Task-induced brain activity patterns in type 2 diabetes: a potential biomarker for cognitive decline

Alterations in Brain Morphometric Networks and Their Relationship with Memory Dysfunction in Patients with Type 2 Diabetes Mellitus

Cognitive dysfunction, a significant complication of type 2 diabetes mellitus (T2DM), can potentially manifest even from the early stages of the disease. Despite evidence of global brain atrophy and related cognitive dysfunction in early-stage T2DM patients, specific regions vulnerable to these changes have not yet been identified. The study enrolled patients with T2DM of less than five years' duration and without chronic complications (T2DM group, n=100) and demographically similar healthy controls (control group, n=50). High-resolution T1-weighted magnetic resonance imaging data were subjected to independent component analysis to identify structurally significant components indicative of morphometric networks. Within these networks, the groups' gray matter volumes were compared, and distinctions in memory performance were assessed. In the T2DM group, the relationship between changes in gray matter volume within these networks and declines in memory performance was examined. Among the identified morphometric networks, the T2DM group exhibited reduced gray matter volumes in both the precuneus (Bonferroni-corrected p=0.003) and insular-opercular (Bonferroni-corrected p=0.024) networks relative to the control group. Patients with T2DM demonstrated significantly lower memory performance than the control group (p=0.001). In the T2DM group, reductions in gray matter volume in both the precuneus (r=0.316, p=0.001) and insular-opercular (r=0.199, p=0.047) networks were correlated with diminished memory performance. Our findings indicate that structural alterations in the precuneus and insular-opercular networks, along with memory dysfunction, can manifest within the first 5 years following a diagnosis of T2DM.

Functional reorganization of memory processing in the hippocampus is associated with neuroprotector GLP-1 levels in type 2 diabetes

Type 2 diabetes (T2D) often impairs memory functions, suggesting specific vulnerability of the hippocampus. In vivo neuroimaging studies relating encoding and retrieval of memory information with endogenous neuroprotection are lacking. The neuroprotector glucagon-like peptide (GLP-1) has a high receptor density in anterior/ventral hippocampus, as shown by animal models. Using an innovative event-related fMRI design in 34 participants we investigated patterns of hippocampal activity in T2D (n = 17) without mild cognitive impairment (MCI) versus healthy controls (n = 17) during an episodic memory task. We directly measured neurovascular coupling by estimating the hemodynamic response function using event-related analysis related to encoding and retrieval of episodic information in the hippocampus. We applied a mixed-effects general linear model analysis and a two-factor ANOVA to test for group differences. Significant between-group differences were found for memory encoding, showing evidence for functional reorganization: T2D patients showed an augmented activation in the posterior hippocampus while anterior activation was reduced. The latter was negatively correlated with both GLP-1 pre- and post-breakfast levels, in the absence of grey matter changes. These results suggest that patients with T2D without MCI have pre-symptomatic functional reorganization in brain regions underlying episodic memory, as a function of the concentration of the neuroprotective neuropeptide GLP-1.

Neuroprotective effect of salidroside on hippocampal neurons in diabetic mice via PI3K/Akt/GSK-3β signaling pathway

BACKGROUND

Diabetic encephalopathy is manifested by cognitive dysfunction. Salidroside, a nature compound isolated from Rhodiola rosea L, has the effects of anti-inflammatory and antioxidant, hypoglycemic and lipid-lowering, improving insulin resistance, inhibiting cell apoptosis, and protecting neurons. However, the mechanism by which salidroside alleviates neuronal degeneration and improves learning and memory impairment in diabetic mice remains unclear.

OBJECTIVE

To investigate the effects and mechanisms of salidroside on hippocampal neurons in streptozotocin-induced diabetic mice.

MATERIALS AND METHODS

C57BL/6 mice were randomly divided into 4 groups to receive either sham (control group (CON)), diabetes mellitus (diabetes group (DM)), diabetes mellitus + salidroside (salidroside group (DM + SAL)), and diabetes mellitus + salidroside + phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (diabetes mellitus + salidroside + LY294002 group (DM + SAL + LY294002)). After 12 weeks of diabetes onset, the cognitive behaviors were tested using Morris water maze. The number of hippocampal neurons was detected by Nissl staining. The expressions of PI3K, p-PI3K, Akt, p-Akt, GSK-3β, p-GSK-3β, cleaved caspase-3, caspase-3, Bax, Bcl-2, MAP2, and SYN in the hippocampus were detected by Western blot. Moreover, the expression of MAP2 and SYN in the hippocampus was further confirmed by immunofluorescence staining.

RESULTS

Salidroside increased the time of diabetic mice in the platform quadrant and reduced the escape latency of diabetic mice. Salidroside also increased the expression of p-PI3K, p-Akt, p-GSK-3β, MAP2, SYN, Bcl-2, while suppressed the expression of cleaved caspase-3, caspase3, and Bax in the DM + SAL group compared with the DM group (P < 0.05). The Nissl staining showed that the number of hippocampus neurons in the DM + SAL group was increased with the intact, compact, and regular arrangement, compared with the DM groups (P < 0.05). Interestingly, the protective effects of salidroside on diabetic cognitive dysfunction, hippocampal morphological alterations, and protein expressions were abolished by inhibition of PI3K with LY294002.

CONCLUSIONS

Salidroside exerts neuroprotective properties in diabetic cognitive dysfunction partly via activating the PI3K/Akt/GSK-3β signaling pathway.

