Biology subjects

Skotheim, J. M.

Publications and source records attributed to Skotheim, J. M..

At least 19 recordsLinked to original sources

bioRxiv2026

Nuclear size is genetically controlled and influences cell fate

The proportional scaling between nuclear and cell size was first described more than 150 years ago and is among the most conserved features of cellular organization. Yet the mechanisms that establish this scaling and its physiological significance have remained unresolved. Here, we address both questions by combining image-enabled cell sorting with genome-wide CRISPR screening, transcriptomics and functional analyses. We identify more than 180 regulators of the nuclear-to-cell ratio and show that distinct classes of genes independently control nuclear and cell size. RNA metabolism predominantly regulates nuclear size, whereas protein synthesis and degradation primarily regulate cell size. This supports a model in which differences in macromolecular partitioning between the nucleus and cytoplasm contribute to osmotic regulation of nuclear size together with mechanical constraints imposed by chromatin, the cytoskeleton, and the nuclear envelope. Changes in nuclear size cause widespread transcriptional remodeling that is independent of changes in cell size. Cells with smaller nuclei exhibit reduced PRC2-dependent H3K27 trimethylation, activation of developmental gene-expression programmes and repression of cell-cycle genes. Consistent with these changes, mouse embryonic stem cells with smaller nuclei show an increased propensity to exit pluripotency and initiate differentiation in response to retinoic acid. Together, our findings provide a mechanistic framework for nuclear size scaling and establish nuclear size as a physical regulator of gene expression and cell-state transitions, linking cellular architecture to cell fate.

cell biology↗

bioRxiv2026

Retrograde signalling mediates cellular adaptation to mitochondrial DNA copy number alterations

Eukaryotic cells maintain multiple copies of the mitochondrial genome, which is essential for cellular metabolism. Accordingly, alterations in the mitochondrial DNA (mtDNA) copy number are associated with severe human diseases and ageing. However, the mechanisms through which cells regulate mtDNA copy number and the cellular consequences of altered copy number remain poorly understood. Here, using budding yeast as a model, we show that mtDNA copy number is determined by the amount of three limiting factors, Mip1, Abf2, and Rim1. By synthetically tuning the concentrations of only these three proteins, we can modulate mtDNA dosage inside the cell. This revealed that cells are surprisingly robust to mtDNA copy number alterations, with increased copy numbers even accelerating cell growth. Our findings suggest that this robustness is due to protein dosage compensation and independent regulation of mitochondrial morphology. Mechanistically, we identified a critical role of the retrograde signalling pathway for this adaptation. We show that signalling from mitochondria to the nucleus is upregulated in cells with higher mtDNA copy number, and disruption of this regulation diminishes their faster growth. Taken together, our work reveals regulatory principles that enable cells to adapt to mtDNA copy number alterations.

cell biology↗

bioRxiv2026

Cell Size Modulates SBF and Whi5 Chromatin Binding to Regulate the Start of the Budding Yeast Cell Cycle

Cell growth and division are tightly coordinated to cell size. In budding yeast, increasing cell size promotes the G1/S transition, called Start, by activating the transcription factor SBF, which drives a large fraction of cell-cycle-dependent gene expression. Part of this regulation arises because the concentration of the SBF inhibitor Whi5 decreases as cells grow. However, cells lacking Whi5 can still maintain a relatively accurate size when the SBF activator Cln3 is also removed, indicating that there are additional size control mechanisms. To understand how cell size is mechanistically translated into the activity of SBF-regulated promoters, we quantified the binding kinetics of Whi5 and SBF in live cells using single-molecule fluorescence microscopy. We found that increasing cell size is associated with both a decreased chromatin affinity of Whi5 and an increased chromatin affinity of SBF, accompanied by a higher SBF:Whi5 cell copy-number ratio. Chromatin-binding trends under basal and Whi5 overexpression conditions indicate that Whi5 restricts SBF association with chromatin. The transition point at which SBF binding overtakes Whi5 binding coincides with the onset of the expression of the G1 cyclins CLN1 and CLN2, two SBF targets that are important for committing cells to division. Reduced Whi5 binding reflects changes in its chromatin-association rate, as Whi5 and SBF dwell times on chromatin remain [~]10 s and are largely independent of cell size. Together, these results show how changes in SBF and Whi5 abundance and chromatin association transmit cell size information to the genome to regulate the size-dependent Start transition in budding yeast.

