Reduction of oxytocin-containing neurons and enhanced glymphatic activity in the hypothalamic paraventricular nucleus of patients with type 2 diabetes mellitus.

Evidence from animal experiments has shown that the hypothalamic paraventricular nucleus (PVN) plays a key role in regulating body weight and blood glucose levels. However, it is unclear whether neuron populations in the human PVN are involved in the development of type 2 diabetes mellitus (T2DM). To address this, we investigated the neuronal and glial populations in the PVN of 26 T2DM patients and 20 matched controls. Our findings revealed a significant reduction in oxytocin (Oxt) neuron density in the PVN of T2DM patients compared to controls, while other neuronal populations remained unchanged. This suggests that Oxt neurons may play a specific role in the pathophysiology of T2DM. Interestingly, the reduction in Oxt neurons was accompanied by a decreased melanocortinergic input in to the PVN as reflected by a reduction in alpha-MSH immunoreactivity. We also analysed two glial cell populations, as they are important for maintaining a healthy neural microenvironment. We found that microglial density, phagocytic capacity, and their proximity to neurons were not altered in T2DM patients, indicating that the loss of Oxt neurons is independent of changes in microglial immunity. However, we did observe a reduction in the number of astrocytes, which are crucial for providing trophic support to local neurons. Moreover, a specific subpopulation of astrocytes characterized by aquaporin 4 expression was overrepresented in T2DM patients. Since this subset of astrocytes is linked to the glymphatic system, their overrepresentation might point to alterations in the hypothalamic waste clearance system in T2DM. Our study shows selective loss of Oxt neurons in the PVN of T2DM individuals in association with astrocytic reduction and gliovascular remodelling. Therefore, hypothalamic Oxt neurons may represent a potential target for T2DM treatment modalities.

ASB4 modulates central melanocortinergic neurons and calcitonin signaling to control satiety and glucose homeostasis.

Variants in the gene encoding ankyrin repeat and SOCS box-containing 4 (ASB4) are linked to human obesity. Here, we characterized the pathways underlying the metabolic functions of ASB4. Hypothalamic Asb4 expression was suppressed by fasting in wild-type mice but not in mice deficient in AgRP, which encodes Agouti-related protein (AgRP), an appetite-stimulating hormone, suggesting that ASB4 is a negative target of AgRP. Many ASB4 neurons in the brain were adjacent to AgRP terminals, and feeding induced by AgRP neuronal activation was disrupted in Asb4-deficient mice. Acute knockdown of Asb4 in the brain caused marked hyperphagia due to increased meal size, and Asb4 deficiency led to increased meal size and food intake at the onset of refeeding, when very large meals were consumed. Asb4-deficient mice were resistant to the meal-terminating effects of exogenously administered calcitonin and showed decreased neuronal expression of Calcr, which encodes the calcitonin receptor. Pro-opiomelanocortin (POMC) neurons in the arcuate nucleus in mice are involved in glucose homeostasis, and Asb4 deficiency specifically in POMC neurons resulted in glucose intolerance that was independent of obesity. Furthermore, individuals with type 2 diabetes showed reduced ASB4 abundance in the infundibular nuclei, the human equivalent of the arcuate nucleus. Together, our results indicate that ASB4 acts in the brain to improve glucose homeostasis and to induce satiety after substantial meals, particularly those after food deprivation.

Loss of Microglial Insulin Receptor Leads to Sex-Dependent Metabolic Disorders in Obese Mice.

Obesity and type 2 diabetes mellitus (T2DM) are highly prevalent disorders, associated with insulin resistance and chronic inflammation. The brain is key for energy homeostasis and contains many insulin receptors. Microglia, the resident brain immune cells, are known to express insulin receptors (InsR) and to be activated by a hypercaloric environment. The aim of this study was to evaluate whether microglial insulin signaling is involved in the control of systemic energy homeostasis and whether this function is sex-dependent. We generated a microglia-specific knockout of the InsR gene in male and female mice and exposed them to control or obesogenic dietary conditions. Following 10 weeks of diet exposure, we evaluated insulin tolerance, energy metabolism, microglial morphology and phagocytic function, and neuronal populations. Lack of microglial InsR resulted in increased plasma insulin levels and insulin resistance in obese female mice. In the brain, loss of microglial InsR led to a decrease in microglial primary projections in both male and female mice, irrespective of the diet. In addition, in obese male mice lacking microglial InsR the number of proopiomelanocortin neurons was decreased, compared to control diet, while no differences were observed in female mice. Our results demonstrate a sex-dependent effect of microglial InsR-signaling in physiology and obesity, and stress the importance of a heterogeneous approach in the study of diseases such as obesity and T2DM.

Specific Silencing of Microglial Gene Expression in the Rat Brain by Nanoparticle-Based Small Interfering RNA Delivery.

