Rapid and quantitative imaging of excitation polarized fluorescence reveals ordered septin dynamics in live yeast

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

High frame-rate fluorescence confocal angle-resolved linear dichroism microscopy

Angle-resolved linear dichroism is a recent technique that exploits images recorded using an illumination field whose polarization angle is sequentially rotated during acquisition. It allows to retrieve orientation information of the fluorescent molecules, namely the average orientation angle and the amplitude of the fluctuations around this average. In order to boost up the acquisition speed without sacrificing the axial sectioning, we propose to combine a spinning disk confocal excitation scheme together with an electrooptical polarization switching and a camera acquisition. The polarization distortions induced when passing through the spinning disk system have been quantified and effectively compensated. The signal to noise features of the camera have been analyzed in detail so that the precision of the method can be quantified. The technique has been successfully tested on giant unilamellar vesicles and on living cells labeled with different fluorescent lipid probes, DiIC18 and di-8-ANEPPQ. It was able to acquire precise orientation images at full frame rates in the range of a second, ultimately limited by the fluorophore brightness and the camera sensitivity.

Thioflavine-T and Congo Red reveal the polymorphism of insulin amyloid fibrils when probed by polarization-resolved fluorescence microscopy

Amyloid fibrils are protein misfolding structures that involve a β-sheet structure and are associated with the pathologies of various neurodegenerative diseases. Here we show that Thioflavine-T and Congo Red, two major dyes used to image fibrils by fluorescence assays, can provide deep structural information when probed by means of polarization-resolved fluorescence microscopy. Unlike fluorescence anisotropy or fluorescence detected linear dichroism imaging, this technique allows to retrieve simultaneously both mean orientation and orientation dispersion of the dye, used here as a reporter of the fibril structure. We have observed that insulin amyloid fibrils exhibit a homogeneous behavior over the fibrils' length, confirming their structural uniformity. In addition, these results reveal the existence of various structures among the observed fibrils' population, in spite of a similar aspect when imaged with conventional fluorescence microscopy. This optical nondestructive technique opens perspectives for in vivo structural analyses or high throughput screening.