Thermotherapy has Sexually Dimorphic Responses in APP/PS1 Mice

A thermoregulatory decline occurs with age due to changes in muscle mass, vasoconstriction, and metabolism that lowers core body temperature (Tc). Although lower Tc is a biomarker of successful aging, we have previously shown this worsens cognitive performance in the APP/PS1 mouse model of Alzheimer's disease (AD) [1]. We hypothesized that elevating Tc with thermotherapy would improve metabolism and cognition in APP/PS1 mice. From 6-12 months of age, male and female APP/PS1 and C57BL/6 mice were chronically housed at 23 or 30°C. At 12 months of age, mice were assayed for insulin sensitivity, glucose tolerance, and spatial cognition. Plasma, hippocampal, and peripheral (adipose, hepatic, and skeletal muscle) samples were procured postmortem and tissue-specific markers of amyloid accumulation, metabolism, and inflammation were assayed. Chronic 30°C exposure increased Tc in all groups except female APP/PS1 mice. All mice receiving thermotherapy had either improved glucose tolerance or insulin sensitivity, but the underlying processes responsible for these effects varied across sexes. In males, glucose regulation was influenced predominantly by hormonal signaling in plasma and skeletal muscle glucose transporter 4 expression, whereas in females, this was modulated at the tissue level. Thermotherapy improved spatial navigation in male C57BL/6 and APP/PS1 mice, with the later attributed to reduced hippocampal soluble amyloid-β (Aβ)42. Female APP/PS1 mice exhibited worse spatial memory recall after chronic thermotherapy. Together, the data highlights the metabolic benefits of passive thermotherapy with potential nonpharmacological management for some individuals with AD, and provides further evidence for the necessity of adopting personalized patient care.

Prodromal Glutamatergic Modulation with Riluzole Impacts Glucose Homeostasis and Spatial Cognition in Alzheimer's Disease Mice

BACKGROUND

Prior research supports a strong link between Alzheimer's disease (AD) and metabolic dysfunction that involves a multi-directional interaction between glucose, glutamatergic homeostasis, and amyloid pathology. Elevated soluble amyloid-β (Aβ) is an early biomarker for AD-associated cognitive decline that contributes to concurrent glutamatergic and metabolic dyshomeostasis in humans and male transgenic AD mice. Yet, it remains unclear how primary time-sensitive targeting of hippocampal glutamatergic activity may impact glucose regulation in an amyloidogenic mouse model. Previous studies have illustrated increased glucose uptake and metabolism using a neuroprotective glutamate modulator (riluzole), supporting the link between glucose and glutamatergic homeostasis.

OBJECTIVE

We hypothesized that targeting early glutamatergic hyperexcitation through riluzole treatment could aid in attenuating co-occurring metabolic and amyloidogenic pathologies with the intent of ameliorating cognitive decline.

METHODS

We conducted an early intervention study in male and female transgenic (AβPP/PS1) and knock-in (APPNL - F/NL - F) AD mice to assess the on- and off-treatment effects of prodromal glutamatergic modulation (2-6 months of age) on glucose homeostasis and spatial cognition through riluzole treatment.

RESULTS

Results indicated a sex- and genotype-specific effect on glucose homeostasis and spatial cognition with riluzole intervention that evolved with disease progression and time since treatment.

CONCLUSION

These findings support the interconnected nature of glucose and glutamatergic homeostasis with amyloid pathology and petition for further investigation into the targeting of this relationship to improve cognitive performance.

Health disparities in aging: Improving dementia care for Black women

In the United States, 80% of surveyed Black patients report experiencing barriers to healthcare for Alzheimer's disease and related dementias (ADRD), delaying the time-sensitive treatment of a progressive neurodegenerative disease. According to the National Institute on Aging, Black study participants are 35% less likely to be given a diagnosis of ADRD than white participants, despite being twice as likely to suffer from ADRD than their white counterparts. Prior analysis of prevalence for sex, race, and ethnicity by the Centers for Disease Control indicated the highest incidence of ADRD in Black women. Older (≥65 years) Black women are at a disproportionately high risk for ADRD and yet these patients experience distinct inequities in obtaining clinical diagnosis and treatment for their condition. To that end, this perspective article will review a current understanding of biological and epidemiological factors that underlie the increased risk for ADRD in Black women. We will discuss the specific barriers Black women face in obtaining access to ADRD care, including healthcare prejudice, socioeconomic status, and other societal factors. This perspective also aims to evaluate the performance of intervention programs targeted toward this patient population and offer possible solutions to promote health equity.

