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Cell compartmentalization, individuation and diversity

Final Report Summary - BARRAGE (Cell compartmentalization, individuation and diversity)

Asymmetric cell division is key for generating cell diversity in eukaryotes, driving the differential inheritance of fate determinants, and/or irreparable damages between daughters. We investigated how daughter cells individualize them-selves from each other. This involved establishing 1- what are the different classes of asymmetrically segregated determinants? 2- what is the role and mechanisms of diffusion barriers in confining these determinants to only one of the two daughters?

Lateral diffusion barriers form in the membrane of the endoplasmic reticulum (ER) and the envelope of the dividing nucleus of yeast cells, at their future plane of cleavage. We have now dissected their genetic requirements, thereby identifying their composition and revealing their mode of action. Both yeast barriers comprise a specialized lipid domain compose of long-fatty-acid-chain lipids. This domain contributes to barrier function by sorting the molecules that are allowed to enter it and pass the barrier. These studies reveal new and unexpected role of lipids in asymmetric cell division.

Remarkably, despite their similar composition assembly of the two yeast barriers follows distinct mechanisms. This provides an ideal setting to investigate variations of barrier forms and identify core principles of their assembly and function. Furthermore, barriers strength is modulated in response to stress and nutrient limitation. The two barriers are regulated independently and their down-regulation show distinct effects on ageing and memory transmission. These observations reveal unexpected and highly differentiated roles for the ER and the nuclear envelope (NE) in epigenetic regulation. Finally, our studies in C. elegans oocytes and mammalian cells establish that diffusion barriers are conserved in metazoans, where they also contribute to the asymmetric segregation of fate determinants. Thus, our data establish diffusion barriers as a conserved and highly plastic mechanism for promoting the individuation of single cells, impinging on ageing and its regulation.

Our studies on the nature of the molecular entities retained by the diffusion barriers have revealed that the barriers herd at least the two following types of fate determinants.

First, the nuclear barrier confines extrachromosomal DNA circles to the yeast mother cell. In order for circles to be retained, they must be attached to nuclear pores, such that the barrier in the nuclear envelope can limit their passage to the bud. Remarkably, the anchorage machinery distinguishes between chromosomal and non-chromosomal DNA molecules. This ensures that the unanchored genome can segregate symmetrically, while anchored extrachromosomal molecules are confined and prevented from propagating in the population. In turn, accumulating circles play a central role in the ageing phenotypes of yeast mother cells. How cells distinguish between chromosomal and non-chromosomal molecules is a focus of our current research and might reveal an ancient form of genomic immunity by which even unicellular organisms prevent intrusion of foreign DNA molecules.

Second, cells memorize past events to optimally adapt to their environment. This includes memorizing occurrences of stress episodes and signals from neighboring cells. Memory encoding depended on the conversion of a novel class of prion-like molecules, which we called mnemons. Confinement of mnemons to the yeast mother cell allows her to keep records of the past, while generating naïve daughters. Like for DNA circles, the action of the barrier on mnemons required anchorage of the mnemons to the ER-membrane by dedicated receptors.