Angiotensin-converting enzyme 2 (ACE2) is an important component of the renin-angiotensin system (RAS). ACE2 is thought to have a renoprotective effect in diabetic nephropathy because it is capable of degrading profibrotic angiotensin II to potentially protective angiotensin-(1-7). Podocyte death and detachment is a key component of diabetic nephropathy. ACE2 is localized in the podocyte and during a diabetic state, podocyte ACE2 expression is reduced. The purpose of this study was to determine the effects of podocyte-specific ACE2 overexpression on the course of diabetic nephropathy. Diabetes was induced using streptozotocin in transgenic mice with podocyte-specific overexpression of human ACE2. The following parameters were assessed: systolic blood pressure, glomerular filtration rate, urinary albumin excretion, mesangial and glomerular area, and podocyte number. Transgenic diabetic mice showed a significant transient attenuated increase in albuminuria, an attenuated increase in mesangial area, decreased glomerular area, and preserved podocyte number, compared to wildtype diabetic mice. This was independent of a change in blood pressure. This study showed that the podocyte-specific overexpression of human ACE2 attenuates the development of diabetic nephropathy.

Angiotensin-converting enzyme 2 (ACE2) is an important component of the renin-angiotensin system (RAS). ACE2 is thought to have a renoprotective effect in diabetic nephropathy because it is capable of degrading profibrotic angiotensin II to potentially protective angiotensin-(1-7). Podocyte death and detachment is a key component of diabetic nephropathy. ACE2 is localized in the podocyte and during a diabetic state, podocyte ACE2 expression is reduced. The purpose of this study was to determine the effects of podocyte-specific ACE2 overexpression on the course of diabetic nephropathy. Diabetes was induced using streptozotocin in transgenic mice with podocyte-specific overexpression of human ACE2. The following parameters were assessed: systolic blood pressure, glomerular filtration rate, urinary albumin excretion, mesangial and glomerular area, and podocyte number. Transgenic diabetic mice showed a significant transient attenuated increase in albuminuria, an attenuated increase in mesangial area, decreased glomerular area, and preserved podocyte number, compared to wildtype diabetic mice. This was independent of a change in blood pressure. This study showed that the podocyte-specific overexpression of human ACE2 attenuates the development of diabetic nephropathy.

Angiotensin-converting enzyme 2 (ACE2) is an important component of the renin-angiotensin system (RAS). ACE2 is thought to have a renoprotective effect in diabetic nephropathy because it is capable of degrading profibrotic angiotensin II to potentially protective angiotensin-(1-7). Podocyte death and detachment is a key component of diabetic nephropathy. ACE2 is localized in the podocyte and during a diabetic state, podocyte ACE2 expression is reduced. The purpose of this study was to determine the effects of podocyte-specific ACE2 overexpression on the course of diabetic nephropathy. Diabetes was induced using streptozotocin in transgenic mice with podocyte-specific overexpression of human ACE2. The following parameters were assessed: systolic blood pressure, glomerular filtration rate, urinary albumin excretion, mesangial and glomerular area, and podocyte number. Transgenic diabetic mice showed a significant transient attenuated increase in albuminuria, an attenuated increase in mesangial area, decreased glomerular area, and preserved podocyte number, compared to wildtype diabetic mice. This was independent of a change in blood pressure. This study showed that the podocyte-specific overexpression of human ACE2 attenuates the development of diabetic nephropathy.

