Evaluation of a SNP map of 6q24-27 confirms diabetic nephropathy loci and identifies novel associations in type 2 diabetes patients with nephropathy from an African-American population. Hum Genet. 2008;124:63-71. [PMID: 18560894 DOI: 10.1007/s00439-008-0523-7]

The E3 ubiquitin ligase parkin is dispensable for metabolic homeostasis in murine pancreatic β cells and adipocytes

The E3 ubiquitin ligase parkin is a critical regulator of mitophagy and has been identified as a susceptibility gene for type 2 diabetes (T2D). However, its role in metabolically active tissues that precipitate T2D development is unknown. Specifically, pancreatic β cells and adipocytes both rely heavily on mitochondrial function in the regulation of optimal glycemic control to prevent T2D, but parkin's role in preserving quality control of β cell or adipocyte mitochondria is unclear. Although parkin has been reported previously to control mitophagy, here we show that, surprisingly, parkin is dispensable for glucose homeostasis in both β cells and adipocytes during diet-induced insulin resistance in mice. We observed that insulin secretion, β cell formation, and islet architecture were preserved in parkin-deficient β cells and islets, suggesting that parkin is not necessary for control of β cell function and islet compensation for diet-induced obesity. Although transient parkin deficiency mildly impaired mitochondrial turnover in β cell lines, parkin deletion in primary β cells yielded no deficits in mitochondrial clearance. In adipocyte-specific deletion models, lipid uptake and β-oxidation were increased in cultured cells, whereas adipose tissue morphology, glucose homeostasis, and beige-to-white adipocyte transition were unaffected in vivo In key metabolic tissues where mitochondrial dysfunction has been implicated in T2D development, our experiments unexpectedly revealed that parkin is not an essential regulator of glucose tolerance, whole-body energy metabolism, or mitochondrial quality control. These findings highlight that parkin-independent processes maintain β cell and adipocyte mitochondrial quality control in diet-induced obesity.

MitoNEET-Parkin Effects in Pancreatic α- and β-Cells, Cellular Survival, and Intrainsular Cross Talk

Mitochondrial metabolism plays an integral role in glucose-stimulated insulin secretion (GSIS) in β-cells. In addition, the diabetogenic role of glucagon released from α-cells plays a major role in the etiology of both type 1 and type 2 diabetes because unopposed hyperglucagonemia is a pertinent contributor to diabetic hyperglycemia. Titrating expression levels of the mitochondrial protein mitoNEET is a powerful approach to fine-tune mitochondrial capacity of cells. Mechanistically, β-cell-specific mitoNEET induction causes hyperglycemia and glucose intolerance due to activation of a Parkin-dependent mitophagic pathway, leading to the formation of vacuoles and uniquely structured mitophagosomes. Induction of mitoNEET in α-cells leads to fasting-induced hypoglycemia and hypersecretion of insulin during GSIS. MitoNEET-challenged α-cells exert potent antiapoptotic effects on β-cells and prevent cellular dysfunction associated with mitoNEET overexpression in β-cells. These observations identify that reduced mitochondrial function in α-cells exerts potently protective effects on β-cells, preserving β-cell viability and mass.

Evidence for a common biological pathway linking three Parkinson's disease-causing genes: parkin, PINK1 and DJ-1

Parkinson's disease (PD) is characterised by the loss of dopaminergic neurons in the midbrain. Autosomal recessive, early-onset cases of PD are predominantly caused by mutations in the parkin, PINK1 and DJ-1 genes. Animal and cellular models have verified a direct link between parkin and PINK1, whereby PINK1 phosphorylates and activates parkin at the outer mitochondrial membrane, resulting in removal of dysfunctional mitochondria via mitophagy. Despite the overwhelming evidence for this interaction, few studies have been able to identify a link for DJ-1 with parkin or PINK1. The aim of this review is to summarise the functions of these three proteins, and to analyse the existing evidence for direct and indirect interactions between them. DJ-1 is able to rescue the phenotype of PINK1-knockout Drosophila models, but not of parkin-knockouts, suggesting that DJ-1 may act in a parallel pathway to that of the PINK1/parkin pathway. To further elucidate a commonality between these three proteins, bioinformatics analysis established that Miro (RHOT1) interacts with parkin and PINK1, and HSPA4 interacts with all three proteins. Furthermore, 30 transcription factors were found to be common amongst all three proteins, with many of them being involved in transcriptional regulation. Interestingly, expression of these proteins and their associated transcription factors are found to be significantly down-regulated in PD patients compared to healthy controls. In summary, this review provides insight into common pathways linking three PD-causing genes and highlights some key questions, the answers to which may provide critical insight into the disease process.

