Deficient Purposeful Use of Forepaws in Female Mice Modelling Rett Syndrome

Treatment with the Bacterial Toxin CNF1 Selectively Rescues Cognitive and Brain Mitochondrial Deficits in a Female Mouse Model of Rett Syndrome Carrying a MeCP2-Null Mutation

Rett syndrome (RTT) is a rare neurological disorder caused by mutations in the X-linked MECP2 gene and a major cause of intellectual disability in females. No cure exists for RTT. We previously reported that the behavioural phenotype and brain mitochondria dysfunction are widely rescued by a single intracerebroventricular injection of the bacterial toxin CNF1 in a RTT mouse model carrying a truncating mutation of the MeCP2 gene (MeCP2-308 mice). Given the heterogeneity of MECP2 mutations in RTT patients, we tested the CNF1 therapeutic efficacy in a mouse model carrying a null mutation (MeCP2-Bird mice). CNF1 selectively rescued cognitive defects, without improving other RTT-related behavioural alterations, and restored brain mitochondrial respiratory chain complex activity in MeCP2-Bird mice. To shed light on the molecular mechanisms underlying the differential CNF1 effects on the behavioural phenotype, we compared treatment effects on relevant signalling cascades in the brain of the two RTT models. CNF1 provided a significant boost of the mTOR activation in MeCP2-308 hippocampus, which was not observed in the MeCP2-Bird model, possibly explaining the differential effects of CNF1. These results demonstrate that CNF1 efficacy depends on the mutation beared by MeCP2-mutated mice, stressing the need of testing potential therapeutic approaches across RTT models.

Chronic Treatment with Cannabidiolic Acid (CBDA) Reduces Thermal Pain Sensitivity in Male Mice and Rescues the Hyperalgesia in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a rare neurologic disorder, characterized by severe behavioural and physiological symptoms. RTT is caused by mutations in the MECP2 gene in about 95% of cases and to date no cure is available. Recent evidence suggests that non-euphoric phytocannabinoids (pCBs) extracted from Cannabis sativa may represent innovative therapeutic molecules for RTT, with the cannabinoid cannabidivarin having beneficial effects on behavioural and brain molecular alterations in RTT mouse models. The present study evaluated the potential therapeutic efficacy for RTT of cannabidiolic acid (CBDA; 0.2, 2, 20 mg/kg through intraperitoneal injections for 14 days), a pCB that has proved to be effective for the treatment of nausea and anxiety in rodents. This study demonstrates that systemic treatment with the low dose of CBDA has anti-nociceptive effects and reduces the thermal hyperalgesia in 8 month-old MeCP2-308 male mice, a validated RTT mouse model. CBDA did not affect other behavioural or molecular parameters. These results provide support to the antinociceptive effects of CBDA and stress the need for further studies aimed at clarifying the mechanisms underlying the abnormal pain perception in RTT.

Deficits in skilled motor and auditory learning in a rat model of Rett syndrome

BACKGROUND

Rett syndrome is an X-linked neurodevelopmental disorder caused by a mutation in the gene MECP2. Individuals with Rett syndrome display developmental regression at an early age, and develop a range of motor, auditory, cognitive, and social impairments. Several studies have successfully modeled some aspects of dysfunction and Rett syndrome-like phenotypes in transgenic mouse and rat models bearing mutations in the MECP2 gene. Here, we sought to extend these findings and characterize skilled learning, a more complex behavior known to be altered in Rett syndrome.

METHODS

We evaluated the acquisition and performance of auditory and motor function on two complex tasks in heterozygous female Mecp2 rats. Animals were trained to perform a speech discrimination task or a skilled forelimb reaching task.

RESULTS

Our results reveal that Mecp2 rats display slower acquisition and reduced performance on an auditory discrimination task than wild-type (WT) littermates. Similarly, Mecp2 rats exhibit impaired learning rates and worse performance on a skilled forelimb motor task compared to WT.

CONCLUSIONS

Together, these findings illustrate novel deficits in skilled learning consistent with clinical manifestation of Rett syndrome and provide a framework for development of therapeutic strategies to improve these complex behaviors.

