Nutrients control cell size

Decoupled transcript and protein concentrations ensure histone homeostasis in different nutrients

To maintain protein homeostasis in changing nutrient environments, cells must precisely control the amount of their proteins, despite the accompanying changes in cell growth and biosynthetic capacity. As nutrients are major regulators of cell cycle length and progression, a particular challenge arises for the nutrient-dependent regulation of 'cell cycle genes', which are periodically expressed during the cell cycle. One important example are histones, which are needed at a constant histone-to-DNA stoichiometry. Here we show that budding yeast achieves histone homeostasis in different nutrients through a decoupling of transcript and protein abundance. We find that cells downregulate histone transcripts in poor nutrients to avoid toxic histone overexpression, but produce constant amounts of histone proteins through nutrient-specific regulation of translation efficiency. Our findings suggest that this allows cells to balance the need for rapid histone production under fast growth conditions with the tight regulation required to avoid toxic overexpression in poor nutrients.

A conserved nutrient responsive axis mediates autophagic degradation of miRNA-mRNA hybrids in blood cell progenitors

In animals, microRNAs are amongst the primary non-coding RNAs involved in regulating the gene expression of a cell. Most mRNAs in a cell are targeted by one or many miRNAs. Although several mechanisms can be attributed to the degradation of miRNA and mRNA within a cell, but the involvement of autophagy in the clearance of miRNA and its target mRNA is not known. We discover a leucine-responsive axis in blood cell progenitors that can mediate an autophagy-directed degradation of miRNA-bound mRNA in Drosophila melanogaster and Homo sapiens. This previously unknown miRNA clearance axis is activated upon amino acid deprivation that can traffic miRNA-mRNA-loaded Argonaute for autophagic degradation in a p62-dependent manner. Thus, our research not only reports a novel axis that can address the turnover of a catalytically active miRISC but also elucidates a slicer-independent mechanism through which autophagy can selectively initiate the clearance of target mRNA.

A multiscale modelling approach for Haematococcus pluvialis cultivation under different environmental conditions

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We develop a novel multiscale model for microalgal photoautotrophic growth.

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The model is segregated-structured type based on Population Balance Equations.

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We combine the model with cultivation experiments of Haematococcus pluvialis.

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We successfully predict cell number, average volume and density distribution dynamics.

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Model can accurately describe the nutrient depletion phase including cell lysis.

Haematococcus pluvialis can produce significant amounts of industrially important compounds belonging to lipids and starch classes, including various specific pigments such as β-carotene, lutein and astaxanthin, as well as lipids, carbohydrates and proteins. Their production can vary depending on environmental stress conditions like nutrient starvation. However, stress conditions lead also to undesired phenomena such as cell lysis, which is likely to be related to products loss. The microorganism develops towards smaller single cell volumes during the growth process, and eventually, more likely towards lysis when fission (i.e. cell division) slows down. The lysis process takes place simultaneously with nutrient depletion, so both growth and lysis are linked to the change of environmental conditions. In this work, we develop a novel multiscale segregated-structured model based on Population Balance Equations (PBEs) to describe the photoautotrophic growth of H.pluvialis, in particular cell growth, and lysis, making possible the description of the relationship between cell volume/transition, cell loss, and metabolic product availability. Cell volume is the internal coordinate of the population balance model, and its link with intrinsic concentrations is also presented. The model parameters are fitted against experimental data, extensive sensitivity analysis is performed and the model predictive capabilities are tested in terms of cell density distributions, as well as 0th and 1st order moments.

The cooperative interplay among inflammation, necroptosis and YAP pathway contributes to the folate deficiency-induced liver cells enlargement

Change in cell size may bring in profound impact to cell function and survival, hence the integrity of the organs consisting of those cells. Nevertheless, how cell size is regulated remains incompletely understood. We used the fluorescent zebrafish transgenic line Tg-GGH/LR that displays inducible folate deficiency (FD) and hepatomegaly upon FD induction as in vivo model. We found that FD caused hepatocytes enlargement and increased liver stiffness, which could not be prevented by nucleotides supplementations. Both in vitro and in vivo studies indicated that RIPK3/MLKL-dependent necroptotic pathway and Hippo signaling interactively participated in this FD-induced hepatocytic enlargement in a dual chronological and cooperative manner. FD also induced hepatic inflammation, which convenes a dialog of positive feedback loop between necroptotic and Hippo pathways. The increased MMP13 expression in response to FD elevated TNFα level and further aggravated the hepatocyte enlargement. Meanwhile, F-actin was circumferentially re-allocated at the edge under cell membrane in response to FD. Our results substantiate the interplay among intracellular folate status, pathways regulation, inflammatory responses, actin cytoskeleton and cell volume control, which can be best observed with in vivo platform. Our data also support the use of this Tg-GGH/LR transgenic line for the mechanistical and therapeutic research for the pathologic conditions related to cell size alteration.

