Nakhoul

Gitelman's syndrome: a pathophysiological and clinical update

AbstractGitelman’s syndrome (GS), also known as familial hypokalemic hypomagnesemia, is a rare autosomal recessive hereditary salt-losing tubulopathy, characterized by hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria, which is usually caused by mutations in the SLC12A3 gene encoding the thiazide-sensitive sodium chloride contrasporter. Because 18–40% of suspected GS patients carry only one SLC12A3 mutant allele, large genomic rearrangements must account for unidentified mutations. The clinical manifestations of GS are highly variable in terms of age at presentation, severity of symptoms, and biochemical abnormalities. Molecular analysis in our sibling’s patients revealed compound heterozygous mutations in the coding region of SLC12A3 as underlying their disease. Such compound heterozygosity can result in disease phenotype for such loss of function mutations in the absence of homozygosis through consanguineous inheritance of mutant alleles, identical by descent. Missense mutations account for approximately 70% of the mutations in GS, and there is a predisposition to large rearrangements caused by the presence of repeated sequences within the SLC12A3. We report two adult male siblings of Jewish origin with late onset GS, who presented in their fifth decade of life with muscle weakness, hypokalemia, hypomagnesaemia, and metabolic alkalosis. Rapid clinical and biochemical improvement was achieved by replacement therapy with potassium and magnesium.

Pharmacogenomic effect of vitamin E on kidney structure and function in transgenic mice with the haptoglobin 2-2 genotype and diabetes mellitus

Polymorphic loci regulating oxidative stress are potential susceptibility genes for diabetic nephropathy (DN). Haptoglobin (Hp) is an antioxidant protein which serves to protect against oxidative stress induced by extracorpuscular hemoglobin. There are two alleles at the Hp locus, 1 and 2. The Hp 1 protein is a superior antioxidant to the Hp 2 protein. The Hp 2 allele has been associated with increased prevalence of DN and appears to be associated with a more rapid progression to end-stage renal disease. We sought to recapitulate this association between Hp genotype and DN in mice genetically modified at the Hp locus. We assessed morphometric, histologic, and functional parameters involved in the development and progression of DN in mice with diabetes mellitus (DM) with either the Hp 2-2 or Hp 1-1 genotype. Morphometric analysis demonstrated that glomerular and proximal tubular hypertrophy were significantly increased in Hp 2-2 DM mice. Histological analysis demonstrated that Hp 2-2 DM mice had significantly more collagen type IV, smooth muscle actin, and increased renal iron deposition. Studies of renal function demonstrated creatinine clearance time and albuminuria were increased in Hp 2-2 DM mice. Vitamin E provided significant protection against the development of functional and histological features characteristic of DN to Hp 2-2 DM but not to Hp 1-1 DM mice. These studies serve to strengthen the association between the Hp 2-2 genotype and diabetic renal disease and suggest a pharmacogenomic interaction may exist between the Hp genotype and vitamin E.

Vitamin E and diabetic nephropathy in mice model and humans.

Diabetes mellitus (DM) is associated with increased oxidative stress due to elevated glucose levels in the plasma. Glucose promotes glycosylation of both plasma and cellular proteins with increased risk for vascular events. Diabetic patients suffer from a higher incidence of cardiovascular complications such as diabetic nephropathy. Haptoglobin (Hp) is an antioxidant plasma protein which binds free hemoglobin, thus preventing heme-iron mediated oxidation. Two alleles exist at the Hp gene locus (1 and 2) encoding three possible Hp genotypes that differ in their antioxidant ability, and may respond differently to vitamin E treatment. Several clinical studies to have shown that Hp 1-1 genotype is a superior antioxidant to the Hp 2-2 genotype and Hp 2-2 genotype is associated with a higher incidence of cardiovascular disease. Vitamin E was found to have beneficial effect in patient and mice with Hp 2-2 genotype. In this review we have summarized the results of our studies in patients with diabetic nephropathy treated with vitamin E and in diabetic mice with different haptoglobin genotypes.

