Drugs with a negative impact on cognitive function (Part 1): chronic kidney disease as a risk factor.

People living with chronic kidney disease (CKD) frequently suffer from mild cognitive impairment and/or other neurocognitive disorders. This review in two parts will focus on adverse drug reactions resulting in cognitive impairment as a potentially modifiable risk factor in CKD patients. Many patients with CKD have a substantial burden of comorbidities leading to polypharmacy. A recent study found that patients seen by nephrologists were the most complex to treat because of their high number of comorbidities and medications. Due to polypharmacy, these patients may experience a wide range of adverse drug reactions. Along with CKD progression, the accumulation of uremic toxins may lead to blood-brain barrier (BBB) disruption and pharmacokinetic alterations, increasing the risk of adverse reactions affecting the central nervous system (CNS). In patients on dialysis, the excretion of drugs that depend on kidney function is severely reduced such that adverse and toxic levels of a drug or its metabolites may be reached at relatively low doses, unless dosing is adjusted. This first review will discuss how CKD represents a risk factor for adverse drug reactions affecting the CNS via (i) BBB disruption associated with CKD and (ii) the impact of reduced kidney function and dialysis itself on drug pharmacokinetics.

Physiological regulation of phosphate homeostasis.

Phosphate homeostasis is dependent on the interaction and coordination of four main organ systems: thyroid/parathyroids, gastrointestinal tract, bone and kidneys, and three key hormonal regulators, 1,25-hydroxyvitamin D3, parathyroid hormone and FGF23 with its co- factor klotho. Phosphorus is a critical nutritional element for normal cellular function, but in excess can be toxic to tissues, particularly the vasculature. As phosphate, it also has an important interaction and inter-dependence with calcium and calcium homeostasis sharing some of the same controlling hormones, although this is not covered in our article. We have chosen to provide a current overview of phosphate homeostasis only, focusing on the role of two major organ systems, the gastrointestinal tract and kidneys, and their contribution to the control of phosphate balance. We describe in some detail the mechanisms of intestinal and renal phosphate transport, and compare and contrast their regulation. We also consider a significant example of phosphate imbalance, with phosphate retention, which is chronic kidney disease; why consequent hyperphosphatemia is important, and some of the newer means of managing it.

Why is diagnosis, investigation, and improved management of kidney stone disease important? Non-pharmacological and pharmacological treatments for nephrolithiasis.

INTRODUCTION: Progress in the medical treatment and management of nephrolithiasis has been limited to date and continues to depend on urinary metabolic screening to assess excretion of the main stone constituents, factors determining stone solubility and precipitation, and on dietary and lifestyle recommendations. AREAS COVERED: In this review, we try to highlight some of the broader aspects of kidney stone disease in relation to recent epidemiological and pathophysiological findings, and emerging new treatments. Specifically, this review will cover recent evidence on the association between metabolic risk factors and kidney stone disease, dietary risk factors, and dietary interventions to prevent kidney stones, and how genomics, metabolomics, and proteomics may improve diagnosis and treatment of this troublesome, if rarely fatal, condition. PubMed was used to identify the most suitable references according to our search strategy; only full manuscripts were included. EXPERT OPINION: What is emerging is that kidney stone disease is not an isolated disorder but is systemic in nature with links to important and common comorbidities such as diabetes, hypertension, cardiovascular disease, and chronic kidney disease. These associations support the need to take nephrolithiasis seriously as a medical condition and to adopt a more holistic approach to its investigation and treatment.

Exploration of a panel of urine biomarkers of kidney disease in two paediatric cohorts with Type 1 diabetes mellitus of differing duration.

