Actin-driven transport of chromosomes

In meiotic oocytes microtubules have similar lengths to those in dividing somatic cells. These microtubules are able to capture chromosomes from within a range of approximately 30 μm that is sufficient to reliably collect all chromosomes in a somatic cell. However, these microtubules are short relative to the 180 μm diameter of the starfish oocyte, in which individual chromosomes can be located as far as 80 μm away from each other. Indeed, we could show that microtubules alone are not able to capture distal chromosomes in the large oocyte. Thus, we searched for the mechanism moving chromosomes that led us to the discovery of a novel, actin-driven mechanism that functions to transport chromosomes to within the 30 μm capture range of microtubules (Lenart et al., 2005).

chromosomes in the actin network

To understand how actin drives chromosome transport, we established conditions for imaging actin filaments and chromosomes at high spatial and temporal resolution in live oocytes, and quantitatively analyzed filament dynamics and chromosome motion in these image series. This revealed an extensive network of actin filaments that forms in the nuclear region after the breakdown of the nuclear envelope. Once the network forms, it begins to contract. We showed that the contraction itself is isotropic, but the network connects to the cell cortex that serve as localized anchors. These anchors provide directionality to the contraction, moving the network towards the cell cortex. The contraction towards the cortex transports chromosomes at least in part by passive sieving: chromosomes are larger than the mesh size and therefore are captured by the contractile actin mesh. This we showed by injecting inert beads of comparable dimensions to chromosomes that are gathered in with a similar efficiency to chromosomes. Taken together, we could reveal the mechanistic design principles of a novel and potentially versatile mode of long-range, directed intracellular transport that is based on sieving by an anchored, homogeneously contracting actin network (Mori et al., 2011).

Currently, we are developing methods for visualizing the actin filament network at even higher resolution in order to quantitatively analyze and thereby understand its structural organization. Secondly, we are using biochemical methods to identify the molecular components, such as motor proteins and filament nucleation factors, that are involved in this contractile mechanism.

 

The mechanism of polar body extrusion

Additionally, we have recently started working on the mechanism of the extremely asymmetric division of oocytes, referred to as polar body extrusion. We are interested in how conserved mechanisms of cytokinesis adapted to this very unusual geometry, and/or whether specific mechanisms are required to mediate this specialized form of cell division.

polarbody

 

Starfish

starfish

 

We are using the oocytes of bat stars (Patiria miniata) as experimental model system. This species is common along the West Coast of the United States. We obtain the animals once or twice a year from providers in California, the Southern California Sea Urchin Co., Marinus Scientific or Monterey Abalone, dependent on the season. We keep the animals at EMBL's marine facility in temperature, light, pH and salinity controlled aquariums supplied with natural sea water from the North Sea. The facility provides us with experimental material year around.