Cognitive flexibility deficits in a mouse model for the absence of full-length dystrophin

Sex differences in spontaneous behavior and cognition in mice using an automated behavior monitoring system

Isolation of sex differences as a key characteristic underlying neurobehavioral differentiation is an essential component of studies in neuroscience. The current study sought to address this concern by observing behavioral differences using an automated home cage system for neurobehavioral assessment, a method rapidly increasing in use due to advances in technology and advantages such as reduced handling stress and cross-lab variability. Sex differences in C57BL/6 mice arose for motor activity and circadian-linked behavior, with females being more active compared to males, and males having a stronger anticipatory increase in activity leading up to the onset of the light phase compared to females. These activity differences were observed not only across the lifespan, but also in different genetic background mouse strains across different testing sites showing the generalizability and robustness of these observed effects. Activity differences were also observed in performance on a spatial learning and reversal task with females making more responses and receiving a corresponding elevation in reward pellets. Notably, there were no sex differences in learning nor achieved accuracy, suggesting these observed effects were predominantly in activity. The outcomes of this study align with previous reports showcasing differences in activity between males and females. The comparison across strains and testing sites showed robust and reproducible differences in behavior between female and male mice that are relevant to consider when designing behavioral studies. Furthermore, the observed sex differences in performance on the learning and reversal procedure raise concern for interpretation of behavior differences between sexes due to the attribution of these differences to motor activity rather than cognition.

Learning, memory and blood-brain barrier pathology in Duchenne muscular dystrophy mice lacking Dp427, or Dp427 and Dp140

Duchenne muscular dystrophy is a severe neuromuscular disorder that is caused by mutations in the DMD gene, resulting in a disruption of dystrophin production. Next to dystrophin expression in the muscle, different isoforms of the protein are also expressed in the brain and lack of these isoforms leads to cognitive and behavioral deficits in patients. It remains unclear how the loss of the shorter dystrophin isoform Dp140 affects these processes. Using a variety of behavioral tests, we found that mdx and mdx4cv mice (which lack Dp427 or Dp427 + Dp140, respectively) exhibit similar deficits in working memory, movement patterns and blood-brain barrier integrity. Neither model showed deficits in spatial learning and memory, learning flexibility, anxiety or spontaneous behavior, nor did we observe differences in aquaporin 4 and glial fibrillary acidic protein. These results indicate that in contrast to Dp427, Dp140 does not play a crucial role in processes of learning, memory and spontaneous behavior.

The unconditioned fear response in vertebrates deficient in dystrophin

Dystrophin loss due to mutations in the Duchenne muscular dystrophy (DMD) gene is associated with a wide spectrum of neurocognitive comorbidities, including an aberrant unconditioned fear response to stressful/threat stimuli. Dystrophin-deficient animal models of DMD demonstrate enhanced stress reactivity that manifests as sustained periods of immobility. When the threat is repetitive or severe in nature, dystrophinopathy phenotypes can be exacerbated and even cause sudden death. Thus, it is apparent that enhanced sensitivity to stressful/threat stimuli in dystrophin-deficient vertebrates is a legitimate cause of concern for patients with DMD that could impact neurocognition and pathophysiology. This review discusses our current understanding of the mechanisms and consequences of the hypersensitive fear response in preclinical models of DMD and the potential challenges facing clinical translatability.

Altered striatal actin dynamics drives behavioral inflexibility in a mouse model of fragile X syndrome

The proteome of glutamatergic synapses is diverse across the mammalian brain and involved in neurodevelopmental disorders (NDDs). Among those is fragile X syndrome (FXS), an NDD caused by the absence of the functional RNA-binding protein FMRP. Here, we demonstrate how the brain region-specific composition of postsynaptic density (PSD) contributes to FXS. In the striatum, the FXS mouse model shows an altered association of the PSD with the actin cytoskeleton, reflecting immature dendritic spine morphology and reduced synaptic actin dynamics. Enhancing actin turnover with constitutively active RAC1 ameliorates these deficits. At the behavioral level, the FXS model displays striatal-driven inflexibility, a typical feature of FXS individuals, which is rescued by exogenous RAC1. Striatal ablation of Fmr1 is sufficient to recapitulate behavioral impairments observed in the FXS model. These results indicate that dysregulation of synaptic actin dynamics in the striatum, a region largely unexplored in FXS, contributes to the manifestation of FXS behavioral phenotypes.

Brain magnetic resonance imaging in the DE50-MD dog model of Duchenne muscular dystrophy reveals regional reductions in cerebral gray matter

BACKGROUND

Duchenne muscular dystrophy is a X-linked disease characterized by severe and progressive muscle weakness, alongside cognitive impairment and a range of neurobehavioral disorders secondary to brain dystrophin deficiency. Duchenne muscular dystrophy patients have reduced cerebral gray matter and altered white matter ultrastructure (detected by magnetic resonance imaging) compared to age-matched controls.

METHODS

We studied the DE50-MD canine model of Duchenne muscular dystrophy, which is deficient in full length brain dystrophin (Dp427) isoforms and has a neurocognitive phenotype. Eight DE50-MD and 6 age-matched littermate wild type male dogs underwent serial brain magnetic resonance imaging from 14 to 33 months of age.