Chronic, Mild Hypothermic Environmental Temperature does not Ameliorate Cognitive Deficits in an Alzheimer's Disease Mouse

Metabolic dysfunction increases with age and is a contributing factor to Alzheimer's disease (AD) development. We have previously observed impaired insulin sensitivity and glucose homeostasis in the APP/PS1 model of AD. To improve these parameters, we chronically exposed male and female mice to mild hypothermic environmental temperature (eT), which positively modulates metabolism. Although a hypothermic eT normalized insulin sensitivity, glucose tolerance was still impaired in both sexes of AD mice. We observed increased plasma glucagon and BAFF in both sexes, but additional sexually dimorphic mechanisms may explain the impaired glucose homeostasis in AD mice. Hepatic Glut2 was decreased in female while visceral adipose tissue TNFα was increased in male APP/PS1 mice. A mild hypothermic eT did not improve spatial learning and memory in either sex and increased amyloid plaque burden in male APP/PS1 mice. Overall, plasma markers of glucose homeostasis and AD pathology were worse in female compared to male APP/PS1 mice suggesting a faster disease progression. This could affect therapeutic outcome if interventional strategies are administered at the same chronological age to male and female APP/PS1 mice. Furthermore, this data suggests a dichotomy exists between mechanisms to improve metabolic function and cognitive health that may be further impaired in AD.

Riluzole attenuates glutamatergic tone and cognitive decline in AβPP/PS1 mice

We have previously demonstrated hippocampal hyperglutamatergic signaling occurs prior to plaque accumulation in AβPP/PS1 mice. Here, we evaluate 2-Amino-6-(trifluoromethoxy) benzothiazole (riluzole) as an early intervention strategy for Alzheimer's disease (AD), aimed at restoring glutamate neurotransmission prior to substantial Beta amyloid (Aβ) plaque accumulation and cognitive decline. Male AβPP/PS1 mice, a model of progressive cerebral amyloidosis, were treated with riluzole from 2-6 months of age. Morris water maze, in vivo electrochemistry, and immunofluorescence were performed to assess cognition, glutamatergic neurotransmission, and pathology, respectively, at 12 months. Four months of prodromal riluzole treatment in AβPP/PS1 mice resulted in long-lasting procognitive effects and attenuated glutamatergic tone that was observed six months after discontinuing riluzole treatment. Riluzole-treated AβPP/PS1 mice had significant improvement in long-term memory compared to vehicle-treated AβPP/PS1 mice that was similar to normal aging C57BL/6J control mice. Furthermore, basal glutamate concentration and evoked-glutamate release levels, which were elevated in vehicle-treated AβPP/PS1 mice, were restored to levels observed in age-matched C57BL/6J mice in AβPP/PS1 mice receiving prodromal riluzole treatment. Aβ plaque accumulation was not altered with riluzole treatment. This study supports that interventions targeting the glutamatergic system during the early stages of AD progression have long-term effects on disease outcome, and importantly may prevent cognitive decline. Our observations provide preclinical support for targeting glutamate neurotransmission in patients at risk for developing AD. Read the Editorial Highlight for this article on page 399.

