Microglial Rac1 is essential for experience-dependent brain plasticity and cognitive performance

Microglia, the largest population of brain immune cells, continuously interact with synapses to maintain brain homeostasis. In this study, we use conditional cell-specific gene targeting in mice with multi-omics approaches and demonstrate that the RhoGTPase Rac1 is an essential requirement for microglia to sense and interpret the brain microenvironment. This is crucial for microglia-synapse crosstalk that drives experience-dependent plasticity, a fundamental brain property impaired in several neuropsychiatric disorders. Phosphoproteomics profiling detects a large modulation of RhoGTPase signaling, predominantly of Rac1, in microglia of mice exposed to an environmental enrichment protocol known to induce experience-dependent brain plasticity and cognitive performance. Ablation of microglial Rac1 affects pathways involved in microglia-synapse communication, disrupts experience-dependent synaptic remodeling, and blocks the gains in learning, memory, and sociability induced by environmental enrichment. Our results reveal microglial Rac1 as a central regulator of pathways involved in the microglia-synapse crosstalk required for experience-dependent synaptic plasticity and cognitive performance.

Postnatal Overfeeding in Rodents Induces a Neurodevelopment Delay and Anxious-like Behaviour Accompanied by Sex- and Brain-Region-Specific Synaptic and Metabolic Changes

Nutritional disturbances during the early postnatal period can have long-lasting effects on neurodevelopment and may be related to behavioural changes at adulthood. While such neuronal connection disruption can contribute to social and behaviour alterations, the dysregulation of the neuroendocrine pathways involved in nutrient-sensing balance may also cause such impairments, although the underlying mechanisms are still unclear. We aimed to evaluate sex-specific neurodevelopmental and behavioural changes upon postnatal overfeeding and determine the potential underpinning mechanisms at the central nervous system level, with a focus on the interconnection between synaptic and neuroendocrine molecular alterations. At postnatal day 3 (PND3) litters were culled to three animals (small litter procedure). Neurodevelopmental tests were conducted at infancy, whereas behavioural tests to assess locomotion, anxiety, and memory were performed at adolescence, together with molecular analysis of the hippocampus, hypothalamus, and prefrontal cortex. At infancy, females presented impaired acquisition of an auditory response, eye opening, olfactory discrimination, and vestibular system development, suggesting that female offspring neurodevelopment/maturation was deeply affected. Male offspring presented a transitory delay in locomotor performance., while both offspring had lower upper limb strength. At adolescence, both sexes presented anxious-like behaviour without alterations in short-term memory retention. Both males and females presented lower NPY1R levels in a region-specific manner. Furthermore, both sexes presented synaptic changes in the hippocampus (lower GABAA in females and higher GABAA levels in males), while, in the prefrontal cortex, similar higher GABAA receptor levels were observed. At the hypothalamus, females presented synaptic changes, namely higher vGLUT1 and PSD95 levels. Thus, we demonstrate that postnatal overfeeding modulates offspring behaviour and dysregulates nutrient-sensing mechanisms such as NPY and GABA in a sex- and brain-region-specific manner.

Maternal diabetes affects rat offspring retinal structure and function: Sex-specific vulnerabilities at infancy

AIMS

Maternal diabetes negatively impacts the offspring's brain, but little is known about its effects on the retina, which is also part of the central nervous system. We hypothesized that maternal diabetes adversely influences offspring retina development leading to structural and functional deficits.

MAIN METHODS

Retinal structure and function were evaluated at infancy, by optical coherence tomography and electroretinography, in male and female offspring of control, diabetic and diabetic-treated with insulin Wistar rats.

KEY FINDINGS

Maternal diabetes induced a delay in male and female offspring eye-opening, while insulin treatment expedited it. Structural analysis showed that maternal diabetes decreased the thickness of the inner and outer segment layer of photoreceptors in male offspring. Electroretinography also revealed that maternal diabetes decreased the amplitude of scotopic b-wave and flicker response in males, suggesting bipolar cells and cone photoreceptor dysfunction, an effect not observed in females. Conversely, maternal diabetes decreased cone arrestin protein levels in female retinas, while not affecting cone photoreceptor number. Dam insulin therapy was efficient in preventing the offspring photoreceptor changes.

SIGNIFICANCE

Our results suggest that photoreceptors are affected by maternal diabetes, which may account for visual impairments at infancy. Notably, both male and female offspring presented specific vulnerabilities to hyperglycemia in this sensitive period of development.

Sex-specificities in offspring neurodevelopment and behaviour upon maternal glycation: Putative underlying neurometabolic and synaptic changes

AIM

Lactation is an important programming window for metabolic disease and neuronal alterations later in life. We aimed to study the effect of maternal glycation during lactation on offspring neurodevelopment and behaviour, assessing possible sex differences and underpinning molecular players.

METHODS

Female Wistar rats were treated with the Glyoxalase-1 inhibitor S-p-Bromobenzylguthione cyclopentyl diester (BBGC 5 mg/kg). A control and vehicle group treated with dimethyl sulfoxide were considered. Male and female offspring were tested at infancy for neurodevelopment hallmarks. After weaning, triglycerides and total antioxidant capacity were measured in breast milk. At adolescence, offspring were tested for locomotor ability, anxious-like behaviour, and recognition memory. Metabolic parameters were assessed, and the hippocampus and prefrontal cortex were collected for molecular analysis.

KEY FINDINGS

Maternal glycation reduced triglycerides and total antioxidant capacity levels in breast milk. At infancy, both male and female offspring presented an anticipation on the achievement of neurodevelopmental milestones. At adolescence, male offspring exposed to maternal glycation presented hyperlocomotion, whereas offspring of both sexes presented a risk-taking phenotype, accompanied by GABA_A receptor upregulation in the hippocampus. Females also demonstrated GABA_A and PSD-95 changes in prefrontal cortex. Furthermore, lower levels of GLO1 and consequently higher accumulation of AGES were also observed in both male and female offspring hippocampus.

SIGNIFICANCE

Early exposure to maternal glycation induces changes in milk composition leading to neurodevelopment changes at infancy, and sex-specific behavioural and neurometabolic changes at adolescence, further evidencing that lactation period is a critical metabolic programming window and in sculpting behaviour.

