CKD is a significant health concern with an underlying genetic component. Multiple genome-wide association studies (GWASs) strongly associated CKD with the shroom family member 3 (SHROOM3) gene, which encodes an actin-associated protein important in epithelial morphogenesis. However, the role of SHROOM3 in kidney development and function is virtually unknown. Studies in zebrafish and rat showed that alterations in Shroom3 can result in glomerular dysfunction. Furthermore, human SHROOM3 variants can induce impaired kidney function in animal models. Here, we examined the temporal and spatial expression of Shroom3 in the mammalian kidney. We detected Shroom3 expression in the condensing mesenchyme, Bowman's capsule, and developing and mature podocytes in mice. Shroom3 null (Shroom3Gt/Gt ) mice showed marked glomerular abnormalities, including cystic and collapsing/degenerating glomeruli, and marked disruptions in podocyte arrangement and morphology. These podocyte-specific abnormalities are associated with altered Rho-kinase/myosin II signaling and loss of apically distributed actin. Additionally, Shroom3 heterozygous (Shroom3 Gt/+ ) mice showed developmental irregularities that manifested as adult-onset glomerulosclerosis and proteinuria. Taken together, our results establish the significance of Shroom3 in mammalian kidney development and progression of kidney disease. Specifically, Shroom3 maintains normal podocyte architecture in mice via modulation of the actomyosin network, which is essential for podocyte function. Furthermore, our findings strongly support the GWASs that suggest a role for SHROOM3 in human kidney disease.
The mammalian kidney undergoes cell interactions between the epithelium and mesenchyme to form the essential filtration unit of the kidney, termed the nephron. A third cell type, the kidney stroma, is a population of fibroblasts located in the kidney capsule, cortex and medulla and is ideally located to affect kidney formation. We found β-catenin, a transcriptional co-activator, is strongly expressed in distinctive intracellular patterns in the capsular, cortical, and medullary renal stroma. We investigated β-catenin function in the renal stroma using a conditional knockout strategy that genetically deleted β-catenin specifically in the renal stroma cell lineage (β-cats-/-). β-cats-/- mutant mice demonstrate marked kidney abnormalities, and surprisingly we show β-catenin in the renal stroma is essential for regulating the condensing mesenchyme cell population. We show that the population of induced mesenchyme cells is significantly reduced in β-cats-/- mutants and exhibited decreased cell proliferation and a specific loss of Cited 1, while maintaining the expression of other essential nephron progenitor proteins. Wnt9b, the key signal for the induction of nephron progenitors, was markedly reduced in adjacent ureteric epithelial cells in β-cats-/-. Analysis of Wnt9b-dependent genes in the neighboring nephron progenitors was significantly reduced while Wnt9b-independent genes remained unchanged. In contrast mice overexpressing β-catenin exclusively in the renal stroma demonstrated massive increases in the condensing mesenchyme population and Wnt9b was markedly elevated. We propose that β-catenin in the renal stroma modulates a genetic program in ureteric epithelium that is required for the induction of nephron progenitors.
Background: Ischemia induced acute kidney injury (AKI) resulting in tubular damage can often progress to chronic kidney disease (CKD) and is a common cause of nephrology consultation. Following renal tubular epithelial damage, molecular and cellular mechanisms are activated to repair and regenerate the damaged epithelium. If these mechanisms are impaired, AKI can progress to CKD. Even in patients whose kidney function returns to normal baseline are more likely to develop CKD. Genome-wide association studies have provided robust evidence that genetic variants in SHROOM3, which encodes an actin-associated protein, are associated with CKD and poor outcomes in transplanted kidneys. Here, we sought to further understand the associations of Shroom3 in CKD. Methods: Kidney ischemia was induced in wild-type and Shroom3 heterozygous null mice (Shroom3Gt/+) and the mechanisms of cellular recovery and repair were examined. Results: A 28-minute bilateral ischemia in Shroom3Gt/+ mice resulted in 100% mortality within 24 hours. After 22-minute ischemic injury, Shroom3Gt/+ mice had a 16% increased mortality, worsened kidney function, and significantly worse histopathology, apoptosis, proliferation, inflammation, and fibrosis after injury. The cortical tubular damage in Shroom3Gt/+ was associated with disrupted epithelial redifferentiation, disrupted Rho-kinase/myosin signaling, and disorganized apical F-actin. Analysis of Madin Darby Canine Kidney Cells showed the levels of Shroom3 are directly correlated to apical organization of actin and actomyosin regulators. Conclusion: These findings establish that Shroom3 is required for epithelial repair and redifferentiation through the organization of actomyosin regulators and could explain why genetic variants in Shroom3 are associated with CKD and allograft rejection.
