Impaired Glucose Homeostasis in a Tau Knock-In Mouse Model

Alzheimer’s disease (AD) is the leading cause of dementia. While impaired glucose homeostasis has been shown to increase AD risk and pathological loss of tau function, the latter has been suggested to contribute to the emergence of the glucose homeostasis alterations observed in AD patients. However, the links between tau impairments and glucose homeostasis, remain unclear. In this context, the present study aimed at investigating the metabolic phenotype of a new tau knock-in (KI) mouse model, expressing, at a physiological level, a human tau protein bearing the P301L mutation under the control of the endogenous mouse Mapt promoter. Metabolic investigations revealed that, while under chow diet tau KI mice do not exhibit significant metabolic impairments, male but not female tau KI animals under High-Fat Diet (HFD) exhibited higher insulinemia as well as glucose intolerance as compared to control littermates. Using immunofluorescence, tau protein was found colocalized with insulin in the β cells of pancreatic islets in both mouse (WT, KI) and human pancreas. Isolated islets from tau KI and tau knock-out mice exhibited impaired glucose-stimulated insulin secretion (GSIS), an effect recapitulated in the mouse pancreatic β-cell line (MIN6) following tau knock-down. Altogether, our data indicate that loss of tau function in tau KI mice and, particularly, dysfunction of pancreatic β cells might promote glucose homeostasis impairments and contribute to metabolic changes observed in AD.

Farnesoid X Receptor Activation in Brain Alters Brown Adipose Tissue Function via the Sympathetic System

The nuclear bile acid (BA) receptor farnesoid X receptor (FXR) is a major regulator of metabolic/energy homeostasis in peripheral organs. Indeed, enterohepatic-expressed FXR controls metabolic processes (BA, glucose and lipid metabolism, fat mass, body weight). The central nervous system (CNS) regulates energy homeostasis in close interaction with peripheral organs. While FXR has been reported to be expressed in the brain, its function has not been studied so far. We studied the role of FXR in brain control of energy homeostasis by treating wild-type and FXR-deficient mice by intracerebroventricular (ICV) injection with the reference FXR agonist GW4064. Here we show that pharmacological activation of brain FXR modifies energy homeostasis by affecting brown adipose tissue (BAT) function. Brain FXR activation decreases the rate-limiting enzyme in catecholamine synthesis, tyrosine hydroxylase (TH), and consequently the sympathetic tone. FXR activation acts by inhibiting hypothalamic PKA-CREB induction of TH expression. These findings identify a function of brain FXR in the control of energy homeostasis and shed new light on the complex control of energy homeostasis by BA through FXR.

CDKN2A/p16INK4a suppresses hepatic fatty acid oxidation through the AMPKα2-SIRT1-PPARα signaling pathway

In addition to their well-known role in the control of cellular proliferation and cancer, cell cycle regulators are increasingly identified as important metabolic modulators. Several GWAS have identified SNPs near CDKN2A, the locus encoding for p16INK4a (p16), associated with elevated risk for cardiovascular diseases and type-2 diabetes development, two pathologies associated with impaired hepatic lipid metabolism. Although p16 was recently shown to control hepatic glucose homeostasis, it is unknown whether p16 also controls hepatic lipid metabolism. Using a combination of in vivo and in vitro approaches, we found that p16 modulates fasting-induced hepatic fatty acid oxidation (FAO) and lipid droplet accumulation. In primary hepatocytes, p16-deficiency was associated with elevated expression of genes involved in fatty acid catabolism. These transcriptional changes led to increased FAO and were associated with enhanced activation of PPARα through a mechanism requiring the catalytic AMPKα2 subunit and SIRT1, two known activators of PPARα. By contrast, p16 overexpression was associated with triglyceride accumulation and increased lipid droplet numbers in vitro, and decreased ketogenesis and hepatic mitochondrial activity in vivo Finally, gene expression analysis of liver samples from obese patients revealed a negative correlation between CDKN2A expression and PPARA and its target genes. Our findings demonstrate that p16 represses hepatic lipid catabolism during fasting and may thus participate in the preservation of metabolic flexibility.