Hippocampal alterations in glutamatergic signaling during amyloid progression in AβPP/PS1 mice

Our previous research demonstrated that soluble amyloid-β (Aβ)₄₂, elicits presynaptic glutamate release. We hypothesized that accumulation and deposition of Aβ altered glutamatergic neurotransmission in a temporally and spatially dependent manner. To test this hypothesis, a glutamate selective microelectrode array (MEA) was used to monitor dentate (DG), CA3, and CA1 hippocampal extracellular glutamate levels in 2–4, 6–8, and 18–20 month-old male AβPP/PS1 and age-matched C57BL/6J control mice. Starting at 6 months of age, AβPP/PS1 basal glutamate levels are elevated in all three hippocampal subregions that becomes more pronounced at the oldest age group. Evoked glutamate release was elevated in all three age groups in the DG, but temporally delayed to 18–20 months in the CA3 of AβPP/PS1 mice. However, CA1 evoked glutamate release in AβPP/PS1 mice was elevated at 2–4 months of age and declined with age. Plaque deposition was anatomically aligned (but temporally delayed) with elevated glutamate levels; whereby accumulation was first observed in the CA1 and DG starting at 6–8 months that progressed throughout all hippocampal subregions by 18–20 months of age. The temporal hippocampal glutamate changes observed in this study may serve as a biomarker allowing for time point specific therapeutic interventions in Alzheimer’s disease patients.

LY379268 Does Not Have Long-Term Procognitive Effects nor Attenuate Glutamatergic Signaling in AβPP/PS1 Mice

Chronically elevated basal glutamate levels are hypothesized to attenuate detection of physiological signals thereby inhibiting memory formation and retrieval, while inducing excitotoxicity-mediated neurodegeneration observed in Alzheimer's disease (AD). However, current medication targeting the glutamatergic system, such as memantine, shows limited efficacy and is unable to decelerate disease progression, possibly because it modulates postsynaptic N-methyl-D-aspartate receptors rather than glutamate release or clearance. To determine if decreasing presynaptic glutamate release leads to long-term procognitive effects, we treated AβPP/PS1 mice with LY379268 (3.0 mg/kg; i.p.), a metabotropic glutamate receptor (mGluR)2/3 agonist from 2-6 months of age when elevated glutamate levels are first observed but cognition is unaffected. C57BL/6J genetic background control mice and another cohort of AβPP/PS1 mice received normal saline (i.p.) as vehicle controls. After 6 months off treatment, mice receiving LY379268 did not show long-term improvement as assessed by the Morris water maze (MWM) spatial learning and memory paradigm. Following MWM, mice were isoflurane anesthetized and a glutamate selective microelectrode was used to measure in vivo basal and stimulus-evoked glutamate release and clearance independently from the dentate, CA3, and CA1 hippocampal subregions. Immunohistochemistry was used to measure hippocampal astrogliosis and plaque pathology. Similar to previous studies, we observed elevated basal glutamate, stimulus evoked glutamate release, and astrogliosis in AβPP/PS1 vehicle mice versus C57BL/6J mice. Treatment with LY379268 did not attenuate these responses nor diminish plaque pathology. The current study builds upon previous research demonstrating hyperglutamatergic hippocampal signaling in AβPP/PS1 mice; however, long-term therapeutic efficacy of LY379268 in AβPP/PS1 was not observed.

Amyloid Beta-Related Alterations to Glutamate Signaling Dynamics During Alzheimer's Disease Progression

Alzheimer’s disease (AD) ranks sixth on the Centers for Disease Control and Prevention Top 10 Leading Causes of Death list for 2016, and the Alzheimer’s Association attributes 60% to 80% of dementia cases as AD related. AD pathology hallmarks include accumulation of senile plaques and neurofibrillary tangles; however, evidence supports that soluble amyloid beta (Aβ), rather than insoluble plaques, may instigate synaptic failure. Soluble Aβ accumulation results in depression of long-term potentiation leading to cognitive deficits commonly characterized in AD. The mechanisms through which Aβ incites cognitive decline have been extensively explored, with a growing body of evidence pointing to modulation of the glutamatergic system. The period of glutamatergic hypoactivation observed alongside long-term potentiation depression and cognitive deficits in later disease stages may be the consequence of a preceding period of increased glutamatergic activity. This review will explore the Aβ-related changes to the tripartite glutamate synapse resulting in altered cell signaling throughout disease progression, ultimately culminating in oxidative stress, synaptic dysfunction, and neuronal loss.