APOL1 kidney disease risk variants cause cytotoxicity by depleting cellular potassium and inducing stress-activated protein kinases.

Two specific genetic variants of the apolipoprotein L1 (APOL1) gene are responsible for the high rate of kidney disease in people of recent African ancestry. Expression in cultured cells of these APOL1 risk variants, commonly referred to as G1 and G2, results in significant cytotoxicity. The underlying mechanism of this cytotoxicity is poorly understood. We hypothesized that this cytotoxicity is mediated by APOL1 risk variant-induced dysregulation of intracellular signaling relevant for cell survival. To test this hypothesis, we conditionally expressed WT human APOL1 (G0), the APOL1 G1 variant, or the APOL1 G2 variant in human embryonic kidney cells (T-REx-293) using a tetracycline-mediated (Tet-On) system. We found that expression of either G1 or G2 APOL1 variants increased apparent cell swelling and cell death compared with G0-expressing cells. These manifestations of cytotoxicity were preceded by G1 or G2 APOL1-induced net efflux of intracellular potassium as measured by X-ray fluorescence, resulting in the activation of stress-activated protein kinases (SAPKs), p38 MAPK, and JNK. Prevention of net K(+) efflux inhibited activation of these SAPKs by APOL1 G1 or G2. Furthermore, inhibition of SAPK signaling and inhibition of net K(+) efflux abrogated cytotoxicity associated with expression of APOL1 risk variants. These findings in cell culture raise the possibility that nephrotoxicity of APOL1 risk variants may be mediated by APOL1 risk variant-induced net loss of intracellular K(+) and subsequent induction of stress-activated protein kinase pathways.

Feasibility of repairing glomerular basement membrane defects in Alport syndrome.

Alport syndrome is a hereditary glomerular disease that leads to kidney failure. It is caused by mutations affecting one of three chains of the collagen α3α4α5(IV) heterotrimer, which forms the major collagen IV network of the glomerular basement membrane (GBM). In the absence of the α3α4α5(IV) network, the α1α1α2(IV) network substitutes, but it is insufficient to maintain normal kidney function. Inhibition of angiotensin-converting enzyme slows progression to kidney failure in patients with Alport syndrome but is not a cure. Restoration of the normal collagen α3α4α5(IV) network in the GBM, by either cell- or gene-based therapy, is an attractive and logical approach toward a cure, but whether or not the abnormal GBM can be repaired once it has formed and is functioning is unknown. Using a mouse model of Alport syndrome and an inducible transgene system, we found that secretion of α3α4α5(IV) heterotrimers by podocytes into a preformed, abnormal, filtering Alport GBM is effective at restoring the missing collagen IV network, slowing kidney disease progression, and extending life span. This proof-of-principle study demonstrates the plasticity of the mature GBM and validates the pursuit of therapeutic approaches aimed at normalizing the GBM to prolong kidney function.

Forced expression of laminin beta1 in podocytes prevents nephrotic syndrome in mice lacking laminin beta2, a model for Pierson syndrome.

Pierson syndrome is a congenital nephrotic syndrome with ocular and neurological defects caused by mutations in LAMB2, the gene encoding the basement membrane protein laminin ß2 (Lamß2). It is the kidney glomerular basement membrane (GBM) that is defective in Pierson syndrome, as Lamß2 is a component of laminin-521 (LM-521; α5ß2γ1), the major laminin in the mature GBM. In both Pierson syndrome and the Lamb2(-/-) mouse model for this disease, laminin ß1 (Lamß1), a structurally similar homolog of Lamß2, is marginally increased in the GBM, but it fails to fully compensate for the loss of Lamß2, leading to the filtration barrier defects and nephrotic syndrome. Here we generated several lines of Lamß1 transgenic mice and used them to show that podocyte-specific Lamß1 expression in Lamb2(-/-) mice abrogates the development of nephrotic syndrome, correlating with a greatly extended lifespan. In addition, the more Lamß1 was expressed, the less urinary albumin was excreted. Transgenic Lamß1 expression increased the level of Lamα5 in the GBM of rescued mice, consistent with the desired increased deposition of laminin-511 (α5ß1γ1) trimers. Ultrastructural analysis revealed occasional knob-like subepithelial GBM thickening but intact podocyte foot processes in aged rescued mice. These results suggest the possibility that up-regulation of LAMB1 in podocytes, should it become achievable, would likely lessen the severity of nephrotic syndrome in patients carrying LAMB2 mutations.

Wnt/ß-catenin pathway in podocytes integrates cell adhesion, differentiation, and survival.

