Cell Polarity and Organelle Positioning

The highly reproducible response of the actin cytoskeleton to the topology of adhesive micro-patterns together with the sensitivity of the microtubule network dynamics  to cortical actin activity result into a highly reproducible and polarized cell internal organization. 

CYTOO micropatterns demonstrate that restricting the distribution of adhesions reduces the variability of cell internal organization (below the example of centrosome distribution) and the variability of mitotic spindle orientation.

Cell shape and mean positioning of centrosomes are identical on both types of adhesive  micro-patterns but centrosome distribution is much broader on fully adhesive micro-pattern on the right.

The self-centering properties of the microtuble network  maintain a central positioning of the centrosome in all cases. The centrosome-nucleus connection results in a nucleus positioning  off-centred towards non-adhesive and contractile cell edges, whereas the centrosome is facing adhesive edges. Nuclear positioning governs the positioning of endomembranes that extend from the nuclear enveloppe.  The distribution and positioning of endomembranes in the cytoplasm depends in turn on the centrosome-centered microtubule network and of direct interaction between the centrosome and the Golgi apparatus.

Thus, the orientation of the nucleus-centrosome-Golgi apparatus axis is governed by the actin-dependent cortical polarity in response to  the topology of adhesion. This defines an intrinsic  cell polarity in which the whole cell activity is polarized. 

A Reference Cell can be obtained for any type of micro-pattern. 

Example of adhesive control of organelle positioning and polarity in cells with similar shape

Another outcome of  micropatterns is that they demonstrate directly that cell adhesion governs not only the positioning but also the shape of all internal compartments.

For example the Golgi apparatus in cells cultured in classical conditions usually appears stretched and spread around the nucleus. Micropatterns demonstrate that this is due to extensive spreading of cells on unlimited substrate. Cells that are grown on micropatterns demonstrate a compact Golgi apparatus, concentric to the centrosome, similar to what is observed in non adherent lymphoid cells.

Golgi apparatus (in red) in RPE1 cells in standard culture conditions	Golgi apparatus (in red) in RPE1  cells on fibronectin micropatterns (in green)