MeCP2 Mutation Results in Compartment-Specific Reductions in Dendritic Branching and Spine Density in Layer 5 Motor Cortical Neurons of YFP-H Mice David P. Stuss1*, Jamie D. Boyd 2 , David B. Levin3, Kerry R. Delaney1 1 Department of Biology, University of Victoria, Victoria, Canada, 2 Department of Psychiatry, Faculty of Medicine, University of British Columbia, Vancouver, Canada, 3 Department of Biosystems Engineering, University of Manitoba, Winnipeg, Canada Abstract Rett Syndrome (RTT) is a neurodevelopmental disorder predominantly caused by mutations in the X-linked gene MECP2. A primary feature of the syndrome is the impaired maturation and maintenance of excitatory synapses in the central nervous system (CNS). Different RTT mouse models have shown that particular Mecp2 mutations have highly variable effects on neuronal architecture. Distinguishing MeCP2 mutant cellular phenotypes therefore demands analysis of specific mutations in well-defined neuronal subpopulations. We examined a transgenically labeled subset of cortical neurons in YFP-H mice crossed with the Mecp2tm1.1Jae mutant line. YFP+ Layer 5 pyramidal neurons in the motor cortex of wildtype and hemizygous mutant male mice were examined for differences in dendrite morphology and spine density. Total basal dendritic length was decreased by 18.6% due to both shorter dendrites and reduced branching proximal to the soma. Tangential dendrite lengths in the apical tuft were reduced by up to 26.6%. Spine density was reduced by 47.4% in the apical tuft and 54.5% in secondary apical dendrites, but remained unaffected in primary apical and proximal basal dendrites. We also found that MeCP2 mutation reduced the number of YFP+ cells in YFP-H mice by up to 72% in various cortical regions without affecting the intensity of YFP expression in individual cells. Our results support the view that the effects of MeCP2 mutation are highly context-dependent and cannot be generalized across mutation types and cell populations. Citation: Stuss DP, Boyd JD, Levin DB, Delaney KR (2012) MeCP2 Mutation Results in Compartment-Specific Reductions in Dendritic Branching and Spine Density in Layer 5 Motor Cortical Neurons of YFP-H Mice. PLoS ONE 7(3): e31896. doi:10.1371/journal.pone.0031896 Editor: Mark R. Cookson, National Institutes of Health, United States of America Received September 8, 2011; Accepted January 17, 2012; Published March 7, 2012 Copyright: 2012 Stuss et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by the Rett Syndrome Research Foundation (now the IRSF, below) (to Dr. Levin and Dr. Delaney; grant title ‘‘MeCP2 Replacement Therapy for Rett Syndrome: A mouse mode’’l). The International Rett Syndrome Foundation (http://www.rettsyndrome.org/) (to Dr. Delaney; grant title ‘‘Cell autonomous versus environmental effects on neuronal morphology in brains of a Rett syndrome mouse model), and by graduate scholarships (to Dr. Stuss) from Canadian Institutes of Health Research (http://www.cihr-irsc.gc.ca/e/193.html) and the Michael Smith Foundation for Health Research (http://www. msfhr.org/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: stussd@uvic.ca Introduction Rett Syndrome (RTT) is neurodevelopmental disorder primar- ily caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2) [1]. RTT has broad phenotypic effects that include cognitive, motor, language, and social deficits, as well as autonomic dysregulation [2,3]. Concordant with this wide spectrum of neurological symptoms, MeCP2 appears to regulate activity-dependent synaptic maturation and maintenance [4–6]. The typical RTT brain has been characterized as developmentally arrested at approximately one year of age, although this effect is highly variable [7]. Global brain volume is reduced but gross morphology is preserved, with frontal regions tending to exhibit more severe effects, which correlate with measures of clinical severity [8,9]. Early human studies indicated that reductions in brain volume were not caused by neurodegeneration but were instead a consequence of increased neuronal density arising from reduced soma size and dendritic arborization [10–15]. Mapping the impact of MeCP2 mutations on the nervous system has been complicated by a high degree of phenotypic variability in both human patients and animal models. At the cellular level, the emerging perspective is that RTT neuronal phenotypes potentially depend on several interacting factors, including X-chromosome inactivation (XCI) ratio, specific muta- tion type, brain region, and cellular subtype. Human phenotype- genotype correlation studies have shown that both skewed XCI ratios and mutation type correlate with variations in clinical severity [16–19]. A similar spectrum of variability has been observed in several murine RTT models carrying different Mecp2 mutations [20–24]. Neuroanatomical studies in these different mouse lines have revealed both overlapping and divergent effects on brain region volumes, neuronal density, dendritic and axonal morphology, dendritic spine density, and spine morphology [6,25– 34]. Both cell-autonomous and non-cell-autonomous effects have been reported and different mutations selectively affect specific morphological features while leaving others intact [29,32]. Collectively, these studies point to a crucial role for cellular context in modulating the effects of different mutations. The use of precisely defined cellular subtypes is therefore necessary for resolving the manner in which MeCP2 mutation effects may be generalized across different cell subpopulations in the CNS. Motor cortex is of particular interest because of the frontal volume reductions and prominent motor dysfunctions observed in RTT, which include apraxia, ataxia, repetitive stereotyped hand PLoS ONE | www.plosone.org 1 March 2012 | Volume 7 | Issue 3 | e31896