Circadian Interventions in Preclinical Models of Huntington’s Disease: A Narrative Review

Environmental enrichment (EE) can be understood as modifications in the living conditions of the animals with the aim of providing activities that enhance their motor or cognitive function. In HD, the benefits of EE in preclinical models are mostly credited to morphological and neuroplastic changes in the brain; including increased expression of proteins [45], neurotrophic and growth factors, changes in genetic and epigenetic factors, in the hypothalamic-pituitary-adrenal (HPA) axis functions and hippocampal cytoarchitecture [46,47]. In animal models, including the R6/2 mouse model of HD and developing pigs, it has been demonstrated that EE can advance circadian phase, delay circadian disintegration, and improve synchronization of activity rhythms to the light-dark (LD) cycles [48,49,50]. Exercise, in particular, can facilitate and promote entrainment of free-running behavioural rhythms and alignment to the LD cycle while influencing the duration of endogenous periods in humans as well as in both diurnal and nocturnal rodents [51]. Moreover, exercise can alleviate age-related circadian desynchrony, as well as influence spontaneous firing rate rhythms and gene expression in the SCN [52,53]. It is possible that these circadian-related benefits are influenced by morphological and neuroplastic changes in the brain, for instance, Manno and colleagues [50] reported that circadian advancements elicited by EE in naïve C57BL/6j mice were associated with increased CA1 neuronal spiking and functional connectivity between the hippocampus and the isocortex. Congruently, Plácido and colleagues [54] found that EE elicited differences in the levels of monoamines, cell proliferation and neuronal cytoarchitecture in the hippocampus of the YAC128 HD mouse model. In HD models, such as the R6/1 mouse, EE has been shown to ameliorate deficits in brain-derived neurotrophic factor (BDNF) levels, known to greatly modulate synaptic plasticity [55,56,57]. Upregulating BDNF levels with intracortical injections has been shown to positively influence homeostatic sleep regulation [58] in rats. Therefore, EE may indirectly modulate sleep and circadian rhythms through its propensity to cause structural and functional changes in neuronal circuits.

Mouse models of HD have been pivotal to demonstrate that different forms of enrichment have beneficial influences on cognitive and motor impairments and improve survival. Exposure to stimulating environments, with for instance, regular behavioural testing, wheel running or treadmill, improved motor performance and slowed the decline in cognitive capabilities in several HD animal models, such as the Q140, R6/1 and R6/2 mouse models, as well as a quinolic-induced HD rat model relative to mutants that did not receive any enrichment [55,59,60,61,62]. Even limited EE was enough to slow the decline in motor function and ameliorate the performance of R6/2 and YAC128 mice in the rotarod test [54,63]. Moreover, exposure to EE from a young age significantly assisted in delaying loss of cerebral volume, motor symptoms and the onset of general health decline in both the R6/1 and R6/2 mice [63,64,65]. In addition, exposure to multiple forms of EE prevented the loss of body weight and improved survival in these two models [2,56,61].

Photoentrainment of mammalian circadian rhythms is mediated by a subset of retinal ganglion cells that are intrinsically photosensitive, shortly known as ipRGCs. These cells express the photopigment melanopsin [66], whose functions, including the suppression of melatonin secretion in the pineal gland [67]), play a fundamental role in regulating circadian rhythms [68] as well as pupil dilation, hormonal secretion and affective processes [69,70]. While sensitive to a broad spectrum of light, melanopsin is optimally responsive to blue light with a wavelength of 480 nanometers [71]. As ipRGCs project to the SCN, exposure to bright light results in spikes of neuronal activity in the SCN and increased expression of genes that modulate circadian rhythms, thus aligning the organism’s clock with the timing of the exposure to photoperiods [71,72]. Moreover, changes in the timing of the photoperiod exposures result in shifts in circadian phase [73]. Most importantly and relevant to this review, bright light therapy has been reported to ameliorate circadian deficits in the R6/2 model [48] as well as in clinical research [74]. How bright light improves behavioural rhythms and motor performance is still under investigation.