Podocytes play an important role in maintaining the structural integrity of glomerular capillaries. There is a limited capacity of podocytes to reproduce; therefore, they are required to support a greater capillary surface area in states of glomerular hypertrophy (i.e., reduced podocyte density). Such reductions in podocyte density can lead to podocyte dysfunction and is thought to substantially increase the susceptibility to hypertension-induced renal injury and progression of chronic kidney disease. However, the extent to which reduced podocyte density increases the susceptibility to hypertension-induced renal injury remains unknown. The goal of this study was to determine the susceptibility to hypertension-induced renal injury in a rodent model of chronic kidney disease with podocyte dysfunction. Male Sprague-Dawley rats were subjected to normotensive renal mass reduction and instrumented with a radiotelemeter for the continuous measurement of blood pressure. After a two week recovery from surgery to allow for completion of renal compensatory increases in size and function, groups of rats were administered a single dose of puromycin aminonucleoside (PAN, 75 mg/kg i.p.), which acutely injures podocytes, or saline (sham). Rats were followed for 4 weeks following PAN or saline injection. At one week post PAN injection, some groups of rats were administered antihypertensive regimens consisting of hydralazine (50-300 mg/L) + hydrochlorothiazide (HCTZ, 25-75 mg/L) or enalapril (50-300 mg/L) + HCTZ (25-75 mg/L) via the drinking water for the remainder of the study. At the end of the study, glomerulosclerosis (GS) was assessed in a blinded fashion and podocyte density was determined using immunofluorescence detection of the podocyte marker Wilms Tumor 1 (WT1) on paraffin-embedded sections from perfused-fixed kidneys. No differences were observed between antihypertensive groups, thus these data are presented as a singled group. The average systolic BP (mmHg) was higher (P<0.05) in rats administered PAN (144±3, n=12) vs. PAN + antihypertensives (127±2, n=20) and saline (130±4, n=17). The magnitude of glomerulosclerosis (GS, % glomeruli exhibiting GS in 100 evaluated) was greater (P<0.05) in rats administered PAN (64±7%) vs. PAN + antihypertensives (30±6%) vs. saline (6±2%). The slope of the relationship between BP and GS (Δ%GS vs. ΔmmHg) was significantly greater (P<0.05) in rats administered PAN (1.1) and PAN + antihypertensives (1.2) vs. saline (0.4). Podocyte density (podocytes/µm3x10-6) was assessed in non-injured glomeruli from a subset of rats from each group and was significantly greater (P<0.05) in those administered PAN + antihypertensives (63±3, n=8) and tended to be greater (P=0.09) in those administered saline (56±4, n=7) as compared to those administered PAN (47±4, n=7). The number of podocytes per glomerular tuft tended to be lower and the average glomerular tuft area tended to be higher in rats administered PAN vs. the other groups; however, these differences were not statistically significant. These data demonstrate that acute podocyte dysfunction increases the susceptibility to BP-induced GS by ~ 3-fold and that lowering BP in such states mitigates further reductions in podocyte density and progression of kidney disease.

Introduction Podocytes are terminally differentiated, highly specialized glomerular cells that form the final barrier to protein loss. Podocyte injury is characterised by proteinuria. Proteinuria is an important prognostic marker in kidney diseases, and lowering proteinuria has become a principal clinical goal. Compelling evidence supports the notion that continuing loss of podocytes plays a major role in the initiation and progression of glomerular diseases. It is my hypothesis that interventions that reduce the disruption by rescuing susceptible podocytes next to injured ones are potential therapies to restore podocyte phenotype and filtration behaviour, thereby protecting the kidney from progressive deterioration. Prevention of this damage, or ways to aid its recovery, could therefore be important to improving the management of human kidney diseases. Methods Transgenic mice expressing the human diphtheria toxin receptor on podocytes had been previously generated in our laboratory. Characterization of two lines showed that graded specific podocyte injury could be induced by single intraperitoneal injection of diphtheria toxin. Eight-week intervention studies involved administration of oral drug in water or food from 24h after toxin injection. Two control groups received no drug or were non-transgenic (wild-type) littermates. Primary endpoints were glomerulosclerosis and kidney function (serum creatinine). Other readouts included blood pressure, albuminuria, serum albumin, podocyte quantification and collagen staining of kidney. The angiotensin converting enzyme inhibitor (ACEi) captopril was tested because of its proven protective effect on renal function in patients with proteinuria. Subsequently another proteinuria-reducing drug, the endothelin receptor A antagonist sitaxsentan was tested alone and in combination with captopril. Results Captopril reduced proteinuria and ameliorated scarring, with matrix accumulation and glomerulosclerosis falling almost to baseline. Podocyte counts were reduced after toxin administration and showed no significant recovery irrespective of captopril treatment. In the following sitaxsentan and captopril combined intervention study, glomerular scarring was significantly reduced in all drug-treated groups either alone or in combination, but only combination drug treatment reduced glomerular damage to levels comparable to wild-type controls, demonstrating a synergistic effect of the two agents. Similarly, serum creatinine was lowered further in combined but not single drug-treated groups. Blood pressure of all drug treated mice was lowered compared to the placebo group. Surprisingly in this second study there were no significant differences in proteinuria between treated and untreated groups. Conclusion These results support the hypothesis that continuing podocyte dysfunction is a key abnormality in proteinuric disease, and plays a major role in progressive glomerulosclerosis. Both captopril and sitaxsentan alone or in combination provided protection without substantial preservation or restoration of podocyte numbers at the degree of injury induced in these experiments. Combined therapy showed a synergistic effect in protecting the kidney from progressive damage. These results suggest that protection may be at least partly due to change in podocyte phenotype. The model is ideal for studying strategies to protect the kidney from progressive damage following specific podocyte injury. Further elucidations on the mechanism of action of the drugs may aid development of superior future therapeutic treatments in the field of renal diseases.