Diabetic kidney disease (DKD) is the single most common cause of albuminuria and end-stage kidney disease in the United States. We found increased expression of Wnt/ß-catenin (Ctnnb1) pathway transcripts and proteins in glomeruli and podocytes of patients and mouse models of DKD. Mice with podocyte-specific expression of stabilized Ctnnb1 exhibited basement membrane abnormalities, albuminuria, and increased susceptibility to glomerular injury. Mice with podocyte-specific deletion of Ctnnb1 or podocyte-specific expression of the canonical Wnt inhibitor Dickkopf-related protein 1 (Dkk1) also showed increased susceptibility to DKD. Podocytes with stabilized Ctnnb1 were less motile and less adhesive to different matrices. Deletion of Ctnnb1 in cultured podocytes increased the expression of podocyte differentiation markers and enhanced cell motility; however, these cells were more susceptible to apoptosis. These results indicate that Wnt/Ctnnb1 signaling in podocytes plays a critical role in integrating cell adhesion, motility, cell death, and differentiation. Balanced Ctnnb1 expression is critical for glomerular filtration barrier maintenance.

Biophysical properties of normal and diseased renal glomeruli.

The mechanical properties of tissues and cells including renal glomeruli are important determinants of their differentiated state, function, and responses to injury but are not well characterized or understood. Understanding glomerular mechanics is important for understanding renal diseases attributable to abnormal expression or assembly of structural proteins and abnormal hemodynamics. We use atomic force microscopy (AFM) and a new technique, capillary micromechanics, to measure the elastic properties of rat glomeruli. The Young's modulus of glomeruli was 2,500 Pa, and it was reduced to 1,100 Pa by cytochalasin and latunculin, and to 1,400 Pa by blebbistatin. Cytochalasin or latrunculin reduced the F/G actin ratios of glomeruli but did not disrupt their architecture. To assess glomerular biomechanics in disease, we measured the Young's moduli of glomeruli from two mouse models of primary glomerular disease, Col4a3(-/-) mice (Alport model) and Tg26(HIV/nl) mice (HIV-associated nephropathy model), at stages where glomerular injury was minimal by histopathology. Col4a3(-/-) mice express abnormal glomerular basement membrane proteins, and Tg26(HIV/nl) mouse podocytes have multiple abnormalities in morphology, adhesion, and cytoskeletal structure. In both models, the Young's modulus of the glomeruli was reduced by 30%. We find that glomeruli have specific and quantifiable biomechanical properties that are dependent on the state of the actin cytoskeleton and nonmuscle myosins. These properties may be altered early in disease and represent an important early component of disease. This increased deformability of glomeruli could directly contribute to disease by permitting increased distension with hemodynamic force or represent a mechanically inhospitable environment for glomerular cells.

Recessive, gain-of-function toxicity in an APOL1 BAC transgenic mouse model mirrors human APOL1 kidney disease.

People of recent sub-Saharan African ancestry develop kidney failure much more frequently than other groups. A large fraction of this disparity is due to two coding sequence variants in the APOL1 gene. Inheriting two copies of these APOL1 risk variants, known as G1 and G2, causes high rates of focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy and hypertension-associated end-stage kidney disease. Disease risk follows a recessive mode of inheritance, which is puzzling given the considerable data that G1 and G2 are toxic gain-of-function variants. We developed coisogenic bacterial artificial chromosome (BAC) transgenic mice harboring either the wild-type (G0), G1 or G2 forms of human APOL1. Expression of interferon gamma (IFN-γ) via plasmid tail vein injection results in upregulation of APOL1 protein levels together with robust induction of heavy proteinuria and glomerulosclerosis in G1/G1 and G2/G2 but not G0/G0 mice. The disease phenotype was greater in G2/G2 mice. Neither heterozygous (G1/G0 or G2/G0) risk variant mice nor hemizygous (G1/-, G2/-) mice had significant kidney injury in response to IFN-γ, although the heterozygous mice had a greater proteinuric response than the hemizygous mice, suggesting that the lack of significant disease in humans heterozygous for G1 or G2 is not due to G0 rescue of G1 or G2 toxicity. Studies using additional mice (multicopy G2 and a non-isogenic G0 mouse) supported the notion that disease is largely a function of the level of risk variant APOL1 expression. Together, these findings shed light on the recessive nature of APOL1-nephropathy and present an important model for future studies.

Hes1 stimulates transcriptional activity of Runx2 by increasing protein stabilization during osteoblast differentiation.

Runt-related transcription factor 2 (Runx2) is a key transcription factor for osteogenic gene expression and osteoblast differentiation. In order to maintain bone homeostasis, the transcriptional activity of Runx2 is tightly modulated by many intra- and extra-cellular factors. Here, we reveal the mechanism by which Hairy Enhancer of Split1 (Hes1) regulates the transcriptional activity of Runx2, and elucidate the potential role of Hes1 during osteoblast differentiation. Coexpression of Hes1 with Runx2 promoted an increase in Runx2 protein levels by increasing the half-life of Runx2; Hes1 thereby augmented the formation of a Runx2-DNA complex at Runx2 target sites. During osteoblast differentiation, the retroviral overexpression of Hes1 accelerated osteogenesis and stimulated the expression of osteogenic marker genes, including osteopontin and type 1 collagen. Taken together, these results suggest that Hes1 augments the protein level and transcriptional activity of Runx2, resulting in the stimulation of osteoblast differentiation.

