Integrins are transmembrane heteromeric receptors that mediate interactions between cells and extracellular matrix (ECM). beta1, the most abundantly expressed integrin subunit, binds at least 12 alpha subunits. beta1 containing integrins are highly expressed in the glomerulus of the kidney; however their role in glomerular morphogenesis and maintenance of glomerular filtration barrier integrity is poorly understood. To study these questions we selectively deleted beta1 integrin in the podocyte by crossing beta1(flox/flox) mice with podocyte specific podocin-cre mice (pod-Cre), which express cre at the time of glomerular capillary formation. We demonstrate that podocyte abnormalities are visualized during glomerulogenesis of the pod-Cre;beta1(flox/flox) mice and proteinuria is present at birth, despite a grossly normal glomerular basement membrane. Following the advent of glomerular filtration there is progressive podocyte loss and the mice develop capillary loop and mesangium degeneration with little evidence of glomerulosclerosis. By 3 weeks of age the mice develop severe end stage renal failure characterized by both tubulointerstitial and glomerular pathology. Thus, expression of beta1 containing integrins by the podocyte is critical for maintaining the structural integrity of the glomerulus.

Tubulointerstitial fibrosis (TIF) is the hallmark of chronic kidney disease and best predictor of renal survival. Many different cell types contribute to TIF progression including tubular epithelial cells, myofibroblasts, endothelia, and inflammatory cells. Previously, most of the attention has centered on myofibroblasts given their central importance in extracellular matrix production. However, emerging data focuses on how the response of the proximal tubule, a specialized epithelial segment vulnerable to injury, plays a central role in TIF progression. Several proximal tubular responses such as de-differentiation, cell cycle changes, autophagy, and metabolic changes may be adaptive initially, but can lead to maladaptive responses that promote TIF both through autocrine and paracrine effects. This review discusses the current paradigm of TIF progression and the increasingly important role of the proximal tubule in promoting TIF both in tubulointerstitial and glomerular injuries. A better understanding and appreciation of the role of the proximal tubule in TIF has important implications for therapeutic strategies to halt chronic kidney disease progression.

The kidney collecting system develops from branching morphogenesis of the ureteric bud (UB). This process requires signaling by growth factors such as glial cell line derived neurotrophic factor (GDNF) and fibroblast growth factors (FGFs) as well as cell extracellular matrix interactions mediated by integrins. The importance of integrin signaling in UB development was investigated by deleting integrin β1 at initiation (E10.5) and late (E18.5) stages of development. Deletion at E10.5 resulted in a severe branching morphogenesis phenotype. Deletion at E18.5 did not alter renal development but predisposed the collecting system to severe injury following ureteric obstruction. β1 integrin was required for renal tubular epithelial cells to mediate GDNF-and FGF-dependent signaling despite normal receptor localization and activation in vitro. Aberrations in the same signaling molecules were present in the β1-null UBs in vivo. Thus β1 integrins can regulate organ branching morphogenesis during development by mediating growthfactor-dependent signaling in addition to their well-defined role as adhesion receptors.

Tubulointerstitial fibrosis, tubular atrophy and peritubular capillary rarefaction are major hallmarks of chronic kidney disease. The tubulointerstitium consists of multiple cell components including tubular epithelia, mesenchymal (fibroblasts and pericytes), endothelial, and inflammatory cells. Crosstalk among these cell components is a key component in the pathogenesis of this complex disease. Following severe or recurrent injury, the renal tubular epithelial cells undergo changes in structure and cell cycle which are accompanied by altered expression and productions of cytokines. These cytokines contribute to the initiation of the fibrotic response by favoring activation of fibroblasts, recruitment of inflammatory cells, and loss of endothelial cells. This review focuses on how augmented growth factor and cytokine production induces epithelial crosstalk with cells in the interstitium to promote progressive tubulointerstitial fibrosis after renal injury.

TGF-b is a profibrotic growth factor in CKD, but its role in modulating the kidney's response to AKI is not well understood. The proximal tubule epithelial cell, which is the main cellular target of AKI, expresses high levels of both TGF-b and its receptors. To determine how TGF-b signaling in this tubular segment affects the response to AKI, we selectively deleted the TGF-b type II receptor in the proximal tubules of mice. This deletion attenuated renal impairment and reduced tubular apoptosis in mercuric chloride-induced injury. In vitro, deficiency of the TGF-b type II receptor protected proximal tubule epithelial cells from hydrogen peroxide-induced apoptosis, which was mediated in part by Smad-dependent signaling. Taken together, these results suggest that TGF-b signaling in the proximal tubule has a detrimental effect on the response to AKI as a result of its proapoptotic effects.

Conflict of interest: JVB and TI are co-inventors on KIM-1 patents (Molecules and methods for inhibiting shedding of KIM-1, patent no. 7696321; Kidney injury-related molecules, patent no. 6664385), which have been assigned to Partners Healthcare and licensed to several companies. JVB and RM are co-inventors on patents (PCT/ US16/52350) on organoid technologies that are assigned to Partners Healthcare. JVB is a consultant to Aldeyra, Angion, Goldilocks, and Medimmune. He is also a consultant to and holds equity in MediBeacon, Sentien Biotech, Thrasos Therapeutics, and Goldfinch Bio and has received grant support from Boehringer Ingelheim.

Transforming growth factor-β (TGF-β) strongly promotes renal tubulointerstitial fibrosis, but the cellular target that mediates its profibrotic actions has not been clearly identified. While in vitro data suggest that TGF-β-induced matrix production is mediated by renal fibroblasts, the role of these cells in TGF-β-dependent tubulointerstitial fibrosis following renal injury is not well defined. To address this, we deleted the TGF-β type II receptor in matrix-producing interstitial cells using two different inducible Cre models: COL1A2-Cre with a mesenchymal enhancer element and tenascin-Cre which targets medullary interstitial cells and either the mouse unilateral ureteral obstruction or aristolochic acid renal injury model. Renal interstitial cells lacking the TGF-β receptor had significantly impaired collagen I production, but unexpectedly, overall tissue fibrosis was unchanged in the conditional knockouts after renal injury. Thus, abrogating TGF-β signaling in matrix-producing interstitial cells is not sufficient to reduce fibrosis after renal injury.

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