Association of vitamin D and vitamin D receptor gene polymorphisms with chronic inflammation, insulin resistance and metabolic syndrome components in type 2 diabetic Egyptian patients

Background

To date the published data concerning the possible interplay between vitamin D (VitD) and Vit D receptor (VDR) gene polymorphism with the immune/inflammatory mediators in type 2 diabetes mellitus (DM) is insufficient. Some of the immune non-classical actions of vitamin D may point to its role in the pathogenesis of type 2 DM through down-regulation of cytokines (IL-6). Although there is evidence to support a relationship among vitamin D status, chronic inflammation and insulin resistance, the underlying mechanism requires further exploration. We aimed to investigate the role of vitamin D in chronic inflammation and insulin resistance in type 2 DM. Moreover, to examine the association of VDR gene polymorphisms [VDR 2228570 C > T (FokI); VDR 1544410 A > G (BsmI)] with the components of metabolic syndrome (MetSyn) in type 2 diabetic Egyptian patients .

Subjects and methods

A total of 190 subjects were enrolled in this study, 60 controls and 130 type 2 diabetic patients (Group II). Group II was subdivided into 63 patients without MetSyn (subgroup IIa) and 67 patients with MetSyn (subgroup IIb). Genetic analysis for VDR gene polymorphisms was done in all subjects. VitD and IL-6 plasma levels were estimated.

Results

The TT genotype for the VDR FokI was significantly more frequent in subgroup IIb than in subgroup IIa and controls (X² = 6.83, P = 0.03 and X² = 16.592, P = 0.000) respectively. The T allele was more frequent in the MetSyn group as compared to diabetics without MetSyn (p = 0.001), odds ratio (OR) and 95% CI for the T allele of C > T (FokI) = 2.30 (1.37–3.86). We did not detect any significant difference in VDR BsmI genotypes between patients and control groups (P = 0.947). FokI VDR was significantly associated with the lipid profile parameters, VitD and IL-6 plasma levels in subgroup IIa and associated with HOMA-IR, insulin, VitD, IL-6 levels, waist circumference (WC) and body mass index (BMI) in subgroup IIb while BsmI VDR variant was associated only with VitD values in both subgroups.

Conclusion

The present study suggests an interaction between VDR polymorphisms and important components of MetSyn, VitD and pro-inflammatory cytokines (IL-6). FokI VDR polymorphisms may be linked to mild inflammation and insulin resistance and might represent a genetic determinant for developing MetSyn in type 2 diabetic Egyptian patients. The challenge is determining the mechanisms of VitD action for recommendation of VitD supplementation that reduces the risks of MetSyn, insulin resistance and progression to type 2 diabetes.

Chronic kidney disease prediction is an inexact science: The concept of “progressors” and “nonprogressors”

