CNS expression of murine fragile X protein (FMRP) as a function of CGG-repeat size.

Large expansions of a CGG-repeat element (>200 repeats; full mutation) in the fragile X mental retardation 1 (FMR1) gene cause fragile X syndrome (FXS), the leading single-gene form of intellectual disability and of autism spectrum disorder. Smaller expansions (55-200 CGG repeats; premutation) result in the neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). Whereas FXS is caused by gene silencing and insufficient FMR1 protein (FMRP), FXTAS is thought to be caused by 'toxicity' of expanded-CGG-repeat mRNA. However, as FMRP expression levels decrease with increasing CGG-repeat length, lowered protein may contribute to premutation-associated clinical involvement. To address this issue, we measured brain Fmr1 mRNA and FMRP levels as a function of CGG-repeat length in a congenic (CGG-repeat knock-in) mouse model using 57 wild-type and 97 expanded-CGG-repeat mice carrying up to ~250 CGG repeats. While Fmr1 message levels increased with repeat length, FMRP levels trended downward over the same range, subject to significant inter-subject variation. Human comparisons of protein levels in the frontal cortex of 7 normal and 17 FXTAS individuals revealed that the mild FMRP decrease in mice mirrored the more limited data for FMRP expression in the human samples. In addition, FMRP expression levels varied in a subset of mice across the cerebellum, frontal cortex, and hippocampus, as well as at different ages. These results provide a foundation for understanding both the CGG-repeat-dependence of FMRP expression and for interpreting clinical phenotypes in premutation carriers in terms of the balance between elevated mRNA and lowered FMRP expression levels.

Transplanted Dentate Progenitor Cells Show Increased Survival in an Enriched Environment But Do Not Exert a Neurotrophic Effect on Spatial Memory Within 2 Weeks of Engraftment.

Cyclin D2 knockout mice show decreased levels of endogenous dentate neurogenesis. We investigated whether transplanted dentate progenitor cells from wild-type mice respond in vivo to an enriched environment and whether they improve deficient dentate neurogenesis through a neurotrophic effect. Adult cyclin D2 knockout mice were transplanted with passaged adult progenitor cells and kept in an enriched environment or under standard housing conditions in isolation. After 1 week, animals living in an enriched environment underwent water maze testing. Progenitor cells grown on a laminin/poly-d-lysine monolayer expressed Sox2 and nestin and could be differentiated in vitro into neurons and astrocytes. After transplantation into the dentate gyrus, cells preferentially survived along the laminin-rich ependymal lining of the basal cistern or basal membrane of capillaries. A subpopulation of transplanted cells migrated into the interstitial space of the hippocampus and was not associated with laminin. Environmental enrichment led to a significant increase in the survival of transplanted progenitor cells on laminin in the dentate gyrus after 2 weeks. However, animals did not show an enhanced performance in the Morris water maze, and transplantation failed to exert a neurotrophic effect on endogenous neurogenesis after 2 weeks. However, a major limitation of the study is the short-term period of investigation, which may have been insufficient to capture functional effects. In conclusion, initial survival of transplanted neural progenitor cells was dependent on the presence of laminin and was significantly enhanced by environmental enrichment. Further studies are needed to address whether an enriched environment continues to promote graft survival over longer periods of time.