Social stress is lethal in the mdx model of Duchenne muscular dystrophy

Functional cardiac consequences of β-adrenergic stress-induced injury in the mdx mouse model of Duchenne muscular dystrophy

Cardiomyopathy is the leading cause of death in Duchenne muscular dystrophy (DMD), however, in the mdx mouse model of DMD, the cardiac phenotype differs from that seen in DMD-associated cardiomyopathy. Although some have used pharmacologic stress to enhance the cardiac phenotype in the mdx model, many methods lead to high mortality, variable cardiac outcomes, and do not recapitulate the structural and functional cardiac changes seen in human disease. Here, we describe a simple and effective method to enhance the cardiac phenotype model in mdx mice using advanced 2D and 4D high-frequency ultrasound to monitor cardiac dysfunction progression in vivo. For our study, mdx and wild-type (WT) mice received daily low-dose (2 mg/kg/day) isoproterenol injections for 10 days. Histopathologic assessment showed that isoproterenol treatment increased myocyte injury, elevated serum cardiac troponin I levels, and enhanced fibrosis in mdx mice. Ultrasound revealed reduced ventricular function, decreased wall thickness, increased volumes, and diminished cardiac reserve in mdx mice compared to wild-type. Our findings highlight the utility of low-dose isoproterenol in mdx mice as a valuable model for exploring therapies targeting DMD-associated cardiac complications.

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.

Effects of Geometric Sound on Brainwave Activity Patterns, Autonomic Nervous System Markers, Emotional Response, and Faraday Wave Pattern Morphology

This study introduces Geometric Sound as a subfield of spatial sound featuring audio stimuli which are sonic holograms of mathematically defined 3D shapes. The effects of Geometric Sound on human physiology were investigated through EEG, heart rate, blood pressure, and a combination of questionnaires monitoring 50 healthy participants in two separate experiments. The impact of Geometric Sound on Faraday wave pattern morphology was further studied. The shapes examined, pyramid, cube, and sphere, exhibited varying significant effects on autonomic nervous system markers, brainwave power amplitude, topology, and connectivity patterns, in comparison to both the control (traditional stereo), and recorded baseline where no sound was presented. Brain activity in the Alpha band exhibited the most significant results, additional noteworthy results were observed across analysis paradigms in all frequency bands. Geometric Sound was found to significantly reduce heart rate and blood pressure and enhance relaxation and general well-being. Changes in EEG, heart rate, and blood pressure were primarily shape-dependent, and to a lesser extent sex-dependent. Pyramid Geometric Sound yielded the most significant results in most analysis paradigms. Faraday Waves patterns morphology analysis indicated that identical frequencies result in patterns that correlate with the excitation Geometric Sound shape. We suggest that Geometric Sound shows promise as a noninvasive therapeutic approach for physical and psychological conditions, stress-related disorders, depression, anxiety, and neurotrauma. Further research is warranted to elucidate underlying mechanisms and expand its applications.

Retention of stress susceptibility in the mdx mouse model of Duchenne muscular dystrophy after PGC-1α overexpression or ablation of IDO1 or CD38

Duchenne muscular dystrophy (DMD) is a lethal degenerative muscle wasting disease caused by the loss of the structural protein dystrophin with secondary pathological manifestations including metabolic dysfunction, mood and behavioral disorders. In the mildly affected mdx mouse model of DMD, brief scruff stress causes inactivity, while more severe subordination stress results in lethality. Here, we investigated the kynurenine pathway of tryptophan degradation and the nicotinamide adenine dinucleotide (NAD+) metabolic pathway in mdx mice and their involvement as possible mediators of mdx stress-related pathology. We identified downregulation of the kynurenic acid shunt, a neuroprotective branch of the kynurenine pathway, in mdx skeletal muscle associated with attenuated peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) transcriptional regulatory activity. Restoring the kynurenic acid shunt by skeletal muscle-specific PGC-1α overexpression in mdx mice did not prevent scruff -induced inactivity, nor did abrogating extrahepatic kynurenine pathway activity by genetic deletion of the pathway rate-limiting enzyme, indoleamine oxygenase 1. We further show that reduced NAD+ production in mdx skeletal muscle after subordination stress exposure corresponded with elevated levels of NAD+ catabolites produced by ectoenzyme cluster of differentiation 38 (CD38) that have been implicated in lethal mdx response to pharmacological β-adrenergic receptor agonism. However, genetic CD38 ablation did not prevent mdx scruff-induced inactivity. Our data do not support a direct contribution by the kynurenine pathway or CD38 metabolic dysfunction to the exaggerated stress response of mdx mice.