Greatwall-Endosulfine: A Molecular Switch that Regulates PP2A/B55 Protein Phosphatase Activity in Dividing and Quiescent Cells

During the cell cycle, hundreds of proteins become phosphorylated and dephosphorylated, indicating that protein kinases and protein phosphatases play a central role in its regulation. It has been widely recognized that oscillation in cyclin-dependent kinase (CDK) activity promotes DNA replication, during S-phase, and chromosome segregation, during mitosis. Each CDK substrate phosphorylation status is defined by the balance between CDKs and CDK-counteracting phosphatases. In fission yeast and animal cells, PP2A/B55 is the main protein phosphatase that counteracts CDK activity. PP2A/B55 plays a key role in mitotic entry and mitotic exit, and it is regulated by the Greatwall-Endosulfine (ENSA) molecular switch that inactivates PP2A/B55 at the onset of mitosis, allowing maximal CDK activity at metaphase. The Greatwall-ENSA-PP2A/B55 pathway is highly conserved from yeast to animal cells. In yeasts, Greatwall is negatively regulated by nutrients through TORC1 and S6 kinase, and couples cell growth, regulated by TORC1, to cell cycle progression, driven by CDK activity. In animal cells, Greatwall is phosphorylated and activated by Cdk1 at G2/M, generating a bistable molecular switch that results in full activation of Cdk1/CyclinB. Here we review the current knowledge of the Greatwall-ENSA-PP2A/B55 pathway and discuss its role in cell cycle progression and as an integrator of nutritional cues.

Nutrient restriction causes reversible G2 arrest in Xenopus neural progenitors

Nutrient status affects brain development; however, the effects of nutrient availability on neural progenitor cell proliferation in vivo are poorly understood. Without food, Xenopus laevis tadpoles enter a period of stasis during which neural progenitor proliferation is drastically reduced, but resumes when food becomes available. Here, we investigate how neural progenitors halt cell division in response to nutrient restriction and subsequently re-enter the cell cycle upon feeding. We demonstrate that nutrient restriction causes neural progenitors to arrest in G2 of the cell cycle with increased DNA content, and that nutrient availability triggers progenitors to re-enter the cell cycle at M phase. Initiation of the nutrient restriction-induced G2 arrest is rapamycin insensitive, but cell cycle re-entry requires mTOR. Finally, we show that activation of insulin receptor signaling is sufficient to increase neural progenitor cell proliferation in the absence of food. A G2 arrest mechanism provides an adaptive strategy to control brain development in response to nutrient availability by triggering a synchronous burst of cell proliferation when nutrients become available. This may be a general cellular mechanism that allows developmental flexibility during times of limited resources.

Conserved NDR/LATS kinase controls RAS GTPase activity to regulate cell growth and chronological lifespan

Adaptation to the nutritional environment is critical for all cells. RAS GTPase is a highly conserved GTP-binding protein with crucial functions for cell growth and differentiation in response to environmental conditions. Here, we describe a novel mechanism connecting RAS GTPase to nutrient availability in fission yeast. We report that the conserved NDR/LATS kinase Orb6 responds to nutritional cues and regulates Ras1 GTPase activity. Orb6 increases the protein levels of an Ras1 GTPase activator, the guanine nucleotide exchange factor Efc25, by phosphorylating Sts5, a protein bound to efc25 mRNA. By manipulating the extent of Orb6-mediated Sts5 assembly into RNP granules, we can modulate Efc25 protein levels, Ras1 GTPase activity, and, as a result, cell growth and cell survival. Thus, we conclude that the Orb6-Sts5-Ras1 regulatory axis plays a crucial role in promoting cell adaptation, balancing the opposing demands of promoting cell growth and extending chronological lifespan.

Cell size and cell cycle progression: the cyclin-dependent kinase link in green algae

This article comments on the following paper: Zachleder V, Ivanov I, Vítová M, and Bišová K. 2019. Effects of cyclin-dependent kinase activity on the coordination of growth and the cell cycle in green algae at different temperatures. Journal of Experimental Botany 70, 845–858.

The TOR Signaling Pathway in Spatial and Temporal Control of Cell Size and Growth

Cell size is amenable by genetic and environmental factors. The highly conserved nutrient-responsive Target of Rapamycin (TOR) signaling pathway regulates cellular metabolic status and growth in response to numerous inputs. Timing and duration of TOR pathway activity is pivotal for both cell mass built up as well as cell cycle progression and is controlled and fine-tuned by the abundance and quality of nutrients, hormonal signals, growth factors, stress, and oxygen. TOR kinases function within two functionally and structurally discrete multiprotein complexes, TORC1 and TORC2, that are implicated in temporal and spatial control of cell size and growth respectively; however, recent data indicate that such functional distinctions are much more complex. Here, we briefly review roles of the two complexes in cellular growth and cytoarchitecture in various experimental model systems.