factors that play critical roles in inflammation, immunity, cell proliferation, differentiation, and survival [99]. One of the factors known to activate NF-κB is an elevation in [Ca²⁺]_i [100, 101]. There are several papers linking Ca²⁺-permeable TRP channels to NF-κB-mediated inflammatory reactions; suppression of TRPC1-mediated Ca²⁺ entry inhibited NF-κB activation, which is associated with immunosuppressive mechanism in helminth infections [34]; pharmacological inhibition of TRPM7 channel suggested its involvement in LPS-induced EC migration via the TLR-NF-κB signaling [37]; endotoxin-induced lung injury involves TLR4-mediated NF-κB activation in a manner dependent on TRPC6-mediated Ca²⁺ entry [21].

Therapeutic potential of targeting NF-κB has been validated by its decoy treatments in a number of inflammatory CV diseases, including myocarditis, post-stenting coronary restenosis, and coronary hypertophic proliferation [102‐104].

The STAT3 has a dual role: both transducing signals through the cytoplasm and functioning as a transcription factor in the nucleus. STAT3 can be activated by extrinsic pathways, i.e., environmental factors, such as ultraviolet (UV) radiation, chemical carcinogens, infection, stress, and cigarette smoke, through growth factor, cytokine, Toll-like, adrenergic, and nicotinic receptors, respectively [95]. Persistently activated STAT3 mediates cell proliferation, survival, and invasion during inflammasome activation. Several studies suggest that TRP channels may regulate STAT3 activity.

Receptor-induced phosphorylation of cellular Janus kinase 2 (JAK2) or c-Jun and STAT3 are regulated by TRPC1- and TRPC6-mediated Ca²⁺ influxes [105‐108]. This could cause persistent inflammation, resulting in chronic inflammation with global tissue changes and injury. The Ca²⁺ transporting activity of TRPM7 is closely associated with the activation of the JAK2/STAT3 and/or Notch signaling pathways, which, in turn, induces ischemic neuronal cell death, metastatic transformation of breast cancer cells or proliferation, migration and invasion of glioma stem cells, and the fingerprints of sustained or chronic inflammation [109‐111]. Albeit little evidence, it is tempting to speculate that constitutive Ca²⁺ permeating activity TRPM7 channel might effectively drive, via Ca²⁺-dependent activation of STAT3, a feedback cycle of persistent activation of inflammasome toward chronic inflammation. A different line of evidence suggests that TRPV1 activates the STAT3 and NF-κB signaling pathways and thereby facilitates the expression of anti-inflammatory neuropeptides [112]. This hints a unique protective role of TRPV1 against inflammation, which is rarely seen for the other types of TRP channels.

HIFs are transcription factors that respond to changes in available oxygen in the cellular environment. However, a recent research suggests that, in certain pathological settings, HIF induction in normoxia likely causes serious consequences encompassing chronic inflammatory components. Chronic inflammation can be a self-perpetuating process so that it may continuously distort cellular microenvironments as the result of aberrantly active transcription factors. Consequent alterations in growth factors, chemokines, cytokines, and redox balance occur within the cellular milieu that, in turn, provide the axis of growth and survival needed for de novo development of cancer and metastasis [113]. In addition, a recent study suggested that cAMP/PKA/CREB/HIF-1α pathway is important for sustained inflammasome activity [114]. Considering that TRP channels serve as important Ca²⁺ entry routes associated with cellular stresses, it is possible that abnormal TRP channel activity would damage the intracellular microorganelles and disturb the redox balance. In fact, there are many reports found for the linkage between HIF-1 and TRP channels (e.g., TRPC1 [115‐117], TRPC3 [118], TRPC5 [119], TRPC6 [120, 121], TRPM2 [92, 122], and TRPM7 [123]).