Wang and colleagues [75] exposed two HD models, the BACHD and Q175 mice, to 6 h of blue-enriched illumination at the start of the light/resting phase for 3 months. While a decline in the power of the locomotor activity rhythms along with increased fragmentation was observed in untreated mutants, treated mice of both strains did not demonstrate such decrements and displayed significantly improved motor performance. In addition to the timing of the exposure, the duration of a light photoperiod can be a major factor in circadian photoentrainment.

The effects and relationships of seasonal variations in day-light length with symptoms presentations have been explored in psychiatric syndromes [76], but scarcely in HD or other neurodegenerative disorders, except for one paper reporting an effect of temperature fluctuations during winter and summer on the age at onset [77]. Humans are not normally considered photoperiodic animals; however, many find themselves living in a long day photoperiod due to the extensive use of artificial lighting, and preclinical evidence suggests that exposure to longer photoperiods might be beneficial. For instance, R6/2 mice held under different photoperiods ultimately showed a significantly delayed disruption in rest-activity rhythms when exposed to a long-day photoperiod (16 h light:8 h dark) relative to mutant mice exposed to a standard light-dark (12 h light:12 h dark, LD) cycle, a short-day cycle (8 h light:16 h dark) or held in constant darkness [78]. Furthermore, R6/2 females exposed to the long-day photoperiod displayed a smaller decrease in the amplitude of rest-activity rhythms over time, along with improved survival and delayed weight loss, suggesting sex divergent effects in the response to light [78]. Differences in the length of the photoperiod (long-days vs. short-days) can result in changes in physiological phenomena that in turn regulate SCN physiology [79,80] and gene expression [81,82,83] with likely implications for regulation of the circadian outputs including the HPA axis [84]. As such, it appears that, as a zeitgeber (“time-giver”), light exposure can influence and align an organism’s circadian rhythms over constant and prolonged periods. The ideal model for this type of work could be a larger mammal in sheep as this species has a robust seasonal physiology and metabolism.

For a variety of practical reasons, mice have become the main driver of preclinical biomedical research; hence, there are numerous transgenic mouse models of HD. Studying neurodegenerative diseases in mouse models comes with some caveats and there are real advances to using large brained, long-lived animals to complement the work done with rodent models [10]. Case in point would be the transgenic sheep model of HD that carries the full-length human cDNA with a CAG repeat of 73 [9]. These animals exhibit disruptions in their activity rhythms [85] as well as in EEG-defined sleep [86], along with alterations in the pattern of melatonin secretion at presymptomatic stages [87].

In mammals, a bidirectional relationship exists between circadian rhythms and metabolism, as circadian disruptions predict poor metabolic outcomes [88] and genetically-induced obesity in mice predicts debilitating arrhythmias [89]. The underlying mechanisms of this relationship are not well understood, however, many aspects of metabolism are under circadian control, such as daily variations in glucose levels and tolerance, which appear to be at least in part modulated by the SCN and clock genes [90]. In addition, the SCN appears to indirectly modulate the rhythmicity of peripheral organ clocks involved in metabolic function through efferent targets in the hypothalamus involved in hunger and satiety [91], as well as food-anticipatory behaviour [92]. Additionally, the SCN communicates with organs involved in metabolic function, such as the pancreas [27], regulate metabolic enzymes and production of metabolites through efferent targets and the autonomic nervous system. Roughly 20% of the liver-soluble proteins show strong oscillations that suggest a direct circadian control [26]. Metabolic signals and factors, such as hormones, from peripheral organs and metabolites have functions that affect clocks in the brain, such as those of the pituitary gland, the periventricular nucleus, and the dorsomedial hypothalamus [26,27]. Peripheral clocks can become uncoupled from the SCN, which remains entrained to light rather than food-intake, suggesting the existence of a food-entrainable oscillator (FEO) in the brain, which some argue is within the dorsomedial hypothalamus [93], albeit without consensus. Further research is needed to develop a more comprehensive understanding of the mechanisms that influence the relationship between circadian rhythms and metabolic processes.

