Carbon monoxide another key product from the breakdown of he

Carbon monoxide, another key product from the breakdown of heme by HO-1, also plays an important role in the vasculature. Like NO, CO is endogenously derived from the endothelium [68] and can weakly activate sGC by binding to its heme moiety [69]. CO was found to have vasodepressor activity in rats [70]. Rapid increases in arterial blood pressure arising from pharmacological inhibition of HO-1 are likely a consequence of the diminished production of CO rather than lower biliverdin, since the latter would ordinarily decrease blood pressure over an extended period of time [70]. In young spontaneously hypertensive rats, the addition of an HO-1 inducer, hemin, significantly increased HO-1 activity and cGMP levels; however, the same results were not obtained in adult spontaneously hypertensive rats [71]. The authors hypothesized that the adult rats may have compensatory overexpression of the HO-1/CO-sGC-cGMP system, which could lead to the desensitization of downstream targets of cGMP, preventing vasodilatory effects and increasing oxidative stress [71]. Despite studies that have shown vasodilatory effects of CO, another study has demonstrated that CO production from HO-1 interferes with the NO-sGC pathway and decreases cGMP [72]. Exogenous CO also decreased cGMP levels [72]. These seemingly divergent findings contrast with older studies that CO led to vasodilation, indicating that more research is needed in understanding the effects of CO in vasculature.
Localization of sGC to membrane microdomains The localization of sGC and its signaling partners in cell membrane microdomains is important for assembling effectors, receptors and targets for optimal cellular signaling and responses. In terms of downstream effects, sGC localization to the plasma membrane of tip AMG-208 helps them control directionality of angiogenic migration [73]. Neuronal NOS-mediated maintenance of cardiomyocyte intercalated disc morphology [74] and spontaneous contraction of rat distal colon by neuronal calcium (Ca2+) activated potassium (K+) channels [75] are processes heavily reliant on sGC localization in the cell membrane. Indeed, the cardioprotection afforded by beta-blocker therapy, as was shown in a canine model of cardiac volume overload, occurs by preservation of sGC localization in caveolae [76]. In the heart, sGC localized at the plasma membrane of cardiomyocytes within caveolae obtains its primary source of NO from nearby endothelial nitric oxide synthase (eNOS) [77]. From this platform, sGC-produced cGMP can easily reach protein kinase G (PKG) for phosphorylation of VASP to effect cardiomyocyte relaxation. Zabel et al. showed that Ca2+ is important in translocating sGC to the cell membrane, which is required for the cyclase to assume functionality [78]. Meurer et al. have also suggested a role for the guanosine triphosphatase (GTPase)-like AGAP1 (membrane-trafficking ArfGAP protein) in a reversible phosphorylation-dependent translocation of sGC to the membrane [79], a process that can be inhibited by G protein regulators, such as LGN (Leu-Gly-Asn repeat-enriched protein) and AGS3 (activator of G-protein signaling 3) [80]. Depressed sGC activity in pressure-overloaded hearts has been shown to follow sGC delocalization from membrane microdomains, wherein caveolin 3 (Cav3) protects the sGC heme from oxidation and thereby facilitates NO signaling [81]. Membrane localization of sGC is a recurring physiological phenomenon across multiple cell types. The role of interacting proteins in sGC's localization to membrane microdomains, and its activation state, partly hinge on complexing sGC to eNOS. Co-immunoprecipitation studies showed sGC complexed with Hsp70 and Hsp90 in COS7 and aortic smooth muscle cells (SMC), respectively [77,82]. Consistent with the stabilizing role of heat shock proteins, Hsp90 was shown to restore heme-depleted sGC to its heme-replete state in aortic SMC [83]. Given the tendency for heme-deficient or heme-Fe3+ (NO-insensitive oxidized heme) sGC to be released from plasma membrane and targeted for degradation, one might argue that Hsp90 also plays an important role in keeping the cyclase targeted to the plasma membrane. In the central nervous system, sGC was found targeted to the plasma membrane of postsynaptic cells by postsynaptic density protein 95 (PSD95) [84], a membrane-associated protein that is speculated to facilitate multimeric scaffolding with potassium channels and NMDA receptors [85].