The calcineurin-NFAT signaling plays a pivotal role in the transcription of cytokine genes and other genes critical for the T cell-mediated adaptive immune responses [124]. However, later studies identified its more ubiquitous roles in other organ systems [125]. For example, an established role of NFAT in the CV system is a hub mediator of hypertrophic signaling. In the heart, both mechanical and neurohormonal stresses activate the calcineurin-NFAT signaling to induce prominent pathological hypertrophy. The members of TRPC subfamily have been implicated in the activation step of this signaling as the stress-sensing Ca²⁺-permeating channels [126‐128]. In blood vessels, therapeutic benefits of an immunosuppressant cyclosporin A is known for transplant vasculopathy, part of which is causally related to the inhibition of calcineurin-NFAT signaling in ECs [129]. In diabetic mice, pharmacological inhibition of NFAT (by A-285222) reduced atherosclerotic lesion by inhibiting cytokine release and adhesion molecule expression [130]. Albeit scanty information, involvement of TRPC members has been suggested for inflammatory responses in renal podocyte injury and structural and functional remodeling after myocardial infarction via activation of the calcineurin-NFAT signaling [131, 132]. There is however little evidence yet obtained that directly links TRP channels to chronic inflammatory diseases (e.g., atherosclerosis) through this signaling pathway.

Atherosclerosis is a progressive inflammatory disease that disintegrates the structure and function of blood vessels through lipid deposition and activation of innate/adaptive immune reactions. There are good evidence to suggest that TRP channels are involved in the respective stages of atherosclerosis. This will be described below, in a stage-by-stage manner.

The main initiation factor for atherosclerosis is the oxidized low-density lipoprotein (ox-LDL), which affects the endothelial surface and thereby induces EC dysfunction. But, other inflammatory components/elements (virus, bacteria, toxins, toxic nutrients and metabolites, ambient particles, autoantibody, heat shock protein [187]; see also above) also promote atherosclerosis. Among TRP channels expressed in the CVS, two reports suggest proatherogenic potential of TRPC6 channel. It is reported that a cardiovascular risk factor, lysophosphatidylcholine (LPC), facilitates TRPC6 translocation to cell membrane causing a rapid Ca²⁺ influx in EC. This, in turn, induces the externalization of TRPC5, thereby allowing sustained Ca²⁺ influx through TRPC6/TRPC5 complex. This cascaded TRPC6/TRPC5 activation causes the inhibition of EC migration which is essential for the healing of atherosclerotic arteries [147]. Endothelial expression of microRNA miR-26a is reduced in the aortic intima of atherogenic ApoE (−/−) mice and ox-LDL-treated human ECs, whereas overexpression of miR-26a induces EC apoptosis. miR-26a acts as a negative regulator of TRPC6, activation of which allows Ca²⁺ influx activating the mitochondrial apoptotic pathway associated with atherosclerosis. Thus, abnormal TRPC6 activity can induce apoptotic EC death, and application of miR-26a may be able to reduce the atherosclerotic lesion [148]. These results provide, albeit indirect, evidence to support non-trivial roles of TRPC6 in the progression of atherosclerosis.

At the site of injury, ECs allow the entry of monocytes and lymphocytes into the vessel wall. In the early stage of this process, the adhesion of monocytes to the endothelial surface is supported by increased intracellular Ca²⁺ concentration in ECs. When the monocyte contacts EC, Ca²⁺ influx into EC is activated, which further strengthens the adherence of monocytes. Pharmacological inhibition of TRPV1 reduces the number of adherent monocytes, so that activation of TRPV1-mediated Ca²⁺ influx likely enables strong adhesion of monocytes to EC. In a similar context, TRPC3-mediated Ca²⁺ influx has been implicated in ATP-induced expression of vascular cell adhesion molecule-1 (VCAM-1) which is critical to recruit monocytes to EC [145, 150].

After injury, monocytes transmigrate across the damaged endothelium and enter the intimal layer of the vessel wall. Thus, reducing monocyte infiltration is thought to be one of the powerful strategies to attenuate the progression of atherosclerosis. A major atherogenic agent LPC can induce a strong chemotaxis of monocytes which appears to require Ca²⁺ influx. Pharmacological characterization of LPC-activated Ca²⁺-permeable currents strongly suggested that activation of both TRPC and TRPV1 is necessary for the optimal chemotaxic activity [149]. In this regard, these two TRP channels could be good molecular targets for anti-atherosclerotic therapy.

