Functional interactions within striatal microcircuit in animal models of Huntington's disease

Mitochondrial Bioenergy in Neurodegenerative Disease: Huntington and Parkinson

Strong evidence suggests a correlation between degeneration and mitochondrial deficiency. Typical cases of degeneration can be observed in physiological phenomena (i.e., ageing) as well as in neurological neurodegenerative diseases and cancer. All these pathologies have the dyshomeostasis of mitochondrial bioenergy as a common denominator. Neurodegenerative diseases show bioenergetic imbalances in their pathogenesis or progression. Huntington's chorea and Parkinson's disease are both neurodegenerative diseases, but while Huntington's disease is genetic and progressive with early manifestation and severe penetrance, Parkinson's disease is a pathology with multifactorial aspects. Indeed, there are different types of Parkinson/Parkinsonism. Many forms are early-onset diseases linked to gene mutations, while others could be idiopathic, appear in young adults, or be post-injury senescence conditions. Although Huntington's is defined as a hyperkinetic disorder, Parkinson's is a hypokinetic disorder. However, they both share a lot of similarities, such as neuronal excitability, the loss of striatal function, psychiatric comorbidity, etc. In this review, we will describe the start and development of both diseases in relation to mitochondrial dysfunction. These dysfunctions act on energy metabolism and reduce the vitality of neurons in many different brain areas.

Striatal network modeling in Huntington's Disease

Medium spiny neurons (MSNs) comprise over 90% of cells in the striatum. In vivo MSNs display coherent burst firing cell assembly activity patterns, even though isolated MSNs do not burst fire intrinsically. This activity is important for the learning and execution of action sequences and is characteristically dysregulated in Huntington's Disease (HD). However, how dysregulation is caused by the various neural pathologies affecting MSNs in HD is unknown. Previous modeling work using simple cell models has shown that cell assembly activity patterns can emerge as a result of MSN inhibitory network interactions. Here, by directly estimating MSN network model parameters from single unit spiking data, we show that a network composed of much more physiologically detailed MSNs provides an excellent quantitative fit to wild type (WT) mouse spiking data, but only when network parameters are appropriate for the striatum. We find the WT MSN network is situated in a regime close to a transition from stable to strongly fluctuating network dynamics. This regime facilitates the generation of low-dimensional slowly varying coherent activity patterns and confers high sensitivity to variations in cortical driving. By re-estimating the model on HD spiking data we discover network parameter modifications are consistent across three very different types of HD mutant mouse models (YAC128, Q175, R6/2). In striking agreement with the known pathophysiology we find feedforward excitatory drive is reduced in HD compared to WT mice, while recurrent inhibition also shows phenotype dependency. We show that these modifications shift the HD MSN network to a sub-optimal regime where higher dimensional incoherent rapidly fluctuating activity predominates. Our results provide insight into a diverse range of experimental findings in HD, including cognitive and motor symptoms, and may suggest new avenues for treatment.

A biophysical model of striatal microcircuits suggests gamma and beta oscillations interleaved at delta/theta frequencies mediate periodicity in motor control

Striatal oscillatory activity is associated with movement, reward, and decision-making, and observed in several interacting frequency bands. Local field potential recordings in rodent striatum show dopamine- and reward-dependent transitions between two states: a "spontaneous" state involving β (∼15-30 Hz) and low γ (∼40-60 Hz), and a state involving θ (∼4-8 Hz) and high γ (∼60-100 Hz) in response to dopaminergic agonism and reward. The mechanisms underlying these rhythmic dynamics, their interactions, and their functional consequences are not well understood. In this paper, we propose a biophysical model of striatal microcircuits that comprehensively describes the generation and interaction of these rhythms, as well as their modulation by dopamine. Building on previous modeling and experimental work suggesting that striatal projection neurons (SPNs) are capable of generating β oscillations, we show that networks of striatal fast-spiking interneurons (FSIs) are capable of generating δ/θ (ie, 2 to 6 Hz) and γ rhythms. Under simulated low dopaminergic tone our model FSI network produces low γ band oscillations, while under high dopaminergic tone the FSI network produces high γ band activity nested within a δ/θ oscillation. SPN networks produce β rhythms in both conditions, but under high dopaminergic tone, this β oscillation is interrupted by δ/θ-periodic bursts of γ-frequency FSI inhibition. Thus, in the high dopamine state, packets of FSI γ and SPN β alternate at a δ/θ timescale. In addition to a mechanistic explanation for previously observed rhythmic interactions and transitions, our model suggests a hypothesis as to how the relationship between dopamine and rhythmicity impacts motor function. We hypothesize that high dopamine-induced periodic FSI γ-rhythmic inhibition enables switching between β-rhythmic SPN cell assemblies representing the currently active motor program, and thus that dopamine facilitates movement in part by allowing for rapid, periodic shifts in motor program execution.