BACKGROUND: The pathogenesis of diabetic kidney disease (DKD) is complex and involves both glomerular and tubular dysfunction. A global assessment of kidney function is necessary to stage DKD, a progressive kidney disease that is likely to begin in childhood. The present study evaluated whether kidney injury biomarkers identified as early DKD biomarkers in adults have any prognostic value in the very early stages of childhood diabetes. METHODS: We measured urine free Retinol-binding protein 4 (UfRBP4), albumin (UAlb), Kidney injury molecule-1 (KIM-1) and the microRNAs miR-155, miR-126 and miR-29b in two cohorts of paediatric T1DM patients without evidence of DKD, but with diabetes of short-duration, ≤ 2.5 years (SD, n = 25) or of long-duration, ≥ 10 years (LD, n = 29); non-diabetic siblings (H, n = 26) were recruited as controls. A p value < 0.05 was considered significant for all results. RESULTS: UfRBP4 and UAlb were not significantly different across the three groups. No differences were found in KIM-1 excretion between any of the three groups. UfRBP4 was correlated with UAlb in all three groups (r 0.49; p < 0.001), whereas KIM-1 showed no correlation with albumin excretion. Among microRNAs, miR-29b was higher in all diabetic children compared with the H control group (p = 0.03), whereas miR-155 and miR-126 were not significantly different. No differences were found between the SD and LD groups for all three microRNAs. No associations were identified between these biomarkers with sex, age, BMI, eGFR, T1DM duration or glycaemic control. CONCLUSIONS: UfRBP4, KIM-1, miR-155, and miR-126 were unaffected by the presence and duration of diabetes, whereas miR-29b showed a modest elevation in diabetics, regardless of duration. These data support the specificity of a panel of urine biomarkers as DKD biomarkers, rather than any relationship to diabetes per se or its duration, and not as early DKD biomarkers in a paediatric setting.

Assessment of Measurement of Salivary Urea by ATR-FTIR Spectroscopy to Screen for CKD.

Stages of CKD are currently defined by eGFR and require measurement of serum creatinine concentrations. Previous studies have shown a good correlation between salivary and serum urea levels and the stage of CKD. However, quantitative salivary urea assays in current clinical use require costly and labor-intensive commercial kits, which restricts the advantage of using saliva and limits wider applicability as a quick and easy means of assessing renal function. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy has been shown to provide a potentially straightforward, reagent-free method for the identification of a range of disease-related biomarkers and is in current clinical use for analyses of the chemical composition of kidney stones. We assessed the feasibility of ATR-FTIR spectroscopy as an alternative method to measure salivary urea in patients with different stages of CKD. The ATR-FTIR spectra of dried saliva samples from six healthy controls and 20 patients with CKD (stages 1-5) were analyzed to provide their urea concentrations. The lower limit of detection of salivary urea by the ATR-FTIR spectroscopy method was 1-2 mM, at the lower end of the clinically relevant range. Statistically significant differences in salivary urea concentrations were demonstrated between healthy subjects (4.1±0.5 mM) and patients with CKD stages 3-5 (CKD stage 3, 6.8±0.7 mM; CKD stage 4, 9.1±1 mM; CKD stage 5, 14.8±1.6 mM). These salivary urea concentrations correlated well with serum urea levels in the same patients measured by an automated analyzer (Spearman rank correlation coefficient of 0.71; P<0.001). The ability of the method to detect and stage CKD was assessed from the sensitivity and specificity parameters of a receiver operating characteristics (ROC) curve analysis. This proof-of-concept study demonstrates that quantitation of salivary urea by ATR-FTIR spectroscopy could provide a viable tool for rapid and cost-effective diagnosis of stages 3-5 CKD.

Toxic nephropathy: Adverse renal effects caused by drugs.

This is a brief overview of toxic nephropathy, which is an increasingly recognised problem with the continual introduction of new drugs and novel drug modalities, especially in oncology, and the risks associated with polypharmacy in many patients; although it is important to remember that it may not always be caused by a drug. It is also important to note that several possibly harmful drugs are now available without prescription ('over-the-counter') and can be purchased easily over the internet, including some poorly characterised herbal remedies. Knowing exactly what our patients are taking as medication is not always easy and patients often fail to mention drugs that may not have been prescribed by a doctor or recommended by a pharmacist. Moreover, patients with several comorbidities often require care from more than one doctor in other specialties, which can also lead to drug prescribing in isolation. This article will summarise some key aspects of drug nephrotoxicity and provide a few clinical pointers to consider, bearing in mind that there is rarely any antidote available, and effective treatment relies on early detection, prompt drug withdrawal, and supportive care. This short review is intended only as a primer to highlight some of the more practical aspects of toxic nephropathy; its content is based on a lecture delivered during the 2021 European Congress of Internal Medicine.

Urinary metabolic profile and stone composition in kidney stone formers with and without heart disease.

