Permeation of macromolecules into the renal glomerular basement membrane and capture by the tubules.

How the kidney prevents urinary excretion of plasma proteins continues to be debated. Here, using unfixed whole-mount mouse kidneys, we show that fluorescent-tagged proteins and neutral dextrans permeate into the glomerular basement membrane (GBM), in general agreement with Ogston's 1958 equation describing how permeation into gels is related to molecular size. Electron-microscopic analyses of kidneys fixed seconds to hours after injecting gold-tagged albumin, negatively charged gold nanoparticles, and stable oligoclusters of gold nanoparticles show that permeation into the lamina densa of the GBM is size-sensitive. Nanoparticles comparable in size with IgG dimers do not permeate into it. IgG monomer-sized particles permeate to some extent. Albumin-sized particles permeate extensively into the lamina densa. Particles traversing the lamina densa tend to accumulate upstream of the podocyte glycocalyx that spans the slit, but none are observed upstream of the slit diaphragm. At low concentrations, ovalbumin-sized nanoparticles reach the primary filtrate, are captured by proximal tubule cells, and are endocytosed. At higher concentrations, tubular capture is saturated, and they reach the urine. In mouse models of Pierson's or Alport's proteinuric syndromes resulting from defects in GBM structural proteins (laminin ß2 or collagen α3 IV), the GBM is irregularly swollen, the lamina densa is absent, and permeation is increased. Our observations indicate that size-dependent permeation into the lamina densa of the GBM and the podocyte glycocalyx, together with saturable tubular capture, determines which macromolecules reach the urine without the need to invoke direct size selection by the slit diaphragm.

Prolactin alters blood pressure by modulating the activity of endothelial nitric oxide synthase.

Increased levels of a cleaved form of prolactin (molecular weight 16 kDa) have been associated with preeclampsia. To study the effects of prolactin on blood pressure (BP), we generated male mice with a single-copy transgene (Tg; inserted into the hypoxanthine-guanine phosphoribosyltransferase locus) that enables inducible hepatic production of prolactin and its cleavage product. The Tg is driven by the indole-3-carbinol (I3C)-inducible rat cytochrome P450 1A1 promoter. When the Tg mice were fed normal chow (NC), plasma prolactin concentrations were comparable to those in female WT mice in the last third of pregnancy, and BP was lower than in WT mice (∼95 mm Hg vs. ∼105 mm Hg). When the Tg mice were fed chow containing IC3, plasma prolactin concentrations increased threefold, BP increased to ∼130 mm Hg, and cardiac function became markedly impaired. IC3 chow did not affect the WT mice. Urinary excretion of nitrite/nitrate and the amount of Ser1177-phosphorylated endothelial nitric oxide (NO) synthase (eNOS) were significantly greater in the Tg mice fed NC than in WT mice, as they are during pregnancy. However, when I3C was fed, these indicators of NO production became significantly less in the Tg mice than in WT mice. The effects of increased plasma prolactin were abolished by a genetic absence of eNOS. Thus, a threefold increase in plasma prolactin is sufficient to increase BP significantly and to markedly impair cardiac function, with effects mediated by NO produced by eNOS. We suggest that pregnant women with abnormally high prolactin levels may need special attention.

High Elmo1 expression aggravates and low Elmo1 expression prevents diabetic nephropathy.

Human genome-wide association studies have demonstrated that polymorphisms in the engulfment and cell motility protein 1 gene (ELMO1) are strongly associated with susceptibility to diabetic nephropathy. However, proof of causation is lacking. To test whether modest changes in its expression alter the severity of the renal phenotype in diabetic mice, we have generated mice that are type 1 diabetic because they have the Ins2(Akita) gene, and also have genetically graded expression of Elmo1 in all tissues ranging in five steps from ∼30% to ∼200% normal. We here show that the Elmo1 hypermorphs have albuminuria, glomerulosclerosis, and changes in the ultrastructure of the glomerular basement membrane that increase in severity in parallel with the expression of Elmo 1. Progressive changes in renal mRNA expression of transforming growth factor ß1 (TGFß1), endothelin-1, and NAD(P)H oxidase 4 also occur in parallel with Elmo1, as do the plasma levels of cystatin C, lipid peroxides, and TGFß1, and erythrocyte levels of reduced glutathione. In contrast, Akita type 1 diabetic mice with below-normal Elmo1 expression have reduced expression of these various factors and less severe diabetic complications. Remarkably, the reduced Elmo1 expression in the 30% hypomorphs almost abolishes the pathological features of diabetic nephropathy, although it does not affect the hyperglycemia caused by the Akita mutation. Thus, ELMO1 plays an important role in the development of type 1 diabetic nephropathy, and its inhibition could be a promising option for slowing or preventing progression of the condition to end-stage renal disease.

