The ECHO of Cell Polarity - Revealing intra- and extracellular factors involved in polarity and axis formation de novo in tobacco cells

Abstract

Within all prokaryotic kingdoms cell polarity plays an indispensable role for various cellular functions. Failure in cell polarity can often result in serious diseases. Thus, understanding the mechanism behind polarity formation assigns a fundamental task to basic research. In contrast to mammalian cells, directionality is a fixed characteristic of plant cells. However, after division, this “direction” must be re-established. How cells acquire polarity and axis presents a central question of plant morphogenesis and represents the aim of this study. Therefore, the role of extracellular as well as intracellular candidates for axis formation was analyzed by using an experimental system based on regenerating protoplasts, where the induction of a cell axis de novo can be followed by quantification of specific regeneration stages. Nuclear migration and positioning are crucial for the morphogenesis of plant cells. The potential role of nuclear positioning for polarity induction was addressed by using overexpression of fluorescently tagged extranuclear (perinuclear actin basket, KCH kinesins) as well as intranuclear (histone H2B) factors of nuclear positioning. Time-lapse series of the early stages of regeneration showed that a central nuclear position is not a prerequisite for axis formation. Additionally, sophisticated quantification methods combined with computational analysis of nuclear proteome indicated differences in histone abundance of these transgenic overexpression lines, where nuclear migration was altered compared to the non-transgenic nuclei. Although nuclear positioning and cell axis formation were uncoupled, both phenomena are clearly dependent on the extra- and intranuclear factors affecting cytoskeletal tensegrity. Via different overexpression lines and pharmacological approaches, this study revealed how (i) cytoskeletal dynamics and (ii) motor proteins, as well as (iii) histones and (iv) nuclear membrane proteins are involved in axis formation. Further, it was demonstrated for the first time in plants that the structural organization of extracellular factors such as RGD peptides is significantly involved in axis formation, indicated by using aligned and unaligned nanofibers. Together, these findings were integrated into a model where retrograde signals are required for polarity induction. These signals travel via the cytoskeleton from the nucleus towards targets at the plasma membrane and back from the cell wall towards the nucleus. To refine this model, advanced microscopy techniques such as color recovery after photoconversion should be used in the future, as it would unravel local differences in dynamics of the cytoskeleton and motor proteins. Drawing the bigger picture, the results of this thesis lead to a general concept that could be valid for all eukaryotic kingdoms. It claims that cell polarity might be regulated by the close interplay of extracellular matrix, cytoskeleton, hormones and other signaling molecules, and organelles (abbreviated as the ECHO-principle) based on a mechanical tensegral mechanism together with chemical signaling. Received at the nucleus, extracellular signals could be “echoed” back to the plasma membrane.