Dynamics, scaling behavior, and control of nuclear wrinkling

The cell nucleus is enveloped by a complex membrane, whose wrinkling has been implicated in disease and cellular aging. The biophysical dynamics and spectral evolution of nuclear wrinkling during multicellular development remain poorly understood due to a lack of direct quantitative measurements. Here, we combine live-imaging experiments, theory, and simulations to characterize the onset and dynamics of nuclear wrinkling during egg development in the fruit fly, Drosophila melanogaster, when nurse cell nuclei increase in size and display stereotypical wrinkling behavior. A spectral analysis of three-dimensional high-resolution data from several hundred nuclei reveals a robust asymptotic power-law scaling of angular fluctuations consistent with renormalization and scaling predictions from a nonlinear elastic shell model. We further demonstrate that nuclear wrinkling can be reversed through osmotic shock and suppressed by microtubule disruption, providing tunable physical and biological control parameters for probing mechanical properties of the nuclear envelope. Our findings advance the biophysical understanding of nuclear membrane fluctuations during early multicellular development.Comment: Main text: 10 pages, 3 figures. SI: 19 pages, 10 figures, 1 tabl

Collective Growth in a Small Multicellular Structure

All living cells must regulate their size and shape. Studies of single cell systems have identified key signaling pathways and molecular actors that orchestrate these complex processes. Yet progress in our understanding of growth regulation in multicellular systems has lagged. This work establishes the Drosophila melanogaster egg chamber as a simple, yet powerful model system for mechanistic studies of collective growth phenomena in a multicellular context. We found that throughout oogenesis, both tissues comprising the Drosophila egg chamber grow dramatically, through cell division and without, and do so nonuniformly. In the germline cluster, 16 cells diverge in size in a predictable manner due to asymmetric transport through arrested cleavage furrows known as ring canals. In the overlying somatic epithelium, it is clone sizes that diverge. In this tissue, cell of a lineage also remain connected through ring canals that facilitate diffusion, and clones grow through propagation of local mitotic waves. Both tissues grow jointly and exhibit collective growth phenomena at the tissue-level, yet both can be visualized and studied mechanistically, with single cell resolution. We also report on findings that are tangentially related to growth regulation, but arose naturally from studying ovarian development in Drosophila. We have found that the three-dimensional arrangement of the 16 cells in the germline cluster within its epithelial enclosure is highly diverse, and that entropic constraints favor particular configurations. Furthermore, our studies of oocyte selection reveal that the early localization and autoregulatory activity of a particular oocyte fate determinant are critical for cell fate specification and maintenance. In all projects, we combined experiments, microscopy and image processing, and theory to address basic biological questions in a highly tractable experimental system. Our work adds to the expanding repertoire of known mechanisms and model systems in the realm of growth regulation, and brings the field a step closer to a more quantitative and predictive understanding of these processes in a uniquely suited multicellular experimental system

ZnUMBA Crosses the Border

Breaks in tight junctions cause transient tissue leaks. In this issue of Developmental Cell, Stephenson et al. (2019) show that local RhoA activation and actomyosin contractions concentrate tight junction proteins at the breach, which repairs the leak

Fusome topology and inheritance during insect gametogenesis

From insects to mammals, oocytes and sperm develop within germline cysts comprising cells connected by intercellular bridges (ICBs). In numerous insects, formation of the cyst is accompanied by growth of the fusome—a membranous organelle that permeates the cyst. Fusome composition and function are best understood in Drosophila melanogaster: during oogenesis, the fusome dictates cyst topology and size and facilitates oocyte selection, while during spermatogenesis, the fusome synchronizes the cyst’s response to DNA damage. Despite its distinct and sex-specific roles during insect gametogenesis, elucidating fusome growth and inheritance in females and its structure and connectivity in males has remained challenging. Here, we take advantage of advances in three-dimensional (3D) confocal microscopy and computational image processing tools to reconstruct the topology, growth, and distribution of the fusome in both sexes. In females, our experimental findings inform a theoretical model for fusome assembly and inheritance and suggest that oocyte selection proceeds through an ‘equivalency with a bias’ mechanism. In males, we find that cell divisions can deviate from the maximally branched pattern observed in females, leading to greater topological variability. Our work consolidates existing disjointed experimental observations and contributes a readily generalizable computational approach for quantitative studies of gametogenesis within and across species. Author summary The ubiquity of germline cysts across animals and accelerating advances in microscopy call for quantitative and highly resolved studies of their developmental dynamics. Here we use Drosophila melanogaster gametogenesis as a model system, alongside a supervised learning algorithm to study a shared organelle that arises during sperm and oocyte development—the fusome. The fusome is a highly specialized membranous organelle that permeates the cyst in both sexes. Our three-dimensional (3D) reconstructions of the fusome and quantitative measurements at successive stages of cyst development during oogenesis shed light on the evolution of cell fate asymmetry within the germline cyst in females, where the cyst gives rise to a single oocyte. In males, where each cell of the cyst goes on to form sperm, the fusome fragments and exhibits topologies that deviate from the stereotypic maximally branched topology found in females. Our findings can be interpreted in the context of the divergent outcomes of gametogenesis in both sexes and highlight the centrality of quantitative measurements in evaluating hypotheses in biological sciences