OBJECTIVE: Kidney stone disease seems to be associated with an increased risk of incident cardiovascular outcomes; the aim of this study is to identify differences in 24-h urine excretory profiles and stone composition among stone formers with and without cardiovascular disease (CVD). METHODS: Data from patients attending the Department of Renal Medicine's metabolic stone clinic from 1995 to 2012 were reviewed. The sample was divided according to the presence or absence of CVD (myocardial infarction, angina, coronary revascularization, or surgery for calcified heart valves). Univariable and multivariable regression models, adjusted for age, sex, BMI, hypertension, diabetes, eGFR, plasma bicarbonate and potential renal acid load of foods were used to investigate differences across groups. RESULTS: 1826 patients had available data for 24-h urine analysis. Among these, 108 (5.9%) had a history of CVD. Those with CVD were older, have higher prevalence of hypertension and diabetes and lower eGFR. Univariable analysis showed that patients with CVD had significantly lower 24-h urinary excretions for citrate (2.4 vs 2.6 mmol/24 h, p = 0.04), magnesium (3.9 vs 4.2 mmol/24 h, p = 0.03) and urinary pH (6.1 vs 6.2, p = 0.02). After adjustment for confounders, differences in urinary citrate and magnesium excretions remained significant. No differences in the probability of stone formation or stone compositions were found. CONCLUSIONS: Stone formers with CVD have lower renal alkali excretion, possibly suggesting higher acid retention in stone formers with cardiovascular comorbidities. Randomized clinical trials including medications and a controlled diet design are needed to confirm the results presented here.

Drug toxicity in the proximal tubule: new models, methods and mechanisms.

The proximal tubule (PT) reabsorbs most of the glomerular filtrate and plays an important role in the uptake, metabolism and excretion of xenobiotics. Some therapeutic drugs are harmful to the PT, and resulting nephrotoxicity is thought to be responsible for approximately 1 in 6 of cases of children hospitalized with acute kidney injury (AKI). Clinically, PT dysfunction leads to urinary wasting of important solutes normally reabsorbed by this nephron segment, leading to systemic complications such as bone demineralization and a clinical scenario known as the renal Fanconi syndrome (RFS). While PT defects can be diagnosed using a combination of blood and urine markers, including urinary excretion of low molecular weight proteins (LMWP), standardized definitions of what constitutes clinically significant toxicity are lacking, and identifying which patients will go on to develop progressive loss of kidney function remains a major challenge. In addition, much of our understanding of cellular mechanisms of drug toxicity is still limited, partly due to the constraints of available cell and animal models. However, advances in new and more sophisticated in vitro models of the PT, along with the application of high-content analytical methods that can provide readouts more relevant to the clinical manifestations of nephrotoxicity, are beginning to extend our knowledge. Such technical progress should help in discovering new biomarkers that can better detect nephrotoxicity earlier and predict its long-term consequences, and herald a new era of more personalized medicine.

Brain dysfunction in tubular and tubulointerstitial kidney diseases.

Kidney function has two important elements: glomerular filtration and tubular function (secretion and reabsorption). A persistent decrease in glomerular filtration rate (GFR), with or without proteinuria, is diagnostic of chronic kidney disease (CKD). While glomerular injury or disease is a major cause of CKD and usually associated with proteinuria, predominant tubular injury, with or without tubulointerstitial disease, is typically non-proteinuric. CKD has been linked with cognitive impairment, but it is unclear how much this depends on a decreased GFR, altered tubular function or the presence of proteinuria. Since CKD is often accompanied by tubular and interstitial dysfunction, we explore here for the first time the potential role of the tubular and tubulointerstitial compartments in cognitive dysfunction. To help address this issue we selected a group of primary tubular diseases with preserved GFR in which to review the evidence for any association with brain dysfunction. Cognition, mood, neurosensory and motor disturbances are not well characterized in tubular diseases, possibly because they are subclinical and less prominent than other clinical manifestations. The available literature suggests that brain dysfunction in tubular and tubulointerstitial diseases is usually mild and is more often seen in disorders of water handling. Brain dysfunction may occur when severe electrolyte and water disorders in young children persist over a long period of time before the diagnosis is made. We have chosen Bartter and Gitelman syndromes and nephrogenic diabetes insipidus as examples to highlight this topic. We discuss current published findings, some unanswered questions and propose topics for future research.

