Epithelial rotation is preceded by planar symmetry breaking of actomyosin and protects epithelial tissue from cell deformations

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

A Low-Tech Flow Chamber for Live Imaging of Drosophila Egg Chambers During Drug Treatments

The Drosophila egg chamber is a powerful system to study epithelial cell collective migration and polarized basement membrane secretion. A strength of this system is the ability to capture these dynamic processes in ex vivo organ culture using high-resolution live imaging. Ex vivo culture also allows acute pharmacological or labeling treatments, extending the versatility of the system. However, many current ex vivo egg chamber culture setups do not permit easy medium exchange, preventing researchers from following individual egg chambers through multiple treatments. Here we present a method to immobilize egg chambers in an easy-to-construct flow chamber that permits imaging of the same egg chamber through repeated solution exchanges. This will allow researchers to take greater advantage of the wide variety of available pharmacological perturbations and other treatments like dyes to study dynamic processes in the egg chamber.

Electrochemical gradients are involved in regulating cytoskeletal patterns during epithelial morphogenesis in the Drosophila ovary

BACKGROUND

During Drosophila oogenesis, the follicular epithelium differentiates into several morphologically distinct follicle-cell populations. Characteristic bioelectrical properties make this tissue a suitable model system for studying connections between electrochemical signals and the organisation of the cytoskeleton. Recently, we have described stage-specific transcellular antero-posterior and dorso-ventral gradients of intracellular pH (pH_i) and membrane potential (V_mem) depending on the asymmetrical distribution and/or activity of various ion-transport mechanisms. In the present study, we analysed the patterns of basal microfilaments (bMF) and microtubules (MT) in relation to electrochemical signals.

RESULTS

The bMF- and MT-patterns in developmental stages 8 to 12 were visualised using labelled phalloidin and an antibody against acetylated α-tubulin as well as follicle-cell specific expression of GFP-actin and GFP-α-tubulin. Obviously, stage-specific changes of the pH_i- and V_mem-gradients correlate with modifications of the bMF- and MT-organisation. In order to test whether cytoskeletal modifications depend directly on bioelectrical changes, we used inhibitors of ion-transport mechanisms that have previously been shown to modify pH_i and V_mem as well as the respective gradients. We inhibited, in stage 10b, Na⁺/H⁺-exchangers and Na⁺-channels with amiloride, V-ATPases with bafilomycin, ATP-sensitive K⁺-channels with glibenclamide, voltage-dependent L-type Ca²⁺-channels with verapamil, Cl^--channels with 9-anthroic acid and Na⁺/K⁺/2Cl^--cotransporters with furosemide, respectively. The correlations between pH_i, V_mem, bMF and MT observed in different follicle-cell types are in line with the correlations resulting from the inhibition experiments. While relative alkalisation and/or hyperpolarisation stabilised the parallel transversal alignment of bMF, acidification led to increasing disorder and to condensations of bMF. On the other hand, relative acidification as well as hyperpolarisation stabilised the longitudinal orientation of MT, whereas alkalisation led to loss of this arrangement and to partial disintegration of MT.

CONCLUSIONS

We conclude that the pH_i- and V_mem-changes induced by inhibitors of ion-transport mechanisms simulate bioelectrical changes occurring naturally and leading to the cytoskeletal changes observed during differentiation of the follicle-cell epithelium. Therefore, gradual modifications of electrochemical signals can serve as physiological means to regulate cell and tissue architecture by modifying cytoskeletal patterns.

Extracellular matrix stiffness cues junctional remodeling for 3D tissue elongation

Organs are sculpted by extracellular as well as cell-intrinsic forces, but how collective cell dynamics are orchestrated in response to environmental cues is poorly understood. Here we apply advanced image analysis to reveal extracellular matrix-responsive cell behaviors that drive elongation of the Drosophila follicle, a model system in which basement membrane stiffness instructs three-dimensional tissue morphogenesis. Through in toto morphometric analyses of wild type and round egg mutants, we find that neither changes in average cell shape nor oriented cell division are required for appropriate organ shape. Instead, a major element is the reorientation of elongated cells at the follicle anterior. Polarized reorientation is regulated by mechanical cues from the basement membrane, which are transduced by the Src tyrosine kinase to alter junctional E-cadherin trafficking. This mechanosensitive cellular behavior represents a conserved mechanism that can elongate edgeless tubular epithelia in a process distinct from those that elongate bounded, planar epithelia.