Nicotinamide benefits both mothers and pups in two contrasting mouse models of preeclampsia.

Preeclampsia (PE) complicates ∼5% of human pregnancies and is one of the leading causes of pregnancy-related maternal deaths. The only definitive treatment, induced delivery, invariably results in prematurity, and in severe early-onset cases may lead to fetal death. Many currently available antihypertensive drugs are teratogenic and therefore precluded from use. Nonteratogenic antihypertensives help control maternal blood pressure in PE, but results in preventing preterm delivery and correcting fetal growth restriction (FGR) that also occurs in PE have been disappointing. Here we show that dietary nicotinamide, a nonteratogenic amide of vitamin B3, improves the maternal condition, prolongs pregnancies, and prevents FGR in two contrasting mouse models of PE. The first is caused by endotheliosis due to excess levels in the mothers of a soluble form of the receptor for vascular endothelial growth factor (VEGF), which binds to and inactivates VEGF. The second is caused by genetic absence of Ankiryn-repeat-and-SOCS-box-containing-protein 4, a factor that contributes to the differentiation of trophoblast stem cells into the giant trophoblast cells necessary for embryo implantation in mice; its absence leads to impaired placental development. In both models, fetal production of ATP is impaired and FGR is observed. We show here that nicotinamide decreases blood pressure and endotheliosis in the mothers, probably by inhibiting ADP ribosyl cyclase (ADPRC), and prevents FGR, probably by normalizing fetal ATP synthesis via the nucleotide salvage pathway. Because nicotinamide benefits both dams and pups, it merits evaluation for preventing or treating PE in humans.

A conversation with Oliver Smithies.

Professor Oliver Smithies is the Weatherspoon Eminent Distinguished Professor of Pathology and Laboratory Medicine at the University of North Carolina, Chapel Hill. Along with Mario Capecchi and Martin Evans, Oliver was awarded the Nobel Prize in Medicine in Physiology or Medicine in 2007 for his contributions to the development of gene targeting using homologous recombination in embryonic stem cells. This technique has had an immense impact on biomedical research over the past two decades. Professor Smithies has had a long and distinguished career as a researcher and mentor. Here, he provides an entertaining and enlightening discussion of his life in science.

Low TGFß1 expression prevents and high expression exacerbates diabetic nephropathy in mice.

Nephropathy develops in many but not all patients with long-standing type 1 diabetes. Substantial efforts to identify genotypic differences explaining this differential susceptibility have been made, with limited success. Here, we show that the expression of the transforming growth factor ß1 gene (Tgfb1) affects the development of diabetic nephropathy in mice. To do this we genetically varied Tgfb1 expression in five steps, 10%, 60%, 100%, 150%, and 300% of normal, in mice with type 1 diabetes caused by the Akita mutation in the insulin gene (Ins2(Akita)). Although plasma glucose levels were not affected by Tgfb1 genotype, many features of diabetic nephropathy (mesangial expansion, elevated plasma creatinine and urea, decreased creatinine clearance and albuminuria) were progressively ameliorated as Tgfb1 expression decreased and were progressively exacerbated when expression was increased. The diabetic 10% hypomorphs had comparable creatinine clearance and albumin excretion to wild-type mice and no harmful changes in renal morphology. The diabetic 300% hypermorphs had ∼1/3 the creatinine clearance of wild-type mice, >20× their albumin excretion, ∼3× thicker glomerular basement membranes and severe podocyte effacement, matching human diabetic nephropathy. Switching Tgfb1 expression from low to high in the tubules of the hypomorphs increased their albumin excretion more than 10-fold but creatinine clearance remained high. Switching Tgfb1 expression from low to high in the podocytes markedly decreased creatinine clearance, but minimally increased albumin excretion. Decreasing expression of Tgfb1 could be a promising option for preventing loss of renal function in diabetes.

Endothelin-1 critically influences cardiac function via superoxide-MMP9 cascade.

We have generated low-expressing and high-expressing endothelin-1 genes (L and H) and have bred mice with four levels of expression: L/L, ∼20%; L/+, ∼65%; +/+ (wild type), 100%; and H/+, ∼350%. The hypomorphic L allele can be spatiotemporally switched to the hypermorphic H allele by Cre-loxP recombination. Young adult L/L and L/+ mice have dilated cardiomyopathy, hypertension, and increased plasma volumes, together with increased ventricular superoxide levels, increased matrix metalloproteinase 9 (Mmp9) expression, and reduced ventricular stiffness. H/+ mice have decreased plasma volumes and significantly heavy stiff hearts. Global or cardiomyocyte-specific switching expression from L to H normalized the abnormalities already present in young adult L/L mice. An epithelial sodium channel antagonist normalized plasma volume and blood pressure, but only partially corrected the cardiomyopathy. A superoxide dismutase mimetic made superoxide levels subnormal, reduced Mmp9 overexpression, and substantially improved cardiac function. Genetic absence of Mmp9 also improved cardiac function, but increased superoxide remained. We conclude that endothelin-1 is critical for maintaining normal contractile function, for controlling superoxide and Mmp9 levels, and for ensuring that the myocardium has sufficient collagen to prevent overstretching. Even a modest (∼35%) decrease in endothelin-1 gene (Edn1) expression is sufficient to cause cardiac dysfunction.

