Huntingtin Aggregation Impairs Autophagy, Leading to Argonaute-2 Accumulation and Global MicroRNA Dysregulation

Many neurodegenerative diseases are characterized by the presence of intracellular protein aggregates, resulting in alterations in autophagy. However, the consequences of impaired autophagy for neuronal function remain poorly understood. In this study, we used cell culture and mouse models of huntingtin protein aggregation as well as post-mortem material from patients with Huntington’s disease to demonstrate that Argonaute-2 (AGO2) accumulates in the presence of neuronal protein aggregates and that this is due to impaired autophagy. Accumulation of AGO2, a key factor of the RNA-induced silencing complex that executes microRNA functions, results in global alterations of microRNA levels and activity. Together, these results demonstrate that impaired autophagy found in neurodegenerative diseases not only influences protein aggregation but also directly contributes to global alterations of intracellular post-transcriptional networks

Psychiatric and metabolic disturbances in experimental models of Huntington’s disease

Non-motor symptoms and signs such as metabolic and psychiatric disturbances have been reported to occur early in Huntington’s disease (HD), a fatal neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene. However, there is a lack of understanding of the underlying neurobiological mechanisms responsible for the early non-motor features. The hypothalamus has emerged as an important site of pathology in HD and since this brain region is well known to regulate metabolism and emotions we hypothesized that expression of mutant huntingtin in this area would be involved in causing metabolic and psychiatric aspects of HD. Therefore, the general aim of the work in this thesis was to investigate such potential relationships using different experimental models of HD. This could be of particular importance since no disease-modifying treatments exist today and the elucidation of the early pathogenesis in HD could facilitate the identification of new targets for therapeutic intervention. In this thesis, we showed that the BACHD mouse, a model engineered to allow conditional inactivation of mutant huntingtin expression, displays early psychiatric-like behaviors in addition to metabolic disturbances. The development of metabolic alterations could be prevented and depressive-like behavior reduced in young BACHD mice by selective inactivation of mutant huntingtin expression in the hypothalamus. In a second mouse model, viral vector-mediated expression of mutant huntingtin in the hypothalamus was sufficient to recapitulate the metabolic phenotype observed in the BACHD mice. Importantly, both studies revealed adverse effects on the hypothalamic orexin system. By contrast, inactivation of mutant huntingtin in leptin receptor-expressing neurons in the BACHD mice had no effect on the observed phenotypes, indicating that leptin is unlikely to be involved. Taken together, these results suggest a link between mutant huntingtin acting in the hypothalamus and the development of psychiatric and metabolic disturbances in HD

Huntington's Disease - New Perspectives Based on Neuroendocrine Changes in Rodent Models.

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene. Although it is characterized by progressive motor impairments, cognitive changes and psychiatric disturbances are major components of the disease. In addition, recent studies have shown that other non-motor symptoms such as alterations in sleep pattern, disruption of the circadian rhythm and increased energy metabolism are common and occur early. Emerging evidence suggests that the latter symptoms are likely results of disturbed functions of the hypothalamus and neuroendocrine circuits, which are known to be central in the regulation of emotion, sleep and metabolism. Whereas clinical data are essential to define key pathological features of HD, animal models that can recapitulate the neurobiological and behavioral features of the disorder are critical tools to elucidate the underlying pathogenic mechanisms. Recent studies employing different HD rodent models have been instrumental in identifying a number of neuroendocrine alterations as well as in highlighting novel potential disease pathways. This review summarizes the current state of knowledge derived from neuroendocrine studies in rodent models of HD in light of clinical relevance and points to future implications for this emerging field

Expression of Mutant Huntingtin in Leptin Receptor-Expressing Neurons Does Not Control the Metabolic and Psychiatric Phenotype of the BACHD Mouse.

Metabolic and psychiatric disturbances occur early on in the clinical manifestation of Huntington's disease (HD), a neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin (HTT) gene. Hypothalamus has emerged as an important site of pathology and alterations in this area and its neuroendocrine circuits may play a role in causing early non-motor symptoms and signs in HD. Leptin is a hormone that controls energy homeostasis by signaling through leptin receptors in the hypothalamus. Disturbed leptin action is implicated in both obesity and depression and altered circulating levels of leptin have been reported in both clinical HD and rodent models of the disease. Pathological leptin signaling may therefore be involved in causing the metabolic and psychiatric disturbances of HD. Here we tested the hypothesis that expression of mutant HTT in leptin receptor carrying neurons plays a role in the development of the non-motor phenotype in the BACHD mouse model. Our results show that inactivation of mutant HTT in leptin receptor-expressing neurons in the BACHD mouse using cross-breeding based on a cre-loxP system did not have an effect on the metabolic phenotype or anxiety-like behavior. The data suggest that mutant HTT disrupts critical hypothalamic pathways by other mechanisms than interfering with intracellular leptin signaling

Expression of mutant huntingtin in leptin receptor-expressing neurons does not control the metabolic and psychiatric phenotype of the BACHD mouse.