The monocytes that have migrated into the intima differentiate into macrophages and uptake ox-LDL. These lipid-laden macrophages are known as foam cells. Although the main foam cells are derived from macrophages, VSMCs can also transform to foam cells. Interestingly, activation of TRPV1 by capsaicin impedes the transformation of ox-LDL-treated VSMCs to foam cells by rescuing otherwise impaired autophagy by ox-LDL [151]. Moreover, TRPV1 activation protected macrophages from ox-LDL-induced lipid accumulation and TNFα-induced inflammation. These results indicate that activation of TRPV1 by capsaicin is another effective strategy to inhibit atherosclerosis.

After transformation, foam cells start to produce inflammatory cytokines. The released inflammatory cytokines trigger further transformation of VSMCs which gain the ability to migrate from the medial to intimal layer. These inflammatory responses make atherosclerotic plaques through necrosis and apoptosis, completing the clinical picture of atherosclerosis. In cultured RAW264 macrophages, LPS-induced production of cytokines (TNFα, IL-6) was shown to depend on TRPV2-mediated Ca²⁺ influx. Thus, in this late stage of atherosclerosis, targeting TRPV2 may be another novel strategy to suppress inflammatory cytokine production and thus formation of atherosclerotic lesions [188].

Bypass surgery and angioplasty with stenting often causes the vascular injury. Injured VSMCs then undergo transformation from quiescent and contractile to invasive and proliferative states. Such phenotype switching is an important process to recover vascular contractility but can also be part of the cause of occlusive vascular diseases including atherosclerosis and adverse responses accompanying neointimal hyperplasia. Several lines of evidence suggest that excessive (constitutively active) TRPC1 and (ROS-induced) TRPM2 activities in VSMCs are involved in the neointimal hyperplasia induced after vascular cuff injury in rat and mouse models, respectively. These changes were accompanied by enhanced cell cycle activity and inhibited by specific antibodies raised against the third extracellular loop of TRPC1 or TRPM2 channel (E3 antibodies). Similar increased expression of TRPC1 was also observed in the intimal layer of human vein graft samples. Thus, these channels may be novel therapeutic targets for occlusive vascular diseases [154, 155].

In cardiomyocytes, Ca²⁺ transients convey information to both contraction and gene transcription. Changes of cytoplasmic Ca²⁺ in cardiomyocytes are controlled by a variety of ion channels, the Na⁺/Ca²⁺ exchanger, Ca²⁺ pumps, and Ca²⁺-binding proteins. Several recent studies using animal models have implicated TRP channels in the development of hypertrophy. It has been shown that hypertrophic agonists upregulate the expression of TRPC1, TRPC3, and TRPC6 channels in cardiac myocytes, leading to activation of the Ca²⁺-(and ROS)–calmodulin–calcineurin-NFAT pathway which eventually results in cardiac hypertrophy. These hypertrophic changes are a pathological process basically involving inflammatory reactions [136, 189] and ultimately reduce the cardiac function increasing the mortality of experimental animals [118, 158‐168].

More recent studies have added a new connection of TRP channels (TRPV2 and TRPM4) to cardiomyopathy. In the patients of cardiomyopathy as well as in its genetic and chemically induced animal models which typically show ventricular dilation, fibrosis, and severely compromised heart function, the expression of TRPV2 was found to be concentrated on the ventricular sarcolemma. Specific abrogation of TRPV2 activity by either overexpression of the amino-terminal domain of TRPV2 (amino acids 1–387) or treatment with chemical inhibitors ameliorated these pathological changes and the contractile function of the heart and improved the survival of the affected animals [171]. Since the excessive activity of TRPV2 was associated with increased production of ROS and phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMK II), participation of inflammatory process is strongly suggested for these pathological changes. Thus, targeting TRPV2 may become a new clinical option to treat cardiomyopathy-associated heart failure.