The extracellular matrix can shape developing organs, but how external forces direct intercellular morphogenesis is unclear. Here, the authors use 3D imaging to show that elongation of the Drosophila egg chamber involves polarized cell reorientation signalled by changes in stiffness of the surrounding extracellular matrix.

Electrochemical patterns during Drosophila oogenesis: ion-transport mechanisms generate stage-specific gradients of pH and membrane potential in the follicle-cell epithelium

Background

Alterations of bioelectrical properties of cells and tissues are known to function as wide-ranging signals during development, regeneration and wound-healing in several species. The Drosophila follicle-cell epithelium provides an appropriate model system for studying the potential role of electrochemical signals, like intracellular pH (pH_i) and membrane potential (V_mem), during development. Therefore, we analysed stage-specific gradients of pH_i and V_mem as well as their dependence on specific ion-transport mechanisms.

Results

Using fluorescent indicators, we found distinct alterations of pH_i- and V_mem-patterns during stages 8 to 12 of oogenesis. To determine the roles of relevant ion-transport mechanisms in regulating pH_i and V_mem and in establishing stage-specific antero-posterior and dorso-ventral gradients, we used inhibitors of Na⁺/H⁺-exchangers and Na⁺-channels (amiloride), V-ATPases (bafilomycin), ATP-sensitive K⁺-channels (glibenclamide), voltage-dependent L-type Ca²⁺-channels (verapamil), Cl⁻-channels (9-anthroic acid) and Na⁺/K⁺/2Cl⁻-cotransporters (furosemide). Either pH_i or V_mem or both parameters were affected by each tested inhibitor. While the inhibition of Na⁺/H⁺-exchangers (NHE) and amiloride-sensitive Na⁺-channels or of V-ATPases resulted in relative acidification, inhibiting the other ion-transport mechanisms led to relative alkalisation. The most prominent effects on pH_i were obtained by inhibiting Na⁺/K⁺/2Cl⁻-cotransporters or ATP-sensitive K⁺-channels. V_mem was most efficiently hyperpolarised by inhibiting voltage-dependent L-type Ca²⁺-channels or ATP-sensitive K⁺-channels, whereas the impact of the other ion-transport mechanisms was smaller. In case of very prominent effects of inhibitors on pH_i and/or V_mem, we also found strong influences on the antero-posterior and dorso-ventral pH_i- and/or V_mem-gradients. For example, inhibiting ATP-sensitive K⁺-channels strongly enhanced both pH_i-gradients (increasing alkalisation) and reduced both V_mem-gradients (increasing hyperpolarisation). Similarly, inhibiting Na⁺/K⁺/2Cl⁻-cotransporters strongly enhanced both pH_i-gradients and reduced the antero-posterior V_mem-gradient. To minor extents, both pH_i-gradients were enhanced and both V_mem-gradients were reduced by inhibiting voltage-dependent L-type Ca²⁺-channels, whereas only both pH_i-gradients were reduced (increasing acidification) by inhibiting V-ATPases or NHE and Na⁺-channels.

Conclusions

Our data show that in the Drosophila follicle-cell epithelium stage-specific pH_i- and V_mem-gradients develop which result from the activity of several ion-transport mechanisms. These gradients are supposed to represent important bioelectrical cues during oogenesis, e.g., by serving as electrochemical prepatterns in modifying cell polarity and cytoskeletal organisation.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0192-x) contains supplementary material, which is available to authorized users.

