Elevated Insulin and Insulin Resistance are Associated with Altered Myelin in Cognitively Unimpaired Middle-Aged Adults

OBJECTIVE

Insulin regulates metabolism and influences neural health. Insulin resistance (IR) and type II diabetes have been identified as risk factors for Alzheimer disease (AD). Evidence has also suggested that myelinated white matter alterations may be involved in the pathophysiology of AD; however, it is unknown whether insulin or IR affect the underlying myelin microstructure. The relationships between insulin, IR, and myelin were examined, with the hypothesis that IR would be associated with reduced myelin.

METHODS

Cognitively unimpaired adults enriched for risk factors for AD underwent multicomponent driven equilibrium single pulse observation of T1 and T2 imaging, a myelin-sensitive neuroimaging technique. Linear regressions were used to test the relationship between homeostatic model assessment of IR, insulin, and myelin water fraction (MWF) as well as interactions with APOE ε4.

RESULTS

Both IR and insulin level were associated with altered myelin content, wherein a significant negative association with MWF was observed in white matter regions and a positive association with MWF was observed in gray matter.

CONCLUSIONS

The results suggest that insulin and IR influence white matter myelination in a cognitively unimpaired population. Additional studies are needed to determine the extent to which this may contribute to cognitive decline or vulnerability to neurodegenerative disease.

Insulin Resistance is Associated with Increased Levels of Cerebrospinal Fluid Biomarkers of Alzheimer's Disease and Reduced Memory Function in At-Risk Healthy Middle-Aged Adults

BACKGROUND

Type 2 diabetes is associated with an increased risk for Alzheimer's disease (AD). Regulation of normal insulin function may be important in reducing the prevalence of dementia due to AD, particularly in individuals who harbor genetic risk for or have a parental family history of AD. The relationship between insulin resistance (IR) and AD pathology remains poorly understood, particularly in midlife prior to the onset of clinical metabolic disease or cognitive decline.

OBJECTIVE

We examined associations between IR as indexed by HOMA-IR, cerebrospinal fluid (CSF) biomarkers of AD pathology, and memory in middle-aged adults enriched for AD. We postulated that higher HOMA-IR and APOEɛ4 carriage would be associated with greater CSF AD pathology and poor memory performance.

METHODS

Cognitively asymptomatic middle-aged adults (N = 70, mean age = 57.7 years) from the Wisconsin Alzheimer's Disease Research Center with a parental family history of dementia due to AD underwent lumbar puncture, blood draw, and neuropsychological testing. CSF AD biomarkers including soluble amyloid-β protein precursor β (sAβPPβ), amyloid-β42 (Aβ42), and phosphorylated tau (P-tau181) were examined with respect to HOMA-IR and APOEɛ4 status. Delayed memory performance was examined with respect to HOMA-IR, CSF AD biomarkers, and APOEɛ4 status.

RESULTS

Higher HOMA-IR was associated with higher sAβPPβ and Aβ42 . APOEɛ4 carriers had significantly higher levels of sAβPPα, sAβPPβ, and P-tau181/Aβ42 compared to noncarriers. The concurrent presence of higher HOMA-IR and CSF AD pathology predicted worse delayed memory performance.

CONCLUSION

Overall, the findings suggest that IR and APOEɛ4 are contributing factors to the development of AD pathology in midlife, and provide support for targeting insulin function as a potentially modifiable risk factor for AD.

Insulin resistance is associated with lower arterial blood flow and reduced cortical perfusion in cognitively asymptomatic middle-aged adults

Insulin resistance (IR) is associated with poor cerebrovascular health and increased risk for dementia. Little is known about the unique effect of IR on both micro- and macrovascular flow particularly in midlife when interventions against dementia may be most effective. We examined the effect of IR as indexed by the Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) on cerebral blood flow in macro- and microvessels utilizing magnetic resonance imaging (MRI) among cognitively asymptomatic middle-aged individuals. We hypothesized that higher HOMA-IR would be associated with reduced flow in macrovessels and lower cortical perfusion. One hundred and twenty cognitively asymptomatic middle-aged adults (57 ± 5 yrs) underwent fasting blood draw, phase contrast-vastly undersampled isotropic projection reconstruction (PC VIPR) MRI, and arterial spin labeling (ASL) perfusion. Higher HOMA-IR was associated with lower arterial blood flow, particularly within the internal carotid arteries (ICAs), and lower cerebral perfusion in several brain regions including frontal and temporal lobe regions. Higher blood flow in bilateral ICAs predicted greater cortical perfusion in individuals with lower HOMA-IR, a relationship not observed among those with higher HOMA-IR. Findings provide novel evidence for an uncoupling of macrovascular blood flow and microvascular perfusion among individuals with higher IR in midlife.

Positive affect predicts cerebral glucose metabolism in late middle-aged adults

Positive affect is associated with a number of health benefits; however, few studies have examined the relationship between positive affect and cerebral glucose metabolism, a key energy source for neuronal function and a possible index of brain health. We sought to determine if positive affect was associated with cerebral glucose metabolism in late middle-aged adults (n = 133). Participants completed the positive affect subscale of the Center for Epidemiological Studies Depression Scale at two time points over a two-year period and underwent 18F-fluorodeoxyglucose-positron emission tomography scanning. After controlling for age, sex, perceived health status, depressive symptoms, anti-depressant use, family history of Alzheimer’s disease, APOE ε4 status and interval between visits, positive affect was associated with greater cerebral glucose metabolism across para-/limbic, frontal, temporal and parietal regions. Our findings provide evidence that positive affect in late midlife is associated with greater brain health in regions involved in affective processing and also known to be susceptible to early neuropathological processes. The current findings may have implications for interventions aimed at increasing positive affect to attenuate early neuropathological changes in at-risk individuals.