Metabolic and psychiatric disturbances occur early on in the clinical manifestation of Huntington's disease (HD), a neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin (HTT) gene. Hypothalamus has emerged as an important site of pathology and alterations in this area and its neuroendocrine circuits may play a role in causing early non-motor symptoms and signs in HD. Leptin is a hormone that controls energy homeostasis by signaling through leptin receptors in the hypothalamus. Disturbed leptin action is implicated in both obesity and depression and altered circulating levels of leptin have been reported in both clinical HD and rodent models of the disease. Pathological leptin signaling may therefore be involved in causing the metabolic and psychiatric disturbances of HD. Here we tested the hypothesis that expression of mutant HTT in leptin receptor carrying neurons plays a role in the development of the non-motor phenotype in the BACHD mouse model. Our results show that inactivation of mutant HTT in leptin receptor-expressing neurons in the BACHD mouse using cross-breeding based on a cre-loxP system did not have an effect on the metabolic phenotype or anxiety-like behavior. The data suggest that mutant HTT disrupts critical hypothalamic pathways by other mechanisms than interfering with intracellular leptin signaling

Hypothalamic and Neuroendocrine Changes in Huntington's Disease.

Huntington's disease (HD) is a fatal hereditary neurodegenerative disorder without satisfactory treatments nor a cure. It is caused by a CAG repeat expansion in the huntingtin gene. The clinical symptoms involve motor-, cognitive- and psychiatric disturbances. Recent studies have shown that non-motor symptoms and signs, such as mood changes, sleep disturbances and metabolic alterations often occur before the onset of overt motor impairments. The hypothalamus is one of the main regulators of emotion, sleep and metabolism, and it is therefore possible that dysfunction of the hypothalamus and neuroendocrine circuits may, at least partly, be responsible for these non-motor symptoms in HD. Several hypothalamic and neuroendocrine changes have now been identified in clinical HD as well as in rodent models of the disease. These changes could be important both in the pathogenesis of HD, constitute biomarkers to track disease progression as well as to provide novel therapeutic targets for this devastating disease. The current state of knowledge in the area of hypothalamic and neuroendocrine changes in both patients and rodent models of HD is summarized in this review, and their potential as targets for novel treatment paradigms are discussed

Hypothalamic expression of mutant huntingtin contributes to the development of depressive-like behavior in the BAC transgenic mouse model of Huntingtons disease

Psychiatric symptoms such as depression and anxiety are important clinical features of Huntingtons disease (HD). However, the underlying neurobiological substrate for the psychiatric features is not fully understood. In order to explore the biological origin of depression and anxiety in HD, we used a mouse model that expresses the human full-length mutant huntingtin, the BACHD mouse. We found that the BACHD mice displayed depressive- and anxiety-like features as early as at 2 months of age as assessed using the Porsolt forced swim test (FST), the sucrose preference test and the elevated plus maze (EPM). BACHD mice subjected to chronic treatment with the anti-depressant sertraline were not different to vehicle-treated BACHD mice in the FST and EPM. The behavioral manifestations occurred in the absence of reduced hippocampal cell proliferation/neurogenesis or upregulation of the hypothalamicpituitaryadrenal axis. However, alterations in anxiety- and depression-regulating genes were present in the hypothalamus of BACHD mice including reduced mRNA expression of neuropeptide Y, tachykinin receptor 3 and vesicular monoamine transporter type 2 as well as increased expression of cocaine and amphetamine regulated transcript. Interestingly, the orexin neuronal population in the hypothalamus was increased and showed cellular atrophy in old BACHD mice. Furthermore, inactivation of mutant huntingtin in a subset of the hypothalamic neurons prevented the development of the depressive features. Taken together, our data demonstrate that the BACHD mouse recapitulates clinical HD with early psychiatric aspects and point to the role of hypothalamic dysfunction in the development of depression and anxiety in the disease

Body weight in BACHD×LepR-cre offspring.

<p>Body weight at 2- and 6-months in males and females from the two different breedings between BACHD and BACHD/LepR-cre (n = 4–10/genotype/sex/breeding). Both BACHD and BACHD/LepR-cre developed early onset obesity but no significant differences between BACHD and BACHD/LepR-cre could be detected in any of the breedings or sexes. (A) F BACHD×M LepR-cre, females, (B) F BACHD×M LepR-cre, males, (C) F LepR-cre×M BACHD, females, and (D) F LepR-cre×M BACHD, males. All data are expressed as means ± SEM. Significant differences from WT and/or LepR-cre mice: * p<0.05 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051168#pone.0051168.s002" target="_blank">Statistical Results S1</a>).</p

Endocrine measurements in BACHD×LepR-cre offspring.

<p>Circulating levels of leptin and insulin were measured in 6-month old female mice from the second breeding (n = 9–10/genotype). (A) Both BACHD and BACHD/LepR-cre showed elevated leptin levels but no significant differences between BACHD and BACHD/LepR-cre could be detected. (B) Only BACHD mice displayed significantly higher insulin levels than wt mice. However, no significant differences could be detected between BACHD and BACHD/LepR-cre. All data are expressed as means ± SEM. Significant differences from WT and/or LepR-cre mice: * p<0.05 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051168#pone.0051168.s002" target="_blank">Statistical Results S1</a>).</p

Body composition in BACHD×LepR-cre offspring.

<p>The graphs show percentage body fat as assessed with dual energy x-ray absorptiometry (DEXA) at 6-months in males and females from the two different breedings (n = 4–10/genotype/sex/breeding). Both BACHD and BACHD/LepR-cre showed increased percentage of body fat but there were no significant differences between BACHD and BACHD/LepR-cre in the two breedings and sexes. (A) F BACHD×M LepR-cre, females, (B) F BACHD×M LepR-cre, males, (C) F LepR-cre×M BACHD, females, and (D) F LepR-cre×M BACHD, males. All data are expressed as means ± SEM. Significant differences from WT and/or LepR-cre mice: * p<0.05 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051168#pone.0051168.s002" target="_blank">Statistical Results S1</a>).</p