Neonatal testosterone voids sexually differentiated microglia morphology and behavior

The involvement of immunity in psychiatric disorders, such as anxiety, is typified by the morphologic adaptation of microglia, immune cells of the brain, to anxiogenic stimuli. We previously reported sexually differentiated microglia morphology in adult rodents, in brain locations implicated in anxiety, including the pre-frontal cortex. These physiologic differences likely drive sex-dependent patterns of microglia morphologic remodeling in response to varied stress conditions in different periods of life, that correlate with sex-dependent behavioral adaptation to anxiogenic stimuli. The time-window of appearance of sex differences in microglia, correlating with sex-specific behavioral performance in anxiogenic conditions are still unknown. In rodents, a postnatal peak of the sexual hormone testosterone is determinant for the so-called brain masculinization and sex-determined behavioral traits. In the present work we aim to clarify if differences in microglia morphology are present at birth or can be driven by postnatal testosterone and impacts on the ability to deal with an anxiogenic context. Differences in microglia morphology are not present at birth, but are observable at adolescence (increased complexity of male microglia, particularly in branches more proximal to the soma), when differences in behavior are also observed. Our data also show that adolescent females neonatally treated with testosterone exhibit masculinized microglia and behavior. Importantly, between adolescence and adulthood, a sex-determined shift in the pattern of complexity takes place and microglia from females become more complex. When testosterone is administered, this morphological effect is partially abolished, approximating microglia and behavior to the male phenotype.

The Duration of Stress Determines Sex Specificities in the Vulnerability to Depression and in the Morphologic Remodeling of Neurons and Microglia

Stress exposure has been shown to induce a variety of molecular and functional alterations associated with anxiety and depression. Some studies suggest that microglia, the immune cells of the brain, play a significant role in determining neuronal and behavioral responses to chronic stress and also contribute to the development of stress-related psychopathologies. However, little is known about the impact of the duration of stress exposure upon microglia and neurons morphology, particularly considering sex differences. This issue deserves particular investigation, considering that the process of morphologic remodeling of neurons and microglia is usually accompanied by functional changes with behavioral expression. Here, we examine the effects of short and long unpredictable chronic mild stress (uCMS) protocols on behavior, evaluating in parallel microglia and neurons morphology in the dorsal hippocampus (dHIP) and in the nucleus accumbens (NAc), two brain regions involved in the etiology of depression. We report that long-term uCMS induced more behavioral alterations in males, which present anxiety and depression-like phenotypes (anhedonia and helplessness behavior), while females only display anxiety-like behavior. After short-term uCMS, both sexes presented anxiety-like behavior. Microglia cells undergo a process of morphologic adaptation to short-term uCMS, dependent on sex, in the NAc: we observed a hypertrophy in males and an atrophy in females, transient effects that do not persist after long-term uCMS. In the dHIP, the morphologic adaptation of microglia is only observed in females (hypertrophy) and after the protocol of long uCMS. Interestingly, males are more vulnerable to neuronal morphological alterations in a region-specific manner: dendritic atrophy in granule neurons of the dHIP and hypertrophy in the medium spiny neurons of the NAc, both after short- or long-term uCMS. The morphology of neurons in these brain regions were not affected in females. These findings raise the possibility that, by differentially affecting neurons and microglia in dHIP and NAc, chronic stress may contribute for differences in the clinical presentation of stress-related disorders under the control of sex-specific mechanisms.

Sex-specific changes in peripheral metabolism in a model of chronic anxiety induced by prenatal stress

BACKGROUND

Prenatal stress is associated with increased susceptibility to psychiatric and metabolic disorders later in life. Prenatal exposure to stress mediators may have sex-dependent effects on offspring brain and metabolic function, promoting a sex-specific vulnerability to psychopathology and metabolic alterations at adulthood. In this work, the impact of prenatal stress on glucose homeostasis and peripheral metabolism of male and female offspring was investigated in a chronic anxiety animal model.

METHODS

Pregnant Wistar rats were injected with saline or glucocorticoid (dexamethasone: 1 mg/kg, subcutaneous) at gestational days 18 and 19. Male and female offspring weight was monitored, and anxious-like behaviour and peripheral insulin-sensitive tissues were analysed at adulthood.

RESULTS

At birth, females and males prenatally exposed to stress presented decreased body weight which remained low in females. At adulthood, a morphological disorganization of the Langerhans islets was observed in both sexes prenatally exposed to stress, yet not changes in insulin levels were detected. Also, prenatal stress increased glucose transporter 4 (GLUT-4) levels in female and male adipose tissues and decreased insulin receptor levels in the liver and skeleton muscle but only in females.

CONCLUSIONS

Exposure to stress mediators in critical periods of development negatively affects behaviour and metabolism. Prenatal stress programmes offspring peripheral metabolism in a sex-specific manner, emphasizing that the response to stress in critical periods of development may be sex-specific having each sex different vulnerabilities to psychiatric and metabolic disorders. Considering sex-specificities may provide critical clues for the design of preventive strategies and for early therapeutic intervention.

Microglia Dysfunction Caused by the Loss of Rhoa Disrupts Neuronal Physiology and Leads to Neurodegeneration

Nervous tissue homeostasis requires the regulation of microglia activity. Using conditional gene targeting in mice, we demonstrate that genetic ablation of the small GTPase Rhoa in adult microglia is sufficient to trigger spontaneous microglia activation, producing a neurological phenotype (including synapse and neuron loss, impairment of long-term potentiation [LTP], formation of β-amyloid plaques, and memory deficits). Mechanistically, loss of Rhoa in microglia triggers Src activation and Src-mediated tumor necrosis factor (TNF) production, leading to excitotoxic glutamate secretion. Inhibiting Src in microglia Rhoa-deficient mice attenuates microglia dysregulation and the ensuing neurological phenotype. We also find that the Rhoa/Src signaling pathway is disrupted in microglia of the APP/PS1 mouse model of Alzheimer disease and that low doses of Aβ oligomers trigger microglia neurotoxic polarization through the disruption of Rhoa-to-Src signaling. Overall, our results indicate that disturbing Rho GTPase signaling in microglia can directly cause neurodegeneration.