Macrophage cytokine production is inhibited by cholinergic signals transmitted via the vagus nerve, an α7 nicotinic acetylcholine receptor (α7nAChR) dependent pathway termed the “cholinergic anti‐inflammatory pathway” (Nature 420:853–9, 2002). Here we addressed the hypothesis that this pathway is capable of providing short‐term memory by downregulating macrophage responses to endotoxin for up to 48 hr. Vagus nerve stimulation (VNS) (5V, 2 ms, 1Hz, 2.5 min) in rats 24 h prior to endotoxin (LPS) significantly reduced serum TNF (sham =767 pg/ml; VNS =358 pg/ml, p<0.01); this effect persisted for 48 h (sham TNF = 1327; VNS TNF = 824, p<0.05). The molecular mechanism is attributable to α7nAChR signaling, because VNS fails to reduce TNF levels in α7nAChR KO mice. This molecular memory is recapitulated in cultured human macrophages pulsed with acetylcholine (ACh) (60 min pulse 24 h prior to LPS). LPS‐induced TNF release was reduced 54% as compared with vehicle controls (p<0.05); pulsing also significantly suppressed activation of NF‐κB (74%). Ach‐pulsing significantly altered the macrophage transcriptome response to endotoxin, indicating that vagus nerve signals “train” macrophages as a short‐term (48 hr) memory response. Evidence that discrete neural signals can mediate short‐term memory in macrophages via α7nAChR has surprising implications for understanding innate immunity. Supported in part by NIGMS.
Shroom3 is an actin‐binding protein that modulates actomyosin dynamics to alter epithelial cell morphology. In adult kidneys, Shroom3 is apically localized in tubular epithelial cells of cortical nephron segments, such as S1 of proximal tubules, distal tubules, and collecting ducts. Genomic variants in Shroom3 are strongly associated with poor renal function and poor outcomes in kidney transplant recipients. After a kidney insult such as ischemia reperfusion injury from renal or cardiac surgeries, the damaged renal epithelium undergoes cell morphology changes to regenerate normal renal epithelial structure. This repair process includes actomyosin remodelling, allowing damaged epithelial cells to undergo de‐differentiation into a mesenchymal cell type, proliferate and re‐differentiate to tubular epithelial cells. In this study, our objective is to determine whether Shroom3 plays a role in renal epithelial repair after an ischemic kidney insult. We performed bilateral renal ischemia/reperfusion (I/R) on 3‐month‐old Shroom3 heterozygous mutant mice (Shrm3+/‐)and wild type (WT) mice. Compared to WT at 10 days post‐I/R, most of the epithelial cells in cortical tubules of Shrm3+/‐ kidneys had F‐actin and phospho‐Myosin Light Chain‐2 (pMLC‐2) sporadically distributed in the cytoplasm rather than being apically localized. In these cortical tubular cells, immunohistochemistry (IHC) demonstrated that mesenchymal marker Vimentin was highly expressed while epithelial markers N‐cadherin and E‐cadherin were significantly lower than in WT. These findings suggest that Shroom3 regulates tubular repair in an actin dependent manner that facilitates proper epithelial re‐differentiation. The inability to complete tubular epithelial re‐differentiation would result in a worsened kidney histopathology. Analysis of Shrm3+/‐ kidneys compared to WT at 10 days post‐I/R showed higher levels of Kidney Injury Molecule‐1 in the apical surface of cortical tubular cells as shown by IHC, a 2.5‐fold increase in cell proliferation shown by KI‐67 (Shrm3+/‐=16.5±0.8 vs. WT=6.6±1.8), a 4.4‐fold increase in apoptosis shown by Caspase‐3 (Shrm3+/‐ =1.4±0.09 vs. WT=0.32±0.04), a 2.4‐fold increase in inflammation levels shown by F4/80 (Shrm3+/‐ =64.5±2.2 vs. WT=26.4±2.5), and a 4.3‐fold increase in fibrosis levels shown by Picrosirius red (Shrm3+/‐ =10.3±0.73 vs. WT=2.4±0.15). These histopathological changes translated to severe impairments in kidney function: Shrm3+/‐ mice had a 4.0‐fold increase in serum creatinine levels at 24 hours post‐I/R and did not return to baseline levels until after 7 days, while WT mice had a 2.0‐fold increase after 24 hours and returned to baseline by 48 hours. Shrm3+/‐ mice also had higher mortality rates over a 10‐day period (Shrm3+/‐ =34.6% vs. WT=14.3%). Taken together, our findings demonstrate that Shroom3 plays key roles in renal epithelium repair after ischemia reperfusion injury likely by regulating actin cytoskeleton remodeling during the repair phase and thus modulating epithelial re‐differentiation. Our results could...
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