The 1918 Influenza Pandemic: Back to the Future?

BACKGROUND: It is just over a century since the 1918 flu pandemic, sometimes referred to as the "mother" of pandemics. This brief retrospective of the 1918 pandemic is taken from the viewpoint of the current SARS-CoV-2/COVID-19 pandemic and is based on a short lecture given during the 2021 Virtual Congress of the ERA-EDTA. SUMMARY: This review summarizes and highlights some of the earlier pandemic's salient features, some parallels with today, and some potential learnings, bearing in mind that the flu pandemic occurred over 100 years ago at a time of major turmoil during the climax to WWl, and with limited medical expertise and knowledge, research facilities, or well-structured and resourced healthcare services. While there is little or no information on renal complications at the time, or an effective treatment, some observations in relation to COVID-19 and vaccination are included. Key Messages: Lessons are difficult to draw from 1918 other than the importance and value of non-pharmaceutical measures to limit viral transmission. While the economic impact of the 1918 pandemic was significant, as it is now with COVID-19, subsequent economic analysis has shown that protecting public health and preserving economic activity are not mutually exclusive. Both H1N1 and SARS-CoV-2 viruses are neurotropic and may cause chronically debilitating neurological diseases, including conditions such as encephalitis lethargica (still debated) and myalgic encephalomyelitis (chronic fatigue syndrome), respectively. Although coronavirus and influenza viral infections have some similarities, they are certainly not the same, as we are realising, and future infectious pandemics may still surprise us, but being "forewarned is forearmed."

Sirtuin 5 depletion impairs mitochondrial function in human proximal tubular epithelial cells.

Ischemia is a major cause of kidney damage. Proximal tubular epithelial cells (PTECs) are highly susceptible to ischemic insults that frequently cause acute kidney injury (AKI), a potentially life-threatening condition with high mortality. Accumulating evidence has identified altered mitochondrial function as a central pathologic feature of AKI. The mitochondrial NAD+-dependent enzyme sirtuin 5 (SIRT5) is a key regulator of mitochondrial form and function, but its role in ischemic renal injury (IRI) is unknown. SIRT5 expression was increased in murine PTECs after IRI in vivo and in human PTECs (hPTECs) exposed to an oxygen/nutrient deprivation (OND) model of IRI in vitro. SIRT5-depletion impaired ATP production, reduced mitochondrial membrane potential, and provoked mitochondrial fragmentation in hPTECs. Moreover, SIRT5 RNAi exacerbated OND-induced mitochondrial bioenergetic dysfunction and swelling, and increased degradation by mitophagy. These findings suggest SIRT5 is required for normal mitochondrial function in hPTECs and indicate a potentially important role for the enzyme in the regulation of mitochondrial biology in ischemia.

Purinergic signalling in the kidney - A beginning with Geoffrey Burnstock.

This not an original publication or a current and up-to-date review of purinergic signalling and kidney function, but rather a tribute to Professor Geoffrey Burnstock, written as a short and personal memoir of our early collaborative work together on this topic: our beginnings and the subsequent journey we took with our many valued collaborators along the way.

Further evidence for functional recovery of AQP2 mutations associated with nephrogenic diabetes insipidus.

Aquaporin-2 (AQP2) is a homotetrameric water channel responsible for the final water reuptake in the kidney. Disease-causing AQP2 mutations induce nephrogenic diabetes insipidus (NDI), a condition that challenges the bodily water balance by producing large urinary volumes. In this study, we characterize three new AQP2 mutations identified in our lab from NDI patients (A120D, A130V, T179N) along the previously reported A47V variant. Using Xenopus oocytes, we compared the key functional and biochemical features of these mutations against classical recessive (R187C) and dominant (R254Q) forms, and once again found clear functional recovery features (increased protein stability and function) for all mutations under study. This behaviour, attributed to heteromerization to wt-AQP2, challenge the classical model to NDI which often depicts recessive mutations as ill-structured proteins unable to oligomerize. Consequently, we propose a revised model to the cell pathophysiology of AQP2-related NDI which accounts for the functional recovery of recessive AQP2 mutations.

Diet-induced iron deficiency in rats impacts small intestinal calcium and phosphate absorption.

