To study nucleolar involvement in brain development, the nuclear and nucleolar
proteomes from the rat cerebral cortex at postnatal day 7 were analyzed using LC-MS/iTRAQ methodology. Data of the analysis are available via ProteomeXchange with identifier PXD002188. Among 504 candidate
nucleolar proteins, the overrepresented gene ontology terms included such cellular compartmentcategories as "nucleolus", "ribosome" and "
chromatin". Consistent with such classification, the most overrepresented functional gene ontology terms were related to
RNA metabolism/ribosomal biogenesis, translation, and
chromatin organization. Sixteen putative
nucleolar proteins were associated with neurodevelopmental phenotypes in humans.
Microcephaly and/or
cognitive impairment were the most common phenotypic manifestations. Although several such
proteins have links to ribosomal biogenesis and/or
genomic stability/
chromatin structure (e.g. EMG1, RPL10, DKC1, EIF4A3, FLNA, SMC1, ATRX, MCM4, NSD1, LMNA, or CUL4B), others including ADAR, LARP7, GTF2I, or TCF4 have no such connections known. Although neither the Alazami syndrome-associated LARP7nor the
Pitt-Hopkins syndrome-associated TCF4 were reported in nucleoli of non-neural cells, in neurons, their nucleolar localization was confirmed by immunostaining. In cultured rat hippocampal neurons, knockdown of LARP7 reduced both perikaryal ribosome content and general
protein synthesis. Similar anti-ribosomal/anti-translation effects were observed after knockdown of the ribosomal biogenesis factor EMG1 whose deficiency underlies
Bowen-Conradi syndrome. Finally, moderate reduction of ribosome content and general
protein synthesis followed overexpression of two
Pitt-Hopkins syndrome mutant variants of TCF4. Therefore, dysregulation of ribosomal biogenesis and/or other functions of the nucleolus may disrupt neurodevelopment resulting in such phenotypes as
microcephaly and/or
cognitive impairment.