Hypothalamic expression of huntingtin causes distinct metabolic changes in Huntington's disease mice

Abstract

Objective In Huntington's disease (HD), the disease-causing huntingtin (HTT) protein is ubiquitously expressed and causes both central and peripheral pathology. In clinical HD, a higher body mass index has been associated with slower disease progression, indicating the role of metabolic changes in disease pathogenesis. Underlying mechanisms of metabolic changes in HD remain poorly understood, but recent studies suggest the involvement of hypothalamic dysfunction. The present study aimed to investigate whether modulation of hypothalamic HTT levels would affect metabolic phenotype and disease features in HD using mouse models. Methods We used the R6/2 and BACHD mouse models that express different lengths of mutant HTT to develop lean- and obese phenotypes, respectively. We utilized adeno-associated viral vectors to overexpress either mutant or wild-type HTT in the hypothalamus of R6/2, BACHD, and their wild-type littermates. The metabolic phenotype was assessed by body weight measurements over time and body composition analysis using dual-energy x-ray absorptiometry at the endpoint. R6/2 mice were further characterized using behavioral analyses, including rotarod, nesting-, and hindlimb clasping tests during early- and late-time points of disease progression. Finally, gene expression analysis was performed in R6/2 mice and wild-type littermates in order to assess transcriptional changes in the hypothalamus and adipose tissue. Results Hypothalamic overexpression of mutant HTT induced significant gender-affected body weight gain in all models, including wild-type mice. In R6/2 females, early weight gain shifted to weight loss during the corresponding late stage of disease despite increased fat accumulation. Body weight changes were accompanied by behavioral alterations. During the period of early weight gain, R6/2 mice displayed a comparable locomotor capacity to wild-type mice. When assessing behavior just prior to weight loss onset in R6/2 mice, decreased locomotor performance was observed in R6/2 females with hypothalamic overexpression of mutant HTT. Transcriptional downregulation of beta-3 adrenergic receptor (B3AR), adipose triglyceride lipase (ATGL), and peroxisome proliferator-activated receptor-gamma (PPAR γ ) in gonadal white adipose tissue was accompanied by distinct alterations in hypothalamic gene expression profiles in R6/2 females after mutant HTT overexpression. No significant effect on metabolic phenotype in R6/2 was seen in response to wild-type HTT overexpression. Conclusions Taken together, our findings provide further support for the role of HTT in metabolic control via hypothalamic neurocircuits. Understanding the specific central neurocircuits and their peripheral link underlying metabolic imbalance in HD may open up avenues for novel therapeutic interventions.

Microarray profiling of hypothalamic gene expression changes in Huntington’s disease mouse models

“…In line with previous work [26,27,55], we see that mHTT (79Q) overexpression in the hypothalamus elicits a more pronounced effect on disease mechanisms, such as the local loss of neuropeptides, compared to wtHTT (18Q) overexpression. Among shared top FC-ranked genes for downregulation, Hdc encoding the rate-limiting enzyme of L-histidine conversion to histamine was significantly downregulated in 79Q compared to the 18Q group.…”

“…The extensive use of AAVs for gene therapy and gene transfer strategies [72] is partly due to eliciting minimal inflammation and immunogenicity especially following the virus delivery, compared to other vector types [73][74][75]. As we in the present study chose to analyze the AAV groups at an earlier time point of phenotype progression (4 weeks post-injection) than prior studies [26,27], we did expect to see an inflammatory response to the early stage of transgene expression [76]. Therefore, in addition to the 18Q vs. WT and 79Q vs. WT datasets, we included a third dataset assessing specific differences, if any, between the injected mice (79Q vs. 18Q).…”

“…Using Affymetrix microarray profiling for large-scale gene expression analysis, this study aimed to assess changes in hypothalamic transcriptome profiles in mice with overexpression of wtHTT or mHTT in the hypothalamus and the full-length mHTT-expressing transgenic BACHD model of HD. These HD mouse models share a similar metabolic phenotype, namely an increased food intake and body weight [26, 27].…”

“…Metabolic alterations and other non-motor symptoms with a hypothalamic link are present throughout all stages of HD [21][22][23][24] and a higher baseline body mass index (BMI) is associated with slower disease progression [25]. We previously established a causal link between mHTT expression in the hypothalamus and metabolic and depressive-like phenotypes by the use of adeno-associated viral vectors (AAVs) to selectively overexpress HTT fragments, using groups injected with either a wild-type HTT construct of 18 CAG representing normal HTT (wtHTT 18Q) or a HTT fragment of 79 CAG (mHTT 79Q) [26][27][28]. Recently, we found that hypothalamus-targeted mHTT 79Q overexpression caused an initial rapid weight gain in the R6/2 mice, but it was insufficient to alter progression to end-stage disease features.…”

“…Recently, we found that hypothalamus-targeted mHTT 79Q overexpression caused an initial rapid weight gain in the R6/2 mice, but it was insufficient to alter progression to end-stage disease features. In injected R6/2 mice, regardless of whether wtHTT 18Q or mHTT 79Q was overexpressed, the weight loss and loss of key hypothalamic populations were present in R6/2 mice with characteristic disease end-stage phenotype [26]. In addition to prior studies on high caloric diets and the use of transgenic mice to induce weight gain [29-34]; increases in body weight in HD appear to be insufficient to significantly modify disease features in animal models.…”