Insulin Resistance and Cognitive Impairment: Evidence From Neuroimaging

Insulin is a peptide well known for its role in regulating glucose metabolism in peripheral tissues. Emerging evidence from human and animal studies indicate the multifactorial role of insulin in the brain, such as neuronal and glial metabolism, glucose regulation, and cognitive processes. Insulin resistance (IR), defined as reduced sensitivity to the action of insulin, has been consistently proposed as an important risk factor for developing neurodegeneration and cognitive impairment. Although the exact mechanism of IR-related cognitive impairment still awaits further elucidation, neuroimaging offers a versatile set of novel contrasts to reveal the subtle cerebral abnormalities in IR. These imaging contrasts, including but not limited to brain volume, white matter (WM) microstructure, neural function and brain metabolism, are expected to unravel the nature of the link between IR, cognitive decline, and brain abnormalities, and their changes over time. This review summarizes the current neuroimaging studies with multiparametric techniques, focusing on the cerebral abnormalities related to IR and therapeutic effects of IR-targeting treatments. According to the results, brain regions associated with IR pathophysiology include the medial temporal lobe, hippocampus, prefrontal lobe, cingulate cortex, precuneus, occipital lobe, and the WM tracts across the globe. Of these, alterations in the temporal lobe are highly reproducible across different imaging modalities. These structures have been known to be vulnerable to Alzheimer's disease (AD) pathology and are critical in cognitive processes such as memory and executive functioning. Comparing to asymptomatic subjects, results are more mixed in patients with metabolic disorders such as type 2 diabetes and obesity, which might be attributed to a multifactorial mechanism. Taken together, neuroimaging, especially MRI, is beneficial to reveal early abnormalities in cerebral structure and function in insulin-resistant brain, providing important evidence to unravel the underlying neuronal substrate that reflects the cognitive decline in IR. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 2.

Impact of dysglycemia and obesity on the brain in adolescents with and without type 2 diabetes: A pilot study

OBJECTIVE

Both diabetes and obesity can affect the brain, yet their impact is not well characterized in children with type 2 (T2) diabetes and obesity. This pilot study aims to explore differences in brain function and cognition in adolescents with T2 diabetes and obesity and nondiabetic controls with obesity and lean controls.

RESEARCH DESIGN AND METHODS

Participants were 12-17 years old (5 T2 diabetes with obesity [mean HgbA1C 10.9%], 6 nondiabetic controls with obesity and 10 lean controls). Functional MRI (FMRI) during hyperglycemic/euglycemic clamps was performed in the T2 diabetes group.

RESULTS

When children with obesity, with and without diabetes, were grouped (mean BMI 98.8%), cognitive scores were lower than lean controls (BMI 58.4%) on verbal, full scale, and performance IQ, visual-spatial and executive function tests. Lower scores correlated with adiposity and insulin resistance but not HgbA1C. No significant brain activation differences during task based and resting state FMRI were noted between children with obesity (with or without diabetes) and lean controls, but a notable effect size for the visual-spatial working memory task and resting state was observed.

CONCLUSIONS

In conclusion, our pilot study suggests that obesity, insulin resistance, and dysglycemia may contribute to relatively poorer cognitive function in adolescents with T2 diabetes and obesity. Further studies with larger sample size are needed to assess if cognitive decline in children with obesity, with and without T2 diabetes, can be prevented or reversed.

Neurovascular coupling in patients with type 2 diabetes mellitus

Functional and metabolic neural changes in Type 2 diabetes mellitus (T2DM) can be associated with poor cognitive performances. Here we analyzed the functional-metabolic neurovascular coupling (NVC) in the brain of T2DM patients. Thirty-three patients (70 ± 6 years, 15 males) with recent T2DM diagnosis and 18 healthy control (HC) subjects (65 ± 9 years, 9 males) were enrolled in a brain MRI study to identify the potential effects of T2DM on NVC. T2DM patients were either drug-naive (n = 19) or under treatment with metformin (n = 14) since less than 6 months. Arterial spin labeling and blood oxygen level dependent resting-state functional MRI (RS-fMRI) images were combined to derive NVC measures in brain regions and large-scale networks in a standard brain parcelation. Altered NVC values in T2DM patients were correlated with cognitive performances spanning several neurological domains using Spearman correlation coefficients. Compared to HC, T2DM patients had reduced NVC in the default mode network (DMN) and increased NVC in three regions of the dorsal (DAN) and salience-ventral (SVAN) attention networks. NVC abnormalities in DAN and SVAN were associated with reduced visuo-spatial cognitive performances. A spatial pattern of NVC reduction in the DMN, accompanied by isolated regional NVC increases in DAN and SVAN, could reflect the emergence of (defective) compensatory processes in T2DM patients in response to altered neurovascular conditions. Overall, this pattern is reminiscent of neural abnormalities previously observed in Alzheimer’s disease, suggesting that similar neurobiological mechanisms, secondary to insulin resistance and manifesting as NVC alterations, might be developing in T2DM pathology.

Impairments in intrinsic functional networks in type 2 diabetes: A meta-analysis of resting-state functional connectivity

Type 2 diabetes mellitus (T2DM) is associated with abnormal communication among large-scale brain networks, revealed by resting-state functional connectivity (rsFC), with inconsistent results between studies. We performed a meta-analysis of seed-based rsFC studies to identify consistent network connectivity alterations. Thirty-three datasets from 30 studies (1014 T2DM patients and 902 healthy controls [HC]) were included. Seed coordinates and between-group effects were extracted, and the seeds were divided into networks based on their location. Compared to HC, T2DM patients showed hyperconnectivity and hypoconnectivity within the DMN, DMN hypoconnectivity with the affective network (AN), ventral attention network (VAN) and frontal parietal network, and DMN hyperconnectivity with the VAN and visual network. T2DM patients also showed AN hypoconnectivity with the somatomotor network and hyperconnectivity with the VAN. T2DM illness durations negatively correlated with within-DMN rsFC. These DMN-centered impairments in large-scale brain networks in T2DM patients may help to explain the cognitive deficits associated with T2DM.

