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Is it changes in perivascular cells that causes impaired regeneration in aged kidneys?
The kidney is susceptible to ageing. Injuries in old patients more rapidly progress to end stage renal disease, and mouse models show increased injury in aged animals, yet the reasons for this are unknown. As in other organs, pericytes play a key role in the kidney’s injury response, and a minority of pericytes expressing Gli-1, a sonic hedgehog (Shh) receptor, contribute the majority of renal myofibroblasts and fibrosis following injury in mice. We hypothesise that differences in pericyte subtypes between old and young organs can explain the age-linked deterioration in tissue repair. This project aimed to interrogate pericyte subpopulations using surface marker expression, to identify potential differences in the starting or responsive pool of cells between ages, and whether such differences are meaningful to the wound healing response, with the ultimate aim of defining beneficial/pathological pericyte subpopulations. The effects of Shh inhibitor Vismodegib during injury were also investigated. Pericytes were detected in baseline old and young kidneys using the markers platelet derived growth factor receptor β (PDGFR-β) and CD146, which together show differential expression across kidney regions. Old kidneys had more CD146 positivity in the outer stripe of the outer medulla (OuS). Old and young animals were subjected to unilateral ischemia reperfusion injury and experienced similar levels of tubular death in the OuS. However although both age groups had increased fibrosis in injured kidneys old animals also had increased fibrosis in contralateral uninjured kidneys. When PDGFR-β and -α were observed together, the PDGFR-α+ ratio increased in old animals. Other cellular differences detected include a population of CD45+ ICAM2+ cells exclusive to old kidneys. When treated with Vismodegib following a unilateral ureterial obstruction no difference in fibrosis was detected. Together these results report pericyte marker heterogeneity between kidney regions, and increased fibrosis in uninjured organs of old mice, but no influence of Shh inhibition on myofibroblast activity. Further elucidating changes in cell populations of aged animals, which have increased susceptibility to injury and impaired recovery, should improve understanding of optimal repair processes and produce novel therapeutic targets.
Abstract: Peritubular capillary rarefaction is hypothesized to contribute to the increased risk of future CKD after AKI. Here, we directly tested the role of Gli1(+) kidney pericytes in the maintenance of peritubular capillary health, and the consequences of pericyte loss during injury. Using bigenic Gli1-CreER(t2); R26tdTomato reporter mice, we observed increased distance between Gli1(+) pericytes and endothelial cells after AKI (mean±SEM: 3.3±0.1 µm before injury versus 12.5±0.2 µm after injury; P<0.001). Using a genetic ablation model, we asked whether pericyte loss alone is sufficient for capillary destabilization. Ten days after pericyte ablation, we observed endothelial cell damage by electron microscopy. Furthermore, pericyte loss led to significantly reduced capillary number at later time points (mean±SEM capillaries/high-power field: 67.6±4.7 in control versus 44.1±4.8 at 56 days; P<0.05) and increased cross-sectional area (mean±SEM: 21.9±0.4 µm(2) in control versus 24.1±0.6 µm(2) at 10 days; P<0.01 and 24.6±0.6 µm(2) at 56 days; P<0.001). Pericyte ablation also led to hypoxic focal and subclinical tubular injury, reflected by transient expression of Kim1 and vimentin in scattered proximal tubule segments. This analysis provides direct evidence that AKI causes pericyte detachment from capillaries, and that pericyte loss is sufficient to trigger transient tubular injury and permanent peritubular capillary rarefaction.
Pub.: 15 Sep '16, Pinned: 08 Jun '17
Abstract: Chronic liver scarring from any cause leads to cirrhosis, portal hypertension, and a progressive decline in renal blood flow and renal function. Extreme renal vasoconstriction characterizes hepatorenal syndrome, a functional and potentially reversible form of acute kidney injury in patients with advanced cirrhosis, but current therapy with systemic vasoconstrictors is ineffective in a substantial proportion of patients and is limited by ischemic adverse events. Serelaxin (recombinant human relaxin-2) is a peptide molecule with anti-fibrotic and vasoprotective properties that binds to relaxin family peptide receptor-1 (RXFP1) and has been shown to increase renal perfusion in healthy human volunteers. We hypothesized that serelaxin could ameliorate renal vasoconstriction and renal dysfunction in patients with cirrhosis and portal hypertension.To establish preclinical proof of concept, we developed two independent rat models of cirrhosis that were characterized by progressive reduction in renal blood flow and glomerular filtration rate and showed evidence of renal endothelial dysfunction. We then set out to further explore and validate our hypothesis in a phase 2 randomized open-label parallel-group study in male and female patients with alcohol-related cirrhosis and portal hypertension. Forty patients were randomized 1:1 to treatment with serelaxin intravenous (i.v.) infusion (for 60 min at 80 μg/kg/d and then 60 min at 30 μg/kg/d) or terlipressin (single 2-mg i.v. bolus), and the regional hemodynamic effects were quantified by phase contrast magnetic resonance angiography at baseline and after 120 min. The primary endpoint was the change from baseline in total renal artery blood flow. Therapeutic targeting of renal vasoconstriction with serelaxin in the rat models increased kidney perfusion, oxygenation, and function through reduction in renal vascular resistance, reversal of endothelial dysfunction, and increased activation of the AKT/eNOS/NO signaling pathway in the kidney. In the randomized clinical study, infusion of serelaxin for 120 min increased total renal arterial blood flow by 65% (95% CI 40%, 95%; p < 0.001) from baseline. Administration of serelaxin was safe and well tolerated, with no detrimental effect on systemic blood pressure or hepatic perfusion. The clinical study's main limitations were the relatively small sample size and stable, well-compensated population.Our mechanistic findings in rat models and exploratory study in human cirrhosis suggest the therapeutic potential of selective renal vasodilation using serelaxin as a new treatment for renal dysfunction in cirrhosis, although further validation in patients with more advanced cirrhosis and renal dysfunction is required.ClinicalTrials.gov NCT01640964.
Pub.: 01 Mar '17, Pinned: 08 Jun '17
Abstract: Pericytes, perivascular cells embedded in the microvascular wall, are crucial for vascular homeostasis. These cells also play diverse roles in tissue development and regeneration as multi-lineage progenitors, immunomodulatory cells and as sources of trophic factors. Here, we establish that pericytes are renin producing cells in the human kidney. Renin was localized by immunohistochemistry in CD146 and NG2 expressing pericytes, surrounding juxtaglomerular and afferent arterioles. Similar to pericytes from other organs, CD146(+)CD34(-)CD45(-)CD56(-) renal fetal pericytes, sorted by flow cytometry, exhibited tri-lineage mesodermal differentiation potential in vitro. Additionally, renin expression was triggered in cultured kidney pericytes by cyclic AMP as confirmed by immuno-electron microscopy, and secretion of enzymatically functional renin, capable of generating angiotensin I. Pericytes derived from second trimester human placenta also expressed renin in an inducible fashion although the renin activity was much lower than in renal pericytes. Thus, our results confirm and extend the recently discovered developmental plasticity of microvascular pericytes, and may open new perspectives to the therapeutic regulation of the renin-angiotensin system.
Pub.: 30 Sep '16, Pinned: 08 Jun '17