Resilience to stress and sex-specific remodeling of microglia and neuronal morphology in a rat model of anxiety and anhedonia

Prenatal exposure to stress or glucocorticoids (GC) is associated with the appearance of psychiatric diseases later in life. Microglia, the immune cells of the brain, are altered in stress-related disorders.

Synthetic GC such as dexamethasone (DEX) are commonly prescribed in case of preterm risk labour in order to promote fetal lung maturation. Recently, we reported long-lasting differences in microglia morphology in a model of in utero exposure to DEX (iuDEX), that presents an anxious phenotype. However, it is still unclear if stress differentially affects iuDEX males and females.

In this work, we evaluated how iuDEX animals of both sexes cope with chronic mild stress for 2 weeks. We evaluated emotional behavior and microglia and neuronal morphology in the dorsal hippocampus (dHIP) and nucleus accumbens (NAc), two brain regions involved in emotion-related disorders. We report that males and females prenatally exposed to DEX have better performance in anxiety- and depression-related behavioral tests after chronic stress exposure in adulthood than non-exposed animals. Interestingly, iuDEX animals present sex-dependent changes in microglia morphology in the dHIP (hypertrophy in females) and in the NAc (atrophy in females and hypertrophy in males). After chronic stress, these cells undergo sex-specific morphological remodeling. Paralleled to these alterations in cytoarchitecture of microglia, we report inter-regional differences in dendritic morphology in a sex-specific manner. iuDEX females present fewer complex neurons in the NAc, whereas iuDEX males presented less complex neuronal morphology in the dHIP. Interestingly, these alterations were modified by stress exposure.

Our work shows that stressful events during pregnancy can exert a preserved sex-specific effect in adulthood. Although the role of the observed cellular remodeling is still unknown, sex-specific differences in microglia plasticity induced by long-term stress exposure may anticipate differences in drug efficacy in the context of stress-induced anxiety- or depression-related behaviors.

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iuDEX induces anxiety- and depression-related behavioral in both sexes.

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iuDEX induces sex dependent fine structural alterations in neurons and microglia morphology in the dHIP and in the NAc.

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uCMS in combination to iuDEX normalize the behavior as well the morphology of neurons in the NAc.

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Stressful events during pregnancy can exert a preserved sex-specific effect in adulthood.

Transient gain of function of cannabinoid CB1 receptors in the control of frontocortical glucose consumption in a rat model of Type-1 diabetes

Here we aimed to unify some previous controversial reports on changes in both cannabinoid CB₁ receptor (CB₁R) expression and glucose metabolism in the forebrain of rodent models of diabetes. We determined how glucose metabolism and its modulation by CB₁R ligands evolve in the frontal cortex of young adult male Wistar rats, in the first 8 weeks of streptozotocin-induced type-1 diabetes (T1D). We report that frontocortical CB₁R protein density was biphasically altered in the first month of T1D, which was accompanied with a reduction of resting glucose uptake ex vivo in acute frontocortical slices that was normalized after eight weeks in T1D. This early reduction of glucose uptake in slices was also restored by ex vivo treatment with both the non-selective CB₁R agonists, WIN55212-2 (500 nM) and the CB₁R-selective agonist, ACEA (3 μM) while it was exacerbated by the CB₁R-selective antagonist, O-2050 (500 nM). These results suggest a gain-of-function for the cerebrocortical CB₁Rs in the control of glucose uptake in diabetes. Although insulin and IGF-1 receptor protein densities remained unaffected, phosphorylated GSKα and GSKβ levels showed different profiles 2 and 8 weeks after T1D induction in the frontal cortex. Altogether, the biphasic response in frontocortical CB₁R density within a month after T1D induction resolves previous controversial reports on forebrain CB₁R levels in T1D rodent models. Furthermore, this study also hints that cannabinoids may be useful to alleviate impaired glucoregulation in the diabetic cortex.

Sex differences in offspring neurodevelopment, cognitive performance and microglia morphology associated with maternal diabetes: Putative targets for insulin therapy

Diabetes during pregnancy has been shown to affect the central nervous system (CNS) of the offspring, resulting in short- and long-term adverse effects. Children of diabetic mothers are more likely to develop cognitive impairment, also having increased susceptibility to psychiatric disorders. Microglia, the immune cells of the CNS, work as sensors of environmental changes, namely metabolic challenges, as early as the intrauterine period. During this period, microglia is actively involved in processes of neurogenesis, synaptic pruning and detection of any environmental alteration that may impact brain development. The remarkable sex dimorphism in neurodevelopment, as well as sex differences in the morphology and immune function of microglia during development, led us to clarify if maternal diabetes affects specific behavioral traits and microglia morphology during infancy in a sex-specific manner. Another important goal of this study was to clarify if insulin, the gold standard treatment of diabetes during gestation, could prevent maternal diabetes-induced behavioral changes, as well as microglia morphology, also considering sex specificities. Other molecular and cellular players potentially involved in the link between changes in metabolism and behavior were also analyzed in the hippocampus, a brain region implicated in cognition and other behavioral outcomes. Diabetes during pregnancy globally delayed female and male offspring development and was associated with impairments in recognition memory, but only in female offspring. In line with these results, at early and late infancy, some molecular and cellular markers were altered in offspring hippocampus in a sex-specific manner. The strict control of glycemia by insulin during pregnancy prevented most of the negative effects induced by uncontrolled hyperglycemia. Notably, insulin administration to diabetic dams may also modulate offspring development in a way that differs from what is observed in physiological conditions, since it promoted the expedited acquisition of developmental milestones and of discrimination ability at memory test, also inducing a hyper-ramification of male and female hippocampal microglia. Importantly, this study highlights the importance of analyzing the impact of maternal diabetes and insulin therapy, taking into account sex differences, since male and female present different vulnerabilities to hyperglycemia in this critical period of life.