cell biology↗

bioRxiv2025

Cell size-dependent mRNA transcription drives proteome remodeling

Increasing cell size drives proteomic changes that impact cell physiology. However, the molecular basis of size-dependent proteome remodeling has remained unclear. Here, we develop an inducible Cyclin D1 expression system in human cells to generate populations of proliferating cells spanning over a two-fold size range. We use this genetic system to make comprehensive genome-wide measurements of mRNA and protein concentrations and stability. We find that protein and mRNA turnover rates are weakly related to cell size, but that mRNA concentrations are strongly size-dependent. This establishes that transcriptional regulation is the basis of proteome remodeling. Live-cell imaging of endogenous mRNAs using MS2 fluorescent protein binding motifs is used to measure how transcriptional dynamics change with cell size. Larger cells prolong transcriptional bursts and shorten inactive periods between bursts but maintain similar burst amplitudes to achieve transcriptional scaling. Taken together, our results show how transcription is modulated by cell size to remodel the proteome and alter cell physiology.

cell biology↗

bioRxiv2025

Division Asymmetry Drives Cell Size Variability in Budding Yeast

Cell size variability within proliferating populations reflects the interdependent regulation of cell growth and division as well as intrinsically stochastic effects. In budding yeast, the G1/S transition exerts strong size control in daughter cells, which manifests as the inverse correlation between how big a cell is when it is born and how much it grows in G1. However, mutations affecting this size control checkpoint only modestly influence population-wide size variability, often altering the coefficient of variation (CV) only by [~]10%. To resolve this paradox, we combine computational modeling and live-cell imaging to identify the principal determinants of cell size variability. Using an experimentally validated stochastic model of the yeast cell cycle, we perform parameter sensitivity analysis and find that division asymmetry between mothers and daughters is the dominant driver of CV, outweighing the effects of G1/S size control. Experimental measurements across genetic perturbations and growth conditions confirm a strong correlation between mother-daughter size asymmetry and population CV. These findings reconcile previous observations and show how asymmetric division operates in concert with G1/S size control to govern cell size heterogeneity.

cell biology↗

bioRxiv2025

Defined human tri-lineage brain microtissues

Microglia are the immune cells of the central nervous system and are thought to be key players in both physiological and disease conditions. Several microglial features are poorly conserved between mice and human, such as the function of the neurodegeneration-associated immune receptor Trem2. Induced pluripotent stem cell (iPSC)-derived microglia offer a powerful opportunity to generate and study human microglia. However, human iPSC-derived microglia often exhibit activated phenotypes in vitro, and assessing their impact on other brain cell types remains challenging due to limitations in current co-culture systems. Here, we developed fully defined brain microtissues, composed of human iPSC-derived neurons, astrocytes, and microglia, co-cultured in 2D or 3D formats. Our microtissues are stable and self-sufficient over time, requiring no exogenous cytokines or growth factors. All three cell types exhibit morphologies characteristic of their in vivo environment and show functional properties. Co-cultured microglia develop more homeostatic phenotypes compared to microglia exposed to exogenous cytokines. Hence, these tri-cultures provide a unique approach to investigate cell-cell interactions between brain cell types. We found that astrocytes and not neurons are sufficient for microglial survival and maturation, and that astrocyte-derived M-CSF is essential for microglial survival. Single-cell and single-nucleus RNA sequencing analyses nominated a network of reciprocal communication between cell types. Brain microtissues faithfully recapitulated pathogenic -synuclein seeding and aggregation, suggesting their usefulness as human cell models to study not only normal but also pathological cell biological processes.

neuroscience↗

bioRxiv2024

The Product neutrality function defining genetic interactions emerges from mechanistic models of cell growth

Genetic analyses, which examine the phenotypic effects of mutations both individually and in combination, have been fundamental to our understanding of cellular functions. Such analyses rely on a neutrality function that predicts the expected phenotype for double mutants based on the phenotypes of the two individual non-interacting mutations. In this study, we examine fitness, the most fundamental cellular phenotype, through an analysis of the extensive colony growth rate data for budding yeast. Our results confirm that the Product neutrality function describes the colony growth rate, or fitness, of a double mutant as the product of the fitnesses of the individual single mutants. This Product neutrality function performs better than additive or minimum neutrality functions, supporting its continued use in genetic interaction studies. Furthermore, we explore the mechanistic origins of this neutrality function by analyzing two theoretical models of cell growth. We perform a computational genetic analysis to show that in both models the product neutrality function naturally emerges due to the interdependence of cellular processes that maximize growth rates. Thus, our findings provide mechanistic insight into how the Product neutrality function arises and affirm its utility in predicting genetic interactions affecting cell growth and proliferation.