Brain insulin response and peripheral metabolic changes in a Tau transgenic mouse model

Accumulation of hyper-phosphorylated and aggregated Tau proteins is a neuropathological hallmark of Alzheimer's Disease (AD) and Tauopathies. AD patient brains also exhibit insulin resistance. Whereas, under normal physiological conditions insulin signaling in the brain mediates plasticity and memory formation, it can also regulate peripheral energy homeostasis. Thus, in AD, brain insulin resistance affects both cognitive and metabolic changes described in these patients. While a role of Aβ oligomers and APOE4 towards the development of brain insulin resistance emerged, contribution of Tau pathology has been largely overlooked. Our recent data demonstrated that one of the physiological function of Tau is to sustain brain insulin signaling. We postulated that under pathological conditions, hyper-phosphorylated/aggregated Tau is likely to lose this function and to favor the development of brain insulin resistance. This hypothesis was substantiated by observations from patient brains with pure Tauopathies. To address the potential link between Tau pathology and brain insulin resistance, we have evaluated the brain response to insulin in a transgenic mouse model of AD-like Tau pathology (THY-Tau22). Using electrophysiological and biochemical evaluations, we surprisingly observed that, at a time when Tau pathology and cognitive deficits are overt and obvious, the hippocampus of THY-Tau22 mice exhibits enhanced response to insulin. In addition, we demonstrated that the ability of i.c.v. insulin to promote body weight loss is enhanced in THY-Tau22 mice. In line with this, THY-Tau22 mice exhibited a lower body weight gain, hypoleptinemia and hypoinsulinemia and finally a metabolic resistance to high-fat diet. The present data highlight that the brain of transgenic Tau mice exhibit enhanced brain response to insulin. Whether these observations are ascribed to the development of Tau pathology, and therefore relevant to human Tauopathies, or unexpectedly results from the Tau transgene overexpression is debatable and discussed.

Tau deletion promotes brain insulin resistance

The molecular pathways underlying tau pathology-induced synaptic/cognitive deficits and neurodegeneration are poorly understood. One prevalent hypothesis is that hyperphosphorylation, misfolding, and fibrillization of tau impair synaptic plasticity and cause degeneration. However, tau pathology may also result in the loss of specific physiological tau functions, which are largely unknown but could contribute to neuronal dysfunction. In the present study, we uncovered a novel function of tau in its ability to regulate brain insulin signaling. We found that tau deletion leads to an impaired hippocampal response to insulin, caused by altered IRS-1 and PTEN (phosphatase and tensin homologue on chromosome 10) activities. Our data also demonstrate that tau knockout mice exhibit an impaired hypothalamic anorexigenic effect of insulin that is associated with energy metabolism alterations. Consistently, we found that tau haplotypes are associated with glycemic traits in humans. The present data have far-reaching clinical implications and raise the hypothesis that pathophysiological tau loss-of-function favors brain insulin resistance, which is instrumental for cognitive and metabolic impairments in Alzheimer's disease patients.

Cdkn2a/p16Ink4a regulates fasting-induced hepatic gluconeogenesis through the PKA-CREB-PGC1α pathway

Type 2 diabetes (T2D) is hallmarked by insulin resistance, impaired insulin secretion, and increased hepatic glucose production. The worldwide increasing prevalence of T2D calls for efforts to understand its pathogenesis in order to improve disease prevention and management. Recent genome-wide association studies have revealed strong associations between the CDKN2A/B locus and T2D risk. The CDKN2A/B locus contains genes encoding cell cycle inhibitors, including p16(Ink4a), which have not yet been implicated in the control of hepatic glucose homeostasis. Here, we show that p16(Ink4a) deficiency enhances fasting-induced hepatic glucose production in vivo by increasing the expression of key gluconeogenic genes. p16(Ink4a) downregulation leads to an activation of PKA-CREB-PGC1α signaling through increased phosphorylation of PKA regulatory subunits. Taken together, these results provide evidence that p16(Ink4a) controls fasting glucose homeostasis and could as such be involved in T2D development.

