[The kidney, its anatomy and main functions]

The kidney performs several major functions: it eliminates toxins produced by cellular or xenobiotic metabolism, regulates the homeostasis of the internal environment and plays a hormonal role, producing erythropoietin, calcitriol and renin. Maintaining the body's homeostasis (hydric, ionic [sodium, potassium, calcium, phosphorus, etc.] or acid-base balance) requires the successive action of plasma filtration, followed by reabsorption/secretion mechanisms, which take place in the various portions of the kidney's functional unit known as the nephron. The initial part of the nephron, the glomerulus, is the site of filtration, while the tubule, which collects the glomerular filtrate, is the site of reabsorption/secretion, leading to the composition of the final urine. It's important to understand how these different structures work, before tackling the various disorders that can affect the kidney.

The Role of Macula Densa Nitric Oxide Synthase 1 Beta Splice Variant in Modulating Tubuloglomerular Feedback

Abnormalities in renal electrolyte and water excretion may result in inappropriate salt and water retention, which facilitates the development and maintenance of hypertension, as well as acid-base and electrolyte disorders. A key mechanism by which the kidney regulates renal hemodynamics and electrolyte excretion is via tubuloglomerular feedback (TGF), an intrarenal negative feedback between tubules and arterioles. TGF is initiated by an increase of NaCl delivery at the macula densa cells. The increased NaCl activates luminal Na-K-2Cl cotransporter (NKCC2) of the macula densa cells, which leads to activation of several intracellular processes followed by the production of paracrine signals that ultimately result in a constriction of the afferent arteriole and a tonic inhibition of single nephron glomerular filtration rate. Neuronal nitric oxide (NOS1) is highly expressed in the macula densa. NOS1β is the major splice variant and accounts for most of NO generation by the macula densa, which inhibits TGF response. Macula densa NOS1β-mediated modulation of TGF responses plays an essential role in control of sodium excretion, volume and electrolyte hemostasis, and blood pressure. In this article, we describe the mechanisms that regulate macula densa-derived NO and their effect on TGF response in physiologic and pathologic conditions. © 2023 American Physiological Society. Compr Physiol 13:4215-4229, 2023.

A glomerulus and proximal tubule microphysiological system simulating renal filtration, reabsorption, secretion, and toxicity

Microphysiological systems (MPS) are powerful predictive tools for assessing drug-induced kidney injuries. Previous MPS have examined single regions of the nephron, but lack simultaneous filtration, reabsorption, and secretion functionality. Here, we developed a partially open MPS that structurally and functionally recapitulated the glomerular filtration barrier, proximal tubular reabsorption, and secretion for seven days. The system introduced a recirculation circuit and an open filtrate output as a source of functional testing. As a proof-of-concept, a tri-culture of immortalized podocytes, umbilical vein endothelial cells, and proximal tubule (PCT) cells were housed in a single MPS: T-junction, glomerulus housing unit, and PCT chip. The MPS successfully retained blood serum protein, reabsorbed glucose, secreted creatinine, and expressed cell-type specific proteins (VE-cadherin, nephrin, and ZO-1). To simulate drug-induced kidney injuries, the system was perfused with cisplatin and adriamycin, and then tested using serum albumin filtration, glucose clearance, and lactate dehydrogenase release. The glomerulus and PCT MPS demonstrated a complex, dynamic microenvironment and recreated some in vivo-like functions in basal and drug-induced conditions, offering a novel prototype for preclinical testing.

Laminar flow substantially affects the morphology and functional phenotype of glomerular endothelial cells

Shear stress induced by laminar blood flow has a profound effect on the morphology and functional phenotype of macrovascular endothelial cells. The influence of laminar flow on the glomerular microvascular endothelium, however, remains largely elusive. The glomerular endothelium, including its glycocalyx, is a crucial part of the glomerular filtration barrier, which is involved in blood filtration. We therefore investigated the influence of laminar flow-induced shear stress on the glomerular endothelium. Conditionally immortalized mouse glomerular endothelial cells were cultured for 7 days under a laminar flow of 5 dyn/cm2 to mimic the glomerular blood flow. The cells were subsequently analysed for changes in morphology, expression of shear stress-responsive genes, nitric oxide production, glycocalyx composition, expression of anti-oxidant genes and the inflammatory response. Culture under laminar flow resulted in cytoskeletal rearrangement and cell alignment compared to static conditions. Moreover, production of nitric oxide was increased and the expression of the main functional component of the glycocalyx, Heparan Sulfate, was enhanced in response to shear stress. Furthermore, glomerular endothelial cells demonstrated a quiescent phenotype under flow, characterized by a decreased expression of the pro-inflammatory gene ICAM-1 and increased expression of the anti-oxidant enzymes HO-1 and NQO1. Upon exposure to the inflammatory stimulus TNFα, however, glomerular endothelial cells cultured under laminar flow showed an enhanced inflammatory response. In conclusion, laminar flow extensively affects the morphology and functional phenotype of glomerular endothelial cells in culture. Furthermore, glomerular endothelial cells respond differently to shear stress compared to macrovascular endothelium. To improve the translation of future in vitro studies with glomerular endothelial cells to the in vivo situation, it appears therefore crucial to culture glomerular endothelial cells under physiological flow conditions.