While the mechanisms underlying the relationship between circadian rhythms and metabolic function are still under investigation, the benefits of time-restricted feeding (TRF), in which food availability is restricted to a select period of hours without significant alteration of the content or calories of food ingested, are well documented. For instance, Hatori and colleagues [94] found that mice treated with TRF consumed just as many calories from a high-fat diet as those fed ad libitum (ALF) yet displayed better motor coordination and were better protected against obesity and other metabolic disorders. Additionally, TRF appears to be most beneficial when the restricted feeding period aligns with the active phase of one’s circadian cycle. In fact, Arble et al. [95] demonstrated that mice fed exclusively during their active/dark phase are better protected from weight gain and obesity than mice whose food availability is restricted to their inactive/light phase. Moreover, obese Histamine H(1)-R knockout mice with arrhythmic feeding patterns displayed improvements in both obesity and related metabolic disorders when solely fed at night [96]. Strikingly, C56BL/6J mice subjected to a TRF regimen that aligned feeding to the active/dark stage had significantly extended lifespans by roughly 35% relative to mice on ALF [97].

For those with HD, the most pertinent benefit of TRF is its potential to establish and entrain circadian rhythms, likely through entraining a FEO that can act independently of the SCN, as dysfunction of the SCN is a characteristic of HD that results in circadian and sleep disruptions that predict a more rapid rate of physical and cognitive decline. By restricting food availability to align with the active phase of R6/2 mice, Maywood et al. [98] demonstrated that activity rhythms are significantly rescued and peak during the time in which the mice anticipate food and that the disintegration of circadian gene expression in hepatic cells are reversed. Additionally, the implication of TRF in male, but not female, BACHD mice significantly corrected inappropriate wake rhythms, improved the temporal patterning of rapid eye-movement (REM) and non-REM (NREM) sleep, and reduced the occurrence of sleep bouts. Moreover, a cosiner analysis found that a higher percentage of both wildtype and BACHD mice treated with TRF had significant diurnal rhythms relative to those with ALF schedules [99]. Furthermore, Northeast et al. [100] showed that, following sleep deprivation between ZT3-6, TRF, but not ALF, augmented slow wave activity, suggesting that this regimen maintains sleep homeostasis. Moreover, the benefits of TRF as a circadian intervention in HD mice models extend to multiple aspects of physical health. For instance, in the R6/2 model, mice treated with TRF displayed improved thermal regulation, locomotor behaviour, and a slower loss of body weight [64]. In both Q175 and BACHD models, the restriction of food availability to solely the peak of the active phase (ZT 15–21) over a period of three or six months resulted in improved locomotor activity, heart rate variability, behavioural sleep patterns and motor improvements that were positively associated with improved circadian outputs [3,4].

The exact mechanisms underlying the benefits of TRF are unknown, however, there are findings that provide insight into possible mechanisms. For instance, deficits in BDNF expression levels, which play a critical role in the modulation of both cell survival and synaptic plasticity [57], are rescued. When circadian patterns of food availability are misaligned, long-term potentiation is inhibited and hippocampal total CREB levels are reduced [101]. Conversely, TRF upregulates BDNF expression [4] and induces structural and functional changes in the hippocampal regions and the septum, which appear to be paralleled by the SCN’s rhythms [102]. Moreover, in HD mouse models, methods that increase the production, release, or signalling of BDNF have resulted in improvements of HD phenotypes [57]. In addition, mice treated with TRF display improved rhythmic cycles in genes associated with autophagy [103] and lower mHTT levels associated with the upregulation of autophagy [42], suggesting that autophagy may play a significant role in improvements seen through TRF. There are also indications that TRF downregulates inflammatory signaling and normalizes the daily patterns of expression for genes involved in inflammation [44,103]. Moreover, prolonged periods of fasting induce depletion of hepatic glycogen that results in ketone bodies being the primary modulator of energy metabolism, which some believe benefits those with neurodegenerative disorders [104]. Albeit the wealth of evidence, further research is still needed to pin down some players in the underlying mechanism(s) for more targeted interventions.