AIMS: Recent reports suggest that iron deficiency impacts both intestinal calcium and phosphate absorption, although the exact transport pathways and intestinal segment responsible have not been determined. Therefore, we aimed to systematically investigate the impact of iron deficiency on the cellular mechanisms of transcellular and paracellular calcium and phosphate transport in different regions of the rat small intestine. METHODS: Adult, male Sprague-Dawley rats were maintained on a control or iron-deficient diet for 2 weeks and changes in intestinal calcium and phosphate uptake were determined using the in situ intestinal loop technique. The circulating levels of the hormonal regulators of calcium and phosphate were determined by ELISA, while the expression of transcellular calcium and phosphate transporters, and intestinal claudins were determined using qPCR and western blotting. RESULTS: Diet-induced iron deficiency significantly increased calcium absorption in the duodenum but had no impact in the jejunum and ileum. In contrast, phosphate absorption was significantly inhibited in the duodenum and to a lesser extent the jejunum, but remained unchanged in the ileum. The changes in duodenal calcium and phosphate absorption in the iron-deficient animals were associated with increased claudin 2 and 3 mRNA and protein levels, while levels of parathyroid hormone, fibroblast growth factor-23 and 1,25-dihydroxy vitamin D3 were unchanged. CONCLUSION: We propose that iron deficiency alters calcium and phosphate transport in the duodenum. This occurs via changes to the paracellular pathway, whereby upregulation of claudin 2 increases calcium absorption and upregulation of claudin 3 inhibits phosphate absorption.

Renal tubular acidosis (RTA) and kidney stones: Diagnosis and management. / Acidosis tubular renal (ATR) y litiasis renal: Diagnóstico y manejo.

Renal tubular acidosis (RTA) is a set of raredis orders in which the renal tubule is unable to excreteacid normally and there by maintain normal acid-basebalance, resulting in a complete or incomplete metabolicacidosis. In distal RTA (dRTA, also known as classicalor type 1 RTA), there is a defect in excreting H+ ionsalong the distal nephron (distal tubule and collectingduct), leading to an alkaline urinary pH with calcium phosphate precipitation and stones. Causes of dRTAinclude genetic mutations, autoimmune disease, and some drugs.Clinical manifestations of the genetic forms of dRTA typically occur during childhood and may vary from mildclinical symptoms, such as a mild metabolic acidosis, hypokalaemia,and incidental detection of kidney stones, to more serious manifestations such as failure to thrive,severe metabolic acidosis, rickets and nephrocalcinosis.Progressive hearing loss may develop in patients withrecessive dRTA, which, depending the causative genemutation, can be present at birth or develop later in adolescence or early adulthood. Diagnosis of dRTA can be challenging, since it requires a high index of suspicion and/or measurement of urinary pH after an acid load, usually in the form of oral ammonium chloride; this should normally acidify the urine to pH below 5.3. In dRTA, urinary citrate levels a real so low and patients are at increased risk of for mingkidney stones from a combination of alkaline urine and low citrate. Ideally, affected patients need regular outpatient follow-up by a urologist and nephrologist. Thus, any patient found to have a calcium phosphate kidney stone, low urinary citrate, and raised urinary pH, especially with an early morning pH >5.5, should be evaluated for underlying dRTA. Patients with complete dRTA will have a low (<20 mmol/L) plasma or serum bicarbonate concentration, whereas in those with incomplete dRTA, bicarbonate levels are usually normal. Oral alkali as potassiumcitrate is still the mainstay of treatment in dRTA.