Renin Cells, the Kidney, and Hypertension

Renin cells are essential for survival perfected throughout evolution to ensure normal development and defend the organism against a variety of homeostatic threats. During embryonic and early postnatal life, they are progenitors that participate in the morphogenesis of the renal arterial tree. In adult life, they are capable of regenerating injured glomeruli, control blood pressure, fluid-electrolyte balance, tissue perfusion, and in turn, the delivery of oxygen and nutrients to cells. Throughout life, renin cell descendants retain the plasticity or memory to regain the renin phenotype when homeostasis is threatened. To perform all of these functions and maintain well-being, renin cells must regulate their identity and fate. Here, we review the major mechanisms that control the differentiation and fate of renin cells, the chromatin events that control the memory of the renin phenotype, and the major pathways that determine their plasticity. We also examine how chronic stimulation of renin cells alters their fate leading to the development of a severe and concentric hypertrophy of the intrarenal arteries and arterioles. Lastly, we provide examples of additional changes in renin cell fate that contribute to equally severe kidney disorders.

Effects of the nitric oxide synthase inhibitor ronopterin (VAS203) on renal function in healthy volunteers

AIMS

Reduced nitric oxide (NO) availability may adversely affect renal perfusion and glomerular filtration. The aim of the present study was to characterize in detail the pharmacological effects of VAS203, an inhibitor of NO synthase, on renal haemodynamics in humans.

METHODS

This double-blind, randomized, placebo-controlled, cross-over phase-I-study comprised 18 healthy men. Renal haemodynamics were assessed with constant-infusion input-clearance technique with p-aminohippurate and inulin for renal plasma flow (RPF) and glomerular filtration rate (GFR), respectively. After baseline measurement, a constant infusion of the tetrahydrobiopterin analogue ronopterin (VAS203, total 10 mg/kg body weight) or placebo was administered at random order for 6 hours additionally. After a wash-out phase of 28 days, the second course was applied. In parallel, markers of early kidney injury and renal function were assessed repeatedly up to 48 hours after starting VAS203/placebo-infusion.

RESULTS

VAS203-infusion resulted in a significant decrease of RPF (P < .0001) and GFR (P < .001) compared to placebo, but magnitude was within the physiological range. RPF and GFR recovered partly 2 hours after end of VAS203-infusion and was normal at beginning of the second infusion period. Compared to placebo, preglomerular resistance (P < .0001), and to lesser extent postglomerular resistance (P < .0001) increased, resulting in a decrease of intraglomerular pressure (P < .01). No treatment related effect on markers of early kidney injury, and on renal function (P for all >.20) have been observed.

CONCLUSIONS

Our phase-I-study in healthy humans indicates that VAS203 (10 mg/kg body weight) reduces renal perfusion and glomerular function within the physiological range mainly due to vasoconstriction at the preglomerular site.

Progenitor Renin Lineage Cells are not involved in the regeneration of glomerular endothelial cells during experimental renal thrombotic microangiopathy

Endothelial cells (EC) frequently undergo primary or secondary injury during kidney disease such as thrombotic microangiopathy or glomerulonephritis. Renin Lineage Cells (RLCs) serve as a progenitor cell niche after glomerular damage in the adult kidney. However, it is not clear whether RLCs also contribute to endothelial replenishment in the glomerulus following endothelial injury. Therefore, we investigated the role of RLCs as a potential progenitor niche for glomerular endothelial regeneration. We used an inducible tet-on triple-transgenic reporter strain mRen-rtTAm2/LC1/LacZ to pulse-label the renin-producing RLCs in adult mice. Unilateral kidney EC damage (EC model) was induced by renal artery perfusion with concanavalin/anti-concanavalin. In this model glomerular EC injury and depletion developed within 1 day while regeneration occurred after 7 days. LacZ-labelled RLCs were restricted to the juxtaglomerular compartment of the afferent arterioles at baseline conditions. In contrast, during the regenerative phase of the EC model (day 7) a subset of LacZ-tagged RLCs migrated to the glomerular tuft. Intraglomerular RLCs did not express renin anymore and did not stain for glomerular endothelial or podocyte cell markers, but for the mesangial cell markers α8-integrin and PDGFRβ. Accordingly, we found pronounced mesangial cell damage parallel to the endothelial injury induced by the EC model. These results demonstrated that in our EC model RLCs are not involved in endothelial regeneration. Rather, recruitment of RLCs seems to be specific for the repair of the concomitantly damaged mesangium.

Cannabinoid-1 receptor deletion in podocytes mitigates both glomerular and tubular dysfunction in a mouse model of diabetic nephropathy

AIMS

To determine the specific role of podocyte-expressed cannabinoid-1 receptor (CB₁ R) in the development of diabetic nephropathy (DN), relative to CB₁ R in other renal cell types.

MATERIAL AND METHODS

We developed a mouse model with a podocyte-specific deletion of CB₁ R (pCB1Rko) and challenged this model with streptozotocin (STZ)-induced type-1 DN. We also assessed the podocyte response to high glucose in vitro and its effects on CB₁ R activation.

RESULTS

High glucose exposure for 48 hours led to an increase in CB₁ R gene expression (CNR1) and endocannabinoid production in cultured human podocytes. This was associated with podocyte injury, reflected by decreased podocin and nephrin expression. These changes could be prevented by Cnr1-silencing, thus identifying CB1R as a key player in podocyte injury. After 12 weeks of chronic hyperglycaemia, STZ-treated pCB1Rko mice showed elevated blood glucose similar to that of their wild-type littermates. However, they displayed less albuminuria and less podocyte loss than STZ-treated wild-type mice. Unexpectedly, pCB1Rko mice also have milder tubular dysfunction, fibrosis and reduction of cortical microcirculation compared to wild-type controls, which is mediated, in part, by podocyte-derived endocannabinoids acting via CB₁ R on proximal tubular cells.

