Allometric scaling of metabolic rate results in lower total mitochondrial oxygen consumption with increasing organismal size. This is considered a universal law in biology. Here, we discuss how ...allometric laws impose size-dependent limits to mitochondrial activity at the cellular level. This cell-size-dependent mitochondrial metabolic activity results in nonlinear scaling of metabolism in proliferating cells, which can explain size homeostasis. The allometry in mitochondrial activity can be controlled through mitochondrial fusion and fission machinery, suggesting that mitochondrial connectivity can bypass transport limitations, the presumed biophysical basis for allometry. As physical size affects cellular functionality, cell-size-dependent metabolism becomes directly relevant for development, metabolic diseases, and aging.
Most of the drugs currently prescribed for cancer treatment are riddled with substantial side effects. In order to develop more effective and specific strategies to treat cancer, it is of importance ...to understand the biology of drug targets, particularly the newly emerging ones. A comprehensive evaluation of these targets will benefit drug development with increased likelihood for success in clinical trials. The folate-mediated one-carbon (1C) metabolism pathway has drawn renewed attention as it is often hyperactivated in cancer and inhibition of this pathway displays promise in developing anticancer treatment with fewer side effects. Here, we systematically review individual enzymes in the 1C pathway and their compartmentalization to mitochondria and cytosol. Based on these insight, we conclude that (1) except the known 1C targets (DHFR, GART, and TYMS), MTHFD2 emerges as good drug target, especially for treating hematopoietic cancers such as CLL, AML, and T-cell lymphoma; (2) SHMT2 and MTHFD1L are potential drug targets; and (3) MTHFD2L and ALDH1L2 should not be considered as drug targets. We highlight MTHFD2 as an excellent therapeutic target and SHMT2 as a complementary target based on structural/biochemical considerations and up-to-date inhibitor development, which underscores the perspectives of their therapeutic potential.
Eukaryotic cells attempt to maintain an optimal size, resulting in size homeostasis. While cellular content scales isometrically with cell size, allometric laws indicate that metabolism per mass unit ...should decline with increasing size. Here we use elutriation and single-cell flow cytometry to analyze mitochondrial scaling with cell size. While mitochondrial content increases linearly, mitochondrial membrane potential and oxidative phosphorylation are highest at intermediate cell sizes. Thus, mitochondrial content and functional scaling are uncoupled. The nonlinearity of mitochondrial functionality is cell size, not cell cycle, dependent, and it results in an optimal cell size whereby cellular fitness and proliferative capacity are maximized. While optimal cell size is controlled by growth factor signaling, its establishment and maintenance requires mitochondrial dynamics, which can be controlled by the mevalonate pathway. Thus, optimization of cellular fitness and functionality through mitochondria can explain the requirement for size control, as well as provide means for its maintenance.
Display omitted
•Mitochondrial functionality is highest in intermediate-sized cells in a population•Mitochondrial membrane potential changes with cell size, not cell cycle•Evidence for an optimal cell size, whereby functionality and fitness are maximized•Mitochondrial dynamics and mevalonate pathway required for the optimal cell size
Organelle content scales linearly with cell size. Miettinen and Björklund investigate how this relates to organelle function and show that mitochondrial functionality and cellular fitness are highest at intermediate cell sizes, suggesting the existence of an optimal cell size. The mevalonate pathway contributes to cell size scaling of mitochondrial function.
Overexpression screens are used to explore gene functions in Drosophila, but this strategy suffers from the lack of comprehensive and systematic fly strain collections and efficient methods for ...generating such collections. Here, we present a strategy that could be used efficiently to generate large numbers of transgenic Drosophila strains, and a collection of 1149 UAS-ORF fly lines that were created with the site-specific ΦC31 integrase method. For this collection, we used a set of 655 genes that were cloned as two variants, either as an open reading frame (ORF) with a native stop codon or with a C-terminal 3xHA tag. To streamline the procedure for transgenic fly generation, we demonstrate the utility of injecting pools of plasmids into embryos, each plasmid containing a randomised sequence (barcode) that serves as a unique identifier for plasmids and, subsequently, fly strains. We also developed a swapping technique that facilitates the rapid exchange of promoters and epitope tags in vivo, expanding the versatility of the ORF collection. The work described here serves as the basis of a systematic library of Gal4/UAS-regulated transgenes.
The maintenance of cell size homeostasis has been studied for years in different cellular systems. With the focus on ‘what regulates cell size’, the question ‘why cell size needs to be maintained’ ...has been largely overlooked. Recent evidence indicates that animal cells exhibit nonlinear cell size dependent growth rates and mitochondrial metabolism, which are maximal in intermediate sized cells within each cell population. Increases in intracellular distances and changes in the relative cell surface area impose biophysical limitations on cells, which can explain why growth and metabolic rates are maximal in a specific cell size range. Consistently, aberrant increases in cell size, for example through polyploidy, are typically disadvantageous to cellular metabolism, fitness and functionality. Accordingly, cellular hypertrophy can potentially predispose to or worsen metabolic diseases. We propose that cell size control may have emerged as a guardian of cellular fitness and metabolic activity.
