Research & Funding
Introduction
Epithelial cells are the first morphologically distinct cell type to be formed during metazoan embryo development. We are interested in understanding the cellular mechanisms which regulate their organization and differentiation using Drosophila oogenesis and embryogenesis as a model system. An epithelial cell has a polarized plasma membrane with an apical domain containing microvilli and a lateral domain containing junctional proteins. Organelles are also present in discrete locations and our interest is to study the distribution and dynamics of mitochondria during differentiation. The evolution of epithelial cells is synonymous with organ system formation.
Figure 1: The epithelial cell contains an apical domain with microvilli and a lateral domain with various junctional complexes such as the subapical complex, the adherens complex and the septate junction.
The Model System
We use Drosophila stem cell differentiation and embryogenesis as model systems to understand the cellular mechanisms dictating the determination and development of epithelial cells.
Oogenesis gives rise to the oocyte which when fertilized by sperm develops into an embryo. In Drosophila, the germ line stem divides 4 times to form 16 cells, one of which gets determined to form the oocyte. The oocyte moves towards the posterior and increases in size while obtaining nutrients from the nurse cells and the follicle cells. Nurse cells and follicle cells are responsible for the polarization of the oocyte and formation of the dorsoventral and the anteroposterior axis of the oocyte and the future embryo. The follicle cells are epithelial in nature and differentiate into anterior, posterior and main body cells with the help of patterning cues such as Notch, EGFR and JAK-Stat signaling. We have found that mitochondrial morphology is controlled during different stages of development and a perturbation of mitochondrial morphology affects differentiation of follicle cells. We are also using neuroblast differentiation as a model system to understand the role of mitochondrial fusion and fission proteins in regulating differentiation.
Figure 2: The posterior oocyte gets nutrients from the nurse and the follicle cells of an egg chamber.
Drosophila embryogenesis starts as a syncytium with nuclear division cycles 1-13 occurring without division of the cytoplasm. Complete cells are formed in the interphase of nuclear cycle 14 where the plasma membrane extends around each cell in a process called cellularization. As the name suggests, this process combines cell formation with polarization of the plasma membrane to form epithelial cells.
Recently we have found that polarized plasma membrane domains form even before cellularization during the syncytial division cycles. Nuclear division cycles 1-9 occur deep within the interior of the embryo. During cycle 10 the nuclei arrive at the periphery and cycle 10-13 occur just beneath the plasma membrane. The plasma membrane surrounds each nucleus incompletely and has an apical domain containing microvilli and a lateral domain in between nuclei containing cytoskeletal remodeling, junctional and membrane trafficking proteins. These domains show restricted diffusion of proteins in the membrane surrounding one nucleus and do not spread into the adjacent domain.
Figure 3: The syncytial Drosophila embryo contains nuclei in a single layer at the periphery incompletely surrounded by polarized plasma membrane domains.
Methods
1. Genetics: We use the powerful tools of Drosophila genetics to manipulate developmental signaling pathways to assess the mechanisms that dictate cellular dynamics during oogenesis and embryogenesis.
2. Live imaging: We use fluorescently tagged transgenes for visualizing dynamics of proteins during development.
3. Photobleaching and photoactivation: We are interesting in assessing distribution of proteins during processes in embryo development by further using photobleaching and photoactivation tools.
Current Questions of Interest
Function of Bar domain proteins in plasma membrane organization and remodelling in the embryogenesis.
Actin cytoskeletal remodeling and its role in plasma membrane polarity in the syncytial blastoderm embryo.
Cytoplasmic organization during Drosophila embryogenesis, in collaboration with Amitabha Nandi and Mithun Mitra at Physics, IIT, Mumbai.
Mitochondrial organization and function in embryogenesis.
Mitochondrial morphology in stem cell differentiation during oogenesis.
Mitochondrial morphology in Drosophila neuroblast differentiation.
Funding
We are funded by the Department of Biotechnology (DBT), Department of Science and Engineering (DST) and the Wellcome Trust DBT India Alliance. I am a Wellcome Trust DBT India Alliance Senior Fellow.