Individuals with HD, like those with other neurodegenerative disorders, suffer from deficits in glucose metabolism in the brain that precede the onset of symptoms and could be directly associated with the severity of the disease [105,106,107,108,109,110,111,112]. Multiple metabolic processes, including glucose uptake and mitochondrial functions, are significantly impaired in all types of neural cells (neurons and glia) in neurodegenerative disorders, along with exacerbated oxidative stress and consequent cell loss in specific brain areas, for instance, the medium spiny neurons in the striatum in HD or the dopaminergic neurons in the substantia nigra in Parkinson’s Disease [104]. As such, the upregulation of alternative energy sources, for instance ketones, could serve as crucial agents in alleviating such “hypometabolism”. This idea is further supported by data in preclinical models of HD in which manipulations aimed at increasing ketone body availability in the circulatory system have been shown to improve survival, slow the loss of body weight, ameliorate deficits in motor symptoms [113,114], and, in a case-study, improve a patient’s quality of life [115]. The use of a high-in-fat and low-in-carbohydrates ketogenic diet (KD), with the goal of artificially upregulating ketone body production and inducing prolonged periods of ketosis, has been proposed and tested in a number of neurodegenerative disorders at both the clinical and preclinical level, including HD [115].

In the R6/2 model, Ruskin et al. [116] reported that the onset of weight loss was significantly delayed in mice fed a KD, while motor symptoms were not further impaired or ameliorated in the mutants. Impaired working memory was reversed in the mutant females but not in the males. Moreover, BACHD mice fed a KD ad libitum displayed significantly altered expressions of genes associated with neurodegenerative disorders, inflammation and sleep, along with strongly improved rhythms in activity, increased power and reduced activity onset variability. Conversely, KD had little effects on sleep parameters, albeit the mutants presented with less fragmented sleep, and an advanced sleep onset relative to their counterpart fed with normal chow. Additionally, circadian enhancement was accompanied by improved motor performance on the rotarod and challenging beam tasks [117]. The mechanisms underlying the observed benefits of upregulating ketone body production are unknown, however, there is evidence suggesting that ketone bodies might influence and downregulate the signalling of inflammatory pathways whilst upregulating autophagy [118]. Furthermore, KD has been shown to elicit substantial changes in the composition of the gut microbiota, which has also been suggested to play a role in modulating HD pathogenesis [119]. In accord, we have recently reported that KD evoked drastic changes in the microbiome of BACHD mice, in particular, in the levels of the probiotic Akkermansia Muciniphila [117], which has been associated with improved metabolic health and reductions in inflammation [120,121]. A significant increase in the gene expression levels of BDNF was also observed in the BACHD mice, likely mediated by the increase in β-hydroxybutyrate [122] elicited by the KD as compared to mutants on normal chow. Interestingly, the increase in β-hydroxybutyrate was significantly higher in the mutants as compared to wild-types [117]. These findings support the use of KD as neuroprotective and to ameliorate and, perhaps, delay the symptoms and disease progression in HD individuals [115], but further research is still needed.

Quality sleep and properly synchronized sleep/wake cycles are important to maintain whole body homeostatic functions, with devastating consequence in case of malalignment or low sleep quality. Although the sleep and circadian benefits of the KD are not well understood, it has been suggested that it may function via increased activation of orexinergic neurons, involved in feeding, sleep and wakefulness or upregulation of NAD+ and sirtuin, both associated with longevity and implicated in the sleep/wake cycle decay during aging [123,124]. In a recent study using a modified KD containing medium-chain triglycerides to avoid eliciting carbohydrate starvation, it was found that only REM sleep was significantly impacted, with no changes in NREM sleep or wake, as well as in the molecular clock, rhythms in locomotor activity or in core body temperature [125]. Too few studies in humans and animal models presently available to surmise a possible mechanistic view, however, some connections and conjectures can be made since: i. both sleep and KD modulate energy homeostasis and metabolism; ii. improving sleep or using a KD can ameliorate cognitive and metabolic dysfunctions; iii. albeit very few, some studies suggest that slow wave sleep (SWS) is modulated by fasting and associated ketogenic phase, which in turn could mediate some of the KD effects. SWS is important for memory consolidation and cognitive functions and is impacted in individuals with metabolic syndromes and linked to cognitive decline in neurodegenerative disorders and in aging [126,127,128].

Not applicable.