Fluoxetine rescues rotarod motor deficits in Mecp2 heterozygous mouse model of Rett syndrome via brain serotonin

Motor skill is a specific area of disability of Rett syndrome (RTT), a rare disorder occurring almost exclusively in girls, caused by loss-of-function mutations of the X-linked methyl-CpG-binding protein2 (MECP2) gene, encoding the MECP2 protein, a member of the methyl-CpG-binding domain nuclear proteins family. Brain 5-HT, which is defective in RTT patients and Mecp2 mutant mice, regulates motor circuits and SSRIs enhance motor skill learning and plasticity. In the present study, we used heterozygous (Het) Mecp2 female and Mecp2-null male mice to investigate whether fluoxetine, a SSRI with pleiotropic effects on neuronal circuits, rescues motor coordination deficits. Repeated administration of 10 mg/kg fluoxetine fully rescued rotarod deficit in Mecp2 Het mice regardless of age, route of administration or pre-training to rotarod. The motor improvement was confirmed in the beam walking test while no effect was observed in the hanging-wire test, suggesting a preferential action of fluoxetine on motor coordination. Citalopram mimicked the effects of fluoxetine, while the inhibition of 5-HT synthesis abolished the fluoxetine-induced improvement of motor coordination. Mecp2 null mice, which responded poorly to fluoxetine in the rotarod, showed reduced 5-HT synthesis in the prefrontal cortex, hippocampus and striatum, and reduced efficacy of fluoxetine in raising extracellular 5-HT as compared to female mutants. No sex differences were observed in the ability of fluoxetine to desensitize 5-HT_1A autoreceptors upon repeated administration. These findings indicate that fluoxetine rescues motor coordination in Mecp2 Het mice through its ability to enhance brain 5-HT and suggest that drugs enhancing 5-HT neurotransmission may have beneficial effects on motor symptoms of RTT.

Manual dexterity of mice during food-handling involves the thumb and a set of fast basic movements

The small first digit (D1) of the mouse's hand resembles a volar pad, but its thumb-like anatomy suggests ethological importance for manipulating small objects. To explore this possibility, we recorded high-speed close-up video of mice eating seeds and other food items. Analyses of ethograms and automated tracking with DeepLabCut revealed multiple distinct microstructural features of food-handling. First, we found that mice indeed made extensive use of D1 for dexterous manipulations. In particular, mice used D1 to hold food with either of two grip types: a pincer-type grasp, or a "thumb-hold" grip, pressing with D1 from the side. Thumb-holding was preferentially used for handling smaller items, with the smallest items held between the two D1s alone. Second, we observed that mice cycled rapidly between two postural modes while feeding, with the hands positioned either at the mouth (oromanual phase) or resting below (holding phase). Third, we identified two highly stereotyped D1-related movements during feeding, including an extraordinarily fast (~20 ms) "regrip" maneuver, and a fast (~100 ms) "sniff" maneuver. Lastly, in addition to these characteristic simpler movements and postures, we also observed highly complex movements, including rapid D1-assisted rotations of food items and dexterous simultaneous double-gripping of two food fragments. Manipulation behaviors were generally conserved for different food types, and for head-fixed mice. Wild squirrels displayed a similar repertoire of D1-related movements. Our results define, for the mouse, a set of kinematic building-blocks of manual dexterity, and reveal an outsized role for D1 in these actions.

Sensory Integration and Functional Reaching in Children With Rett Syndrome/Rett-Related Disorders

Background

The loss of functional hand skills is a primary characteristic of Rett syndrome. Stereotypies, dyspraxia, and other sensory processing issues severely limit the individual's ability to reach toward and sustain grasp on objects. This loss of functional reach and grasp severely limits their ability to participate in self-help, play, and school-related activities. We proposed that Ayres Sensory Integration (ASI) treatment would improve sensory processing and motor planning, which would lay the sensory-motor groundwork for improving grasp of objects, an important first step in developing functional hand use.

Objective

We examined effects of ASI treatment on rate of reaching and grasping for children with Rett syndrome/Rett-related disorders.

Methods

We used an interrupted time series design to measure changes in outcome variables occurring after intervention initiation and cessation. We analyzed daily video observations during baseline, intervention, and post-intervention periods, over a span of 7 months.