RESULTS

Reduced regional gray matter was detected in DE50-MD dogs compared with wildtype, including the piriform lobe, hippocampus and cingulate gyrus. Lateral ventricle volume was larger in DE50-MD dogs. Differences did not progress over time. White matter volume did not differ between DE50-MD and wildtype dogs. There was no difference in brain nor cranial vault volume between DE50-MD and wildtype dogs.

CONCLUSION

Dystrophin deficiency in the canine brain results in structural changes that likely contribute to the neurocognitive phenotype.

Startle responses in Duchenne muscular dystrophy: a novel biomarker of brain dystrophin deficiency

Duchenne muscular dystrophy (DMD) is characterized by loss of dystrophin in muscle, however patients also have variable degree of intellectual disability and neurobehavioural co-morbidities. In contrast to muscle, in which a single full-length dystrophin isoform (Dp427) is produced, multiple isoforms are produced in the brain, and their deficiency accounts for the variability of CNS manifestations, with increased risk of comorbidities in patients carrying mutations affecting the 3' end of the gene, which disrupt expression of shorter Dp140 and Dp71 isoforms. A mouse model (mdx mouse) lacks Dp427 in muscle and CNS and exhibits exaggerated startle responses to threat, linked to the deficiency of dystrophin in limbic structures such as the amygdala, which normalize with postnatal brain dystrophin-restoration therapies. A pathological startle response is not a recognized feature of DMD, and its characterization has implications for improved clinical management and translational research. To investigate startle responses in DMD, we used a novel fear-conditioning task in an observational study of 56 males aged 7-12 years (31 affected boys, mean age 9.7 ± 1.8 years; 25 controls, mean age 9.6 ± 1.4 years). Trials of two neutral visual stimuli were presented to participants: one 'safe' cue presented alone; one 'threat' cue paired with an aversive noise to enable conditioning of physiological startle responses (skin conductance response and heart rate). Retention of conditioned physiological responses was subsequently tested by presenting both cues without the aversive noise in an 'Extinction' phase. Primary outcomes were the initial unconditioned skin conductance and change in heart rate responses to the aversive 'threat' and acquisition and retention of conditioned responses after conditioning. Secondary and exploratory outcomes were neuropsychological measures and genotype associations. The mean unconditioned skin conductance response was greater in the DMD group than controls [mean difference 3.0 µS (1.0, 5.1); P = 0.004], associated with a significant threat-induced bradycardia only in the patient group [mean difference -8.7 bpm (-16.9, -0.51); P = 0.04]. Participants with DMD found the task more aversive than controls, with increased early termination rates during the Extinction phase (26% of DMD group versus 0% of controls; P = 0.007). This study provides the first evidence that boys with DMD show similar increased unconditioned startle responses to threat to the mdx mouse, which in the mouse respond to brain dystrophin restoration. Our study provides new insights into the neurobiology underlying the complex neuropsychiatric co-morbidities in DMD and defines an objective measure of this CNS phenotype, which will be valuable for future CNS-targeted dystrophin-restoration studies.

Kynurenine metabolism is altered in mdx mice: A potential muscle to brain connection

NEW FINDINGS

Research has shown that promoting muscle health with regular aerobic exercise can improve mental health through a kynurenine metabolic pathway. With regular aerobic exercise, kynurenine metabolism is favourably altered towards a neuroprotective pathway that promotes kynurenic acid production through increased expression of PGC-1α, kynurenine amino acid transferase (KAT) enzymes, and lowered inflammation. Whether conditions of muscle disease such as muscular dystrophy can negatively influence this pathway remains unknown. We show that the DBA/2J mdx model of Duchenne muscular dystrophy exhibit altered kynurenine metabolism with less KYNA and PGC-1α and the highest level of TNF-a mRNA - results associated with anxiety-like behaviour.

ABSTRACT

Regular exercise can direct muscle kynurenine (KYN) metabolism toward the neuroprotective branch of the kynurenine pathway thereby limiting the accumulation of neurotoxic metabolites in the brain and contributing to mental resilience. However, the effect of muscle disease on KYN metabolism has not yet been investigated. Previous work has highlighted anxiety-like behaviors in approximately 25% of patients with Duchenne muscular dystrophy (DMD), possibly due to altered KYN metabolism. Here, we characterized KYN metabolism in mdx mouse models of DMD. Young (8-10 week old) DBA/2J (D2) mdx mice, but not age-matched C57BL/10 (C57) mdx mice, had lower levels of circulating KYNA and KYNA:KYN ratio compared with their respective wild-type (WT) controls. While both C57 and D2 mdx mice displayed signs of anxiety-like behaviour, spending more time in the corners of the arena during a novel object recognition test, this effect was more prominent in D2 mdx mice. Correlational analysis detected a significant negative association between KYNA:KYN levels and time spent in corners in D2 mice, but not C57 mice. In extensor digitorum longus muscles from D2 mdx mice, but not C57 mdx mice, we found lowered protein levels of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha and kynurenine amino transferase-1 enzyme when compared with WT. Furthermore, D2 mdx quadricep muscles had the highest level of TNF-α expression, which is suggestive of enhanced inflammation. Thus, our pilot work shows that KYN metabolism is altered in D2 mdx mice, with a potential contribution from altered muscle health. This article is protected by copyright. All rights reserved.