Detrimental effects of diet-induced obesity on τ pathology are independent of insulin resistance in τ transgenic mice

The τ pathology found in Alzheimer disease (AD) is crucial in cognitive decline. Midlife development of obesity, a major risk factor of insulin resistance and type 2 diabetes, increases the risk of dementia and AD later in life. The impact of obesity on AD risk has been suggested to be related to central insulin resistance, secondary to peripheral insulin resistance. The effects of diet-induced obesity (DIO) on τ pathology remain unknown. In this study, we evaluated effects of a high-fat diet, given at an early pathological stage, in the THY-Tau22 transgenic mouse model of progressive AD-like τ pathology. We found that early and progressive obesity potentiated spatial learning deficits as well as hippocampal τ pathology at a later stage. Surprisingly, THY-Tau22 mice did not exhibit peripheral insulin resistance. Further, pathological worsening occurred while hippocampal insulin signaling was upregulated. Together, our data demonstrate that DIO worsens τ phosphorylation and learning abilities in τ transgenic mice independently from peripheral/central insulin resistance.

Control of metabolism by nutrient-regulated nuclear receptors acting in the brain

Today, we are witnessing a rising incidence of obesity worldwide. This increase is due to a sedentary life style, an increased caloric intake and a decrease in physical activity. Obesity contributes to the appearance of type 2 diabetes, dyslipidemia and cardiovascular complications due to atherosclerosis, and nephropathy. Therefore, the development of new therapeutic strategies may become a necessity. Given the metabolism controlling properties of nuclear receptors in peripheral organs (such as liver, adipose tissues, pancreas) and their implication in various processes underlying metabolic diseases, they constitute interesting therapeutic targets for obesity, dyslipidemia, cardiovascular disease and type 2 diabetes. The recent identification of the central nervous system as a player in the control of peripheral metabolism opens new avenues to our understanding of the pathophysiology of obesity and type 2 diabetes and potential novel ways to treat these diseases. While the metabolic functions of nuclear receptors in peripheral organs have been extensively investigated, little is known about their functions in the brain, in particular with respect to brain control of energy homeostasis. This review provides an overview of the relationships between nuclear receptors in the brain, mainly at the hypothalamic level, and the central regulation of energy homeostasis. In this context, we will particularly focus on the role of PPARα, PPARγ, LXR and Rev-erbα.

Bone marrow p16INK4a-deficiency does not modulate obesity, glucose homeostasis or atherosclerosis development

Objective

A genomic region near the CDKN2A locus, encoding p16^INK4a, has been associated to type 2 diabetes and atherosclerotic vascular disease, conditions in which inflammation plays an important role. Recently, we found that deficiency of p16^INK4a results in decreased inflammatory signaling in murine macrophages and that p16^INK4a influences the phenotype of human adipose tissue macrophages. Therefore, we investigated the influence of immune cell p16^INK4a on glucose tolerance and atherosclerosis in mice.

Methods and Results

Bone marrow p16^INK4a-deficiency in C57Bl6 mice did not influence high fat diet-induced obesity nor plasma glucose and lipid levels. Glucose tolerance tests showed no alterations in high fat diet-induced glucose intolerance. While bone marrow p16^INK4a-deficiency did not affect the gene expression profile of adipose tissue, hepatic expression of the alternative markers Chi3l3, Mgl2 and IL10 was increased and the induction of pro-inflammatory Nos2 was restrained on the high fat diet. Bone marrow p16^INK4a-deficiency in low density lipoprotein receptor-deficient mice did not affect western diet-induced atherosclerotic plaque size or morphology. In line, plasma lipid levels remained unaffected and p16^INK4a-deficient macrophages displayed equal cholesterol uptake and efflux compared to wild type macrophages.

