Identification of a mechanosensor in juxtaglomerular cells for the regulation of renin synthesis and secretion

PROJECT SUMMARY Juxtaglomerular (JG) cells are a cluster of specialized vascular smooth muscle (VSM) cells located in the wall of each afferent arteriole (Af-Art) at the entrance to the glomerulus. On one hand, JG cells preserve most of the characteristics of VSM cells, but on the other hand, they are distinguished from VSM cells by their unique endocrine characteristics, specifically the synthesis and secretion of renin. Renin released from JG cells into the circulation catalyzes the conversion of angiotensinogen to angiotensin I, the first and rate-limiting step of the renin-angiotensin-aldosterone system (RAAS) cascade. Commensurate with the central role of RAAS in fluid and electrolyte homeostasis, renin synthesis and secretion by JG cells is regulated by multiple inputs that generally reflect the status of extracellular fluid volume. The most important among the control mechanisms of renin release is a pressure-sensing mechanism, denominated as the intrarenal baroreceptor. It is believed that JG cells can detect slight changes in perfusion pressure through the Af-Art and respond with precise amounts of renin synthesized and secreted to the circulation in sync, therefore functioning as a mechano-sensitive rheostat to maintain a steady-state level of Af-Art perfusion pressure and thus systemic blood pressure. However, the nature of the intrarenal baroreceptor and in particular, the central questions about this pressure-sensing mechanism: what the exact mechano-sensor is in JG cells and how it controls renin synthesis and secretion, remain elusive. A novel technique, single-cell RNA sequencing (scRNA-seq), provides a new tool to further understand the genetic and molecular basis of the intrarenal baroreceptor in JG cells. Our preliminary studies of scRNA-seq analysis have revealed that a gene Sdc1 is not only highly and differentially expressed in JG cells vs. VSM cells, but also positively correlated with Ren1 expression. Sdc1 encodes syndecan-1, a transmembrane proteoglycan, which allows for the transfer of information about extracellular environment changes into the cell that consequently affects intracellular activities. Thus, in this project, we aim to determine whether syndecan-1 is a key mechano-sensor of the intrarenal baroreceptor mechanism in JG cells that controls renin synthesis and secretion, regulates the circulating RAAS, and thus contributes to the maintenance of fluid and electrolyte homeostasis and blood pressure. Specifically, in Aim 1, we will determine the expression of syndecan-1 in the kidney, especially in JG cells; moreover, we will examine the effects of different perfusion pressures on the renin mRNA and protein expression levels in JG cells with the in vitro model of isolated/perfused juxtaglomerular apparatus and in vivo model of aortic coarctation in wild-type (WT), global Sdc1 knockout (KO), and JG cell- specific Sdc1KO mice. In Aim 2, we will examine the volume depletion (low perfusion pressure) or volume expansion (high perfusion pressure)-induced changes in the renin expression levels in JG cells, circulating levels of RAAS, renal vascular resistance, urinary sodium excretion, and blood pressure in WT, global Sdc1KO, and JG cell-specific Sdc1KO mice.