Partial restoration of brain dystrophin and behavioral deficits by exon skipping in the muscular dystrophy X-linked (mdx) mouse

Objectives

Duchenne muscular dystrophy is associated with various degrees of cognitive impairment and behavioral disturbances. Emotional and memory deficits also constitute reliable outcome measures to assess efficacy of treatments in the mdx mouse lacking the muscle and neuronal full‐length dystrophins. The present study aimed to evaluate whether these deficits could be alleviated by the restoration of brain dystrophin.

Methods

We performed intracerebroventricular administration of a new potent tricyclo‐DNA antisense oligonucleotide (tcDNA‐ASO) containing a full phosphodiester backbone conjugated to a palmitic acid moiety (tcDNA‐ASO), designed to skip the mutated exon 23 of mdx mice.

Results

We first show that the tcDNA‐ASO rescues expression of brain dystrophin to 10–30% of wild‐type levels and significantly reduces the abnormal unconditioned fear responses in mdx mice in a dose‐dependent manner, 5 weeks post‐injection. Exon skipping efficiency, ASO biodistribution, protein restoration and effect on the fear response were optimal with a dose of 400 μg at 6–7 weeks post‐injection, with synaptic‐like expression in brain tissues such as the hippocampus and amygdala. Furthermore, this dose of tcDNA‐ASO restored long‐term memory retention of mdx mice in an object recognition task, but only had minor effects on fear conditioning.

Interpretation

These results suggest for the first time that postnatal re‐expression of brain dystrophin could reverse or at least alleviate some cognitive deficits associated with Duchenne muscular dystrophy. ANN NEUROL 2022;92:213–229

Dendritic autophagy degrades postsynaptic proteins and is required for long-term synaptic depression in mice

The pruning of dendritic spines during development requires autophagy. This process is facilitated by long-term depression (LTD)-like mechanisms, which has led to speculation that LTD, a fundamental form of synaptic plasticity, also requires autophagy. Here, we show that the induction of LTD via activation of NMDA receptors or metabotropic glutamate receptors initiates autophagy in the postsynaptic dendrites in mice. Dendritic autophagic vesicles (AVs) act in parallel with the endocytic machinery to remove AMPA receptor subunits from the membrane for degradation. During NMDAR-LTD, key postsynaptic proteins are sequestered for autophagic degradation, as revealed by quantitative proteomic profiling of purified AVs. Pharmacological inhibition of AV biogenesis, or conditional ablation of atg5 in pyramidal neurons abolishes LTD and triggers sustained potentiation in the hippocampus. These deficits in synaptic plasticity are recapitulated by knockdown of atg5 specifically in postsynaptic pyramidal neurons in the CA1 area. Conducive to the role of synaptic plasticity in behavioral flexibility, mice with autophagy deficiency in excitatory neurons exhibit altered response in reversal learning. Therefore, local assembly of the autophagic machinery in dendrites ensures the degradation of postsynaptic components and facilitates LTD expression.

Therapeutic Application of Extracellular Vesicles-Capsulated Adeno-Associated Virus Vector via nSMase2/Smpd3, Satellite, and Immune Cells in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by loss-of-function mutations in the dystrophin gene on chromosome Xp21. Disruption of the dystrophin–glycoprotein complex (DGC) on the cell membrane causes cytosolic Ca²⁺ influx, resulting in protease activation, mitochondrial dysfunction, and progressive myofiber degeneration, leading to muscle wasting and fragility. In addition to the function of dystrophin in the structural integrity of myofibers, a novel function of asymmetric cell division in muscular stem cells (satellite cells) has been reported. Therefore, it has been suggested that myofiber instability may not be the only cause of dystrophic degeneration, but rather that the phenotype might be caused by multiple factors, including stem cell and myofiber functions. Furthermore, it has been focused functional regulation of satellite cells by intracellular communication of extracellular vesicles (EVs) in DMD pathology. Recently, a novel molecular mechanism of DMD pathogenesis—circulating RNA molecules—has been revealed through the study of target pathways modulated by the Neutral sphingomyelinase2/Neutral sphingomyelinase3 (nSMase2/Smpd3) protein. In addition, adeno-associated virus (AAV) has been clinically applied for DMD therapy owing to the safety and long-term expression of transduction genes. Furthermore, the EV-capsulated AAV vector (EV-AAV) has been shown to be a useful tool for the intervention of DMD, because of the high efficacy of the transgene and avoidance of neutralizing antibodies. Thus, we review application of AAV and EV-AAV vectors for DMD as novel therapeutic strategy.

