Chronic kidney disease (CKD) is estimated to affect approximately 11 to 13% of the global population [1]. Progression of CKD is affected by a number of conditions such as age, chronic inflammation, diabetes, autoimmune disorders, and severe infection in the kidney [2]. One of the final pathological outcomes of CKD is renal fibrosis [2,3]. Renal fibrosis is the result of excessive accumulation of extracellular matrix (ECM) that disrupts renal function [2,4,5,6]. Some of the major hallmarks of long-term fibrosis include tubular atrophy, tubular dilation, increased fibrogenesis, and increased scar formation [1,3]. Renal fibrosis can be categorized based on the affected renal structures: fibrosis in the glomerulus is referred to as glomerulosclerosis; fibrosis in the proximal and distal tubules is referred to as tubulointerstitial fibrosis; and fibrosis around the vasculature is referred to as perivascular fibrosis [7]. Common diagnostics of CKD and renal fibrosis rely on end-stage markers of renal failure, measured by decreased glomerular filtration (eGFR) and increased proteinuria (albumin concentration in the urine), signifying a loss of transport capabilities and disruptions in the glomerular filtration barrier [4,8]. Although the mechanisms that initiate fibrosis are essential for tissue repair, prolonged activation of these mechanisms leads to chronic fibrosis and renal failure. Current research has focused on identifying biomarkers that can identify and treat renal fibrosis prior to end-stage renal failure.

The primary feature of renal fibrosis is the excess deposition and assembly of ECM. This increased ECM deposition causes changes in both the chemical and mechanical environments within the tissue, altering cellular function and exacerbating renal fibrosis. Notably, remodeling in the basement membrane and interstitial space encourage malfunction of the renal system [5]. ECM proteins are assembled into scaffold-like structures by the surrounding renal cells. In turn, cells bind to this de novo ECM, inducing altered signal transduction and cell behavior, that contributes to this exacerbated ECM assembly [2,5]. As such, understanding the bi-directional interactions between renal cells and the ECM is essential to identify therapeutic approaches to disrupt this cycle. The following sections will highlight major ECM-secreting and assembling cells in the kidney implicated in renal fibrosis. 

Renal pericytes and renal fibroblasts are mesenchymal cells that play key roles in maintaining the physiological structure of the kidney. Renal fibroblasts are key producers of ECM in the tubulointerstitium and glomerulus. They are essential in providing structural and mechanical support to the kidney by maintaining the basement membrane surrounding the tubules and vasculature [9]. Pericytes are crucial in the development and stabilization of the vascular network, covering 10 to 50% of the entire surface [10,11,12]. They regulate oxygen transport by producing renal hormones renin and erythropoietin (EPO) and are characterized through their increased expression of platelet-derived growth factor receptor-β (PDGFRβ) [9,10]. In the glomerulus, mesangial cells are a specialized form of pericytes found in the juxtaglomerular compartment that interact with surrounding endothelial cells and podocytes to regulate glomerular filtration in response to vascular stretch [12].

Previous work using single-cell RNA sequencing has shown that renal fibroblasts and pericytes are the primary source of myofibroblasts in renal fibrosis [11,13,14]. Myofibroblasts are the main promoter of fibrotic progression, exhibiting a pro-migratory phenotype through increased expression of α-Smooth Muscle Actin (α-SMA) and deposition of ECM components including collagen, fibronectin, and glycosaminoglycans to repair injured tissue [9,11,12]. After the injured tissue has been repaired, differentiated myofibroblasts undergo apoptosis and decrease inflammatory signals in the repaired tissue. However, in a fibrotic environment, these myofibroblasts fail to undergo apoptosis and continue secreting ECM components and remodeling the surrounding microenvironment [15]. Chronic inflammation also leads to continued differentiation of progenitors into new myofibroblasts, increasing the damage to the tissue [9]. During fibrosis, pericytes exhibit increased myofibroblast markers, including increased expression of α-SMA, upregulated collagen deposition, and increased migration away from the vasculature [13,16,17]. This active pericyte differentiation results in their detachment from the vasculature [12,18]. Along with a loss of vascular stability, the decreased pericyte population drives a reduction in the secretion of both renin and EPO that impairs blood flow through the renal vasculature [19]. In addition to fibroblasts and pericytes, bone marrow-derived fibroblasts are a proposed source of myofibroblast population in fibrotic diseases; however, their contribution is not fully understood [13].