Poor lysosomal membrane integrity in proximal tubule cells of haptoglobin 2-2 genotype mice with diabetes mellitus

The haptoglobin (Hp) genotype is a major determinant of progression of nephropathy in individuals with diabetes mellitus (DM). The major function of the Hp protein is to bind and modulate the fate of extracorpuscular hemoglobin and its iron cargo. We have previously demonstrated an interaction between the Hp genotype and the DM on the accumulation of iron in renal proximal tubule cells. The primary objective of this study was to determine the intracellular localization of this iron in the proximal tubule cell and to assess its potential toxicity. Transmission electron microscopy demonstrated a marked accumulation of electron-dense deposits in the lysosomes of proximal tubules cells in Hp 2-2 DM mice. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy were used to perform elemental analysis of these deposits and demonstrated that these deposits were iron rich. These deposits were associated with lysosomal membrane lipid peroxidation and loss of lysosomal membrane integrity. Vitamin E administration to Hp 2-2 DM mice resulted in a significant decrease in both intralysosomal iron-induced oxidation and lysosomal destabilization. Iron-induced renal tubular injury may play a major role in the development of diabetic nephropathy and may be a target for slowing the progression of renal disease.

Interaction between the Haptoglobin 2 Phenotype and Diabetes Mellitus on Systolic Pulmonary Arterial Pressure and Nitric Oxide Bioavailability in Hemodialysis Patients.

Elevated systolic pulmonary artery pressure (s-PAP, ≥35 mmHg) serves as an independent predictor of mortality in hemodialysis (HD) and diabetic (DM) patients. A polymorphism in the antioxidant Haptoglobin (Hp) gene has been shown to regulate the bioavailability of nitric oxide (NO), a major mediator of pulmonary vascular tone. We therefore set out to test the hypothesis that the Hp polymorphism may be a determinant of developing elevated s-PAP specifically in the DM state due to a decreased bioavailability of NO. To test our hypothesis we Hp typed and performed transthoracic echocardiography on a series of HD patients and stratified them into elevated and normal s-PAP groups and then evaluated whether there was a significant association between the Hp type, elevated s-PAP, and decreased NO bioavailability as defined by low plasma nitrite. We found a statistically significant interaction between the Hp type and DM on the prevalence of elevated s-PAP and lower mean nitrite levels with the combination of elevated s-PAP and low nitrite levels being significantly more prevalent in Hp 2-2 DM individuals. We conclude that the Hp 2 type is associated with elevated s-PAP levels and low plasma nitrite levels in HD patients specifically in the DM state.

Is the Hp 2-2 diabetic mouse model a good model to study diabetic nephropathy?

Diabetic nephropathy (DN) is the leading cause of end stage renal disease and dialysis worldwide. Despite aggressive treatment, the number of patients on hemodialysis due to type 1 and type 2 diabetes mellitus is increasing annually. The lack of reliable animal models that mimic human disease has delayed the identification of specific factors that cause or predict DN. Different investigators around the world are testing different murine models. Validation criteria for early and advanced DN, phenotypic methods, background strain have recently been developed. Establishment of an authentic mouse model of DN will undoubtedly facilitate the understanding of the underlying genetic mechanisms that contribute to the development of DN and to study new treatments. Here we describe the characteristics of our new mouse model with type 1 diabetes mellitus and different haptoglobin genotypes that can mimic human DN.

The Non Mineral Axis Klotho-Vitamin D in Diabetic Nephropathy: Review

Diabetes Mellitus (DM) is metabolic disease associated with hyperlipidemia and arterial hypertension that are known as major risk factors for coronary heart disease. Diabetic complications are related to increased oxidative stress due to elevated glucose levels in the plasma. DM patients suffer from higher incidence of micro vascular complications such as Diabetic Nephropathy (DN). DN is the leading cause of End Stage Renal Disease (ESRD) and accounts for approximately 40% of all patients who require replacement therapy. Recently, the proximal tubular damage caused by increased deposition of iron in the lysosomes accompanied with oxidative stress, was described as another mechanism involved in the pathogenesis of DN. The well-known risk factors for DN are uncontrolled glucose levels, genetic factors such as Haptoglobin (Hp) and Angiotensin Converting Enzyme (ACE) genes polymorphism. Despite the availability of various drugs for DN such as converting enzyme inhibitors (ACEIs), angiotensin receptor blockers (ARB) and renin inhibitors (RI), 40% of type 2 DM (T2D) patients will develop ESRD. Hence, the aim of this review is to understand the importance of klotho protein and vitamin D in the pathogenesis of DN and as a potential future target for treatment.

