Quantitative evolutionary cell biology of cell motility
Cells from lineages across the eukaryotic tree crawl over surfaces and through three-dimensional environments. Different organisms crawl using one or more of a handful of distinct modes of migration, driven by unique molecular mechanisms. Understanding these mechanisms and their evolutionary relationships first requires narrowly defining mechanical modes of locomotion, and then identifying phenotypic and molecular markers of each. We are currently defining gene complements required for distinct modes of cell migration, and using the resulting data to trace the evolution of these complex behaviors, and to even predict crawling motility in new species. Current projects (see below) focus on the migration of chytrid fungi, Naegleria amoebae, and human neutrophils.
Bradiochytrium dendrobatidis is a fungal disease devastating amphibian populations world-wide. Based on evolutionary conservation of actin regulators, we predicted that this organism is capable of fast, pseudopod-based crawling motility. This prediction proved correct, but only during a few hours of its lifecycle, potentially during amphibian infection. These “zoospore” cells lack cell walls, build dynamic pseudopods, and crawl at ~40 microns per minute in confined environments. Chytrid pseudopods are filled with polymerized actin that requires Arp2/3 activity for assembly. Pseudopod-building cells also construct actin-rich cortical networks. Within several hours after birth, zoospores lose the ability to build pseudopods and begin to assemble chitinous cell walls. This transition appears to coincide with disassembly of the majority of cortical actin, leaving regions of polymerized actin reminiscent of the actin patches of walled fungal cells. We are currently investigating this transition between what appears to be an actin cytoskeleton similar to that of professionally migratory amoeboid cells (e.g. Dictyostelium and white blood cells), to an actin cytoskeleton which seems to echo that of yeast.
Naegleria gruberi takes on two extremely different forms during its lifecycle: an amoeba that crawls using actin, and a flagellate that swims with two flagella. The rapid differentiation between these forms makes Naegleria a prime model for understanding both types of cell motility. The differentiation from crawling amoebae to swimming flagellates involves assembling an entire microtubule cytoskeleton de novo, including two basal bodies (centrioles), the two flagella (cilia) that they pattern, and an entire cortical microtubule array. The process of differentiation includes transcribing and translating all of the microtubule cytoskeletal components—including tubulin— yet takes only 60-90 minutes. In the laboratory, we can induce cells to undergo this differentiation extremely synchronously. We are currently developing molecular tools for genetic engineering of Naegleria, as well as using transcriptional profiling to understand global changes occuring during the transition between the two forms of motility.
Not only is there variation between how single-celled organisms crawl, individual cells of multicellular organisms also use different molecular mechanisms to move. Many labs study the movement of highly adherent cells (including fibroblasts and epithelial cells) that use well defined structures called focal adhesions. But not all human cells use focal adhesion to crawl. We are studying the movement of other human cells that use expanding actin networks within pseudopods to rapidly migrate through tissues. The mechanisms used by these cells, including human neutrophils, appear similar to those used by free-living amoebae, and because of the wealth of molecular tools available, we are using them as a model this type of movement. In particular, we are using differentiated HL-60 cells to test involvement of novel genes that we identified through comparative genomics in pseudopod-based cell crawling.