Nucleolar Enrichment of Brain Proteins with Critical Roles in Human Neurodevelopment

Abstract

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 compartment categories 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 LARP7 nor 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/antitranslation 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. Molecular & Cellular

A Unified Approach to Targeting the Lysosome's Degradative and Growth Signaling Roles

“…The unique ability of DQ661 to disrupt the mTOR-Rheb interaction is therefore not shared by allosteric or catalytic inhibitors of mTORC1, nor by other prototypical lysosomally directed cationic amphiphilic drugs (CADs). Next, incubation of cells with O-propargyl-puromycin followed by click chemistry-enabled attachment of a fluorophore was used to compare the effects of mTOR and lysosomal inhibitors on de novo protein translation (29). Catalytic inhibition of mTORC1 decreased protein translation to a significantly greater degree than allosteric inhibition of mTORC1.…”

Ribosomal biogenesis as an emerging target of neurodevelopmental pathologies

“…Indeed, nucleolar enrichment of RPL10 was detected in brain cell nuclei from 7‐day‐old rat pups (Slomnicki et al . ). However, observations of increased apoptosis in brains of RPL10‐depleted fish embryos suggest that besides translational deficits, RS may also contribute to the RPL10 deficiency‐associated microcephaly (Brooks et al .…”

“…While it remains to be determined whether cohesins play a similar, pro‐ribosomal role in neurons, these proteins were found enriched in the nucleolar proteome from the developing rat brain (Slomnicki et al . ). Therefore, at least some neurological symptoms of cohesinopathies, including ID and/or microcephaly, may be caused by insufficient ribosome supply and translational deficits that affect neuronal connectivity as well as RS‐mediated impairment of neurogenesis.…”

The La and related RNA‐binding proteins (LARPs): structures, functions, and evolving perspectives

“…Another study proposed that some symptoms of Alazami syndrome (specific PD with LARP7 mutations) may result from translational and/or nucleolar stress in neural cells . Although LARP7 is generally nucleoplasmic, it was convincingly found among proteins specifically enriched in nucleoli of rat neurons . Interestingly, in hippocampal neurons, LARP7 knockdown reduced perikaryal ribosome content and protein synthesis .…”

“…Although LARP7 is generally nucleoplasmic, it was convincingly found among proteins specifically enriched in nucleoli of rat neurons . Interestingly, in hippocampal neurons, LARP7 knockdown reduced perikaryal ribosome content and protein synthesis . How LARP7 executes this function had been unclear but is explained by recent findings that show that the 7SK snRNP plays a positive role in promoting the transcription of snRNAs and the small nucleolar (sno)RNAs, U3, U8, and U13 which are involved in processing the precursors of large ribosomal RNA during ribosome biogenesis .…”