Microglia are the major innate immune cells in the brain and are essential for maintaining homeostasis in a neuronal microenvironment. Currently, a genetic tool to modify microglial gene expression in specific brain regions is not available. In this report, we introduce a tailor-designed method that uses lipid and polymer hybridized nanoparticles (LPNPs) for the local delivery of small interfering RNAs (siRNAs), allowing the silencing of specific microglial genes in the hypothalamus. Our physical characterization proved that this LPNP-siRNA was uniform and stable. We demonstrated that, due to their natural phagocytic behavior, microglial cells are the dominant cell type taking up these LPNPs in the hypothalamus of rats. We then tested the silencing efficiency of LPNPs carrying a cluster of differentiation molecule 11b (CD11b) or Toll-like receptor 4 (TLR4) siRNA using different in vivo and in vitro approaches. In cultured microglial cells treated with LPNP-CD11b siRNA or LPNP-TLR4 siRNA, we found a silencing efficiency at protein expression levels of 65 or 77%, respectively. In line with this finding, immunohistochemistry and western blotting results from in vivo experiments showed that LPNP-CD11b siRNA significantly inhibited microglial CD11b protein expression in the hypothalamus. Furthermore, following lipopolysaccharide (LPS) stimulation of cultured microglial cells, gene expression of the TLR4 downstream signaling component myeloid differentiation factor 88 and its associated cytokines was significantly inhibited in LPNP-TLR4 siRNA-treated microglial cells compared with cells treated with LPNP-scrambled siRNA. Finally, after LPNP-TLR4 siRNA injection into the rat hypothalamus, we observed a significant reduction in microglial activation in response to LPS compared with the control rats injected with LPNP-scrambled siRNA. Our results indicate that LPNP-siRNA is a promising tool to manipulate microglial activity locally in the brain and may serve as a prophylactic approach to prevent microglial dysfunction-associated diseases.

The impact of antidiabetic treatment on human hypothalamic infundibular neurons and microglia.

Animal studies indicate that hypothalamic dysfunction plays a major role in type 2 diabetes mellitus (T2DM) development, and that insulin resistance and inflammation are important mechanisms involved in this disorder. However, it remains unclear how T2DM and antidiabetic treatments affect the human hypothalamus. Here, we characterized the proopiomelanocortin (POMC) immunoreactive (-ir) neurons, the neuropeptide-Y-ir (NPY-ir) neurons, the ionized calcium-binding adapter molecule 1-ir (iba1-ir) microglia, and the transmembrane protein 119-ir (TMEM119-ir) microglia in the infundibular nucleus (IFN) of human postmortem hypothalamus of 32 T2DM subjects with different antidiabetic treatments and 17 matched nondiabetic control subjects. Compared with matched control subjects, T2DM subjects showed a decrease in the number of POMC-ir neurons, but no changes in NPY-ir neurons or microglia. Interestingly, T2DM subjects treated with the antidiabetic drug metformin had fewer NPY-ir neurons and microglia than T2DM subjects not treated with metformin. We found that the number of microglia correlated with the number of NPY-ir neurons, but only in T2DM subjects. These results indicate that different changes in POMC and NPY neurons and microglial cells in the IFN accompany T2DM. In addition, T2DM treatment modality is associated with highly selective changes in hypothalamic neurons and microglial cells.

Time-Restricted Feeding Improves Glucose Tolerance in Rats, but Only When in Line With the Circadian Timing System.

Epidemiological studies indicate that shift-workers have an increased risk of type 2 diabetes mellitus (T2DM). Glucose tolerance and insulin sensitivity both are dependent on the circadian timing system (i.e., the time-of-day) and fasting duration, in rodents as well as humans. Therefore, question is whether manipulation of the circadian timing system, for example by changing the timing of feeding and fasting, is a potential preventive treatment for T2DM. Time-restricted feeding (TRF) is well-known to have profound effects on various metabolic measures, including glucose metabolism. However, experiments that directly measure the effects of TRF on glucose tolerance and/or insulin sensitivity at different time points throughout the 24 h cycle are lacking. Here we show, in rats, that TRF in line with the circadian timing system (i.e., feeding during the active phase) improves glucose tolerance during intravenous glucose tolerance tests (ivGTT) in the active phase, as lower insulin levels were observed with similar levels of glucose clearance. However, this was not the case during the inactive phase in which more insulin was released but only a slightly faster glucose clearance was observed. Contrasting, TRF out of sync with the circadian timing system (i.e., feeding during the inactive phase) worsened glucose tolerance, although only marginally, likely because of adaptation to the 4 week TRF regimen. Our results show that TRF can improve glucose metabolism, but strict adherence to the time-restricted feeding period is necessary, as outside the regular eating hours glucose tolerance is worsened.

Diet-Induced Obesity Disturbs Microglial Immunometabolism in a Time-of-Day Manner.

