Matthew A. Sparks

AT1A Angiotensin Receptors in the Renal Proximal Tubule Regulate Blood Pressure

Hypertension affects more than 1.5 billion people worldwide but the precise cause of elevated blood pressure (BP) cannot be determined in most affected individuals. Nonetheless, blockade of the renin-angiotensin system (RAS) lowers BP in the majority of patients with hypertension. Despite its apparent role in hypertension pathogenesis, the key cellular targets of the RAS that control BP have not been clearly identified. Here we demonstrate that RAS actions in the epithelium of the proximal tubule have a critical and nonredundant role in determining the level of BP. Abrogation of AT(1) angiotensin receptor signaling in the proximal tubule alone is sufficient to lower BP, despite intact vascular responses. Elimination of this pathway reduces proximal fluid reabsorption and alters expression of key sodium transporters, modifying pressure-natriuresis and providing substantial protection against hypertension. Thus, effectively targeting epithelial functions of the proximal tubule of the kidney should be a useful therapeutic strategy in hypertension.

Response to Lack of Specificity of Commercial Antibodies Leads to Misidentification of Angiotensin Type-1 Receptor Protein

The angiotensin II type 1 receptor (AT1R) mediates most hypertensive actions of angiotensin II. To understand the molecular regulation of the AT1R in normal physiology and pathophysiology, methods for sensitive and specific detection of AT1R protein are required. Here, we examined the specificity of a panel of putative AT1R antibodies that are commonly used by investigators in the field. For these studies, we carried out Western blotting and immunohistochemistry with kidney tissue from wild-type mice and genetically modified mice lacking the major murine AT1R isoform, AT1A (AT1AKO), or with combined deficiency of both the AT1A and AT1B isoforms (AT1ABKO). For the 3 antibodies tested, Western blots of protein homogenates from wild-type kidneys yielded distinct bands with the expected size range for AT1R. In addition, these bands appeared identical in samples from mice lacking 1 or both murine AT1R isoforms. Additionally, the pattern of immunohistochemical staining in kidneys, liver, and adrenal glands of wild-type mice was very similar to that of AT1ABKO mice completely lacking all AT1R. We verified the absence of AT1R subtypes in each mouse line by the following: (1) quantitative polymerase chain reaction documenting the absence of mRNA species, and (2) functionally by assessing angiotensin II–dependent vasoconstriction, which was substantially blunted in both AT1AKOs and AT1ABKOs. Finally, these antibodies failed to detect epitope-tagged AT1AR protein overexpressed in human embryonic kidney cells. We conclude that anti-AT1R antibodies available from commercial sources and commonly used in published studies exhibit nonspecific binding in mouse tissue that may lead to erroneous results.

TRPC6 enhances angiotensin II-induced albuminuria.

Mutations in the canonical transient receptor potential cation channel 6 (TRPC6) are responsible for familial forms of adult onset focal segmental glomerulosclerosis (FSGS). The mechanisms by which TRPC6 mutations cause kidney disease are not well understood. We used TRPC6-deficient mice to examine the function of TRPC6 in the kidney. We found that adult TRPC6-deficient mice had BP and albumin excretion rates similar to wild-type animals. Glomerular histomorphology revealed no abnormalities on both light and electron microscopy. To determine whether the absence of TRPC6 would alter susceptibility to hypertension and renal injury, we infused mice with angiotensin II continuously for 28 days. Although both groups developed similar levels of hypertension, TRPC6-deficient mice had significantly less albuminuria, especially during the early phase of the infusion; this suggested that TRPC6 adversely influences the glomerular filter. We used whole-cell patch-clamp recording to measure cell-membrane currents in primary cultures of podocytes from both wild-type and TRPC6-deficient mice. In podocytes from wild-type mice, angiotensin II and a direct activator of TRPC6 both augmented cell-membrane currents; TRPC6 deficiency abrogated these increases in current magnitude. Our findings suggest that TRPC6 promotes albuminuria, perhaps by promoting angiotensin II-dependent increases in Ca(2+), suggesting that TRPC6 blockade may be therapeutically beneficial in proteinuric kidney disease.

Rare hereditary COL4A3/COL4A4 variants may be mistaken for familial focal segmental glomerulosclerosis.

