Dopaminergic manipulations and its effects on neurogenesis and motor function in a transgenic mouse model of Huntington's disease

Selective neurodegeneration generated by intravenous α-synuclein pre-formed fibril administration is not associated with endogenous α-synuclein levels in the rat brain

Selective loss of discrete neuronal populations is a prominent feature of many neurodegenerative conditions, but the molecular basis of this is poorly understood. A central role of α-synuclein in the selective neurodegeneration of Parkinson's disease has been speculated, as its level of expression critically determines the propensity of this protein to misfold. To investigate whether the propensity of neuronal cell loss is associated with the level of endogenous α-synuclein expression, non-transgenic rats were given a single intravenous administration of α-synuclein pre-formed fibrils (PFFs) reversibly complexed with the rabies virus glycoprotein peptide (RVG9R). The number of surviving cells in different neuronal populations was systematically quantified using unbiased stereology. Our data demonstrated that a non-selective, transvascular delivery of α-synuclein PFFs led to a time-dependent loss of specific populations of midbrain (but not olfactory) dopaminergic neurons, medullary (but not pontine) cholinergic neurons, and brainstem serotonergic neurons. Contrary to the central role of endogenous α-synuclein expression in determining the seeding and aggregation propensity of pathological α-synuclein, we did not observe an association between the levels of α-synuclein expression in different regions of the rodent brain (although did not ascertain this at the individual cell level) and neurodegenerative propensity. The results from our study highlight the complexity of the neurodegenerative process generated by α-synuclein seeding. Further investigations are therefore required to elucidate the molecular basis of neurodegeneration driven by exogenous pathogenic α-synuclein spread.

Modulatory Activity of the Endocannabinoid System in the Development and Proliferation of Cells in the CNS

The Endocannabinoid System (ECS, also known as Endocannabinoidome) plays a key role in the function of the Central Nervous System, though the participation of this system on the early development - specifically in neuroprotection and proliferation of nerve cells - has been poorly studied. Here, we collect and describe evidence regarding how cannabinoid receptors CB1R and CB2R regulate several cell markers related to proliferation. While CB1R participates in the modulation of neuronal and glial proliferation, CB2R is involved in the proliferation of glial cells. The endocannabinoids anandamide (AEA) and 2-arachidonoylglycerol (2-AG) exert significant effects on nerve cell proliferation. AEA generated during embryogenesis induces major effects on the differentiation of neuronal progenitor cells, whereas 2-AG participates in modulating cell migration events rather than affecting the neural proliferation rate. However, although the ECS has been demonstrated to participate in neuroprotection, more characterization on its role in neuronal and glial proliferation and differentiation is needed, especially in brain areas with recognized high neurogenesis rates. This has encouraged scientists to elucidate and propose specific mechanisms related with these cell proliferation mechanisms to better understand some neurodegenerative disorders such as Parkinson, Huntington and Alzheimer diseases, in which neuronal loss and poor neurogenesis are crucial factors for their onset and progression. In this review, we collect and present recent evidence published pointing to an active role of the ECS in the development and proliferation of nerve cells.

Reduced expression of dopamine D2 receptors on astrocytes in R6/1 HD mice and HD post-mortem tissue

Dysfunction of the central dopaminergic system is thought to contribute to some of the clinical features of Huntington's disease (HD), and dopamine (DA) receptor antagonists are commonly used to good effect in its treatment. It is well established that there is an early significant reduction in neuronal D2 receptors in HD, considered to be a compensatory response to increased dopaminergic activity. However, no studies have examined the expression of D2 receptors on astrocytes which is important given that these cells have been shown to play a role in the pathogenesis of HD, as well as express dopamine receptors and modulate DA homeostasis in the normal brain. We therefore sought to investigate the expression of D2 receptors on astrocytes in HD, and found them to be reduced in both the R6/1 HD mouse model, and in human post-mortem brain in comparison to controls, suggesting that astrocytes may be important in DA-dependent aspects of HD. Further studies are needed to determine the functional significance of this finding.

Dopamine as a growth differentiation factor in the mammalian brain

The catecholamine, dopamine, plays an important role in the central nervous system of mammals, including executive functions, motor control, motivation, arousal, reinforcement, and reward. Dysfunctions of the dopaminergic system lead to diseases of the brains, such as Parkinson's disease, Tourette's syndrome, and schizophrenia. In addition to its fundamental role as a neurotransmitter, there is evidence for a role as a growth differentiation factor during development. Recent studies suggest that dopamine regulates the development of γ-aminobutyric acidergic interneurons of the cerebral cortex. Moreover, in adult brains, dopamine increases the production of new neurons in the hippocampus, suggesting the promoting effect of dopamine on proliferation and differentiation of neural stem cells and progenitor cells in the adult brains. In this mini-review, I center my attention on dopaminergic functions in the cortical interneurons during development and further discuss cell therapy against neurodegenerative diseases.