Hyperglycemia and hyperinsulinemia effects on anterior cingulate cortex myoinositol-relation to brain network functional connectivity in healthy adults

Brain mechanisms underlying the association of diabetes metabolic disorders-hyperglycemia and insulin resistance-with cognitive impairment are unknown. Myoinositol is a brain metabolite involved in cell osmotic balance, membrane phospholipid turnover, and second messenger neurotransmission, which affect brain function. Increased brain myoinositol and altered functional connectivity have been found in diabetes, mild cognitive impairment, and Alzheimer's disease, but the independent effects of plasma glucose and insulin on brain myoinositol and function are not characterized. We measured myoinositol concentrations in the pregenual anterior cingulate cortex (ACC), a region involved in self-reflective awareness and decision making, using proton magnetic resonance spectroscopy, and whole brain resting-state functional connectivity using fMRI, during acute hyperglycemia (with attendant hyperinsulinemia) and euglycemic-hyperinsulinemia compared with basal fasting-euglycemia (EU) in 11 healthy nondiabetic participants (5 women/6 men, means ± SD, age: 27 ± 7 yr, fasting-glucose: 5.2 ± 0.4 mmol/L, fasting-insulin: 4.9 ± 4.4 μU/mL). Brain MR data were acquired during two separate visits: 1) EU followed by a 60-min hyperglycemic-clamp (glucose: 10.7 ± 0.2 mmol/L, insulin: 33 ± 6 μU/mL); 2) EU followed by a hyperinsulinemic-euglycemic-clamp (glucose: 5.3 ± 0.1 mmol/L, insulin: 27 ± 5 μU/mL) designed to match individual insulin levels achieved during the visit 1 hyperglycemic-clamp. Myoinositol decreased by 14% during the hyperglycemic-clamp (from 7.7 ± 1.5 mmol/kg to 6.6 ± 0.8 mmol/kg, P = 0.031), and by 9% during the hyperinsulinemic-euglycemic-clamp (from 7.1 ± 0.7 mmol/kg to 6.5 ± 0.7 mmol/kg, P = 0.014), with no significant difference between the two clamps. Lower myoinositol was associated with higher functional connectivity of the thalamus and precentral cortex with insula-ACC-related networks, suggesting myoinositol is involved in insulin modulation of cognitive/emotional network function in healthy adults. Regional brain myoinositol levels may be useful biomarkers for monitoring cognitive and mood-enhancing treatment responses.NEW & NOTEWORTHY Hyperinsulinemia-related decreases of brain anterior cingulate cortex (ACC) myoinositol independent of plasma glucose levels and the association of low ACC myoinositol with increased functional connectivity between sensorimotor regions and ACC/insula-related networks suggest involvement of myoinositol in insulin-modulated brain network function in healthy adults. In diabetes, elevated brain myoinositol may be due to reduced brain insulin levels or action, rather than hyperglycemia, and may be involved in brain network dysfunctions leading to cognitive or mood disorders.

New Horizons-Cognitive Dysfunction Associated With Type 2 Diabetes

The prevalence of type 2 diabetes (T2D) and cognitive dysfunction increases with age. As society ages, clinicians will be increasingly tasked with managing older people who have both T2D and cognitive dysfunction. T2D is associated with an increased risk of cognitive dysfunction and hence there is increasing interest in whether T2D is a causal factor in the pathogenesis of cognitive decline and dementia. Recent advances in the use of sensitive measures of in vivo brain dysfunction in life-course studies can help understand potential mechanistic pathways and also help guide recommendations for clinical practice. In this article we will describe new horizons in the understanding of cognitive dysfunction associated with T2D. Coming from a clinical perspective, we discuss potential mechanisms and pathways linking the 2 conditions and the contribution of multimodal neuroimaging and study designs to advancing understanding in the field. We also highlight the important issues on the horizon that will need addressing in clinical identification, management, and risk reduction for people with coexistent T2D and cognitive dysfunction.

Altered Structural and Functional MRI Connectivity in Type 2 Diabetes Mellitus Related Cognitive Impairment: A Review

Type 2 diabetes mellitus (T2DM) is associated with cognitive impairment in many domains. There are several pieces of evidence that changes in neuronal neuropathies and metabolism have been observed in T2DM. Structural and functional MRI shows that abnormal connections and synchronization occur in T2DM brain circuits and related networks. Neuroplasticity and energy metabolism appear to be principal effector systems, which may be related to amyloid beta (Aβ) deposition, although there is no unified explanation that includes the complex etiology of T2DM with cognitive impairment. Herein, we assume that cognitive impairment in diabetes may lead to abnormalities in neuroplasticity and energy metabolism in the brain, and those reflected to MRI structural connectivity and functional connectivity, respectively.

Risk for Alzheimer's disease: A review of long-term episodic memory encoding and retrieval fMRI studies

Many risk factors have been identified that predict future progression to Alzheimer's disease (AD). However, clear links have yet to be made between these risk factors and how they affect brain functioning in early stages of AD. We conducted a narrative review and a quantitative analysis to better understand the relationship between nine categories of AD risk (i.e., brain pathology, genetics/family history, vascular health, head trauma, cognitive decline, engagement in daily life, late-life depression, sex/gender, and ethnoracial group) and task-evoked fMRI activity during episodic memory in cognitively-normal older adults. Our narrative review revealed widespread regional alterations of both greater and lower brain activity with AD risk. Nevertheless, our quantitative analysis revealed that a subset of studies converged on two patterns: AD risk was associated with (1) greater brain activity in frontal and parietal regions, but (2) reduced brain activity in hippocampal and occipital regions. The brain regions affected depended on the assessed memory stage (encoding or retrieval). Although the results clearly indicate that AD risks impact brain activity, we caution against using fMRI as a diagnostic tool for AD at the current time because the above consistencies were present among much variability, even among the same risk factor.

