Genetically modified bone morphogenetic protein signalling Alters traumatic brain injury‐induced gene expression responses in the adult mouse

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Abstract
Three genetic mouse models were examined to define effects of bone morphogenetic protein (BMP) signalling on gene expression in normal and injured adult brain. CaMKII‐Cre eliminated the BMP receptor Acvr1 (Alk2) and the common TGFβ superfamily signal mediator Smad4 or activated a constitutively active Acvr1 in postnatal forebrain neurons. All mutants followed mendelian ratios, with no overt phenotypic changes. In situ hybridization demonstrated normal patterns of the dendritic marker MAP2 (Mtap2) throughout cortex despite neuron‐specific losses of Acvr1 or Smad4. However, strong up‐regulation of Mtap2 transcript in these mice was found by quantitative RT‐PCR (qRT‐PCR), indicating that Mtap2 is normally suppressed by BMP. Traumatic brain injury (TBI) resulted in increases of histone‐associated DNA fragments in both control and Smad4‐deficient cortex. Several cell‐type‐specific transcripts known to be involved in injury‐related responses were measured by qRT‐PCR. Gfap mRNA was strongly up‐regulated in controls as well as in the loss‐of‐BMP‐signalling mutants. Notably, activated Acvr1 signalling gave significantly lower TBI‐induced up‐regulations of Gfap and Phox2a mRNA levels, indicating reductions in astroglial and neuronal reactions to injury. Strong impairment in injury‐induced Timp1 transcript up‐regulation was also seen in these mice. In contrast, osteopontin (Spp1) transcript levels in activated microglia were not reduced by Acvr1 signalling. Altogether, the data suggest that BMP signalling is dispensable in adult cortical neurons but that augmented BMP signalling affects molecular changes associated with neuronal lesions.