Microglia cytoarchitecture in the brain of adenosine A2A receptor knockout mice: Brain region and sex specificities

Microglia cells exert a critical role in brain development, mainly supported by their immune functions, which predicts an impact on the genesis of psychiatric disorders. In fact, microglia stress during gestation is, for instance, associated with chronic anxiety and cognitive deficits accompanied by long-lasting, region- and sex-specific changes in microglia morphology. We recently reported that the pattern of microglia morphologic plasticity, which is sex-determined, impacts on anxious-like behaviour and cognition. We also reported that the pharmacologic blockade of adenosine A_2A receptors (A_{2 A} R) is able to reshape microglia morphology, in a sex-specific manner and with behavioural sequelae. In order to better understand the role of A_{2 A} R in the sex differentiation of microglia, we now compared their morphology in wild-type and A_{2 A} R knockout male and female C57BL/6 mice in two cardinal brain regions implicated in anxiety-like behaviour and cognition, the prefrontal cortex (PFC) and the dorsal hippocampus (dHIP). We report interregional differences between PFC and dHIP in a sex-specific manner: while males presented more complex microglia in the dHIP, microglia from females had a more complex morphology in the PFC. Surprisingly, the genetic deletion of A_{2 A} R did not alter these sex differences, but promoted the exclusive remodelling (increase in complexity) in PFC microglia from females. These findings further support the existence of a heterogeneous microglial network, distinct between sexes and brain regions, and help characterizing the role of A_{2 A} R in the sex- and brain region-specific morphologic differentiation of microglia.

The Retina as a Window or Mirror of the Brain Changes Detected in Alzheimer's Disease: Critical Aspects to Unravel

Alzheimer's disease is the most frequent cause of dementia worldwide, representing a global health challenge, with a massive impact on the quality of life of Alzheimer's disease patients and their relatives. The diagnosis of Alzheimer's disease constitutes a real challenge, because the symptoms manifest years after the first degenerative changes occurring in the brain and the diagnosis is based on invasive and/or expensive techniques. Therefore, there is an urgent need to identify new reliable biomarkers to detect Alzheimer's disease at an early stage. Taking into account the evidence for visual deficits in Alzheimer's disease patients, sometimes even before the appearance of the first disease symptoms, and that the retina is an extension of the brain, the concept of the retina as a window to look into the brain or a mirror of the brain has received increasing interest in recent years. However, only a few studies have assessed the changes occurring in the retina and the brain at the same time points. Unlike previous reviews on this subject, which are mainly focused on brain changes, we organized this review by comprehensively summarizing findings related with structural, functional, cellular, and molecular parameters in the retina reported in both Alzheimer's disease patients and animal models. Moreover, we separated the studies that assessed only the retina, and those that assessed both the retina and brain, which are few but allow establishing correlations between the retina and brain. This review also highlights some inconsistent results in the literature as well as relevant missing gaps in this field.

Subtle thinning of retinal layers without overt vascular and inflammatory alterations in a rat model of prediabetes

PURPOSE

Diabetic retinopathy is a neurovascular disease characterized by increased permeability of the blood-retinal barrier, changes in the neural components of the retina, and low-grade chronic inflammation. Diabetic retinopathy is a major complication of diabetes; however, the impact of a prediabetic state on the retina remains to be elucidated. The aim of this study was to assess possible early retinal changes in prediabetic rats, by evaluating changes in the integrity of the blood-retinal barrier, the retinal structure, neural markers, and inflammatory mediators.

METHODS

Several parameters were analyzed in the retinas of Wistar rats that drank high sucrose (HSu; 35% sucrose solution during 9 weeks, the prediabetic animal model) and were compared with those of age-matched controls. The permeability of the blood-retinal barrier was assessed with the Evans blue assay, and the content of the tight junction proteins and neural markers with western blotting. Optical coherence tomography was used to evaluate retinal thickness. Cell loss at the ganglion cell layer was assessed with terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL) assay and by evaluating the immunoreactivity of the Brn3a transcription factor. To assess retinal neuroinflammation, the mRNA expression and protein levels of inducible nitric oxide synthase isoform (iNOS), interleukin-1 beta (IL-1β), and tumor necrosis factor (TNF) were evaluated. Iba1 and MHC-II immunoreactivity and translocator protein (TSPO) mRNA levels were assessed to study the microglial number and activation state.

RESULTS

The thickness of the inner retinal layers of the HSu-treated animals decreased. Nevertheless, no apoptotic cells were observed, and no changes in retinal neural markers were detected in the retinas of the HSu-treated animals. No changes were detected in the permeability of the blood-retinal barrier, as well as the tight junction protein content between the HSu-treated rats and the controls. In addition, the inflammatory parameters remained unchanged in the retina despite the tendency for an increase in the number of retinal microglial cells.

CONCLUSIONS

In a prediabetic rat model, the retinal structure is affected by the thinning of the inner layers, without overt vascular and inflammatory alterations. The results suggest neuronal dysfunction (thinning of the inner retina) that may precede or anticipate the vascular and inflammatory changes. Subtle structural changes might be viewed as early disturbances in an evolving disease, suggesting that preventive strategies (such as the modification of diet habits) could be applied at this stage, before the progression toward irreversible dysfunction and damage to the retina.

Adenosine A2A Receptor Blockade Modulates Glucocorticoid-Induced Morphological Alterations in Axons, But Not in Dendrites, of Hippocampal Neurons

The exposure to supra-physiological levels of glucocorticoids in prenatal life can lead to a long-term impact in brain cytoarchitecture, increasing the susceptibility to neuropsychiatric disorders. Dexamethasone, an exogenous glucocorticoid widely used in pregnant women in risk of preterm delivery, is associated with higher rates of neuropsychiatric conditions throughout life of the descendants. In animal models, prenatal dexamethasone exposure leads to anxious-like behavior and increased susceptibility to depressive-like behavior in adulthood, concomitant with alterations in neuronal morphology in brain regions implicated in the control of emotions and mood. The pharmacologic blockade of the purinergic adenosine A_2A receptor, which was previously described as anxiolytic, is also able to modulate neuronal morphology, namely in the hippocampus. Additionally, recent observations point to an interaction between glucocorticoid receptors (GRs) and adenosine A_2A receptors. In this work, we explored the impact of dexamethasone on neuronal morphology, and the putative implication of adenosine A_2A receptor in the mediation of dexamethasone effects. We report that in vitro hippocampal neurons exposed to dexamethasone (250 nM), in the early phases of development, exhibit a polarized morphology alteration: dendritic atrophy and axonal hypertrophy. While the effect of dexamethasone in the axon is dependent on the activation of adenosine A_2A receptor, the effect in the dendrites relies on the activation of GRs, regardless of the activation of adenosine A_2A receptor. These results support the hypothesis of the interaction between GRs and adenosine A_2A receptors and the potential therapeutic value of modulating adenosine A_2A receptors activation in order to prevent glucocorticoid-induced alterations in developing neurons.

