Mice with podocyte-specific overexpression of wild type alpha-actinin-4 are healthy controls for K256E-alpha-actinin-4 mutant transgenic mice

Angiopoietin-like protein 4 promotes hyperlipidemia-induced renal injury by down-regulating the expression of ACTN4

OBJECTIVE

The molecular mechanism of in hyperlipidemia-induced renal injury has not been elucidated. Angiogenin-like protein 4 (ANGPTL4) is a key regulator of lipid metabolism. The role of ANGPTL4 hyperlipidemia-induced renal injury has not been reported.

METHODS

Wild type C57 mice and gene angptl4 knockout mice were fed with 60% high fat diet or normal diet respectively. The serum lipid, urinary albumin and renal pathology were tested at the 9th, 13th, 17th and 21st week with high fat diet.

RESULTS

Elevated blood lipids in the wild-type mice with high-fat diet were found at 9th week. At the 17th week, the level of urinary albumin in high-fat fed wild type mice were significantly higher than which with normal diet, correspondingly, segmental fusion of podocyte foot process in kidney could be observed in these hyperlipidemia mice. IHC showed that the expression of ANGPTL4 in glomeruli of high-fat fed wild type mice began significant elevated since the 9th week. When given high fat diet, compared to the wild type, the gene angptl4 knockout mice showed significantly alleviated the levels of hyperlipidemia, proteinuria and effacement of podocyte foot process. Finally, the expression of ACTN4 showed remarkably lower in glomeruli podocyte of wild type mice fed high fat diet than that of wild type mice with normal diet at each time-point (P < 0.01). Differently, the expression of ACTN4 in gene angptl4 knockout mice did not happen significantly weaken when given the same dose of high fat diet.

CONCLUSION

ANGPTL4 could play a role in hyperlipidemic-induced renal injury via down-regulating the expression of ACTN4 in kidney podocyte.

Proteostasis as a therapeutic target in glomerular injury associated with mutant α-actinin-4

Mutations in α-actinin-4 (actinin-4) result in hereditary focal segmental glomerulosclerosis (FSGS) in humans. Actinin-4 mutants induce podocyte injury because of dysregulation of the cytoskeleton and proteotoxicity. Injury may be associated with endoplasmic reticulum (ER) stress and polyubiquitination of proteins. We assessed if the chemical chaperone 4-phenylbutyrate (4-PBA) can ameliorate the proteotoxicity of an actinin-4 mutant. Actinin-4 K255E, which causes FSGS in humans (K256E in the mouse), showed enhanced ubiquitination, accelerated degradation, aggregate formation, and enhanced association with filamentous (F)-actin in glomerular epithelial cells (GECs). The mutant disrupted ER function and stimulated autophagy. 4-PBA reduced actinin-4 K256E aggregation and its tight association with F-actin. Transgenic mice that express actinin-4 K256E in podocytes develop podocyte injury, proteinuria, and FSGS in association with glomerular ER stress. Treatment of these mice with 4-PBA in the drinking water over a 10-wk period significantly reduced albuminuria and ER stress. Another drug, celastrol, which enhanced expression of ER and cytosolic chaperones in GECs, tended to reduce actinin-4 aggregation but did not decrease the tight association of actinin-4 K256E with F-actin and did not reduce albuminuria in actinin-4 K256E transgenic mice. Thus, chemical chaperones, such as 4-PBA, may represent a novel therapeutic approach to certain hereditary glomerular diseases.

The role of alpha-actinin-4 in human kidney disease

Mutations in the Alpha-actinin-4 gene (ACTN4) cause a rare form of familial focal segmental glomerulosclerosis in humans. Individuals with kidney disease-associated ACTN4 mutations tend to have mild to moderate proteinuria, with many developing decreased kidney function progressing to end stage kidney disease. All of the disease-causing ACTN4 mutations identified to date are located within the actin-binding domain of the encoded protein, increasing its binding affinity to F-actin and leading to abnormal actin rich cellular aggregates. The identification of ACTN4 mutations as a cause of human kidney disease demonstrates a key cellular pathway by which alterations in cytoskeletal behavior can mediate kidney disease. Here we review the studies relevant to ACTN4 and its role in mediating kidney disease.