An Examination of Brain Abnormalities and Mobility in Individuals with Mild Cognitive Impairment and Alzheimer's Disease

Background: Mobility changes are concerning for elderly patients with cognitive decline. Given frail older individuals' vulnerability to injury, it is critical to identify contributors to limited mobility. Objective: To examine whether structural brain abnormalities, including reduced gray matter volume and white matter hyperintensities, would be associated with limited mobility among individuals with cognitive impairment, and to determine whether cognitive impairment would mediate this relationship. Methods: Thirty-four elderly individuals with mild cognitive impairment (MCI) and Alzheimer's disease underwent neuropsychological evaluation, mobility assessment, and structural brain neuroimaging. Linear regression was conducted with predictors including gray matter volume in six regions of interest (ROI) and white matter hyperintensity (WMH) burden, with mobility measures as outcomes. Results: Lower gray matter volume in caudate nucleus was associated with slower speed on a functional mobility task. Higher cerebellar volume was also associated with slower functional mobility. White matter hyperintensity burden was not significantly associated with mobility. Conclusion: Our findings provide evidence for associations between subcortical gray matter volume and speed on a functional mobility task among cognitively impaired individuals.

Cerebrospinal Fluid Markers of Alzheimer's Disease Pathology and Microglial Activation are Associated with Altered White Matter Microstructure in Asymptomatic Adults at Risk for Alzheimer's Disease

BACKGROUND

The immune response in Alzheimer's disease (AD) involves activation of microglia which may remove amyloid-β (Aβ). However, overproduction of inflammatory compounds may exacerbate neural damage in AD. AD pathology accumulates years before diagnosis, yet the extent to which neuroinflammation is involved in the earliest disease stages is unknown.

OBJECTIVE

To determine whether neuroinflammation exacerbates neural damage in preclinical AD.

METHODS

We utilized cerebrospinal fluid (CSF) and magnetic resonance imaging collected in 192 asymptomatic late-middle-aged adults (mean age = 60.98 years). Neuroinflammatory markers chitinase-3-like protein 1 (YKL-40) and monocyte chemoattractant protein-1 (MCP-1) in CSF were utilized as markers of neuroinflammation. Neural cell damage was assessed using CSF neurofilament light chain protein (NFL), CSF total tau (T-Tau), and neural microstructure assessed with diffusion tensor imaging (DTI). With regard to AD pathology, CSF Aβ42 and tau phosphorylated at threonine 181 (P-Tau181) were used as markers of amyloid and tau pathology, respectively. We hypothesized that higher YKL-40 and MCP-1 in the presence of AD pathology would be associated with higher NFL, T-Tau, and altered microstructure on DTI.

RESULTS

Neuroinflammation was associated with markers of neural damage. Higher CSF YKL-40 was associated with both higher CSF NFL and T-Tau. Inflammation interacted with AD pathology, such that greater MCP-1 and lower Aβ42 was associated with altered microstructure in bilateral frontal and right temporal lobe and that greater MCP-1 and greater P-Tau181 was associated with altered microstructure in precuneus.

CONCLUSION

Inflammation may play a role in neural damage in preclinical AD.

Insulin Resistance is Associated with Higher Cerebrospinal Fluid Tau Levels in Asymptomatic APOEɛ4 Carriers

BACKGROUND

Insulin resistance (IR) is linked with the occurrence of pathological features observed in Alzheimer's disease (AD), including neurofibrillary tangles and amyloid plaques. However, the extent to which IR is associated with AD pathology in the cognitively asymptomatic stages of preclinical AD remains unclear.

OBJECTIVE

To determine the extent to which IR is linked with amyloid and tau pathology in late-middle-age.

METHOD

Cerebrospinal fluid (CSF) samples collected from 113 participants enrolled in the Wisconsin Registry for Alzheimer's Prevention study (mean age = 60.6 years), were assayed for AD-related markers of interest: Aβ₄₂, P-Tau181, and T-Tau. IR was determined using the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR). Linear regression was used to test the effect of IR, and APOEɛ4, on tau and amyloid pathology. We hypothesized that greater IR would be associated with higher CSF P-Tau181 and T-Tau, and lower CSF Aβ₄₂.

RESULTS

No significant main effects of HOMA-IR on P-Tau181, T-Tau, or Aβ₄₂ were observed; however, significant interactions were observed between HOMA-IR and APOEɛ4 on CSF markers related to tau. Among APOEɛ4 carriers, higher HOMA-IR was associated with higher P-Tau181 and T-Tau. Among APOEɛ4 non-carriers, HOMA-IR was negatively associated with P-Tau181 and T-Tau. We found no effects of IR on Aβ₄₂ levels in CSF.

CONCLUSION

IR among asymptomatic APOEɛ4 carriers was associated with higher P-Tau181 and T-Tau in late-middle age. The results suggest that IR may contribute to tau-related neurodegeneration in preclinical AD. The findings may have implications for developing prevention strategies aimed at modifying IR in mid-life.