Low-intensity blast induces acute glutamatergic hyperexcitability in mouse hippocampus leading to long-term learning deficits and altered expression of proteins involved in synaptic plasticity and serine protease inhibitors

Neurocognitive consequences of blast-induced traumatic brain injury (bTBI) pose significant concerns for military service members and veterans with the majority of "invisible injury." However, the underlying mechanism of such mild bTBI by low-intensity blast (LIB) exposure for long-term cognitive and mental deficits remains elusive. Our previous studies have shown that mice exposed to LIB result in nanoscale ultrastructural abnormalities in the absence of gross or apparent cellular damage in the brain. Here we tested the hypothesis that glutamatergic hyperexcitability may contribute to long-term learning deficits. Using brain slice electrophysiological recordings, we found an increase in averaged frequencies with a burst pattern of miniature excitatory postsynaptic currents (mEPSCs) in hippocampal CA3 neurons in LIB-exposed mice at 1- and 7-days post injury, which was blocked by a specific NMDA receptor antagonist AP5. In addition, cognitive function assessed at 3-months post LIB exposure by automated home-cage monitoring showed deficits in dynamic patterns of discrimination learning and cognitive flexibility in LIB-exposed mice. Collected hippocampal tissue was further processed for quantitative global-proteomic analysis. Advanced data-independent acquisition for quantitative tandem mass spectrometry analysis identified altered expression of proteins involved in synaptic plasticity and serine protease inhibitors in LIB-exposed mice. Some were correlated with the ability of discrimination learning and cognitive flexibility. These findings show that acute glutamatergic hyperexcitability in the hippocampus induced by LIB may contribute to long-term cognitive dysfunction and protein alterations. Studies using this military-relevant mouse model of mild bTBI provide valuable insights into developing a potential therapeutic strategy to ameliorate hyperexcitability-modulated LIB injuries.

Validation of DE50-MD dogs as a model for the brain phenotype of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD), a fatal musculoskeletal disease, is associated with neurodevelopmental disorders and cognitive impairment caused by brain dystrophin deficiency. Dog models of DMD represent key translational tools to study dystrophin biology and to develop novel therapeutics. However, characterisation of dystrophin expression and function in the canine brain is lacking. We studied the DE50-MD canine model of DMD that has a missense mutation in the donor splice site of exon 50. Using a battery of cognitive tests, we detected a neurocognitive phenotype in DE50-MD dogs, including reduced attention, problem solving and exploration of novel objects. Through a combination of capillary immunoelectrophoresis, immunolabelling, quantitative PCR and RNAScope in situ hybridisation, we show that regional dystrophin expression in the adult canine brain reflects that of humans, and that the DE50-MD dog lacks full-length dystrophin (Dp427) protein expression but retains expression of the two shorter brain-expressed isoforms, Dp140 and Dp71. Thus, the DE50-MD dog is a translationally relevant pre-clinical model to study the consequences of Dp427 deficiency in the brain and to develop therapeutic strategies for the neurological sequelae of DMD.

Summary: The DE50-MD canine model of Duchenne muscular dystrophy (DMD) shows cognitive and brain biochemical changes secondary to dystrophin deficiency in the brain, and is a relevant pre-clinical model to develop therapies for DMD.

Noncognitive species-typical and home-cage behavioral alterations in conditional presenilin 1/presenilin 2 double knockout mice

Impairments in activities of daily living (ADL) are common clinical symptoms of human Alzheimer's disease (AD). Describing the ADL in AD animal models might provide more insights into the mechanism/treatment of the disease. Here, we demonstrated that the forebrain presenilin 1(Psen1)/presenilin 2 (Psen2) conditional double knockout (DKO) mice exhibited deficits in nest building, marble burying and food burrowing starting at 3 months old and worsening at later ages. At 4 months of age, spontaneous activities in the home cage were also impaired in DKO mice, including physically demanding activities, habituation-like behaviors, and nourishment behaviors during the first two hours in the dark phase. These results indicated that loss of function of Psen1 and Psen2 in mice impaired a series of noncognitive behaviors in the early phase of neurodegeneration. This observation suggests that DKO mice are an ideal model for further mechanistic studies of Psen1 and Psen2 functions in regulating noncognitive behaviors.

Measuring Behavior in the Home Cage: Study Design, Applications, Challenges, and Perspectives

The reproducibility crisis (or replication crisis) in biomedical research is a particularly existential and under-addressed issue in the field of behavioral neuroscience, where, in spite of efforts to standardize testing and assay protocols, several known and unknown sources of confounding environmental factors add to variance. Human interference is a major contributor to variability both within and across laboratories, as well as novelty-induced anxiety. Attempts to reduce human interference and to measure more "natural" behaviors in subjects has led to the development of automated home-cage monitoring systems. These systems enable prolonged and longitudinal recordings, and provide large continuous measures of spontaneous behavior that can be analyzed across multiple time scales. In this review, a diverse team of neuroscientists and product developers share their experiences using such an automated monitoring system that combines Noldus PhenoTyper® home-cages and the video-based tracking software, EthoVision® XT, to extract digital biomarkers of motor, emotional, social and cognitive behavior. After presenting our working definition of a "home-cage", we compare home-cage testing with more conventional out-of-cage tests (e.g., the open field) and outline the various advantages of the former, including opportunities for within-subject analyses and assessments of circadian and ultradian activity. Next, we address technical issues pertaining to the acquisition of behavioral data, such as the fine-tuning of the tracking software and the potential for integration with biotelemetry and optogenetics. Finally, we provide guidance on which behavioral measures to emphasize, how to filter, segment, and analyze behavior, and how to use analysis scripts. We summarize how the PhenoTyper has applications to study neuropharmacology as well as animal models of neurodegenerative and neuropsychiatric illness. Looking forward, we examine current challenges and the impact of new developments. Examples include the automated recognition of specific behaviors, unambiguous tracking of individuals in a social context, the development of more animal-centered measures of behavior and ways of dealing with large datasets. Together, we advocate that by embracing standardized home-cage monitoring platforms like the PhenoTyper, we are poised to directly assess issues pertaining to reproducibility, and more importantly, measure features of rodent behavior under more ethologically relevant scenarios.