Partial restoration of brain dystrophin by tricyclo-DNA antisense oligonucleotides alleviates emotional deficits in mdx52 mice

The mdx52 mouse model recapitulates a frequent mutation profile associated with brain involvement in Duchenne muscular dystrophy. Deletion of exon 52 impedes expression of two dystrophins (Dp427, Dp140) expressed in brain, and is eligible for therapeutic exon-skipping strategies. We previously showed that mdx52 mice display enhanced anxiety and fearfulness, and impaired associative fear learning. In this study, we examined the reversibility of these phenotypes using exon 51 skipping to restore exclusively Dp427 expression in the brain of mdx52 mice. We first show that a single intracerebroventricular administration of tricyclo-DNA antisense oligonucleotides targeting exon 51 restores 5%-15% of dystrophin protein expression in the hippocampus, cerebellum, and cortex, at stable levels between 7 and 11 week after injection. Anxiety and unconditioned fear were significantly reduced in treated mdx52 mice and acquisition of fear conditioning appeared fully rescued, while fear memory tested 24 h later was only partially improved. Additional restoration of Dp427 in skeletal and cardiac muscles by systemic treatment did not further improve the unconditioned fear response, confirming the central origin of this phenotype. These findings indicate that some emotional and cognitive deficits associated with dystrophin deficiency may be reversible or at least improved by partial postnatal dystrophin rescue.

The unconditioned fear response in dystrophin-deficient mice is associated with adrenal and vascular function

Loss of function mutations in the gene encoding dystrophin elicits a hypersensitive fear response in mice and humans. In the dystrophin-deficient mdx mouse, this behaviour is partially protected by oestrogen, but the mechanistic basis for this protection is unknown. Here, we show that female mdx mice remain normotensive during restraint stress compared to a hypotensive and hypertensive response in male mdx and male/female wildtype mice, respectively. Partial dystrophin expression in female mdx mice (heterozygous) also elicited a hypertensive response. Ovariectomized (OVX) female mdx mice were used to explain the normotensive response to stress. OVX lowered skeletal muscle mass and lowered the adrenal mass and zona glomerulosa area (aldosterone synthesis) in female mdx mice. During a restraint stress, OVX dampened aldosterone synthesis and lowered the corticosterone:11-dehydrocorticosterone. All OVX-induced changes were restored with replacement of oestradiol, except that oestradiol lowered the zona fasciculata area of the adrenal gland, dampened corticosterone synthesis but increased cortisol synthesis. These data suggest that oestrogen partially attenuates the unconditioned fear response in mdx mice via adrenal and vascular function. It also suggests that partial dystrophin restoration in a dystrophin-deficient vertebrate is an effective approach to develop an appropriate hypertensive response to stress.

Pharmacological TRPC6 inhibition improves survival and muscle function in mice with Duchenne muscular dystrophy

Gene mutations causing loss of dystrophin result in the severe muscle disease known as Duchenne muscular dystrophy (DMD). Despite efforts at genetic repair, DMD therapy remains largely palliative. Loss of dystrophin destabilizes the sarcolemmal membrane, inducing mechanosensitive cation channels to increase calcium entry and promote cell damage and, eventually, muscle dysfunction. One putative channel is transient receptor potential canonical 6 (TRPC6); we have shown that TRPC6 contributed to abnormal force and calcium stress-responses in cardiomyocytes from mice lacking dystrophin that were haplodeficient for utrophin (mdx/utrn+/- [HET] mice). Here, we show in both the HET mouse and the far more severe homozygous mdx/utrn-/- mouse that TRPC6 gene deletion or its selective pharmacologic inhibition (by BI 749327) prolonged survival 2- to 3-fold, improving skeletal and cardiac muscle and bone defects. Gene pathways reduced by BI 749327 treatment most prominently regulated fat metabolism and TGF-β1 signaling. These results support the testing of TRPC6 inhibitors in human trials for other diseases as a novel DMD therapy.