Minimal change disease (MCD) and primary (idiopathic) focal segmental glomerulosclerosis (1FSGS), referred to collectively as “primary podocytopathies”, are major causes of nephrotic syndrome in children and adults, and are thought to be due to direct podocyte damage visible only at the electron microscopic level. Lupus podocytopathy (LP) is a newly recognized entity that involves severe podocyte injury in the setting of systemic lupus erythematosus, in the complete absence of peripheral capillary wall immune deposits. All of these pathologic diagnoses hinge on the ultrastructural finding of severe podocyte injury and foot process effacement. In addition to these ultrastructural changes, we have observed the presence of fine granular anti-IgG antibody immunoreactivity on podocyte cell bodies in kidney biopsies of patients with MCD, LP, and some patients with the tip lesion variant and NOS variants of 1FSGS. To validate this finding, we compared antibody staining from primary podocytopathy biopsies with those in biopsies from patients with other disease states, including lesions associated with severe podocyte injury in the absence of immune deposits: secondary (adaptive) focal segmental glomerulosclerosis, thin basement membrane disease, diabetic nephropathy, and renal amyloidosis. We found that a fine granular pattern of anti-IgG immunoreactivity on podocyte cell bodies is a specific morphologic feature of the primary podocytopathies, including virtually all cases of MCD that we encountered, some instances of tip lesion variant and NOS variant of 1FSGS, and one cases of LP. The antigen targeted by the anti-IgG immunostaining in these biopsies exhibited one of several oligoclonal IgG heavy chain subtype plus light chain profiles. Ultra-high resolution microscopy revealed fine linear anti-IgG staining along filtration slit diaphragms, suggesting that IgG deposition may potentially be targeting a filtration slit-associated antigen such as podocin. Our findings suggest the possibility of a direct antibody-mediated mechanism of podocyte injury in the primary podocytopathies, one that potentially targets podocyte-specific protein structures, and which may provide a specific and more rapid diagnostic marker for this group of diseases. The findings also suggest an etiologic relationship between MCD and some instances of 1FSGS.