High-fat diet induces a neurometabolic state characterized by changes in glutamate and N-acetylaspartate pools associated with early glucose intolerance: An in vivo multimodal MRI study

BACKGROUND

Type-2 diabetes mellitus (T2DM) is a metabolic disorder with a broad range of complications in the brain that depend on the conditions that precede its onset, such as obesity and metabolic syndromes. It has been suggested that neurotransmitter and metabolic perturbations may emerge even before the early stages of T2DM and that high-caloric intake could adversely influence the brain in such states. Notwithstanding, evidence for neurochemical and structural alterations in these conditions are still sparse and controversial.

PURPOSE

To evaluate the influence of high-fat diet in the neurochemical profile and structural integrity of the rodent brain.

STUDY TYPE

Prospective.

SUBJECTS

Wistar rats (n = 12/group).

FIELD STRENGTH/SEQUENCE

A PRESS, ISIS, RARE, and EPI sequences were performed at 9.4T.

ASSESSMENT

Neurochemical and structural parameters were assessed by magnetic resonance spectroscopy, voxel-based morphometry, volumetry, and diffusion tensor imaging.

STATISTICAL TESTS

Measurements were compared through Student and Mann-Whitney tests. Pearson correlation was used to assess relationships between parameters.

RESULTS

Animals submitted to high-caloric intake gained weight (P = 0.003) and developed glucose intolerance (P < 0.001) but not hyperglycemia. In the hippocampus, the diet induced perturbations in glutamatergic metabolites reflected by increased levels of glutamine (P = 0.016) and glutamatergic pool (Glx) (P = 0.036), which were negatively correlated with glucose intolerance (glutamine, r = -0.804, P = 0.029), suggesting a link with neurometabolic dysregulation. At caudate-putamen, high-fat diet led to a surprising increase in the pool of N-acetylaspartate (P = 0.028). A relation with metabolic changes was again suggested by the negative correlation between glucose intolerance and levels of glutamatergic metabolites in this region (glutamate, r = -0.845, P = 0.014; Glx, r = -0.834, P = 0.020). Neither changes in phosphate compounds nor major structural alterations were observed for both regions.

DATA CONCLUSION

We found evidence that high-fat diet-induced obesity leads to distinct early and region-specific metabolic/neurochemical imbalances in the presence of early glucose intolerance even when structural alterations or T2DM are absent.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018.

Adenosine A2A receptor regulation of microglia morphological remodeling-gender bias in physiology and in a model of chronic anxiety

Developmental risk factors, such as the exposure to stress or high levels of glucocorticoids (GCs), may contribute to the pathogenesis of anxiety disorders. The immunomodulatory role of GCs and the immunological fingerprint found in animals prenatally exposed to GCs point towards an interplay between the immune and the nervous systems in the etiology of these disorders. Microglia are immune cells of the brain, responsive to GCs and morphologically altered in stress-related disorders. These cells are regulated by adenosine A_2A receptors, which are also involved in the pathophysiology of anxiety. We now compare animal behavior and microglia morphology in males and females prenatally exposed to the GC dexamethasone. We report that prenatal exposure to dexamethasone is associated with a gender-specific remodeling of microglial cell processes in the prefrontal cortex: males show a hyper-ramification and increased length whereas females exhibit a decrease in the number and in the length of microglia processes. Microglial cells re-organization responded in a gender-specific manner to the chronic treatment with a selective adenosine A_2A receptor antagonist, which was able to ameliorate microglial processes alterations and anxiety behavior in males, but not in females.

Inside the Diabetic Brain: Role of Different Players Involved in Cognitive Decline

Diabetes mellitus is the most common metabolic disease, and its prevalence is increasing. A growing body of evidence, both in animal models and epidemiological studies, has demonstrated that metabolic diseases like obesity, insulin resistance, and diabetes are associated with alterations in the central nervous system (CNS), being linked with development of cognitive and memory impairments and presenting a higher risk for dementia and Alzheimer's disease. The rising prevalence of diabetes together with its increasing earlier onset suggests that diabetes-related cognitive dysfunction will increase in the near future, causing substantial socioeconomic impact. Decreased insulin secretion or action, dysregulation of glucose homeostasis, impairment in the hypothalamic-pituitary-adrenal axis, obesity, hyperleptinemia, and inflammation may act independently or synergistically to disrupt neuronal homeostasis and cause diabetes-associated cognitive decline. However, the crosstalk between those factors and the mechanisms underlying the diabetes-related CNS complications is still elusive. During the past few years, different strategies (neuroprotective and antioxidant drugs) have emerged as promising therapies for this complication, which still remains to be preventable or treatable. This Review summarizes fundamental past and ongoing research on diabetes-associated cognitive decline, highlighting potential contributors, mechanistic mediators, and new pharmacological approaches to prevent and/or delay this complication.

Long-term exposure to high glucose increases the content of several exocytotic proteins and of vesicular GABA transporter in cultured retinal neural cells