Pregabalin synchronizes the regeneration of nerve and muscle fibers optimizing the gait recovery of MDX dystrophic mice

Duchenne muscular dystrophy (DMD) is an X-linked genetic disorder induced by mutations in the dystrophin gene, leading to a degeneration of muscle fibers, triggering retrograde immunomodulatory, and degenerative events in the central nervous system. Thus, neuroprotective drugs such as pregabalin (PGB) can improve motor function by modulating plasticity, together with anti-inflammatory effects. The present work aimed to study the effects of PGB on axonal regeneration after axotomy in dystrophic and non-dystrophic mice. For that, MDX and C57BL/10 mouse strains were subjected to peripheral nerve damage and were treated with PGB (30 mg/kg/day, i.p.) for 28 consecutive days. The treatment was carried out in mice as soon as they completed 5 weeks of life, 1 week before the lesion, corresponding to the peak period of muscle degeneration in the MDX strain. Six-week-old mice were submitted to unilateral sciatic nerve crush and were sacrificed in the 9th week of age. The ipsi and contralateral sciatic nerves were processed for immunohistochemistry and qRT-PCR, evaluating the expression of proteins and gene transcripts related to neuronal and Schwann cell activity. Cranial tibial muscles were dissected for evaluation of neuromuscular junctions using α-bungarotoxin, and the myelinated axons of the sciatic nerve were analyzed by morphometry. The recovery of motor function was monitored throughout the treatment through tests of forced locomotion (rotarod) and spontaneous walking track test (Catwalk system). The results show that treatment with PGB reduced the retrograde cyclic effects of muscle degeneration/regeneration on the nervous system. This fact was confirmed after peripheral nerve injury, showing better adaptation and response of neurons and glia for rapid axonal regeneration, with efficient muscle targeting and regain of function. No side effects of PGB treatment were observed, and the expression of pro-regenerative proteins in neurons and Schwann cells was upregulated. Morphometry of the axons was in line with the preservation of motor endplates, resulting in enhanced performance of dystrophic animals. Overall, the present data indicate that pregabalin is protective and enhances regeneration of the SNP during the development of DMD, improving motor function, which can, in turn, be translated to the clinic.

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.

Brain Dp140 alters glutamatergic transmission and social behaviour in the mdx52 mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a muscle disorder caused by DMD mutations and is characterized by neurobehavioural comorbidities due to dystrophin deficiency in the brain. The lack of Dp140, a dystrophin short isoform, is clinically associated with intellectual disability and autism spectrum disorders (ASDs), but its postnatal functional role is not well understood. To investigate synaptic function in the presence or absence of brain Dp140, we utilized two DMD mouse models, mdx23 and mdx52 mice, in which Dp140 is preserved or lacking, respectively. ASD-like behaviours were observed in pups and 8-week-old mdx52 mice lacking Dp140. Paired-pulse ratio of excitatory postsynaptic currents, glutamatergic vesicle number in basolateral amygdala neurons, and glutamatergic transmission in medial prefrontal cortex-basolateral amygdala projections were significantly reduced in mdx52 mice compared to those in wild-type and mdx23 mice. ASD-like behaviour and electrophysiological findings in mdx52 mice were ameliorated by restoration of Dp140 following intra-cerebroventricular injection of antisense oligonucleotide drug-induced exon 53 skipping or intra-basolateral amygdala administration of Dp140 mRNA-based drug. Our results implicate Dp140 in ASD-like behaviour via altered glutamatergic transmission in the basolateral amygdala of mdx52 mice.