CONCLUSIONS

Activation of CB₁ R in podocytes contributes to both glomerular and tubular dysfunction in type-1 DN, which highlights the therapeutic potential of peripheral CB₁ R blockade.

Plasma bradykinin and early diabetic nephropathy lesions in type 1 diabetes mellitus

Objective

To examine the association of bradykinin and related peptides with the development of diabetic nephropathy lesions in 243 participants with type 1 diabetes (T1D) from the Renin-Angiotensin System Study who, at baseline, were normoalbuminuric, normotensive and had normal or increased glomerular filtration rate (GFR).

Design

Plasma concentrations of bradykinin and related peptides were measured at baseline by quantitative mass spectrometry. All participants were randomly assigned at baseline to receive placebo, enalapril or losartan during the 5 years between kidney biopsies. Kidney morphometric data were available from kidney biopsies at baseline and after 5 years. Relationships of peptides with changes in morphometric variables were assessed using multiple linear regression after adjustment for age, sex, diabetes duration, HbA1c, mean arterial pressure, treatment assignment and, for longitudinal analyses, baseline structure.

Results

Baseline median albumin excretion rate of study participants was 5.0 μg/min, and mean GFR was 128 mL/min/1.73 m². After multivariable adjustment, higher plasma concentration of bradykinin (1–8) was associated with greater glomerular volume (partial r = 0.191, P = 0.019) and total filtration surface area (partial r = 0.211, P = 0.010), and higher bradykinin (1–7) and hyp3-bradykinin (1–7) were associated with lower cortical interstitial fractional volume (partial r = -0.189, P = 0.011; partial r = -0.164, P = 0.027 respectively). In longitudinal analyses, higher bradykinin was associated with preservation of surface density of the peripheral glomerular basement membrane (partial r = 0.162, P = 0.013), and for participants randomized to losartan, higher hyp3-bradykinin (1–8) was associated with more limited increase in cortical interstitial fractional volume (partial r = -0.291, P = 0.033).

Conclusions

Higher plasma bradykinin and related peptide concentrations measured before clinical onset of diabetic nephropathy in persons with T1D were associated with preservation of glomerular structures, suggesting that elevations of these kinin concentrations may reflect adaptive responses to early renal structural changes in diabetic nephropathy.

Heparanase-driven inflammation from the AGEs-stimulated macrophages changes the functions of glomerular endothelial cells

AIMS

Amounts of macrophages were infiltrated in glomeruli in diabetic nephropathy. Heparanase has been thought to be closely related to proteinuria. Our aims were to determine the effect of heparanase on the inflammation in AGEs-stimulated macrophages and its role on the functions of glomerular endothelial cells (GEnCs).

METHODS

The expression of inflammation cytokines in macrophages were assayed by q-RT PCR, western, and ELISA. Then western was used to measure the expression of RAGE and key proteins in NF-κB pathway in macrophages. The expression of the adherence molecules and tight junction proteins in GEnCs were assessed by western. The adherence of mononuclear cells to GEnCs were observed by HE staining and transendothelial FITC-BSA were tested for the permeability of GEnCs.

RESULTS

HPA siRNA and heparanase inhibitor sulodexide could attenuate the increasing inflammatory factors (TNF-α and IL-1β) in AGEs-stimulated macrophages. NF-κB inhibitor PDTC could also decrease the augmented inflammation cytokines through inhibiting the activation of the NF-κB pathway induced by AGEs. The phosphorylation of NF-κB signaling pathway could be also attenuated by HPA siRNA and sulodexide, the same to the receptor of AGEs RAGE. When the macrophage-conditioned culture medium were added to the glomerular endothelial cells, we found HPA siRNA and sulodexide groups could decrease the increasing adherence and permeability of GEnCs induced by AGEs.

CONCLUSIONS

Heparanase increases the inflammation in AGEs-stimulated macrophages through activating the RAGE-NF-κB pathway. Heparanase driven inflammation from AGEs-stimulated macrophages increases the adherence of GEnCs and augments the permeability of GEnCs.

Involvement of P2 receptors in regulation of glomerular permeability to albumin by extracellular nucleotides of intra-/extra-glomerular origins

Plasma filtration through glomerular filtration barrier (GFB) is a key process to maintain fluid and electrolyte homeostasis. GFB consisting of endothelial cells, podocytes and basement membrane restricts passage of albumin but is permeable for smaller plasma molecules. Various biological agents, such as extracellular nucleotides influence activity of cells, which in turn affects permeability of GFB. Nucleotides are released from cells outside and within the glomeruli that activate the purinoceptors - P2Rs classified into ATP-gated non-selective ion channels, P2X receptors (P2XRs), and G-protein-coupled metabotropic P2Y receptors (P2YRs). P2Rs are expressed on cellular components of GFB. P2Rs activation triggers intracellular calcium concentration and calcium-dependent metabolism with subsequent affect on glomerular permeability to albumin. Purinergic-dependent glomerular cell activation also affects the biophysical properties of acelluar glomerular basement membrane (GMB). Finally, P2Rs stimulation may lead to increased proteins excretion in urine. The involvement of P2Rs in increased GFB permeability to albumin may be expected under pathophysiological conditions characterized by increased albumin excretion in urine.