Focal segmental glomerulosclerosis (FSGS) is a histological lesion with many causes including inherited genetic defects with significant proteinuria being the predominant clinical finding at presentation. Mutations in COL4A3 and COL4A4 are known to cause Alport syndrome, thin basement membrane nephropathy, and to result in pathognomonic glomerular basement membrane findings. Secondary FSGS is known to develop in classic Alport Syndrome at later stages of the disease. Here, we present seven families with rare or novel variants in COL4A3 or COL4A4 (six with single and one with two heterozygous variants) from a cohort of 70 families with a diagnosis of hereditary FSGS. The predominant clinical findings at diagnosis were proteinuria associated with hematuria. In all seven families, there were individuals with nephrotic range proteinuria with histologic features of FSGS by light microscopy. In one family, electron microscopy showed thin glomerular basement membrane, but four other families had variable findings inconsistent with classical Alport nephritis. There was no recurrence of disease after kidney transplantation. Families with COL4A3 and COL4A4 variants that segregated with disease represent 10% of our cohort. Thus, COL4A3 and COL4A4 variants should be considered in the interpretation of next-generation sequencing data from such patients. Furthermore, this study illustrates the power of molecular genetic diagnostics in the clarification of renal phenotypes.

Mutations in the Gene That Encodes the F-Actin Binding Protein Anillin Cause FSGS

FSGS is characterized by segmental scarring of the glomerulus and is a leading cause of kidney failure. Identification of genes causing FSGS has improved our understanding of disease mechanisms and points to defects in the glomerular epithelial cell, the podocyte, as a major factor in disease pathogenesis. Using a combination of genome-wide linkage studies and whole-exome sequencing in a kindred with familial FSGS, we identified a missense mutation R431C in anillin (ANLN), an F-actin binding cell cycle gene, as a cause of FSGS. We screened 250 additional families with FSGS and found another variant, G618C, that segregates with disease in a second family with FSGS. We demonstrate upregulation of anillin in podocytes in kidney biopsy specimens from individuals with FSGS and kidney samples from a murine model of HIV-1-associated nephropathy. Overexpression of R431C mutant ANLN in immortalized human podocytes results in enhanced podocyte motility. The mutant anillin displays reduced binding to the slit diaphragm-associated scaffold protein CD2AP. Knockdown of the ANLN gene in zebrafish morphants caused a loss of glomerular filtration barrier integrity, podocyte foot process effacement, and an edematous phenotype. Collectively, these findings suggest that anillin is important in maintaining the integrity of the podocyte actin cytoskeleton.

A Novel Role for Type 1 Angiotensin Receptors on T Lymphocytes to Limit Target Organ Damage in Hypertension

Rationale: Human clinical trials using type 1 angiotensin (AT1) receptor antagonists indicate that angiotensin II is a critical mediator of cardiovascular and renal disease. However, recent studies have suggested that individual tissue pools of AT1 receptors may have divergent effects on target organ damage in hypertension. Objective: We examined the role of AT1 receptors on T lymphocytes in the pathogenesis of hypertension and its complications. Methods and Results: Deficiency of AT1 receptors on T cells potentiated kidney injury during hypertension with exaggerated renal expression of chemokines and enhanced accumulation of T cells in the kidney. Kidneys and purified CD4+ T cells from “T cell knockout” mice lacking AT1 receptors on T lymphocytes had augmented expression of Th1-associated cytokines including interferon-&ggr; and tumor necrosis factor-&agr;. Within T lymphocytes, the transcription factors T-bet and GATA-3 promote differentiation toward the Th1 and Th2 lineages, respectively, and AT1 receptor-deficient CD4+ T cells had enhanced T-bet/GATA-3 expression ratios favoring induction of the Th1 response. Inversely, mice that were unable to mount a Th1 response due to T-bet deficiency were protected from kidney injury in our hypertension model. Conclusions: The current studies identify an unexpected role for AT1 receptors on T lymphocytes to protect the kidney in the setting of hypertension by favorably modulating CD4+ T helper cell differentiation.

A Role for Angiotensin II Type 1 Receptors on Bone Marrow-Derived Cells in the Pathogenesis of Angiotensin II–Dependent Hypertension

Activation of type 1 angiotensin (AT1) receptors causes hypertension, leading to progressive kidney injury. AT1 receptors are expressed on immune cells, and previous studies have identified a role for immune cells in angiotensin II–dependent hypertension. We, therefore, examined the role of AT1 receptors on immune cells in the pathogenesis of hypertension by generating bone marrow chimeras with wild-type donors or donors lacking AT1A receptors (BMKO). The 2 groups had virtually identical blood pressures at baseline, suggesting that AT1 receptors on immune cells do not make a unique contribution to the determination of baseline blood pressure. By contrast, in response to chronic angiotensin II infusion, the BMKOs had an augmented hypertensive response, suggesting a protective effect of AT1 receptors on immune cells with respect to blood pressure elevation. The BMKOs had 50% more albuminuria after 4 weeks of angiotensin II–dependent hypertension. Angiotensin II–induced pathological injury to the kidney was similar in the experimental groups. However, there was exaggerated renal expression of the macrophage chemokine monocyte chemoattractant protein 1 in the BMKO group, leading to persistent accumulation of macrophages in the kidney. This enhanced mononuclear cell infiltration into the BMKO kidneys was associated with exaggerated renal expression of the vasoactive mediators interleukin-1&bgr; and interleukin-6. Thus, in angiotensin II–induced hypertension, bone marrow–derived AT1 receptors limited mononuclear cell accumulation in the kidney and mitigated the chronic hypertensive response, possibly through the regulation of vasoactive cytokines.