In addition, a recent gene invalidation study showed that the activity of TRPM4 may be a prerequisite to preventing the development of eccentric ventricular hypertrophy of the heart. Indeed, TRPM4 has been shown to negatively regulate angiotensin II-induced cardiac hypertrophy owing its membrane depolarizing ability whereby the magnitude of store-operated Ca²⁺ entry which activates the calcineurin-NFAT hypertrophic pathway is negatively controlled [172, 173]. The correlation of these findings with inflammation is however unclear, since immunohistological examination revealed no obvious sign of fibrosis or hypertrophy but rather hyperplasia of smaller sized cardiomyocytes compared with normal ones.

In infarcted myocardium, necrotic cardiomyocytes release danger signals, activating an intense inflammatory response, by which complex cellular processes associated with injury, repair, and remodeling of the infarcted regions are activated [141]. In the MI period of animal models and human patients, the plasma level of brain-derived neurotrophic factor (BDNF) was found elevated along with tropomyosin-related kinase B (TrkB) and its downstream effector TRPC3 and TRPC6 channels. After BDNF treatment, the infarct size was markedly reduced and cardiac contractility was significantly restored, which seemed to be associated with decreased apoptotic response of cardiomyocytes. Since these beneficial effects of BDNF were reversed by pharmacological or functional inhibition of TRPC3/TRPC6 channels, these channels likely play a protective role against detrimental cardiac remodeling after MI [132, 174]. In a puzzling contrast, however, adenovirus-mediated overexpression of TRPC3/TRPC4/TRPC6 was reported to induce a hypertrophic response via the NFAT-mediated signaling in adult feline cardiomyocytes and accompany reduction of the contractility and catecholamine response due to increased spontaneous Ca²⁺ leak from the SR. In the same study, mice treated with MI procedures showed similarly enhanced TRPC1, TRPC3, TRPC4, and TRPC6 expression and Ca²⁺ channel activity along with induction of hypertrophic genes. These two lines of evidence have been interpreted to indicate the benefits of blocking the TRPC channels that improve post-MI structural remodeling and dysfunction. It is a matter of further investigation of what differences in experimental settings would make such disparate consequences of TRPC channel activation in the post-MI recovery.

In sharp contrast with a protective role of TRPM4 against dilated cardiomyopathy (see above), the deletion of the trpm4 gene in mice rather improved survival and β-adrenergic cardiac reserve after experimentally induced ischemic heart failure [175, 185]. In addition, several lines of evidence support the ameliorative role of TRPV1 in myocardial infarction as found in atherosclerosis [176, 177, 179‐182].

Reperfusion of the ischemic myocardium is essential for rescuing it from the death. However, reperfusion itself causes additional myocardial injury termed “ischemia/reperfusion (I/R) injury” [190]. I/R injury in the heart occurs through innate immune responses involving TLR (TLR2, TLR4) and the Myd88- and Trif-dependent NF-κB-interferon-3 pathway, activation of which induces the release of proinflammatory and immunomodulatory cytokines [142]. Moreover, oxidative stress-induced acute inflammatory response is implicated in the development of I/R injury [191].

There are two conflicting reports linking TRPM2 channel to I/R injury. One study suggested that activation of neutrophil TRPM2 channel by ROS exacerbated myocardial I/R injury by upregulating the expression of endothelial adhesion molecules MAC-1 and LFA-1. This then resulted in a stronger adhesion of neutrophils on the coronary EC surface. Neutrophil accumulation in the myocardium is a key process that induces myocardial injury [191]. Thus, specific inhibition of neutrophil TRPM2 activity may serve as an effective means to mitigate the exacerbation of myocardial infarction. However, in a striking contrast to this view, an independent study proposed an opposing and more complex hypothesis. According to the findings of this study, genetic deletion of TRPM2 exacerbated the extent of myocardial dysfunction after I/R injury via increased generation (upregulation of NADPH oxidase) and decreased scavenging capacity (downregulation of superoxide anion disulmutases) of ROS. As pointed out by the authors themselves, the discrepancies between the above two studies may reflect small but essential differences in experimental designs, as well as the use of global knockout mice with different exon-targeting invalidation strategies [122]. In any case, it is almost undoubted that TRPM2 channel is among central players in immunoinflammatory reactions in the cardiovascular disease and thus a promising therapeutic target.