Evolutionary morphology of podocytes and primary urine-producing apparatus

Excretory organs were acquired in the early phase of metazoan evolution, and they play a crucial role in the maintenance of homeostasis of body fluids. In general, these organs consist of two functional components, the primary-urine producing apparatus and the modulating tubule. This basic organization of the excretory organs is conserved among most metazoans. Herein, we present an overview of the morphological evolution of the primary urine-producing apparatus in metazoans and describe the acquisition of the renal glomerulus—a specialized primary urine-producing apparatus—in vertebrates. We also describe the advancement of the glomerular structure and function in higher vertebrates.

Shear stress blunts tubuloglomerular feedback partially mediated by primary cilia and nitric oxide at the macula densa

The present study tested whether primary cilia on macula densa serve as a flow sensor to enhance nitric oxide synthase 1 (NOS1) activity and inhibit tubuloglomerular feedback (TGF). Isolated perfused macula densa was loaded with calcein red and 4,5-diaminofluorescein diacetate to monitor cell volume and nitric oxide (NO) generation. An increase in tubular flow rate from 0 to 40 nl/min enhanced NO production by 40.0 ± 1.2%. The flow-induced NO generation was blocked by an inhibitor of NOS1 but not by inhibition of the Na/K/2Cl cotransporter or the removal of electrolytes from the perfusate. NO generation increased from 174.8 ± 21 to 276.1 ± 24 units/min in cultured MMDD1 cells when shear stress was increased from 0.5 to 5.0 dynes/cm(2). The shear stress-induced NO generation was abolished in MMDD1 cells in which the cilia were disrupted using a siRNA to ift88. Increasing the NaCl concentration of the tubular perfusate from 10 to 80 mM NaCl in the isolated perfused juxtaglomerular preparation reduced the diameter of the afferent arteriole by 3.8 ± 0.1 μm. This response was significantly blunted to 2.5 ± 0.2 μm when dextran was added to the perfusate to increase the viscosity and shear stress. Inhibition of NOS1 blocked the effect of dextran on TGF response. In vitro, the effects of raising perfusate viscosity with dextran on tubular hydraulic pressure were minimized by reducing the outflow resistance to avoid stretching of tubular cells. These results suggest that shear stress stimulates primary cilia on the macula densa to enhance NO generation and inhibit TGF responsiveness.

The role of membrane parameters and of filtration pressure in the determination of the shape of the polyvinylpyrrolidone sieving curve. An in vitro and in vivo study

The theoretical equations describing the transport of a solute across a porous membrane predict that the shape of the sieving curve (ratio of solute concentrations in filtrate and filtrand versus molecular size) depends not only on the porosity of the membrane but also on the filtration pressure. This has been verified experimentally on an artificial membrane (Amicon XM-50). This model has been used to interpret the effects of angiotensin II on the shape of the glomerular 125I PVP sieving curve. The mean effective filtration pressure is increased by the intrarenal perfusion of angiotensin II.

Cell junctions of the glomerular epithelium in a very early vertebrate (Myxine glutinosa)

The glomerular epithelial cells of the Atlantic hagfish (Myxine glutinosa) lack a slit diaphragm which is replaced by filaments. The epithelial cells show numerous occluding junctions (maculae and fasciae occludentes), septate desmosome-like junctions and desmosomes. The present findings are compared with available data on developing and mature glomerular epithelial cells of mammals.

Studies on glomerular permeability using inert polymers

Dextrans and other macromolecular inert polymers such as polyvinyl pyrrolidone fulfil all the criteria of an ideal test material for measuring glomerular permeability. Following a single intravenous injection of radioactively labelled polydisperse material serial plasma and urine samples can be analysed by gel filtration chromatography and the polymer fractionated into its constituent molecular sizes. Polymer of molecular size 24 A is capable of passing through the glomerulus as readily as water while material larger than 60 A is affectively excluded from the glomerular filtrate and one can determine the clearance of 20--30 intermediate molecular sized fractions. The accumulated data can be analysed by a number of different theoretical mathematical models; furthermore the technique can be employed in the study of proteinuria induced by vasoactive amines such as renin or angiotensin.

Longitudinal study of living kidney donor glomerular dynamics after nephrectomy

BACKGROUND

Over 5,000 living kidney donor nephrectomies are performed annually in the US. While the physiological changes that occur early after nephrectomy are well documented, less is known about the long-term glomerular dynamics in living donors.

METHODS

We enrolled 21 adult living kidney donors to undergo detailed long-term clinical, physiological, and radiological evaluation pre-, early post- (median, 0.8 years), and late post- (median, 6.3 years) donation. A morphometric analysis of glomeruli obtained during nephrectomy was performed in 19 subjects.

RESULTS

Donors showed parallel increases in single-kidney renal plasma flow (RPF), renocortical volume, and glomerular filtration rate (GFR) early after the procedure, and these changes were sustained through to the late post-donation period. We used mathematical modeling to estimate the glomerular ultrafiltration coefficient (Kf), which also increased early and then remained constant through the late post-donation study. Assuming that the filtration surface area (and hence, Kf) increased in proportion to renocortical volume after donation, we calculated that the 40% elevation in the single-kidney GFR observed after donation could be attributed exclusively to an increase in the Kf. The prevalence of hypertension in donors increased from 14% in the early post-donation period to 57% in the late post-donation period. No subjects exhibited elevated levels of albuminuria.