La acidosis tubular renal (ATR) es un conjunto de enfermedades raras en las que el túbulo renal es incapaz de excretar ácido de forma normal y por ello de mantener un balance ácido-base normal, resultando en una acidosis metabólica completa o incompleta. En la ATR distal (ATRd, también conocida como ATR tipo 1 o clásica), hay un defecto en la excreción de iones H+a lo largo de la parte distal de la nefrona (túbulo distal  y tubo colector) que conduce a un pH urinario alcalino con precipitación de fosfato cálcico y litiasis. Las causas de la ATRd incluyen mutaciones genéticas, enfermedad autoinmune y algunos fármacos. Las manifestaciones clínicas de la forma genética de la ATRd ocurren típicamente durante la infancia y pueden variar desde síntomas leves, como acidosis metabólica leve, hipokaliemia y detección accidental de litiasis renal, hasta manifestaciones más graves tales como falta de crecimiento, acidosis metabólica severa, raquitismo y nefrocalcinosis. En pacientes con ATRd recesiva puede desarrollarse una pérdida progresiva de audición que, dependiendo de la causa de la mutación genética, puede estar presente en el momento del nacimientoo desarrollarse más tarde en la adolescencia o edad adulta temprana.El diagnóstico de la ATRd puede ser un reto ya que requiere un alto grado de sospecha y/o la medición del pH urinario tras una carga ácida, normalmente en forma de cloruro amónico oral; esto debería normalmente acidificar la orina a un pH inferior a 5,3. En la ATRd, los niveles de citrato urinario también son bajos y los pacientes tienen un mayor riesgo de formar litiasis renal por una combinación de orina alcalina e hipocitraturia. Lo ideal es que los pacientes afectados sean seguidos de forma regular por un urólogo y un nefrólogo. Así,cualquier paciente con litiasis de fosfato cálcico, hipocitraturia y pH urinario elevado, especialmente con un pH urinario matutino >5,5, debería ser estudiado para descartar una ATRd oculta. Los pacientes con ATRd completa tendrán una concentración plasmática o sérica de bicarbonato baja (<20 mmol/L), mientras que en aquellos con una ATRd incompleta, los niveles de bicarbonato son generalmente normales. Los alcalinizantes orales como el citrato potásico son aún el principal pilar del tratamiento en la ATRd.

Intestinal sodium/glucose cotransporter 3 expression is epithelial and downregulated in obesity.

AIM: We aimed to determine whether the sodium/glucose cotransporter family member SGLT3, a proposed glucose sensor, is expressed in the intestine and/or kidney, and if its expression is altered in mouse models of obesity and in humans before and after weight-loss surgery. MAIN METHODS: We used in-situ hybridization and quantitative PCR to determine whether the Sglt3 isoforms 3a and 3b were expressed in the intestine and kidney of C57, leptin-deficient ob/ob, and diabetic BTBR ob/ob mice. Western blotting and immunohistochemistry were also used to assess SGLT3 protein levels in jejunal biopsies from obese patients before and after weight-loss Roux-en-Y gastric bypass surgery (RYGB), and in lean healthy controls. KEY FINDINGS: Sglt3a/3b mRNA was detected in the small intestine (duodenum, jejunum and ileum), but not in the large intestine or kidneys of mice. Both isoforms were detected in epithelial cells (confirmed using intestinal organoids). Expression of Sglt3a/3b mRNA in duodenum and jejunum was significantly lower in ob/ob and BTBR ob/ob mice than in normal-weight littermates. Jejunal SGLT3 protein levels in aged obese patients before RYGB were lower than in lean individuals, but substantially upregulated 6 months post-RYGB. SIGNIFICANCE: Our study shows that Sglt3a/3b is expressed primarily in epithelial cells of the small intestine in mice. Furthermore, we observed an association between intestinal mRNA Sglt3a/3b expression and obesity in mice, and between jejunal SGLT3 protein levels and obesity in humans. Further studies are required to determine the possible role of SGLT3 in obesity.

Purinergic signalling in the kidney: In physiology and disease.

Historically, the control of renal vascular and tubular function has, for the most part, concentrated on neural and endocrine regulation. However, in addition to these extrinsic factors, it is now appreciated that several complex humoral control systems exist within the kidney that can act in an autocrine and/or paracrine fashion. These paracrine systems complement neuroendocrine regulation by dynamically fine-tuning renal vascular and tubular function to buffer rapid changes in nephron perfusion and flow rate of tubular fluid. One of the most pervasive is the extracellular nucleotide/P2 receptor system, which is central to many of the intrinsic regulatory feedback loops within the kidney such as renal haemodynamic autoregulation and tubuloglomerular feedback (TGF). Although physiological actions of extracellular adenine nucleotides were reported almost 100 years ago, the conceptual framework for purinergic regulation of renal function owes much to the work of Geoffrey Burnstock. In this review, we reflect on our >20-year collaboration with Professor Burnstock and highlight the research that is still unlocking the potential of the renal purinergic system to understand and treat kidney disease.