Histone deacetylase 1-mediated histone modification regulates osteoblast differentiation.

Osteogenesis is a complex process associated with dramatic changes in gene expression. To elucidate whether modifications in chromatin structure are involved in osteoblast differentiation, we examined the expression levels of histone deacetylases (HDACs) and the degree of histone acetylation at the promoter regions of osteogenic genes. During osteogenesis, total HDAC enzymatic activity was decreased with significant reduction in HDAC1 expression. Consistently, recruitment of HDAC1 to the promoters of osteoblast marker genes, including osterix and osteocalcin, was down-regulated, whereas histone H3 and H4 were hyperacetylated at those promoters during osteoblast differentiation. Moreover, suppression of HDAC activity with a HDAC inhibitor, sodium butyrate, accelerated osteogenesis by inducing osteoblast marker genes including osteopontin and alkaline phosphatase. Consistently, knockdown of HDAC1 by the short interference RNA system stimulated osteoblast differentiation. Taken together, these data propose that down-regulation of HDAC1 is an important process for osteogenesis.

The glomerular basement membrane as a barrier to albumin.

The glomerular basement membrane (GBM) is the central, non-cellular layer of the glomerular filtration barrier that is situated between the two cellular components--fenestrated endothelial cells and interdigitated podocyte foot processes. The GBM is composed primarily of four types of extracellular matrix macromolecule--laminin-521, type IV collagen α3α4α5, the heparan sulphate proteoglycan agrin, and nidogen--which produce an interwoven meshwork thought to impart both size-selective and charge-selective properties. Although the composition and biochemical nature of the GBM have been known for a long time, the functional importance of the GBM versus that of podocytes and endothelial cells for establishing the glomerular filtration barrier to albumin is still debated. Together with findings from genetic studies in mice, the discoveries of four human mutations affecting GBM components in two inherited kidney disorders, Alport syndrome and Pierson syndrome, support essential roles for the GBM in glomerular permselectivity. Here, we explain in detail the proposed mechanisms whereby the GBM can serve as the major albumin barrier and discuss possible approaches to circumvent GBM defects associated with loss of permselectivity.

Berberine promotes osteoblast differentiation by Runx2 activation with p38 MAPK.

Berberine (BBR) has been implicated in bone biology. Although BBR reduces osteoporosis by enhancing BMD and inhibiting osteoclast activity, the effects of BBR on osteoblasts during the process of osteogenesis have not been thoroughly studied. In osteoblastic cells, BBR enhanced the expression of osteogenic marker genes including osteopontin and osteocalcin and promoted the transcriptional activity of the key osteogenic transcription factor Runx2. In osteoblasts, BBR increased the binding of Runx2 to the promoter region of osteopontin. The recruitment of co-factors such as p300 and HDAC1 to the promoter regions of osteopontin and osteocalcin was regulated by BBR, resulting in an enhancement in the expression of those genes. Furthermore, BBR activated p38 mitogen-activated protein kinase (MAPK) and increased cyclooxygenase 2 (COX2) expression, which are key factors in osteoblast differentiation. Consistently, a p38 MAPK-specific inhibitor attenuated the effect of BBR on osteogenesis, whereas p38 MAPK overexpression augmented BBR-induced osteogenic gene expression. Moreover, BBR stimulated bone area formation in calvarial organ culture. Taken together, these findings indicate that BBR promotes osteoblast differentiation through activation of Runx2 by p38 MAPK. Therefore, BBR may be a potential therapeutic agent to treat bone-related disorders including osteoporosis.

Transcriptional regulation of mouse 6-phosphogluconate dehydrogenase by ADD1/SREBP1c.

6-Phosphogluconate dehydrogenase (6PGDH) constitutes the pentose phosphate pathway and produces NADPH. 6PGDH is also considered as a lipogenic gene since NADPH is a pivotal cofactor for lipogenesis. Thus, it is important to elucidate how 6PGDH is regulated by various signals related to energy homeostasis. Here, we provide several evidences that ADD1/SREBP1c regulates the expression of mouse 6PGDH gene. DNase I footprinting assay and point mutation studies revealed that the E-box (CANNTG) motif in the promoter of mouse 6PGDH is an important cis-regulatory element for ADD1/SREBP1c. 6PGDH mRNA is highly expressed in white adipose tissue and tightly modulated by nutritional status. Furthermore, we found that ADD1/SREBP1c mediates insulin-dependent 6PGDH expression and that PI3-kinase is an important linker for its regulation. Taken together, these data suggest that ADD1/SREBP1c is a key transcription factor for 6PGDH gene expression and would coordinate glucose metabolism and lipogenesis for energy homeostasis.