Diabetic retinopathy is a leading cause of vision loss and blindness. Increasing evidence has shown that the neuronal components of the retina are affected even before the detection of vascular lesions. Hyperglycemia is considered the main pathogenic factor for the development of diabetic complications. Nevertheless, other factors like neuroinflammation, might also contribute for neural changes. To clarify whether hyperglycemia can be the main trigger of synaptic changes, we evaluated whether prolonged elevated glucose per se, mimicking chronic hyperglycemia, is able to change the content and distribution of several exocytotic proteins and vesicular glutamate and GABA transporters in retinal neurons. Moreover, we also tested the hypothesis that an inflammatory stimulus (interleukin-1β) could exacerbate the effects induced by exposure to elevated glucose, contributing for changes in synaptic proteins in retinal neurons. Rat retinal neural cells were cultured for 9 days. Cells were exposed to elevated D-glucose (30 mM) or D-mannitol (osmotic control), for 7 days, or were exposed to interleukin-1β (10 ng/ml) or LPS (1 μg/ml) for 24 h. The protein content and distribution of SNARE proteins (SNAP-25, syntaxin-1, VAMP-2), synapsin-1, synaptotagmin-1, rabphilin 3a, VGluT-1 and VGAT, were evaluated by western blotting and immunocytochemistry. The protein content and immunoreactivity of syntaxin-1, synapsin-1, rabphilin 3a and VGAT increased in retinal neural cells exposed to high glucose. No changes were detected when cells were exposed to interleukin-1β, LPS or mannitol per se. Particularly, exposure to interleukin-1β for 24 h did not exacerbate the effect of high glucose on the content and immunoreactivity of exocytotic proteins, suggesting the primordial role of hyperglycemia for neuronal changes. In summary, prolonged exposure to elevated glucose alters the total content of several proteins involved in exocytosis, suggesting that hyperglycemia per se is a fundamental factor for neuronal changes caused by diabetes.

Diabetes induces changes in KIF1A, KIF5B and dynein distribution in the rat retina: implications for axonal transport

Diabetic retinopathy is a leading cause of vision loss and blindness. Disruption of axonal transport is associated with many neurodegenerative diseases and might also play a role in diabetes-associated disorders affecting nervous system. We investigated the impact of type 1 diabetes (2 and 8 weeks duration) on KIF1A, KIF5B and dynein motor proteins in the retina. Additionally, since hyperglycemia is considered the main trigger of diabetic complications, we investigated whether prolonged exposure to elevated glucose could affect the content and distribution of motor proteins in retinal cultures. The immunoreactivity of motor proteins was evaluated by immunohistochemistry in retinal sections and by immunoblotting in total retinal extracts from streptozotocin-induced diabetic and age-matched control animals. Primary retinal cultures were exposed to high glucose (30 mM) or mannitol (osmotic control; 24.5 mM plus 5.5 mM glucose), for seven days. Diabetes decreased the content of KIF1A at 8 weeks of diabetes as well as KIF1A immunoreactivity in the majority of retinal layers, except for the photoreceptor and outer nuclear layer. Changes in KIF5B immunoreactivity were also detected by immunohistochemistry in the retina at 8 weeks of diabetes, being increased at the photoreceptor and outer nuclear layer, and decreased in the ganglion cell layer. Regarding dynein immunoreactivity there was an increase in the ganglion cell layer after 8 weeks of diabetes. No changes were detected in retinal cultures. These alterations suggest that axonal transport may be impaired under diabetes, which might contribute to early signs of neural dysfunction in the retina of diabetic patients and animal models.

Role of microglia adenosine A(2A) receptors in retinal and brain neurodegenerative diseases

Neuroinflammation mediated by microglial cells in the brain has been commonly associated with neurodegenerative diseases. Whether this microglia-mediated neuroinflammation is cause or consequence of neurodegeneration is still a matter of controversy. However, it is unequivocal that chronic neuroinflammation plays a role in disease progression and halting that process represents a potential therapeutic strategy. The neuromodulator adenosine emerges as a promising targeting candidate based on its ability to regulate microglial proliferation, chemotaxis, and reactivity through the activation of its G protein coupled A2A receptor (A2AR). This is in striking agreement with the ability of A2AR blockade to control several brain diseases. Retinal degenerative diseases have been also associated with microglia-mediated neuroinflammation, but the role of A2AR has been scarcely explored. This review aims to compare inflammatory features of Parkinson's and Alzheimer's diseases with glaucoma and diabetic retinopathy, discussing the therapeutic potential of A2AR in these degenerative conditions.

Diabetes causes transient changes in the composition and phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and interaction with auxiliary proteins in the rat retina

PURPOSE

The impairment of glutamatergic neurotransmission has been associated with diabetic complications in the central nervous system, such as diabetic retinopathy. Here, we investigated the effect of elevated glucose exposure and diabetes on α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor composition, subunit phosphorylation, and the association of the GluA2 subunit with accessory proteins in the retina.

METHODS

The subunit composition of AMPA receptors and the association of the GluA2 subunit with modulatory proteins were evaluated with coimmunoprecipitation in retinal neural cell cultures and in the retina of experimentally induced-diabetic rats. The phosphorylation status of AMPA receptor subunits was evaluated with western blotting.

RESULTS

In retinal neural cell cultures, elevated glucose did not significantly alter the composition of AMPA receptors, namely, the interactions between the GluA1, GluA2, and GluA4 subunits, but reduced GluA2 association with GRIP1. Moreover, elevated glucose did not cause changes on the level of GluA1 phosphorylated at serine residues 831 and 845. Diabetes induced early transitory changes in the interaction between AMPA receptor subunits GluA1, GluA2, and GluA4. At 8 weeks of diabetes, the content of GluA1 phosphorylated at serine 831 or serine 845 in the retina increased, compared to age-matched controls.

CONCLUSIONS

Taken together, these results suggest that diabetes induces dynamic changes in AMPA receptor subunit composition, which could affect glutamatergic transmission in the rat retina.

Flavonoids as therapeutic compounds targeting key proteins involved in Alzheimer's disease

Alzheimer's disease is characterized by pathological aggregation of protein tau and amyloid-β peptides, both of which are considered to be toxic to neurons. Naturally occurring dietary flavonoids have received considerable attention as alternative candidates for Alzheimer's therapy taking into account their antiamyloidogenic, antioxidative, and anti-inflammatory properties. Experimental evidence supports the hypothesis that certain flavonoids may protect against Alzheimer's disease in part by interfering with the generation and assembly of amyloid-β peptides into neurotoxic oligomeric aggregates and also by reducing tau aggregation. Several mechanisms have been proposed for the ability of flavonoids to prevent the onset or to slow the progression of the disease. Some mechanisms include their interaction with important signaling pathways in the brain like the phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways that regulate prosurvival transcription factors and gene expression. Other processes include the disruption of amyloid-β aggregation and alterations in amyloid precursor protein processing through the inhibition of β-secretase and/or activation of α-secretase, and inhibiting cyclin-dependent kinase-5 and glycogen synthase kinase-3β activation, preventing abnormal tau phosphorylation. The interaction of flavonoids with different signaling pathways put forward their therapeutic potential to prevent the onset and progression of Alzheimer's disease and to promote cognitive performance. Nevertheless, further studies are needed to give additional insight into the specific mechanisms by which flavonoids exert their potential neuroprotective actions in the brain of Alzheimer's disease patients.