Synaptic alterations as a neurodevelopmental trait of Duchenne muscular dystrophy

Dystrophinopaties, e.g., Duchenne muscular dystrophy (DMD), Becker muscular dystrophy and X-linked dilated cardiomyopathy are inherited neuromuscular diseases, characterized by progressive muscular degeneration, which however associate with a significant impact on general system physiology. The more severe is the pathology and its diversified manifestations, the heavier are its effects on organs, systems, and tissues other than muscles (skeletal, cardiac and smooth muscles). All dystrophinopaties are characterized by mutations in a single gene located on the X chromosome encoding dystrophin (Dp427) and its shorter isoforms, but DMD is the most devasting: muscular degenerations manifests within the first 4 years of life, progressively affecting motility and other muscular functions, and leads to a fatal outcome between the 20s and 40s. To date, after years of studies on both DMD patients and animal models of the disease, it has been clearly demonstrated that a significant percentage of DMD patients are also afflicted by cognitive, neurological, and autonomic disorders, of varying degree of severity. The anatomical correlates underlying neural functional damages are established during embryonic development and the early stages of postnatal life, when brain circuits, sensory and motor connections are still maturing. The impact of the absence of Dp427 on the development, differentiation, and consolidation of specific cerebral circuits (hippocampus, cerebellum, prefrontal cortex, amygdala) is significant, and amplified by the frequent lack of one or more of its lower molecular mass isoforms. The most relevant aspect, which characterizes DMD-associated neurological disorders, is based on morpho-functional alterations of selective synaptic connections within the affected brain areas. This pathological feature correlates neurological conditions of DMD to other severe neurological disorders, such as schizophrenia, epilepsy and autistic spectrum disorders, among others. This review discusses the organization and the role of the dystrophin-dystroglycan complex in muscles and neurons, focusing on the neurological aspect of DMD and on the most relevant morphological and functional synaptic alterations, in both central and autonomic nervous systems, described in the pathology and its animal models.

Juvenile exposure to doxorubicin alters the cardiovascular response to adult-onset psychosocial stress in mice

Childhood cancer survivors have a high risk for premature cardiovascular diseases, mainly due to cardiotoxic cancer treatments such as doxorubicin (DOX). Psychosocial stress is a significant cardiovascular risk factor and an enormous burden in childhood cancer survivors. Although observational studies suggest that psychosocial stress is associated with cardiovascular complications in cancer survivors, there is no translationally relevant animal model to study this interaction. We established a "two-hit" model in which juvenile mice were administered DOX (4 mg/kg/week for 3 weeks), paired to a validated model of chronic subordination stress (CSS) 5 weeks later upon reaching adulthood. Blood pressure, heart rate, and activity were monitored by radio-telemetry. At the end of CSS experiment, cardiac function was assessed by echocardiography. Cardiac fibrosis and inflammation were assessed by histopathologic analysis. Gene expressions of inflammatory and fibrotic markers were determined by PCR. Juvenile exposure to DOX followed by adult-onset CSS caused cardiac fibrosis and inflammation as evident by histopathologic findings and upregulated gene expression of multiple inflammatory and fibrotic markers. Intriguingly, juvenile exposure to DOX blunted CSS-induced hypertension but not CSS-induced tachycardia. There were no significant differences in cardiac function parameters among all groups, but juvenile exposure to DOX abrogated the hypertrophic response to CSS. In conclusion, we established a translationally relevant mouse model of juvenile DOX-induced cardiotoxicity that predisposes to adult-onset stress-induced adverse cardiac remodeling. Psychosocial stress should be taken into consideration in cardiovascular risk stratification of DOX-treated childhood cancer survivors.

Sensitivity to behavioral stress impacts disease pathogenesis in dystrophin-deficient mice

Mutation to the gene encoding dystrophin can cause Duchenne muscular dystrophy (DMD) and increase the sensitivity to stress in vertebrate species, including the mdx mouse model of DMD. Behavioral stressors can exacerbate some dystrophinopathy phenotypes of mdx skeletal muscle and cause hypotension-induced death. However, we have discovered that a subpopulation of mdx mice present with a wildtype-like response to mild (forced downhill treadmill exercise) and moderate (scruff restraint) behavioral stressors. These "stress-resistant" mdx mice are more physically active, capable of super-activating the hypothalamic-pituitary-adrenal and renin-angiotensin-aldosterone pathways following behavioral stress and they express greater levels of mineralocorticoid and glucocorticoid receptors in striated muscle relative to "stress-sensitive" mdx mice. Stress-resistant mdx mice also presented with a less severe striated muscle histopathology and greater exercise and skeletal muscle oxidative capacity at rest. Most interestingly, female mdx mice were more physically active following behavioral stressors compared to male mdx mice; a response abolished after ovariectomy and rescued with estradiol. We demonstrate that the response to behavioral stress greatly impacts disease severity in mdx mice suggesting the management of stress in patients with DMD be considered as a therapeutic approach to ameliorate disease progression.

