Bdnf overexpression in hippocampal neurons prevents dendritic atrophy caused by Rett-associated MECP2 mutations

Abstract

The expression of the methylated DNA-binding protein MeCP2 increases during neuronal development, which suggests that this epigenetic factor is crucial for neuronal terminal differentiation. We evaluated dendritic and axonal development in embryonic day-18 hippocampal neurons in culture by measuring total length and counting branch point numbers at 4 days in vitro, well before synapse formation. Pyramidal neurons transfected with a plasmid encoding a small hairpin RNA (shRNA) to knockdown endogenous Mecp2 had shorter dendrites than control untransfected neurons, without detectable changes in axonal morphology. On the other hand, overexpression of wildtype (wt) human MECP2 increased dendritic branching, in addition to axonal branching and length. Consistent with reduced neuronal growth and complexity in Rett syndrome (RTT) brains, overexpression of human MECP2 carrying missense mutations common in RTT individuals (R106W or T158M) reduced dendritic and axonal length. One of the targets of MeCP2 transcriptional control is the Bdnf gene. Indeed, endogenous Mecp2 knockdown increased the intracellular levels of BDNF protein compared to untransfected neurons, suggesting that MeCP2 represses Bdnf transcription. Surprisingly, overexpression of wt MECP2 also increased BDNF levels, while overexpression of RTT-associated MECP2 mutants failed to affect BDNF levels. The extracellular BDNF scavenger TrkB-Fc prevented dendritic overgrowth in wt MECP2-overexpressing neurons, while overexpression of the Bdnf gene reverted the dendritic atrophy caused by Mecp2-knockdown. However, this effect was only partial, since Bdnf increased dendritic length only to control levels in mutant MECP2-overexpressing neurons, but not as much as in Bdnftransfected cells. Our results demonstrate that MeCP2 plays varied roles in dendritic and axonal development during neuronal terminal differentiation, and that some of these effects are mediated by autocrine actions of BDNF.

Epigenetic Regulation of Gene Expression in Physiological and Pathological Brain Processes

“…MeCP2 mutant mice have reduced BDNF levels. This reduction, and the neuronal atrophy characteristic of RTT, can be reversed by forebrain-specific overexpression of Bdnf in vivo (46), an effect that was recently confirmed in cultured hippocampal neurons (180). In addition to Bdnf, a recent study identified seven other genes as direct binding targets of MeCP2 in the mouse brain (346), including myelin-associated proteins and dopamine decarboxylase.…”

“…At the anatomical level, significant reductions in both gray and white matter are observed at the regressive stage (249). At the cellular level, neurons in RTT patients may have a reduced density of dendritic spines (180), and murine RTT models display axonal and dendritic spine abnormalities (20). Moreover, mouse models of RTT exhibit an overall decrease in exploratory activity (49,127), show severe cognitive deficits, and have impaired synaptic plasticity (61).…”

Ethanol deregulates Mecp2/MeCP2 in differentiating neural stem cells via interplay between 5-methylcytosine and 5-hydroxymethylcytosine at the Mecp2 regulatory elements

“…For example, ethanol exposure (Reiter-Funk and Dohrman, 2005;Roivainen et al, 1995a;Zou et al, 1993) and MeCP2 overexpression (Larimore et al, 2009;Rastegar et al, 2009) have been shown to show similar patterns of increased neurite branching. However, ethanol exposure has also been shown to reduce the neurite outgrowth (Camarillo and Miranda, 2008), similar to few studies on MeCP2-deficiency (Nguyen et al, 2012;Rastegar et al, 2009;Wang et al, 2013).…”

Rett Syndrome: Reaching for Clinical Trials

“…Furthermore, conditional overexpression of BDNF in excitatory forebrain neurons of Mecp2-deficient mice-achieved by crossing BDNF STOP mice with Mecp2;cre mice-led to the improvement of some of their RTT-like neurologic phenotypes [81]. In addition, in vitro expression of green fluorescent prot ein-tagged BDNF from a cytomegalovirus-driven plasmid rescued the dendritic atrophy caused by short hairpin RNA-mediated knockdown of Mecp2 in cultured rat hippocampal neurons [82], and the dendritic atrophy and lower spine density in cultured hippocampal neurons from Mecp2 knockout mice (Xu and LP-M, in preparation). Unfortunately, BDNF has very low BBB permeability, which limits the bioavailability of peripherally administered BDNF as a potential therapy.…”