How cells undergo, cell division has fascinated many researchers around the world for over a century. Proper orientation and elongation of the mitotic spindle are critical processes for determining the correct positioning of the cytokinetic furrow and is thus essential for determining the relative size and spatial disposition of the resulting daughter cells during development. These processes further ensure that the cell fate determinants are appropriately segregated in the resulting daughter cells during asymmetric cell division including in the stem cells. In our laboratory, we combine multifaceted approaches of the cell and molecular biology to advance our current understanding in the field of spindle orientation and spindle elongation during mitotic progression in human cells and Caenorhabditis elegans embryos. 

How cells undergo, cell division has fascinated many researchers around the world for over a century. Proper orientation and elongation of the mitotic spindle are critical processes for determining the correct positioning of the cytokinetic furrow and is thus essential for determining the relative size and spatial disposition of the resulting daughter cells during development. These processes further ensure that the cell fate determinants are appropriately segregated in the resulting daughter cells during asymmetric cell division including in the stem cells. In our laboratory, we combine multifaceted approaches of the cell and molecular biology to advance our current understanding in the field of spindle orientation and spindle elongation during mitotic progression in human cells and Caenorhabditis elegans embryos.  

In metazoans, proper spindle orientation is often dictated by an evolutionarily conserved ternary complex (NuMA/ LGN/ Gαi1-3 in Homo sapiens, LIN-5/ GPR-1/2/Gα in C. elegans). This complex anchors the microtubule-dependent minus-end directed motor protein complex dynein at the cell cortex. Such cortically anchored dynein is thought to regulate spindle orientation by exerting a pulling force on the plus-end of astral microtubules. However, despite tremendous efforts in identifying these key components, the underlying mechanism by which these proteins impact the spindle orientation and spindle elongation in time and space during mitotic progression is not well understood. The aim of our research program is to decipher the relationship between mitotic progression and spindle behaviour. We are currently pursuing a couple of major aims to dissect spindle behaviour in time and space.

Uncovering the role of the plasma membrane lipids in mitotic spindle behaviour

Our recent work has revealed that cortical levels of NuMA and dynein increase as human cells transit from metaphase to anaphase, and that this increase is vital for proper spindle elongation during anaphase [Figure 1]. Moreover, we have demonstrated that cortical dynein enrichment during anaphase does not require Gαi1-3 and LGN, but is rather solely dependent on NuMA. Importantly, a novel piece of data which we obtained using several biochemical and cell-biological tools, indicates that NuMA directly interacts with PtIns(4)P [PIP] and PtIns (4,5)P2 [PIP2] phosphoinositides and that the cortical localization of NuMA during anaphase is dependent upon these membrane lipids [Figure 2]. Intriguingly, NuMA is also excluded from the equatorial region of the cell cortex during anaphase in a manner that depends on the centralspindlin complex CYK4/MKLP1. One of the major goal of our research is to unravel the mechanisms by which phosphoinositide-based lipids and centralspindlin complex affect spindlebehaviour in mammalian cells.

Identifying novel modulators of spindle positioning 

Because of its large size (50 μm) and amenability to experimental manipulations, C. elegansone-cell stage embryo is extremely useful for investigating metazoan spindle positioning. In the one-cell stage of C. elegans embryo, under the influence of polarity mitotic spindle is initially set-up in the centre, but is displaced towards the posterior during metaphase and anaphase, resulting in an asymmetric first cell division.

Analogous to the human cells, spindle orientatio in the one-cell stage of C. elegans embryo is dependent on the ternary complex (LIN5/GPR-1/2/Gα) and dynein. However, despite our basic understanding of the spindle positioning mechanisms in the C. elegans embryo, the nature of the factors influencing the cortical localization, as well as the activity of the ternary complex and dynein have not been addressed in a systematic manner. To identify novel factors involved in spindle positioning, we are utilizing cutting-edge genetic and proteomic approaches in C.elegans embryos to advance our understanding in this fascinating process. 

Furthermore, as defective spindle orientation can lead to several pathological disorders such as neurological diseases and tumorigenesis, it is anticipated that knowledge gathered from such studies may lead to the development of novel diagnostic and therapeutic tools for regenerative medicine, neurogenesis and oncology.