CONCLUSIONS

Adaptive hyperfiltration after donor nephrectomy is attributable to hyperperfusion and hypertrophy of the remaining glomeruli. Our findings point away from the development of glomerular hypertension following kidney donation.

TRIAL REGISTRATION

Not applicable. FUNDING. NIH (R01DK064697 and K23DK087937); Astellas Pharma US; the John M. Sobrato Foundation; the Satellite Extramural Grant Foundation; and the American Society of Nephrology.

Benign hyperfiltration after living kidney donation

Almost one-third of transplanted kidneys come from living donors, who sacrifice approximately 30% of their pre-donation glomerular filtration rate (GFR) after they experience compensatory hypertrophy and hyperfiltration in their remaining kidney. Although hyperfiltration can cause glomerular injury, many studies have suggested that donor nephrectomy itself does not cause long-term loss of GFR at a higher rate than what is seen in the normal aging population. However, when post-donation kidney diseases occur in an unfortunate few, recent studies suggest that GFR loss at donor nephrectomy increases the risk of eventual end-stage renal disease (ESRD). In this issue of the JCI, Lenihan and colleagues evaluated glomerular dynamics in a cohort of kidney donors prior to, within 1 year of, and several years after kidney donation. Their results suggest that adaptive hyperfiltration in the remaining kidney occurs without glomerular hypertension, furthering our understanding of the relatively benign renal outcomes for most living kidney donors.

Point: Proposing the electrokinetic model

It is still not fully resolved how the glomerular filter works and why it never clogs. Several models have been proposed. In this review, we will compare the most widely used "pore model" to the more recent and refined "electrokinetic model" of glomerular filtration. The pore model assumes the existence of highly ordered regular pores, but it cannot provide a mechanistic explanation for several of the inherent characteristics of the glomerular filter. The electrokinetic model assumes that streaming potentials generate an electrical field along the filter surface which repels the negatively charged plasma proteins, preventing them from passing across the filter. The electrokinetic model can provide elegant mechanistic solutions for most of the unresolved riddles about the glomerular filter.

Metanephric kidney development in the chicken embryo: Glomerular numbers, characteristics and perfusion

The developing metanephric kidneys and chorioallantoic membrane (CAM) work in unison to ensure ion and water homeostasis in the avian embryo within its egg. This study focused on how avian renal structure and glomerular perfusion change in concert during development, as well as on changes in body fluid compartment osmolalities. White leghorn chicken eggs were incubated at 37.5°C and 55-60% relative humidity and were examined during days (D) 10-18 of development. Alcian blue, a stain that forms solid aggregations in actively perfused glomeruli of the metanephric kidney, was used to identify the proportion of glomeruli actually perfused. Total nephron number increased from 4705±1599 nephrons/kidney on day 12 to 39,825±3051 nephrons/kidney on day 18. Actively perfused nephrons increased ~23-fold from 761±481 nephrons/kidney on day 12 (~16% of total nephrons) to 17,313±2750 nephrons/kidney on 18 (~43% of total nephrons). Glomerular volume increased from days 12 to 14, remaining constant thereafter. Blood and cloacal fluid osmolality ranged from 270 to 280 mOsm/L. Amniotic fluid osmolality changed in a complex fashion during development but was comparable to blood on days 10 and 18. Allantoic fluid had the lowest osmolalities (175-215 mOsm/L) across development. Uric acid increased steadily within the allantoic fluid compartment, from 36±1mmol/L to 63±4mmol/L. The avian metanephric kidney thus shows a dramatic increase in both recruitment of nephrons and potential filtering capacity during the last half of incubation, in preparation for the degeneration of the allantoic membranes prior to internal piping and subsequent hatching.

Direct assessment of tubuloglomerular feedback responsiveness in connexin 40-deficient mice

Participation of connexin 40 (Cx40) in the regulation of renin secretion and in the tubuloglomerular feedback (TGF) component of renal autoregulation suggests that gap junctional coupling through Cx40 contributes to the function of the juxtaglomerular apparatus. In the present experiments, we determined the effect of targeted Cx40 deletion in C57BL/6 and FVB mice on TGF responsiveness. In C57BL/6 mice, stop-flow pressure (PSF) fell from 40.3 ± 2 to 34.5 ± 2 mmHg in wild-type (WT) and from 31 ± 1.06 to 26.6 ± 0.98 mmHg in Cx40-/- mice. PSF changes of 5.85 ± 0.67 mmHg in WT and of 4.3 ± 0.55 mmHg in Cx40-/- mice were not significantly different (P = 0.08). In FVB mice, PSF fell from 37.4 ± 1.5 to 31.6 ± 1.5 mmHg in WT and from 28.1 ± 1.6 to 25.4 ± 1.7 mmHg in Cx40-/-, with mean TGF responses being significantly greater in WT than Cx40-/- (5.5 ± 0.55 vs. 2.7 ± 0.84 mmHg; P = 0.002). In both genetic backgrounds, PSF values were significantly lower in Cx40-/- than WT mice at all flow rates. Arterial blood pressure in the animals prepared for micropuncture was not different between WT and Cx40-/- mice. We conclude that the TGF response magnitude in superficial cortical nephrons is reduced by 30-50% in mice without Cx40, but that with the exception of a small number of nephrons, residual TGF activity is maintained. Thus gap junctional coupling appears to modulate TGF, perhaps by determining the kinetics of signal transmission.

