Institute for Clinical Neurobiology, Zinklesweg 10, 97078 Würzburg, Germany.
Hum Mol Genet. 2010 May 15;19(10):1951-66. doi: 10.1093/hmg/ddq073. Epub 2010 Feb 18.
Axonal transport and translation of beta-actin mRNA plays an important role for axonal growth and presynaptic differentiation in many neurons including hippocampal, cortical and spinal motor neurons. Several beta-actin mRNA-binding and transport proteins have been identified, including ZBP1, ZBP2 and hnRNP-R. hnRNP-R has been found as an interaction partner of the survival motor neuron protein that is deficient in spinal muscular atrophy. Little is known about the function of hnRNP-R in axonal beta-actin translocation. hnRNP-R and beta-actin mRNA are colocalized in axons. Recombinant hnRNP-R interacts directly with the 3'-UTR of beta-actin mRNA. We studied the role of hnRNP-R in motor neurons by knockdown in zebrafish embryos and isolated mouse motor neurons. Suppression of hnRNP-R in developing zebrafish embryos results in reduced axon growth in spinal motor neurons, without any alteration in motor neuron survival. ShRNA-mediated knockdown in isolated embryonic mouse motor neurons reduces beta-actin mRNA translocation to the axonal growth cone, which is paralleled by reduced axon elongation. Dendrite growth and neuronal survival were not affected by hnRNP-R depletion in these neurons. The loss of beta-actin mRNA in axonal growth cones of hnRNP-R-depleted motor neurons resembles that observed in Smn-deficient motor neurons, a model for the human disease spinal muscular atrophy. In particular, hnRNP-R-depleted motor neurons also exhibit defects in presynaptic clustering of voltage-gated calcium channels. Our data suggest that hnRNP-R-mediated axonal beta-actin mRNA translocation plays an essential physiological role for axon growth and presynaptic differentiation.
β-肌动蛋白 mRNA 的轴突运输和翻译在许多神经元（包括海马、皮质和脊髓运动神经元）的轴突生长和突触前分化中起着重要作用。已经鉴定出几种β-肌动蛋白 mRNA 结合和运输蛋白，包括 ZBP1、ZBP2 和 hnRNP-R。hnRNP-R 已被发现是运动神经元存活蛋白的相互作用伙伴，而运动神经元存活蛋白在脊髓性肌萎缩症中缺失。hnRNP-R 在轴突β-肌动蛋白易位中的功能知之甚少。hnRNP-R 和β-肌动蛋白 mRNA 在轴突中共定位。重组 hnRNP-R 与β-肌动蛋白 mRNA 的 3'-UTR 直接相互作用。我们通过在斑马鱼胚胎和分离的小鼠运动神经元中敲低 hnRNP-R 来研究 hnRNP-R 在运动神经元中的作用。在发育中的斑马鱼胚胎中抑制 hnRNP-R 会导致脊髓运动神经元的轴突生长减少，而运动神经元的存活没有任何改变。在分离的胚胎小鼠运动神经元中，shRNA 介导的敲低会减少β-肌动蛋白 mRNA 向轴突生长锥的易位，这与轴突伸长减少相平行。在这些神经元中，hnRNP-R 耗竭不会影响树突生长和神经元存活。hnRNP-R 耗尽的运动神经元的轴突生长锥中β-肌动蛋白 mRNA 的丢失类似于 Smn 缺陷运动神经元（脊髓性肌萎缩症的人类疾病模型）中观察到的丢失。特别是，hnRNP-R 耗尽的运动神经元也表现出电压门控钙通道的突触前簇集缺陷。我们的数据表明，hnRNP-R 介导的轴突β-肌动蛋白 mRNA 易位对轴突生长和突触前分化起着至关重要的生理作用。
