Impaired Refinement of Kinematic Variability in Huntington Disease Mice on an Automated Home Cage Forelimb Motor Task.

The effective development of novel therapies in mouse models of neurologic disorders relies on behavioral assessments that provide accurate read-outs of neuronal dysfunction and/or degeneration. We designed an automated behavioral testing system (PiPaw), which integrates an operant lever-pulling task directly into the mouse home cage. This task is accessible to group-housed mice 24 h per day, enabling high-throughput longitudinal analysis of forelimb motor learning. Moreover, this design eliminates the need for exposure to novel environments and minimizes experimenter interaction, significantly reducing two of the largest stressors associated with animal behavior. Male mice improved their performance of this task over 1 week of testing by reducing intertrial variability of reward-related kinematic parameters (pull amplitude or peak velocity). In addition, mice displayed short-term improvements in reward rate, and a concomitant decrease in movement variability, over the course of brief bouts of task engagement. We used this system to assess motor learning in mouse models of the inherited neurodegenerative disorder, Huntington disease (HD). Despite having no baseline differences in task performance, male Q175-FDN HD mice were unable to modulate the variability of their movements to increase reward on either short or long timescales. Task training was associated with a decrease in the amplitude of spontaneous excitatory activity recorded from striatal medium spiny neurons in the hemisphere contralateral to the trained forelimb in WT mice; however, no such changes were observed in Q175-FDN mice. This behavioral screening platform should prove useful for preclinical drug trials toward improved treatments in HD and other neurologic disorders.SIGNIFICANCE STATEMENT In order to develop effective therapies for neurologic disorders, such as Huntington disease (HD), it is important to be able to accurately and reliably assess the behavior of mouse models of these conditions. Moreover, these behavioral assessments should provide an accurate readout of underlying neuronal dysfunction and/or degeneration. In this paper, we used an automated behavioral testing system to assess motor learning in mice within their home cage. Using this system, we were able to study motor abnormalities in HD mice with an unprecedented level of detail, and identified a specific behavioral deficit associated with an underlying impairment in striatal neuronal plasticity. These results validate the usefulness of this system for assessing behavior in mouse models of HD and other neurologic disorders.

PiDose: an open-source system for accurate and automated oral drug administration to group-housed mice.

Drug treatment studies in laboratory mice typically employ manual administration methods such as injection or gavage, which can be time-consuming to perform over long periods and cause substantial stress in animals. These stress responses may mask or enhance treatment effects, increasing the risk of false positive or negative results and decreasing reliability. To address the lack of an automated method for drug treatment in group-housed mice, we have developed PiDose, a home-cage attached device that weighs individual animals and administers a daily dosage of drug solution based on each animal's bodyweight through their drinking water. Group housed mice are identified through the use of RFID tagging and receive both regular water and drug solution drops by licking at a spout within the PiDose module. This system allows animals to be treated over long periods (weeks to months) in a fully automated fashion, with high accuracy and minimal experimenter interaction. PiDose is low-cost and fully open-source and should prove useful for researchers in both translational and basic research.

The Protein Tyrosine Phosphatase Receptor Delta Regulates Developmental Neurogenesis.

PTPRD is a receptor protein tyrosine phosphatase that is genetically associated with neurodevelopmental disorders. Here, we asked whether Ptprd mutations cause aberrant neural development by perturbing neurogenesis in the murine cortex. We show that loss of Ptprd causes increases in neurogenic transit-amplifying intermediate progenitor cells and cortical neurons and perturbations in neuronal localization. These effects are intrinsic to neural precursor cells since acute Ptprd knockdown causes similar perturbations. PTPRD mediates these effects by dephosphorylating receptor tyrosine kinases, including TrkB and PDGFRß, and loss of Ptprd causes the hyperactivation of TrkB and PDGFRß and their downstream MEK-ERK signaling pathway in neural precursor cells. Moreover, inhibition of aberrant TrkB or MEK activation rescues the increased neurogenesis caused by knockdown or homozygous loss of Ptprd. These results suggest that PTPRD regulates receptor tyrosine kinases to ensure appropriate numbers of intermediate progenitor cells and neurons, suggesting a mechanism for its genetic association with neurodevelopmental disorders.

Direct assessment of presynaptic modulation of cortico-striatal glutamate release in a Huntington's disease mouse model.

