Biology subjects

Amon, A.

Publications and source records attributed to Amon, A..

9 recordsLinked to original sources

bioRxiv2024

A noncanonical GTPase signaling mechanism controls exit from mitosis in budding yeast

In the budding yeast Saccharomyces cerevisiae, exit from mitosis is coupled to spindle position to ensure successful genome partitioning between mother and daughter cell. This coupling occurs through a GTPase signaling cascade known as the mitotic exit network (MEN). The MEN senses spindle position via a Ras-like GTPase Tem1 which localizes to the spindle pole bodies (SPBs, yeast equivalent of centrosomes) during anaphase and signals to its effector protein kinase Cdc15. How Tem1 couples the status of spindle position to MEN activation is not fully understood. Here, we show that Cdc15 has a relatively weak preference for Tem1GTP and Tem1s nucleotide state does not change upon MEN activation. Instead, we find that Tem1s nucleotide cycle establishes a localization-based concentration difference in the cell where only Tem1GTP is recruited to the SPB, and spindle position regulates the MEN by controlling Tem1 localization. SPB localization of Tem1 primarily functions to promote Tem1-Cdc15 interaction for MEN activation by increasing the effective concentration of Tem1. Consistent with this model, we demonstrate that artificially tethering Tem1 to the SPB or concentrating Tem1 in the cytoplasm with genetically encoded multimeric nanoparticles could bypass the requirement of Tem1GTP and correct spindle position for MEN activation. This localization/concentration-based GTPase signaling mechanism for Tem1 differs from the canonical Ras-like GTPase signaling paradigm and is likely relevant to other localization-based signaling scenarios.

cell biology↗

bioRxiv2021

The environmental stress response regulates ribosome content in cell cycle-arrested S. cerevisiae

Temperature sensitive cell division cycle (cdc-ts) cells are unable to progress through the cell cycle at the restrictive temperature due to mutations in genes essential to cell cycle progress. Cells harboring cdc-ts mutations increase in cell volume upon arrest but eventually stop growing. We found that this attenuation in growth was due to selective downregulation of ribosome concentration. We saw similar ribosome downregulation in cells arrested in the cell cycle through alpha factor addition, rapamycin addition, and entrance into stationary phase. In all cell cycle arrests studied, cells activated the Environmental Stress Response (ESR), a key transcriptional response to many stressors in S. cerevisiae. When we combined cell cycle arrest with hyperactivation of the Ras/PKA pathway, ESR activation was prevented, cells were unable to downregulate their ribosomes, and cell viability was decreased. Our work uncovers a key role for the environmental stress response in coupling cell cycle progression to biomass accumulation.

cell biology↗

bioRxiv2021

Decreasing Mitochondrial RNA Polymerase Activity Reverses Biased Inheritance of Hypersuppressive mtDNA

Faithful inheritance of mitochondrial DNA (mtDNA) is crucial for cellular respiration/oxidative phosphorylation and mitochondrial membrane potential. However, how mtDNA is transmitted to progeny is not fully understood. We utilized hypersuppressive mtDNA, a class of respiratory deficient Saccharomyces cerevisiae mtDNA that is preferentially inherited over wild-type mtDNA (rho+), to uncover the factors governing mtDNA inheritance. We found that regions of rho+ mtDNA persisted after hypersuppressive takeover indicating that hypersuppressive preferential inheritance may partially be due to active destruction of rho+ mtDNA. From a multicopy suppression screen, we found that overexpression of putative mitochondrial RNA exonuclease PET127 reduced hypersuppressive biased inheritance. This suppression required PET127 binding to the mitochondrial RNA polymerase RPO41 but not PET127 exonuclease activity. A temperature-sensitive allele of RPO41 improved rho+ mtDNA inheritance relative to hypersuppressive mtDNA at semi-permissive temperatures revealing a previously unknown role for rho+ transcription in promoting hypersuppressive mtDNA inheritance.

