The role played by perivascular cells in kidney interstitial injury

Abstract

Fibrosis of the kidney is a disease affecting millions worldwide and is a harbinger of progressive loss of organ function resulting in organ failure. Recent findings suggest that understanding mechanisms of development and progression of fibrosis will lead to new therapies urgently required to counteract loss of organ function. Recently, little-known cells that line the kidney microvasculature, known as pericytes, were identified as the precursor cells which become the scar-forming myofibroblasts. Kidney pericytes are extensively branched cells located in the wall of capillaries, embedded within the microvascular basement membrane, and incompletely envelope endothelial cells with which they establish focal contacts. In response to kidney injuries, pericytes detach from endothelial cells and migrate into the interstitial space where they undergo a transition into myofibroblasts. Detachment leads to fibrosis but also leaves an unstable endothelium, prone to rarefaction. Endothelial-pericyte crosstalk at the vascular endothelial growth factor receptors and platelet derived growth factor receptors in response to injury have been identified as major new targets for therapeutic intervention.

Figure 1.. A schema showing the relationship between the nephron and the microvasculature of the nephron and showing the attachment of pericytes to capillaries and the close association of perivascular fibroblasts to the arterioles.

Figure 2.. Kidney pericytes and their response to injury. (A,B) Schematic (A) and fluorescence images (B) from Coll-GFP mice of pericytes (PCs, green) and capillary endothelial cells (ECs, immunostained for CD31 as red) in normal kidney and 24 hours after unilateral ureteral obstruction (UUO). In response to kidney injury, PCs detach themselves from ECs, spread, migrate (arrowheads in B) and increase collagen expression (become more green). Progression of this process (A) leads to unstable vasculature, capillary loss and interstitial matrix expansion. (C,D) Schematic (C) and electron microscopy images (human sample) (D) of PC-interaction with EC in normal kidney. PC processes are enveloped in capillary basement membrane (CBM) (arrows) where intimate connections and cell : cell signaling occurs known as “peg and socket” junctions (arrowheads). L = capillary lumen, E = EC, Pp = pericyte process.

Figure 3.. Results of fate mapping of Foxd1 progenitors in adult and injured kidney using the Foxd1-Cre;Rosa26-tdTomatoR mouse. A: Schema showing the cross of Foxd1-Cre recombinase allele with TdTomato reporter allele driven by the universal promoters at the Rosa26 locus. Bigenic mice recombine genomic DNA at the Rosa locus only in cells that have activated Foxd1 gene in nephrogenesis. B: Confocal images of kidney cortex showing in normal adult kidney large numbers of perivascular cells, which all co-express PDGFR-β. VSMCs of the kidney arterioles are also derived from Foxd1-progenitors and co-express αSMA intermediate filament, but none of Foxd1-derived pericytes (arrowheads) or perivascular fibroblasts (arrows) express αSMA. In kidney injury (shown here is UUO Day 7) the pericyte and perivascular fibroblast populations expand and continue to express PDGFR-β. However, now all of the expanded population of interstitial Foxd1-progenitor derived cells co-express αSMA, the marker which defines these cells as myofibroblasts.

Figure 4.. Pericyte detachment, transition to the myofibroblast phenotype and interstitial fibrosis can be blocked by inhibiting the activation of VEGFR2 and PDGFR-β with circulating soluble receptor ectodomains: A: Schema showing the experimental protocol. Using engineered adenoviruses as gene delivery tools, the liver, which is the main target organ for IV delivery of adenoviruses, synthesizes and releases into the circulation high levels of soluble receptors for the duration of the experiments. Three days after viral delivery kidney injuries were performed and analyzed up to 10 days later. B: Western blots detecting sPDGFR-β or sVEGFR2 in 1 µl of mouse plasma. C: Low power immunofluorescence images of kidneys Day 4 after UUO or sham surgery (CON) in Coll-GFP reporter mice that received control Fc-virus or viruses producing sPDGFR-β or sVEGFR2. Note that activated Coll-GFP+ pericytes/myofibroblasts are much more abundant in the interstitium of control (sFc) diseased kidneys compared with kidneys that were exposed to sVEGFR2 or sPDGFR-β (g = glomerulus where podocytes also can be seen). D: High power fluorescence images of pericytes (green) and endothelial cells (red) in kidneys on Day 1 or 2 after UUO injury or sham surgery (CON). Note that in normal kidney pericytes are attached to endothelial cells but 24 h after UUO in kidney exposed to sFc (control virus) there is detachment spreading and migration of pericytes from the endothelium. In the presence of either sPDGFR-β or sVEGFR2 this detachment is almost completely prevented. Similar findings persist 2 days after UUO injury.