Asymmetric inheritance of the apical domain and self-renewal of retinal ganglion cell progenitors depend on Anillin function.

Divisions that generate one neuronal lineage-committed and one self-renewing cell maintain the balance of proliferation and differentiation for the generation of neuronal diversity. The asymmetric inheritance of apical domains and components of the cell division machinery has been implicated in this process, and might involve interactions with cell fate determinants in regulatory feedback loops of an as yet unknown nature. Here, we report the dynamics of Anillin - an essential F-actin regulator and furrow component - and its contribution to progenitor cell divisions in the developing zebrafish retina. We find that asymmetrically dividing retinal ganglion cell progenitors position the Anillin-rich midbody at the apical domain of the differentiating daughter. anillin hypomorphic conditions disrupt asymmetric apical domain inheritance and affect daughter cell fate. Consequently, the retinal cell type composition is profoundly affected, such that the ganglion cell layer is dramatically expanded. This study provides the first in vivo evidence for the requirement of Anillin during asymmetric neurogenic divisions. It also provides insights into a reciprocal regulation between Anillin and the ganglion cell fate determinant Ath5, suggesting a mechanism whereby the balance of proliferation and differentiation is accomplished during progenitor cell divisions in vivo.

Decoupling from yolk sac is required for extraembryonic tissue spreading in the scuttle fly Megaselia abdita.

Extraembryonic tissues contribute to animal development, which often entails spreading over embryo or yolk. Apart from changes in cell shape, the requirements for this tissue spreading are not well understood. Here, we analyze spreading of the extraembryonic serosa in the scuttle fly Megaselia abdita. The serosa forms from a columnar blastoderm anlage, becomes a squamous epithelium, and eventually spreads over the embryo proper. We describe the dynamics of this process in long-term, whole-embryo time-lapse recordings, demonstrating that free serosa spreading is preceded by a prolonged pause in tissue expansion. Closer examination of this pause reveals mechanical coupling to the underlying yolk sac, which is later released. We find mechanical coupling prolonged and serosa spreading impaired after knockdown of M. abdita Matrix metalloprotease 1. We conclude that tissue-tissue interactions provide a critical functional element to constrain spreading epithelia.

Folded gastrulation and T48 drive the evolution of coordinated mesoderm internalization in flies.

Gastrulation constitutes a fundamental yet diverse morphogenetic process of metazoan development. Modes of gastrulation range from stochastic translocation of individual cells to coordinated infolding of an epithelial sheet. How such morphogenetic differences are genetically encoded and whether they have provided specific developmental advantages is unclear. Here we identify two genes, folded gastrulation and t48, which in the evolution of fly gastrulation acted as a likely switch from an ingression of individual cells to the invagination of the blastoderm epithelium. Both genes are expressed and required for mesoderm invagination in the fruit fly Drosophila melanogaster but do not appear during mesoderm ingression of the midge Chironomus riparius. We demonstrate that early expression of either or both of these genes in C.riparius is sufficient to invoke mesoderm invagination similar to D.melanogaster. The possible genetic simplicity and a measurable increase in developmental robustness might explain repeated evolution of similar transitions in animal gastrulation.

Probing compressibility of the nuclear interior in wild-type and lamin deficient cells using microscopic imaging and computational modeling.

Mechanical properties of the cell nucleus play an important role in maintaining the integrity of the genome and controlling the cellular force balance. Irregularities in these properties have been related to disruption of a variety of force-dependent processes in the cell, such as migration, division, growth or differentiation. Characterizing mechanical properties of the cell nucleus in situ and relating these parameters to cellular phenotypes remain challenging tasks, as conventional micromanipulation techniques do not allow direct probing of intracellular structures. Here, we present a framework based on light microscopic imaging and automated mechanical modeling that enables characterization of the compressibility of the nuclear interior in situ. Based entirely on optical methods, our approach does not require application of destructive or contacting techniques and it enables measurements of a significantly larger number of cells. Compressibility, in this paper represented by Poisson's ratio ν, is determined by fitting a numerical model to experimentally observed time series of microscopic images of fluorescent cell nuclei in which bleached patterns are introduced. In a proof-of-principle study, this framework was applied to estimate ν in wild type cells and cells lacking important structural proteins of the nuclear envelope (LMNA(-/-)). Based on measurements of a large number of cells, our study revealed distinctive changes in compressibility of the nuclear interior between these two cell types. Our method allows an automated, contact-free estimation of mechanical properties of intracellular structures. Combined with knockdown and overexpression screens, it paves the way towards a high-throughput measurement of intracellular mechanical properties in functional phenotyping screens.