Conclusion

Bone marrow p16^INK4a-deficiency does not affect plasma lipids, obesity, glucose tolerance or atherosclerosis in mice.

PPARα activation differently affects microparticle content in atherosclerotic lesions and liver of a mouse model of atherosclerosis and NASH

BACKGROUND

Atherosclerosis and non-alcoholic fatty liver disease (NAFLD) are complex pathologies characterized by lipid accumulation, chronic inflammation and extensive tissue remodelling. Microparticles (MPs), small membrane vesicles produced by activated and apoptotic cells, might not only be biomarkers, but also functional actors in these pathologies. The apoE2-KI mouse is a model of atherosclerosis and NAFLD. Activation of the nuclear receptor PPARα decreases atherosclerosis and components of non-alcoholic steatohepatitis (NASH) in the apoE2-KI mouse.

OBJECTIVES

(1) To determine whether MPs are present in atherosclerotic lesions, liver and plasma during atherosclerosis and NASH progression in apoE2-KI mice, and (2) to study whether PPARα activation modulates MP concentrations.

METHODS

ApoE2-KI mice were fed a Western diet to induce atherosclerosis and NASH. MPs were isolated from atherosclerotic lesions, liver and blood and quantified by flow cytometry.

RESULTS

An increase of MPs was observed in the atherosclerotic lesions and in the liver of apoE2-KI mice upon Western diet feeding. PPARα activation with fenofibrate decreased MP levels in the atherosclerotic lesions in a PPARα-dependent manner, but did not influence MP concentrations in the liver.

CONCLUSION

Here we report that MPs are present in atherosclerotic lesions and in the liver of apoE2-KI mice. Their concentration increased during atherosclerosis and NASH development. PPARα activation differentially modulates MP levels in a tissue-specific manner.

[Mechanisms of neuronal death in Huntington's disease. Second part: therapeutic challenges]

Huntington's disease is caused by an abnormal CAG expansion within the gene encoding Huntingtin which induces a major cortico-striatal degeneration as well as motor and cognitive impairments. Since the discovery of the present mutation, a number of experimental data have been collected to uncover the physiopathological consequences of mutated Huntingtin expression. Here, we review the therapeutic challenges of Huntington's disease.

[Mechanisms of neuronal death in Huntington's disease. First part: general considerations and histopathological features]

Huntington's disease is caused by an abnormal CAG expansion within the gene encoding Huntingtin which induces a major cortico-striatal degeneration as well as motor and cognitive impairments. Since the discovery of the present mutation, a number of experimental data have been collected to uncover the physiopathological consequences of mutated Huntingtin expression. Here, we review the molecular and cellular mechanisms underlying and show how this better knowledge can be translate to clinical trials in patients.

Effects of the adenosine A2A receptor antagonist SCH 58621 on cyclooxygenase-2 expression, glial activation, and brain-derived neurotrophic factor availability in a rat model of striatal neurodegeneration

Inhibition of adenosine A2A receptors (A2ARs) is neuroprotective in several experimental models of striatal diseases. However, the mechanisms elicited by A2AR blockade are only partially known, and critical aspects about the potential beneficial effects of A2AR antagonism in models of neurodegeneration still await elucidation. In the present study, we analyzed the influence of the selective A2AR antagonist SCH 58261 in a rat model of striatal excitotoxicity obtained by unilateral intrastriatal injection of quinolinic acid (QA). We found that SCH 58261 differently affected the expression of cyclooxygenase-2 (COX-2) induced by QA in cortex and striatum. The antagonist enhanced COX-2 expression in cortical neurons and prevented it in striatal microglia-like cells. Similarly, SCH 58261 differently regulated astrogliosis and microglial activation in the 2 brain regions. In addition, the A2AR antagonist prevented the QA-induced increase in striatal brain-derived neurotrophic factor levels. Because COX-2 activity has been linked to excitotoxic processes and because brain-derived neurotrophic factor depletion has been observed in mouse models as well as in patients with Huntington disease, we suggest that the final outcome of A2AR blockade (namely neuroprotection vs neurodegeneration) is likely to depend on the balance among its various and region-specific effects.