Results

During baseline, rate of grasping declined moderately. There was a 15% increase in grasping from the end of baseline to end of the post-intervention period. There was no significant change in rate of reaching.

Conclusions

This study provides preliminary data showing very small improvements in hand grasp of children with Rett syndrome following ASI treatment; larger studies in diverse settings are needed to establish the effectiveness of this approach. This study shows that an interrupted time series research design provides a valid template for evaluating interventions for children with rare disorders.

Rescue of prepulse inhibition deficit and brain mitochondrial dysfunction by pharmacological stimulation of the central serotonin receptor 7 in a mouse model of CDKL5 Deficiency Disorder

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene cause CDKL5 Deficiency Disorder (CDD), a rare neurodevelopmental syndrome characterized by severe behavioural and physiological symptoms. No cure is available for CDD. CDKL5 is a kinase that is abundantly expressed in the brain and plays a critical role in neurodevelopmental processes, such as neuronal morphogenesis and plasticity. This study provides the first characterization of the neurobehavioural phenotype of 1 year old Cdkl5-null mice and demonstrates that stimulation of the serotonin receptor 7 (5-HT₇R) with the agonist molecule LP-211 (0.25 mg/kg once/day for 7 days) partially rescues the abnormal phenotype and brain molecular alterations in Cdkl5-null male mice. In particular, LP-211 treatment completely normalizes the prepulse inhibition defects observed in Cdkl5-null mice and, at a molecular level, restores the abnormal cortical phosphorylation of rpS6, a downstream target of mTOR and S6 kinase, which plays a direct role in regulating protein synthesis. Moreover, we demonstrate for the first time that mitochondria show prominent functional abnormalities in Cdkl5-null mouse brains that can be restored by pharmacological stimulation of brain 5-HT₇R.

Tsix-Mecp2 female mouse model for Rett syndrome reveals that low-level MECP2 expression extends life and improves neuromotor function

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused by a mutation in the X-linked methyl-CpG-binding protein 2 (MECP2). There is currently no disease-specific treatment, but MECP2 restoration through reactivation of the inactive X (Xi) has been of considerable interest. Progress toward an Xi-reactivation therapy has been hampered by a lack of suitable female mouse models. Because of cellular mosaicism due to random X-chromosome inactivation (XCI), Mecp2^+/- heterozygous females develop only mild RTT. Here, we create an improved female mouse model by introducing a mutation in Tsix, the antisense regulator of XCI allelic choice. Tsix-Mecp2 mice show reduced MECP2 mosaicism and closely phenocopy the severely affected Mecp2-null males. Tsix-Mecp2 females demonstrate shortened lifespan, motor weakness, tremors, and gait disturbance. Intriguingly, they also exhibit repetitive behaviors, as is often seen in human RTT, including excessive grooming and biting that result in self-injury. With a Tsix allelic series, we vary MECP2 levels in brain and demonstrate a direct, but nonlinear correlation between MECP2 levels and phenotypic improvement. As little as 5-10% MECP2 restoration improves neuromotor function and extends lifespan five- to eightfold. Our study thus guides future pharmacological strategies and suggests that partial MECP2 restoration could have disproportionate therapeutic benefit.

Motor, Somatosensory, Viscerosensory and Metabolic Impairments in a Heterozygous Female Rat Model of Rett Syndrome

Rett Syndrome (RTT), an autism-related disorder caused by mutation of the X-linked Methyl CpG-binding Protein 2 (MECP2) gene, is characterized by severe cognitive and intellectual deficits. While cognitive deficits are well-documented in humans and rodent models, impairments of sensory, motor and metabolic functions also occur but remain poorly understood. To better understand non-cognitive deficits in RTT, we studied female rats heterozygous for Mecp2 mutation (Mecp2^−/x); unlike commonly used male Mecp2^−/y rodent models, this more closely approximates human RTT where males rarely survive. Mecp2^−/x rats showed rapid, progressive decline of motor coordination through six months of age as assessed by rotarod performance, accompanied by deficits in gait and posture. Mecp2^−/x rats were hyper-responsive to noxious pressure and cold, but showed visceral hyposensitivity when tested by colorectal distension. Mecp2^−/x rats ate less, drank more, and had more body fat resulting in increased weight gain. Our findings reveal an array of progressive non-cognitive deficits in this rat model that are likely to contribute to the compromised quality of life that characterizes RTT.

