Brain-Derived Neurotrophic Factor Prevents Depressive-Like Behaviors in Early-Symptomatic YAC128 Huntington’s Disease Mice

Brain-Derived Neurotrophic Factor (BDNF) in Huntington's Disease: Neurobiology and Therapeutic Potential

Huntington's disease is a hereditary neurodegenerative disorder marked by severe neurodegeneration in the striatum and cortex. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of growth factors. It plays a crucial role in maintaining the survival and proper function of striatal neurons. Depletion of BDNF has been linked to impairment and death of striatal neurons, leading to the manifestation of motor, cognitive, and behavioral dysfunctions characteristic of Huntington's disease. This review highlights the current update on the neurobiology of BDNF in the pathogenesis of Huntington's disease. The molecular evidence and the affected signaling pathways are also discussed. In addition, the impact of experimental manipulation of BDNF levels and its pharmaceutical potential for Huntington's disease treatment are explicitly reviewed.

The pathobiology of depression in Huntington's disease: an unresolved puzzle

Huntington's disease (HD) is an autosomal-dominant progressive neurodegenerative disease that manifests with a triad of symptoms including motor dysfunctions, cognitive deficits, and prominent neuropsychiatric symptoms, the most common of which is depression, with a prevalence between 30 and 70%. Depressive symptoms occur in all stages of HD, beginning in presymptomatic HD gene carriers, and are strongly associated with suicidal ideation and suicidality, but their relationship with other clinical dimensions in HD is controversial and the underlying pathophysiology is poorly understood. Analysis of the available literature until November 2023 concerned the prevalence, clinical manifestations, neuroimaging, transgenic models, and treatment options of HD depression. While it was believed that depression in HD is due to psychosomatic factors in view of the fatal disease, studies in transgenic models of HD demonstrated molecular changes including neurotrophic and serotonergic dysregulation and disorders of the hypothalamic-pituitary-adrenal axis inducing depression-like changes. While relevant neuropathological data are missing, recent neuroimaging studies revealed correlations between depressive symptoms and dysfunctional connectivities in the default mode network, basal ganglia and prefrontal cortex, and changes in limbic and paralimbic structures related to the basic neurodegenerative process. The impact of response to antidepressants in HD patients is controversial; selective serotonin reuptake inhibitors are superior to serotonin-norepinephrine reuptake inhibitors, while electroconvulsive therapy may be effective for pharmacotherapy resistant cases. Since compared to major depressive disorder and depression in other neurodegenerative diseases, our knowledge of the molecular basis in HD depression is limited, further studies to elucidate the heterogeneous pathogenesis in this fatal disorder are warranted.

Pathomechanisms of behavioral abnormalities in Huntington disease: an update

Huntington disease (HD), a devastating autosomal-dominant neurodegenerative disease caused by an expanded CAG trinucleotide repeat, is clinically characterized by a triad of symptoms including involuntary motions, behavior problems and cognitive deficits. Behavioral symptoms with anxiety, irritability, obsessive-compulsive behaviors, apathy and other neuropsychiatric symptoms, occurring in over 50% of HD patients are important features of this disease and contribute to impairment of quality of life, but their pathophysiology is poorly understood. Behavior problems, more frequent than depression, can be manifest before obvious motor symptoms and occur across all HD stages, usually correlated with duration of illness. While specific neuropathological data are missing, the relations between gene expression and behavior have been elucidated in transgenic models of HD. Disruption of interneuronal communications, with involvement of prefronto-striato-thalamic networks and hippocampal dysfunctions produce deficits in multiple behavioral domains. These changes that have been confirmed by multistructural neuroimaging studies are due to a causal cascade linking molecular pathologies (glutamate-mediated excitotoxicity, mitochondrial dysfunctions inducing multiple biochemical and structural alterations) and deficits in multiple behavioral domains. The disruption of large-scale connectivities may explain the variability of behavior profiles and is useful in understanding the biological backgrounds of functional decline in HD. Such findings offer new avenues for targeted treatments in terms of minimizing neurobehavioral impairment in HD.