In 2002, the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (NKF KDOQI) instituted new guidelines that established a novel chronic kidney disease (CKD) staging paradigm. This set of guidelines, since updated, is now very widely accepted around the world. Nevertheless, the authoritative United States Preventative Task Force had in August 2012 acknowledged that we know surprisingly little about whether screening adults with no signs or symptoms of CKD improve health outcomes and that we deserve better information on CKD. More recently, the American Society of Nephrology and the American College of Physicians, two very well respected United States professional physician organizations were strongly at odds coming out with exactly opposite recommendations regarding the need or otherwise for ”CKD screening” among the asymptomatic population. In this review, we revisit the various angles and perspectives of these conflicting arguments, raise unanswered questions regarding the validity and veracity of the NKF KDOQI CKD staging model, and raise even more questions about the soundness of its evidence-base. We show clinical evidence, from a Mayo Clinic Health System Renal Unit in Northwestern Wisconsin, United States, of the pitfalls of the current CKD staging model, show the inexactitude and unpredictable vagaries of current CKD prediction models and call for a more cautious and guarded application of CKD staging paradigms in clinical practice. The impacts of acute kidney injury on CKD initiation and CKD propagation and progression, the effects of such phenomenon as the syndrome of late onset renal failure from angiotensin blockade and the syndrome of rapid onset end stage renal disease on CKD initiation, CKD propagation and CKD progression to end stage renal disease all demand further study and analysis. Yet more research on CKD staging, CKD prognostication and CKD predictions is warranted. Finally and most importantly, cognizant of the very serious limitations and drawbacks of the NKF K/DOQI CKD staging model, the need to individualize CKD care, both in terms of patient care and prognostication, cannot be overemphasized.

The PARK2 gene is involved in the maintenance of pancreatic β-cell functions related to insulin production and secretion

Several association studies have implicated the PARK2 gene that encodes parkin--the key molecule orchestrating the mitochondrial quality control system--as a candidate susceptibility gene for diabetes. A total of 7551 unrelated Korean KARE cohort subjects were analyzed to investigate the association between the PARK2 single nucleotide polymorphism (SNP) and quantitative glycemic traits. Two SNPs, rs10455889 and rs9365294, were significantly associated with fasting plasma glucose level (p=∼1.2×10(-4)) and insulin secretion indices (p=∼7.4×10(-5)) in male KARE subjects. Parkin was expressed predominantly in the rat pancreatic islets. Downregulation of the Park2 gene in rat INS-1 β-cells resulted in a significant decrease in the glucose-stimulated insulin secretion, intracellular insulin gene expression, and intracellular ATP level. The Park2-depleted β-cells also exhibited increased mitochondrial fragmentation and ROS production and decreased mitochondrial membrane potential. Both population-based statistical evaluation and experimental evidence demonstrated a fundamental role of the PARK2 gene in the maintenance of β-cell function.

Renoprotection and the Bardoxolone Methyl Story - Is This the Right Way Forward? A Novel View of Renoprotection in CKD Trials: A New Classification Scheme for Renoprotective Agents

In the June 2011 issue of the New England Journal of Medicine, the BEAM (Bardoxolone Methyl Treatment: Renal Function in CKD/Type 2 Diabetes) trial investigators rekindled new interest and also some controversy regarding the concept of renoprotection and the role of renoprotective agents, when they reported significant increases in the mean estimated glomerular filtration rate (eGFR) in diabetic chronic kidney disease (CKD) patients with an eGFR of 20-45 ml/min/1.73 m(2) of body surface area at enrollment who received the trial drug bardoxolone methyl versus placebo. Unfortunately, subsequent phase IIIb trials failed to show that the drug is a safe alternative renoprotective agent. Current renoprotection paradigms depend wholly and entirely on angiotensin blockade; however, these agents [angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs)] have proved to be imperfect renoprotective agents. In this review, we examine the mechanistic limitations of the various previous randomized controlled trials on CKD renoprotection, including the paucity of veritable, elaborate and systematic assessment methods for the documentation and reporting of individual patient-level, drug-related adverse events. We review the evidence base for the presence of putative, multiple independent and unrelated pathogenetic mechanisms that drive (diabetic and non-diabetic) CKD progression. Furthermore, we examine the validity, or lack thereof, of the hyped notion that the blockade of a single molecule (angiotensin II), which can only antagonize the angiotensin cascade, would veritably successfully, consistently and unfailingly deliver adequate and qualitative renoprotection results in (diabetic and non-diabetic) CKD patients. We clearly posit that there is this overarching impetus to arrive at the inference that multiple, disparately diverse and independent pathways, including any veritable combination of the mechanisms that we examine in this review, and many more others yet to be identified, do concurrently and asymmetrically contribute to CKD initiation and propagation to end-stage renal disease (ESRD) in our CKD patients. We conclude that current knowledge of CKD initiation and progression to ESRD, the natural history of CKD and the impacts of acute kidney injury on this continuum remain in their infancy and call for more research. Finally, we suggest a new classification scheme for renoprotective agents: (1) the single-pathway blockers that block a single putative pathogenetic pathway involved in CKD progression, as typified by ACE inhibitors and/or ARBs, and (2) the multiple-pathway blockers that are able to block or antagonize the effects of multiple pathogenetic pathways through their ability to simultaneously block, downstream, the effects of several pathways or mechanisms of CKD to ESRD progression and could therefore concurrently interfere with several unrelated upstream pathways or mechanisms. We surmise that maybe the ideal and truly renoprotective agent, clearly a multiple-pathway blocker, is on the horizon. This calls for more research efforts from all.