Assessment of Behavioral Characteristics With Procedures of Minimal Human Interference in the mdx Mouse Model for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe, progressive neuromuscular disorder caused by mutations in the DMD gene resulting in loss of functional dystrophin protein. The muscle dystrophin isoform is essential to protect muscles from contraction-induced damage. However, most dystrophin isoforms are expressed in the brain. In addition to progressive muscle weakness, many DMD patients therefore also exhibit intellectual and behavioral abnormalities. The most commonly used mouse model for DMD, the mdx mouse, lacks only the full-length dystrophin isoforms and has been extensively characterized for muscle pathology. In this study, we assessed behavioral effects of a lack of full-length dystrophins on spontaneous behavior, discrimination and reversal learning, anxiety, and short-term spatial memory and compared performance between male and female mdx mice. In contrast to our previous study using only female mdx mice, we could not reproduce the earlier observed reversal learning deficit. However, we did notice small differences in the number of visits made during the Y-maze and dark-light box. Results indicate that it is advisable to establish standard operating procedures specific to behavioral testing in mdx mice to allow the detection of the subtle phenotypic differences and to eliminate inter and intra laboratory variance.

The nSMase2/Smpd3 gene modulates the severity of muscular dystrophy and the emotional stress response in mdx mice

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive, degenerative muscular disorder and cognitive dysfunction caused by mutations in the dystrophin gene. It is characterized by excess inflammatory responses in the muscle and repeated degeneration and regeneration cycles. Neutral sphingomyelinase 2/sphingomyelin phosphodiesterase 3 (nSMase2/Smpd3) hydrolyzes sphingomyelin in lipid rafts. This protein thus modulates inflammatory responses, cell survival or apoptosis pathways, and the secretion of extracellular vesicles in a Ca²⁺-dependent manner. However, its roles in dystrophic pathology have not yet been clarified.

METHODS

To investigate the effects of the loss of nSMase2/Smpd3 on dystrophic muscles and its role in the abnormal behavior observed in DMD patients, we generated mdx mice lacking the nSMase2/Smpd3 gene (mdx:Smpd3 double knockout [DKO] mice).

RESULTS

Young mdx:Smpd3 DKO mice exhibited reduced muscular degeneration and decreased inflammation responses, but later on they showed exacerbated muscular necrosis. In addition, the abnormal stress response displayed by mdx mice was improved in the mdx:Smpd3 DKO mice, with the recovery of brain-derived neurotrophic factor (Bdnf) expression in the hippocampus.

CONCLUSIONS

nSMase2/Smpd3-modulated lipid raft integrity is a potential therapeutic target for DMD.

The Role of Dystrophin Gene Mutations in Neuropsychological Domains of DMD Boys: A Longitudinal Study

Background: Duchenne Muscular Dystrophy (DMD) is a fatal muscular dystrophy of pediatric population coupled with other secondary comorbidities including mental retardation and neuropsychological impairments. Mutation location in the dystrophin gene, have been associated with neuropsychological functioning in DMD.

Purpose: We investigated temporal changes in the neuropsychological functioning of DMD subjects, hitherto understudied.

Methods: Subjects with suspected DMD were enrolled according to the ethical guidelines. Genetic confirmation by Multiplex Ligation Dependent Probe Amplification was carried out to identify pathogenic deletion or duplication in dystrophin gene. Intellectual and neuropsychological functioning was assessed by using standardized batteries. Investigated neuropsychological domains included visual, verbal and working memory, selective and sustained attention, executive functioning, verbal fluency, and visuo-constructive and visuo-spatial abilities. The assessments were carried out at baseline and followed for one time point in 30 cases.

Result: The follow-up assessment revealed that neuropsychological functioning did not worsen with time. Improvements were seen in block designing task (p = 0.050), serial positioning primacy effect (p = 0.002), Stroop incongruent task (p = 0.006), visual long-term memory (p = 0.003) and attention (p = 0.001). DMD cases with mutation location affecting short dystrophin isoform (Dp140) also showed improvement in these domains.

Conclusion: No temporal alterations were found in DMD subjects, though improvements in few domains were observed. Neuropsychological rehabilitation may be useful in improving the quality of life in DMD subjects.