Dynamics of hydraulic and contractile wave-mediated fluid transport during Drosophila oogenesis

© 2021 National Academy of Sciences. All rights reserved. From insects to mice, oocytes develop within cysts alongside nurse-like sister germ cells. Prior to fertilization, the nurse cells’ cytoplasmic contents are transported into the oocyte, which grows as its sister cells regress and die. Although critical for fertility, the biological and physical mechanisms underlying this transport process are poorly understood. Here, we combined live imaging of germline cysts, genetic perturbations, and mathematical modeling to investigate the dynamics and mechanisms that enable directional and complete cytoplasmic transport in Drosophila melanogaster egg chambers. We discovered that during “nurse cell (NC) dumping” most cytoplasm is transported into the oocyte independently of changes in myosin-II contractility, with dynamics instead explained by an effective Young–Laplace law, suggesting hydraulic transport induced by baseline cell-surface tension. A minimal flow-network model inspired by the famous two-balloon experiment and motivated by genetic analysis of a myosin mutant correctly predicts the directionality, intercellular pattern, and time scale of transport. Long thought to trigger transport through “squeezing,” changes in actomyosin contractility are required only once NC volume has become comparable to nuclear volume, in the form of surface contractile waves that drive NC dumping to completion. Our work thus demonstrates how biological and physical mechanisms cooperate to enable a critical developmental process that, until now, was thought to be mainly biochemically regulated

Mapping parameter spaces of biological switches.

Since the seminal 1961 paper of Monod and Jacob, mathematical models of biomolecular circuits have guided our understanding of cell regulation. Model-based exploration of the functional capabilities of any given circuit requires systematic mapping of multidimensional spaces of model parameters. Despite significant advances in computational dynamical systems approaches, this analysis remains a nontrivial task. Here, we use a nonlinear system of ordinary differential equations to model oocyte selection in Drosophila, a robust symmetry-breaking event that relies on autoregulatory localization of oocyte-specification factors. By applying an algorithmic approach that implements symbolic computation and topological methods, we enumerate all phase portraits of stable steady states in the limit when nonlinear regulatory interactions become discrete switches. Leveraging this initial exact partitioning and further using numerical exploration, we locate parameter regions that are dense in purely asymmetric steady states when the nonlinearities are not infinitely sharp, enabling systematic identification of parameter regions that correspond to robust oocyte selection. This framework can be generalized to map the full parameter spaces in a broad class of models involving biological switches

Entropic effects in cell lineage tree packings

Optimal packings [1, 2] of unconnected objects have been studied for centuries [3–6], but the packing principles of linked objects, such as topologically complex polymers [7, 8] or cell lineages [9, 10], are yet to be fully explored. Here, we identify and investigate a generic class of geometrically frustrated tree packing problems, arising during the initial stages of animal development when interconnected cells assemble within a convex enclosure [10]. Using a combination of 3D imaging, computational image analysis, and mathematical modelling, we study the tree packing problem in Drosophila egg chambers, where 16 germline cells are linked by cytoplasmic bridges to form a branched tree. Our imaging data reveal non-uniformly distributed tree packings, in agreement with predictions from energy-based computations. This departure from uniformity is entropic and affects cell organization during the first stages of the animal’s development. Considering mathematical models of increasing complexity, we investigate spherically confined tree packing problems on convex polyhedrons [11] that generalize Platonic and Archimedean solids. Our experimental and theoretical results provide a basis for understanding the principles that govern positional ordering in linked multicellular structures, with implications for tissue organization and dynamics [12, 13]