Predicting the protein composition of human urine in normal and pathological states: Quantitative description based on Dent1 disease (CLCN5 mutation).

KEY POINTS: The presence of plasma proteins in urine is difficult to interpret quantitatively. It may be a result of impaired glomerular filtration or impaired proximal tubule (PT) reabsorption, or both. Dent1 disease (CLCN5 mutation) abolishes PT protein reabsorption leaving glomerular function intact. Using urine protein measurements from patients with Dent1 disease and normal individuals, we devised a mathematical model that incorporates two PT transport processes with distinct kinetics. This model predicts albumin, α1 -microglobulin (α1 -m), ß2 -microglobulin (ß2 -m) and retinol-binding protein 4 (RBP4) urine concentrations. Our results indicate that the urinary excretion of ß2 -m and RBP4 differs from that of albumin and α1 -m in their sensitivity to changes in the glomerular filtration rate, glomerular protein leak, tubular protein uptake via endocytosis and PT water reabsorption. The model predicts quantitatively how hyperfiltration and glomerular leak interact to promote albuminuria. Our model should contribute to improved understanding and interpretation of urine protein measurements in renal disease. ABSTRACT: To clarify the relative contributions of glomerular filtration and tubular uptake to urinary protein excretion, we developed a mathematical model of protein reabsorption in the human proximal tubule (PT) using Michaelis-Menten kinetics and molar urinary protein measurements taken from human Dent1 disease (CLCN5 loss-of-function mutation). ß2 -Microglobulin (ß2 -m) and retinol-binding protein 4 (RBP4) are normally reabsorbed with 'very high' efficiency uptake kinetics and fractional urinary excretion of 0.025%, whereas albumin and α1 -microglobulin (α1 -m) are reabsorbed by 'high' efficiency uptake kinetics and 50-fold higher fractional urinary excretion of 1.15%. Our model correctly predicts the urinary ß2 -m, RBP4 and α1 -m content in aristolochic acid nephropathy, and elevated ß2 -m excretion with increased single nephron glomerular filtration rate (SNGFR) following unilateral-nephrectomy. We explored how altered endocytic uptake, water reabsorption, SNGFR and glomerular protein filtration affect excretion. Our results help to explain why ß2 -m and RBP4 are more sensitive markers of PT dysfunction than albumin or α1 -m, and suggest that reduced PT sodium and water reabsorption in Fanconi syndrome may contribute to proteinuria. Transition of albumin excretion from normal to microalbuminuria, a 5-fold increase, corresponds to a 3.5-fold elevation in albumin glomerular filtration, supporting the use of microalbuminuria screening to detect glomerular leak in diabetes. In macroalbuminuria, small albumin permeability changes produce large changes in excretion. However, changes in SNGFR can alter protein excretion, and hyperfiltration with glomerular leak can combine to increase albuminuria. Our model provides a validated quantitative description of the transport processes underlying the protein composition of human urine in normal and pathophysiological states.

Multiparametric imaging reveals that mitochondria-rich intercalated cells in the kidney collecting duct have a very high glycolytic capacity.

Alpha intercalated cells (αICs) in the kidney collecting duct (CD) belong to a family of mitochondria rich cells (MRCs) and have a crucial role in acidifying the urine via apical V-ATPase pumps. The nature of metabolism in αICs and its relationship to transport was not well-understood. Here, using multiphoton live cell imaging in mouse kidney tissue, FIB-SEM, and other complementary techniques, we provide new insights into mitochondrial structure and function in αICs. We show that αIC mitochondria have a rounded structure and are not located in close proximity to V-ATPase containing vesicles. They display a bright NAD(P)H fluorescence signal and low uptake of voltage-dependent dyes, but are energized by a pH gradient. However, expression of complex V (ATP synthase) is relatively low in αICs, even when stimulated by metabolic acidosis. In contrast, anaerobic glycolytic capacity is surprisingly high, and sufficient to maintain intracellular calcium homeostasis in the presence of complete aerobic inhibition. Moreover, glycolysis is essential for V-ATPase-mediated proton pumping. Key findings were replicated in narrow/clear cells in the epididymis, also part of the MRC family. In summary, using a range of cutting-edge techniques to investigate αIC metabolism in situ, we have discovered that these mitochondria dense cells have a high glycolytic capacity.