Brain-Derived Neurotrophic Factor Dysregulation as an Essential Pathological Feature in Huntington's Disease: Mechanisms and Potential Therapeutics

Brain-derived neurotrophic factor (BDNF) is a major neurotrophin whose loss or interruption is well established to have numerous intersections with the pathogenesis of progressive neurological disorders. There is perhaps no greater example of disease pathogenesis resulting from the dysregulation of BDNF signaling than Huntington's disease (HD)-an inherited neurodegenerative disorder characterized by motor, psychiatric, and cognitive impairments associated with basal ganglia dysfunction and the ultimate death of striatal projection neurons. Investigation of the collection of mechanisms leading to BDNF loss in HD highlights this neurotrophin's importance to neuronal viability and calls attention to opportunities for therapeutic interventions. Using electronic database searches of existing and forthcoming research, we constructed a literature review with the overarching goal of exploring the diverse set of molecular events that trigger BDNF dysregulation within HD. We highlighted research that investigated these major mechanisms in preclinical models of HD and connected these studies to those evaluating similar endpoints in human HD subjects. We also included a special focus on the growing body of literature detailing key transcriptomic and epigenetic alterations that affect BDNF abundance in HD. Finally, we offer critical evaluation of proposed neurotrophin-directed therapies and assessed clinical trials seeking to correct BDNF expression in HD individuals.

Beyond Motor Deficits: Environmental Enrichment Mitigates Huntington's Disease Effects in YAC128 Mice

Huntington's disease (HD) is a neurodegenerative genetic disorder characterized by motor, psychiatric, cognitive, and peripheral symptoms without effective therapy. Evidence suggests that lifestyle factors can modulate disease onset and progression, and environmental enrichment (EE) has emerged as a potential approach to mitigate the progression and severity of neurodegenerative processes. Wild-type (WT) and yeast artificial chromosome (YAC) 128 mice were exposed to different EE conditions. Animals from cohort 1 were exposed to EE between postnatal days 21 and 60, and animals from cohort 2 were exposed to EE between postnatal days 60 and 120. Motor and non-motor behavioral tests were employed to evaluate the effects of EE on HD progression. Monoamine levels, hippocampal cell proliferation, neuronal differentiation, and dendritic arborization were also assessed. Here we show that EE had an antidepressant-like effect and slowed the progression of motor deficits in HD mice. It also reduced monoamine levels, which correlated with better motor performance, particularly in the striatum. EE also modulated neuronal differentiation in the YAC128 hippocampus. These results confirm that EE can impact behavior, hippocampal neuroplasticity, and monoamine levels in YAC128 mice, suggesting this could be a therapeutic strategy to modulate neuroplasticity deficits in HD. However, further research is needed to fully understand EE's mechanisms and long-term effects as an adjuvant therapy for this debilitating condition.

A systematic review and meta-analysis of depression and apathy frequency in adult-onset Huntington's disease

Depression and apathy are associated with decreased functional capacity in Huntington's disease (HD) but frequency of depression and apathy in HD is largely unknown. Systematic literature searching was conducted across 21 databases until 30 June 2021. Inclusion criteria was limited to clinician-rated assessments of depression and apathy and adult-onset HD. Inverse-variance heterogeneity meta-analyses were conducted exploring depression and apathy frequency within individuals from families affected by HD, and within individuals with confirmed HD gene-positive status. Screening identified 289 articles for full-text review; nine remained for meta-analysis. Depression frequency in the lifetime in adults affected by or at-risk for HD was 38%, I² = 99%. Apathy frequency in the lifetime in adults affected by or at-risk for HD was 40%, I² = 96%. The robustness of the findings improved when limiting the analysis to gene-positive individuals only where apathy was found to be slightly more common than depression, 48% and 43% respectively. Future studies may consider reporting results from juvenile-onset HD and adult-onset HD cohorts separately to further explore phenotypic profiles.

Therapeutic Strategies in Huntington’s Disease: From Genetic Defect to Gene Therapy

Despite the identification of an expanded CAG repeat on exon 1 of the huntingtin gene located on chromosome 1 as the genetic defect causing Huntington’s disease almost 30 years ago, currently approved therapies provide only limited symptomatic relief and do not influence the age of onset or disease progression rate. Research has identified various intricate pathogenic cascades which lead to neuronal degeneration, but therapies interfering with these mechanisms have been marked by many failures and remain to be validated. Exciting new opportunities are opened by the emerging techniques which target the mutant protein DNA and RNA, allowing for “gene editing”. Although some issues relating to “off-target” effects or immune-mediated side effects need to be solved, these strategies, combined with stem cell therapies and more traditional approaches targeting specific pathogenic cascades, such as excitotoxicity and bioavailability of neurotrophic factors, could lead to significant improvement of the outcomes of treated Huntington’s disease patients.