Insulin-like growth factor-II is produced by, signals to and is an important survival factor for the mature podocyte in man and mouse

Podocytes are crucial for preventing the passage of albumin into the urine and, when lost, are associated with the development of albuminuria, renal failure and cardiovascular disease. Podocytes have limited capacity to regenerate, therefore pro-survival mechanisms are critically important. Insulin-like growth factor-II (IGF-II) is a potent survival and growth factor; however, its major function is thought to be in prenatal development, when circulating levels are high. IGF-II has only previously been reported to continue to be expressed in discrete regions of the brain into adulthood in rodents, with systemic levels being undetectable. Using conditionally immortalized human and ex vivo adult mouse cells of the glomerulus, we demonstrated the podocyte to be the major glomerular source and target of IGF-II; it signals to this cell via the IGF-I receptor via the PI3 kinase and MAPK pathways. Functionally, a reduction in IGF signalling causes podocyte cell death in vitro and glomerular disease in vivo in an aged IGF-II transgenic mouse that produces approximately 60% of IGF-II due to a lack of the P2 promoter of this gene. Collectively, this work reveals the fundamental importance of IGF-II in the mature podocyte for glomerular health across mammalian species.

Role of guanine-nucleotide exchange factor Epac in renal physiology and pathophysiology

Exchange proteins directly activated by cAMP [Epac(s)] were discovered more than a decade ago as new sensors for the second messenger cAMP. The Epac family members, including Epac1 and Epac2, are guanine nucleotide exchange factors for the Ras-like small GTPases Rap1 and Rap2, and they function independently of protein kinase A. Given the importance of cAMP in kidney homeostasis, several molecular and cellular studies using specific Epac agonists have analyzed the role and regulation of Epac proteins in renal physiology and pathophysiology. The specificity of the functions of Epac proteins may depend upon their expression and localization in the kidney as well as their abundance in the microcellular environment. This review discusses recent literature data concerning the involvement of Epac in renal tubular transport physiology and renal glomerular cells where various signaling pathways are known to be operative. In addition, the potential role of Epac in kidney disorders, such as diabetic kidney disease and ischemic kidney injury, is discussed.

Signal transduction in a compliant short loop of Henle

To study the transformation of fluctuations in filtration rate into tubular fluid chloride concentration oscillations alongside the macula densa, we have developed a mathematical model for tubuloglomerular feedback (TGF) signal transduction along the pars recta, the descending limb, and the thick ascending limb (TAL) of a short-looped nephron. The model tubules are assumed to have compliant walls and, thus, a tubular radius that depends on the transmural pressure difference. Previously, it has been predicted that TGF transduction by the TAL is a generator of nonlinearities: if a sinusoidal oscillation is added to a constant TAL flow rate, then the time required for a fluid element to traverse the TAL is oscillatory in time but nonsinusoidal. The results from the new model simulations presented here predict that TGF transduction by the loop of Henle is also, in the same sense, a generator of nonlinearities. Thus, this model predicts that oscillations in tubular fluid alongside the macula densa will be nonsinusoidal and will exhibit harmonics of sinusoidal perturbations of pars recta flow. Model results also indicate that the loop acts as a low-pass filter in the transduction of the TGF signal.

Modeling TGF-mediated flow dynamics in a system of three coupled nephrons

This paper focuses on a mathematical model of a system of three closely coupled nephrons and accompanying analytical and computational analysis. In our previous modeling efforts, we have shown how coupling magnifies the tendency of many coupled identical nephrons to oscillate owing to tubuloglomerular feedback (TGF) mechanism. However, in this study, our focus is on the coupled nonidentical nephrons and their dynamics due to the TGF system. Our detailed analytical and computational results suggest that systems of three nonidentical nephrons coupled to their nearest neighbors are prone to be found in an oscillatory state, relative to a single-nephron case with the same properties; however, their steady-state regions are not necessarily as small as it was predicted from the system of many coupled identical nephrons cases.

The role of tubuloglomerular feedback in the pathogenesis of acute kidney injury

The mechanisms involved in reduction in glomerular filtration rate (GFR) in prerenal and intrarenal acute kidney injury (AKI) are not mutually exclusive and prerenal mechanisms continue to play a role in the pathogenesis of established intrarenal AKI. In nearly all forms of AKI, glomeruli are morphologically normal; therefore, the investigative efforts have focused on systemic and intrarenal mechanisms that lead to the failure of filtration at the glomerulus. There is observed and/or deductive evidence supporting the role of tubuloglomerular feedback in mediating the reduction in GFR in various forms of AKI. In prerenal AKI, the activation of various neurohormonal renal vasoconstrictors can increase the sensitivity and responsiveness of tubuloglomerular feedback. In different forms of intrarenal AKI, the varying degree of tubular injury is linked to filtration failure directly by mechanisms such as tubular obstruction or tubular backleak of solutes, or indirectly via the activation of tubuloglomerular feedback. Tubular obstruction or backleak of solutes, while readily understood, do not appear to be consistent features in experimental AKI and have a limited role in explaining the degree of impairment of GFR in human AKI. The functional connection between tubular damage and filtration failure mediated by tubuloglomerular feedback via alterations in nephron plasma flow and glomerular capillary hydrostatic pressure is more consistently observed or deduced from experimental data. It also explains the principal abnormality of increased preglomerular resistances, a pathogenic characteristic of both experimental and human AKI.

