lethal giant larvae is required with the par genes for the early polarization of the Drosophila oocyte

A Progressive Somatic Cell Niche Regulates Germline Cyst Differentiation in the Drosophila Ovary

In the germarium of the Drosophila ovary, developing germline cysts are surrounded by a population of somatic escort cells that are known to function as the niche cells for germline differentiation;¹ however, the underlying molecular mechanisms of this niche function remain poorly understood. Through single-cell gene expression profiling combined with genetic analyses, we here demonstrate that the escort cells can be spatially and functionally divided into two successive domains. The anterior escort cells (aECs) specifically produce ecdysone, which acts on the cystoblast to promote synchronous cell division, whereas the posterior escort cells (pECs) respond to ecdysone signaling and regulate soma-germline cell adhesion to promote the transition from 16-cell cyst-to-egg chamber formation. The patterning of the aEC and pEC domains is independent of the germline but is dependent on JAK/STAT signaling activity, which emanates from the posterior. Thus, a heterogeneous population of escort cells constitutes a stepwise niche environment to orchestrate cystoblast division and differentiation toward egg chamber formation.

Collective Cell Sorting Requires Contractile Cortical Waves in Germline Cells

Encapsulation of germline cells by layers of somatic cells forms the basic unit of female reproduction called primordial follicles in mammals and egg chambers in Drosophila. How germline and somatic tissues are coordinated for the morphogenesis of each separated unit remains poorly understood. Here, using improved live imaging of Drosophila ovaries, we uncovered periodic actomyosin waves at the cortex of germ cells. These contractile waves are associated with pressure release blebs, which project from germ cells into somatic cells. We demonstrate that these cortical activities, together with cadherin-based adhesion, are required to sort each germline cyst as one collective unit. Genetic perturbations of cortical contractility, bleb protrusion, or adhesion between germline and somatic cells induced encapsulation defects resulting from failures to encapsulate any germ cells, or the inclusion of too many germ cells per egg chamber, or even the mechanical split of germline cysts. Live-imaging experiments revealed that reducing contractility or adhesion in the germline reduced the stiffness of germline cysts and their proper anchoring to the somatic cells. Germline cysts can then be squeezed and passively pushed by constricting surrounding somatic cells, resulting in cyst splitting and cyst collisions during encapsulation. Increasing germline cysts activity or blocking somatic cell constriction movements can reveal active forward migration of germline cysts. Our results show that germ cells play an active role in physical coupling with somatic cells to produce the female gamete.

Lgl cortical dynamics are independent of binding to the Scrib-Dlg complex but require Dlg-dependent restriction of aPKC

Apical-basal polarity underpins the formation of epithelial barriers that are crucial for metazoan physiology. Although apical-basal polarity is long known to require the basolateral determinants Lethal Giant Larvae (Lgl), Discs Large (Dlg) and Scribble (Scrib), mechanistic understanding of their function is limited. Lgl plays a role as an aPKC inhibitor, but it remains unclear whether Lgl also forms complexes with Dlg or Scrib. Using fluorescence recovery after photobleaching, we show that Lgl does not form immobile complexes at the lateral domain of Drosophila follicle cells. Optogenetic depletion of plasma membrane PIP₂ or dlg mutants accelerate Lgl cortical dynamics. However, Dlg and Scrib are required only for Lgl localization and dynamic behavior in the presence of aPKC function. Furthermore, light-induced oligomerization of basolateral proteins indicates that Lgl is not part of the Scrib-Dlg complex in the follicular epithelium. Thus, Scrib and Dlg are necessary to repress aPKC activity in the lateral domain but do not provide cortical binding sites for Lgl. Our work therefore highlights that Lgl does not act in a complex but in parallel with Scrib-Dlg to antagonize apical determinants.

Subcellular Specialization and Organelle Behavior in Germ Cells

Gametes, eggs and sperm, are the highly specialized cell types on which the development of new life solely depends. Although all cells share essential organelles, such as the ER (endoplasmic reticulum), Golgi, mitochondria, and centrosomes, germ cells display unique regulation and behavior of organelles during gametogenesis. These germ cell-specific functions of organelles serve critical roles in successful gamete production. In this chapter, I will review the behaviors and roles of organelles during germ cell differentiation.

The lethal giant larvae gene in Tribolium castaneum: molecular properties and roles in larval and pupal development as revealed by RNA interference

We identified and characterized the TcLgl gene putatively encoding lethal giant larvae (Lgl) protein from the red flour beetle (Tribolium castaneum). Analyses of developmental stage and tissue-specific expression patterns revealed that TcLgl was constitutively expressed. To examine the role of TcLgl in insect development, RNA interference was performed in early (1-day) larvae, late (20-day) larvae, and early (1-day) pupae. The early larvae injected with double-stranded RNA of TcLgl (dsTcLgl) at 100, 200, and 400 ng/larva failed to pupate, and 100% mortality was achieved within 20 days after the injection or before the pupation. The late larvae injected with dsTcLgl at these doses reduced the pupation rates to only 50.3%, 36.0%, and 18.2%, respectively. The un-pupated larvae gradually died after one week, and visually unaffected pupae failed to emerge into adults and died during the pupal stage. Similarly, when early pupae were injected with dsTcLgl at these doses, the normal eclosion rates were reduced to only 22.5%, 18.0%, and 11.2%, respectively, on day 7 after the injection, and all the adults with abnormal eclosion died in two days after the eclosion. These results indicate that TcLgl plays an essential role in insect development, especially during their metamorphosis.