Temporal Characterization of Behavioral and Hippocampal Dysfunction in the YAC128 Mouse Model of Huntington’s Disease

Huntington’s disease (HD) is a genetic neurodegenerative disease characterized by motor, psychiatric, and cognitive symptoms. Emerging evidence suggests that emotional and cognitive deficits seen in HD may be related to hippocampal dysfunction. We used the YAC128 HD mouse model to perform a temporal characterization of the behavioral and hippocampal dysfunctions. Early and late symptomatic YAC128 mice exhibited depressive-like behavior, as demonstrated by increased immobility times in the Tail Suspension Test. In addition, YAC128 mice exhibited cognitive deficits in the Swimming T-maze Test during the late symptomatic stage. Except for a reduction in basal mitochondrial respiration, no significant deficits in the mitochondrial respiratory rates were observed in the hippocampus of late symptomatic YAC128 mice. In agreement, YAC128 animals did not present robust alterations in mitochondrial ultrastructural morphology. However, light and electron microscopy analysis revealed the presence of dark neurons characterized by the intense staining of granule cell bodies and shrunken nuclei and cytoplasm in the hippocampal dentate gyrus (DG) of late symptomatic YAC128 mice. Furthermore, structural alterations in the rough endoplasmic reticulum and Golgi apparatus were detected in the hippocampal DG of YAC128 mice by electron microscopy. These results clearly show a degenerative process in the hippocampal DG in late symptomatic YAC128 animals.

New Avenues for the Treatment of Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by a CAG expansion in the HD gene. The disease is characterized by neurodegeneration, particularly in the striatum and cortex. The first symptoms usually appear in mid-life and include cognitive deficits and motor disturbances that progress over time. Despite being a genetic disorder with a known cause, several mechanisms are thought to contribute to neurodegeneration in HD, and numerous pre-clinical and clinical studies have been conducted and are currently underway to test the efficacy of therapeutic approaches targeting some of these mechanisms with varying degrees of success. Although current clinical trials may lead to the identification or refinement of treatments that are likely to improve the quality of life of those living with HD, major efforts continue to be invested at the pre-clinical level, with numerous studies testing novel approaches that show promise as disease-modifying strategies. This review offers a detailed overview of the currently approved treatment options for HD and the clinical trials for this neurodegenerative disorder that are underway and concludes by discussing potential disease-modifying treatments that have shown promise in pre-clinical studies, including increasing neurotropic support, modulating autophagy, epigenetic and genetic manipulations, and the use of nanocarriers and stem cells.

Why Woody got the blues: The neurobiology of depression in Huntington's disease

Huntington's disease (HD) is an extraordinary disorder that usually strikes when individuals are in the prime of their lives, as was the case for the influential 20th century musician Woody Guthrie. HD demonstrates the exceptionally fine line between life and death in such 'genetic diseases', as the only difference between those who suffer horribly and die slowly of this disease is often just a handful of extra tandem repeats (beyond the normal polymorphic range) in a genome that constitutes over 3 billion paired nucleotides of DNA. Furthermore, HD presents as a complex and heterogenous combination of psychiatric, cognitive and motor symptoms, so can appear as an unholy trinity of 'three disorders in one'. The autosomal dominant nature of the disorder is also extremely challenging for affected families, as a 'flip of a coin' dictates which children inherit the mutation from their affected parent, and the gene-negative family members bear the burden of caring for the other half of the family that is affected. In this review, we will focus on one of the earliest, and most devastating, symptoms associated with HD, depression, which has been reported to affect approximately half of gene-positive HD family members. We will discuss the pathogenesis of HD, and depressive symptoms in particular, including molecular and cellular mechanisms, and potential genetic and environmental modifiers. This expanding understanding of HD pathogenesis may not only lead to novel therapeutic options for HD families, but may also provide insights into depression in the wider population, which has the greatest burden of disease of any disorder and an enormous unmet need for new therapies.