Sodium-Glucose Transporter Inhibitors and Diabetic Nephropathy in Humans and Animal Model

Diabetic nephropathy as the leading cause for end stage renal disease and replacement therapy is increasing every year. Treatment of T2DM with the present oral blood glucose lowering drugs and insulin is challenging, with an enormous number of patients are able to achieve the target glycaemic control (HbA1C<6.5%). Despite the use of new Insulin compounds and different recommended combination of oral anti-diabetic drugs, the benefits of these recommendations are offset by side effects such as weight gain and recurrent hypoglycaemia. Therefore, the need for new agents that control blood glucose strictly and have other proactive cellular pathways is challenging. The sodium glucose transporter protein 2 (SGLT2) inhibitors, are recently being widely used. The main therapeutic effect of these new drugs, SGLT2 inhibitors (SGLT2-I), is lowering the blood glucose levels via inhibitory effect on the transport of glucose and sodium in the proximal tubular cells by sodium glucose transport 1. SGLT2-I reduce plasma sodium level by natriuretic and diuresis, with decreasing blood pressure and body weight. These new medications can be used as first and second lines of treatment especially in patients with normal glomerular filtration rate, with or without cardiovascular complications. The most effective combination of SGLT2I is Metformin especially in albuminuria and slowing the progression of diabetic nephropathy especially if initiated in early stages of DM. The new class of medication (SGLT2I) are less effective in patients with moderate CKD (eGFR<45 ml/min). This review will focus on the new pathways such autophagy as a new pathway where SGLT2 are involved with protective effects.

The Iron-Klotho-VDR Axis Is a Major Determinant of Proximal Convoluted Tubule Injury in Haptoglobin 2-2 Genotype Diabetic Nephropathy Patients and Mice

The haptoglobin (Hp) genotype (1-1 and 2-2) is a major determinant of nephropathy progression in diabetes mellitus patients. Hp 2-2 diabetic mice have impaired Hb clearance and increased iron deposits and oxidative stress in the proximal tubules (PCT), leading to increased renal injury. However, the precise mechanism of the PCT injury in diabetic nephropathy (DN) remains elusive. In the kidney, 1,25(OH)2D3 suppresses the inflammatory response to renal tubular injury and requires normal renal expression of the α-klotho protein. In this study, we set out to test the hypothesis that the increased renal iron deposits in the PCT of Hp 2-2 DN affect the α-klotho-vitamin D receptor (VDR) axis and thereby exacerbates the PCT injury generated by the iron deposits. Immunohistochemical analysis of human and mouse kidney biopsies along with western blot analysis showed that the increased iron deposits in the PCT of the Hp 2-2 genotype were accompanied with significantly decreased α-klotho and VDR renal expression but significantly increased 1-α-hydroxylase renal expression. In conclusion, the iron-klotho-VDR axis is a major player in the mechanism contributing to iron-mediated PCT injury in diabetic Hp 2-2 mice and patients. Targeting this axis may open the way for new ideas regarding the pathogenesis and treatment of DN.

Diabetic Nephropathy from RAAS to Autophagy: The Era for New Players

Diabetic nephropathy is a leading cause of end-stage renal disease (ESRD) and increased cardiovascular morbidity and mortality worldwide in patients with type 2 diabetes mellitus. The mechanisms behind the pathophysiology of DN are complex and continue to be not fully understood. Both metabolic (hyperglycaemia) and haemodynamic alterations interact synergistically, and have been reported to activate local RAAS resulting in increased angiotensin-2. In spite the early and chronic treatment with converting enzyme inhibitors and angiotensin receptor blocking drugs, the number of patients reaching end stage renal disease and replacement therapy are increasing. Recently different pathways were proposed to be involved in the pathogenesis of diabetic nephropathy, including the autophagy process, Klotho and the selective agonist vitamin D and his receptor. Under hyperglycemic stress especially in podocytes and proximal convolute tubule cells, there is decrease in the protective autophagic process and increase in cellular damage. α-Klotho is a multifunctional protein highly expressed in the kidney. The klotho protein has endogenous anti fibrotic function via antagonism of Wnt/β-catenin signaling, which promotes fibrogenesis, suggesting that loss of Klotho in early stage of diabetic nephropathy may contribute to the progression of DN by accelerated fibrogenesis. The selective vitamin D agonist and his receptor, play a protective pathway in diabetic nephropathy. A key reno-protective function of vitamin D is to reduce albuminuria or proteinuria, major risk factors for CKD progression, renal failure, cardiovascular events, and death. This anti-proteinuric effect and the deceleration of DN progression is mediated primarily via the blocking of the renin angiotensin aldosterone system. In this review we will discuss the different new mechanisms involved in diabetic nephropathy and future therapeutic agents like the mTorc1 blocker, Rapamycin, that can upregulate the autophagy process, the new sodium-glucose transport inhibitors and Paricalcitol, the selective active vitamin D.