Diabetes alters KIF1A and KIF5B motor proteins in the hippocampus

Diabetes mellitus is the most common metabolic disorder in humans. Diabetic encephalopathy is characterized by cognitive and memory impairments, which have been associated with changes in the hippocampus, but the mechanisms underlying those impairments triggered by diabetes, are far from being elucidated. The disruption of axonal transport is associated with several neurodegenerative diseases and might also play a role in diabetes-associated disorders affecting nervous system. We investigated the effect of diabetes (2 and 8 weeks duration) on KIF1A, KIF5B and dynein motor proteins, which are important for axonal transport, in the hippocampus. The mRNA expression of motor proteins was assessed by qRT-PCR, and also their protein levels by immunohistochemistry in hippocampal slices and immunoblotting in total extracts of hippocampus from streptozotocin-induced diabetic and age-matched control animals. Diabetes increased the expression and immunoreactivity of KIF1A and KIF5B in the hippocampus, but no alterations in dynein were detected. Since hyperglycemia is considered a major player in diabetic complications, the effect of a prolonged exposure to high glucose on motor proteins, mitochondria and synaptic proteins in hippocampal neurons was also studied, giving particular attention to changes in axons. Hippocampal cell cultures were exposed to high glucose (50 mM) or mannitol (osmotic control; 25 mM plus 25 mM glucose) for 7 days. In hippocampal cultures incubated with high glucose no changes were detected in the fluorescence intensity or number of accumulations related with mitochondria in the axons of hippocampal neurons. Nevertheless, high glucose increased the number of fluorescent accumulations of KIF1A and synaptotagmin-1 and decreased KIF5B, SNAP-25 and synaptophysin immunoreactivity specifically in axons of hippocampal neurons. These changes suggest that anterograde axonal transport mediated by these kinesins may be impaired in hippocampal neurons, which may lead to changes in synaptic proteins, thus contributing to changes in hippocampal neurotransmission and to cognitive and memory impairments.

Heme oxygenase-1 protects retinal endothelial cells against high glucose- and oxidative/nitrosative stress-induced toxicity

Diabetic retinopathy is a leading cause of visual loss and blindness, characterized by microvascular dysfunction. Hyperglycemia is considered the major pathogenic factor for the development of diabetic retinopathy and is associated with increased oxidative/nitrosative stress in the retina. Since heme oxygenase-1 (HO-1) is an enzyme with antioxidant and protective properties, we investigated the potential protective role of HO-1 in retinal endothelial cells exposed to high glucose and oxidative/nitrosative stress conditions. Retinal endothelial cells were exposed to elevated glucose, nitric oxide (NO) and hydrogen peroxide (H(2)O(2)). Cell viability and apoptosis were assessed by MTT assay, Hoechst staining, TUNEL assay and Annexin V labeling. The production of reactive oxygen species (ROS) was detected by the oxidation of 2',7'-dichlorodihydrofluorescein diacetate. The content of HO-1 was assessed by immunobloting and immunofluorescence. HO activity was determined by bilirubin production. Long-term exposure (7 days) of retinal endothelial cells to elevated glucose decreased cell viability and had no effect on HO-1 content. However, a short-time exposure (24 h) to elevated glucose did not alter cell viability, but increased both the levels of intracellular ROS and HO-1 content. Moreover, the inhibition of HO with SnPPIX unmasked the toxic effect of high glucose and revealed the protection conferred by HO-1. Oxidative/nitrosative stress conditions increased cell death and HO-1 protein levels. These effects of elevated glucose and HO inhibition on cell death were confirmed in primary endothelial cells (HUVECs). When cells were exposed to oxidative/nitrosative stress conditions there was also an increase in retinal endothelial cell death and HO-1 content. The inhibition of HO enhanced ROS production and the toxic effect induced by exposure to H(2)O(2) and NOC-18 (NO donor). Overexpression of HO-1 prevented the toxic effect induced by H(2)O(2) and NOC-18. In conclusion, HO-1 exerts a protective effect in retinal endothelial cells exposed to hyperglycemic and oxidative/nitrosative stress conditions.

Elevated glucose concentration changes the content and cellular localization of AMPA receptors in the retina but not in the hippocampus

Diabetic retinopathy and diabetic encephalopathy are two common complications of diabetes mellitus. The impairment of glutamatergic neurotransmission in the retina and hippocampus has been suggested to be involved in the pathogenesis of these diabetic complications. In this study, we investigated the effect of elevated glucose concentration and diabetes on the protein content and surface expression of AMPA receptor subunits in the rat retina and hippocampus. We have used two models, cultured retinal and hippocampal cells exposed to elevated glucose concentration and an animal model of streptozotocin-induced type 1 diabetes. The immunoreactivity of GluA1, GluA2 and GluA4 was evaluated by Western blot and immunocytochemistry. The levels of these subunits at the plasma membrane were evaluated by biotinylation and purification of plasma membrane-associated proteins. Elevated glucose concentration increased the total levels of GluA2 subunit of AMPA receptors in retinal neural cells, but not of the subunits GluA1 or GluA4. However, at the plasma membrane, elevated glucose concentration induced an increase of all AMPA receptor subunits. In cultured hippocampal neurons, elevated glucose concentration did not induce significant alterations in the levels of AMPA receptor subunits. In the retinas of diabetic rats there were no persistent changes in the levels of AMPA receptor subunits comparing to aged-matched control retinas. Also, no consistent changes were detected in the levels of GluA1, GluA2 or GluA4 in the hippocampus of diabetic rats. We demonstrate that elevated glucose concentration induces early changes in AMPA receptor subunits, mainly in GluA2 subunit, in retinal neural cells. Conversely, hippocampal neurons seem to remain unaffected by elevated glucose concentration, concerning the expression of AMPA receptors, suggesting that AMPA receptors are more susceptible to the stress caused by elevated glucose concentration in retinal cells than in hippocampal neurons.

