Endothelial cell SMAD6 balances Alk1 function to regulate adherens junctions and hepatic vascular development

Area of Science:

• Vascular Biology
• Cell Signaling
• Developmental Biology

Background:

• Bone morphogenetic protein (BMP) signaling is vital for blood vessel formation and function.
• The precise mechanisms by which BMP pathway components regulate vascular development remain unclear.

Purpose of the Study:

• To investigate the role of inhibitory SMAD6 in endothelial cells during embryonic vascular development.
• To elucidate how SMAD6 modulates activin receptor-like kinase 1 (ALK1)-mediated signaling in endothelial cells.

Main Methods:

• In vivo studies using mouse models with genetic manipulation of Smad6 and Alk1 in endothelial cells.
• Cellular assays examining endothelial cell junctions, barrier function, and signaling pathways (PI3K, actomyosin contractility).

Main Results:

• SMAD6 acts as a negative regulator of ALK1 signaling in endothelial cells, preventing dysmorphogenesis and hemorrhage in the embryonic liver vasculature.
• Loss of SMAD6 leads to destabilized endothelial cell junctions and impaired barrier function, which can be rescued by reducing Alk1 gene dosage.
• Mechanistically, SMAD6 loss disrupts endothelial cell junctions by altering PI3K signaling and actomyosin contractility.

Conclusions:

• SMAD6 is essential for maintaining proper ALK1 signaling amplitude in endothelial cells, crucial for vascular integrity.
• ALK1 functions as a 'Goldilocks' pathway, requiring balanced signaling for normal vascular development, with SMAD6 acting as a key modulator.
• Dysregulation of this SMAD6-ALK1 axis contributes to vascular defects, highlighting its importance in developmental and potentially pathological angiogenesis.