Mechanical Force Alters Morphogenetic Movements and Segmental Gene Expression Patterns during Drosophila Embryogenesis

Abhishek Kumar,G V Shivashankar,Johannes Jaeger

doi:10.1371/journal.pone.0033089

Abstract

The development of an organism is accompanied by various cellular morphogenetic movements, changes in cellular as well as nuclear morphology and transcription programs. Recent evidence suggests that intra and inter-cellular connections mediated by various adhesion proteins contribute to defining nuclear morphology. In addition, three dimensional organization of the cell nucleus regulate the transcription programs. However the link between cellular morphogenetic movements and its coupling to nuclear function in a developmental context is poorly understood. In this paper we use a point perturbation by tissue level laser ablation and sheet perturbation by application of force using magnetic tweezers to alter cellular morphogenetic movements and probe its impact on nuclear morphology and segmental gene expression patterns. Mechanical perturbations during blastoderm stage in a developing Drosophila embryo resulted in localized alterations in nuclear morphology and cellular movement. In addition, global defects in germ-band (GB) extension and retraction are observed when external force is applied during morphogenetic movements, suggesting a long-range physical coupling within the GB layer of cells. Further local application of force resulted in redistribution of non muscle myosin-II in the GB layer. Finally these perturbations lead to altered segmental gene (engrailed) expression patterns later during the development. Our observations suggest that there exists a tight regulation between nuclear morphology and cellular adhesive connections during morphogenetic movement of cells in the embryo. The observed spatial changes in patterning genes, with perturbation, highlight the importance of nuclear integrity to cellular movement in establishing gene expression program in a developmental system.

Highlights

During early Drosophila embryogenesis, cells within an embryo are repositioned to different spatial regions in three dimensions and experience varied chemical gradients, established during cellular blastoderm stage by maternal proteins [1,2,3,4]
Cellularization begins which is followed by germ band extension (GBE) and the GB front reaches 2/3rd egg length
During GBE, centroid positions of the H2B-EGFP nuclei were tracked till the nuclei moved out of the plane of focus; Fig. 1D shows typical tracks of nuclei in an embryo, (Movie S2 -movie of tracks of individual nuclei and Fig. S1A, movie of tracks was obtained from time lapse images using ImageJ plugin - MtrackJ)

Summary

Introduction

During early Drosophila embryogenesis, cells within an embryo are repositioned to different spatial regions in three dimensions and experience varied chemical gradients, established during cellular blastoderm stage by maternal proteins [1,2,3,4]. In Drosophila melanogaster, GB invagination is one of the major morphogenetic movements for gastrulation, during which cells segregate into three germ layers - endoderm, mesoderm and ectoderm At this stage, movement of cells is highly coordinated in both space and time and segmental gene expression patterns emerge [8,9,10]. We propose that during morphogenetic movements there are changes in both cell shape and nuclear organization which might impinge on global transcription programs inside an organism In this context, the link between nuclear morphology and the emergence of segmental gene expression pattern in a developing embryo is poorly understood

Methods

Results

Conclusion