Reorganization of Cajal bodies and nucleolar targeting of coilin in motor neurons of type I spinal muscular atrophy
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Type I spinal muscular atrophy (SMA) is an autosomal recessive disorder caused by loss or mutations of the survival motor neuron 1 (SMN1) gene. The reduction in SMN protein levels in SMA leads to degeneration and death of motor neurons. In this study, we have analyzed the nuclear reorganization of Cajal bodies, PML bodies and nucleoli in type I SMA motor neurons with homozygous deletion of exons 7 and 8 of the SMN1 gene. Western blot analysis revealed a marked reduction of SMN levels compared to the control sample. Using a neuronal dissociation procedure to perform a careful immunocytochemical and quantitative analysis of nuclear bodies, we demonstrated a severe decrease in the mean number of Cajal bodies per neuron and in the proportion of motor neurons containing these structures in type I SMA. Moreover, most Cajal bodies fail to recruit SMN and spliceosomal snRNPs, but contain the proteasome activator PA28γ, a molecular marker associated with the cellular stress response. Neuronal stress in SMA motor neurons also increases PML body number. The existence of chromatolysis and eccentric nuclei in SMA motor neurons correlates with Cajal body disruption and nucleolar relocalization of coilin, a Cajal body marker. Our results indicate that the Cajal body is a pathophysiological target in type I SMA motor neurons. They also suggest the Cajal body-dependent dysfunction of snRNP biogenesis and, therefore, pre-mRNA splicing in these neurons seems to be an essential component for SMA pathogenesis.
KeywordsSpinal muscular atrophy Cajal bodies “Gems” PML bodies Motor neurons Pre-mRNA splicing disease
The authors wish to thank Saray Pereda and Raquel García-Ceballos for technical assistance. We would also like to thank Greg Matera for critical reading of this manuscript. This work was supported by the following grants: “Dirección General de Investigación” (BFU2008-00175 and BFU2011-23983), “Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas” (CIBERNED; CB06/05/0037) Spain, and Instituto de Salud Carlos III, Fondo de Investigación Sanitaria (08-0729) to EFT.
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