Defects of full-length dystrophin trigger retinal neuron damage and synapse alterations by disrupting functional autophagy

Dystrophin (dys) mutations predispose Duchenne muscular disease (DMD) patients to brain and retinal complications. Although different dys variants, including long dys products, are expressed in the retina, their function is largely unknown. We investigated the putative role of full-length dystrophin in the homeostasis of neuro-retina and its impact on synapsis stabilization and cell fate. Retinas of mdx mice, the most used DMD model which does not express the 427-KDa dys protein (Dp427), showed overlapped cell death and impaired autophagy. Apoptotic neurons in the outer plexiform/inner nuclear layer and the ganglion cell layer had an impaired autophagy with accumulated autophagosomes. The autophagy dysfunction localized at photoreceptor axonal terminals and bipolar, amacrine, and ganglion cells. The absence of Dp427 does not cause a severe phenotype but alters the neuronal architecture, compromising mainly the pre-synaptic photoreceptor terminals and their post-synaptic sites. The analysis of two dystrophic mutants of the fruit fly Drosophila melanogaster, the homozygous Dys^E17 and Dys^EP3397, lacking functional large-isoforms of dystrophin-like protein, revealed rhabdomere degeneration. Structural damages were evident in the internal network of retina/lamina where photoreceptors make the first synapse. Both accumulated autophagosomes and apoptotic features were detected and the visual system was functionally impaired. The reactivation of the autophagosome turnover by rapamycin prevented neuronal cell death and structural changes of mutant flies and, of interest, sustained autophagy ameliorated their response to light. Overall, these findings indicate that functional full-length dystrophin is required for synapsis stabilization and neuronal survival of the retina, allowing also proper autophagy as a prerequisite for physiological cell fate and visual properties.

Characterization of brain dystrophins absence and impact in dystrophin-deficient Dmdmdx rat model

Duchenne Muscular Dystrophy (DMD) is a severe muscle-wasting disease caused by mutations in the DMD gene encoding dystrophin, expressed mainly in muscles but also in other tissues like retina and brain. Non-progressing cognitive dysfunction occurs in 20 to 50% of DMD patients. Furthermore, loss of expression of the Dp427 dystrophin isoform in the brain of mdx mice, the most used animal model of DMD, leads to behavioral deficits thought to be linked to insufficiencies in synaptogenesis and channel clustering at synapses. Mdx mice where the locomotor phenotype is mild also display a high and maladaptive response to stress. Recently, we generated Dmd^mdx rats carrying an out-of frame mutation in exon 23 of the DMD gene and exhibiting a skeletal and cardiac muscle phenotype similar to DMD patients. In order to evaluate the impact of dystrophin loss on behavior, we explored locomotion parameters as well as anhedonia, anxiety and response to stress, in Dmd^mdx rats aged from 1.5 to 7 months, in comparison to wild-type (WT) littermates. Pattern of dystrophin expression in the brain of WT and Dmd^mdx rats was characterized by western-blot analyses and immunohistochemistry. We showed that dystrophin-deficient Dmd^mdx rats displayed motor deficits in the beam test, without association with depressive or anxiety-like phenotype. However, Dmd^mdx rats exhibited a strong response to restraint-induced stress, with a large increase in freezings frequency and duration, suggesting an alteration in a functional circuit including the amygdala. In brain, large dystrophin isoform Dp427 was not expressed in mutant animals. Dmd^mdx rat is therefore a good animal model for preclinical evaluations of new treatments for DMD but care must be taken with their responses to mild stress.

Dystrophin Dp71 and the Neuropathophysiology of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by frameshift mutations in the DMD gene that prevent the body-wide translation of its protein product, dystrophin. Besides a severe muscle phenotype, cognitive impairment and neuropsychiatric symptoms are prevalent. Dystrophin protein 71 (Dp71) is the major DMD gene product expressed in the brain and mutations affecting its expression are associated with the DMD neuropsychiatric syndrome. As with dystrophin in muscle, Dp71 localises to dystrophin-associated protein complexes in the brain. However, unlike in skeletal muscle; in the brain, Dp71 is alternatively spliced to produce many isoforms with differential subcellular localisations and diverse cellular functions. These include neuronal differentiation, adhesion, cell division and excitatory synapse organisation as well as nuclear functions such as nuclear scaffolding and DNA repair. In this review, we first describe brain involvement in DMD and the abnormalities observed in the DMD brain. We then review the gene expression, RNA processing and functions of Dp71. We review genotype-phenotype correlations and discuss emerging cellular/tissue evidence for the involvement of Dp71 in the neuropathophysiology of DMD. The literature suggests changes observed in the DMD brain are neurodevelopmental in origin and that their risk and severity is associated with a cumulative loss of distal DMD gene products such as Dp71. The high risk of neuropsychiatric syndromes in Duchenne patients warrants early intervention to achieve the best possible quality of life. Unravelling the function and pathophysiological significance of dystrophin in the brain has become a high research priority to inform the development of brain-targeting treatments for Duchenne.