Focused ultrasound enhanced intranasal delivery of brain derived neurotrophic factor produces neurorestorative effects in a Parkinson's disease mouse model

Focused ultrasound-enhanced intranasal (IN + FUS) delivery is a noninvasive approach that utilizes the olfactory pathway to administer pharmacological agents directly to the brain, allowing for a more homogenous distribution in targeted locations compared to IN delivery alone. However, whether such a strategy has therapeutic values, especially in neurodegenerative disorders such as Parkinson’s disease (PD), remains to be established. Herein, we evaluated whether the expression of tyrosine hydroxylase (TH), the rate limiting enzyme in dopamine catalysis, could be enhanced by IN + FUS delivery of brain-derived neurotrophic factor (BDNF) in a toxin-based PD mouse model. Mice were put on the subacute dosing regimen of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), producing bilateral degeneration of the nigrostriatal pathway consistent with early-stage PD. MPTP mice then received BDNF intranasally followed by multiple unilateral FUS-induced blood-brain barrier (BBB) openings in the left basal ganglia for three consecutive weeks. Subsequently, mice were survived for two months and were evaluated morphologically and behaviorally to determine the integrity of their nigrostriatal dopaminergic pathways. Mice receiving IN + FUS had significantly increased TH immunoreactivity in the treated hemisphere compared to the untreated hemisphere while mice receiving only FUS-induced BBB opening or no treatment at all did not show any differences. Additionally, behavioral changes were only observed in the IN + FUS treated mice, indicating improved motor control function in the treated hemisphere. These findings demonstrate the robustness of the method and potential of IN + FUS for the delivery of bioactive factors for treatment of neurodegenerative disorder.

Cell-Autonomous and Non-cell-Autonomous Pathogenic Mechanisms in Huntington's Disease: Insights from In Vitro and In Vivo Models

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion in the trinucleotide CAG repeat in exon-1 in the huntingtin gene, located on chromosome 4. When the number of trinucleotide CAG exceeds 40 repeats, disease invariably is manifested, characterized by motor, cognitive, and psychiatric symptoms. The huntingtin (Htt) protein and its mutant form (mutant huntingtin, mHtt) are ubiquitously expressed but although multiple brain regions are affected, the most vulnerable brain region is the striatum. Striatal medium-sized spiny neurons (MSNs) preferentially degenerate, followed by the cortical pyramidal neurons located in layers V and VI. Proposed HD pathogenic mechanisms include, but are not restricted to, excitotoxicity, neurotrophic support deficits, collapse of the protein degradation mechanisms, mitochondrial dysfunction, transcriptional alterations, and disorders of myelin. Studies performed in cell type-specific and regionally selective HD mouse models implicate both MSN cell-autonomous properties and cell-cell interactions, particularly corticostriatal but also with non-neuronal cell types. Here, we review the intrinsic properties of MSNs that contribute to their selective vulnerability and in addition, we discuss how astrocytes, microglia, and oligodendrocytes, together with aberrant corticostriatal connectivity, contribute to HD pathophysiology. In addition, mHtt causes cell-autonomous dysfunction in cell types other than MSNs. These findings have implications in terms of therapeutic strategies aimed at preventing neuronal dysfunction and degeneration.

Prophylactic effect of physical exercise on Aβ1-40-induced depressive-like behavior: Role of BDNF, mTOR signaling, cell proliferation and survival in the hippocampus

Alzheimer's disease (AD) is characterized by progressive cognitive impairments as well as non-cognitive symptoms such as depressed mood. Physical exercise has been proposed as a preventive strategy against AD and depression, an effect that may be related, at least partially, to its ability to prevent impairments on cell proliferation and neuronal survival in the hippocampus, a structure implicated in both cognition and affective behavior. Here, we investigated the ability of treadmill exercise (4 weeks) to counteract amyloid β_1-40 peptide-induced depressive-like and anxiety-like behavior in mice. Moreover, we addressed the role of the BDNF/mTOR intracellular signaling pathway as well as hippocampal cell proliferation and survival in the effects of physical exercise and/or Aβ_1-40. Aβ_1-40 administration (400 pmol/mouse, i.c.v.) increased immobility time and reduced the latency to immobility in the forced swim test, a finding indicative of depressive-like behavior. In addition, Aβ_1-40 administration also decreased time spent in the center of the open field and increased grooming and defecation, alterations indicative of anxiety-like behavior. These behavioral alterations were accompanied by a reduction in the levels of mature BDNF and mTOR (Ser²⁴⁴⁸) phosphorylation in the hippocampus. In addition, Aß_1-40 administration reduced cell proliferation and survival in the ventral, dorsal and entire dentate gyrus of the hippocampus. Importantly, most of these behavioral, neurochemical and structural impairments induced by Aβ_1-40 were not observed in mice subjected to 4 weeks of treadmill exercise. These findings indicate that physical exercise has the potential to prevent the occurrence of early emotional disturbances associated with AD and this appears to be mediated, at least in part, by modulation of hippocampal BDNF and mTOR signaling as well as through promotion of cell proliferation and survival in the hippocampal DG.