Tubulovascular Cross-Talk by Vascular Endothelial Growth Factor A Maintains Peritubular Microvasculature in Kidney

Vascular endothelial growth factor A (VEGFA) production by podocytes is critical for glomerular endothelial health. VEGFA is also expressed in tubular epithelial cells in kidney; however, its physiologic role in the tubule has not been established. Using targeted transgenic mouse models, we found that Vegfa is expressed by specific epithelial cells along the nephron, whereas expression of its receptor (Kdr/Vegfr2) is largely restricted to adjacent peritubular capillaries. Embryonic deletion of tubular Vegfa did not affect systemic Vegfa levels, whereas renal Vegfa abundance was markedly decreased. Excision of Vegfa from renal tubules resulted in the formation of a smaller kidney, with a striking reduction in the density of peritubular capillaries. Consequently, elimination of tubular Vegfa caused pronounced polycythemia because of increased renal erythropoietin (Epo) production. Reducing hematocrit to normal levels in tubular Vegfa-deficient mice resulted in a markedly augmented renal Epo production, comparable with that observed in anemic wild-type mice. Here, we show that tubulovascular cross-talk by Vegfa is essential for maintenance of peritubular capillary networks in kidney. Disruption of this communication leads to increased renal Epo production and resulting polycythemia, presumably to counterbalance microvascular losses.

Why Not Nephrology? A Survey of US Internal Medicine Subspecialty Fellows

BACKGROUND There is a decreased interest in nephrology such that the number of trainees likely will not meet the upcoming workforce demands posed by the projected number of patients with kidney disease. We conducted a survey of US internal medicine subspecialty fellows in fields other than nephrology to determine why they did not choose nephrology. METHODS A web-based survey with multiple choice, yes/no, and open-ended questions was sent in summer 2011 to trainees reached through internal medicine subspecialty program directors. RESULTS 714 fellows responded to the survey (11% response rate). All non-nephrology internal medicine subspecialties were represented, and 90% of respondents were from university-based programs. Of the respondents, 31% indicated that nephrology was the most difficult physiology course taught in medical school, and 26% had considered nephrology as a career choice. Nearly one-fourth of the respondents said they would have considered nephrology if the field had higher income or the subject were taught well during medical school and residency training. The top reasons for not choosing nephrology were the belief that patients with end-stage renal disease were too complicated, the lack of a mentor, and that there were insufficient procedures in nephrology. CONCLUSIONS Most non-nephrology internal medicine subspecialty fellows never considered nephrology as a career choice. A significant proportion were dissuaded by factors such as the challenges of the patient population, lack of role models, lack of procedures, and perceived difficulty of the subject matter. Addressing these factors will require the concerted effort of nephrologists throughout the training community.

Angiotensin II Type 1A Receptors in Vascular Smooth Muscle Cells Do Not Influence Aortic Remodeling in Hypertension

Vascular injury and remodeling are common pathological sequelae of hypertension. Previous studies have suggested that the renin-angiotensin system acting through the type 1 angiotensin II (AT1) receptor promotes vascular pathology in hypertension. To study the role of AT1 receptors in this process, we generated mice with cell-specific deletion of AT1 receptors in vascular smooth muscle cells using Cre/Loxp technology. We crossed the SM22&agr;-Cre transgenic mouse line expressing Cre recombinase in smooth muscle cells with a mouse line bearing a conditional allele of the Agtr1a gene (Agtr1aflox), encoding the major murine AT1 receptor isoform (AT1A). In SM22&agr;-Cre+Agtr1aflox/flox (SMKO) mice, AT1A receptors were efficiently deleted from vascular smooth muscle cells in larger vessels but not from resistance vessels such as preglomerular arterioles. Thus, vasoconstrictor responses to angiotensin II were preserved in SMKO mice. To induce hypertensive vascular remodeling, mice were continuously infused with angiotensin II for 4 weeks. During infusion of angiotensin II, blood pressures increased significantly and to a similar extent in SMKO and control mice. In control mice, there was evidence of vascular oxidative stress indicated by enhanced nitrated tyrosine residues in segments of aorta; this was significantly attenuated in SMKO mice. Despite these differences in oxidative stress, the extent of aortic medial expansion induced by angiotensin II infusion was virtually identical in both groups. Thus, vascular AT1A receptors promote oxidative stress in the aortic wall but are not required for remodeling in angiotensin II–dependent hypertension.