Home-cage monitoring ascertains signatures of ictal and interictal behavior in mouse models of generalized seizures

Epilepsy is a significant contributor to worldwide disability. In epilepsy, disability can be broadly divided into two components: ictal (pertaining to the burden of unpredictable seizures and associated medical complications including death) and interictal (pertaining to more pervasive debilitating changes in cognitive and emotional behavior). In this study, we objectively and noninvasively appraise aspects of ictal and interictal behavior in mice using instrumented home-cage chambers designed to assay kinematic and appetitive behavioral measures. Through daily intraperitoneal injections of the chemoconvulsant pentylenetetrazole (PTZ) applied to C57BL/6J mice, we coordinately measure how “behavioral severity” (complex dynamic changes in movement and sheltering behavior) and convulsive severity (latency and occurrence of convulsive seizures) evolve or kindle with repeated injections. By closely studying long epochs between PTZ injections, we identify an interictal syndrome of nocturnal hypoactivity and increased sheltering behavior which remits with the cessation of seizure induction. We observe elements of this interictal behavioral syndrome in seizure-prone DBA/2J mice and in mice with a pathogenic Scn1a mutation (modeling Dravet syndrome). Through analyzing their responses to PTZ, we illustrate how convulsive severity and “behavioral” severity are distinct and independent aspects of the overall severity of a PTZ-induced seizure. Our results illustrate the utility of an ethologically centered automated approach to quantitatively appraise murine expressions of disability in mouse models of seizures and epilepsy. In doing so, this study highlights the very unique psychopharmacological profile of PTZ.

Visual Discrimination, Serial Reversal, and Extinction Learning in the mdx Mouse

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy and the most common neuromuscular disorder. In addition to neuromuscular consequences, some individuals with DMD experience global intellectual dysfunction and executive dysfunction of unknown mechanistic origin. The cognitive profile of the mdx mouse, the most commonly used mouse model of DMD, has been incompletely characterized and has never been assessed using the touchscreen operant conditioning paradigm. The touchscreen paradigm allows the use of protocols that are virtually identical to those used in human cognitive testing and may, therefore, provide the most translational paradigm for quantifying mouse cognitive function. In the present study, we used the touchscreen paradigm to assess the effects of the mdx mutation on visual discrimination learning, serial reversal learning, and extinction learning. To enable measuring task-dependent learning and memory processes while holding demands on sensory-driven information processing constant, we developed equally salient visual stimuli and used them on all experimental stages. Acquisition of the initial pairwise visual discrimination was facilitated in mdx mice relative to wildtype littermates; this effect was not explained by genotypic differences in impulsivity, motivation, or motor deficits. The mdx mutation had no effect on serial reversal or extinction learning. Together, findings from this study and previous studies suggest that mdx effects on cognitive function are task-specific and may be influenced by discrimination type (spatial, visual), reward type (food, escape from a non-preferred environment), sex, and genetic background.

Silica nanoparticle exposure during the neonatal period impairs hippocampal precursor proliferation and social behavior later in life

Introduction

Silica nanoparticles (SiO₂-NPs) are currently among the most widely used nanomaterials, but their potentially adverse effects on brain development remain unknown. The developing brain is extremely sensitive to NP neurotoxicity during the early postnatal period.

Materials and methods

Herein, we investigated the effects of SiO₂-NPs (doses of 10, 20, or 50 mg with a particle size of ~91 nm, equivalent to aerosol mass concentrations 55.56, 111.11, and 277.78 mg/m³, respectively) exposure from postnatal day (P) 1 to P7 on hippocampal precursor proliferation at P8 and long-term neurobehavior in adults.

Results

SiO₂-NP exposure resulted in inflammatory cell infiltration in lung tissue, microglia over-activation in the hippocampal dentate gyrus (DG), and decreased hippocampal precursor proliferation in the DG-subgranular zone at P8. Moreover, after exposure to 20 mg of SiO_2-NPs, mice exhibited social interaction deficits and slight anxiety-like behaviors in adulthood, but this exposure did not induce locomotor activity impairment, depression-like behavior, or short-term memory impairment.

Discussion

These findings suggest that early-age SiO₂-NP exposure induced inflammation and inhibited precursor proliferation in the DG in a dose-dependent manner, which might be related to the social dysfunction observed in adulthood.

Altered release and uptake of gamma-aminobutyric acid in the cerebellum of dystrophin-deficient mice

Dystrophin deficiency caused by mutations of the related gene leads to muscle wasting in Duchenne muscular dystrophy (DMD). Some patients with DMD also present with intellectual disability and various degrees of neurological disorders, which have been related to a decreased number of postsynaptic gamma-aminobutyric acid type A receptors (GABA_ARs) in the hippocampus (HPC) and cerebellum (CBL). The aim of this study was to examine the relevance of dystrophin in the presynaptic GABAergic function in brain regions in which this protein is normally abundant. [³H]-GABA release, induced by nicotinic receptor (nAChR) activation or K⁺ depolarization, and [³H]-GABA uptake were determined using synaptosomes extracted from the cortex (CTX), HPC, and CBL of littermate control and mdx mice. Superfusion of the synaptosomes with nicotine or high K⁺ solutions led to a concentration-dependent and Ca²⁺-dependent [³H]-GABA release in control and mdx synaptosomes. [³H]-GABA release induced by 10 μM nicotine in mdx CBL synaptosomes was 47% less than that in control mice. K⁺-induced [³H]-GABA release did not differ between control and mdx synaptosomes. α7-containing and β2-containing nAChRs were involved in nicotine-induced [³H]-GABA release in control and mdx synaptosomes. Kinetic analysis of [³H]-GABA uptake in mdx CBL synaptosomes showed a reduced (50%) half-maximal uptake time (t_1/2) and increased (44%) rate of [³H]-GABA uptake (V_max) compared to controls. The apparent transporter affinity (K_m) for GABA was not altered. Our findings show that dystrophin deficiency in mdx mice is associated with significant changes in the release and uptake of GABA in the CBL. These presynaptic alterations may be related to the reported decrease in postsynaptic GABA_AR in the same brain region. The results indicate possible dysfunction of GABAergic synapses associated with dystrophin deficiency in the CBL, which may contribute to the cognitive and neurobehavioral disorders in mdx mice and patients with DMD.

