However, in contrast to ALS, the number of Gems does not decrease in the spinal motor neurons in other motor neuron diseases.[34] Thus, in human spinal motor neurons, the nonspecific alteration of Gems resulting from the suppression of transcriptional activity is less likely. Therefore, we speculate that the alteration of TDP-43 directly decreases the number of Gems. Another important question is how TDP-43 is associated with the formation of Gems. Two hypotheses have been proposed for the formation of nuclear bodies: (i) ordered assembly

buy CT99021 of the component proteins; or (ii) stochastic assembly, in which component proteins accumulate in an unordered manner at specific RNA or the complex of core proteins.[27-29, 44, 45] Although the process of how nuclear bodies are formed remains unclear, there are several indispensable FK506 price component

proteins in each body. Thus, two possible molecular mechanisms exist for decreasing the number of Gems by depletion of TDP-43: (i) the depletion of TDP-43 alters the mRNA of the component proteins of Gem; or (ii) TDP-43 directly contributes to the formation of Gems, such that its depletion results in fewer Gems. With regard to the first possibility, it has been reported that TDP-43 regulates the alternative splicing of SMN. The depletion of TDP-43 increased the SMN splicing variant excluding exon 7 in a reporter system.[46] The SMN excluding exon 7 is less stable than SMN with exon 7, resulting in less SMN product.[47] Indeed, we found that the amount of SMN proteins decreased due to the depletion of TDP-43.[34] However, we were unable to confirm the increase in the SMN splicing variants excluding exon 7 in intrinsic SMN mRNA by depletion of TDP-43. Instead of the alteration of splicing variants, we found that the SMN mRNA decreased in the

cells with depleted TDP-43, suggesting that the depletion of TDP-43 induces additional splicing, and the splicing isoform may be less stable than canonical SMN mRNA. However, we were unable to detect the additional splicing variants, which may contribute to the reduced amount of SMN mRNA. Moreover, researchers have not fully evaluated whether the SMN protein Aurora Kinase or mRNA are reduced in tissues affected with ALS.[48] Therefore, although the intrinsic SMN protein is reduced in cultured cells with the depletion of TDP-43, it is not clear that this is the mechanism underlying the reduction of SMN in tissue affected by ALS. Next, we hypothesized that TDP-43 binds to the component proteins of Gem and increases their stability. Indeed, the protein–protein interaction between TDP-43 and SMN has been reported in a forced expression system,[9, 37, 49] although the result is controversial.[34] In addition, comprehensive analysis of binding proteins to TDP-43 using mass spectrometry failed to identify SMN or other component proteins of Gem.