Cilia function as calcium-mediated mechanosensors that instruct left-right asymmetry

Abstract

The breaking of bilateral symmetry in most vertebrates is critically dependent upon the motile cilia of the embryonic left-right organizer (LRO), which generate a directional fluid flow; however, it remains unclear how this flow is sensed. Here, we demonstrated that immotile LRO cilia are mechanosensors for shear force using a methodological pipeline that combines optical tweezers, light sheet microscopy, and deep learning to permit in vivo analyses in zebrafish. Mechanical manipulation of immotile LRO cilia activated intraciliary calcium transients that required the cation channel Polycystin-2. Furthermore, mechanical force applied to LRO cilia was sufficient to rescue and reverse cardiac situs in zebrafish that lack motile cilia. Thus, LRO cilia are mechanosensitive cellular levers that convert biomechanical forces into calcium signals to instruct left-right asymmetry.

Going with the flow

In most vertebrates, left-right differences are specified during early embryogenesis by a small cluster of cells called the left-right organizer. Within this organizer, motile cilia move rapidly to create a leftward directional flow of extracellular fluid that is the first sign of a left-right difference, but how this flow is sensed and transduced into later molecular and anatomical left-right asymmetry has been unclear. Working with mouse embryos, Katoh et al . found that immotile cilia sense the mechanical force generated by the flow and suggest a biophysical mechanism by which the direction of the flow is sensed. Independently, working in zebrafish, Djenoune et al . used optical tweezers and live imaging to show that immotile cilia in the organizer function as mechanosensors that translate extracellular fluid flow into calcium signals. When motile cilia were paralyzed and normal flow stopped, mechanical manipulation of the cilia could rescue, or even reverse, left-right patterning. Thus, ciliary force sensing is necessary, sufficient, and instructive for embryonic laterality. —SMH

Abstract

Applying oscillatory force on cilia in zebrafish embryos reveals that they are mechanosensors shaping cardiac left-right asymmetry.