[Asymptomatic proteinuria in children]

Asymptomatic proteinuria is a common finding in primary care practice. Most children with asymptomatic proteinuria, diagnosed at screening urinalysis, do not have kidney disease. When proteinuria is detected, it is important to determine whether it is transient, orthostatic or persistent. Transient proteinuria is most often associated with fever, exercise or stress and it resolves on urine testing when the cause is withdrawn. Orthostatic proteinuria is a benign and common condition in school-age children. Persistent proteinuria should be carefully evaluated because it is a marker of renal damage and associated with kidney disease. It is not necessary to extensively investigate all children found to have proteinuria. Children with persistent proteinuria should be referred to a pediatric nephrologist to get a diagnosis and start treatment when necessary.

Association of kidney function and metabolic risk factors with density of glomeruli on renal biopsy samples from living donors

OBJECTIVE

To test the hypothesis that kidney function and metabolic risk factors are associated with glomerular density on renal biopsy samples from healthy adults.

PATIENTS AND METHODS

This study compared glomerular density with predonation kidney function, blood pressure, and metabolic risk factors in living kidney donors at Mayo Clinic in Rochester, MN, from May 10, 1999, to February 4, 2009. During implantation of the kidney allograft, an 18-gauge core needle biopsy sample of the renal cortex was obtained, sectioned, and examined by pathologists. Glomerular density was determined by the number of glomeruli (normal and sclerotic) divided by area of cortex.

RESULTS

The study sample of 1046 kidney donors had a mean of 21 glomeruli (0.8 sclerotic glomeruli) and a glomerular density of 2.3 glomeruli per square millimeter. In a subset of 54 donors, glomerular density inversely correlated with the mean glomerular area (r(s) = -0.28). Independent predictors of decreased glomerular density were older age, increased glomerular filtration rate, family history of end-stage renal disease, increased serum uric acid, and increased body mass index. Increased urine albumin excretion, hypertension, decreased high-density lipoprotein cholesterol, and metabolic syndrome were also associated with decreased glomerular density after age-sex adjustment. These associations were not explained by the presence of glomerulosclerosis, tubular atrophy, interstitial fibrosis, or arteriosclerosis on the renal biopsy sample. In older donors, decreased glomerular density was attenuated by an increased prevalence of glomerulosclerosis and tubular atrophy.

CONCLUSION

Decreased glomerular density is associated with many different kidney function and metabolic risk factors among relatively healthy adults and may represent an early state of increased risk of parenchymal injury.

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.

Electrical forces determine glomerular permeability

There is ongoing controversy about the mechanisms that determine the characteristics of the glomerular filter. Here, we tested whether flow across the glomerular filter generates extracellular electrical potential differences, which could be an important determinant of glomerular filtration. In micropuncture experiments in Necturus maculosus, we measured a potential difference across the glomerular filtration barrier that was proportional to filtration pressure (-0.045 mV/10 cm H₂O). The filtration-dependent potential was generated without temporal delay and was negative within Bowman's space. Perfusion with the cationic polymer protamine abolished the potential difference. We propose a mathematical model that considers the relative contributions of diffusion, convection, and electrophoretic effects on the total flux of albumin across the filter. According to this model, potential differences of -0.02 to -0.05 mV can induce electrophoretic effects that significantly influence the glomerular sieving coefficient of albumin. This model of glomerular filtration has the potential to provide a mechanistic theory, based on experimental data, about the filtration characteristics of the glomerular filtration barrier. It provides a unique approach to the microanatomy of the glomerulus, renal autoregulation, and the pathogenesis of proteinuria.

Connecting tubule glomerular feedback antagonizes tubuloglomerular feedback in vivo

In vitro experiments showed that the connecting tubule (CNT) sends a signal that dilates the afferent arteriole (Af-Art) when Na(+) reabsorption in the CNT lumen increases. We call this process CNT glomerular feedback (CTGF) to differentiate it from tubuloglomerular feedback (TGF), which is a cross talk between the macula densa (MD) and the Af-Art. In TGF, the MD signals the Af-Art to constrict when NaCl transport by the MD is enhanced by increased luminal NaCl. CTGF is mediated by CNT Na(+) transport via epithelial Na(+) channels (ENaC). However, we do not know whether CTGF occurs in vivo or whether it opposes the increase in Af-Art resistance caused by TGF. We hypothesized that CTGF occurs in vivo and opposes TGF. To test our hypothesis, we conducted in vivo micropuncture of individual rat nephrons, measuring stop-flow pressure (P(SF)) as an index of glomerular filtration pressure. To test whether activation of CTGF opposes TGF, we used benzamil to block CNT Na(+) transport and thus CTGF. CTGF inhibition with the ENaC blocker benzamil (1 μM) potentiated the decrease in P(SF) at 40 and 80 nl/min. Next, we tested whether we could augment CTGF by inhibiting NaCl reabsorption in the distal convoluted tubule with hydrochlorothiazide (HCTZ, 1 mM) to enhance NaCl delivery to the CNT. In the presence of HCTZ, benzamil potentiated the decrease in P(SF) at 20, 40, and 80 nl/min. We concluded that in vivo CTGF occurs and opposes the vasoconstrictor effect of TGF.

