Duchenne muscular dystrophy (DMD) is a lethal X-linked disease characterized by muscular wasting due to lack of dystrophin (Dp427), a cytoskeletal protein expressed in muscle and selected brain regions (i.e. hippocampus). Dp427 binds to the Dystrophin Glycoprotein Complex (DGC) that spans the plasma membrane and modulates structural and several intracellular signaling properties (i.e. protein nytrosylation by neuronal nitric oxide synthase activation). The presence of the dystrophin-DGC in areas involved in motor, emotional and cognitive functions suggest that lack of Dp427 may be responsible for neurological disturbances described in DMD patients. These could be further aggravated by a condition of chronic stress, to which patients are subjected, and by cyclic therapeutic treatments with glucocorticoid (GC) for reducing muscular inflammation. Aim of the project is to uncover the effects that chronic treatment with GC and stress behavioral tests exert on brain regions susceptible to stressors and already compromised in DMD, as the hippocampus. In the first part of the study, we will perform a chronic treatment with GC of wild type (WT) and genetically dystrophic mdx mice, animal model of the pathology, in order to evaluate changes in hippocampal neurogenesis, a process highly stress-modulated. Moreover, as GC signaling is implicated in the pathogenesis of stress-related psychiatric disorders, i.e. post-traumatic disorder (PTSD), short and long term behavioral responses to a traumatic stress will be examined. Finally, since the lack of Dp427 has been linked with aberrant activity of histone deacetylases, due to an impaired nytrosylation, we will also analyze the global levels of histone acetylation in the hippocampus of both control and experimental WT and mdx mice. We expect to acquire information on the role of Dp427 in nervous system physiology, fundamental for prospecting therapeutic treatments for preventing or reducing, DMD-related neurological disorders.
Structural and functional abnormalities in several brain regions, among which the hippocampus, have been described in DMD patients and mdx mice, often correlating with a wide range of cognitive disturbances. More specifically, mdx mice have altered presynaptic structure in excitatory hippocampal synapses (43), and both acquisition and long-term retention of cued and trace fear memories were impaired, suggesting alterations in a functional circuit including the amygdala (44). These regions are also particularly susceptible to stress, which in the long run can determine severe behavioral abnormalities, as PTSD.
Major pharmacological treatment of DMD patients is repetitive administration of high doses of corticosteroids, directed at lowering recurrent muscular inflammatory events. However, besides the fact that prolonged exposures to GCs determines several adverse side effects, the signaling that these molecules elicit in the brain of dystrophic patients, already compromised, has never been investigated. Moreover, another obscure aspect of the pathology is whether lack of Dp427 impacts histone acetylation and consequent gene expression in the nervous system, as described in muscles.
We previously demonstrated that dystrophic mdx mice indeed have significant changes in the expression of genes coding for proteins involved in neuron development, survival and differentiation (33). In addition, in a previous financed project (prot. C26A15PJLW) we demonstrated that hippocampal neurons of mdx mice respond less efficiently than WT to acute corticosterone (CORT) treatment, both in vitro and in vivo. It is, therefore, important to complete these observations by investigating long-term responses of hippocampus to chronic GC treatments, as those administered to DMD patients.
Understanding how brain regions of the limbic system, structrally and functionally compromised from development by the lack of Dp427, respond to stressful events and anti-inflammatory treatments is important in order to prevent further brain damages.
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