Renin release: role of SNAREs

Juxtaglomerular apparatus-mediated homeostatic mechanisms: therapeutic implication for chronic kidney disease

INTRODUCTION

Juxtaglomerular apparatus (JGA)-mediated homeostatic mechanism links to how sodium-glucose cotransporter 2 inhibitors (SGLT2is) slow progression of chronic kidney disease (CKD) and may link to how tolvaptan slows renal function decline in autosomal dominant polycystic kidney disease (ADPKD).

AREA COVERED

JGA-mediated homeostatic mechanism has been hypothesized based on investigations of tubuloglomerular feedback and renin-angiotensin system. We reviewed clinical trials of SGLT2is and tolvaptan to assess the relationship between this mechanism and these drugs.

EXPERT OPINION

When sodium load to macula densa (MD) increases, MD increases adenosine production, constricting afferent arteriole (Af-art) and protecting glomeruli. Concurrently, MD signaling suppresses renin secretion, increases urinary sodium excretion, and counterbalances reduced sodium filtration. However, when there is marked increase in sodium load per-nephron, as in advanced CKD, MD adenosine production increases, relaxing Af-art and maintaining sodium homeostasis at the expense of glomeruli. The beneficial effects of tolvaptan on renal function in ADPKD may also depend on the JGA-mediated homeostatic mechanisms since tolvaptan inhibits sodium reabsorption in the thick ascending limb.The JGA-mediated homeostatic mechanism regulates Af-arts, constricting to relaxing according to homeostatic needs. Understanding this mechanism may contribute to the development of pharmacotherapeutic compounds and better care for patients with CKD.

Exogenous H2S Attenuates Hypertension by Regulating Renin Exocytosis under Hyperglycaemic and Hyperlipidaemic Conditions

Obesity, along with type 2 diabetes mellitus (T2DM), is a major contributor to hypertension. The renin-angiotensin-aldosterone system is involved in the occurrence of diabetes and hypertension. However, the mechanism by which obesity is related to T2DM induced hypertension is unclear. In this study, we observed that blood pressure and serum renin content were increased in patients with diabetes and hypertension. Hydrogen sulfide (H₂S), as an endogenous bioactive molecule, has been shown to be a vasodilator. Db/db mice, characterized by obesity and T2DM, and juxtaglomerular (JG) cells, which line the afferent arterioles at the entrance of the glomeruli to produce renin, treated with glucose, palmitic acid (PA) and oleic acid (OA), were used as animal and cellular models. NaHS, the H₂S donor, was administered to db/db mice through intraperitoneal injection. NaHS significantly alleviated blood pressure in db/db mice, decreased the renin content in the serum of db/db mice and reduced renin secretion from JG cells. NaHS modulated renin release via cAMP and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), including synaptosome-associated protein 23 (SNAP23) and vesicle-associated membrane protein 2 (VAMP2), which mediate renin exocytosis. Furthermore, NaHS increased the levels of autophagy-related proteins and colocalization with EGFP-LC3 puncta with renin-containing granules and VAMP2 to consume excessive renin to maintain intracellular homeostasis. Therefore, exogenous H₂S attenuates renin release and promotes renin-vesicular autophagy to relieve diabetes-induced hypertension.

Arachidonic Acid as Mechanotransducer of Renin Cell Baroreceptor

For normal maintenance of blood pressure and blood volume a well-balanced renin-angiotensin-aldosterone system (RAS) is necessary. For this purpose, renin is secreted as the situation demands by the juxtaglomerular cells (also called as granular cells) that are in the walls of the afferent arterioles. Juxtaglomerular cells can sense minute changes in the blood pressure and blood volume and accordingly synthesize, store, and secrete appropriate amounts of renin. Thus, when the blood pressure and blood volume are decreased JGA cells synthesize and secrete higher amounts of renin and when the blood pressure and blood volume is increased the synthesis and secretion of renin is decreased such that homeostasis is restored. To decipher this important function, JGA cells (renin cells) need to sense and transmit the extracellular physical forces to their chromatin to control renin gene expression for appropriate renin synthesis. The changes in perfusion pressure are sensed by Integrin β1 that is transmitted to the renin cell’s nucleus via lamin A/C that produces changes in the architecture of the chromatin. This results in an alteration (either increase or decrease) in renin gene expression. Cell membrane is situated in an unique location since all stimuli need to be transmitted to the cell nucleus and messages from the DNA to the cell external environment can be conveyed only through it. This implies that cell membrane structure and integrity is essential for all cellular functions. Cell membrane is composed to proteins and lipids. The lipid components of the cell membrane regulate its (cell membrane) fluidity and the way the messages are transmitted between the cell and its environment. Of all the lipids present in the membrane, arachidonic acid (AA) forms an important constituent. In response to pressure and other stimuli, cellular and nuclear shape changes occur that render nucleus to act as an elastic mechanotransducer that produces not only changes in cell shape but also in its dynamic behavior. Cell shape changes in response to external pressure(s) result(s) in the activation of cPLA2 (cytosolic phospholipase 2)-AA pathway that stretches to recruit myosin II which produces actin-myosin cytoskeleton contractility. Released AA can undergo peroxidation and peroxidized AA binds to DNA to regulate the expression of several genes. Alterations in the perfusion pressure in the afferent arterioles produces parallel changes in the renin cell membrane leading to changes in renin release. AA and its metabolic products regulate not only the release of renin but also changes in the vanilloid type 1 (TRPV1) expression in renal sensory nerves. Thus, AA and its metabolites function as intermediate/mediator molecules in transducing changes in perfusion and mechanical pressures that involves nuclear mechanotransduction mechanism. This mechanotransducer function of AA has relevance to the synthesis and release of insulin, neurotransmitters, and other soluble mediators release by specialized and non-specialized cells. Thus, AA plays a critical role in diseases such as diabetes mellitus, hypertension, atherosclerosis, coronary heart disease, sepsis, lupus, rheumatoid arthritis, and cancer.