Renal structure and function are regulated by specialized epithelial and endothelial cells. A single layer of epithelial cells known as podocytes form the Bowman’s Capsule in the glomerulus, and are continguous with tubular epithelial cells (TECs) that form the Loop of Henle and the collecting duct [7]. Endothelial cells line the vasculature from the afferent arterioles and form the capillary structures within the glomerulus, at which point they merge to form the lining of the efferent arterioles and maintain the glomerular filtration barrier [7].

Epithelial and endothelial cells differentiate into a mesenchymal phenotype through the process of epithelial-mesenchymal transition (EMT) or endothelial-mesenchymal transition (EndoMT), respectively [20,21]. EMT and EndoMT are transdifferentiation processes where cells lose key phenotypic markers such as strong cell–cell adhesions, apicobasal polarity, and a cobblestone morphology, and acquire mesenchymal characteristics [22]. Although EMT has been implicated in in vitro research, there are few in vivo studies that have shown a major contribution of epithelial or endothelial cells to the myofibroblast population [11,23] in renal fibrosis. Instead, renal epithelial and endothelial cells are thought to undergo partial EMT, where they express both epithelial and mesenchymal markers without losing their cobblestone morphology, cell–cell contacts, or tubular epithelial structure [23,24]. Instead of acting as direct myofibroblast progenitors like fibroblasts and pericytes, these cells play major roles in directing myofibroblast differentiation and maintaining activity during fibrotic progression through cytokine secretion and excess secretion of ECM proteins [20]. This is in contrast to other pathological events, such as tumor metastasis, where epithelial cells undergo full EMT to promote cancer cell migration and invasion [25,26].

As stated before, pericytes play an important role in healthy tissue in stabilizing the vasculature [17]. However, when the kidney is injured, pericytes will transdifferentiate into myofibroblasts, and detach from the vasculature. This migration away from the vasculature leaves the endothelial cells vulnerable to further injury that leads to vascular rarefaction and injury to surrounding tubules [16]. Increased TGF-β signaling and cytokine interaction also lead to higher rates of apoptosis of endothelial cells, causing further degradation that leads to the full dissolution of the vessels [18].

One of the key diagnostic criteria of renal fibrosis is decreased eGFR, signifying damage to the glomerular filtration barrier. Increased damage to the barrier promotes the infiltration of immune cells such as monocytes and lymphocytes [27]. Monocytes differentiate into macrophages to promote repair in injured tissue [27]. In acute healing, macrophages change from a pro-inflammatory (M1) to anti-inflammatory (M2) phenotype [28]. However, fibrosis promotes increased pro-inflammatory macrophage activity including the production of reactive oxygen species (ROS), inflammatory cytokine secretion, and synthesis of pro-fibrotic matrix metalloproteinases (MMPs) [27,29]. The release of inflammatory signals induces changes in surrounding cell morphology such as myofibroblast differentiation, EMT, and EndoMT [29]. Mediation of M1 polarization of macrophages in injured kidneys reduces the release of inflammatory cytokines and excessive ECM deposition [30]. Along with M1 polarization, treatment of UUO (unilateral ureteral obstruction) mice with the anti-inflammatory quercetin downregulated M2 macrophage polarization in the injured tubulointerstitium [30], suggesting that targeting overall macrophage infiltration and polarization in the kidney may ameliorate chronic inflammation associated with renal fibrosis.

Macrophages have also recently been proposed as a source of the myofibroblast population in cases of progressive fibrosis and end-stage renal failure [28]. Increased bone-marrow derived macrophages are present in fibrotic kidneys and exhibit increased co-expression of macrophage markers and myofibroblast markers such α-SMA [30]. The depletion of macrophages in glomerulonephritis has been shown to reduce immune complex-mediated glomerulonephritis and overall fibrosis progression [28]. Given that macrophage accumulation and macrophage–mesenchymal transition (MMT) promote fibrosis, recent studies have suggested targeting macrophage activation as a way to ameliorate fibrosis and associated autoimmune renal diseases [30].