Opposite regulation of Wnt/ß-catenin and Shh signaling pathways by Rack1 controls mammalian cerebellar development.

The development of the cerebellum depends on intricate processes of neurogenesis, migration, and differentiation of neural stem cells (NSCs) and progenitor cells. Defective cerebellar development often results in motor dysfunctions and psychiatric disorders. Understanding the molecular mechanisms that underlie the complex development of the cerebellum will facilitate the development of novel treatment options. Here, we report that the receptor for activated C kinase (Rack1), a multifaceted signaling adaptor protein, regulates mammalian cerebellar development in a cell type-specific manner. Selective deletion of Rack1 in mouse NSCs or granule neuron progenitors (GNPs), but not Bergmann glial cells (BGs), causes severe defects in cerebellar morphogenesis, including impaired folia and fissure formation. NSCs and GNPs lacking Rack1 exhibit enhanced Wnt/ß-catenin signaling but reduced Sonic hedgehog (Shh) signaling. Simultaneous deletion of ß-catenin in NSCs, but not GNPs, significantly rescues the Rack1 mutant phenotype. Interestingly, Rack1 controls the activation of Shh signaling by regulating the ubiquitylation and stability of histone deacetylase 1 (HDAC1)/HDAC2. Suppression of HDAC1/HDAC2 activity in the developing cerebellum phenocopies the Rack1 mutant. Together, these results reveal a previously unknown role of Rack1 in controlling mammalian cerebellar development by opposite regulation of Wnt/ß-catenin and Shh signaling pathways.

Dysregulation of histone acetylation pathways in hippocampus and frontal cortex of Alzheimer's disease patients.

Memory impairment is the main feature of Alzheimer's disease (AD). Initial impairments originate in the temporal lobe area and propagate throughout the brain in a sequential manner. Epigenetic mechanisms, especially histone acetylation, regulate plasticity and memory processes. These may be dismantled during the disease. The aim of this work was to establish changes in the acetylation-associated pathway in two key brain regions affected in AD: the hippocampus and the F2 area of frontal cortex in end-stage AD patients and age-matched controls. We found that the F2 area was more affected than the hippocampus. Indeed, CREB-Binding Protein (CBP), P300/CBP-associated protein (PCAF), Histone Deacetylase 1 (HDAC1) and HDAC2 (but not HDAC3) levels were strongly decreased in F2 area of AD compared to controls patients, whereas only HDAC1 was decreased and CBP showed a downward trend in the hippocampus. At the histone level, we detected a substantial increase in total (H3 and H2B) histone levels in the frontal cortex, but these were decreased in nuclear extracts, pointing to a dysregulation in histone trafficking/catabolism in this brain region. Histone H3 acetylation levels were increased in cell nuclei mainly in the frontal cortex. These findings provide evidence for acetylation dysfunctions at the level of associated enzymes and of histones in AD brains, which may underlie transcriptional dysregulations and AD-related cognitive impairments. They further point to stronger dysregulations in the F2 area of the frontal cortex than in the hippocampus at an end-stage of the disease, suggesting a differential vulnerability and/or compensatory mechanisms efficiency towards epigenetic alterations.