Disrupted endothelial cell-cell dynamics in ALK1 and SMAD4 deficiency drive arteriovenous malformations

Arteriovenous malformations (AVMs) are a hallmark of hereditary haemorrhagic telangiectasia (HHT) and arise from abnormal vascular remodelling. Although AVM formation has been associated with disruptions in the BMP9/10 signalling pathway, the distinct contributions of its components remain unclear. Here, we combine in vitro mosaic endothelial cell (EC) cultures with agent-based modelling (ABM) to investigate how knockdown of the BMP9/10 pathway components ALK1 and SMAD4 alters cell-cell interactions and collective vascular organisation. Using cell tracking data, we inferred the mechanical interactions between neighbouring ECs in 2D monolayers by applying Approximate Bayesian Computation to an ABM of cell migration. SMAD4 knockdown increased the motile forces generated by ECs, while ALK1 knockdown weakened the ability of cells to push apart and rearrange with their neighbours, both resulting in greater mixing and fluidity within the cell layer. When these altered interaction profiles were incorporated into an ABM of collective EC dynamics during vascular remodelling, they gave rise to distinct AVM-prone phenotypes. SMAD4-deficient populations exhibited local blockages as highly polarised cells migrated excessively, whereas ALK1-deficient populations disrupted vascular patterning by impairing coordinated polarity and neighbour separation, favouring flow reversal. Our findings suggest that canonical BMP9/10 pathway defects destabilise the biomechanical balance of endothelial interactions, leading to emergent collective behaviours that predispose vessels to AVM formation. Significance StatementArteriovenous malformations (AVMs) are abnormal connections between arteries and veins that disrupt blood flow and can cause stroke or life-threatening bleeding. They are a hallmark of hereditary haemorrhagic telangiectasia (HHT), a genetic vascular disorder, but the cellular events that initiate AVMs remain unclear. Using a combination of endothelial cell cultures and computational modelling, we show that loss of two key HHT-linked genes, ALK1 and SMAD4, disrupts the coordinated movement and interactions of endothelial cells. Although both deficiencies lead to AVM-like behaviours, they do so through distinct mechanisms. SMAD4 loss drives excessive collective migration, whereas ALK1 loss impairs polarity and neighbour separation. These findings provide a mechanistic framework for how genetic defects could contribute to cell-cell dynamics observed in vascular malformations.