Emotional behavior and brain anatomy of the mdx52 mouse model of Duchenne muscular dystrophy

The exon-52-deleted mdx52 mouse is a critical model of Duchenne muscular dystrophy (DMD), as it features a deletion in a hotspot region of the DMD gene, frequently mutated in patients. Deletion of exon 52 impedes expression of several brain dystrophins (Dp427, Dp260 and Dp140), thus providing a key model for studying the cognitive impairment associated with DMD and testing rescuing strategies. Here, using in vivo magnetic resonance imaging and neurohistology, we found no gross brain abnormalities in mdx52 mice, suggesting that the neural dysfunctions in this model are likely at the level of brain cellular functionalities. Then, we investigated emotional behavior and fear learning performance of mdx52 mice compared to mdx mice that only lack Dp427 to focus on behavioral phenotypes that could be used in future comparative preclinical studies. mdx52 mice displayed enhanced anxiety and a severe impairment in learning an amygdala-dependent Pavlovian association. These replicable behavioral outcome measures are reminiscent of the internalizing problems reported in a quarter of DMD patients, and will be useful for preclinical estimation of the efficacy of treatments targeting brain dysfunctions in DMD.

5-HTT Deficiency in Male Mice Affects Healing and Behavior after Myocardial Infarction

Anxiety disorders and depression are common comorbidities in cardiac patients. Mice lacking the serotonin transporter (5-HTT) exhibit increased anxiety-like behavior. However, the role of 5-HTT deficiency on cardiac aging, and on healing and remodeling processes after myocardial infarction (MI), remains unclear. Cardiological evaluation of experimentally naïve male mice revealed a mild cardiac dysfunction in ≥4-month-old 5-HTT knockout (−/−) animals. Following induction of chronic cardiac dysfunction (CCD) by MI vs. sham operation 5-HTT−/− mice with infarct sizes >30% experienced 100% mortality, while 50% of 5-HTT+/− and 37% of 5-HTT+/+ animals with large MI survived the 8-week observation period. Surviving (sham and MI < 30%) 5-HTT−/− mutants displayed reduced exploratory activity and increased anxiety-like behavior in different approach-avoidance tasks. However, CCD failed to provoke a depressive-like behavioral response in either 5-Htt genotype. Mechanistic analyses were performed on mice 3 days post-MI. Electrocardiography, histology and FACS of inflammatory cells revealed no abnormalities. However, gene expression of inflammation-related cytokines (TGF-β, TNF-α, IL-6) and MMP-2, a protein involved in the breakdown of extracellular matrix, was significantly increased in 5-HTT−/− mice after MI. This study shows that 5-HTT deficiency leads to age-dependent cardiac dysfunction and disrupted early healing after MI probably due to alterations of inflammatory processes in mice.