Diabetes and the link between neuroplasticity and glutamate in the aging human motor cortex

OBJECTIVES

In older adults, type-2 diabetes mellitus (T2DM) impacts cognition and increases dementia risk. Prior studies suggest that impaired neuroplasticity may contribute to the cognitive decline in T2DM, but the underlying mechanisms of altered neuroplasticity are unclear. We investigated the relationship of the concentration of glutamatergic metabolites with measures of cortical plasticity in older adults across the spectrum of glucose intolerance/insulin resistance.

METHODS

Forty adults (50-87 years: 17-T2DM, 14-pre-diabetes, 9-controls) underwent magnetic resonance spectroscopy to quantify glutamate and other key metabolites within a 2 cm3 region around the hand knob of the left primary motor cortex. Thirty-six also underwent a separate transcranial magnetic stimulation (TMS) assessment of cortical excitability and plasticity using single-pulse TMS and intermittent theta-burst stimulation targeting the same brain region.

RESULTS

Group differences were observed in relative concentrations of glutamine (p = .028), glucose (p = .008), total cholines (p = .048), and the glutamine/glutamate ratio (p = .024). Cortical plasticity was reduced in both T2DM and pre-diabetes groups relative to controls (p-values < .05). Only the T2DM group showed a significant positive association between glutamate concentration and plasticity (r = .56, p = .030).

CONCLUSIONS

Neuroplastic mechanisms are already impaired in pre-diabetes. In T2DM, reduced cortico-motor plasticity is associated with lower cortical glutamate concentration.

SIGNIFICANCE

Impaired plasticity in T2DM is associated with low glutamatergic metabolite levels. The glutamatergic neurotransmission system constitutes a potential therapeutic target for cognitive problems linked to plasticity-related deficiencies in T2DM.

Pathological Between-Network Positive Connectivity in Early Type 2 Diabetes Patients Without Cerebral Small Vessel Diseases

Background and Purpose

Previous neuroimaging studies have demonstrated type 2 diabetes (T2D)-related brain structural and functional changes are partly associated with cognitive decline. However, less is known about the underlying mechanisms. Chronic hyperglycemia and microvascular complications are the two of most important risk factors related to cognitive decline in diabetes. Cerebral small vessel diseases (CSVDs), such as those defined by lacunar infarcts, white matter hyperintensities (WMHs) and microhemorrhages, are also associated with an increased risk of cognitive decline and dementia. In this study, we examined brain magnetic resonance imaging (MRI) changes in patients in the early stages of T2D without CSVDs to focus on glucose metabolism factors and to avoid the interference of vascular risk factors on T2D-related brain damage.

Methods

T2D patients with disease durations of less than 5 years and without any signs of CSVDs (n = 34) were compared with healthy control subjects (n = 24). Whole-brain region-based functional connectivity was analyzed with network-based statistics (NBS), and brain surface morphology was examined. In addition, the Montreal Cognitive Assessment (MoCA) was conducted for all subjects.

Results

At the whole-brain region-based functional connectivity level, thirty-three functional connectivities were changed in T2D patients relative to those in controls, mostly manifested as pathological between-network positive connectivity and located mainly between the sensory-motor network and auditory network. Some of the connectivities were positively correlated with blood glucose level, insulin resistance, and MoCA scores in the T2D group. The network-level analysis showed between-network hyperconnectivity in T2D patients, but no significant changes in within-network connectivity. In addition, there were no significant differences in MoCA scores or brain morphology measures, including cortical thickness, surface area, mean curvature, and gray/white matter volume, between the two groups.

Conclusion

The findings indicate that pathological between-network positive connectivity occurs in the early stages of T2D without CSVDs. The abnormal connectivity may indicate that the original balance of mutual antagonistic/cooperative relationships between the networks is broken, which may be a neuroimaging basis for predicting cognitive decline in early T2D patients.

Neuroimaging signatures of brain plasticity in adults with prenatal exposure to polychlorinated biphenyls: Altered functional connectivity on functional MRI

Prenatal exposure to polychlorinated biphenyls (PCBs), persistent organic pollutants in food chains and environment, exerts negative effects on children's cognitive function. To study the long-term effects, we examined cognitive functions in the male children of women with substantial PCB exposure in Taiwan during 1978-1979 and investigated neural basis of cognitive function changes through structural magnetic resonance imaging (MRI) and functional MRI (fMRI), which included resting-state and task-activated fMRI with two paradigms: a 2-back task and a picture rotation task. Ten men aged 30.0 ± 3.7 years with prenatal exposure to PCBs and 11 unexposed controls aged 28.1 ± 3.1 years participated. Both groups had similar cognitive phenotypes and behavioral results. Structural MRI analysis results showed that the PCB group had increased cortical thickness over the right inferior parietal lobule. In the resting-state study, the PCB group showed alterations in the default mode network. During the tasks, the PCB group showed decreased task-induced deactivation signals in cognition-associated brain areas during the 2-back task but enhanced deactivations during the picture rotation task. This study demonstrated altered structural MRI as well as resting and task-related fMRI in men with prenatal PCB exposure, suggesting altered brain plasticity and compensatory neuropsychological performance.