Thromboxane receptor (TPr) activity is elevated in diabetes and contributes to complications of diabetic kidney disease (DKD). TPr blockade appears to have therapeutic potential. Several rodent models of DKD show attenuation of renal damage and proteinuria upon administration of the TPr antagonist, S18886. However, the cellular targets that underlie the injurious effects of TPr activation in DKD remain to be elucidated. A pilot study in our laboratory subjected a conditionally-immortalized mouse podocyte cell line to high glucose (25 mM D-glucose) and equibiaxial mechanical stretch (an in vitro simulator of increased glomerular capillary pressure associated with glomerular hyperfiltration in early diabetes). qRT-PCR revealed that exposure of podocytes to mechanical stretch (10% elongation) and high glucose for 6 hours yielded a 9-fold increase in TPr mRNA levels vs. controls (non-stretch, 5mM D-glucose + 25mM L-glucose) (p<0.05, n=5). We hypothesized that TPr expression and activity are increased in podocytes during the onset of DKD resulting in maladaptive effects on this key glomerular filtration barrier cell type. We showed that enhanced TPr signaling threatens podocytes viablility. Cultured podocytes treated with the TPr agonist, U-46619 (1 μM) for 24 hours are more vulnerable to apoptosis as quantified by Hoescht 33342 (20% cell death p<0.001, n=3) , TUNEL (30-fold increase, ns, n=3) and Annexin-V labeling (3-fold increase, p <0.001, n=3). To further support these in vitro findings, we developed a transgenic mouse with podocyte-specific overexpression of TPr. A construct consisting of a desensitization resistant mutant of the human TPr with both N- and C-terminal HA-epitope tags under the control of an 8.3 kb fragment of the immediate 5’ mouse NPHS1 promoter was cloned, isolated and injected into FVB/n oocytes that were implanted into pseudopregnant CD1 females. Founders were characterized for TPr transgene expression, and TPr transgene mRNA levels were detected by qRT-PCR. Our in vitro results suggest that increased TPr expression in podocytes of diabetic mice may contribute to filtration barrier damage and have important implications in the development and progression of DKD.

Thromboxane receptor (TPr) activity is elevated in diabetes and contributes to complications of diabetic kidney disease (DKD). TPr blockade appears to have therapeutic potential. Several rodent models of DKD show attenuation of renal damage and proteinuria upon administration of the TPr antagonist, S18886. However, the cellular targets that underlie the injurious effects of TPr activation in DKD remain to be elucidated. A pilot study in our laboratory subjected a conditionally-immortalized mouse podocyte cell line to high glucose (25 mM D-glucose) and equibiaxial mechanical stretch (an in vitro simulator of increased glomerular capillary pressure associated with glomerular hyperfiltration in early diabetes). qRT-PCR revealed that exposure of podocytes to mechanical stretch (10% elongation) and high glucose for 6 hours yielded a 9-fold increase in TPr mRNA levels vs. controls (non-stretch, 5mM D-glucose + 25mM L-glucose) (p<0.05, n=5). We hypothesized that TPr expression and activity are increased in podocytes during the onset of DKD resulting in maladaptive effects on this key glomerular filtration barrier cell type. We showed that enhanced TPr signaling threatens podocytes viablility. Cultured podocytes treated with the TPr agonist, U-46619 (1 μM) for 24 hours are more vulnerable to apoptosis as quantified by Hoescht 33342 (20% cell death p<0.001, n=3) , TUNEL (30-fold increase, ns, n=3) and Annexin-V labeling (3-fold increase, p <0.001, n=3). To further support these in vitro findings, we developed a transgenic mouse with podocyte-specific overexpression of TPr. A construct consisting of a desensitization resistant mutant of the human TPr with both N- and C-terminal HA-epitope tags under the control of an 8.3 kb fragment of the immediate 5’ mouse NPHS1 promoter was cloned, isolated and injected into FVB/n oocytes that were implanted into pseudopregnant CD1 females. Founders were characterized for TPr transgene expression, and TPr transgene mRNA levels were detected by qRT-PCR. Our in vitro results suggest that increased TPr expression in podocytes of diabetic mice may contribute to filtration barrier damage and have important implications in the development and progression of DKD.

