Cell division | EPFL Graph Search

Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis), producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in the diploid parent cell to one of each type in the daughter cells. In cell biology, mitosis (/maɪˈtoʊsɪs/) is a part of the cell cycle, in which, replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA replication occurs) and is often followed by t

Characterization of RGS-7 as a regulator of forces involved in C. elegans early cell divisions

Simon Germann

Proper regulation of cell division is crucial to most organisms. Asymmetric division and unequal cleavage are one of the many mechanisms by which spatial regulation of cell division might be achieved. Indeed, by varying its polarity cues or by modulating the size of its daughters, a cell can potentially influence its cellular neighborhood and possibly act upon it. In C. elegans, asymmetric cell division occurs at early stages of development, and is induced by an asymmetric positioning of the spindle during mitosis. RGS-7, a member of the RGS protein family, is a potential candidate as a regulator of this process, and was shown to affect the balance of pulling forces responsible for the asymmetric position of the spindle, possibly acting through Gα and/or Gβ, which are heterotrimeric G-protein complex subunits involved in force generation in C. elegans early cell divisions. However, many aspects of the role this protein may have in this process remain to be elucidated. Using available rgs-7 mutants, RNA interference, spindle severing experiments and immunofluorescence assays, this study highlights the potential functions RGS-7 may provide to a dividing C. elegans embryo

2008

Bacterial cell cycle dynamics: size regulation during exponential growth and role of polyphosphate during starvation response

Aster Louis K Vanhecke

Bacteria are the most diverse and abundant kingdom of life and have adapted to survive and thrive in habitats around the globe. When provided with ample nutrients they grow and divide at staggering rates, increasing their population exponentially. Upon nutrient depletion on the other hand, they quickly adapt by drastically altering their metabolism, halting growth to survive for a very long time. Since bacteria are tiny -about a few micrometers-, visualizing these processes requires microscopy. To measure the dynamics of their shape and inner structures precisely, one needs to choose a technique that balances spatial resolution, temporal resolution and photo-toxicity. In this thesis, I present two projects using advanced dynamic microscopy, first to study cell size regulation during exponential growth in the abundance of nutrients and then to elucidate the role and positioning of polyphosphate granules during cell cycle exit in response to nutrient starvation. During exponential growth, bacteria balance growth and division to regulate their size, resulting in a narrow size distribution, referred to as cell size homeostasis. Recent work tried to uncover what cells sense to decide to divide in order to achieve size homeostasis: time, size, growth or a combination of those. Control of cell division is often equated to control of constriction onset; however, the constriction period still accounts for up to 40% of cell growth and could thus contribute significantly to cell size regulation. We used SIM microscopy to measure constriction kinetics and their impact on cell size regulation in Caulobacter crescentus. We found that constriction rate regulation can determine cell size. Moreover, constriction rate modulation compensates for variability in elongation before constriction, allowing a higher fidelity cell size homeostasis. We suggest a parsimonious model where excess cell wall precursors accumulate proportionally to elongation before constriction and set the rate of constriction. This is the first direct demonstration that constriction rate can contribute to cell size control and homeostasis in bacteria. Upon nutrient starvation, bacteria exit their cell cycle to preserve energy and nutrients. In many bacteria, such as Pseudomonas aeruginosa, this is associated with the accumulation of polyphosphate (polyP) in intracellular granules. PolyP is created by polyphosphate kinases (ppkâs), which are required for successful cell cycle exit and survival of and recovery from long-term starvation. Interestingly, these polyP granules are regularly spaced within the nucleoid. To date, it is not known during which stage polyP is required for cell cycle exit, and what causes the spacing of the granules. Here, we use fluorescence microscopy to probe the cell cycle stage of Îppk cells arrested during nutrient starvation as well as the localization and dynamics of ppkâs. We show that a majority of Îppk cells are arrested with open replication forks. Furthermore, we find that ppkâs localize in distinct patterns, already prior to starvation and polyP granule production, which could be responsible for the positioning of polyP granules. To this end, we developed a background subtraction algorithm to remove cytoplasmic fluorescence, improving accuracy of spot detection and localization.

EPFL2019

Asymmetric Cell Division in the Skin

Wan-Hung Fan

An asymmetric cell division (ACD) is defined as a division that gives rise to two daughter cells with different fates. It is the most common type of division during development. Analogous to Drosophila neuroblasts, proteins involved in ACD such as Insc and LGN are found to be asymmetrically localized during perpendicular cell divisions in mammalian skin, suggesting that the cutaneous tissue homeostasis may be controlled by using similar mechanisms. Most cell divisions in the basal layer of the skin are perpendicular to the basal membrane (BM), but when a skin tumor is formed, most cell divisions would become parallel to the BM. Previous studies in our lab have shown that regressing skin tumor caused by β-catenin depletion have increased perpendicular cell divisions accompanied by reduced stem cell numbers and enhanced mTrim2 expression. Furthermore, induced expression of mTRIM2 in skin tumors led to tumor regression. Taken together, this suggests that progenitor cells in the basal layer of the epidermis may use vertical ACDs to maintain progenitor cell number while using horizontal SCDs to expand progenitor cell number and mTrim2 may function as a cell fate determinant in skin. To validate the hypothesis that vertical divisions of basal keratinocytes to the BM are ACDs, I utilized light sheet based fluorescence microscopy (LSFM) to perform the time lapse recording of murine embryonic skin development between E14.5 and E17.5 and to monitor individual dividing cells and to track their fates after division. For visualizing the dividing cells, protein localization and differentiating cells, I have generated several transgenic mouse lines and combined them together. My study showed that ACD of basal keratinocytes can be either vertical or horizontal to the BM and divisions in the suprabasal layer can be both ACDs and SCDs. To study the potential function of mTRIMs in skin, I generated the knock-out (KO) mice. Neither mTrim2 KO, mTrim3 KO or mTrim32 KO mice showed any phenotype in skin, therefore I combined these mice to generate mTrim2/3 double KO and mTrim2/3/32 triple KO mice. Still, no skin phenotype was observed in these mice, indicating that mTrim2/3/32 might not function as a cell fate determinant in skin.