HD is a progressive neurodegenerative disease for which there is no effective therapy. Because HD is caused by a mutation in the gene encoding Htt, genetic testing can identify patients before they become symptomatic, thus offering the possibility of early interventions that delay or prevent the onset of the disease. In the present study, we show that once-weekly ECS treatments delay disease onset and improve survival by an average of 2 weeks which is comparable with the reported increase in survival in N171-82Q HD mice that were either maintained on an intermittent fasting dietary restriction regimen (10
) or treated with antidepressant drugs (38
). Histological analyses demonstrated a reduction in the neuron loss in the striatum of ECS-treated HD mice compared with sham-treated HD mice. These results in an animal model of HD suggest that ECS treatment can counteract the pathogenic actions of mutant Htt, thereby preserving the viability and function of striatal neurons.
To elucidate the mechanism by which ECS treatment ameliorates cellular pathology and extends lifespan, we examined neuroprotective proteins that inhibit apoptotic biochemical cascades and preserve the cellular ability to adapt to stress. ECS treatment resulted in elevated levels of the protein chaperones Hsp70 and Hsp40 in the striatum of HD mice. Studies in which levels of Hsp70 or Hsp40 are selectively increased or decreased have shown that these two chaperones can protect neurons against insults relevant to HD including excitotoxins, metabolic stress and mutant Htt (40
The expanded polyglutamine stretch in mutant Htt is thought to trigger a conformational change that leads to partial unfolding or misfolding which, if not corrected by molecular chaperones, can lead to abnormal proteolysis and protein aggregation (42
). Histopathological comparison showed that ECS treatment resulted in a significant reduction in striatal and cortical Htt aggregation. Misfolding and aggregation of mutant Htt are therapeutic targets since they are early molecular events in the pathogenic cascades that underlie the neurological dysfunction in transgenic HD mice (32
). A variety of molecular chaperones have been demonstrated to exert therapeutic effects against various experimental models of the polyglutamine diseases, including HD (44
). Here we showed that once-weekly ECT treatment increases Hsp70 and Hsp40 that likely played a significant role in suppressing Htt misfolding and aggregation. Indeed, we found that the ECS treatment reduces Htt aggregation in striatal (Fig. ) and cortical (data not shown) cells of HD mice suggesting that ECS interrupts the disease process at an early stage. The ECS-induced increase of Hsp70 and Hsp40 is likely mediated by HSF1, a stress-responsive transcriptional regulator that has been shown to suppress polyglutamine aggregate formation in cellular and mouse models (47
We found that striatal BDNF levels were markedly reduced in HD mice compared with WT mice, and that BDNF levels were significantly increased in HD mice that had been maintained on long-term ECS treatment compared with sham control HD mice. BDNF is a neurotrophin that plays critical roles in synaptic plasticity and neuronal survival in many brain regions including those affected in HD. Previous studies have shown that endogenous BDNF (16
), and BDNF delivered directly or by transgenesis (11
), can protect striatal and cortical neurons in experimental models of HD. Because BDNF transcription has been reported to be impeded by the misfolded Htt protein and because Hsps reduce the accumulation of misfolded Htt protein, our data suggest that elevation of Hsps may contribute in part to the restoration of the BDNF level in ECS-treated HD mice. Furthermore, as the majority of striatal BDNF is synthesized by cortical neurons (48
), the data also implies that mitigation of cortical aggregate formation may lessen the HD's adverse effects on the basal ganglia.
The elevation of BDNF levels may mediate, at least in part, the retardation of disease onset and extension of survival by ECS in N171-82Q HD mice. Consistent with the latter possibility, it was reported that paroxetine and seratraline, two other anti-depressant treatments that increase BDNF levels in the striatum and cortex of HD mice, also delay disease onset and extend survival in N171-82Q HD mice (38
). BDNF may protect neurons against excitotoxic, metabolic and oxidative stress believed to be involved in the death of neurons in HD. Indeed, BDNF can protect neurons against glutamate receptor-mediated excitotoxicity (14
) energetic/mitochondrial stress (12
) and oxidative insults (50
). By maintaining the survival and function of striatal neurons, BDNF levels could improve motor control in HD mice as reduction in BDNF levels advances the age of onset and exacerbates the severity of motor dysfunction (16
). Hence, although ECS treatment may induce the expression of a plethora of plasticity-related genes (51
), the elevation of BDNF is likely responsible for ameliorating the behavioral and neuropathological phenotype in N171-82Q HD mice.
