MeCP2 Mutations Results in Cell-autonomous Abnormalities of the Dendritic Complexity of Cortical Projection Neurons
N.Kishi; J.D.Macklis.
Abstract
Rett syndrome is a neurodevelopmental disorder and the second most common cause of mental retardation in girls. The identification in 1999 of mutation of the methyl-CpG binding protein 2 (MECP2) gene (nominally a transcriptional repressor) on the X chromosome as the cause of Rett syndrome enabled a new era of cellular and molecular analysis and understanding of Rett syndrome pathophysiology.
Our previous work shows that layer II/III cortical projection neurons increasingly express MECP2 as they mature, and MECP2 mutation affects their soma size and dendritic complexity (Kishi and Macklis, Mol Cell Neurosci, 2004), indicating that MECP2 is involved in the maturation and maintenance of neurons, particularly cerebral cortex callosal projection neurons (CPN), including dendritic arborization, rather than in cell fate decisions.
To further investigate whether these abnormalities are cell autonomous, we developed neuronal transplantation experiments to generate physical chimeras. We performed transplantation into P2 neocortex during neocortical development, placing dual mutant/transgenic MECP2-null/GFP+ E17.5 neurons into wild-type (wt) cortex, and wt/GFP+ neurons into MECP2-null cortex. Transplanted MECP2 -/y layer II/III pyramidal neurons are smaller and less complex than those of transplanted wt neurons, even in the wt environment, while wt neurons are not similarly affected in MECP2 -/y cortex, indicating that the phenotype of MECP2 -/y pyramidal neurons results predominantly via cell-autonomous mechanisms.
To identify target genes of MECP2, we have initiated two complementary approaches by both microarray and chromatin immunoprecipitation (ChIP) approaches, using FACS-purified CPN (Arlotta, Molyneaux et al., Neuron, 2005). We have identified candidate MECP2-binding sequences by ChIP based screening. Secondary screens and confirmation are ongoing.
Lay Summary
MECP2 is involved in the maturation and maintenance of neurons, especially with regards to neuronal size and shape. MeCP2-deficient neurons are known to express an altered morphology. To further investigate whether these abnormalities are cell autonomous - that is, independent of the cell's environment - the authors performed elegant neuronal transplantation experiments. They transplanted MeCP2-null neurons into non-affected mouse brains, and conversely transplanted non-affected neurons into MeCP2-null mouse brains. The transplanted neurons were tagged with a fluorescent marker, for easy and specific identification in their new environments. Interestingly, the neurons retained the morphology of their donor environment, rather than taking on characteristics of their new host environments. Indeed, this suggests that the abnormalities of MeCP2-null morphology occur independently of its environment. RSRF has funded Dr. Kishi for 3 years and just awarded Dr. Macklis a research grant.