cell biology↗

bioRxiv2020

Cell size is a determinant of stem cell potential during aging

Stem cells are remarkably small in size. Whether small size is important for stem cell function is unknown. We find that murine hematopoietic stem cells (HSCs) enlarge under conditions known to decrease stem cell function. This decreased fitness of large HSCs is due to reduced proliferative potential. Preventing HSC enlargement by inhibiting macromolecule biosynthesis or reducing large HSCs size by shortening G1 averts the loss of stem cell potential under conditions causing stem cell exhaustion. Finally, we show that a fraction of murine and human HSCs enlarge during aging. Preventing this age-dependent enlargement improves HSC function. We conclude that small cell size is important for stem cell function in vivo and propose that stem cell enlargement contributes to their functional decline during aging. One Sentence SummarySize increase drives stem cell aging.

cell biology↗

bioRxiv2020

Cross-compartment signal propagation in the Mitotic Exit Network

In budding yeast, the Mitotic Exit Network (MEN), a GTPase signaling cascade integrates spatial and temporal cues to promote exit from mitosis. This signal integration requires transmission of a signal generated on the cytoplasmic face of spindle pole bodies (SPBs; yeast equivalent of centrosomes) to the nucleolus, where the MEN effector protein Cdc14 resides. Here, we show that the MEN activating signal at SPBs is relayed to Cdc14 in the nucleolus through the dynamic localization of its terminal kinase complex Dbf2-Mob1. Cdc15, the protein kinase that activates Dbf2-Mob1 at SPBs, also regulates its nuclear access. Once in the nucleus, priming phosphorylation of Cfi1/Net1, the nucleolar anchor of Cdc14, by the Polo-like kinase Cdc5 targets Dbf2-Mob1 to the nucleolus. Nucleolar Dbf2-Mob1 then phosphorylates Cfi1/Net1 and Cdc14, activating Cdc14. The kinase-primed transmission of the MEN signal from the cytoplasm to the nucleolus exemplifies how signaling cascades can bridge distant inputs and responses.

cell biology↗

bioRxiv2020

Aneuploid cells activate NF-κB to promote their immune clearance by NK cells

SummaryThe immune system plays a major role in the protection against cancer. Identifying and characterizing the pathways mediating this immune surveillance is thus critical for understanding how cancer cells are recognized and eliminated. We previously found that untransformed cells that had undergone senescence due to highly abnormal karyotypes are eliminated by Natural Killer (NK) cells in vitro. Here we show that this is also true for aneuploid untransformed cells that had not lost their ability to proliferate. Their elimination by NK cells, like that of aneuploid senescent cells, is predominantly mediated by non-cell autonomous mechanisms. Our data further indicate that NF-κB signaling in aneuploid cells is central to eliciting this immune response. Inactivating NF-κB abolishes NK-cell mediated clearance in aneuploid cells in vitro. In cancer cell lines, NF-κB signaling correlates with degree of aneuploidy, raising the possibility that aneuploidy-induced immune recognition is partially retained in cancer.Competing Interest StatementThe authors have declared no competing interest.View Full Text

cancer biology↗

bioRxiv2020

The environmental stress response causes ribosome loss in aneuploid yeast cells.