Parcellation of the human hippocampus based on gray matter volume covariance: Replicable results on healthy young adults

The hippocampus is a key brain region that participates in a range of cognitive and affective functions, and is involved in the etiopathogenesis of numerous neuropsychiatric disorders. The structural complexity and functional diversity of the hippocampus suggest the existence of structural and functional subdivisions within this structure. For the first time, we parcellated the human hippocampus with two independent data sets, each of which consisted of 198 T1-weighted structural magnetic resonance imaging (sMRI) images of healthy young subjects. The method was based on gray matter volume (GMV) covariance, which was quantified by a bivariate voxel-to-voxel linear correlation approach, as well as a multivariate masked independent component analysis approach. We subsequently interrogated the relationship between the GMV covariance patterns and the functional connectivity patterns of the hippocampal subregions using sMRI and resting-state functional MRI (fMRI) data from the same participants. Seven distinct GMV covariance-based subregions were identified for bilateral hippocampi, with robust reproducibility across the two data sets. We further demonstrated that the structural covariance patterns of the hippocampal subregions had a correspondence with the intrinsic functional connectivity patterns of these subregions. Together, our results provide a topographical configuration of the hippocampus with converging structural and functional support. The resulting subregions may improve our understanding of the hippocampal connectivity and functions at a subregional level, which provides useful parcellations and masks for future neuroscience and clinical research on the structural and/or functional connectivity of the hippocampus.

Investigating the impact of overnight fasting on intrinsic functional connectivity: a double-blind fMRI study

The human brain depends mainly on glucose supply from circulating blood as an energy substrate for its metabolism. Most of the energy produced by glucose catabolism in the brain is used to support intrinsic communication purposes in the absence of goal-directed activity. This intrinsic brain function can be detected with fMRI as synchronized fluctuations of the BOLD signal forming functional networks. Here, we report results from a double-blind, placebo controlled, cross-over study addressing changes in intrinsic brain activity in the context of very low, yet physiological, blood glucose levels after overnight fasting. Comparison of four major resting state networks in a fasting state and a state of elevated blood glucose levels after glucagon infusion revealed altered patterns of functional connectivity only in a small region of the posterior default mode network, while the rest of the networks appeared unaffected. Furthermore, low blood glucose was associated with changes in the right frontoparietal network after cognitive effort. Our results suggest that fasting has only limited impact on intrinsic brain activity, while a detrimental impact on a network related to attention is only observable following cognitive effort, which is in line with ego depletion and its reliance on glucose.

Brain functional imaging in obese and diabetic patients

Obesity and type 2 diabetes are associated with greater risk of brain damage. Over the last decade, functional imaging techniques (functional magnetic resonance imaging, fMRI, positron emission tomography, PET, electroencephalography, magnetoencephalography, near infrared spectroscopy) have been exploited to better characterize behavioral and cognitive processes, by addressing cerebral reactions to a variety of stimuli or tasks, including hormones and substrates (e.g., glucose, insulin, gut peptides), environmental cues (e.g., presentation of sensory stimuli), and cognitive tasks. Among these techniques, fMRI and PET are most commonly used, and this review focuses on results obtained with these techniques in relation to brain substrate metabolism, appetite control and food intake, and cognitive decline in obesity and type 2 diabetes. The available knowledge indicates that there are a series of cerebral abnormalities associating with, or preceding obesity and type 2 diabetes, including impaired substrate handling, insulin resistance, disruption of inter-organ cross-talk and of resting state networking. Some of these abnormalities are reversed by metabolic interventions, suggesting that they are partly a consequence rather than cause of disease. Therefore, causal implications and mechanisms remain to be determined.

Youth-Onset Type 2 Diabetes and the Developing Brain

PURPOSE OF REVIEW

This review describes the literature evaluating the potential adverse effects of youth-onset type 2 diabetes on the developing brain. A summary of recently published articles and the current state of knowledge are covered succinctly in this manuscript.

RECENT FINDINGS

Current literature suggests both cognitive and brain structural differences are found in youth with type 2 diabetes. Studies have shown poorer scores in a number of neurocognitive domains, particularly in areas of executive functioning and memory. Additionally, imaging studies have found differences in brain gray matter volume, white matter volume, and microstructural integrity. These findings are largely consistent with the adult literature. Youth with type 2 diabetes demonstrate lower cognitive scores and structural brain differences. Although causality has not yet been established, these findings are important because these individuals are still undergoing neurodevelopmental maturation.

What magnetic resonance imaging reveals - A systematic review of the relationship between type II diabetes and associated brain distortions of structure and cognitive functioning

Due to its increasing prevalence, Type 2 diabetes mellitus (T2DM) represents a major health challenge for modern society. Despite it being of fundamental interest, only a few MRI studies have conducted statistical analyses to draw scientifically valid conclusions about the complex interplay of T2DM and its associated clinical, structural, functional, metabolite, as well as cognitive distortions. Therefore, a systematic review of 68 manuscripts, following the PRISMA guidelines, was conducted. Notably, although the associations between imaging, clinical, and cognitive variables are not fully homogeneous, findings show a clear trend towards a link between altered brain structure and a decline in cognitive processing ability. The results of the review highlight the heterogeneity of the methods used across manuscripts in terms of assessed clinical variables, imaging, and data analysis methods. This is particularly significant as, if the subjects' criteria are not carefully considered, results are easily prone to confounding factors.

Biomarkers for cognitive decline in patients with diabetes mellitus: evidence from clinical studies

Diabetes mellitus is considered as an important factor for cognitive decline and dementia in recent years. However, cognitive impairment in diabetic patients is often underestimated and kept undiagnosed, leading to thousands of diabetic patients suffering from worsening memory. Available reviews in this field were limited and not comprehensive enough. Thus, the present review aimed to summarize all available clinical studies on diabetic patients with cognitive decline, and to find valuable biomarkers that might be applied as diagnostic and therapeutic targets of cognitive impairment in diabetes. The biomarkers or risk factors of cognitive decline in diabetic patients could be classified into the following three aspects: serum molecules or relevant complications, functional or metabolic changes by neuroimaging tools, and genetic variants. Specifically, factors related to poor glucose metabolism, insulin resistance, inflammation, comorbid depression, micro-/macrovascular complications, adipokines, neurotrophic molecules and Tau protein presented significant changes in diabetic patients with cognitive decline. Besides, neuroimaging platform could provide more clues on the structural, functional and metabolic changes during the cognitive decline progression of diabetic patients. Genetic factors related to cognitive decline showed inconsistency based on the limited studies. Future studies might apply above biomarkers as diagnostic and treatment targets in a large population, and regulation of these parameters might shed light on a more valuable, sensitive and specific strategy for the diagnosis and treatment of cognitive decline in diabetic patients.