systems biology↗

bioRxiv2024

Somatic polyploidy supports biosynthesis and tissue function by increasing transcriptional output

Cell size and biosynthetic capacity generally increase with increased DNA content. Polyploidy has therefore been proposed to be an adaptive strategy to increase cell size in specialized tissues with high biosynthetic demands. However, if and how DNA concentration limits cellular biosynthesis in vivo is not well understood, and the impacts of polyploidy in non-disease states is not well studied. Here, we show that polyploidy in the C. elegans intestine is critical for cell growth and yolk biosynthesis, a central role of this organ. Artificially lowering the DNA/cytoplasm ratio by reducing polyploidization in the intestine gave rise to smaller cells with more dilute mRNA. Highly-expressed transcripts were more sensitive to this mRNA dilution, whereas lowly-expressed genes were partially compensated - in part by loading more RNA Polymerase II on the remaining genomes. DNA-dilute cells had normal total protein concentration, which we propose is achieved by increasing production of translational machinery at the expense of specialized, cell-type specific proteins.

cell biology↗

bioRxiv2024

Genome concentration limits cell growth and modulates proteome composition in Escherichia coli

Defining the cellular factors that drive growth rate and proteome composition is essential for understanding and manipulating cellular systems. In bacteria, ribosome concentration is known to be a constraining factor of cell growth rate, while gene concentration is usually assumed not to be limiting. Here, using single-molecule tracking, quantitative single-cell microscopy, and modeling, we show that genome dilution in Escherichia coli cells arrested for DNA replication limits total RNA polymerase activity within physiological cell sizes across tested nutrient conditions. This rapid-onset limitation on bulk transcription results in sub-linear scaling of total active ribosomes with cell size and sub-exponential growth. Such downstream effects on bulk translation and cell growth are near-immediately detectable in a nutrient-rich medium, but delayed in nutrient-poor conditions, presumably due to cellular buffering activities. RNA sequencing and tandem-mass-tag mass spectrometry experiments further reveal that genome dilution remodels the relative abundance of mRNAs and proteins with cell size at a global level. Altogether, our findings indicate that chromosome concentration is a limiting factor of transcription and a global modulator of the transcriptome and proteome composition in E. coli. Experiments in Caulobacter crescentus and comparison with eukaryotic cell studies identify broadly conserved DNA concentration-dependent scaling principles of gene expression.

cell biology↗

bioRxiv2023

Genome dilution by cell growth drives starvation-like proteome remodeling in mammalian and yeast cells

Cell size is tightly controlled in healthy tissues and single-celled organisms, but it remains unclear how size influences cell physiology. Increasing cell size was recently shown to remodel the proteomes of cultured human cells, demonstrating that large and small cells of the same type can be biochemically different. Here, we corroborate these results in mouse hepatocytes and extend our analysis using yeast. We find that size-dependent proteome changes are highly conserved and mostly independent of metabolic state. As eukaryotic cells grow larger, the dilution of the genome elicits a starvation-like proteome phenotype, suggesting that growth in large cells is limited by the genome in a manner analogous to a limiting nutrient. We also find that this phenomenon explains many proteomic changes ascribed to yeast aging. Overall, our data suggest that genome concentration is a universal determinant of proteome content in growing cells.

cell biology↗

bioRxiv2022

Cell size contributes to single-cell proteome variation

Accurate measurements of the molecular composition of single cells will be necessary for understanding the relationship between gene expression and function in diverse cell types. One of the most important phenotypes that differs between cells is their size, which was recently shown to be an important determinant of proteome composition in populations of similarly sized cells. We therefore sought to test if the effects of cell size on protein concentrations were also evident in single cell proteomics data. Using the relative concentrations of a set of reference proteins to estimate a cells DNA-to-cell volume ratio, we found that differences in cell size explain a significant amount of cell-to-cell variance in two published single cell proteome datasets.

