Abstract. Injury to a target organ can be sensed by bone marrow stem cells that migrate to the site of damage, undergo differentiation, and promote structural and functional repair. This remarkable stem cell capacity prompted an investigation of the potential of mesenchymal and hematopoietic stem cells to cure acute renal failure. The model of renal injury induced in mice by the anticancer agent cisplatin was chosen.

In mice with cisplatin-induced acute kidney injury, administration of bone marrow-derived mesenchymal stem cells (MSC) restores renal tubular structure and improves renal function, but the underlying mechanism is unclear. Here, we examined the process of kidney cell repair in co-culture experiments with MSC and cisplatin-injured proximal tubular epithelial cells (PTEC). Exposure of PTEC to cisplatin markedly reduced cell viability at 4 days, but co-culture with MSC provided a protective effect by promoting tubular cell proliferation. This effect was mediated by insulin-like growth factor-1 (IGF-1), highly expressed by MSC as mRNA and protein, since blocking the growth factor's function with a specific antibody attenuated cell proliferation of PTEC. Confirming this, knocking down IGF-1 expression in MSC by small interfering-RNA also resulted in a significant decrease in PTEC proliferation and increased apoptosis. Furthermore, in the murine model of cisplatin-induced kidney injury, administering IGF-1 gene-silenced MSC limited their protective effect on renal function and tubular structure. These findings indicate that MSC exert beneficial effects on tubular cell repair in acute kidney injury by producing the mitogenic and pro-survival factor IGF-1.

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Background Focal segmental glomerulosclerosis (FSGS) is a kidney disease that presents with nephrotic syndrome and is often resistant to glucocorticosteroids and progresses to end-stage kidney disease in 50–70% of patients. Genetic studies in familial FSGS indicate that it is a disease of the podocytes, major components of the glomerular filtration barrier. However the molecular cause of over half of primary FSGS is unknown, and effective treatments have been elusive. Methods We performed whole-genome linkage analysis followed by high-throughput sequencing of the positive linkage area in a family with autosomal recessive FSGS and sequenced a newly discovered gene in 52 unrelated FSGS patients. Immunohistochemistry was performed in human kidney biopsies and cultured podocytes. Expression studies in vitro were performed to characterize the functional consequences of the mutations identified. Results Two mutations (A159P and Y695X) in MYO1E, encoding the non-muscle class I myosin, myosin 1E (Myo1E), which segregated with FSGS in two independent pedigrees were identified. Patients were homozygous for the mutations and were resistant to glucocorticosteroids. Electron microscopy showed thickening and disorganization of the glomerular basement membrane. Normal expression of Myo1E was documented in control human kidney biopsies in vivo and in glomerular podocytes in vitro. Transfection studies revealed abnormal subcellular localization and function of A159P-Myo1E mutant. The Y695X mutation causes loss of calmodulin binding and the tail domains of Myo1E. Conclusions MYO1E mutations lead to childhood onset steroid-resistant FSGS. These data support a role of Myo1E in podocyte function and the consequent integrity of the glomerular permselectivity barrier.

Chronic diseases of the kidney have a progressive course toward organ failure. Common pathway mechanisms of progressive injury, irrespectively of the etiology of the underlying diseases, include glomerular capillary hypertension and enhanced passage of plasma proteins across the glomerular capillary barrier because of impaired permselective function. These changes are associated with podocyte injury and glomerular sclerosis. Direct evidence for causal roles is lacking, particularly for the link between intraglomerular protein deposition and sclerosing reaction. Because transforming growth factor-beta1 (TGF-beta1) is the putative central mediator of scarring, we hypothesized that TGF-beta1 can be up-regulated by protein overload of podocytes thereby contributing to sclerosis. In rats with renal mass reduction, protein accumulation in podocytes as a consequence of enhanced transcapillary passage preceded podocyte dedifferentiation and injury, increase in TGF-beta1 expression in podocytes, and TGF-beta1-dependent activation of mesangial cells. Angiotensin-converting enzyme inhibitor prevented both accumulation of plasma proteins and TGF-beta1 overexpression in podocytes and sclerosis. Albumin load on podocytes in vitro caused loss of the synaptopodin differentiation marker and enhanced TGF-beta1 mRNA and protein. Conditioned medium of albumin-stimulated podocytes induced a sclerosing phenotype in mesangial cells, an effect mimicked by TGF-beta1 and blocked by anti-TGF-beta1 antibodies. Thus, the passage of excess plasma proteins across the glomerular capillary wall is the trigger of podocyte dysfunction and of a TGF-beta1-mediated mechanism underlying sclerosis. Agents to reduce TGF-beta1, possibly combined with angiotensin blockade, should have priority in novel approaches to treatment of progressive nephropathies.

Profibrogenic signaling from both proximal tubular cells on challenge with filtered protein and inflammatory cells is implicated as a key candidate trigger of progressive tubulointerstitial injury.

Down-regulation of nephrin and loss of the electron dense structure of slit diaphragm indicate a novel mechanism accounting for proteinuria in diabetic nephropathy. To the extent that glomerular protein trafficking contributes to renal disease progression, our findings may have clinical relevance. Reduction of nephrin in the context of normal expression of CD2AP and podocin can be taken reasonably as a specific marker of renal disease in diabetes. Therapies targeted at correcting podocyte nephrin might be of value for diabetic medicine.

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