Cyclin-Dependent Kinase 5 Dysfunction Contributes to Depressive-like Behaviors in Huntington's Disease by Altering the DARPP-32 Phosphorylation Status in the Nucleus Accumbens

BACKGROUND

Depression is the most common psychiatric condition in Huntington's disease (HD), with rates more than twice those found in the general population. At the present time, there is no established molecular evidence to use as a basis for depression treatment in HD. Indeed, in some patients, classic antidepressant drugs exacerbate chorea or anxiety. Cyclin-dependent kinase 5 (Cdk5) has been involved in processes associated with anxiety and depression. This study evaluated the involvement of Cdk5 in the development and prevalence of depressive-like behaviors in HD and aimed to validate Cdk5 as a target for depression treatment.

METHODS

We evaluated the impact of pharmacological inhibition of Cdk5 in depressive-like and anxiety-like behaviors in Hdh^+/Q111 knock-in mutant mice by using a battery of behavioral tests. Biochemical and morphological studies were performed to define the molecular mechanisms acting downstream of Cdk5 activation. A double huntingtin/DARPP-32 (dopamine- and cAMP-regulated phosphoprotein 32) knock-in mutant mouse was generated to analyze the role of DARPP-32 in HD depression.

RESULTS

We found that Hdh^+/Q111 mutant mice exhibited depressive-like, but not anxiety-like, behaviors starting at 2 months of age. Cdk5 inhibition by roscovitine infusion prevented depressive-like behavior and reduced DARPP-32 phosphorylation at Thr75 in the nucleus accumbens. Hdh^+/Q111 mice heterozygous for DARPP-32 Thr75Ala point mutation were resistant to depressive-like behaviors. We identified β-adducin phosphorylation as a Cdk5 downstream mechanism potentially mediating structural spine plasticity changes in the nucleus accumbens and depressive-like behavior.

CONCLUSIONS

These results point to Cdk5 in the nucleus accumbens as a critical contributor to depressive-like behaviors in HD mice by altering DARPP-32/β-adducin signaling and disrupting the dendritic spine cytoskeleton.

Antidepressant Effects of Probucol on Early-Symptomatic YAC128 Transgenic Mice for Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a trinucleotide expansion in the HD gene, resulting in an extended polyglutamine tract in the protein huntingtin. HD is traditionally viewed as a movement disorder, but cognitive and neuropsychiatric symptoms also contribute to the clinical presentation. Depression is one of the most common psychiatric disturbances in HD, present even before manifestation of motor symptoms. Diagnosis and treatment of depression in HD-affected individuals are essential aspects of clinical management in this population, especially owing to the high risk of suicide. This study investigated whether chronic administration of the antioxidant probucol improved motor and affective symptoms as well as hippocampal neurogenic function in the YAC128 transgenic mouse model of HD during the early- to mild-symptomatic stages of disease progression. The motor performance and affective symptoms were monitored using well-validated behavioral tests in YAC128 mice and age-matched wild-type littermates at 2, 4, and 6 months of age, after 1, 3, or 5 months of treatment with probucol (30 mg/kg/day via water supplementation, starting on postnatal day 30). Endogenous markers were used to assess the effect of probucol on cell proliferation (Ki-67 and proliferation cell nuclear antigen (PCNA)) and neuronal differentiation (doublecortin (DCX)) in the hippocampal dentate gyrus (DG). Chronic treatment with probucol reduced the occurrence of depressive-like behaviors in early- and mild-symptomatic YAC128 mice. Functional improvements were not accompanied by increased progenitor cell proliferation and neuronal differentiation. Our findings provide evidence that administration of probucol may be of clinical benefit in the management of early- to mild-symptomatic HD.