Nucleolar Disruption and Cajal Body Disassembly are Nuclear Hallmarks of DNA Damage‐Induced Neurodegeneration in Purkinje Cells

Abstract

The Purkinje cell (PC) degeneration (pcd) phenotype results from mutation in nna1 gene and is associated with the degeneration and death of PCs during the postnatal life. Although the pcd mutation is a model of the ataxic mouse, it shares clinical and pathological characteristics of inherited human spinocerebellar ataxias. PC degeneration in pcd mice provides a useful neuronal system to study nuclear mechanisms involved in DNA damage-dependent neurodegeneration, particularly the contribution of nucleoli and Cajal bodies (CBs). Both nuclear structures are engaged in housekeeping functions for neuronal survival, the biogenesis of ribosomes and the maturation of snRNPs and snoRNPs required for pre-mRNA and pre-rRNA processing, respectively. In this study, we use ultrastructural analysis, in situ transcription assay and molecular markers for DNA damage, nucleoli and CB components to demonstrate that PC degeneration involves the progressive accumulation of nuclear DNA damage associated with disruption of nucleoli and CBs, disassembly of polyribosomes into monoribosomes, ribophagy and shut down of nucleolar and extranucleolar transcription. Microarray analysis reveals that four genes encoding repressors of nucleolar rRNA synthesis (p53, Rb, PTEN and SNF2) are upregulated in the cerebellum of pcd mice. Collectively, these data support that nucleolar and CB alterations are hallmarks of DNA damage-induced neurodegeneration.

Purkinje Cell Degeneration in pcd Mice Reveals Large Scale Chromatin Reorganization and Gene Silencing Linked to Defective DNA Repair

“…As PC degeneration progressed, an increase in the ␥H2AX-positive nuclear domains was observed by immunofluorescence, as reported previously (24). The signal of this histone mark colocalized with the H4K20me3-positive heterochromatin domains (Fig.…”

“…In a previous study (24), we have demonstrated that the pcd mutation causes disruption of the nucleolus in PCs, with transcriptional inhibition of ribosomal genes and consequently a deficit in the biogenesis of ribosomes. Our present results indicate that the pcd mutation also induces the repression of protein-coding genes in extensive domains of the genome of PCs.…”

“…They include endoplasmic reticulum stress (20), decreased peptide turnover downstream from the proteasome (21), mitochondrial dysfunction and altered proteolytic processing of Nna1-interacting proteins (22), a dysfunction of microtubule stability (23), and disruption of nucleoli and Cajal bodies (24). Furthermore, our previous studies have demonstrated that the pcd mutation induces the formation of nuclear foci of DNA damage in both mitral and Purkinje cells (16,24). Moreover, defects in DNA repair pathways have recently emerged as a new fundamental pathology linked to several spinocerebellar ataxias, as well as other neurodegenerative disorders (9,(25)(26)(27).…”

Cajal bodies in neurons

“…76 A cellular hallmark of the disease is the progressive accumulation of DNA damage associated with a defective DNA repair, resulting in a severe inhibition of transcription of both ribosomal and protein-coding genes. 60 Gene silencing in Purkinje cells clearly correlates with progressive disruption of both nucleoli and CBs. 60 Interestingly, in addition to a severe loss of CBs, coilin was redistributed as a thin shell around the nucleolus (Fig.…”

“…31,32 Several studies have shown that CB number correlates with neuronal size and global transcriptional activity. 31,57,59,60 For example, sensory ganglion neurons include 3 cell-size categories related to their different transcriptional and bioelectrical activities: small, medium and large neurons. In rats, the mean CB number increases from 1.1 per cell in small neurons, to 1.8 and 2.9 in medium and large neurons, respectively, indicating a close relationship between cell body size, a parameter directly related to global transcriptional and translational activity of the neuron, 61 and CB formation.…”