We aim to identify the molecular and cellular mechanisms by which cells sense and respond to physical forces, with a particular focus on mechanosensitive ion channels. We are studying the structure, function, regulation, and evolution of mechanosensitive ion channels in the green lineage, using molecular genetics, single-channel patch clamp electrophysiology, live-imaging, and computational modeling approaches. We are also engaged in functional and genetic screens designed to identify novel mechanosensory proteins, and in the development of new tools for the non-invasive analysis of membrane forces. You can watch Liz give a Plantae Presents seminar here about “MS ion channels in the Green Lineage” (about 30 minutes).
The pollen grain as a model system for plant cell mechanobiology
Pollen provides an ideal system in which to study independent, single-cell mechanics. In addition to being essential for the propagation of flowering plants, pollen grains are easily isolated and assays for in vitro and in vivo hydration, germination, and fertilization are well-established. In collaboration with colleagues at WUSTL and Case Western, we have developed a mathematical model of the effects of turgor pressure and tension-dependent osmolyte release on cell-wall expansion after rehydration that predicts the behavior of wild type pollen and pollen lacking MS channels. Without tension-gated channel opening, the cell continues to expand indefinitely, unlike wild-type cells. We are currently investigating genetic links between MSL8 and the previously established cell wall integrity-signaling pathway in pollen grains. Taken together, these results implicate MSL8 in a homeostatic cycle that serves to maintain the proper level of turgor during hydration, germination, and tube growth. We were recently awarded a grant from NSF MCB to further this project.
MS ion channels as a nexus for mechanical decision-making
Our understanding of MSL channels collected over the past decade show that they can operate in two independent modes in response to mechanical stimulation. These are osmotic homeostasis (“mechanostasis”, wherein MS channels serve in a negative feedback loop to keep cells from swelling); and signal transduction (“mechanotransduction”, wherein MS channels activate a linear pathway to a stable outcome, such as programmed cell death). MSL10 is required for the plant to respond properly to abiotic (cell swelling) stresses. We hypothesize that MSL10 detects any events that alter lateral membrane tension, releasing osmolytes under transient or mild stimuli but triggering adaptive mechanotransduction pathways under extended or extreme stimuli. We are taking genetic, proteomic, biochemical, and modeling approaches to elucidate the molecular mechanisms of these mechanosensory pathways, and to determine the parameters that determine when MSL10 switches between the two modes of action. This project is funded by the Center for Engineering Mechanobiology.
Evolution of MS ion channels in the green lineage and beyond
In a way, it is surprising that the use of MS ion channels has been conserved across all kingdoms, given that the parameters governing mechanoperceptive events are so different in walled and un-walled cells. To understand how MS channels evolved to function in diverse mechanical contexts, we are comparing the structure and function of MS ion channel family members in the dicot Arabidopsis thaliana, the moss Physcomitrella patens, and the alga Chlamydomonas reinhardtii. In animals, PIEZO channels mediate cation influx across the plasma membrane, are required for a wide array of mechanoperceptive events, and are linked to multiple human diseases. We made the unexpected discovery that PIEZO homologs in moss have diverged from their animal counterparts both in subcellular localization and in function. We are now working to understand their physiological role in moss, algae, and dicots. This project is funded by a Faculty Scholar grant from HHMI and the Simons Foundation.