The D2 Dopamine Receptor Interferes With the Protective Effect of the A2A Adenosine Receptor on TDP-43 Mislocalization in Experimental Models of Motor Neuron Degeneration

The A2A adenosine receptor (A2AR) and D2 dopamine receptor (D2R) are two G-protein-coupled receptors that can form dimers and negatively regulate their partners. TAR DNA-binding protein (TDP-43) is a nuclear protein that has been implicated in amyotrophic lateral sclerosis (ALS). Mislocalization of TDP-43 from the nucleus to the cytoplasm is an early step of TDP-43 proteinopathy. Our previous studies indicated that A2AR is a potential drug target for ALS because treatment with an A2AR agonist (JMF1907; a T1-11 analog) prevents reactive oxygen species (ROS)-induced TDP-43 mislocalization in a motor neuron cell line (NSC34) and delays motor impairment in a TDP-43 transgenic ALS mouse model. Here, we set out to assess whether activation of D2R interferes with the beneficial effects of an A2AR agonist on motor neurons. We first demonstrated that A2AR and D2R are both located in motor neurons of mouse and human spinal cords and human iPSC-derived motor neurons. Expression of A2AR and D2R in NSC34 cells led to dimer formation without affecting the binding affinity of A2AR toward T1-11. Importantly, activation of D2R reduced T1-11-mediated activation of cAMP/PKA signaling and subsequent inhibition of TDP-43 mislocalization in NSC34 cells. Treatment with quinpirole (a D2 agonist) blunted the rescuing effect of T1-11 on TDP-43 mislocalization and impaired grip strength in a mouse model of ALS. Our findings suggest that D2R activation may limit the beneficial responses of an A2AR agonist in motor neurons and may have an important role in ALS pathogenesis.

Synaptopathic mechanisms of neurodegeneration and dementia: Insights from Huntington's disease

Dementia encapsulates a set of symptoms that include loss of mental abilities such as memory, problem solving or language, and reduces a person's ability to perform daily activities. Alzheimer's disease is the most common form of dementia, however dementia can also occur in other neurological disorders such as Huntington's disease (HD). Many studies have demonstrated that loss of neuronal cell function manifests pre-symptomatically and thus is a relevant therapeutic target to alleviate symptoms. Synaptopathy, the physiological dysfunction of synapses, is now being approached as the target for many neurological and psychiatric disorders, including HD. HD is an autosomal dominant and progressive degenerative disorder, with clinical manifestations that encompass movement, cognition, mood and behaviour. HD is one of the most common tandem repeat disorders and is caused by a trinucleotide (CAG) repeat expansion, encoding an extended polyglutamine tract in the huntingtin protein. Animal models as well as human studies have provided detailed, although not exhaustive, evidence of synaptic dysfunction in HD. In this review, we discuss the neuropathology of HD and how the changes in synaptic signalling in the diseased brain lead to its symptoms, which include dementia. Here, we review and discuss the mechanisms by which the 'molecular orchestras' and their 'synaptic symphonies' are disrupted in neurodegeneration and dementia, focusing on HD as a model disease. We also explore the therapeutic strategies currently in pre-clinical and clinical testing that are targeted towards improving synaptic function in HD.

Adult neurogenesis and neurodegenerative diseases: A systems biology perspective

New neurons are generated throughout adulthood in two regions of the brain, the olfactory bulb and dentate gyrus of the hippocampus, and are incorporated into the hippocampal network circuitry; disruption of this process has been postulated to contribute to neurodegenerative diseases including Alzheimer's disease and Parkinson's disease. Known modulators of adult neurogenesis include signal transduction pathways, the vascular and immune systems, metabolic factors, and epigenetic regulation. Multiple intrinsic and extrinsic factors such as neurotrophic factors, transcription factors, and cell cycle regulators control neural stem cell proliferation, maintenance in the adult neurogenic niche, and differentiation into mature neurons; these factors act in networks of signaling molecules that influence each other during construction and maintenance of neural circuits, and in turn contribute to learning and memory. The immune system and vascular system are necessary for neuronal formation and neural stem cell fate determination. Inflammatory cytokines regulate adult neurogenesis in response to immune system activation, whereas the vasculature regulates the neural stem cell niche. Vasculature, immune/support cell populations (microglia/astrocytes), adhesion molecules, growth factors, and the extracellular matrix also provide a homing environment for neural stem cells. Epigenetic changes during hippocampal neurogenesis also impact memory and learning. Some genetic variations in neurogenesis related genes may play important roles in the alteration of neural stem cells differentiation into new born neurons during adult neurogenesis, with important therapeutic implications. In this review, we discuss mechanisms of and interactions between these modulators of adult neurogenesis, as well as implications for neurodegenerative disease and current therapeutic research. © 2016 Wiley Periodicals, Inc.