Nephropathy and elevated BP in mice with podocyte-specific NADPH oxidase 5 expression

NADPH oxidase (Nox) enzymes are a significant source of reactive oxygen species, which contribute to glomerular podocyte dysfunction. Although studies have implicated Nox1, -2, and -4 in several glomerulopathies, including diabetic nephropathy, little is known regarding the role of Nox5 in this context. We examined Nox5 expression and regulation in kidney biopsies from diabetic patients, cultured human podocytes, and a novel mouse model. Nox5 expression increased in human diabetic glomeruli compared with nondiabetic glomeruli. Stimulation with angiotensin II upregulated Nox5 expression in human podocyte cultures and increased reactive oxygen species generation. siRNA-mediated Nox5 knockdown inhibited angiotensin II-stimulated production of reactive oxygen species and altered podocyte cytoskeletal dynamics, resulting in an Rac-mediated motile phenotype. Because the Nox5 gene is absent in rodents, we generated transgenic mice expressing human Nox5 in a podocyte-specific manner (Nox5(pod+)). Nox5(pod+) mice exhibited early onset albuminuria, podocyte foot process effacement, and elevated systolic BP. Subjecting Nox5(pod+) mice to streptozotocin-induced diabetes further exacerbated these changes. Our data show that renal Nox5 is upregulated in human diabetic nephropathy and may alter filtration barrier function and systolic BP through the production of reactive oxygen species. These findings provide the first evidence that podocyte Nox5 has an important role in impaired renal function and hypertension.

Podocyte Injury Associated with Mutant α-Actinin-4

Focal segmental glomerulosclerosis (FSGS) is an important cause of proteinuria and nephrotic syndrome in humans. The pathogenesis of FSGS may be associated with glomerular visceral epithelial cell (GEC; podocyte) injury, leading to apoptosis, detachment, and "podocytopenia", followed by glomerulosclerosis. Mutations in α-actinin-4 are associated with FSGS in humans. In cultured GECs, α-actinin-4 mediates adhesion and cytoskeletal dynamics. FSGS-associated α-actinin-4 mutants show increased binding to actin filaments, compared with the wild-type protein. Expression of an α-actinin-4 mutant in mouse podocytes in vivo resulted in proteinuric FSGS. GECs that express mutant α-actinin-4 show defective spreading and motility, and such abnormalities could alter the mechanical properties of the podocyte, contribute to cytoskeletal disruption, and lead to injury. The potential for mutant α-actinin-4 to injure podocytes is also suggested by the characteristics of this mutant protein to form microaggregates, undergo ubiquitination, impair the ubiquitin-proteasome system, enhance endoplasmic reticulum stress, and exacerbate apoptosis.

Podocyte-specific VEGF down-regulation and pathophysiological development

It is well-known that vascular endothelial growth factor (VEGF) plays a key role in development and pathology, but its function in normal adult tissues is rarely understood. Increased use of anti-angiogenic therapies targeting VEGF in human pathologies have shown more and more adverse effects. In this report, a conditional expression model (Tet-On system) was used to down-regulate podocyte VEGF in adult mice, which resulted in many kidney problems, characterized by glomerular morphological changes, proteinuria, reduced water consumption and urination, increased urine electro-conductivity, as well as high susceptibility to BSA stress. Our findings indicated that podocyte-specific VEGF down-regulation resulted in poor kidney performance and led mice to be more susceptible to further kidney damages.

Using standard nomenclature to adequately name transgenes, knockout gene alleles and any mutation associated to a genetically modified mouse strain

Mice provide an unlimited source of animal models to study mammalian gene function and human diseases. The powerful genetic modification toolbox existing for the mouse genome enables the creation of, literally, thousands of genetically modified mouse strains, carrying spontaneous or induced mutations, transgenes or knock-out/knock-in alleles which, in addition, can exist in hundreds of different genetic backgrounds. Such an immense diversity of individuals needs to be adequately annotated, to ensure that the most relevant information is kept associated with the name of each mouse line, and hence, the scientific community can correctly interpret and benefit from the reported animal model. Therefore, rules and guidelines for correctly naming genes, alleles and mouse strains are required. The Mouse Genome Informatics Database is the authoritative source of official names for mouse genes, alleles, and strains. Nomenclature follows the rules and guidelines established by the International Committee on Standardized Genetic Nomenclature for Mice. Herewith, both from the International Society for Transgenic Technologies (ISTT) and from the scientific journal Transgenic Research, we would like to encourage all our colleagues to adhere and follow adequately the standard nomenclature rules when describing mouse models. The entire scientific community using genetically modified mice in experiments will benefit.