Division orientation: disentangling shape and mechanical forces

Oriented cell divisions are essential for the generation of cell diversity and for tissue shaping during morphogenesis. Cells in tissues are mechanically linked to their neighbors, upon which they impose, and from which they experience, physical force. Recent work in multiple systems has revealed that tissue-level physical forces can influence the orientation of cell division. A long-standing question is whether forces are communicated to the spindle orienting machinery via cell shape or directly via mechanosensing intracellular machinery. In this article, we review the current evidence from diverse model systems that show spindles are oriented by tissue-level physical forces and evaluate current models and molecular mechanisms proposed to explain how the spindle orientation machinery responds to extrinsic force.

The dPix-Git complex is essential to coordinate epithelial morphogenesis and regulate myosin during Drosophila egg chamber development

How biochemical and mechanical information are integrated during tissue development is a central question in morphogenesis. In many biological systems, the PIX-GIT complex localises to focal adhesions and integrates both physical and chemical information. We used Drosophila melanogaster egg chamber formation to study the function of PIX and GIT orthologues (dPix and Git, respectively), and discovered a central role for this complex in controlling myosin activity and epithelial monolayering. We found that Git's focal adhesion targeting domain mediates basal localisation of this complex to filament structures and the leading edge of migrating cells. In the absence of dpix and git, tissue disruption is driven by contractile forces, as reduction of myosin activators restores egg production and morphology. Further, dpix and git mutant eggs closely phenocopy defects previously reported in pak mutant epithelia. Together, these results indicate that the dPix-Git complex controls egg chamber morphogenesis by controlling myosin contractility and Pak kinase downstream of focal adhesions.

The migration and fusion events related to ROCK activity strongly influence the morphology of chicken embryo intestinal organoids

The method of organoid culture has become a tool widely used in gastrointestinal research, but so far, the migration of organoids derived from gut epithelium and formed in 3D Matrigel matrix has not been reported and studied. The intestinal epithelial tissue derived from 19-day-old chicken embryo was cultured in Matrigel and the dynamic properties of the forming organoids were analyzed by time-lapse image analysis. It was observed that about one in ten organoids actively moved through the matrix, at a speed of 10-20 μm/h. Moreover, rotation was observed in the majority of organoids that did not migrate long distances. The fusion events took place between organoids, which collided during the movement or growth. In our previous paper, we showed that the presence of Toll-like receptor 4 ligand, Escherichia coli lipopolysaccharide (LPS, 1 μg/ml), increased the mean organoid diameter. Here, we confirm this result and demonstrate that the Rho-associated protein kinase (ROCK) inhibitor Y-27632 (10 μM) did not completely abolish organoid migration, but prevented the fusion events, in both LPS-treated and untreated cultures. In consequence, in the presence of Y-27632, the differences between cultures incubated with and without LPS were not visible. We conclude that migration and fusion of organoids may influence their morphology and suggest that these phenomena should be taken into account during the design of experimental settings.

Tissue tension and not interphase cell shape determines cell division orientation in the Drosophila follicular epithelium

We investigated the cell behaviors that drive morphogenesis of the Drosophila follicular epithelium during expansion and elongation of early-stage egg chambers. We found that cell division is not required for elongation of the early follicular epithelium, but drives the tissue toward optimal geometric packing. We examined the orientation of cell divisions with respect to the planar tissue axis and found a bias toward the primary direction of tissue expansion. However, interphase cell shapes demonstrate the opposite bias. Hertwig's rule, which holds that cell elongation determines division orientation, is therefore broken in this tissue. This observation cannot be explained by the anisotropic activity of the conserved Pins/Mud spindle-orienting machinery, which controls division orientation in the apical-basal axis and planar division orientation in other epithelial tissues. Rather, cortical tension at the apical surface translates into planar division orientation in a manner dependent on Canoe/Afadin, which links actomyosin to adherens junctions. These findings demonstrate that division orientation in different axes-apical-basal and planar-is controlled by distinct, independent mechanisms in a proliferating epithelium.