Beyond the Paradigm: Novel Functions of Renin-Producing Cells

The juxtaglomerular renin-producing cells (RPC) of the kidney are referred to as the major source of circulating renin. Renin is the limiting factor in renin-angiotensin system (RAS), which represents a proteolytic cascade in blood plasma that plays a central role in the regulation of blood pressure. Further cells disseminated in the entire organism express renin at a low level as part of tissue RASs, which are thought to locally modulate the effects of systemic RAS. In recent years, it became increasingly clear that the renal RPC are involved in developmental, physiological, and pathophysiological processes outside RAS. Based on recent experimental evidence, a novel concept emerges postulating that next to their traditional role, the RPC have non-canonical RAS-independent progenitor and renoprotective functions. Moreover, the RPC are part of a widespread renin lineage population, which may act as a global stem cell pool coordinating homeostatic, stress, and regenerative responses throughout the organism. This review focuses on the RAS-unrelated functions of RPC - a dynamic research area that increasingly attracts attention.

Urinary Renin in Patients and Mice With Diabetic Kidney Disease

In patients with diabetic kidney disease (DKD), plasma renin activity is usually decreased, but there is limited information on urinary renin and its origin. Urinary renin was evaluated in samples from patients with longstanding type I diabetes mellitus and mice with streptozotocin-induced diabetes mellitus. Renin-reporter mouse model (Ren1d-Cre;mT/mG) was made diabetic with streptozotocin to examine whether the distribution of cells of the renin lineage was altered in a chronic diabetic environment. Active renin was increased in urine samples from patients with DKD (n=36), compared with those without DKD (n=38; 3.2 versus 1.3 pg/mg creatinine; P<0.001). In mice with streptozotocin-induced diabetes mellitus, urine renin was also increased compared with nondiabetic controls. By immunohistochemistry, in mice with streptozotocin-induced diabetes mellitus, juxtaglomerular apparatus and proximal tubular renin staining were reduced, whereas collecting tubule staining, by contrast, was increased. To examine the role of filtration and tubular reabsorption on urinary renin, mice were either infused with either mouse or human recombinant renin and lysine (a blocker of proximal tubular protein reabsorption). Infusion of either form of renin together with lysine markedly increased urinary renin such that it was no longer different between nondiabetic and diabetic mice. Megalin mRNA was reduced in the kidney cortex of streptozotocin-treated mice (0.70±0.09 versus 1.01±0.04 in controls, P=0.01) consistent with impaired tubular reabsorption. In Ren1d-Cre;mT/mG with streptozotocin-induced diabetes mellitus, the distribution of renin lineage cells within the kidney was similar to nondiabetic renin-reporter mice. No evidence for migration of cells of renin linage to the collecting duct in diabetic mice could be found. Renin mRNA in microdissected collecting ducts from streptozotocin-treated mice, moreover, was not significantly different than in controls, whereas in kidney cortex, largely reflecting juxtaglomerular apparatus renin, it was significantly reduced. In conclusion, in urine from patients with type 1 diabetes mellitus and DKD and from mice with streptozotocin-induced diabetes mellitus, renin is elevated. This cannot be attributed to production from cells of the renin lineage migrating to the collecting duct in a chronic hyperglycemic environment. Rather, the elevated levels of urinary renin found in DKD are best attributed to altered glomerular filteration and impaired proximal tubular reabsorption.

Apelin/APJ system: A novel therapeutic target for oxidative stress-related inflammatory diseases (Review)

Apelin, the endogenous ligand of APJ which is a member of G protein-coupled receptors, has been shown to be expressed in a variety of tissues in vivo and to exert significant biological effects. Studies have indicated that the apelin/APJ system is involved in the regulation of a variety of physiological functions and pathological processes, and that it is associated with cardiovascular diseases (such as atherosclerosis, hypertension, heart failure and myocardial injury), diabetes with microvascular complications, ischemia reperfusion injury, tumors, pre-eclampsia, as well as others. The occurrence of these diseases is closely related to endothelial dysfunction and the local inflammatory response; however, the occurrence of oxidative stress is related to vascular injury, due to the excessive generation of reactive oxygen species (ROS) and can lead to vascular damage and a series of inflammatory reactions. Therefore, this review summarizes the association between apelin/APJ, oxidative stress and inflammation-related diseases. In addition, drugs targeting the apelin/APJ system are recommended, thus providing a novel therapeutic strategy for oxidative stress-related inflammatory diseases.

Five years of data diuresis: what have WEH learned?

This year represents the fifth annual Data Diuresis session of the Water and Electrolyte Homeostasis (WEH) section of the American Physiological Society (APS) at the 2015 Experimental Biology meeting. As opposed to taking a single organ approach to the study of physiology, the WEH section employs an integrative approach to encompass how the different organ systems interact to regulate numerous physiological and pathophysiological processes. The goal of this minireview is to highlight the broad spectrum of research themes that were presented over the first five years of Data Diuresis. Presentation topics include (but are not limited to) oxidative stress, inflammation, obesity, pregnancy, and hypertension spanning the brain, heart and vasculature, and kidney. WEH researchers continue to impact and help drive the direction of physiological research across multiple disciplines, leaving us excited to see what the next five years of Data Diuresis will bring.