Pregabalin-induced neuroprotection and gait improvement in dystrophic MDX mice

Duchenne muscular dystrophy (DMD) is a genetic disease linked to the X chromosome induced by mutations in the dystrophin gene. Neuroprotective drugs, such as pregabalin (PGB), can improve motor function through the modulation of excitatory synapses, together with anti-apoptotic and anti-inflammatory effects. The present work studied the effects of PGB in the preservation of dystrophic peripheral nerves, allowing motor improvements in MDX mice. Five weeks old MDX and C57BL/10 mice were treated with PGB (30 mg/kg/day, i.p.) or vehicle, for 28 consecutive days. The mice were sacrificed on the 9th week, the sciatic nerves were dissected out and processed for immunohistochemistry and qRT-PCR, for evaluating the expression of proteins and gene transcripts related to neuronal activity and Schwann cell function. The lumbar spinal cords were also processed for qRT-PCR to evaluate the expression of neurotrophic factors and pro- and anti-inflammatory cytokines. Cranial tibial muscles were dissected out for endplate evaluation with α-bungarotoxin. The recovery of motor function was monitored throughout the treatment, using a spontaneous walking track test (Catwalk system) and a forced locomotion test (Rotarod). The results showed that treatment with PGB reduced the retrograde effects of muscle degeneration/regeneration on the nervous system from the 5th to the 9th week in MDX mice. Thus, PGB induced protein expression in neurons and Schwann cells, protecting myelinated fibers. In turn, better axonal morphology and close-to-normal motor endplates were observed. Indeed, such effects resulted in improved motor coordination of dystrophic animals. We believe that treatment with PGB improved the balance between excitatory and inhibitory inputs to spinal motoneurons, increasing motor control. In addition, PGB enhanced peripheral nerve homeostasis, by positively affecting Schwann cells. In general, the present results indicate that pregabalin is effective in protecting the PNS during the development of DMD, improving motor coordination, indicating possible translation to the clinic.

Some dystrophy phenotypes of dystrophin-deficient mdx mice are exacerbated by mild, repetitive daily stress

Psychosocial stressors can cause physical inactivity, cardiac damage, and hypotension-induced death in the mdx mouse model of Duchenne muscular dystrophy (DMD). Because repeated exposure to mild stress can lead to habituation in wild-type mice, we investigated the response of mdx mice to a mild, daily stress to determine whether habituation occurred. Male mdx mice were exposed to a 30-sec scruff restraint daily for 12 weeks. Scruff restraint induced immediate physical inactivity that persisted for at least 60 minutes, and this inactivity response was just as robust after 12 weeks as it was after one day. Physical inactivity in the mdx mice was not associated with acute skeletal muscle contractile dysfunction. However, skeletal muscle of mdx mice that were repeatedly stressed had slow-twitch and tetanic relaxation times and trended toward high passive stiffness, possibly due to a small but significant increase in muscle fibrosis. Elevated urinary corticosterone secretion, adrenal hypertrophy, and a larger adrenal cortex indicating chronic activation of the hypothalamic-pituitary-adrenal (HPA) axis were measured in 12-week stressed mdx mice relative to those unstressed. However, pharmacological inhibition of the HPA axis did not affect scruff-induced physical inactivity and acute corticosterone injection did not recapitulate the scruff-induced phenotype, suggesting the HPA axis is not the driver of physical inactivity. Our results indicate that the response of mdx mice to an acute mild stress is non-habituating and that when that stressor is repeated daily for weeks, it is sufficient to exacerbate some phenotypes associated with dystrophinopathy in mdx mice.

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.

Psychosocial Stress Hastens Disease Progression and Sudden Death in Mice with Arrhythmogenic Cardiomyopathy

Physiological stressors, such as exercise, can precipitate sudden cardiac death or heart failure progression in patients with arrhythmogenic cardiomyopathy (ACM). Yet, whether and to what extent a highly prevalent and more elusive environmental factor, such as psychosocial stress (PSS), can also increase ACM disease progression is unexplored. Here, we first quantified perceived stress levels in patients with ACM and found these levels correlated with the extent of arrhythmias and cardiac dysfunction. To determine whether the observed correlation is due to causation, we inflicted PSS-via the resident-intruder (RI) paradigm—upon Desmoglein-2 mutant mice, a vigorously used mammalian model of ACM. We found that ACM mice succumbed to abnormally high in-trial, PSS mortality. Conversely, no sudden deaths occurred in wildtype (WT) counterparts. Desmoglein-2 mice that survived RI challenge manifested markedly worse cardiac dysfunction and remodeling, namely apoptosis and fibrosis. Furthermore, WT and ACM mice displayed similar behavior at baseline, but Desmoglein-2 mice exhibited heightened anxiety following RI-induced PSS. This outcome correlated with the worsening of cardiac phenotypes. Our mouse model demonstrates that in ACM-like subjects, PSS is incisive enough to deteriorate cardiac structure and function per se, i.e., in the absence of any pre-existing anxious behavior. Hence, PSS may represent a previously underappreciated risk factor in ACM disease penetrance.