Corticostriatal synaptic plasticity alterations in the R6/1 transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative condition characterized by abnormal dopamine (DA)-glutamate interactions, severe alterations in motor control, and reduced behavioral flexibility. Experimental models of disease show that during symptomatic phases, HD shares with other hyperkinetic disorders the loss of synaptic depotentiation in the striatal spiny projection neurons (SPNs). Here we test the hypothesis that corticostriatal long-term depression (LTD), a well-conserved synaptic scaling down response to environmental stimuli, is also altered in symptomatic male R6/1 mice, a HD model with gradual development of symptoms. In vitro patch-clamp and intracellular recordings of corticostriatal slices from R6/1 mice confirm that, similar to other models characterized by hyperkinesia and striatal DA D1 receptor pathway dysregulation, once long-term potentiation (LTP) is induced, synaptic depotentiation is lost. Our new observations show that activity-dependent LTD was abolished in SPNs of mutant mice. In an experimental condition in which N-methyl-d-aspartate (NMDA) receptors are normally not recruited, in vitro bath application of DA revealed an abnormal response of D1 receptors that caused a shift in synaptic plasticity direction resulting in an NMDA-dependent LTP. Our results demonstrate that corticostriatal LTD is lost in R6/1 mouse model and confirm the role of aberrant DA-glutamate interactions in the alterations of synaptic scaling down associated with HD symptoms.

Pathway-Specific Control of Striatal Neuron Vulnerability by Corticostriatal Cannabinoid CB1 Receptors

The vast majority of neurons within the striatum are GABAergic medium spiny neurons (MSNs), which receive glutamatergic input from the cortex and thalamus, and form two major efferent pathways: the direct pathway, expressing dopamine D1 receptor (D1R-MSNs), and the indirect pathway, expressing dopamine D2 receptor (D2R-MSNs). While molecular mechanisms of MSN degeneration have been identified in animal models of striatal damage, the molecular factors that dictate a selective vulnerability of D1R-MSNs or D2R-MSNs remain unknown. Here, we combined genetic, chemogenetic, and pharmacological strategies with behavioral and neurochemical analyses, and show that the pool of cannabinoid CB1 receptor (CB1R) located on corticostriatal terminals efficiently safeguards D1R-MSNs, but not D2R-MSNs, from different insults. This cell-specific response relies on the regulation of glutamatergic signaling, and is independent from the CB1R-dependent control of astroglial activity in the striatum. These findings define cortical CB1R as a pivotal synaptic player in dictating a differential vulnerability of D1R-MSNs versus D2R-MSNs, and increase our understanding of the role of coordinated cannabinergic-glutamatergic signaling in establishing corticostriatal circuits and its dysregulation in neurodegenerative diseases.

Corticostriatal network dysfunction in Huntington's disease: Deficits in neural processing, glutamate transport, and ascorbate release

AIMS

This review summarizes evidence for dysfunctional connectivity between cortical and striatal neurons in Huntington's disease (HD), a fatal neurodegenerative condition caused by a single gene mutation. The focus is on data derived from recording of electrophysiological signals in behaving transgenic mouse models.

DISCUSSIONS

Firing patterns of individual neurons and the frequency oscillations of local field potentials indicate a disruption in corticostriatal processing driven, in large part, by interactions between cells that contain the mutant gene rather than the mutant gene alone. Dysregulation of glutamate, an excitatory amino acid released by cortical afferents, plays a key role in the breakdown of corticostriatal communication, a process modulated by ascorbate, an antioxidant vitamin found in high concentration in striatum. Up-regulation of glutamate transport by drug administration or viral-vector delivery improves ascorbate homeostasis and neurobehavioral processing in HD mice. Further analysis of electrophysiological data, including the use of sophisticated computational strategies, is required to discern how behavioral demands modulate the flow of corticostriatal information and its disruption by HD.

CONCLUSIONS

Long before massive cell loss occurs, HD impairs the mechanisms by which cortical and striatal neurons communicate. A key problem identified in transgenic animal models is dysregulation of the dynamic changes in extracellular glutamate and ascorbate. Improved understanding of how these neurochemical systems impact corticostriatal communication is necessary before an effective therapeutic strategy can emerge.

