Our research for cellular mechanosensing and mechanotransduction consists of:
1. Platelet Mechanosensing and Activation
The platelet glycoprotein (GP) Ib-IX complex is the primary platelet mechanosensor. It senses blood flow through its engagement with the VWF A1 domain and transmits a signal into the platelet. Collaborating with Dr. Li, I recently identified a quasi-stable mechanosensitive domain (MSD) of approximately 60 residues between the macroglycopeptide region and the transmembrane helix of the GPIba subunit. The MSD unfolds at 5–20 pN when subjected to mechanical stretch by the engaged A1. Unfolding the MSD triggers receptor signaling and stimulates platelets. This new mechanosensing mechanism may have important implications for the platelet clearance mechanism and the pathogenesis of a number of bleeding disorders such as immune thrombocytopenia.
2. Endothelial Mechanosensing and Mechanotransduction
The second project on this theme is about the mechanism underlying endothelial mechanosensing and mechanotransduction, or how cells on the vascular wall sense and respond to mechanical stimuli from blood flow. The overarching goal of this program is to elucidate the molecular mechanisms of endothelial surface glycocalyx (ESG)–mediated mechanotransduction. ESG is a carbohydrate-rich layer found on the vascular endothelium that provides a multifunctional protective coating of the inner lumen of the vasculature. Composed of membrane glycoproteins, glycosaminoglycans, and proteoglycans, the ESG forms a bulky, matrix-like structure, serving critical functions in the mechanotransduction of blood flow, the maintenance of endothelial permeability, and the control of leukocyte adhesion and inflammation (14). Although it has been known for decades that ESG senses blood flow and transduces the mechanical signal into the endothelial production of nitric oxide (NO) — an essential signaling molecule to regulate vascular tone — the molecular pathways of ESG-mediated mechanotransduction, as well as the biophysical properties that enable ESG to serve as a mechanosensor and mechanotransducer, have not yet been discovered. To address this important gap in mechanobiology, we have developed a multiscale approach that includes single-molecule and single-cell force spectroscopy, fluorescence microscopy, parallel flow devices, and in vitro primary cell cultures. We used a home-built atomic force microscope, mounted on an inverted fluorescent microscope, in order to apply a precisely controlled force onto a single endothelial cell and to record calcium entry and NO production simultaneously.
Inspired by our preliminary results, we hypothesize that there is a functional mechanotransduction unit on the endothelial surface. This mechanotransduction unit consists of ESG and its core proteins (e.g., glypicans or syndecans), mechanosensitive channels (e.g., transient receptor potential [TRP] channels), and endothelial Nitric Oxide Synthase (eNOS), with all these components co-localizing within the caveolae. This hypothesis is being actively tested in my group.