Developmental and maintenance defects in Rett syndrome neurons identified by a new mouse staging system in vitro

Abstract

Rett Syndrome (RTT) is a neurodevelopmental disorder associated with intellectual disability, mainly caused by loss-of-function mutations in the MECP2 gene. RTT brains display decreased neuronal size and dendritic arborization possibly caused by either a developmental failure or a deficit in the maintenance of dendritic arbor structure. To distinguish between these two hypotheses, the development of Mecp2-knockout mouse hippocampal neurons was analyzed in vitro. Since a staging system for the in vitro development of mouse neurons was lacking, mouse and rat hippocampal neurons development was compared between 1–15 days in vitro (DIV) leading to a 6-stage model for both species. Mecp2-knockout hippocampal neurons displayed reduced growth of dendritic branches from stage 4 (DIV4) onwards. At stages 5–6 (DIV9-15), synapse number was lowered in Mecp2-knockout neurons, suggesting increased synapse elimination. These results point to both a developmental and a maintenance setback affecting the final shape and function of neurons in RTT.

Distinct effects on the dendritic arbor occur by microbead versus bath administration of brain-derived neurotrophic factor

“…Previous studies indicate that dendrites undergo frequent extensions and retractions from DIV 8–10 [65,66]. Moreover, it is important to recognize that the arbor is in a natural state of flux, and the results from this study shed light not on how BDNF stimulation affects a mature, stable dendritic arbor but rather how it shapes an immature, developing dendritic arbor.…”

Dendritic spine dysgenesis in autism related disorders

“…Mouse models of RTT are widely used and recapitulate many of the behavioral and anatomical abnormalities associated with the human disorder [111, 112] and additionally show impairments in both LTP and LTD [113, 114]. Commonly used strains of Mecp2 KO mice have impaired dendritic complexity [115, 116] and lower dendritic spine density [117–120] as well as a lack of mushroom spines in cortical and hippocampal neurons [121, 122]. …”

Gene Therapy for Rett Syndrome: Prospects and Challenges

“…RTT is thought to be a consequence of lack of neuronal maturation (to complete or to maintain maturation) and MeCP2 has been shown to play important roles in both neuronal development and maintenance processes [85][86][87]. In a mouse model, early restoration of Mecp2 expression in presymptomatic mice led to apparent normal development [8].…”

“…However, the extent of amelioration of existing phenotypes after treatment of fully adult (∼10 months) mice would predict that later administration to older girls or adults with RTT Syndrome would also be effective [8,12,89]. Regardless of the time of intervention, an important requirement is that transgene expression should be long-lasting, as deletion of Mecp2 from mature neurons is deleterious [87,90]. Indeed, there have been many reports of AAV-mediated transgenes persisting for long periods, including reports of continued expression >6 years after delivery in primates [91] and after >8 years in dogs [92,93].…”

“…The dominant neurological manifestation of the disorder indicates that neurons are more severely affected than other cell types [87]. However, there is growing recognition of the possible role of glial cells in the development of the phenotype [109,110] and its recovery [111].…”