VEGF is a potent angiogenic and endothelial cell growth factor that is key for the development of the glomerulus, the main filtration unit of the kidney. It is continued to be expressed in the mature glomerulus, with podocytes being the major site of production. VEGF binds to two receptors, VEGFR-1 and VEGFR-2, which are expressed by the adjacent endothelial cells (ECs). VEGFR-2 is the primary mediator of VEGF signaling while VEGFR-1is thought to function as a ‘decoy’ receptor, sequestering VEGF away from VEGFR-2. Gene targeting studies in mice show that VEGF loss from the podocyte results in profound defects of the ECs, consistent with a paracrine signaling loop. However, the identification of VEGF receptors on podocytes in vitro suggests an additional autocrine signaling pathway for VEGF may exist. To further study the role of VEGF in the glomerulus and to address whether a VEGF autocrine loop is functional in vivo, we generated a transgenic mouse model with inducible VEGF upregulation in the podocyte and genetically deleted VEGFR-2 and VEGFR-1 from the podocyte using the Cre-loxP system. Increased VEGF production from the podocyte leads to increased glomerular permeability and ultrastructural changes in the glomerular filtration barrier depending on the time and length of induction. Podocyte-selective deletion of VEGFR-2 did not cause glomerular disease. In contrast, VEGFR-1 loss from the podocyte led to proteinuria and glomerular defects at 6 weeks of age with extensive podocyte foot process effacement. In keeping with the model that VEGFR-1 functions as a VEGF trap, similarities were observed between the glomerular lesions of VEGFR-1 mutant mice and transgenic mice that overexpress VEGF within podocytes. Strikingly, in vitro studies also revealed an increase in podocyte cell adhesion to sVEGFR-1, suggesting additional roles for sVEGFR-1. Together, these data suggest that a tight regulation of VEGF must be maintained in the adult glomerulus. Furthermore, these findings provide the first genetic evidence that VEGF autocrine signaling loop through VEGFR-2 is dispensable in normal glomeruli. In addition, podocytes express sVEGFR-1 and is required in podocytes in vivo to maintain glomerular integrity by regulating VEGF availability and podocyte cell adhesive properties.

VEGF is a potent angiogenic and endothelial cell growth factor that is key for the development of the glomerulus, the main filtration unit of the kidney. It is continued to be expressed in the mature glomerulus, with podocytes being the major site of production. VEGF binds to two receptors, VEGFR-1 and VEGFR-2, which are expressed by the adjacent endothelial cells (ECs). VEGFR-2 is the primary mediator of VEGF signaling while VEGFR-1is thought to function as a ‘decoy’ receptor, sequestering VEGF away from VEGFR-2. Gene targeting studies in mice show that VEGF loss from the podocyte results in profound defects of the ECs, consistent with a paracrine signaling loop. However, the identification of VEGF receptors on podocytes in vitro suggests an additional autocrine signaling pathway for VEGF may exist. To further study the role of VEGF in the glomerulus and to address whether a VEGF autocrine loop is functional in vivo, we generated a transgenic mouse model with inducible VEGF upregulation in the podocyte and genetically deleted VEGFR-2 and VEGFR-1 from the podocyte using the Cre-loxP system. Increased VEGF production from the podocyte leads to increased glomerular permeability and ultrastructural changes in the glomerular filtration barrier depending on the time and length of induction. Podocyte-selective deletion of VEGFR-2 did not cause glomerular disease. In contrast, VEGFR-1 loss from the podocyte led to proteinuria and glomerular defects at 6 weeks of age with extensive podocyte foot process effacement. In keeping with the model that VEGFR-1 functions as a VEGF trap, similarities were observed between the glomerular lesions of VEGFR-1 mutant mice and transgenic mice that overexpress VEGF within podocytes. Strikingly, in vitro studies also revealed an increase in podocyte cell adhesion to sVEGFR-1, suggesting additional roles for sVEGFR-1. Together, these data suggest that a tight regulation of VEGF must be maintained in the adult glomerulus. Furthermore, these findings provide the first genetic evidence that VEGF autocrine signaling loop through VEGFR-2 is dispensable in normal glomeruli. In addition, podocytes express sVEGFR-1 and is required in podocytes in vivo to maintain glomerular integrity by regulating VEGF availability and podocyte cell adhesive properties.