Glutamate is the main excitatory neurotransmitter in the brain, and impairments in its signaling are associated with many neurological disorders, including Huntington's disease (HD). Previous studies in HD mouse models demonstrate altered glutamate receptor distribution and signaling at cortico-striatal synapses, and some studies suggest that glutamate release is altered; however, traditional methods to study synaptic glutamate release are indirect or have poor temporal resolution. Here we utilize iGluSnFR, a modified green fluorescent protein reporter for real-time imaging of glutamate transmission, to study presynaptic modulation of cortical glutamate release in the striatum of the YAC128 HD mouse model. We determined that iGluSnFR can be used to accurately measure short- and long-term changes in glutamate release caused by modulation of extracellular Ca2+ levels, activation of presynaptic receptors, and high-frequency stimulation (HFS) protocols. We also confirmed a difference in the expression of HFS-induced long-term depression in YAC128. Together, this research demonstrates the utility of iGluSnFR in studying presynaptic modulation of glutamate release in healthy mice and disease models that display impairments in glutamate signaling. NEW & NOTEWORTHY We use iGluSnFR to directly assess presynaptic modulation of cortico-striatal glutamate release in brain slice and compare changes in glutamate release between wild type and a Huntington's disease mouse model, YAC128. We observed reductions in glutamate release after low extracellular Ca2+ and activation of various presynaptic receptors. We also demonstrate a presynaptic mechanism of reduced glutamate release in high-frequency stimulation-induced long-term depression and show this to be altered in YAC128.

An Automated Home-Cage System to Assess Learning and Performance of a Skilled Motor Task in a Mouse Model of Huntington's Disease.

Behavioral testing is a critical step in assessing the validity of rodent models of neurodegenerative disease, as well as evaluating the efficacy of pharmacological interventions. In models of Huntington's disease (HD), a gradual progression of impairments is observed across ages, increasing the need for sensitive, high-throughput and longitudinal assessments. Recently, a number of automated systems have been developed to perform behavioral profiling of animals within their own home-cage, allowing for 24-h monitoring and minimizing experimenter interaction. However, as of yet, few of these have had functionality for the assessment of skilled motor learning, a relevant behavior for movement disorders such as HD. To address this, we assess a lever positioning task within the mouse home-cage. Animals first acquire a simple operant response, before moving to a second phase where they must learn to hold the lever for progressively longer in a rewarded position range. Testing with this paradigm has revealed the presence of distinct phenotypes in the YAC128 mouse model of HD at three early symptomatic time points. YAC128 mice at two months old, but not older, had a motor learning deficit when required to adapt their response to changes in task requirements. In contrast, six-month-old YAC128 mice had disruptions of normal circadian activity and displayed kinematic abnormalities during performance of the task, suggesting an impairment in motor control. This system holds promise for facilitating high throughput behavioral assessment of HD mouse models for preclinical therapeutic screening.

Real-time imaging of glutamate clearance reveals normal striatal uptake in Huntington disease mouse models.

It has become well accepted that Huntington disease (HD) is associated with impaired glutamate uptake, resulting in a prolonged time-course of extracellular glutamate that contributes to excitotoxicity. However, the data supporting this view come largely from work in synaptosomes, which may overrepresent nerve-terminal uptake over astrocytic uptake. Here, we quantify real-time glutamate dynamics in HD mouse models by high-speed imaging of an intensity-based glutamate-sensing fluorescent reporter (iGluSnFR) and electrophysiological recordings of synaptically activated transporter currents in astrocytes. These techniques reveal a disconnect between the results obtained in synaptosomes and those in situ. Exogenous glutamate uptake is impaired in synaptosomes, whereas real-time measures of glutamate clearance in the HD striatum are normal or even accelerated, particularly in the aggressive R6/2 model. Our results highlight the importance of quantifying glutamate dynamics under endogenous release conditions, and suggest that the widely cited uptake impairment in HD does not contribute to pathogenesis.

Transient maternal IL-6 mediates long-lasting changes in neural stem cell pools by deregulating an endogenous self-renewal pathway.

The mechanisms that regulate the establishment of adult stem cell pools during normal and perturbed mammalian development are still largely unknown. Here, we asked whether a maternal cytokine surge, which occurs during human maternal infections and has been implicated in cognitive disorders, might have long-lasting consequences for neural stem cell pools in adult progeny. We show that transient, maternally administered interleukin-6 (IL-6) resulted in an expanded adult forebrain neural precursor pool and perturbed olfactory neurogenesis in offspring months after fetal exposure. This increase is likely the long-term consequence of acute hyperactivation of an endogenous autocrine/paracrine IL-6-dependent self-renewal pathway that normally regulates the number of forebrain neural precursors. These studies therefore identify an IL-6-dependent neural stem cell self-renewal pathway in vivo, and support a model in which transiently increased maternal cytokines can act through this pathway in offspring to deregulate neural precursor biology from embryogenesis throughout life.