Summary Mutations in the X-linked MECP2, which encodes the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2) cause Rett syndrome (RTT) and several neurodevelopmental disorders including cognitive disorders, autism, juvenile-onset schizophrenia, and encephalopathy with early lethality. RTT is characterized by apparently normal early development followed by regression, motor abnormalities, seizures, and features of autism, especially stereotyped behaviors. The mechanisms mediating these striking features are poorly understood. Here we show that mice lacking Mecp2 from γ-amino-butyric-acid-(GABA)-ergic neurons recapitulate numerous RTT and autistic features, including repetitive behaviors. Loss of MeCP2 from a subset of forebrain GABAergic neurons also recapitulates many features of RTT. MeCP2-deficient GABAergic neurons show reduced inhibitory quantal size consistent with presynaptic reduction in glutamic acid decarboxylase-1 and -2 levels and GABA immunoreactivity. These data demonstrate that MeCP2 is critical for normal GABAergic neuronal function and that subtle dysfunction of GABAergic neurons contributes to numerous neuropsychiatric phenotypes.

Rett syndrome (RTT) is characterized by specific motor, cognitive, and behavioral deficits. Because several of these abnormalities occur in other disease states associated with alterations in aminergic neurotransmitters, we investigated the contribution of such alterations to RTT pathogenesis. We found that both individuals with RTT and Mecp2-null mice have lower-than-normal levels of aminergic metabolites and content. Deleting Mecp2 from either TH-positive dopaminergic and noradrenergic neurons or PET1-positive serotonergic neurons in mice decreased corresponding neurotransmitter concentration and specific phenotypes, likely through MeCP2 regulation of rate-limiting enzymes involved in aminergic neurotransmitter production. These data support a cell-autonomous, MeCP2-dependent mechanism for the regulation of aminergic neurotransmitter synthesis contributing to unique behavioral phenotypes.dopamine ͉ norepinephrine ͉ Rett syndrome ͉ serotonin R ett syndrome (RTT, MIM 312750) is an X-linked neurodevelopmental disorder caused, in the vast majority of cases, by mutations in Methyl-CpG Binding Protein 2 (MECP2) (1). RTT primarily affects females, with a milder clinical phenotype correlating with late truncating and some missense mutations (2, 3). It has been suspected that aminergic neurons malfunction in RTT, because the heightened anxiety, aggression, and mood alterations seen in RTT and MECP2 disorders are associated with abnormalities of the serotonergic system in other human disorders (4-6). Rigidity and movement abnormalities are associated with dysfunction of dopaminergic system (7-9). Autonomic dysfunction underlying breathing irregularities could be attributable to decreased norepinephrine (7, 10). Although clinical studies have explored the hypothesis that the aminergic neurotransmitter systems might be altered in RTT, no solid conclusions have been reached. Early work suggested decreased aminergic metabolite levels in the spinal fluid of individuals with RTT (11, 12), but subsequent reports failed to identify such changes (13,14). More recently, one study reported instances of both decreased and increased aminergic metabolite levels in a few individuals with RTT (15). The interpretation of these clinical data are hindered by clinical heterogeneity, small sample sizes, and lack of a sufficient number of controls. To circumvent these problems, we explored the role of the aminergic neurotransmitter system by performing both clinical and animal studies. To determine whether the aminergic neurotransmitter system is altered in RTT, we analyzed the levels of the dopamine metabolite homovanillic acid (HVA) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in spinal fluid from women who both met the clinical criteria for RTT and had a diseasecausing MECP2 mutation. We also evaluated neurotransmitter changes in a RTT murine model and studied the neurochemical, molecular, and behavioral consequences of deleting Mecp2 from either TH-positive dopaminergic and noradrenergic neurons or PET1-positive serot...

SUMMARY Mice lacking the proneural transcription factor Math1 (Atoh1) lack multiple neurons of the proprioceptive and arousal systems and die shortly after birth from an apparent inability to initiate respiration. We sought to determine whether Math1 was necessary for the development of hindbrain nuclei involved in respiratory rhythm generation, such as the parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN), defects in which are associated with Congenital Central Hypoventilation Syndrome (CCHS). Using a new Math1-GFP fusion allele, we traced the development of Math1-expressing pFRG/RTN and paratrigeminal neurons and found that loss of Math1 did indeed disrupt their migration and differentiation. We also identified new Math1-dependent neurons and their projections in the pre-Bötzinger complex, a structure critical for respiratory rhythmogenesis, and found that glutamatergic modulation reestablished a rhythm in the absence of Math1. This study identifies Math1-dependent neurons that are critical for perinatal breathing that may link proprioception and arousal with respiration.

Summary Rett Syndrome (RTT) is an autism spectrum disorder caused by mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2). In order to map the neuroanatomic origins of the complex neuropsychiatric behaviors observed in patients with RTT, and to uncover novel endogenous functions of MeCP2, we removed Mecp2 from Sim1-expressing neurons in the hypothalamus using Cre-loxP technology. Loss of MeCP2 in Sim1-expressing neurons resulted in mice that recapitulated the abnormal physiological stress response that is seen upon MeCP2 dysfunction in the entire brain. Surprisingly, we also uncovered a novel role for MeCP2 in the regulation of social and feeding behaviors since the Mecp2 conditional knockout (CKO) mice were aggressive, hyperphagic and obese. This study demonstrates that deleting Mecp2 in a defined brain region is an excellent approach to map the neuronal origins of complex behaviors and provides new insight about the function of MeCP2 in specific neurons.

Rett Syndrome, an X-linked dominant neurodevelopmental disorder characterized by regression of language and hand use, is primarily caused by mutations in methyl-CpG-binding protein 2 (MECP2). Loss of function mutations in MECP2 are also found in other neurodevelopmental disorders such as autism, Angelman-like syndrome and non-specific mental retardation. Furthermore, duplication of the MECP2 genomic region results in mental retardation with speech and social problems. The common features of human neurodevelopmental disorders caused by the loss or increase of MeCP2 function suggest that even modest alterations of MeCP2 protein levels result in neurodevelopmental problems. To determine whether a small reduction in MeCP2 level has phenotypic consequences, we characterized a conditional mouse allele of Mecp2 that expresses 50% of the wild-type level of MeCP2. Upon careful behavioral analysis, mice that harbor this allele display a spectrum of abnormalities such as learning and motor deficits, decreased anxiety, altered social behavior and nest building, decreased pain recognition and disrupted breathing patterns. These results indicate that precise control of MeCP2 is critical for normal behavior and predict that human neurodevelopmental disorders will result from a subtle reduction in MeCP2 expression.