Unilateral induction of progenitors in the spinal cord of hSOD1G93A transgenic rats correlates with an asymmetrical hind limb paralysis

Transgenic rats expressing a mutated form of the human Cu/Zn superoxide dismutase (hSOD1(G93A)) develop an amyotrophic lateral sclerosis (ALS)-like phenotype, including motor neurone degeneration and reactive gliosis in the spinal cord. This study aimed at examining the presence of endogenous neural progenitors in the lumbar spinal cord of these rats at the end-stage of the disease. Immunohistochemical data clearly demonstrated the induced expression of the stem cell factor reported as a chemoattractant and survival factor for neural stem cells as well as nestin (neuro-epithelial stem cell intermediate filament) in the spinal cord sections. While the stem cell factor immunolabelling appeared diffuse throughout the gray matter, nestin labelling was restricted to clusters within the ventral horn. Interestingly, as paralysis regularly develops asymmetrically, induction of nestin was only detected on the ipsilateral side of the predominant symptoms. Finally, immunohistochemical detection of the stem cell factor receptor (c-Kit) revealed its specific induction which coincided with nestin immunolabelling. Together, these results are indicative of endogenous recruitment of neural progenitors within lesioned tissues and could support the development of treatments involving endogenous or exogenous stem cells.

Minocycline in phenotypic models of Huntington's disease

Minocycline has been shown to be neuroprotective in various models of neurodegenerative diseases. However, its potential in Huntington's disease (HD) models characterized by calpain-dependent degeneration and inflammation has not been investigated. Here, we have tested minocycline in phenotypic models of HD using 3-nitropropionic acid (3NP) intoxication and quinolinic acid (QA) injections. In the 3NP rat model, where the development of striatal lesions involves calpain, we found that minocycline was not protective, although it attenuated the development of inflammation induced after the onset of striatal degeneration. The lack of minocycline activity on calpain-dependent cell death was also confirmed in vitro using primary striatal cells. Conversely, we found that minocycline reduced lesions and inflammation induced by QA. In cultured cells, minocycline protected against mutated huntingtin and staurosporine, stimulations known to promote caspase-dependent cell death. Altogether, these data suggested that, in HD, minocycline may counteract the development of caspase-dependent neurodegeneration, inflammation, but not calpain-dependent neuronal death.

Dopamine D3 receptor stimulation promotes the proliferation of cells derived from the post-natal subventricular zone

In the adult mammalian brain, neural stem cells persist in the subventricular zone (SVZ) where dopamine D3 receptors are expressed. Here, we demonstrate that addition of 1 microm apomorphine increases cell numbers in post-natal SVZ cell cultures. This effect was prevented by a co-treatment with haloperidol, sulpiride or U-99194A, a D3-preferring antagonist, and mimicked by the dopamine D3 receptor selective agonist 7-hydroxy-dipropylaminotetralin (7-OH-DPAT). EC50 values were 4.04 +/- 1.54 nm for apomorphine and 0.63 +/- 0.13 nm for 7-OH-DPAT, which fits the pharmacological profile of the D3 receptor. D3 receptors were detected in SVZ cells by RT-PCR and immunocytochemistry. D3 receptors were expressed in numerous beta-III tubulin immunopositive cells. The fraction of apoptotic nuclei remained unchanged following apomorphine treatment, thus ruling out any possible effect on cell survival. In contrast, proliferation was increased as both the proportion of nuclei incorporating bromo-deoxyuridine and the expression of the cell division marker cyclin D1 were enhanced. These findings provide support for a regulatory role of dopamine over cellular dynamics in post-natal SVZ.

