Controlling cell position in heterotypic spheroids: an experimental and computational demonstration

Solid tumors comprise a complex mixture of cells, and the biophysical determinants and consequences of their spatial organization are incompletely understood. We use spheroid tumor models and complementary computational models to predict and control spatial organization in aggregates composed of multiple cell types. We consider cell sorting in the context of available adhesion proteins and cellular contractility both experimentally and in an equilibrium Cellular Potts Model. Experimentally, we characterize the mechanisms of spheroid formation as being primarily cadherin- or integrin-driven. The primary compaction mediator for a given cell type is the major factor dictating preference for interior or exterior position within mixed aggregates. In particular, cadherin-deficient, invasion-competent cells tend to position towards the outside of aggregates, facilitating access to the extracellular matrix. Reducing actomyosin contractility has a differential effect on spheroid formation depending on the compaction mechanism. Inhibition of contractility has a significant stabilizing effect on cell-cell adhesions in integrin-driven aggregation and a mildly destabilizing effect in cadherin-based aggregation. This differential response can be recapitulated in a simple Cellular Potts Model and is exploited experimentally to statically control aggregate organization and dynamically rearrange cells in pre-formed aggregates. Sequestration of invasive cells in the interior of spheroids provides a physical barrier that reduces invasion in three-dimensional culture, revealing a potential strategy for containment of aggressive cancers.