cell biology↗

bioRxiv2022

Chromatin-modifying enzymes as modulators of nuclear size during lineage differentiation

The mechanism of nuclear size determination and alteration during normal lineage development and cancer pathologies which is not fully understood. As recently reported, chromatin modification can change nuclear morphology. Therefore, we screened a range of pharmacological chemical compounds that impact the activity of chromatin-modifying enzymes, in order to get a clue of the specific types of chromatin-modifying enzymes that remarkably effect nuclear size and shape. We found that interrupted activity of chromatin-modifying enzymes is associated with nuclear shape abnormalities. Furthermore, the activity of chromatin-modifying enzymes perturbs cell fate determination in cellular maintenance and lineage commitment. Our results indicated that chromatin-modifying enzyme regulates cell fate decision during lineage differentiation and is associate with nuclear size alteration. SignificanceHere we described for the first time the modulation of chromatin-modifying enzymes remarkably effects nuclear shape, and perturbs cell fate determination in cellular maintenance and lineage fate commitment in normal stem cells and leukemia cancer cells. We found that the irregularities of nuclear contour were highly related to pharmacological inhibition of chromatin-modifying enzyme activity. After manipulating a histone demethylase named GASC1, we found that upregulation of GASC1 impairs the differentiation of hESCs to terminally differentiated neural cells. Moreover, upregulation of GASC1 impairs the proliferation of leukemia cells due to cell cycle arrest in the G0/G1 phase and indicates misshapen nuclei. This study suggested that chromatin-modifying enzyme regulates the nuclear contour related cell fate decision also including cancer cell fate determination.

cancer biology↗

bioRxiv2022

Evolution of cell size control is canalized towards adders or sizers by cell cycle structure and selective pressures

Cell size is controlled to be within a specific range to support physiological function. To control their size, cells use diverse mechanisms ranging from sizers, in which differences in cell size are compensated for in a single cell division cycle, to adders, in which a constant amount of cell growth occurs in each cell cycle. This diversity raises the question why a particular cell would implement one rather than another mechanism? To address this question, we performed a series of simulations evolving cell size control networks. The size control mechanism that evolved was influenced by both cell cycle structure and specific selection pressures. Moreover, evolved networks recapitulated known size control properties of naturally occurring networks. If the mechanism is based on a G1 size control and an S/G2/M timer, as found for budding yeast and some human cells, adders likely evolve. But, if the G1 phase is significantly longer than the S/G2/M phase, as is often the case in mammalian cells in vivo, sizers become more likely. Sizers also evolve when the cell cycle structure is inverted so that G1 is a timer, while S/G2/M performs size control, as is the case for the fission yeast S. pombe. For some size control networks, cell size consistently decreases in each cycle until a burst of cell cycle inhibitor drives an extended G1 phase much like the cell division cycle of the green algae Chlamydomonas. That these size control networks evolved such self-organized criticality shows how the evolution of complex systems can drive the emergence of critical processes.

systems biology↗

bioRxiv2021

RNA polymerase II dynamics and mRNA stability feedback determine mRNA scaling with cell size

A fundamental feature of cellular growth is that protein and RNA amounts scale with cell size so that concentrations remain constant. A key component to this is that global transcription rates increase in larger cells, but the underlying mechanism has remained unknown. Here, we identify RNAPII as the major limiting factor increasing transcription with cell size in budding yeast as transcription is highly sensitive to the dosage of RNAPII but not to other components of the general transcriptional machinery. Our experiments support a dynamic equilibrium model where global transcription at a given size is set by the mass-action recruitment kinetics of unengaged nucleoplasmic RNAPII, and DNA content. This drives a sub-linear increase in transcription with size, which is precisely compensated for by a decrease in mRNA decay rates as cells enlarge. Thus, limiting RNAPII and feedback on mRNA stability work in concert to ensure mRNA concentration homeostasis in growing cells.