Dysregulation of Corticostriatal Connectivity in Huntington's Disease: A Role for Dopamine Modulation

Aberrant communication between striatum, the main information processing unit of the basal ganglia, and cerebral cortex plays a critical role in the emergence of Huntington’s disease (HD), a fatal monogenetic condition that typically strikes in the prime of life. Although both striatum and cortex undergo substantial cell loss over the course of HD, corticostriatal circuits become dysfunctional long before neurons die. Understanding the dysfunction is key to developing effective strategies for treating a progressively worsening triad of motor, cognitive, and psychiatric symptoms. Cortical output neurons drive striatal activity through the release of glutamate, an excitatory amino acid. Striatal outputs, in turn, release γ-amino butyric acid (GABA) and exert inhibitory control over downstream basal ganglia targets. Ample evidence from transgenic rodent models points to dysregulation of corticostriatal glutamate transmission along with corresponding changes in striatal GABA release as underlying factors in the HD behavioral phenotype. Another contributor is dysregulation of dopamine (DA), a modulator of both glutamate and GABA transmission. In fact, pharmacological manipulation of DA is the only currently available treatment for HD symptoms. Here, we review data from animal models and human patients to evaluate the role of DA in HD, including DA interactions with glutamate and GABA within the context of dysfunctional corticostriatal circuitry.

Altered behavioral responses to gamma-aminobutyric acid pharmacological agents in a mouse model of Huntington's disease

BACKGROUND

Disruptions in gamma-aminobutyric (GABA) acid signaling are believed to be involved in Huntington's disease pathogenesis, but the regulation of GABAergic signaling remains elusive. Here we evaluated GABAergic signaling by examining the function of GABAergic drugs in Huntington's disease and the expression of GABAergic molecules using mouse models and human brain tissues from Huntington's disease.

METHODS

We treated wild-type and R6/2 mice (a transgenic Huntington's disease mouse model) acutely with vehicle, diazepam, or gaboxadol (drugs that selectively target synaptic or extrasynaptic GABA_A receptors) and monitored their locomotor activity. The expression levels of GABA_A receptors and a major neuron-specific chloride extruder (potassium-chloride cotransporter-2) were analyzed by real-time quantitative polymerase chain reaction, Western blot, and immunocytochemistry.

RESULTS

The R6/2 mice were less sensitive to the sedative effects of both drugs, suggesting reduced function of GABA_A receptors. Consistently, the expression levels of α1/α2 and δ subunits were lower in the cortex and striatum of R6/2 mice. Similar results were also found in 2 other mouse models of Huntington's disease and in Huntington's disease patients. Moreover, the interaction and expression levels of potassium-chloride cotransporter-2 and its activator (brain-type creatine kinase) were decreased in Huntington's disease neurons. These findings collectively suggest impaired chloride homeostasis, which further dampens GABA_A receptor-mediated inhibitory signaling in Huntington's disease brains.

CONCLUSIONS

The dysregulated GABAergic responses and altered expression levels of GABA_A receptors and potassium-chloride cotransporter-2 in Huntington's disease mice appear to be authentic and may contribute to the clinical manifestations of Huntington's disease patients. © 2017 International Parkinson and Movement Disorder Society.

Differential Alteration in Expression of Striatal GABAAR Subunits in Mouse Models of Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder characterized by progressive motor symptoms that are preceded by cognitive deficits and is considered as a disorder that primarily affects forebrain striatal neurons. To gain a better understanding of the molecular and cellular mechanisms associated with disease progression, we analyzed the expression of proteins involved in GABAergic neurotransmission in the striatum of the R6/1 transgenic mouse model. Western blot, quantitative PCR and immunohistochemical analyses were conducted on male R6/1 mice and age-matched wild type littermates. Analyses were performed on 2 and 6 month-old animals, respectively, before and after the onset of motor symptoms. Expression of GAD 67, GAD 65, NL2, or gephyrin proteins, involved in GABA synthesis or synapse formation did not display major changes. In contrast, expression of α1, α3 and α5 GABA_AR subunits was increased while the expression of δ was decreased, suggesting a change in tonic- and phasic inhibitory transmission. Western blot analysis of the striatum from 8 month-old Hdh Q111, a knock-in mouse model of HD with mild deficits, confirmed the α1 subunit increased expression. From immunohistochemical analyses, we also found that α1 subunit expression is increased in medium-sized spiny projection neurons (MSN) and decreased in parvalbumin (PV)-expressing interneurons at 2 and 6 months in R6/1 mice. Moreover, α2 subunit labeling on the PV and MSN cell membranes was increased at 2 months and decreased at 6 months. Alteration of gene expression in the striatum and modification of GABA_A receptor subtypes in both interneurons and projection neurons suggested that HD mutation has a profound effect on synaptic plasticity at an early stage, before the onset of motor symptoms. These results also indicate that cognitive and other behavioral deficits may be associated with changes in GABAergic neurotransmission that consequently could be a relevant target for early therapeutic treatment.