Death of cortical and striatal neurons induced by mitochondrial defect involves differential molecular mechanisms

An important aspect of Huntington's disease (HD) pathogenesis which may have important therapeutic implications is that the cellular events leading to cell death may be different in cortical and striatal neurons. In the present study, we characterized cellular changes in cortical and striatal neurons treated with the mitochondrial toxin 3-nitropropionic acid (3NP) in culture. Degeneration induced by 3NP was similar in both striatal and cortical neurons as observed using markers of cell viability and DNA fragmentation. However, in striatal neurons, 3NP produced a marked delocalization of Bad, Bax, cytochrome c and Smac while this was not observed in cortical neurons. Death of striatal neurons was preceded by activation of calpain and was blocked by calpain inhibitor I. In cortical neurons, calpain was not activated and calpain inhibitor I was without effect. In both cell types, caspase-9 and -3 were not activated by 3NP and the caspase inhibitor zVAD-fmk did not provide neuroprotective effect. Interestingly, treatment with staurosporine (STS) triggered caspase-9 and -3 in cortical and striatal cells, suggesting that the molecular machinery related to caspase-dependent apoptosis was functional in both cell types even though this machinery was not involved in 3NP toxicity. The present results clearly demonstrate that under mitochondrial inhibition, striatal and cortical neurons die through different pathways. This suggests that mitochondrial defects in HD may trigger the death of cortical and striatal neurons through different molecular events.

A dual role of adenosine A2A receptors in 3-nitropropionic acid-induced striatal lesions: implications for the neuroprotective potential of A2A antagonists

Reduction of A2A receptor expression is one of the earliest events occurring in both Huntington's disease (HD) patients and mice overexpressing the N-terminal part of mutated huntingtin. Interestingly, increased activity of A2A receptors has been found in striatal cells prone to degenerate in experimental models of this neurodegenerative disease. However, the role of A2A receptors in the pathogenesis of HD remains obscure. In the present study, using A2A-/- mice and pharmacological compounds in rat, we demonstrate that striatal neurodegeneration induced by the mitochondrial toxin 3-nitropropionic acid (3NP) is regulated by A2A receptors. Our results show that the striatal outcome induced by 3NP depends on a balance between the deleterious activity of presynaptic A2A receptors and the protective activity of postsynaptic A2A receptors. Moreover, microdialysis data demonstrate that this balance is anatomically determined, because the A2A presynaptic control on striatal glutamate release is absent within the posterior striatum. Therefore, because blockade of A2A receptors has differential effects on striatal cell death in vivo depending on its ability to modulate presynaptic over postsynaptic receptor activity, therapeutic use of A2A antagonists in Huntington's as well as in other neurodegenerative diseases could exhibit undesirable biphasic neuroprotective-neurotoxic effects.

The adenosine A1 receptor agonist adenosine amine congener exerts a neuroprotective effect against the development of striatal lesions and motor impairments in the 3-nitropropionic acid model of neurotoxicity

Huntington's disease is a genetic neurodegenerative disorder characterized clinically by both motor and cognitive impairments and striatal lesions. At present, there are no pharmacological treatments able to prevent or slow its development. In the present study, we report the neuroprotective effect of adenosine amine congener (ADAC), a specific A1 receptor agonist known to be devoid of any of the side effects that usually impair the clinical use of such compounds. Remarkably, in a rat model of Huntington's disease generated by subcutaneous infusion of the mitochondrial inhibitor 3-nitropropionic acid (3NP), we have observed that an acute treatment with ADAC (100 microg x kg(-1) x d(-1)) not only strongly reduces the size of the striatal lesion (-40%) and the remaining ongoing striatal degeneration (-30%), but also prevents the development of severe dystonia of hindlimbs. Electrophysiological recording on corticostriatal brain slices demonstrated that ADAC strongly decreases the field EPSP amplitude by 70%, whereas it has no protective effect up to 1 microm against the 3NP-induced neuronal death in primary striatal cultures. This suggests that ADAC protective effects may be mediated presynaptically by the modulation of the energetic impairment-induced striatal excitotoxicity. Altogether, our results indicate that A1 receptor agonists deserve further experimental evaluation in animal models of Huntington's disease.