Introduction

Guanylyl cyclase (also known as guanylate cyclase) is an enzyme that catalyzes the synthesis of cyclic guanosine 3′,5′-monophosphate (cGMP) from guanosine 5' triphosphate (GTP). Guanylyl cyclase exists in both a membrane-bound and soluble form in the cell. The membrane-bound form is a plasma membrane receptor, while soluble forms of guanylyl cyclase undergo activation by nitric oxide. Nitric oxide then functions as a primary messenger, amplifying the signal intracellularly. This review discusses guanylyl cyclase's biochemistry, function, and clinical relevance.[1] 

Fundamentals

Hormones that use the guanylyl cyclase signaling pathway include atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), and nitric oxide (endothelium-derived relaxing factor, or EDRF). These hormones either bind to their receptors on the cell membrane or their soluble receptor in the cytoplasm. The receptor has intrinsic guanylate cyclase activity, and the guanylyl cyclase converts GTP to cGMP. The cGMP then activates protein kinase G. The activated protein kinase G causes smooth muscle vasodilation.[2]

Issues of Concern

The role of guanylyl cyclase is prominent in congestive heart failure. The ventricles and the atria release atrial natriuretic peptide and B-type natriuretic peptide (BNP) in response to increased stretch. These natriuretic hormones increase guanylyl cyclase activity. The guanylyl cyclase also mediates penile erection by nitric oxide binding to soluble guanylyl cyclase (sGP), as the soluble guanylyl cyclase catalyzes the conversion of GTP to cGMP.[1] 

Research has shown that membrane-bound guanylyl cyclase-C (GC-C) receptors may be novel therapeutic targets for treating inflammatory bowel disease (IBDs).[3] In addition, soluble guanylyl cyclase (SGs) are abundant in the striatum, and their role in neurodegenerative diseases like Parkinson's disease has been discovered.[4]

Cellular Level

Nitric oxide (NO) is a freely diffusible intercellular signaling molecule that mediates vasodilation. Nitric oxide binds to soluble guanylyl cyclase and increases cGMP levels. Nitric oxide binds to a heme prosthetic group on the guanylyl cyclase. Once NO binds, this triggers a conformational change that stimulates cGMP synthesis. The activation of soluble guanylyl cyclase leads to increased cGMP concentration and transmitting the NO signal to downstream proteins in the signaling cascade, including cGMP-dependent protein kinase, cGMP-gated cation channels, and cGMP-regulated phosphodiesterase.

Nitric oxide also binds to the transmembrane receptor guanylyl cyclase; this converts GTP to cGMP, which acts as a second messenger to activate protein kinase G. Activation of protein kinase G causes smooth muscle relaxation and vasodilation.[5]

Molecular Level

Soluble guanylyl cyclase is a heterodimeric protein with alpha and beta subunits; it is a hemoprotein, and NO binds to soluble guanylate cyclase heme and activates the enzyme.[6]

Membrane-bound guanylyl cyclase contains an extracellular domain that binds to hormones such as ANP and BNP, a transmembrane domain, kinase homology domain, hinge region, and guanylyl cyclase domain, which converts GTP to cGMP [7]

Membrane-bound guanylyl cyclase has seven different isozymes GC-A through GC-G. GC-A mediates ANP and BNP, controls blood pressure, and regulates cellular growth in the brain and kidney, angiogenesis, liver regeneration, and lipolysis in adipose tissue. GC-B stimulates the proliferation of chondrocytes by cGMP activation of protein kinase-2 at the growth plate.[7]

GC-A, also called natriuretic peptide receptor-A (NPR-A), expresses throughout the cardiovascular system in vascular smooth muscle, vascular endothelium, heart, and kidney. GC-B, also called NPR-B, is highly expressed in vascular endothelium and smooth muscle. However, guanylyl cyclase's presence in cardiac tissue is predominantly localized to the non-myocyte population and mostly in fibroblasts.[8]

Function

As mentioned earlier, mammalian single membrane-spanning guanylyl cyclases exist in at least seven varieties, including GC-A, B, C, D, E, F, and G. Guanylyl cyclases catalyze the conversion of GTP to cGMP and pyrophosphate. Nitric oxide, bicarbonate, and natriuretic peptides, such as ANP and BNP, activate the guanylyl cyclase enzymes, forming the cyclic GMP.