Pinocembrin alleviates cognition deficits by inhibiting inflammation in diabetic mice

Diabetic encephalopathy (DE) is one of the most common diabetic complications in diabetes mellitus and is characterized by cognitive impairment and neuroinflammation. It has been reported that hyperglycemia can induce hippocampal and cortical neuronal damage, which can result in severe spatial learning and memory impairment. Pinocembrin (Pino) has been widely used in the therapy of cancer and other diseases due to its anti-inflammatory, anti-allergic, anti-oxidant, anti-carcinogenic, and anti-viral activities. However, the effects of Pino on DE-induced cognition deficits and its precise mechanisms remain largely unknown. Therefore, the aim of this study was to investigate the neuroprotective effects of Pino on cognition and its potential mechanisms in a DE mouse model induced by streptozotocin (STZ, 150mg/kg). Here, we demonstrated that Pino significantly improved the behavior and cognitive deficits of DE mice in open field tasks and the Morris water maze. Pino also markedly increased neuronal survival in the frontal cortex and hippocampal CA3 region. Furthermore, western blotting was performed to measure nuclear translocation of nuclear factor-kappaB (NF-κB) and the expression of tumor necrosis factor-α (TNF-α) in the frontal cortex and hippocampus. The results demonstrated that Pino could suppress the nuclear translocation of NF-κB and decrease TNF-α expression in the cerebral cortex and the hippocampus of DE mice. Taken together, the results suggest that Pino alleviates cognition deficits by protecting neurons from inflammation injury in diabetic mice.

Brain functional alterations in Type 2 Diabetes - A systematic review of fMRI studies

Type 2 Diabetes (T2DM) is emerging as a major global health issue. T2DM can adversely affect cognition and increase dementia risk. This systematic review aimed to examine the functional brain changes that may underlie cognitive dysfunction in adults with T2DM. Studies were restricted to those which used functional magnetic resonance imaging (fMRI). Nineteen independent studies were identified, mostly comprised of middle aged or older adults. Resting-state studies demonstrated that compared to controls, connectivity of the Default Mode Network (DMN) was reduced and the majority of task-based studies identified reduced activation in T2DM patients in regions relevant to task performance. Abnormalities of low frequency spontaneous brain activity were observed, particularly in visual regions. As most studies demonstrated that alterations in fMRI were related to poorer neuropsychological task performance, these results indicate that functional brain abnormalities in T2DM have consequences for cognition.

Cerebral Pathology and Cognition in Diabetes: The Merits of Multiparametric Neuroimaging

Type 2 diabetes mellitus is associated with accelerated cognitive decline and various cerebral abnormalities visible on MRI. The exact pathophysiological mechanisms underlying cognitive decline in diabetes still remain to be elucidated. In addition to conventional images, MRI offers a versatile set of novel contrasts, including blood perfusion, neuronal function, white matter microstructure, and metabolic function. These more-advanced multiparametric MRI contrasts and the pertaining parameters are able to reveal abnormalities in type 2 diabetes, which may be related to cognitive decline. To further elucidate the nature of the link between diabetes, cognitive decline, and brain abnormalities, and changes over time thereof, biomarkers are needed which can be provided by advanced MRI techniques. This review summarizes to what extent MRI, especially advanced multiparametric techniques, can elucidate the underlying neuronal substrate that reflects the cognitive decline in type 2 diabetes.

Islet amyloid polypeptide: Another key molecule in Alzheimer's pathogenesis?

Recent epidemiological evidence reveals that patients suffering from type 2 diabetes mellitus (T2DM) often experience a significant decline in cognitive function, and approximately 70% of those cases eventually develop Alzheimer's disease (AD). Although several pathological processes are shared by AD and T2DM, the exact molecular mechanisms connecting these two diseases are poorly understood. Aggregation of human islet amyloid polypeptide (hIAPP), the pathological hallmark of T2DM, has also been detected in brain tissue and is associated with cognitive decline and AD development. In addition, hIAPP and amyloid β protein (Aβ) share many biophysical and physiological properties as well as exert similar cytotoxic mechanisms. Therefore, it is important to examine the possible role of hIAPP in the pathogenesis of AD. In this article, we introduce the basics on this amyloidogenic protein. More importantly, we discuss the potential mechanisms of hIAPP-induced AD development, which will be beneficial for proposing novel and feasible strategies to optimize AD prevention and/or treatment in diabetics.

Dysfunctional organization of default mode network before memory impairments in type 2 diabetes

AIMS

Episodic memory depends on the maintenance of an intact default mode network (DMN), and is one of the earliest cognitive domains to become impaired in type 2 diabetes mellitus (T2DM). Therefore, exploring the alterations in task-related DMN deactivation and functional connectivity that affect episodic memory in T2DM will help to enhance our understanding of the allocation patterns underlying memory function in T2DM.

METHODS

The current study assessed changes in DMN activation and architecture during the encoding of a functional magnetic resonance imaging task in 39 patients with T2DM and 41 healthy controls which were recruited from local communities in Beijing, China.

RESULTS

We found that patients with T2DM and intact episodic memory performance exhibited reduced deactivation in the right precuneus and the middle temporal gyrus during the episodic memory task. Furthermore, an anterior-posterior disconnection phenotype and altered topological configuration of the DMN were observed in patients with T2DM using graph-theoretical approaches. Correlation analysis showed that altered deactivation during the episodic memory task was closely related to connectivity dysfunction and topological properties in the DMN.

