Feng Lin, Wasser Martin
School of Computer Science and Engineering, Nanyang Technological University, Singapore, Singapore.
Imaging Informatics Division, Bioinformatics Institute (BII), Agency for Science, Technology and Research (A*STAR), Singapore, Singapore.
BMC Bioinformatics. 2017 Jul 10;18(1):329. doi: 10.1186/s12859-017-1739-0.
Many human muscle wasting diseases are associated with abnormal nuclear localization. During metamorphosis in Drosophila melanogaster, multi-nucleated larval dorsal abdominal muscles either undergo cell death or are remodeled to temporary adult muscles. Muscle remodeling is associated with anti-polar nuclear migration and atrophy during early pupation followed by polar migration and muscle growth during late pupation. Muscle remodeling is a useful model to study genes involved in myonuclear migration. Previously, we showed that loss of Cathepsin-L inhibited anti-polar movements, while knockdown of autophagy-related genes affected nuclear positioning along the medial axis in late metamorphosis.
To compare the phenotypic effects of gene perturbations on nuclear migration more objectively, we developed new descriptors of myonuclear distribution. To obtain nuclear pattern features, we designed an algorithm to detect and track nuclear regions inside live muscles. Nuclear tracks were used to distinguish between fast moving nuclei associated with fragments of dead muscles (sarcolytes) and slow-moving nuclei inside remodelled muscles. Nuclear spatial pattern features, such as longitudinal (lonNS) and lateral nuclear spread (latNS), allowed us to compare nuclear migration during muscle remodelling in different genetic backgrounds. Anti-polar migration leads to a lonNS decrease. As expected, lack of myonuclear migration caused by the loss of Cp1 was correlated with a significantly lower lonNS decrease. Unexpectedly, the decrease in lonNS was significantly enhanced by Atg9, Atg5 and Atg18 silencing, indicating that the loss of autophagy promotes the migration and clustering of nuclei. Loss of autophagy also caused a scattering of nuclei along the lateral axis, leading to a two-row as opposed to single row distribution in control muscles. Increased latNS resulting from knockdown of Atg9 and Atg18 was correlated with increased muscle diameter, suggesting that the wider muscle fibre promotes lateral displacement of nuclei from the medial axis during polar migration.
We developed new nuclear features to characterize the dynamics of nuclear distribution in time-lapse images of Drosophila metamorphosis. Image quantification improved our understanding of phenotypic abnormalities in nuclear distribution resulting from gene perturbations. Therefore, in vivo imaging and quantitative image analysis of Drosophila metamorphosis promise to provide novel insights into the relationship between muscle wasting and myonuclear positioning.
许多人类肌肉萎缩疾病与异常的核定位有关。在黑腹果蝇变态发育过程中，多核的幼虫腹部背侧肌肉要么经历细胞死亡，要么被重塑为临时的成虫肌肉。肌肉重塑与化蛹早期的反极核迁移和萎缩有关，随后在化蛹后期发生极向迁移和肌肉生长。肌肉重塑是研究参与肌核迁移的基因的有用模型。此前，我们发现组织蛋白酶-L的缺失会抑制反极运动，而自噬相关基因的敲低会影响变态发育后期核沿中轴线的定位。
为了更客观地比较基因扰动对核迁移的表型影响，我们开发了新的肌核分布描述符。为了获得核模式特征，我们设计了一种算法来检测和跟踪活体肌肉内的核区域。核轨迹用于区分与死肌（肌溶细胞）片段相关的快速移动的核和重塑肌肉内缓慢移动的核。核空间模式特征，如纵向（lonNS）和横向核扩散（latNS），使我们能够比较不同遗传背景下肌肉重塑过程中的核迁移。反极迁移导致lonNS降低。正如预期的那样，由Cp1缺失引起的肌核迁移缺乏与显著更低的lonNS降低相关。出乎意料的是，Atg9、Atg5和Atg18沉默显著增强了lonNS的降低，表明自噬的丧失促进了核的迁移和聚集。自噬的丧失还导致核沿横轴分散，导致与对照肌肉中的单排分布相反的双排分布。Atg9和Atg18敲低导致的latNS增加与肌肉直径增加相关，表明更宽的肌纤维在极向迁移过程中促进核从中轴线的横向位移。
我们开发了新的核特征来表征果蝇变态发育延时图像中核分布的动态。图像量化提高了我们对基因扰动导致的核分布表型异常的理解。因此，果蝇变态发育的体内成像和定量图像分析有望为肌肉萎缩与肌核定位之间的关系提供新的见解。