molecular biology↗

bioRxiv2021

Increasing cell size remodels the proteome and promotes senescence

Cell size is tightly controlled in healthy tissues, but it is poorly understood how variations in cell size affect cell physiology. To address this, we employed a high-accuracy mass spectrometry-based approach to measure how the proteome changes with cell size. Protein concentration changes are widespread, measurable in both asynchronous and G1-arrested cell populations, and predicted by subcellular localization, size-dependent changes in mRNA concentrations, and protein turnover. As proliferating cells grow larger, protein concentration changes typically associated with cellular senescence are increasingly pronounced. This suggests that large size is a cause rather than just a consequence of cell senescence. Consistent with this hypothesis, larger cells are prone to replicative, DNA damage-, and CDK4/6i-induced senescence. Size-dependent changes to the proteome, including those associated with senescence, are not observed when an increase in cell size is accompanied by a similar increase in ploidy. This shows that proteome composition is determined by the DNA-to-ploidy ratio rather than cell size per se and that polyploidization is an elegant method to generate large non-senescent cells as is commonly found in nature. Together, our findings show how cell size could impact many aspects of cell physiology through remodeling the proteome, thereby providing a rationale for cell size control and polyploidization.

cell biology↗

bioRxiv2021

Localized phosphorylation of RNA Polymerase II by G1 cyclin-Cdk promotes cell cycle entry

The cell cycle is thought to be initiated by cyclin-dependent kinases (Cdk) inactivating transcriptional inhibitors of cell cycle gene-expression(1, 2). In budding yeast, the G1 cyclin Cln3-Cdk1 complex is thought to directly phosphorylate Whi5, thereby releasing the transcription factor SBF and committing cells to division(3-7). Here, we report that Cln3-Cdk1 does not phosphorylate Whi5, but instead phosphorylates the RNA Polymerase II subunit Rpb1s C-terminal domain (CTD) on S5 of its heptapeptide repeats. Cln3-Cdk1 binds SBF-regulated promoters(8) and Cln3s function can be performed by the canonical S5 kinase(9) Ccl1-Kin28 when synthetically recruited to SBF. Thus, Cln3-Cdk1 triggers cell division by phosphorylating Rpb1 at SBF-regulated promoters to activate transcription. Our findings blur the distinction between cell cycle and transcriptional Cdks to highlight the ancient relationship between these processes.

cell biology↗

bioRxiv2020

Size-independent mRNA synthesis and chromatin-based partitioning mechanisms generate and maintain constant amounts of protein per cell

Biosynthesis scales with cell size such that protein concentrations generally remain constant as cells grow. As an exception, synthesis of the cell-cycle inhibitor Whi5 sub-scales with cell size so that its concentration is lower in larger cells to promote cell-cycle entry. Here, we find that a transcriptional control uncouples Whi5 synthesis from cell size and, screening for similar genes, identify histones as the major class of sub-scaling transcripts besides WHI5. Histone synthesis is thereby matched to genome content rather than cell size. Such sub-scaling proteins are challenged by asymmetric cell division because proteins are typically partitioned in proportion to new-born cell volume. To avoid this fate, Whi5 uses chromatin-binding to partition similar protein amounts to each new-born cell regardless of cell size. Finally, disrupting both Whi5 synthesis and chromatin-based partitioning compromises G1 size control. Thus, specific transcriptional and partitioning mechanisms determine protein sub-scaling to control cell size.

cell biology↗

bioRxiv2020

Whi5 is diluted and protein synthesis does not dramatically increase in pre-Start G1

In their manuscript, Litsios et al.1 report a new model for how cell growth and biosynthetic activity control the G1/S transition in budding yeast. In essence, Litsios et al. claim that Start is driven by an increasing concentration of the G1 cyclin Cln3 due to a dramatic acceleration of protein synthesis in pre-Start G1 and not by the dilution of the cell cycle inhibitor Whi5. While we previously reported that Start was in part driven by cell growth during G1 diluting out the Start inhibitor Whi52, Litsios et al. report that Whi5 remains at constant concentration during G1, and changes in Whi5 concentration therefore do not contribute to Start. Since Litsios et al. directly contradict several key points of our own model of how cell growth triggers Start, we decided to investigate their claims and data. More specifically, we decided to investigate Litsios et al.s three major claims: O_LIWhi5 concentration remains constant during G1 C_LIO_LICln3 concentration strongly increases prior to Start C_LIO_LIGlobal protein synthesis rates increase by 2-3 fold prior to Start C_LI We investigated each of these three claims and found that the evidence presented by Litsios et al. does not support their claims due to inadequate analysis methods and flaws in their experiments.

molecular biology↗