Aneuploidy, a condition characterized by whole chromosome gains and losses, is often associated with significant cellular stress and decreased fitness. However, how cells respond to the aneuploid state has remained controversial. In aneuploid budding yeast, two opposing gene expression patterns have been reported: the "environmental stress response" (ESR) and the "common aneuploidy gene-expression" (CAGE) signature, in which many ESR genes are oppositely regulated. Here, we investigate and bring clarity to this controversy. We show that the CAGE signature is not an aneuploidy-specific gene expression signature but the result of normalizing the gene expression profile of actively proliferating aneuploid cells to that of euploid cells grown into stationary phase. Because growth into stationary phase is amongst the strongest inducers of the ESR, the ESR in aneuploid cells was masked when stationary phase euploid cells were used for normalization in transcriptomic studies. When exponentially growing euploid cells are used in gene expression comparisons with aneuploid cells, the CAGE signature is no longer evident in aneuploid cells. Instead, aneuploid cells exhibit the ESR. We further show that the ESR causes selective ribosome loss in aneuploid cells, providing an explanation for the decreased cellular density of aneuploid cells. We conclude that aneuploid budding yeast cells mount the ESR, rather than the CAGE signature, in response to aneuploidy-induced cellular stresses, resulting in selective ribosome loss. We propose that the ESR serves two purposes in aneuploid cells: protecting cells from aneuploidy-induced cellular stresses and preventing excessive cellular enlargement during slowed cell cycles by downregulating translation capacity.

cell biology↗

bioRxiv2019

Aneuploidy and deregulated DNA damage response define haploinsufficiency in breast tissues of BRCA2 mutation carriers

Women harboring heterozygous germline mutations of BRCA2 have a 50-80% risk of developing breast cancer, yet the early pathogenesis of these cancers is poorly understood. We sought to reveal early steps in BRCA2-associated carcinogenesis through analysis of sorted cell populations from freshly-isolated, non-cancerous breast tissues among a cohort of BRCA2 mutation carriers and matched controls. Single-cell whole-genome sequencing demonstrates that >25% of BRCA2 carrier (BRCA2mut/+) luminal progenitor (LP) cells exhibit sub-chromosomal copy number variations (CNVs), which are rarely observed in non-carriers. Correspondingly, primary BRCA2mut/+ breast epithelia exhibit spontaneous and replication stress-induced DNA damage together with attenuated replication checkpoint and apoptotic responses, associated with an age-associated expansion of the LP compartment in human carrier tissues. These phenotypes are not associated with loss of wild-type BRCA2. Collectively, these findings provide evidence for BRCA2 haploinsufficiency and associated DNA damage in vivo that precede histologic abnormalities. These results provide unanticipated opportunities for new cancer risk assessment and prevention strategies in high-risk patients.

cancer biology↗

bioRxiv2018

A somatic evolutionary model of the dynamics of aneuploid cells during hematopoietic reconstitution

Aneuploidy is associated with many cancers. Recent studies demonstrate that in thehematopoietic stem and progenitor cell (HSPC) compartment aneuploid cells havereduced fitness and are efficiently purged from the bone marrow. However, early phasesof hematopoietic reconstitution following bone marrow transplantation provide awindow of opportunity whereby aneuploid cells rise in frequency, only to decline to basallevels thereafter. Here we demonstrate by Monte Carlo modeling that two mechanismscould underlie this aneuploidy peak: rapid expansion of the engrafted HSPC populationand bone marrow microenvironment degradation caused by pre-transplantationradiation treatment. Both mechanisms reduce the strength of purifying selection actingin early post-transplantation bone marrow. We explore the contribution of other factorssuch as alterations in cell division rates that affect the strength of purifying selection, thebalance of drift and selection imposed by the HSPC population size, and the mutationselectionbalance dependent on the rate of aneuploidy generation per cell division. Wepropose a somatic evolutionary model for the dynamics of cells with aneuploidy or otherfitness-reducing mutations during hematopoietic reconstitution following bone marrowtransplantation.\n\nSignificanceBone marrow transplantations (BMT) following ablative irradiation pose a great health risk. Its been shown that additionally the bone microenvironment is conducive to elevated frequencies of aneuploid cells in mice during bone marrow reconstitution post-BMT. As aneuploidy is linked with many cancers, we explore the reasons of such aberrant cell frequency peaks by Monte Carlo modeling. We demonstrate that elevated rates of aneuploidy early post-BMT are likely to be caused by reduced purifying somatic selection resulting from the expansion of the reconstituting population and the damage to stem cell niches caused by ablative radiation

cell biology↗