The renin-angiotensin-aldosterone system (RAAS) is one of the key players in the regulation of blood volume and blood pressure. Dysfunction of this system is connected with cardiovascular and renal diseases. Regulation of RAAS is under the control of multiple intracellular mechanisms. Cyclic nucleotides and phosphodiesterases are the major regulators of this system since they control expression and activity of renin and aldosterone. In this review, we summarize known mechanisms by which cyclic nucleotides and phosphodiesterases regulate renin gene expression, secretion of renin granules from juxtaglomerular cells and aldosterone production from zona glomerulosa cells of adrenal gland. We also discuss several open questions which deserve future attention.
The renin-angiotensin-aldosterone system is one of the key players in the regulation of blood volume and blood pressure. Dysfunction of this system is connected with cardiovascular and renal diseases. The main regulatory mechanism responsible for the activity of RAAS is a cascade of proteolytic enzymes that cleaves circulating angiotensinogen by renin to generate angiotensin I (Ang I), followed by subsequent cleavage of Ang I by angiotensin converting enzyme (ACE) to angiotensin II (Ang II). Angiotensinogen, the only precursor of all biologically active angiotensin peptides, is predominantly produced by the liver and its production is controlled by several hormones including estrogens, steroids, and thyroid hormones. Historically, Ang II acting through Ang II receptors (AT-Rs), which belong to the G protein coupled receptor family (GPCR) was considered as a main regulator of RAAS system. This concept was revisited after discovery of angiotensin converting enzyme type 2 (ACE2) which generates Ang-(1-7). This short peptide acts at its specific GPCR called Mas receptor and counterbalances the vasoconstrictor role of Ang II.
Renin activity is a rate-limiting step controlling the activity of the RAAS. Juxtaglomerular (JG) cells located at afferent arterioles of the kidney glomeruli are the main source of renin secretion into the blood stream. Generally, renin production from JG cells is controlled by renal perfusion pressure, the tubular sodium chloride concentration sensed by macula densa cells and negative feedback loops involving blood pressure, sodium balance and Ang II concentration. At the cellular level, cyclic nucleotides, 3’,5’-cyclic adenosine monophosphate (cAMP) and 3’,5’-cyclic guanosine monophosphate (cGMP) are strongly involved in the regulation of renin gene expression and release from JG cells.
Aldosterone is the next important player of RAAS which is synthesized and released by zona glomerulosa (ZG) cells of adrenal gland. Aldosterone, in addition to Ang II, is involved in the hemostatic regulation of blood pressure by controlling plasma sodium and potassium concentrations which are the main determinants of blood volume. Two classical pathways are involved in the regulation of aldosterone synthesis and release. Firstly, adrenocorticotropic hormone (ACTH) binds to the Gs-coupled melanocortin type 2 receptor (MC 2 R) and initiates cAMP synthesis. Secondly, Ang II and potassium concentrations in the plasma by different mechanisms can increase intracellular calcium.
In mammalian cells, cAMP is synthesized from ATP by adenylate cyclases (ACs) from 10 different families, regulated by G-protein coupled receptors (GPCRs), intracellular calcium and bicarbonate. cGMP is produced from GTP by two types of guanylate cyclase. The first one is the nitric oxide (NO) sensitive or the so-called soluble guanylate cyclase (sGC), which is directly activated by NO. The second one is a family of particulate guanylyl cyclases (pGCs) which are membrane receptors for natriuretic peptides. cAMP acts in cells primarily by activating cAMP dependent protein kinase (PKA), exchange protein directly activated by cAMP (Epac), or cyclic nucleotide gated (CNG) channels. cGMP can activate cGMP dependent proteins kinase (PKG) or CNG channels. PKA and PKG phosphorylate multiple substrates regulating various intracellular processes.
