Newly STEMming functions of macula densa-derived prostanoids

Macula densa (MD) cells are chief cells within the kidney, playing key sensory and regulatory functions in the maintenance of body fluid and electrolyte homeostasis and blood pressure. MD cells are strategically positioned in the distal nephron at the entrance of the glomerulus as the tubular component of the juxtaglomerular apparatus (JGA), an important renal anatomical site that controls renal hemodynamics, glomerular filtration and renin release (renin-angiotensin system (RAS) activation). Despite their importance, MD cells have been a mysterious renal cell type, mainly because their low number (only about 20 cells per nephron) and relative inaccessibility make them difficult to study. Therefore, our knowledge of the MD is limited to the traditional functions of these cells: the sensing of variations in the distal tubular fluid microenvironment (tubular salt, metabolites, flow) and the generation and release of paracrine mediators for tubulo-vascular crosstalk that controls afferent arteriole vasoconstriction (tubuloglomerular feedback, TGF) and renin secretion.^1–4 Tubular salt sensing by the MD involves apical NaCl transport via the furosemide-sensitive Na:2Cl:K cotransporter (NKCC2), which is the primary NaCl entry mechanism in these cells. In fact, a classic hallmark of MD-mediated renin release is its effective stimulation by furosemide or other loop diuretics.^1,2 The downstream elements of MD-mediated renin release signaling include the low tubular salt-induced and NKCC2-mediated activation of p38 and extracellular-regulated kinase 1/2 (ERK1/2) mitogen-activated protein (MAP) kinases, cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase (mPGES) and the synthesis and release of PGE2 by these cells.⁵ PGE2 is the classic paracrine mediator of MD-mediated renin release, acting mainly on the EP4 receptor subtype of PGE2 receptors on JG renin cells (Figure).²

Figure.

Schematic illustration of the traditional and new functions of macula densa-derived prostaglandin E2 (PGE2). The sensing of reduced tubular [NaCl] via the furosemide-sensitive Na:2Cl:K (NKCC2) cotransporter leads to p38 and ERK1/2 (MAPK) signaling, increased PGE2 synthesis and release via cycloxygenase-2 (COX-2) and microsomal PGE synthase (mPGES) activation in macula densa (MD) cells. The paracrine action of MD-derived PGE2 causes renin release from juxtaglomerular (JG) renin cells (JGC) via the EP4 receptor (classic function). The newly emerging function of this MD/PGE2/EP4 axis is the recruitment of new renin cells into the JG apparatus (JGA) via the activation of CD44+ mesenchymal stem cell-like cells in the renal interstitium and their migration towards the JGA, and their differentiation into renin producing JGCs. AA: afferent arteriole, EA: efferent arteriole, G: glomerulus.

The most important and immediate MD partner cell in the JGA, the renin producing JG cell has received considerable attention in the last few years. A variety of stress stimuli that threaten body fluid and electrolyte homeostasis increase circulating renin and activate the RAS, one of the first lines of systemic defense mechanisms, by increasing the number of renin-expressing and releasing JG cells in the terminal part of the afferent arteriole (JGA). According to the prevailing renal physiology paradigm, JG cell recruitment involves de-differentiation and re-expression of renin in afferent arteriole vascular smooth muscle cells that belong to the renin cell lineage.^6,7 However, this classic paradigm of JG cell recruitment was challenged recently by the demonstration that CD44+ mesenchymal stem cell-like cells exist in the adult kidney, are recruited to the JG area, and differentiate into renin cells in response to loss of body fluid and salt.⁸ Another study showed that cells of the renin lineage are progenitors of podocytes and parietal epithelial cells in glomerular disease, and may enhance glomerular regeneration.⁹ These studies opened a new era in renin cell research, and established new links between renal stem/progenitor cells, renal physiology, and kidney disease that involve the renin cell. One of the many exciting questions stemming from these studies is what controls renal stem cell recruitment to the JGA?

In this issue, Yang et al report their new study that addressed this question.¹⁰ As a logical extension of their recent work mentioned above (regarding CD44+ mesenchymal cell recruitment to the JGA⁸), the same group of investigators hypothesized that chronic sodium deprivation stimulates renal CD44+ cell activation, migration, and differentiation to JG renin cells via MD-derived PGE2. First, they applied an in vitro approach and co-cultured isolated CD44+ cells with a MD cell line. Lowering NaCl content of the culture medium induced PGE2 production by MD cells and the migration of CD44+ cells, which effect was inhibited by the pharmacological blockade of COX-2 or EP4 receptor.¹⁰ Also, the addition of PGE2 to CD44+ cells increased cell migration and induced renin expression via the EP4 receptor.¹⁰ Second, the investigators used an in vivo experimental model and found that the recruitment of renal CD44+ cells to the JGA, which was activated by dietary sodium restriction and furosemide treatment, was attenuated in wild type mice by treatment with the COX-2 inhibitor rofecoxib, and by EP4 receptor deficiency.¹⁰ Altogether, this study provides new insights into the physiologically and pathologically important mechanism of JG cell recruitment, and identifies new key players in this process: MD control of a PGE2/EP4 signaling axis, and renal CD44+ mesenchymal stem cell-like cells as effectors. It should be noted that although the in vitro cell data strongly suggest the role of MD cells, MD cell-specificity and origin of PGE2 was not unequivocally demonstrated in the present in vivo studies. Future experiments need to further clarify the role of MD-derived prostanoids, and likely other factors in renal stem cell-mediated JG cell recruitment in vivo. Regardless, the present findings by Yang et al will significantly advance the fields of renal physiology and renal stem cells.

Since the importance of MD-derived PGE2 in renin release is well established, it makes perfect sense that the MD, via PGE2/EP4 signaling to renal stem cells, controls JG cell recruitment as well. The strategic anatomical localization of the small MD cell plaque at the vascular entrance of the glomerulus, and the MD-specific expression of COX-2 and mPGES that provides a point source of PGE2, are consistent with the development of a PGE2 dose gradient in the renal cortex that activates and directs the migration of renal stem cells towards the JGA epicenter. The findings of several earlier studies are supportive of this new function of MD-derived prostanoids acting on stem cells. For example, the paracrine action of PGE2 via the EP4 receptor on the target cell is a well-established mechanism for stem and progenitor cell trafficking in many tissues.¹¹ Also, COX-2 and its products are known to be important factors in embryonic nephrogenesis. Partial genetic knockout or chemical inhibitors of COX-2 were shown to inhibit glomerulogenesis.¹² It is highly anticipated that future studies will shed more light on these newly stemming functions of the mysterious MD cells.