Persistent Unresolved Inflammation in the Mecp2-308 Female Mutated Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a rare neurodevelopmental disorder usually caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2). Several Mecp2 mutant mouse lines have been developed recapitulating part of the clinical features. In particular, Mecp2-308 female heterozygous mice, bearing a truncating mutation, are a validated model of the disease. While recent data suggest a role for inflammation in RTT, little information on the inflammatory status in murine models of the disease is available. Here, we investigated the inflammatory status by proteomic 2-DE/MALDI-ToF/ToF analyses in symptomatic Mecp2-308 female mice. Ten differentially expressed proteins were evidenced in the Mecp2-308 mutated plasma proteome. In particular, 5 positive acute-phase response (APR) proteins increased (i.e., kininogen-1, alpha-fetoprotein, mannose-binding protein C, alpha-1-antitrypsin, and alpha-2-macroglobulin), and 3 negative APR reactants were decreased (i.e., serotransferrin, albumin, and apolipoprotein A1). CD5 antigen-like and vitamin D-binding protein, two proteins strictly related to inflammation, were also changed. These results indicate for the first time a persistent unresolved inflammation of unknown origin in the Mecp2-308 mouse model.

Loss of MeCP2 in the rat models regression, impaired sociability and transcriptional deficits of Rett syndrome

Mouse models of the transcriptional modulator Methyl-CpG-Binding Protein 2 (MeCP2) have advanced our understanding of Rett syndrome (RTT). RTT is a 'prototypical' neurodevelopmental disorder with many clinical features overlapping with other intellectual and developmental disabilities (IDD). Therapeutic interventions for RTT may therefore have broader applications. However, the reliance on the laboratory mouse to identify viable therapies for the human condition may present challenges in translating findings from the bench to the clinic. In addition, the need to identify outcome measures in well-chosen animal models is critical for preclinical trials. Here, we report that a novel Mecp2 rat model displays high face validity for modelling psychomotor regression of a learned skill, a deficit that has not been shown in Mecp2 mice. Juvenile play, a behavioural feature that is uniquely present in rats and not mice, is also impaired in female Mecp2 rats. Finally, we demonstrate that evaluating the molecular consequences of the loss of MeCP2 in both mouse and rat may result in higher predictive validity with respect to transcriptional changes in the human RTT brain. These data underscore the similarities and differences caused by the loss of MeCP2 among divergent rodent species which may have important implications for the treatment of individuals with disease-causing MECP2 mutations. Taken together, these findings demonstrate that the Mecp2 rat model is a complementary tool with unique features for the study of RTT and highlight the potential benefit of cross-species analyses in identifying potential disease-relevant preclinical outcome measures.

MeCP2 deficiency results in robust Rett-like behavioural and motor deficits in male and female rats

Since the identification of MECP2 as the causative gene in the majority of Rett Syndrome (RTT) cases, transgenic mouse models have played a critical role in our understanding of this disease. The use of additional mammalian RTT models offers the promise of further elucidating critical early mechanisms of disease as well as providing new avenues for translational studies. We have identified significant abnormalities in growth as well as motor and behavioural function in a novel zinc-finger nuclease model of RTT utilizing both male and female rats throughout development. Male rats lacking MeCP2 (Mecp2^ZFN/y) were noticeably symptomatic as early as postnatal day 21, with most dying by postnatal day 55, while females lacking one copy of Mecp2 (Mecp2^ZFN/+) displayed a more protracted disease course. Brain weights of Mecp2^ZFN/y and Mecp2^ZFN/+rats were significantly reduced by postnatal day 14 and 21, respectively. Early motor and breathing abnormalities were apparent in Mecp2^ZFN/y rats, whereas Mecp2^ZFN/+rats displayed functional irregularities later in development. The large size of this species will provide profound advantages in the identification of early disease mechanisms and the development of appropriately timed therapeutics. The current study establishes a foundational basis for the continued utilization of this rat model in future RTT research.