The histidine triad nucleotide-binding protein 1 supports mu-opioid receptor-glutamate NMDA receptor cross-regulation

A series of pharmacological and physiological studies have demonstrated the functional cross-regulation between MOR and NMDAR. These receptors coexist at postsynaptic sites in midbrain periaqueductal grey (PAG) neurons, an area implicated in the analgesic effects of opioids like morphine. In this study, we found that the MOR-associated histidine triad nucleotide-binding protein 1 (HINT1) is essential for maintaining the connection between the NMDAR and MOR. Morphine-induced analgesic tolerance is prevented and even rescued by inhibiting PKC or by antagonizing NMDAR. However, in the absence of HINT1, the MOR becomes supersensitive to morphine before suffering a profound and lasting desensitization that is refractory to PKC inhibition or NMDAR antagonism. Thus, HINT1 emerges as a key protein that is critical for sustaining NMDAR-mediated regulation of MOR signaling strength. Thus, HINT1 deficiency may contribute to opioid-intractable pain syndromes by causing long-term MOR desensitization via mechanisms independent of NMDAR.

Parkin is a lipid-responsive regulator of fat uptake in mice and mutant human cells

It has long been hypothesized that abnormalities in lipid biology contribute to degenerative brain diseases. Consistent with this, emerging epidemiologic evidence links lipid alterations with Parkinson disease (PD), and disruption of lipid metabolism has been found to predispose to α-synuclein toxicity. We therefore investigated whether Parkin, an E3 ubiquitin ligase found to be defective in patients with early onset PD, regulates systemic lipid metabolism. We perturbed lipid levels by exposing Parkin+/+ and Parkin-/- mice to a high-fat and -cholesterol diet (HFD). Parkin-/- mice resisted weight gain, steatohepatitis, and insulin resistance. In wild-type mice, the HFD markedly increased hepatic Parkin levels in parallel with lipid transport proteins, including CD36, Sr-B1, and FABP. These lipid transport proteins were not induced in Parkin-/- mice. The role of Parkin in fat uptake was confirmed by increased oleate accumulation in hepatocytes overexpressing Parkin and decreased uptake in Parkin-/- mouse embryonic fibroblasts and patient cells harboring complex heterozygous mutations in the Parkin-encoding gene PARK2. Parkin conferred this effect, in part, via ubiquitin-mediated stabilization of the lipid transporter CD36. Reconstitution of Parkin restored hepatic fat uptake and CD36 levels in Parkin-/- mice, and Parkin augmented fat accumulation during adipocyte differentiation. These results demonstrate that Parkin is regulated in a lipid-dependent manner and modulates systemic fat uptake via ubiquitin ligase-dependent effects. Whether this metabolic regulation contributes to premature Parkinsonism warrants investigation.

RGSZ2 binds to the neural nitric oxide synthase PDZ domain to regulate mu-opioid receptor-mediated potentiation of the N-methyl-D-aspartate receptor-calmodulin-dependent protein kinase II pathway

Morphine increases the production of nitric oxide (NO) via the phosphoinositide 3-kinase/Akt/neural nitric oxide synthase (nNOS) pathway. Subsequently, NO enhances N-methyl-D-aspartate receptor (NMDAR)/calmodulin-dependent protein kinase II (CaMKII) cascade, diminishing the strength of morphine-activated Mu-opioid receptor (MOR) signaling. During this process, NO signaling is restricted by the association of nNOS to the MOR.

