RESUMO
Degenerated motor neurons in the spinal cord are the pathological hallmark of spinal muscular atrophy (SMA). SMA is caused by mutations in the ubiquitously expressed survival motor neuron 1 (SMN1) gene, which lead to reduced levels of functional SMN protein. Many different functions have been assigned to SMN, including assembly of ribonucleoproteins (RNPs), splicing, transcription and axonal mRNA transport. Recently, tissue from SMA patients and animal models has been used to determine which function of SMN is affected in SMA patients. A surprising picture has emerged: the impaired assembly of RNP subunits of the spliceosome seems to be responsible for SMA pathogenesis. Here, we present a model of how this defect might cause motor-neuron degeneration and consider potential therapies.
Assuntos
Proteína de Ligação ao Elemento de Resposta ao AMP Cíclico , Atrofia Muscular Espinal , Proteínas do Tecido Nervoso , Proteínas de Ligação a RNA , Ribonucleoproteínas/metabolismo , Processamento Alternativo , Animais , Proteína de Ligação ao Elemento de Resposta ao AMP Cíclico/genética , Proteína de Ligação ao Elemento de Resposta ao AMP Cíclico/metabolismo , Humanos , Neurônios Motores/citologia , Neurônios Motores/metabolismo , Neurônios Motores/patologia , Atrofia Muscular Espinal/genética , Atrofia Muscular Espinal/patologia , Atrofia Muscular Espinal/fisiopatologia , Atrofia Muscular Espinal/terapia , Proteínas do Tecido Nervoso/genética , Proteínas do Tecido Nervoso/metabolismo , RNA Mensageiro/metabolismo , Proteínas de Ligação a RNA/genética , Proteínas de Ligação a RNA/metabolismo , Ribonucleoproteínas/genética , Proteínas do Complexo SMN , Proteína 1 de Sobrevivência do Neurônio MotorRESUMO
Spinal muscular atrophy (SMA) is a motoneuron disease caused by reduced levels of survival motoneuron (SMN) protein. Previous studies have assigned SMN to uridine-rich small nuclear ribonucleoprotein particle (U snRNP) assembly, splicing, transcription, and RNA localization. Here, we have used gene silencing to assess the effect of SMN protein deficiency on U snRNP metabolism in living cells and organisms. In HeLa cells, we show that reduction of SMN to levels found in SMA patients impairs U snRNP assembly. In line with this, induced silencing of SMN expression in Xenopus laevis or zebrafish arrested embryonic development. Under less severe knock-down conditions, zebrafish embryos proceeded through development yet exhibited dramatic SMA-like motor axon degeneration. The same was observed after silencing two other essential factors in the U snRNP assembly pathway, Gemin2 and pICln. Importantly, the injection of purified U snRNPs into either SMN- or Gemin2-deficient embryos of Xenopus and zebrafish prevented developmental arrest and motoneuron degeneration, respectively. These findings suggest that motoneuron degeneration in SMA patients is a direct consequence of impaired production of U snRNPs.
Assuntos
Atrofia Muscular Espinal/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Interferência de RNA , Ribonucleoproteínas Nucleares Pequenas/metabolismo , Peixe-Zebra/embriologia , Animais , Axônios/metabolismo , Axônios/patologia , Modelos Animais de Doenças , Embrião não Mamífero/embriologia , Embrião não Mamífero/patologia , Fibroblastos/metabolismo , Células HeLa , Humanos , Atrofia Muscular Espinal/genética , Atrofia Muscular Espinal/patologia , Proteínas do Tecido Nervoso/genética , Proteínas de Ligação a RNA , Ribonucleoproteínas Nucleares Pequenas/genética , Xenopus laevis , Peixe-Zebra/genéticaRESUMO
Although many RNA-protein complexes or ribonucleoproteins (RNPs) assemble spontaneously in vitro, little is known about how they form in the environment of a living cell. Insight into RNP assembly has come unexpectedly from functional analyses of the survival motor neuron (SMN) protein, a gene product that is affected in the neuromuscular disease spinal muscular atrophy. These studies show that the assembly of spliceosomal U-rich small nuclear RNPs in vivo depends on the activity of two large protein complexes, one of which contains the SMN protein. These complexes might also facilitate the assembly of other cellular RNPs.