Cyclic nucleotides are degraded by PDEs which are hydrolyzing enzymes converting cAMP and cGMP to monophosphates. Over 100 PDE isoforms are generated by alternative splicing of more than 20 genes. PDEs have been classified into eleven families based on substrate specificity and regulatory properties. This high versatility leads to very precise regulation of cyclic nucleotide levels in various organ systems. PDEs are also critically involved in the compartmentalization of cAMP and cGMP signaling in various subcellular nano- or microdomains which determine signal output by local substrate regulation. The response by each of the cyclic nucleotides in amplitude, space and time is controlled by their degradation via respective PDEs. PDEs 4, 7 and 8 are highly specific for cAMP hydrolysis, whereas PDEs 5, 6 and 9 are cGMP specific. There are also several dual-specific PDEs such as PDE1, 2, 3, 10 and 11.
Some dual-specific PDEs are also involved into the cross-talk between both cyclic nucleotides. For example, cGMP can activate PDE2 via an N-terminal regulatory domain, thereby promoting cAMP degradation and enabling a negative cGMP-to-cAMP cross-talk. Also, cGMP can bind to the catalytic domain of PDE3 with high affinity causing a negative cGMP-to-cAMP cross-talk and impaired cAMP degradation.
Studies have reported that disruption in the expression of various PDEs or mutations in the PDE genes can lead to diseases including cancers, cardiovascular, neuronal, and other pathologies. Therefore, the role of PDEs and integration into the cAMP and cGMP pathways along with their crosstalk is essential to multiple physiological systems.
Renin concentration in plasma is regulated by the renin gene expression and release of renin from the renin containing granules in JG cells. These processes are regulated by second messengers including cyclic nucleotides and intracellular calcium. It is generally accepted that cAMP downstream of membrane receptors such as β 1-adrenergic and prostaglandin receptors, is the main activatory signal for renin expression and release, whereas increase of calcium concentration inhibits it by so-called “calcium paradox”. The effect of cGMP on JG cells is more complicated and still not fully understood. Several mediators of cGMP including PKG I, PKG II PDE2 and PDE3 are expressed in JG cells and cGMP could stimulate as well as inhibit renin gene expression and secretion. Atrial natriuretic peptide (ANP) cGMP-dependently inhibited renin release from JG cells, whereas inhibition of PDE3 by cGMP activates renin secretion (for details see below).
Renin gene in human and mouse genome is localized in the chromosome 1. The 5’-flanking noncoding region plays a central role in regulating renin gene expression. Activation of the renin transcription is mainly mediated by the cAMP/PKA pathway. In contrast, activation of protein kinase C and increase of intracellular calcium are involved in the suppression of renin expression. cAMP-induced activation of the renin gene expression is initiated by PKA dependent phosphorylation of cAMP responsive transcription factors (CREB, CREM, and ATF1) that in phosphorylated form bind to CRE sequences in the proximal promoter and the kidney enhancer region. cAMP/PKA pathway is also critically involved in the control of basal transcriptional level of renin gene. Deletion of Gs coupled receptors strongly decreased renin expression in mouse models. The question whether and how cGMP is involved in the regulation of the renin gene expression is still open. cGMP by inhibition of PDE3 enhances cAMP effect which in turn activates renin expression. Deletion of PKG II, but not PKG I, in mouse significantly enhanced renin mRNA levels. However, the molecular mechanisms of renin gene activation, whether it relates to the phosphorylation of the transcription factors by PKG II, or some other mechanisms are not clear. Nitric oxide (NO) by activation of soluble guanylate cyclase supports recruitment of renin producing cells to the glomerular arteriole, however the molecular mechanisms underlying these processes also not clear.
cGMP exerts dual effect on renin secretion. Activation of PKG II by 8-pCPT-cGMP, which is specific for PKG II and does not stimulate PDE3 inhibited basal cAMP-stimulated renin secretion. Using PKG I and PKG II knock- out models it was concluded that in mice, cGMP and PKG are involved in the acute regulation of renin release but not in the long-term regulation of renin gene e