AIMS

Here, we examined how nNOS/NO signaling is downregulated by the morphine-activated MOR and how this regulation affects antinociception.

RESULTS

Accordingly, we show that the MOR-NMDAR regulatory loop relies on the negative control of nNOS activity exerted by RGSZ2, a protein physically coupled to the MOR. This regulation requires binding of the nNOS N terminal PDZ domain to the RGSZ2 PDZ binding motifs that lie upstream of the RGS box.

INNOVATION

Indeed, in RGSZ2-deficient mice morphine over-stimulates the nNOS/NO/NMDAR/CaMKII pathway, causing analgesic tolerance to develop rapidly. Recovery of RGSZ2 levels or inhibition of nNOS, protein kinase C, NMDAR, or CaMKII function restores MOR signaling and morphine recovers its full analgesic potency.

CONCLUSION

This RGSZ2-dependent regulation of NMDAR activity is relevant to persistent pain disorders associated with heightened NMDAR-mediated glutamate responses and the reduced antinociceptive capacity of opioids.

Detecting purely epistatic multi-locus interactions by an omnibus permutation test on ensembles of two-locus analyses

BACKGROUND

Purely epistatic multi-locus interactions cannot generally be detected via single-locus analysis in case-control studies of complex diseases. Recently, many two-locus and multi-locus analysis techniques have been shown to be promising for the epistasis detection. However, exhaustive multi-locus analysis requires prohibitively large computational efforts when problems involve large-scale or genome-wide data. Furthermore, there is no explicit proof that a combination of multiple two-locus analyses can lead to the correct identification of multi-locus interactions.

RESULTS

The proposed 2LOmb algorithm performs an omnibus permutation test on ensembles of two-locus analyses. The algorithm consists of four main steps: two-locus analysis, a permutation test, global p-value determination and a progressive search for the best ensemble. 2LOmb is benchmarked against an exhaustive two-locus analysis technique, a set association approach, a correlation-based feature selection (CFS) technique and a tuned ReliefF (TuRF) technique. The simulation results indicate that 2LOmb produces a low false-positive error. Moreover, 2LOmb has the best performance in terms of an ability to identify all causative single nucleotide polymorphisms (SNPs) and a low number of output SNPs in purely epistatic two-, three- and four-locus interaction problems. The interaction models constructed from the 2LOmb outputs via a multifactor dimensionality reduction (MDR) method are also included for the confirmation of epistasis detection. 2LOmb is subsequently applied to a type 2 diabetes mellitus (T2D) data set, which is obtained as a part of the UK genome-wide genetic epidemiology study by the Wellcome Trust Case Control Consortium (WTCCC). After primarily screening for SNPs that locate within or near 372 candidate genes and exhibit no marginal single-locus effects, the T2D data set is reduced to 7,065 SNPs from 370 genes. The 2LOmb search in the reduced T2D data reveals that four intronic SNPs in PGM1 (phosphoglucomutase 1), two intronic SNPs in LMX1A (LIM homeobox transcription factor 1, alpha), two intronic SNPs in PARK2 (Parkinson disease (autosomal recessive, juvenile) 2, parkin) and three intronic SNPs in GYS2 (glycogen synthase 2 (liver)) are associated with the disease. The 2LOmb result suggests that there is no interaction between each pair of the identified genes that can be described by purely epistatic two-locus interaction models. Moreover, there are no interactions between these four genes that can be described by purely epistatic multi-locus interaction models with marginal two-locus effects. The findings provide an alternative explanation for the aetiology of T2D in a UK population.

CONCLUSION

An omnibus permutation test on ensembles of two-locus analyses can detect purely epistatic multi-locus interactions with marginal two-locus effects. The study also reveals that SNPs from large-scale or genome-wide case-control data which are discarded after single-locus analysis detects no association can still be useful for genetic epidemiology studies.