CONCLUSIONS

Our findings demonstrate that the maintenance of memory in patients with T2DM involves reduced deactivation and impaired anterior-posterior connections in the DMN during encoding.

Altered Intranetwork and Internetwork Functional Connectivity in Type 2 Diabetes Mellitus With and Without Cognitive Impairment

Type 2 diabetes mellitus (T2DM) is associated with cognitive impairment. We investigated whether alterations of intranetwork and internetwork functional connectivity with T2DM progression exist, by using resting-state functional MRI. MRI data were analysed from 19 T2DM patients with normal cognition (DMCN) and 19 T2DM patients with cognitive impairment (DMCI), 19 healthy controls (HC). Functional connectivity among 36 previously well-defined brain regions which consisted of 5 resting-state network (RSN) systems [default mode network (DMN), dorsal attention network (DAN), control network (CON), salience network (SAL) and sensorimotor network (SMN)] was investigated at 3 levels (integrity, network and connectivity). Impaired intranetwork and internetwork connectivity were found in T2DM, especially in DMCI, on the basis of the three levels of analysis. The bilateral posterior cerebellum, the right insula, the DMN and the CON were mainly involved in these changes. The functional connectivity strength of specific brain architectures in T2DM was found to be associated with haemoglobin A1c (HbA1c), cognitive score and illness duration. These network alterations in intergroup differences, which were associated with brain functional impairment due to T2DM, indicate that network organizations might be potential biomarkers for predicting the clinical progression, evaluating the cognitive impairment, and further understanding the pathophysiology of T2DM.

Gray matter alterations and correlation of nutritional intake with the gray matter volume in prediabetes

The neurophysiology of prediabetes plays an important role in preventive medicine. The dysregulation of glucose metabolism is likely linked to changes in neuron-related gray matter. Therefore, we designed this study to investigate gray matter alterations in medication-naive prediabetic patients. We expected to find alterations in the gray matter of prediabetic patients.A total of 64 prediabetic patients and 54 controls were enrolled. All subjects received T1 scans using a 3-T magnetic resonance imaging machine. Subjects also completed nutritional intake records at the 24-hour and 3-day time points to determine their carbohydrate, protein, fat, and total calorie intake. We utilized optimized voxel-based morphometry to estimate the gray matter differences between the patients and controls. In addition, the preprandial serum glucose level and the carbohydrate, protein, fat, and total calorie intake levels were tested to determine whether these parameters were correlated with the gray matter volume.Prediabetic patients had lower gray matter volumes than controls in the right anterior cingulate gyrus, right posterior cingulate gyrus, left insula, left super temporal gyrus, and left middle temporal gyrus (corrected P < 0.05; voxel threshold: 33). Gray matter volume in the right anterior cingulate was also negatively correlated with the preprandial serum glucose level gyrus in a voxel-dependent manner (r = -0.501; 2-tailed P = 0.001).The cingulo-temporal and insula gray matter alterations may be associated with the glucose dysregulation in prediabetic patients.

Brain Activation during Memory Encoding in Type 2 Diabetes Mellitus: A Discordant Twin Pair Study

Type 2 diabetes mellitus increases the risk of dementia and neuronal dysfunction may occur years before perceptible cognitive decline. We aimed to study the impact of type 2 diabetes on brain activation during memory encoding in middle-aged people, controlling for age, sex, genes, and early-shared environment. Twenty-two twin pairs discordant for type 2 diabetes mellitus (mean age 60.9 years) without neurological disease were recruited from the Australian Twin Registry (ATR) and underwent functional magnetic resonance imaging (fMRI) during a memory encoding task, cognitive tests, and structural MRI. Type 2 diabetes was associated with significantly reduced activation in left hemisphere temporoparietal regions including angular gyrus, supramarginal gyrus, and middle temporal gyrus and significantly increased activation in bilateral posteriorly distributed regions. These findings were present in the absence of within-pair differences in standard cognitive test scores, brain volumes, or vascular lesion load. Differences in activation were more pronounced among monozygotic (MZ) pairs, with MZ individuals with diabetes also displaying greater frontal activation. These results provide evidence for preclinical memory-related neuronal dysfunction in type 2 diabetes. They support the search for modifiable later-life environmental factors or epigenetic mechanisms linking type 2 diabetes and cognitive decline.

Impact of diabetes on cognitive function and brain structure

Both type 1 and type 2 diabetes have been associated with reduced performance on multiple domains of cognitive function and with structural abnormalities in the brain. With an aging population and a growing epidemic of diabetes, central nervous system-related complications of diabetes are expected to rise and could have challenging future public health implications. In this review, we will discuss the brain structural and functional changes that have been associated with type 1 and type 2 diabetes. Diabetes duration and glycemic control may play important roles in the development of cognitive impairment in diabetes, but the exact underlying pathophysiological mechanisms causing these changes in cognition and structure are not well understood. Future research is needed to better understand the natural history and the underlying mechanisms, as well as to identify risk factors that predict who is at greatest risk of developing cognitive impairment. This information will lead to the development of new strategies to minimize the impact of diabetes on cognitive function.

Poorly controlled cholesterol is associated with cognitive impairment in T2DM: a resting-state fMRI study

BACKGROUND

Debate remains on whether hypercholesterolemia is associated with cognitive impairment. Hence, we investigated whether poorly controlled cholesterol impairs functional connectivity among patients with type 2 diabetes mellitus (T2DM).

