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.

Neuronal cytoskeletal gene dysregulation and mechanical hypersensitivity in a rat model of Rett syndrome.

Children with Rett syndrome show abnormal cutaneous sensitivity. The precise nature of sensory abnormalities and underlying molecular mechanisms remain largely unknown. Rats with methyl-CpG binding protein 2 (MeCP2) mutation, characteristic of Rett syndrome, show hypersensitivity to pressure and cold, but hyposensitivity to heat. They also show cutaneous hyperinnervation by nonpeptidergic sensory axons, which include subpopulations encoding noxious mechanical and cold stimuli, whereas peptidergic thermosensory innervation is reduced. MeCP2 knockdown confined to dorsal root ganglion sensory neurons replicated this phenotype in vivo, and cultured MeCP2-deficient ganglion neurons showed augmented axonogenesis. Transcriptome analysis revealed dysregulation of genes associated with cytoskeletal dynamics, particularly those controlling actin polymerization and focal-adhesion formation necessary for axon growth and mechanosensory transduction. Down-regulation of these genes by topoisomerase inhibition prevented abnormal axon sprouting. We identified eight key affected genes controlling actin signaling and adhesion formation, including members of the Arhgap, Tiam, and cadherin families. Simultaneous virally mediated knockdown of these genes in Rett rats prevented sensory hyperinnervation and reversed mechanical hypersensitivity, indicating a causal role in abnormal outgrowth and sensitivity. Thus, MeCP2 regulates ganglion neuronal genes controlling cytoskeletal dynamics, which in turn determines axon outgrowth and mechanosensory function and may contribute to altered pain sensitivity in Rett syndrome.

Gait analysis at multiple speeds reveals differential functional and structural outcomes in response to graded spinal cord injury.

Open-field behavioral scoring is widely used to assess spinal cord injury (SCI) outcomes, but has limited usefulness in describing subtle changes important for posture and locomotion. Additional quantitative methods are needed to increase the resolution of locomotor outcome assessment. This study used gait analysis at multiple speeds (GAMS) across a range of mild-to-severe intensities of thoracic SCI in the rat. Overall, Basso, Beattie, and Bresnahan (BBB) scores and subscores were assessed, and detailed automated gait analysis was performed at three fixed walking speeds (3.5, 6.0, and 8.5 cm/sec). Variability in hindpaw brake, propel, and stance times were analyzed further by integrating across the stance phase of stepping cycles. Myelin staining of spinal cord sections was used to quantify white matter loss at the injury site. Varied SCI intensity produced graded deficits in BBB score, BBB subscores, and spinal cord white matter and total volume loss. GAMS measures of posture revealed decreased paw area, increased limb extension, altered stance width, and decreased values for integrated brake, propel, and stance. Measures of coordination revealed increased stride frequency concomitant with decreased stride length, resulting in deviation from consistent forelimb/hindlimb coordination. Alterations in posture and coordination were correlated to impact severity. GAMS results correlated highly with functional and histological measures and revealed differential relationships between sets of GAMS dynamics and cord total volume loss versus epicenter myelin loss. Automated gait analysis at multiple speeds is therefore a useful tool for quantifying nuanced changes in gait as an extension of histological and observational methods in assessing SCI outcomes.