Neuromodulation during the progression of Rett syndrome: An immunohistochemistry study of ACh and PV+ innervation in the basal forebrain of a female mouse model of RTT

Rett syndrome (RTT) is a rare neurodevelopmental disorder caused by a mutation of the X-linked MECP2 (methyl-CpG-binding protein 2) gene. It is characterized by severe cognitive and motor impairments, sensory deficits, physical and psychological symptoms. After an apparently typical postnatal development, individuals with RTT undergo a regression period characterized by loss of the previously acquired skills. To study the processes at the basis of this disorder I adopted a circuit-based approach, investigating when and how the dysregulation of the cholinergic (ACh) and GABAergic (GABA) neuromodulatory systems affect neural networks formation and functioning. My hypothesis was that a disrupted ACh neuromodulation and GABA inhibition contribute to the derailment of cortical neuronal network activity and sensory processing observed in this disorder. Additionally, I hypothesized that MECP2 re-expression before critical periods has some on RTT phenotype in the symptomatic stage of the disorder. To answer this question, I used immunohistochemistry to visualize and quantify MeCP2-expressing (MeCP2+) neurons, Parvalbumin-positive interneurons (PV+), and choline acetyltransferase-expressing (ChAT+) neurons in the basal forebrain (BF) of a cohort of female mice models of RTT (Mecp2 heterozygous mice, Mecp2+/-). Mice brains were collected at two stages of the disorder: before regression at postnatal day 30 (P30) and at full regression at P90/110. Moreover, in a larger cohort of Mecp2+/- female mice, I monitored the progression of disease hallmarks and visual acuity. Together with this first line of research, I adopted a complementary approach based on the use of cell-type specific knockout mice in which MECP2 was selectively expressed in ChAT+ neurons. In these mice we monitored RTT phenotype and visual acuity at later stages of the disorder (P150-500). I found that ChAT was differentially regulated over the course of the disorder, being upregulated in the presymptomatic mice and then downregulated during the first phases of disorder onset. The misregulation was non cell autonomous as even cells expressing MeCP2 exhibit an initial significant disruption. In addition, we found that targeting the expression of MECP2 selectively in ChAT positive cells was sufficient to improve RTT phenotypic score and visual acuity at later stages of the disorder. By investigating the effects of a disrupted neuromodulation in RTT, this study aimed at deepening the understanding the neurobiology of the disorder in its various stages. Moreover, it may contribute to take a step forward toward early diagnosis and the development of targeted pharmacological and/or genetic interventions.