Simultaneous assessment of cognitive function, circadian rhythm, and spontaneous activity in aging mice

Cognitive function declines substantially with age in both humans and animal models. In humans, this decline is associated with decreases in independence and quality of life. Although the methodology for analysis of cognitive function in human models is relatively well established, similar analyses in animal models have many technical issues (e.g., unintended experimenter bias, motivational issues, stress, and testing during the light phase of the light dark cycle) that limit interpretation of the results. These caveats, and others, potentially bias the interpretation of studies in rodents and prevent the application of current tests of learning and memory as part of an overall healthspan assessment in rodent models of aging. The goal of this study was to establish the methodology to assess cognitive function in aging animals that addresses many of these concerns. Here, we use a food reward-based discrimination procedure with minimal stress in C57Bl/6J male mice at 6, 21, and 27 months of age, followed by a reversal task to assess behavioral flexibility. Importantly, the procedures minimize issues related to between-experimenter confounds and are conducted during both the dark and light phases of the light dark cycle in a home-cage setting. During cognitive testing, we were able to assess multiple measures of spontaneous movement and diurnal activity in young and aged mice including, distance moved, velocity, and acceleration over a 90-h period. Both initial discrimination and reversal learning significantly decreased with age and, similar to rats and humans, not all old mice demonstrated impairments in learning with age. These results permitted classification of animals based on their cognitive status. Analysis of movement parameters indicated decreases in distance moved as well as velocity and acceleration with increasing age. Based on these data, we developed preliminary models indicating, as in humans, a close relationship exists between age-related movement parameters and cognitive ability. Our results provide a reliable method for assessing cognitive performance with minimal stress and simultaneously provide key information on movement and diurnal activity. These methods represent a novel approach to developing non-invasive healthspan measures in rodent models that allow standardization across laboratories.

Influence of full-length dystrophin on brain volumes in mouse models of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) affects besides muscle also the brain, resulting in memory and behavioral problems. The consequences of dystrophinopathy on gross macroscopic alterations are unclear. To elucidate the effect of full-length dystrophin expression on brain morphology, we used high-resolution post-mortem MRI in mouse models that either express 0% (mdx), 100% (BL10) or a low amount of full-length dystrophin (mdx-XistΔhs). While absence or low amounts of full-length dystrophin did not significantly affect whole brain volume and skull morphology, we found differences in volume of individual brain structures. The results are in line with observations in humans, where whole brain volume was found to be reduced only in patients lacking both full-length dystrophin and the shorter isoform Dp140.

Myelination is delayed during postnatal brain development in the mdx mouse model of Duchenne muscular dystrophy

BACKGROUND

In Duchenne muscular dystrophy (DMD), the loss of the dystrophin component of the dystrophin-glycoprotein complex (DGC) compromises plasma membrane integrity in skeletal muscle, resulting in extensive muscle degeneration. In addition, many DMD patients exhibit brain deficits in which the cellular etiology remains poorly understood. We recently found that dystroglycan, a receptor component of the DGC that binds intracellularly to dystrophin, regulates the development of oligodendrocytes, the myelinating glial cells of the brain.

RESULTS

We investigated whether dystrophin contributes to oligodendroglial function and brain myelination. We found that oligodendrocytes express up to three dystrophin isoforms, in conjunction with classic DGC components, which are developmentally regulated during differentiation and in response to extracellular matrix engagement. We found that mdx mice, a model of DMD lacking expression of the largest dystrophin isoform, have delayed myelination and inappropriate oligodendrocyte progenitor proliferation in the cerebral cortex. When we prevented the expression of all oligodendroglial dystrophin isoforms in cultured oligodendrocytes using RNA interference, we found that later stages of oligodendrocyte maturation were significantly delayed, similar to mdx phenotypes in the developing brain.

CONCLUSIONS

We find that dystrophin is expressed in oligodendrocytes and influences developmental myelination, which provides new insight into potential cellular contributors to brain dysfunction associated with DMD.

Reversal of neurobehavioral social deficits in dystrophic mice using inhibitors of phosphodiesterases PDE5A and PDE9A

Duchenne muscular dystrophy is caused by mutations in the DYSTROPHIN gene. Although primarily associated with muscle wasting, a significant portion of patients (approximately 25%) are also diagnosed with autism spectrum disorder. We describe social behavioral deficits in dystrophin-deficient mice and present evidence of cerebellar deficits in cGMP production. We demonstrate therapeutic potential for selective inhibitors of the cGMP-specific PDE5A and PDE9A enzymes to restore social behaviors in dystrophin-deficient mice.