Striatal synaptic dysfunction and hippocampal plasticity deficits in the Hu97/18 mouse model of Huntington disease

Huntington disease (HD) is a fatal neurodegenerative disorder caused by a CAG repeat expansion in the gene (HTT) encoding the huntingtin protein (HTT). This mutation leads to multiple cellular and synaptic alterations that are mimicked in many current HD animal models. However, the most commonly used, well-characterized HD models do not accurately reproduce the genetics of human disease. Recently, a new ‘humanized’ mouse model, termed Hu97/18, has been developed that genetically recapitulates human HD, including two human HTT alleles, no mouse Hdh alleles and heterozygosity of the HD mutation. Previously, behavioral and neuropathological testing in Hu97/18 mice revealed many features of HD, yet no electrophysiological measures were employed to investigate possible synaptic alterations. Here, we describe electrophysiological changes in the striatum and hippocampus of the Hu97/18 mice. At 9 months of age, a stage when cognitive deficits are fully developed and motor dysfunction is also evident, Hu97/18 striatal spiny projection neurons (SPNs) exhibited small changes in membrane properties and lower amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs); however, release probability from presynaptic terminals was unaltered. Strikingly, these mice also exhibited a profound deficiency in long-term potentiation (LTP) at CA3-to-CA1 synapses. In contrast, at 6 months of age we found only subtle alterations in SPN synaptic transmission, while 3-month old animals did not display any electrophysiologically detectable changes in the striatum and CA1 LTP was intact. Together, these data reveal robust, progressive deficits in synaptic function and plasticity in Hu97/18 mice, consistent with previously reported behavioral abnormalities, and suggest an optimal age (9 months) for future electrophysiological assessment in preclinical studies of HD.

Transient and steady-state selection in the striatal microcircuit

Although the basal ganglia have been widely studied and implicated in signal processing and action selection, little information is known about the active role the striatal microcircuit plays in action selection in the basal ganglia-thalamo-cortical loops. To address this knowledge gap we use a large scale three dimensional spiking model of the striatum, combined with a rate coded model of the basal ganglia-thalamo-cortical loop, to asses the computational role the striatum plays in action selection. We identify a robust transient phenomena generated by the striatal microcircuit, which temporarily enhances the difference between two competing cortical inputs. We show that this transient is sufficient to modulate decision making in the basal ganglia-thalamo-cortical circuit. We also find that the transient selection originates from a novel adaptation effect in single striatal projection neurons, which is amenable to experimental testing. Finally, we compared transient selection with models implementing classical steady-state selection. We challenged both forms of model to account for recent reports of paradoxically enhanced response selection in Huntington's disease patients. We found that steady-state selection was uniformly impaired under all simulated Huntington's conditions, but transient selection was enhanced given a sufficient Huntington's-like increase in NMDA receptor sensitivity. Thus our models provide an intriguing hypothesis for the mechanisms underlying the paradoxical cognitive improvements in manifest Huntington's patients.

Genetic strategies to investigate neuronal circuit properties using stem cell-derived neurons

The mammalian brain is anatomically and functionally complex, and prone to diverse forms of injury and neuropathology. Scientists have long strived to develop cell replacement therapies to repair damaged and diseased nervous tissue. However, this goal has remained unrealized for various reasons, including nascent knowledge of neuronal development, the inability to track and manipulate transplanted cells within complex neuronal networks, and host graft rejection. Recent advances in embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) technology, alongside novel genetic strategies to mark and manipulate stem cell-derived neurons, now provide unprecedented opportunities to investigate complex neuronal circuits in both healthy and diseased brains. Here, we review current technologies aimed at generating and manipulating neurons derived from ESCs and iPSCs toward investigation and manipulation of complex neuronal circuits, ultimately leading to the design and development of novel cell-based therapeutic approaches.