How do our cells know whether to become liver cells or neurons, blood cells or lymphocytes? One reason we have such a diverse range of cells – despite the shared, initial source genetic material – is that the precursors of these cells polarize. Simply stated, these cells decide to have a front end different from a rear. Polarization, which is characteristic of nearly all eukaryotic cells, is key to cell differentiation. Through an understanding of the mechanisms of polarization, applications to a wide array of fields, ranging from stem cell research to neuroscience, can be developed.
Daniel Lew, Ph.D., a professor at Duke University, spoke on the process of polarization in yeast this week at Dartmouth Medical School. Lew began by describing why scientists value yeasts as an ideal model of eukaryotic cells, citing their wide array of cellular regulation molecules, their rapid life cycle, and the vast amount of existing research that provides numerous experimental techniques that researchers can exploit as the primary reasons.
Lew then described his lab’s research on the polarization process in yeast (in which one section of the yeast begins to bud and splits from the original cell), delineating the multiple factors that affect how a yeast cell “knows” which end is which. Lew made clear that this decision is influenced both by external cues, such as pheromones released by a nearby cell before mating, as well as internal cues. Lew proved the importance of the latter in an experiment in which he grew the yeast in a culture free from such external cues; interestingly, each cell arbitrarily decided on an axis of polarity and began budding despite the lack of external orientation markers.
This process of polarization hinges on the aggregation of choice molecules on a section of the cytoskeleton, specifically the GTPase-Cdc42p, and a guanidine-nucleotide exchange factor (GEF) tethered together by a scaffold protein. This tethering creates a GTP cycling area where Cdc42p hydrolyzes GTP, and the resulting GDP is removed and replaced with GTP by the associated GEF. In a positive feedback loop, this process amplifies the concentration of locally bound GTP-Cdc42p. Because of this positive feedback, once one of multiple competing clusters of polarizing factors gains a slight advantage, it “wins out.” The result yields a single cluster of the polarizing factor and a clear axis of polarity.
This final conclusion was called into question when a student working in Lew’s lab named Audrey Howell developed a fast filming method that made the viewing of the competition between clusters possible. It was observed that clusters arise and fade in various locations without polarization before a single cluster finally stabilizes and causes polarization to occur. Based on this result, it became clear that the lab’s positive feedback model needed revision. Incorporating a negative feedback condition into the model yielded interesting predictions about competing clusters. Specifically, less than one percent of cell overexpression of a particular polarizing factors can cause competing clusters to equilibrate and form two buds, an interesting result not previously recorded.
Lew concluded the presentation by addressing the possible reasons behind this observed phenomenon of oscillating cluster location. Lew noted one advantage of this wandering-cluster behavior is that it enables the cell to sample a wider range of its environment; this is particularly useful for a mating cell trying to establish the direction of a pheromone gradient and orient itself to the cell releasing the pheromone. Lew and his lab continue to work on questions of the morphological checkpoint in yeast, specifically how budding and the cell cycle are linked, and cell polarity.