COMT Val158Met Polymorphism Modulates Huntington's Disease Progression

Little is known about the genetic factors modulating the progression of Huntington's disease (HD). Dopamine levels are affected in HD and modulate executive functions, the main cognitive disorder of HD. We investigated whether the Val158Met polymorphism of the catechol-O-methyltransferase (COMT) gene, which influences dopamine (DA) degradation, affects clinical progression in HD. We carried out a prospective longitudinal multicenter study from 1994 to 2011, on 438 HD gene carriers at different stages of the disease (34 pre-manifest; 172 stage 1; 130 stage 2; 80 stage 3; 17 stage 4; and 5 stage 5), according to Total Functional Capacity (TFC) score. We used the Unified Huntington's Disease Rating Scale to evaluate motor, cognitive, behavioral and functional decline. We genotyped participants for COMT polymorphism (107 Met-homozygous, 114 Val-homozygous and 217 heterozygous). 367 controls of similar ancestry were also genotyped. We compared clinical progression, on each domain, between groups of COMT polymorphisms, using latent-class mixed models accounting for disease duration and number of CAG (cytosine adenine guanine) repeats. We show that HD gene carriers with fewer CAG repeats and with the Val allele in COMT polymorphism displayed slower cognitive decline. The rate of cognitive decline was greater for Met/Met homozygotes, which displayed a better maintenance of cognitive capacity in earlier stages of the disease, but had a worse performance than Val allele carriers later on. COMT polymorphism did not significantly impact functional and behavioral performance. Since COMT polymorphism influences progression in HD, it could be used for stratification in future clinical trials. Moreover, DA treatments based on the specific COMT polymorphism and adapted according to disease duration could potentially slow HD progression.

Exercise as a pro-cognitive, pro-neurogenic and anti-inflammatory intervention in transgenic mouse models of Alzheimer's disease

It is now well established, at least in animal models, that exercise elicits potent pro-cognitive and pro-neurogenic effects. Alzheimer's disease (AD) is one of the leading causes of dementia and represents one of the greatest burdens on healthcare systems worldwide, with no effective treatment for the disease to date. Exercise presents a promising non-pharmacological option to potentially delay the onset of or slow down the progression of AD. Exercise interventions in mouse models of AD have been explored and have been found to reduce amyloid pathology and improve cognitive function. More recent studies have expanded the research question by investigating potential pro-neurogenic and anti-inflammatory effects of exercise. In this review we summarise studies that have examined exercise-mediated effects on AD pathology, cognitive function, hippocampal neurogenesis and neuroinflammation in transgenic mouse models of AD. Furthermore, we attempt to identify the optimum exercise conditions required to elicit the greatest benefits, taking into account age and pathology of the model, as well as type and duration of exercise.

Animal models of Huntington's disease for translation to the clinic: best practices

Mouse models of Huntington's disease (HD) recapitulate many aspects of the human disease. These genetically modified mice are powerful tools that are used not only to examine the pathogenesis of the disease, but also to assess the efficacy of potential new treatments. Disappointingly, in the past few years we have seen the success of potential therapies in animal studies, subsequently followed by failure in clinical trials. We discuss here a number of factors that influence the translatability of findings from the preclinical to the clinical realm. In particular, we discuss issues related to sample size, reporting of information regarding experimental protocols and mouse models, assignment to experimental groups, incorporation of cognitive measures for early phases of the disease, environmental enrichment, surrogate measures for survival, and the use of more than one HD mouse model. Although it is reasonable to question the appropriateness of the animal models used, we argue that it is more parsimonious to assume that improvements in experimental design and publication of negative results will lead to improved translatability to the clinic and insights about HD pathophysiology.

Deep brain stimulation in Huntington's disease: assessment of potential targets

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder that has very few effective therapeutic interventions. Since the disease has a defined neural circuitry abnormality, neuromodulation could be an option. Case reports, original research, and animal model studies were selected from the databases of Medline and PubMed. All related studies published up to July 2014 were included in this review. The following search terms were used: "Deep brain stimulation," "DBS," "thalamotomy," "pallidal stimulation," and "Huntington's Disease," "HD," "chorea," or "hyperkinetic movement disorders." This review examines potential nodes in the HD circuitry that could be modulated using deep brain stimulation (DBS) therapy. With rapid evolution of imaging and ability to reach difficult targets in the brain with refined DBS technology, some phenotypes of HD could potentially be treated with DBS in the near future. Further clinical studies are warranted to validate the efficacy of neuromodulation and to determine the most optimal target for HD.