METHODS

Resting-state functional connectivity infers to an interregional cooperation characterized by synchronous and low-frequency (<0.08 Hz) fluctuations on blood oxygen level-dependent functional magnetic resonance imaging (fMRI). We used resting-state fMRI to investigate the functional connectivity of 25 T2DM patients with poorly controlled cholesterol, 22 patients with target cholesterol and 26 healthy controls. Further correlation analysis was conducted between the functional connectivity and clinical data as well as neuropsychological tests.

RESULTS

The three groups did not statistically differ in age, sex, education level, body mass index, blood pressure, fasting C-peptides, and triglyceride. Compared with target cholesterol patients, patients with poorly controlled cholesterol showed significantly increased levels of serum cholesterol, low-density lipoprotein (LDL), and LDL/high-density lipoproteins (HDL) ratio, as well as poor performance in Trail Making Test B (TMT-B) (p<0.05). Disordered functional connectivity of bilateral hippocampus-middle frontal gyrus (MFG) in the poorly controlled group consistently existed when compared with the two other groups. Moreover, the aberrant functional connectivity was associated with the TMT-B scores and the LDL/HDL index in T2DM patients with poorly controlled cholesterol.

CONCLUSIONS

T2DM patients with poorly controlled cholesterol showed impaired attention and executive function. The resting-state connectivity disturbance of the hippocampus-MFG may be involved in this process. Decreasing the LDL/HDL ratio can be taken as precaution against cognitive decrements.

The GLP-1 Receptor Agonist Exendin-4 and Diazepam Differentially Regulate GABAA Receptor-Mediated Tonic Currents in Rat Hippocampal CA3 Pyramidal Neurons

Glucagon-like peptide-1 (GLP-1) is a metabolic hormone that is secreted in a glucose-dependent manner and enhances insulin secretion. GLP-1 receptors are also found in the brain where their signalling affects neuronal activity. We have previously shown that the GLP-1 receptor agonists, GLP-1 and exendin-4 enhanced GABA-activated synaptic and tonic currents in rat hippocampal CA3 pyramidal neurons. The hippocampus is the centre for memory and learning and is important for cognition. Here we examined if exendin-4 similarly enhanced the GABA-activated currents in the presence of the benzodiazepine diazepam. In whole-cell recordings in rat brain slices, diazepam (1 μM), an allosteric positive modulator of GABA_A receptors, alone enhanced the spontaneous inhibitory postsynaptic current (sIPSC) amplitude and frequency by a factor of 1.3 and 1.6, respectively, and doubled the tonic GABA_A current normally recorded in the CA3 pyramidal cells. Importantly, in the presence of exendin-4 (10 nM) plus diazepam (1 μM), only the tonic but not the sIPSC currents transiently increased as compared to currents recorded in the presence of diazepam alone. The results suggest that exendin-4 potentiates a subpopulation of extrasynaptic GABA_A receptors in the CA3 pyramidal neurons.

Aberrant functional connectivity of default-mode network in type 2 diabetes patients

OBJECTIVES

Type 2 diabetes mellitus is associated with increased risk for dementia. Patients with impaired cognition often show default-mode network disruption. We aimed to investigate the integrity of a default-mode network in diabetic patients by using independent component analysis, and to explore the relationship between network abnormalities, neurocognitive performance and diabetic variables.

METHODS

Forty-two patients with type 2 diabetes and 42 well-matched healthy controls were included and underwent resting-state functional MRI in a 3 Tesla unit. Independent component analysis was adopted to extract the default-mode network, including its anterior and posterior components. Z-maps of both sub-networks were compared between the two groups and correlated with each clinical variable.

RESULTS

Patients showed increased connectivity around the medial prefrontal cortex in the anterior sub-network, but decreased connectivity around the posterior cingulate cortex in the posterior sub-network. The decreased connectivity in the posterior part was significantly correlated with the score on Complex Figure Test-delay recall test (r = 0.359, p = 0.020), the time spent on Trail-Making Test-part B (r = -0.346, p = 0.025) and the insulin resistance level (r = -0.404, p = 0.024).

CONCLUSION

Dissociation pattern in the default-mode network was found in diabetic patients, which might provide powerful new insights into the neural mechanisms that underlie the diabetes-related cognitive decline.

KEY POINTS

• Type 2 diabetes mellitus is associated with impaired cognition • Default- mode network plays a central role in maintaining normal cognition • Network connectivity within the default mode was disrupted in type 2 diabetes patients • Decreased network connectivity was correlated with cognitive performance and insulin resistance level • Disrupted default-mode network might explain the impaired cognition in diabetic population.

Brain alterations and clinical symptoms of dementia in diabetes: aβ/tau-dependent and independent mechanisms

Emerging evidence suggests that diabetes affects cognitive function and increases the incidence of dementia. However, the mechanisms by which diabetes modifies cognitive function still remains unclear. Morphologically, diabetes is associated with neuronal loss in the frontal and temporal lobes including the hippocampus, and aberrant functional connectivity of the posterior cingulate cortex and medial frontal/temporal gyrus. Clinically, diabetic patients show decreased executive function, information processing, planning, visuospatial construction, and visual memory. Therefore, in comparison with the characteristics of AD brain structure and cognition, diabetes seems to affect cognitive function through not only simple AD pathological feature-dependent mechanisms but also independent mechanisms. As an Aβ/tau-independent mechanism, diabetes compromises cerebrovascular function, increases subcortical infarction, and might alter the blood-brain barrier. Diabetes also affects glucose metabolism, insulin signaling, and mitochondrial function in the brain. Diabetes also modifies metabolism of Aβ and tau and causes Aβ/tau-dependent pathological changes. Moreover, there is evidence that suggests an interaction between Aβ/tau-dependent and independent mechanisms. Therefore, diabetes modifies cognitive function through Aβ/tau-dependent and independent mechanisms. Interaction between these two mechanisms forms a vicious cycle.