Biology subjects

Kassouf, M.

Publications and source records attributed to Kassouf, M..

5 recordsLinked to original sources

bioRxiv2023

Active regulatory elements recruit cohesin to establish cell-specific chromatin domains.

As the structure of the genome is analysed at ever increasing resolution it is becoming clear that there is considerable variation in the 3D chromatin architecture across different cell types. It has been proposed that this may, in part, be due to increased recruitment of cohesin to activated cis-elements (enhancers and promoters) leading to cell-type specific loop extrusion underlying the formation of new subTADs. Here we show that cohesin correlates well with the presence of active enhancers and this varies in an allele-specific manner with the presence or absence of polymorphic enhancers which vary from one individual to another. Using the alpha globin cluster as a model, we show that when all enhancers are removed, peaks of cohesin disappear from these regions and the erythroid specific subTAD is no longer formed. Re-insertion of the major alpha globin enhancer (R2) is associated with the appearance of a new peak of cohesin at the site of insertion. In complementary experiments insertion of R2 into a "neutral" region of the genome recruits cohesin, induces transcription and creates a new large (75kb) erythroid specific domain. Together these findings support the proposal that active enhancers recruit cohesin, stimulate loop extrusion and promote the formation of cell specific subTADs.

genomics↗

bioRxiv2023

Ancient genomic linkage couples metabolism with erythroid development

Generation of mature cells from progenitors requires tight coupling of differentiation and metabolism. During erythropoiesis, erythroblasts are required to massively upregulate globin synthesis then clear extraneous material and enucleate to produce erythrocytes1-3. Nprl3 has remained in synteny with the -globin genes for >500 million years4, and harbours the majority of the -globin enhancers5. Nprl3 is a highly conserved inhibitor of mTORC1, which controls cellular metabolism. However, whether Nprl3 itself serves an erythroid role is unknown. Here, we show that Nprl3 is a key regulator of erythroid metabolism. Using Nprl3-deficient fetal liver and adult competitive bone marrow - fetal liver chimeras, we show that NprI3 is required for sufficient erythropoiesis. Loss of Nprl3 elevates mTORC1 signalling, suppresses autophagy and disrupts erythroblast glycolysis and redox control. Human CD34+ progenitors lacking NPRL3 produce fewer enucleated cells and demonstrate dysregulated mTORC1 signalling in response to nutrient availability and erythropoietin. Finally, we show that the -globin enhancers upregulate NprI3 expression, and that this activity is necessary for optimal erythropoiesis. Therefore, the anciently conserved linkage of NprI3, -globin and their associated enhancers has enabled coupling of metabolic and developmental control in erythroid cells. This may enable erythropoiesis to adapt to fluctuating nutritional and environmental conditions.

cell biology↗

bioRxiv2023

Loop extrusion by cohesin plays a key role in enhancer-activated gene expression during differentiation

Enhancers and their target promoters often come into close physical proximity when activated. This proximity may be explained by a variety of mechanisms; most recently via cohesin-mediated chromatin loop extrusion. Despite this compelling hypothesis, acute depletion of cohesin does not cause widespread changes in gene expression. We have tested the role of cohesin-mediated loop extrusion on gene expression at the mouse alpha-globin locus during erythropoiesis. Acute depletion of cohesin downregulates alpha-globin expression at early but not late stages of differentiation. When single or multiple CTCF sites are placed between the alpha-globin enhancers and promoters, alpha-gene expression is downregulated. Importantly, the orientation of the CTCF site plays a critical role, suggesting that within this activated domain, cohesin predominantly but not exclusively translocates from the enhancers to the promoters. We find that loop extrusion does play an important role in establishing enhancer-promoter proximity and consequent expression of inducible genes during differentiation.

molecular biology↗

bioRxiv2022

Super-enhancers require a combination of classical enhancers and novel facilitator elements to drive high levels of gene expression

Super-enhancers (SEs) are a class of compound regulatory elements which control expression of key cell-identity genes. It remains unclear whether they are simply clusters of independent classical enhancers or whether SEs manifest emergent properties and should therefore be considered as a distinct class of element. Here, using synthetic biology and genome editing, we engineered the well characterised erythroid -globin SE at the endogenous -globin locus, removing all SE constituent elements in a mouse embryonic stem cell-line, to create a "blank canvas". This has allowed us to re-build the SE through individual and combinatorial reinsertion of its five elements (R1, R2, R3, Rm, R4), to test the importance of each constituents sequence and position within the locus. Each re-inserted element independently creates a region of open chromatin and binds its normal repertoire of transcription factors; however, we found a high degree of functional interdependence between the five constituents. Surprisingly, the two strongest -globin enhancers (R1 and R2) act sub-optimally both on their own and in combination, and although the other three elements (R3, Rm and R4) exhibit no discernible enhancer activity, they each exert a major positive effect in facilitating the activity of the classical enhancers (R1 and R2). This effect depends not simply on the sequence of each element but on their positions within the cluster. We propose that these "facilitators" are a novel form of regulatory element, important for ensuring the full activity of SEs, but distinct from conventional enhancer elements.

genetics↗

bioRxiv2020

Scalable In Vitro Production of Defined Mouse Erythroblasts

Mouse embryonic stem cells (mESCs) can be manipulated in vitro to recapitulate the process of erythropoiesis, during which multipotent cells undergo lineage specification, differentiation and maturation to produce erythroid cells. Although useful for identifying specific progenitors and precursors, this system has not been fully exploited as a source of cells to analyse erythropoiesis. Here, we establish a protocol in which characterised erythroblasts can be isolated in a scalable manner from differentiated embryoid bodies (EBs). Using transcriptional and epigenetic analysis, we demonstrate that this system faithfully recapitulates normal primitive erythropoiesis and fully reproduces the effects of natural and engineered mutations seen in primary cells obtained from mouse models. We anticipate this system to be of great value in reducing the time and costs of generating and maintaining mouse lines in a number of research scenarios. Key PointsO_LIScalable purification of primitive-like erythroid cells from in vitro differentiated mESCs offers tractable tools for genetic studies C_LIO_LIIn vitro derived erythroid cells recapitulate wild type and engineered mutation phenotypes observed in primary cells obtained from mouse models C_LI

molecular biology↗