Neurocardiovascular deficits in the Q175 mouse model of Huntington's disease

Abstract

Cardiovascular dysautonomia as well as the deterioration of circadian rhythms are among the earliest detectable pathophysiological changes in individuals with Huntington's disease (HD). Preclinical research requires mouse models that recapitulate disease symptoms and the Q175 knock-in model offers a number of advantages but potential autonomic dysfunction has not been explored. In this study, we sought to test the dual hypotheses that cardiovascular dysautonomia can be detected early in disease progression in the Q175 model and that this dysfunction varies with the daily cycle. Using radiotelemetry implants, we observed a significant reduction in the diurnal and circadian activity rhythms in the Q175 mutants at the youngest ages. By middle age, the autonomically driven rhythms in core body temperature were highly compromised, and the Q175 mutants exhibited striking episodes of hypothermia that increased in frequency with mutant huntingtin gene dosage. In addition, Q175 mutants showed higher resting heart rate (HR) during sleep and greatly reduced correlation between activity and HR HR variability was reduced in the mutants in both time and frequency domains, providing more evidence of autonomic dysfunction. Measurement of the baroreceptor reflex revealed that the Q175 mutant could not appropriately increase HR in response to a pharmacologically induced decrease in blood pressure. Echocardiograms showed reduced ventricular mass and ejection fraction in mutant hearts. Finally, cardiac histopathology revealed localized points of fibrosis resembling those caused by myocardial infarction. Thus, the Q175 mouse model of HD exhibits cardiovascular dysautonomia similar to that seen in HD patients with prominent sympathetic dysfunction during the resting phase of the activity rhythm.

Keywords: Autonomic nervous system; Huntington's disease; baroreceptor reflex; cardiovascular function; circadian rhythms; core body temperature; echocardiograms; electrocardiograms; fibrosis; heart rate variability; radio telemetry.

Figure 1

Q175 mice exhibited reduced amplitude of the diurnal and circadian activity rhythms as measured by telemetry starting at 3 months of age (WT, n = 8; Het, n = 10; Hom, n = 9 for all of the telemetry measurements). All of the mice exhibited significant activity rhythms in LD and DD conditions. The activity levels during the night were significantly altered by the Q175 mutation in the young mutants. (A) LD conditions: activity was unaltered during the day but significantly reduced throughout the night in the Hom and Het Q175 compared to WT. (B) DD conditions: activity was significantly reduced throughout the subjective night in the Hom and Het Q175 compared to WT. (C) By middle age, activity was significantly reduced only in the Hom Q175. The differences in activity between Het Q175 and WT were no longer significant as the WT mice began exhibiting the typical age‐related decline. (D) In DD conditions, the middle aged Hom Q175 exhibited greatly reduced activity during the night. Please see Table 1 for results of statistical tests. Asterisks indicate P < 0.05 between the genotypes at each of the hourly bins (ZT or CT).

Figure 2

Hom Q175 mice exhibited disrupted rhythms in CBT in middle age. (A, B) The young adult Q175 exhibited a significantly different CBT rhythm in LD and DD conditions. Only a few phases late in the night were impacted. (C, D) By middle age, there were significant differences in the CBT rhythm in LD and DD. The mutant mice exhibited episodes of hypothermia that were most severe in the Hom Q175. Please see Table 1 for results of statistical tests. Asterisks indicate P < 0.05 between the genotypes at each of the hourly bins (ZT or CT).

Figure 3

Q175 mice exhibited altered diurnal and circadian resting HR rhythms. (A, B) The young Q175 exhibited significantly different resting HR rhythms under both LD and DD. The effects were most striking under LD conditions where the mutants showed high resting HR. (C, D) By middle age, this tachycardia reversed and we actually saw low resting HR in the Hom Q175 under DD conditions. Please see Table 1 for results of statistical tests. Asterisks indicate P < 0.05 between the genotypes at each of the hourly bins (ZT or CT).

Figure 4

Q175 mice exhibited an abnormal relationship between HR and activity. (A) The young adult Hom Q175 exhibited high HR even while they exhibited low levels of activity. (B) By middle age, the Hom Q175 exhibited lower HR and lower activity in the night. WT data shown with black lines (HR) and fill (activity); the Het Q175 data are shown with gray lines (HR) and fill (activity); the Hom Q175 data are shown with red lines (HR) and fill (activity). C) Q175 homozygotes exhibited significant loss of correlation of HR with activity levels (P < 0.001) in young mice. (D) By middle age, differences are diminished by the effects of aging and the progression of autonomic pathology in the mutants. WT data shown with black lines (HR) and fill (activity); Het Q175 data shown with gray lines (HR) and fill (activity); Hom Q175 data shown with red lines (HR) and fill (activity). WT, wild‐type.

Figure 5

Q175 mice exhibited low HRV as measured at 3 months. (A) The young Q175 (Hom, Het) exhibited significantly reduced HRV. Most phases of the daily cycle were impacted. (B) The HF domain (1.5–5.0 Hz) was largely unaltered except at 2 phases late in the night. (C) The LF domain (0.2–1.5 Hz) was reduced in the mutants (Hom, Het) throughout the night. Please see Table 1 for results of statistical tests. Asterisks indicate P < 0.05 between the genotypes at each of the hourly bins (ZT or CT).

Figure 6

Baroreceptor evaluations were used to determine the autonomic responsivity to blood pressure perturbations (WT, n = 7; Het, n = 6; Hom, n = 9). As expected, the WT mice had normal reflex responses. (A) When the blood pressure was transiently lowered with nitroprusside (NP, 40 μg/kg), the heart rate was elevated. (B) When the blood pressure was transiently increased with angiotensin II (AT, 4 μg/kg), the heart rate dropped in compensation. The impact of ATII and NP were blocked by pretreatment with sympathetic and parasympathetic receptor antagonists (750 μg/kg propranolol, 75 μg/kg glycopyrrolate). The Hom Q175 mice had very blunted HR responses to NP (n = 9). While some of the Het Q175 (2 out of 6 animals) showed a greatly reduced response to the NP injection, the overall NP‐response was not significantly different from WT. WT, wild‐type, ATII, administration of angiotensin II.

Figure 7

The age‐dependent progression in heart dysfunction was evaluated in a separate cohort of mice using echocardiograms starting at 3 months of age and progressing to 12 months (WT, n = 12; Het, n = 10; Hom, n = 10). Both cardiac structural (Lv mass, EDD, ESD) and functional (Lv % FS, E/A ratio, and Lv EF) deficits were observed and exhibited significant effects of age and genotype (Table 4).

Figure 8

Histological analyses indicated heart pathology in the Q175 mouse. (A) Representative images of Masson's trichrome stained hearts of each genotype showing that the gross dimensions of the Hom Q175 heart were strikingly smaller. (B) Mutants had reduced cardiomyocyte size as well as altered cytoarchitecture as shown by WGA staining. (C) Masson's trichrome stain revealed greatly increased incidence of fibrotic lesions in Q175 Hom hearts. (D) These infarcts were present in each genotype, but were more common (about 3–4 folds) in both ventricles and anteriorly in the interventricular septum of the Hom Q175. Please see Table 5 and 6 for the results of statistical tests. Data are shown as the Mean ± SEM (n = 5–6 animals/genotype). *P < 0.05. The comparison between the number of infarcts was made with Kruskal–Wallis ANOVA followed by Dunn's Multiple Comparison Test.