Diabetes induces early transient changes in the content of vesicular transporters and no major effects in neurotransmitter release in hippocampus and retina

Diabetes induces changes in neurotransmitter release in central nervous system, which depend on the type of neurotransmitter and region studied. In this study, we evaluated the effect of diabetes (two and eight weeks duration) on basal and evoked release of [(14)C]glutamate and [(3)H]GABA in hippocampal and retinal synaptosomes. We also analyzed the effect of diabetes on the protein content of vesicular glutamate and GABA transporters, VGluT-1, VGluT-2 and VGAT, and on the α(1A) subunit of P/Q type calcium channels, which are abundant in nerve terminals. The protein content of vesicular glutamate and GABA transporters, and of the α(1A) subunit, was differently affected by diabetes in hippocampal and retinal synaptosomes. The changes were more pronounced in the retina than in hippocampus. VGluT-1 and VGluT-2 content was not affected in hippocampus. Moreover, changes occurred early, at two weeks of diabetes, but after eight weeks almost no changes were detected, with the exception of VGAT in the retina. Regarding neurotransmitter release, no major changes were detected. After two weeks of diabetes, neurotransmitter release was similar to controls. After eight weeks of diabetes, the basal release of glutamate slightly increased in hippocampus and the evoked GABA release decreased in retina. In conclusion, diabetes induces early transient changes in the content of glutamate and/or GABA vesicular transporters, and on calcium channels subunit, in retinal or hippocampal synaptosomes, but only minor changes in the release of glutamate or GABA. These results point to the importance of diabetes-induced changes in neural tissues at the presynaptic level, which may underlie alterations in synaptic transmission, particularly if they become permanent during the later stages of the disease.

Long-term exposure to high glucose induces changes in the content and distribution of some exocytotic proteins in cultured hippocampal neurons

A few studies have reported the existence of depletion of synaptic vesicles, and changes in neurotransmitter release and in the content of exocytotic proteins in the hippocampus of diabetic rats. Recently, we found that diabetes alters the levels of synaptic proteins in hippocampal nerve terminals. Hyperglycemia is considered the main trigger of diabetic complications, although other factors, such as low insulin levels, also contribute to diabetes-induced changes. Thus, the aim of this work was to evaluate whether long-term elevated glucose per se, which mimics prolonged hyperglycemia, induces significant changes in the content and localization of synaptic proteins involved in exocytosis in hippocampal neurons. Hippocampal cell cultures were cultured for 14 days and were exposed to high glucose (50 mM) or mannitol (osmotic control; 25 mM plus 25 mM glucose), for 7 days. Cell viability and nuclear morphology were evaluated by MTT and Hoechst assays, respectively. The protein levels of vesicle-associated membrane protein-2 (VAMP-2), synaptosomal-associated protein-25 (SNAP-25), syntaxin-1, synapsin-1, synaptophysin, synaptotagmin-1, rabphilin 3a, and also of vesicular glutamate and GABA transporters (VGluT-1 and VGAT), were evaluated by immunoblotting, and its localization was analyzed by immunocytochemistry. The majority of the proteins were not affected. However, elevated glucose decreased the content of SNAP-25 and increased the content of synaptotagmin-1 and VGluT-1. Moreover, there was an accumulation of syntaxin-1, synaptotagmin-1 and VGluT-1 in the cell body of some hippocampal neurons exposed to high glucose. No changes were detected in mannitol-treated cells. In conclusion, elevated glucose per se did not induce significant changes in the content of the majority of the synaptic proteins studied in hippocampal cultures, with the exception of SNAP-25, synaptotagmin-1 and VGluT-1. However, there was an accumulation of some proteins in cell bodies of hippocampal neurons exposed to elevated glucose, suggesting that the trafficking of these proteins to the synapse may be compromised. Moreover, these results also suggest that other factors, in addition to hyperglycemia, certainly contribute to alterations detected in synaptic proteins in diabetic animals.

Diabetes changes the levels of ionotropic glutamate receptors in the rat retina

PURPOSE

Diabetic retinopathy (DR) is a leading cause of vision loss and blindness among adults between the age 20 to 74. Changes in ionotropic glutamate receptor subunit composition can affect retinal glutamatergic neurotransmission and, therefore, contribute to visual impairment. The purpose of this study was to investigate whether diabetes leads to changes in ionotropic glutamate receptor subunit expression at the protein and mRNA level in the rat retina.

METHODS

Changes in the expression of ionotropic glutamate receptor subunits were investigated at the mRNA and protein levels in retinas of streptozotocin (STZ)-induced diabetic and age-matched control rats. Animals were euthanized one, four and 12 weeks after the onset of diabetes. Retinal protein extracts were prepared, and the receptor subunit levels were assessed by western blotting. Transcript levels were assessed by real-time quantitative PCR.

RESULTS

Transcript levels of most ionotropic glutamate receptor subunits were not significantly changed in the retinas of diabetic rats, as compared to age-matched controls but protein levels of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA), kainate, and N-methyl-D-aspartic acid receptors (NMDA) receptors were found to be altered.

CONCLUSIONS

The results provide evidence that diabetes affects the retinal content of ionotropic glutamate receptor subunits at the protein level. The possible implications of these changes on retinal physiology and visual impairment in DR are discussed.

Müller cells do not influence leukocyte adhesion to retinal endothelial cells

PURPOSE

Diabetic retinopathy is associated with inflammation. The authors investigated the influence of Müller cells on leukocyte adhesion to retinal endothelial cells.

METHODS

ICAM-1 levels were assessed by Western blotting and immunocytochemistry. Leukocyte adhesion was quantified using a fluorescence assay.

RESULTS

High glucose and oxidative/nitrosative stress conditions increased ICAM-1 levels in endothelial cells and leukocyte adhesion. Under the influence of Müller cells (co-cultures/conditioned medium), the effects were comparable to those found when endothelial cells were exposed, alone, to similar conditions.

CONCLUSIONS

These results show that Müller cells do not influence leukocyte adhesion under the in vitro conditions used in this study.