Regulation of kidney development by Shp2: an unbiased stereological analysis

Genes that regulate renal branching morphogenesis are likely to indirectly regulate nephron endowment, but few have been validated to do so in vivo. PTPN11, which encodes the nonreceptor protein tyrosine phosphatase Shp2, acts downstream of receptor tyrosine kinases to modulate the Ras-MAPK pathway and has been implicated in branching morphogenesis in vitro and in invertebrates, and is therefore a candidate in vivo regulator of nephron number. In this work, heterozygous null mutant Shp2(+/-) mice at postnatal days 30-35 were compared with their wild-type (WT) littermates using unbiased stereology to determine if, indeed, the former had decreased nephron number due to their 50% decrease in gene/protein dosage. Although there was a trend toward decreases in total glomerular (nephron) number and kidney volume in Shp2(+/-) mice compared with WT, neither difference was statistically significant (11310 vs. 12198 glomeruli, P = 0.22; 62.8 mm(3) vs. 66.0 mm(3) renal volume; P = 0.40). We conclude that loss of 50% gene/protein dosage of PTPN11/Shp2 is insufficient to affect glomerular (and thereby nephron) number in mouse kidneys in vivo.

Renal action of C-type natriuretic peptide: advocating the isolated perfused rat kidney model

C-type natriuretic Peptide (CNP), the third natriuretic peptide (NP) identified, is mainly expressed in the nervous system and endothelial cells. In addition, CNP is believed to be produced locally in tubular cells and glomeruli of normal human kidneys. CNP exerts mainly vasodilatory and antimitogenetic effects rather than regulation of body fluid homeostasis via autocrine or paracrine pathway. Many factors, such as shear, pro-inflammatory cytokines and lipopolysaccharide, can regulate the production and excretion of CNP both in vivo and in vitro. However, little information about the renal action of CNP was obtained in the past from the model of isolated perfused rat kidney in which variables could be changed in a controlled manner and systemic influences could be eliminated. However, reviewing the data from the studies that used this model inspires us to conclude that such model can be a useful tool to probe the undiscovered aspects of the renal actions of CNP and should be advocated for future studies on it.

GTPase-Rac enhances depolarization-induced superoxide production by the macula densa during tubuloglomerular feedback

Superoxide (O(2)(-) ) enhances tubuloglomerular feedback (TGF) by scavenging nitric oxide at the macula densa (MD). The primary source of O(2)(-) in the MD during TGF is NADPH oxidase, which is activated by membrane depolarization. While Rac, a small GTP-binding protein, has been shown to enhance NADPH oxidase activity, its role in O(2)(-) generation by the MD is unknown. We hypothesized that depolarization of the MD leads to translocation of Rac to the apical membrane, and its activation, in turn, augments O(2)(-) generation during TGF. We tested this by measuring membrane potential and increased O(2)(-) levels during TGF responses in isolated, perfused tubules containing the intact MD plaque. Switching tubular NaCl from 10 to 80 mM, which induces TGF, depolarized membrane potential by 28.4 + or - 4.5% from control (P < 0.05) and O(2)(-) levels from 124 + or - 19 to 361 + or - 27 U/min. This NaCl-induced depolarization and O(2)(-) generation were blocked by a Cl(-) channel blocker, 5-nitro-2(3-phenylpropylamino) benzoic acid (NPPB; 10(-6) M). Inhibition of Rac blunted NaCl-induced O(2)(-) generation by 47%. When the NaCl content of the MD perfusate was increased from 10 to 80 mM, immunointensity of Rac on the apical side increased from 32 + or - 3.1 to 46 + or - 2.5% of the total immunofluorescence in the MD, indicating that high NaCl induces the translocation of Rac to the apical membrane. This NaCl-induced Rac translocation was blocked by a Cl(-) channel blocker, NPPB, indicating that depolarization of the MD induced Rac translocation. In conclusion, we found that depolarization of the MD during TGF leads to translocation of Rac to the apical membrane, which enhances O(2)(-) generation by the MD.

Altered whole kidney blood flow autoregulation in a mouse model of reduced beta-ENaC

Renal blood flow (RBF) autoregulation is mediated by at least two mechanisms, the fast acting myogenic response (approximately 5 s) and slow acting tubuloglomerular feedback (TGF; approximately 25 s). Previous studies suggest epithelial Na(+) channel (ENaC) family proteins, beta-ENaC in particular, mediate myogenic constriction in isolated renal interlobar arteries. However, it is unknown whether beta-ENaC-mediated myogenic constriction contributes to RBF autoregulation in vivo. Therefore, the goal of this investigation was to determine whether the myogenic mediated RBF autoregulation is inhibited in a mouse model of reduced beta-ENaC (m/m). To address this goal, we evaluated the temporal response of RBF and renal vascular resistance (RVR) to a 2-min step increase in mean arterial pressure (MAP). Pressure-induced changes in RBF and RVR at 0-5, 6-25, and 110-120 s after step increase in MAP were used to assess the contribution of myogenic and TGF mechanisms and steady-state autoregulation, respectively. The rate of the initial increase in RVR, attributed to the myogenic mechanism, was reduced by approximately 50% in m/m mice, indicating the speed of the myogenic response was inhibited. Steady-state autoregulation was similar between beta-ENaC +/+ and m/m mice. Although the rate of the secondary increase in RVR, attributed to TGF, was similar in beta-ENaC +/+ and m/m mice, however, it occurred over a longer period (+10 s), which may have allowed TGF to compensate for a loss in myogenic autoregulation. Our findings suggest beta-ENaC is an important mediator of renal myogenic constriction-mediated RBF autoregulation in vivo.