Background: Disturbance of immunometabolic signaling is a key process involved in the progression of obesity. Microglia-the resident immune cells in the brain, initiate local immune responses. It is known that hypercaloric diets lead to microglial activation. Previously, we observed that hypothalamic microglial cells from mice fed high-fat diet (HFD) lose their day/night rhythm and are constantly activated. However, little is known about daily rhythmicity in microglial circadian, immune and metabolic functions, either in lean or obese conditions. Therefore, we hypothesized that HFD disturbs microglial immunometabolism in a day/night-dependent manner. Methods: Obesity was induced in Wistar rats by feeding them HFD ad libitum for the duration of 8 weeks. Microglia were isolated from HFD- and chow-fed control animals at six time points during 24 h [every 4 h starting 2 h after lights on, i.e., Zeitgeber Time 2 (ZT2)]. Gene expression was evaluated using quantitative RT-PCR. JTK_Cycle software was used to estimate daily rhythmicity. Statistical analysis was performed with two-way ANOVA test. Results: Consumption of the obesogenic diet resulted in a 40 g significantly higher body weight gain in week 8, compared to chow diet (p < 0.0001), associated with increased adiposity. We observed significant rhythmicity of circadian clock genes in microglia under chow conditions, which was partially lost in diet-induced obesity (DIO). Microglial immune gene expression also showed time-of-day differences, which were disrupted in HFD-fed animals. Microglia responded to the obesogenic conditions by a shift of substrate utilization with decreased glutamate and glucose metabolism in the active period of the animals, and an overall increase of lipid metabolism, as indicated by gene expression evaluation. Additionally, data on mitochondria bioenergetics and dynamics suggested an increased energy production in microglia during the inactive period on HFD. Finally, evaluation of monocyte functional gene expression showed small or absent effect of HFD on peripheral myeloid cells, suggesting a cell-specific microglial inflammatory response in DIO. Conclusions: An obesogenic diet affects microglial immunometabolism in a time-of-day dependent manner. Given the central role of the brain in energy metabolism, a better knowledge of daily rhythms in microglial immunometabolism could lead to a better understanding of the pathogenesis of obesity.

Loss of arginine vasopressin- and vasoactive intestinal polypeptide-containing neurons and glial cells in the suprachiasmatic nucleus of individuals with type 2 diabetes.

AIMS/HYPOTHESIS: The central pacemaker of the mammalian biological timing system is located within the suprachiasmatic nucleus (SCN) in the anterior hypothalamus. Together with the peripheral clocks, this central brain clock ensures a timely, up-to-date and proper behaviour for an individual throughout the day-night cycle. A mismatch between the central and peripheral clocks results in a disturbance of daily rhythms in physiology and behaviour. It is known that the number of rhythmically expressed genes is reduced in peripheral tissue of individuals with type 2 diabetes mellitus. However, it is not known whether the central SCN clock is also affected in the pathogenesis of type 2 diabetes. In the current study, we compared the profiles of the SCN neurons and glial cells between type 2 diabetic and control individuals. METHODS: We collected post-mortem hypothalamic tissues from 28 type 2 diabetic individuals and 12 non-diabetic control individuals. We performed immunohistochemical analysis for three SCN neuropeptides, arginine vasopressin (AVP), vasoactive intestinal polypeptide (VIP) and neurotensin (NT), and for two proteins expressed in glial cells, ionised calcium-binding adapter molecule 1 (IBA1, a marker of microglia) and glial fibrillary acidic protein (GFAP, a marker of astroglial cells). RESULTS: The numbers of AVP immunoreactive (AVP-ir) and VIP-ir neurons and GFAP-ir astroglial cells in the SCN of type 2 diabetic individuals were significantly decreased compared with the numbers in the SCN of the control individuals. In addition, the relative intensity of AVP immunoreactivity was reduced in the individuals with type 2 diabetes. The number of NT-ir neurons and IBA1-ir microglial cells in the SCN was similar in the two groups. CONCLUSIONS/INTERPRETATION: Our data show that type 2 diabetes differentially affects the numbers of AVP- and VIP-expressing neurons and GFAP-ir astroglial cells in the SCN, each of which could affect the daily rhythmicity of the SCN biological clock machinery. Therefore, for effectively treating type 2 diabetes, lifestyle changes and/or medication to normalise central biological clock functioning might be helpful.

Deficiency of leptin receptor in myeloid cells disrupts hypothalamic metabolic circuits and causes body weight increase.

OBJECTIVE: Leptin is a cytokine produced by adipose tissue that acts mainly on the hypothalamus to regulate appetite and energy homeostasis. Previous studies revealed that the leptin receptor is expressed not only in neurons, but also in glial cells. Microglia are resident immune cells in the brain that play an essential role in immune defense and neural network development. Previously we reported that microglial morphology and cytokine production are changed in the leptin receptor deficient db/db mouse, suggesting that leptin's central effects on metabolic control might involve signaling through microglia. In the current study, we aimed to uncover the role of leptin signaling in microglia in systemic metabolic control. METHODS: We generated a mouse model with leptin receptor deficiency, specifically in the myeloid cells, to determine the role of microglial leptin signaling in the development of metabolic disease and to investigate microglial functions. RESULTS: We discovered that these mice have increased body weight with hyperphagia. In the hypothalamus, pro-opiomelanocortin neuron numbers in the arcuate nucleus (ARC) and α-MSH projections from the ARC to the paraventricular nucleus (PVN) decreased, which was accompanied by the presence of less ramified microglia with impaired phagocytic capacity in the PVN. CONCLUSIONS: Myeloid cell leptin receptor deficient mice partially replicate the db/db phenotype. Leptin signaling in hypothalamic microglia is important for microglial function and a correct formation of the hypothalamic neuronal circuit regulating metabolism.