Growing microtubules push the oocyte nucleus to polarize the Drosophila dorsal-ventral axis

The Drosophila dorsal-ventral (DV) axis is polarized when the oocyte nucleus migrates from the posterior to the anterior margin of the oocyte. Prior work suggested that dynein pulls the nucleus to the anterior side along a polarized microtubule cytoskeleton, but this mechanism has not been tested. By imaging live oocytes, we find that the nucleus migrates with a posterior indentation that correlates with its direction of movement. Furthermore, both nuclear movement and the indentation depend on microtubule polymerization from centrosomes behind the nucleus. Thus, the nucleus is not pulled to the anterior but is pushed by the force exerted by growing microtubules. Nuclear migration and DV axis formation therefore depend on centrosome positioning early in oogenesis and are independent of anterior-posterior axis formation.

Poly(ADP-ribose) controls DE-cadherin-dependent stem cell maintenance and oocyte localization

Within the short span of the cell cycle, poly(ADP-ribose) (pADPr) can be rapidly produced by poly(ADP-ribose) polymerases and degraded by poly(ADP-ribose) glycohydrolases. Here we show that changes in association between pADPr and heterogeneous nuclear ribonucleoproteins (hnRNPs) regulate germline stem cell (GSC) maintenance and egg chamber polarity during oogenesis in Drosophila. The association of pADPr and Hrp38, an orthologue of human hnRNPA1, disrupts the interaction of Hrp38 with the 5'-untranslated region of DE-cadherin messenger RNA, thereby diminishing DE-cadherin translation in progenitor cells. Following the reduction of DE-cadherin level, GSCs leave the stem cell niche and differentiate. Defects in either pADPr catabolism or Hrp38 function cause a decrease in DE-cadherin translation, leading to a loss of GSCs and mislocalization of oocytes in the ovary. Taken together, our findings suggest that Hrp38 and its association with pADPr control GSC self-renewal and oocyte localization by regulating DE-cadherin translation.

Adherens junctions in C. elegans embryonic morphogenesis

Caenorhabditis elegans provides a simplified, in vivo model system in which to study adherens junctions (AJs) and their role in morphogenesis. The core AJ components-HMR-1/E-cadherin, HMP-2/β-catenin and HMP-1/α-catenin-were initially identified through genetic screens for mutants with body axis elongation defects. In early embryos, AJ proteins are found at sites of contact between blastomeres, and in epithelial cells AJ proteins localize to the multifaceted apical junction (CeAJ)-a single structure that combines the adhesive and barrier functions of vertebrate adherens and tight junctions. The apically localized polarity proteins PAR-3 and PAR-6 mediate formation and maturation of junctions, while the basolaterally localized regulator LET-413/Scribble ensures that junctions remain apically positioned. AJs promote robust adhesion between epithelial cells and provide mechanical resistance for the physical strains of morphogenesis. However, in contrast to vertebrates, C. elegans AJ proteins are not essential for general cell adhesion or for epithelial cell polarization. A combination of conserved and novel proteins localizes to the CeAJ and works together with AJ proteins to mediate adhesion.

The nucleoporin Seh1 forms a complex with Mio and serves an essential tissue-specific function in Drosophila oogenesis

The nuclear pore complex (NPC) mediates the transport of macromolecules between the nucleus and cytoplasm. Recent evidence indicates that structural nucleoporins, the building blocks of the NPC, have a variety of unanticipated cellular functions. Here, we report an unexpected tissue-specific requirement for the structural nucleoporin Seh1 during Drosophila oogenesis. Seh1 is a component of the Nup107-160 complex, the major structural subcomplex of the NPC. We demonstrate that Seh1 associates with the product of the missing oocyte (mio) gene. In Drosophila, mio regulates nuclear architecture and meiotic progression in early ovarian cysts. Like mio, seh1 has a crucial germline function during oogenesis. In both mio and seh1 mutant ovaries, a fraction of oocytes fail to maintain the meiotic cycle and develop as pseudo-nurse cells. Moreover, the accumulation of Mio protein is greatly diminished in the seh1 mutant background. Surprisingly, our characterization of a seh1 null allele indicates that, although required in the female germline, seh1 is dispensable for the development of somatic tissues. Our work represents the first examination of seh1 function within the context of a multicellular organism. In summary, our studies demonstrate that Mio is a novel interacting partner of the conserved nucleoporin Seh1 and add to the growing body of evidence that structural nucleoporins can have novel tissue-specific roles.

The C. elegans homolog of Drosophila Lethal giant larvae functions redundantly with PAR-2 to maintain polarity in the early embryo

Polarity is essential for generating cell diversity. The one-cell C. elegans embryo serves as a model for studying the establishment and maintenance of polarity. In the early embryo, a myosin II-dependent contraction of the cortical meshwork asymmetrically distributes the highly conserved PDZ proteins PAR-3 and PAR-6, as well as an atypical protein kinase C (PKC-3), to the anterior. The RING-finger protein PAR-2 becomes enriched on the posterior cortex and prevents these three proteins from returning to the posterior. In addition to the PAR proteins, other proteins are required for polarity in many metazoans. One example is the conserved Drosophila tumor-suppressor protein Lethal giant larvae (Lgl). In Drosophila and mammals, Lgl contributes to the maintenance of cell polarity and plays a role in asymmetric cell division. We have found that the C. elegans homolog of Lgl, LGL-1, has a role in polarity but is not essential. It localizes asymmetrically to the posterior of the early embryo in a PKC-3-dependent manner, and functions redundantly with PAR-2 to maintain polarity. Furthermore, overexpression of LGL-1 is sufficient to rescue loss of PAR-2 function. LGL-1 negatively regulates the accumulation of myosin (NMY-2) on the posterior cortex, representing a possible mechanism by which LGL-1 might contribute to polarity maintenance.