Apart from protecting vulnerable neurons, ECS has been shown to facilitate neurogenesis through upregulation of BDNF and other growth factors (52
). Although not addressed in the present study, it is possible that increased neurogenesis which functions to replace lost or damaged striatal neurons may contribute to ECS-induced beneficial effects. It has been demonstrated that induction of neurogenesis can slow disease progression in transgenic HD mice (15
). One functional consequence of neurogenesis is learning and memory (53
), and impaired learning has also been described in different transgenic mouse models of HD (54
). Previous studies showed that ECS treatment increased the total number of synapses in adult male rat hippocampus (55
). Because a reduction in BDNF levels has been described in the hippocampus of transgenic HD mice (57
) and because this neurotrophin has been shown to rescue the deficits in long-term potentiation of synaptic transmission (a cellular correlate of learning and memory) in hippocampal slices from transgenic HD mice (58
), it will be interesting to determine whether ECS treatment prevents cognitive impairments in transgenic HD mice.
Interestingly, ECS did not have significant effects on levels of Hsp70, Hsp40 and BDNF in the striatum of WT mice, although there were clear trends towards elevated levels of each of these proteins. Previous studies have shown that BDNF mRNA levels are increased acutely (hours to a few days) in the hippocampus following ECS (59
). Other studies have found that BDNF mRNA levels in the hippocampus are elevated 2-fold in rats that had received daily ECS treatments for a 10-day period (60
). Another study found that BDNF protein levels were significantly increased in the cortex and striatum of rats after 10 daily ECS treatments (60
). In our study, the mice were treated with ECS once weekly for 8 weeks using a 50 mA, 0.2 s ECS stimulus. Our ECS treatment was therefore less frequent than previous studies, which could explain the lesser effect of our treatment regimen on BDNF levels compared with more frequent ECS treatments. Striatal neurons in HD mice may be more sensitive to a low ECS treatment because of their lower threshold for excitability, and hence exhibit a greater increase in levels of BDNF and protein chaperones.
In addition to mitigating both gross brain and neuronal atrophy, ECS treatment also ameliorated the progressive body weight loss in N171-82Q HD mice. The basis for the weight loss in HD mice is not yet clear. BDNF and its receptor, TrkB, play prominent roles in food intake and energy metabolism regulation through central mechanisms involving the hypothalamus (61
). The potential role of BDNF in regulation of energy metabolism was first discovered through generation of BDNF ± mice which display an obese phenotype (62
). Subsequently, mutations in bdnf
have been identified in some obese patients (64
). Genetic ablation of bdnf
in the hypothalamus of adult mice results in hyperphagic behavior and obesity (66
). Emerging evidence suggests that disturbed functions of the hypothalamus may contribute to some signs and symptoms associated with metabolic alterations in HD patients (67
). Further studies are needed to examine whether the body weight loss and its amelioration by ECS involve altered expression of BDNF in the hypothalamus of HD mice.
A functional BDNF polymorphism (BDNF Val66Met) was reported to influence the vulnerability to various psychiatric disorders (68
). There has been a resurgence of interest in the use of ECS for the treatment of drug-refractive psychiatric disorders, and it is considered safe and effective for the treatment of depression in the elderly, including those with co-morbidities (56
). ECS treatments often result in long-lasting clinical improvements in psychiatric symptoms which are correlated with the increased BDNF level (69
). Several reports describe the beneficial effects of ECS in relieving depression in HD patients (23
) who, in general, have higher suicide rates relative to those with other medical and neurodegenerative diseases (75
). However, there have been no controlled studies of ECS treatment in symptomatic HD patients, nor any attempts to delay the onset of HD with periodic ECS treatments. Our preclinical findings demonstrate for the first time that ECS treatment slows the progression of the neurodegenerative process caused by mutant Htt in an animal model of HD. ECS treatment resulted in increase in the expression of several adaptive cellular stress response proteins which may promote neuronal survival and plasticity, and so forestall the neurodegenerative process resulting in a delay in the disease onset and life extension. The present findings have significant implications for preventive treatment strategies for individuals that carry the mutant Htt