Cellular bases of the RNA metabolism dysfunction in motor neurons of a murine model of spinal muscular atrophy: Role of Cajal bodies and the nucleolus

Abstract

Spinal muscular atrophy (SMA) is caused by a homozygous deletion or mutation in the survival motor neuron 1 (SMN1) gene that leads to reduced levels of SMN protein resulting in degeneration of motor neurons (MNs). The best known functions of SMN is the biogenesis of spliceosomal snRNPs. Linked to this function, Cajal bodies (CBs) are involved in the assembly of spliceosomal (snRNPs) and nucleolar (snoRNPs) ribonucleoproteins required for pre-mRNA and pre-rRNA processing. Recent studies support that the interaction between CBs and nucleoli, which are especially prominent in neurons, is essential for the nucleolar rRNA homeostasis. We use the SMN∆7 murine model of type I SMA to investigate the cellular basis of the dysfunction of RNA metabolism in MNs. SMN deficiency in postnatal MNs produces a depletion of functional CBs and relocalization of coilin, which is a scaffold protein of CBs, in snRNP-free perinucleolar caps or within the nucleolus. Disruption of CBs is the earliest nuclear sign of MN degeneration. We demonstrate that depletion of CBs, with loss of CB-nucleolus interactions, induces a progressive nucleolar dysfunction in ribosome biogenesis. It includes reorganization and loss of nucleolar transcription units, segregation of dense fibrillar and granular components, retention of SUMO-conjugated proteins in intranucleolar bodies and a reactive, compensatory, up-regulation of mature 18S rRNA and genes encoding key nucleolar proteins, such as upstream binding factor, fibrillarin, nucleolin and nucleophosmin. We propose that CB depletion and nucleolar alterations are essential components of the dysfunction of RNA metabolism in SMA.

Accumulation of poly(A) RNA in nuclear granules enriched in Sam68 in motor neurons from the SMNΔ7 mouse model of SMA

“…3A ) and nucleolar segregation of the granular component (Fig. 3B,C ), as we have recently reported 34 . Ultrastructural analysis confirmed the presence of two categories of PARGs: rounded, compact electron-dense bodies (Fig.…”

“…Splicing alterations in the spinal cord in murine SMA models include widespread intron retention, particularly of minor U12 introns, as well as a time-dependent differential expression of a number of exons associated with neurodevelopmental and cell stress pathways 27 – 29 , 31 , 32 . In addition to the dysfunction of pre-mRNA processing, as a pathogenic factor in SMA, recent studies identified ribosome biology and translation efficiency as key processes affected by SMN depletion, which may also contribute to MN degeneration and SMA pathogenesis 33 , 34 .…”

Ribosomal biogenesis as an emerging target of neurodevelopmental pathologies

“…; Tapia et al . ), translation efficiency for multiple RPs was particularly affected (Bernabo et al . ).…”

Interaction of modified oligonucleotides with nuclear proteins, formation of novel nuclear structures and sequence-independent effects on RNA processing

“…In addition, the aberrant inclusions that we speculate occupy interchromosomal 643 spaces may well impose physical constraints upon nuclear organisation, and prevent proper 644 localisation of Cajal bodies and the nucleoli close to their normal chromosomal sites. Tissues 645 with high demand for transcript splicing and ribosome biogenesis, and neurons in particular, 646 have prominent Cajal bodies, juxtaposed to nucleoli (for review see (Lafarga et al 2017 Not surprisingly, considering the altered nuclear architecture and distribution of proteins 668 implicated in RNA biology, transcriptome sequencing showed significant global effects on the 669 expression of transcripts and revealed disturbances to many critical cellular processes in 670 phosphorothioate AO transfected cells. Of particular concern, pathways involved in apoptosis, 671 chromatin silencing, cellular metabolism and a number of signalling pathways, autophagy and 672 nucleotide excision repair were disturbed.…”