Cell size is intimately connected to metabolism and growth rate, which are influenced by mitochondrial activity and biophysical constraints. We discuss that cellular fitness is often cell‐size dependent. While the current evidence relies largely on proliferating cells, this connection from cell size to fitness may also help explain metabolic diseases.
Joint regulation of growth rate and cell division rate determines cell size. Here we discuss how animal cells achieve cell size homeostasis potentially involving multiple signaling pathways ...converging at metabolic regulation of growth rate and cell cycle progression. While several models have been developed to explain cell size control, comparison of the two predominant models shows that size homeostasis is dependent on the ability to adjust cellular growth rate based on cell size. Consequently, maintenance of size homeostasis requires that larger cells can grow slower than small cells in relative terms. We review recent experimental evidence showing that such size adjustment occurs primarily at or immediately before the G1/S transition of the cell cycle. We further propose that bidirectional feedback between growth rate and size results in cell size sensing and discuss potential mechanisms how this may be accomplished.
•Cell size is determined by joint regulation of cell size and cell cycle duration.•Size dependent modulation of growth rate allows maintenance of cell size homeostasis.•Central carbon metabolism is key regulator of both cell size and cell cycle.•Size dependent regulation of metabolism and growth rate implies cell size sensing.
The analgesic and antipyretic compound acetaminophen (paracetamol) is one of the most used drugs worldwide. Acetaminophen overdose is also the most common cause for acute liver toxicity. Here we show ...that acetaminophen and many structurally related compounds bind quinone reductase 2 (NQO2) in vitro and in live cells, establishing NQO2 as a novel off-target. NQO2 modulates the levels of acetaminophen derived reactive oxygen species, more specifically superoxide anions, in cultured cells. In humans, NQO2 is highly expressed in liver and kidney, the main sites of acetaminophen toxicity. We suggest that NQO2 mediated superoxide production may function as a novel mechanism augmenting acetaminophen toxicity.
Balance between cell growth and proliferation determines cell size homeostasis, but little is known about how metabolic pathways are involved in the maintenance of this balance. Here, we perform a ...screen with a library of clinically used drug molecules for their effects on cell size. We find that statins, inhibitors of the mevalonate pathway, reduce cell proliferation and increase cell size and cellular protein density in various cell types, including primary human cells. Mevalonate pathway effects on cell size and protein density are mediated through geranylgeranylation of the small GTPase RAB11, which is required for basal autophagic flux. Our results identify the mevalonate pathway as a metabolic regulator of autophagy and expose a paradox in the regulation of cell size and proteostasis, where inhibition of an anabolic pathway can cause an increase in cell size and cellular protein density.
Display omitted
•Mevalonate pathway regulates cell size, proliferation, and cellular protein density•Geranylgeranylation of RAB11 links mevalonate pathway to autophagy•Mevalonate pathway is required for basal autophagic flux•Cell size and protein density effects are dependent on RAB11 and autophagy
Control of cell size is essential for normal cell functions. Miettinen and Björklund find that the mevalonate pathway regulates basal autophagic flux in mammalian cells, and they show that inhibition of this metabolic pathway increases cell size and, unexpectedly, cellular protein density.
Homozygosity for the G allele of rs6983267 at 8q24 increases colorectal cancer (CRC) risk approximately 1.5 fold. We report here that the risk allele G shows copy number increase during CRC ...development. Our computer algorithm, Enhancer Element Locator (EEL), identified an enhancer element that contains rs6983267. The element drove expression of a reporter gene in a pattern that is consistent with regulation by the key CRC pathway Wnt. rs6983267 affects a binding site for the Wnt-regulated transcription factor TCF4, with the risk allele G showing stronger binding in vitro and in vivo. Genome-wide ChIP assay revealed the element as the strongest TCF4 binding site within 1 Mb of MYC. An unambiguous correlation between rs6983267 genotype and MYC expression was not detected, and additional work is required to scrutinize all possible targets of the enhancer. Our work provides evidence that the common CRC predisposition associated with 8q24 arises from enhanced responsiveness to Wnt signaling.
Overexpression screens can be used to explore gene function in Drosophila melanogaster, but to demonstrate their full potential, comprehensive and systematic collections of fly strains are required. ...Here we provide a protocol for high-throughput cloning of Drosophila open-reading frames (ORFs) that are regulated by upstream activation sequences (UAS sites); the resulting GAL4-inducible UAS-ORF plasmid library is then used to generate Drosophila strains by ΦC31 integrase-mediated site-specific integration. We also provide details for FLP/FRT-mediated in vivo exchange of epitope tags (or regulatory regions) in the ORF library strains, which further extends the potential applications of the library. These transgenic UAS-ORF strains are a useful resource to complement and validate genetic experiments performed with loss-of-function mutants and RNA interference (RNAi) lines. The duration of the complete protocol strongly depends on the number of ORFs required, but embryos can be injected and balanced fly stocks can be established within ∼7-8 weeks for a few genes.
PDF