Transforming growth factor-ß1 and diabetic nephropathy.

Transforming growth factor-ß1 (TGF-ß1) is established to be involved in the pathogenesis of diabetic nephropathy. The diabetic milieu enhances oxidative stress and induces the expression of TGF-ß1. TGF-ß1 promotes cell hypertrophy and extracellular matrix accumulation in the mesangium, which decreases glomerular filtration rate and leads to chronic renal failure. Recently, TGF-ß1 has been demonstrated to regulate urinary albumin excretion by both increasing glomerular permeability and decreasing reabsorption in the proximal tubules. TGF-ß1 also increases urinary excretion of water, electrolytes and glucose by suppressing tubular reabsorption in both normal and diabetic conditions. Although TGF-ß1 exerts hypertrophic and fibrogenic effects in diabetic nephropathy, whether suppression of the function of TGF-ß1 can be an option to prevent or treat the complication is still controversial. This is partly because adrenal production of mineralocorticoids could be augmented by the suppression of TGF-ß1. However, differentiating the molecular mechanisms for glomerulosclerosis from those for the suppression of the effects of mineralocorticoids by TGF-ß1 may assist in developing novel therapeutic strategies for diabetic nephropathy. In this review, we discuss recent findings on the role of TGF-ß1 in diabetic nephropathy.

Primary aldosteronism and impaired natriuresis in mice underexpressing TGFß1.

To uncover the potential cardiovascular effects of human polymorphisms influencing transforming growth factor ß1 (TGFß1) expression, we generated mice with Tgfb1 mRNA expression graded in five steps from 10% to 300% normal. Adrenal expression of the genes for mineralocorticoid-producing enzymes ranged from 50% normal in the hypermorphs at age 12 wk to 400% normal in the hypomorphs accompanied with proportionate changes in plasma aldosterone levels, whereas plasma volumes ranged from 50% to 150% normal accompanied by marked compensatory changes in plasma angiotensin II and renin levels. The aldosterone/renin ratio ranged from 0.3 times normal in the 300% hypermorphs to six times in the 10% hypomorphs, which have elevated blood pressure. Urinary output of water and electrolytes are markedly decreased in the 10% hypomorphs without significant change in the glomerular filtration rate. Renal activities for the Na(+), K(+)-ATPase, and epithelial sodium channel are markedly increased in the 10% hypomorphs. The hypertension in the 10% hypomorphs is corrected by spironolactone or amiloride at doses that do not change blood pressure in wild-type mice. Thus, changes in Tgfb1 expression cause marked progressive changes in multiple systems that regulate blood pressure and fluid homeostasis, with the major effects being mediated by changes in adrenocortical function.

Transforming growth factor beta1 and aldosterone.

PURPOSE OF REVIEW: It is well established that blocking the renin-angiotensin-aldosterone system (RAAS) is effective for the treatment of cardiovascular and renal complications in hypertension and diabetes mellitus. Although the induction of transforming growth factor beta1 (TGFbeta1) by components of the RAAS mediates the hypertrophic and fibrogenic changes in cardiovascular-renal complications, it is still controversial as to whether TGFbeta1 can be a target to prevent such complications. Here, we review recent findings on the role of TGFbeta1 in fluid homeostasis, focusing on the relationship with aldosterone. RECENT FINDINGS: TGFbeta1 suppresses the adrenal production of aldosterone and renal tubular sodium reabsorption. We have generated mice with TGFbeta1 mRNA expression graded in five steps, from 10 to 300% of normal, and found that blood pressure and plasma volume are negatively regulated by TGFbeta1. Notably, the 10% hypomorph exhibits primary aldosteronism and sodium and water retention due to markedly impaired urinary excretion of water and electrolytes. SUMMARY: These results identify TGFbeta signalling as an important counterregulatory system against aldosterone. Understanding the molecular mechanisms for the suppressive effects of TGFbeta1 on adrenocortical and renal function may further our understanding of primary aldosteronism, as well as assist in the development of novel therapeutic strategies for hypertension.