Other activators of guanylyl cyclase enzymes include uroguanylin, guanylin, and guanylyl cyclase activating proteins. The cyclic GMP then serves as an intracellular second messenger that controls blood pressure, cardiac hypertrophy, sexual arousal, gut peristalsis, platelet aggregation, neurotransmission, long bone growth, lipolysis, intestinal fluid secretion, phototransduction, and oocyte maturation.[5]

Mechanism

Phosphodiesterase 3 (PDE3) is an enzyme that regulates the contractility of vascular smooth muscle cells and cardiac myocytes. PDE3 controls the phenotypic switch in vascular smooth muscle cells and the stress response, and it interacts with L-type calcium channels and the cardiac sarcoplasmic reticulum calcium pump (SERCA2) to modulate cardiac myocyte contractility and relaxation.[8]

Phosphodiesterase 5 (PDE5) is an enzyme found in the smooth muscle cells of the corpus cavernosum that cleaves and degrades cGMP to 5’GMP. PDE5 inhibitors and cGMP have similar structures; they bind competitively to PDE 5 and inhibit cGMP hydrolysis, thus enhancing the effects of NO. The erection then prolongs due to increased cGMP concentration in smooth muscle cells. However, after PDE 5 inhibitor administration, adequate sexual stimulation is still necessary for the erection, as PDE 5 inhibitors do not directly affect corpus cavernosum smooth muscle relaxation.[9]

Pathophysiology

cGMP mediates several physiologic processes in different cell types in the cardiovascular system. Dysfunctional signaling at any cascade step, including cGMP synthesis, effector activation, or cGMP breakdown, has been present in many cardiovascular diseases, such as hypertension and atherosclerosis, cardiac hypertrophy, and heart failure. [10]

Guanylyl cyclase plays an important role in cardiac muscle relaxation. ANP and BNP are produced and released by cardiac muscle when there is an increase in myocardial wall stretch and/or pressure and an increase in catecholamines, arginine vasopressin, angiotensin 2, endothelin, and glucocorticoids. ANP and BNP are natriuretic peptides that bind to membrane-associated guanylyl cyclase receptors (NPRs). Of the seven mammalian membrane-associated guanylyl cyclases, the NPR-A and NPR-B participate in natriuretic peptide pathways. ANP and BNP activate NPR-A to regulate body volume homeostasis, blood pressure, and local anti-hypertrophic effects in the myocardium.[11][12]

Guanylyl cyclase plays a prominent role in the process of erection during sexual arousal, at nerve terminals, and in endothelial cells in the corpus cavernosum. NO activates guanylyl cyclase, which converts GTP into cGMP, triggering a cGMP-dependent cascade of events. Smooth muscle relaxation occurs when cGMP accumulates in the corpus cavernosum to increase blood flow to the penis.[9]

Clinical Significance

Nitrates can relieve angina by acting as a vasodilator. Nitrates reduce symptomatic and silent ischemic episodes in patients with ST-segment alterations in coronary heart disease. These anti-ischemic effects can improve prognosis by preventing infarction and deterioration of left ventricular function due to chronic myocardial ischemia. Nitrates decrease the frequency of ischemic episodes and reduce the number of anginal attacks that produce clinical symptoms. They decrease preload as vasodilation pools blood into distal extremities and decrease blood returning to the heart.[11]

Sildenafil, vardenafil, tadalafil, and avanafil are PDE5 inhibitors that are the first-line for managing erectile dysfunction in men. PDE 5 is an enzyme found in the smooth muscle cells of the corpus cavernosum that cleaves and degrades cGMP to 5’GMP. PDE 5 inhibitors bind and inhibit PDE5, inhibiting cGMP hydrolysis, thus enhancing the effects of NO. The erection then prolongs due to increased cGMP in smooth muscle cells.[9]

Nitric oxide-cGMP enhancers are also valuable for treating primary pulmonary hypertension; these act by dilating the pulmonary arteries and bringing the mean pulmonary artery pressure down, thus reducing right ventricle afterload; this has shown functional class improvement, cardiac index improvement, and improvement in 6-minute walking distance. In trials, nitrates have been combined with other agents like endothelin receptor blockers and prostaglandins to show progress. But only FDA-approved combination therapy combines ambrisentan (endothelin receptor blocker) and tadalafil (PDE5 inhibitor). In some studies, nitrates and sildenafil have been used concomitantly for treating pulmonary hypertension in congestive heart failure.[13]

Nitrates can cause excessive hypotension as a side effect. Vasodilation increases blood flow to distal extremities, decreasing blood flow returning to the brain, which can cause symptomatic hypotension and headaches, in which case nitrate therapy should be discontinued. A hypertensive crisis can occur if nitrates are used in patients with acute inferior myocardial infarction associated with right ventricular dysfunction or infarction or with concurrent use of PDE 5 inhibitor or N-acetylcysteine.[14]

Nitrate tolerance can occur with long-term continuous administration as it decreases vasodilatory effects. To prevent tolerance from long-term nitrate therapy, providers should advise a 10 to 12-hour nitrate-free interval; thus, nitrates would be administered only for a portion of each day. Also, during the nitrate-free periods, some patients might develop an increase in angina and require sublingual nitroglycerin for short-term therapeutic relief.[15]