Thermotherapy has Sexually Dimorphic Responses in APP/PS1 Mice

Abstract

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, but future studies are needed to determine therapeutic benefits for those with AD.

Keywords: Alzheimer’s disease; cognition; core body temperature; glucose tolerance; insulin sensitivity; metabolism.

Figure 1:. Effects of thermotherapy on Tc.

Rectal temperature was determined before (23°C) and after one month of thermotherapy treatment (30°C). The number of animals is inset on each bar graph. A two-tailed t-test was used to determine changes in Tc within a genotype. *p<0.05, **p<0.01.

Figure 2:. Blood glucose changes after thermotherapy.

Percent change of blood glucose levels after an ip injection of 1 IU / kg bw of insulin at t=0 minutes (A). The AUC was calculated for the duration of the ITT and compared temperature effects in male (B) and female (C) mice. Percent change of blood glucose levels after an ip injection of 2 g / kg bw of glucose at t=0 minutes (D). The AUC was calculated for both male (E) and female (F) mice. Fed (G) and fasting (H) blood glucose levels were determined prior to ip injection of insulin and glucose, respectively. The number of animals is inset on each bar graph. A two-way ANOVA factor analysis with Sidak’s post hoc was used to determine significant blood glucose changes due to thermotherapy across time intervals. A two-tailed t-test was used to determine blood glucose changes within a genotype and sex. *p<0.05, **p<0.01, ****p<0.0001.

Figure 3:. Plasma concentrations of glucose regulating hormones.

Plasma expression levels of glucagon (A), glucagon-like peptide 1 (GLP1; B), insulin (C), fibroblast growth factor 21 (FGF21; E), and B-cell activating factor (BAFF; E) as detected by multiplex assay. A two-tailed t-test was used to determine plasma concentration changes within a genotype and sex. *p<0.05, **p<0.01, ***p<0.001.

Figure 4:. Hepatic, perigonadal white adipose tissue, and skeletal muscle mRNA expression.

Hepatic mRNA expression of insulin receptor (INSR; A), phosphatidylinositol 3-kinase (PI3K; B), AKT1 (C), glucose transporter 2 (GLUT2; D), glucokinase (GCK; E), and glucose 6-phosphatase (G6PC; F) relative to B2M. Perigonadal WAT mRNA expression changes of uncoupling protein 1 (UCP1; G), PGC-1α (PPARGC1A; H), INSR (I), tumor necrosis factor α (TNFA; J), and interleukin 6 (IL6; K) relative to B2M. GLUT4 (L) and 1 (M) mRNA expression relative to GAPDH in skeletal muscle. A two-tailed t-test was used to determine mRNA expression fold changes within a genotype and sex. The number of animals is either inset or above the error bars on each bar graph. *p<0.05, **p<0.01, ***p<0.001.

Figure 5:. Sexually dimorphic spatial navigation responses to thermotherapy in APP/PS1 mice.

Average latency to the platform during the five MWM training sessions (A). A two-way ANOVA with Sidak’s post hoc was used to determine significant time differences due to thermotherapy across training days. The number of platform entries (B), latency to first platform entry (C), number of annulus 40 entries (D), latency to first annulus 40 entry (E), and swimming speed (F) during the delayed MWM probe challenge. A two-tailed t-test was used to determine thermotherapy effects on probe challenge parameters within a sex and genotype. The number of animals is inset on each bar graph. *p<0.05, **p<0.01.

Figure 6:. Thermotherapy decreases hippocampal soluble Aβ₄₂ in male mice.

Average concentration of hippocampal soluble Aβ₄₂ was determined by ELISA in male and female mice. C57BL/6 mice were used as a negative control as denoted by the segmented y-axis. A two-tailed t test was used to determine thermotherapy effects within a sex and genotype. The number of animals is either inset or above the error bars on each bar graph. *p<0.05.