Biology subjects

White, S. A.

Publications and source records attributed to White, S. A..

8 recordsLinked to original sources

bioRxiv2026

Mean temperature determines whether winter variability accelerates or buffers energy loss

Winter survival in dormant animals depends on conserving finite energy reserves, yet winter temperatures fluctuate around shifting means. In ectotherms, metabolic rate increases exponentially with temperature, so thermal variability is expected to accelerate energy loss, with important consequences for overwinter survival and population persistence under climate change. However, it remains unclear whether dormant ectotherms can compensate physiologically for thermal variability. We overwintered Bombus impatiens queens under constant (2, 3, 4{degrees}C) or variable (2 {+/-} 6{degrees}C or 4 {+/-} 6{degrees}C) regimes for six weeks, then measured metabolic rates across a range of temperatures. The temperature dependence of metabolic rate shifted in response to thermal experience, but the direction of compensation depended on mean temperature: variability centered on 2{degrees}C elevated metabolic rate and increased thermal sensitivity relative to all other conditions, whereas variability centered on 4{degrees}C reduced metabolic rate and dampened thermal sensitivity relative to constant 4{degrees}C. We used these metabolic responses to simulate rates of lipid depletion and found that survival trajectories echoed physiological shifts: experiencing variability around 2{degrees}C would reduce subsequent survival time, whereas experiencing variability around 4{degrees}C would preserve subsequent survival even under variable future conditions. Thus, identical thermal variance produced opposite energetic outcomes depending on the mean temperature around which fluctuations occurred. Integrating both temperature means and variability is, therefore, essential for predicting overwintering survival in a changing world.

physiology↗

bioRxiv2023

Whole Body PBPK Model of Nasal Naloxone Administration to Measure Repeat Dosing Requirements During Fentanyl Overdose

Opioid use in the United States and abroad is an endemic part of culture with yearly increases in overdose rates and deaths. As rates of overdose incidence increases, the use of the safe and effective reversal agent, naloxone, in the form of a nasal rescue spray is being fielded and used by emergency medical technicians (EMTs) at a greater and greater rate. Despite advances in deployment of these rescue products, deaths are continuing to increase. There is evidence that repeated dosing of a naloxone nasal spray (such as Narcan) is becoming more common due to the amount and type of opiate being abused. Despite the benefits of naloxone related to opioid reversals, we lack repeated dosing guidelines as a function of opiate and amount the patient has taken. Goal directed dosing is promising, where respiratory markers are being used as an indication of the patient recovery but require time and understanding by the EMT. We construct a whole-body model of the pharmacokinetics and dynamics of an opiate, fentanyl on respiratory depression. We then construct a model of nasal deposition and administration of naloxone to investigate repeat dosing requirements for large overdoses. We demonstrate that naloxone is highly effective at reversing respiratory symptoms of the patient and recommend dosing requirements as a function of opiate amount administered. By designing the model to include circulation and respiration we investigate physiological markers that may be used in goal directed therapy rescue treatments.

pharmacology and toxicology↗

bioRxiv2021

Proteasome-dependent truncation of the negative heterochromatin regulator Epe1 mediates antifungal resistance

Epe1 histone demethylase restricts H3K9-methylation-dependent heterochromatin, preventing it from spreading over, and silencing, gene-containing regions in fission yeast. External stress induces an adaptive response allowing heterochromatin island formation that confers resistance on surviving wild-type lineages. Here we investigate the mechanism by which Epe1 is regulated in response to stress. Exposure to caffeine or antifungals results in Epe1 ubiquitylation and proteasome-dependent removal of the N-terminal 150 residues from Epe1, generating truncated tEpe1 which accumulates in the cytoplasm. Constitutive tEpe1 expression increases H3K9 methylation over several chromosomal regions, reducing expression of underlying genes and enhancing resistance. Reciprocally, constitutive non- cleavable Epe1 expression decreases resistance. tEpe1-mediated resistance requires a functional JmjC demethylase domain. Moreover, caffeine-induced Epe1-to-tEpe1 cleavage is dependent on an intact cell-integrity MAP kinase stress signalling pathway, mutations in which alter resistance. Thus, environmental changes provoke a mechanism that curtails the function of this key epigenetic modifier, allowing heterochromatin to reprogram gene expression, thereby bestowing resistance to some cells within a population. H3K9me-heterochromatin components are conserved in human and crop plant fungal pathogens for which a limited number of antifungals exist. Our findings reveal how transient heterochromatin-dependent antifungal resistant epimutations develop and thus inform on how they might be countered.

cell biology↗

bioRxiv2020

Large domains of heterochromatin direct the formation of short mitotic chromosome loops

During mitosis chromosomes reorganise into highly compact, rod-shaped forms, thought to consist of consecutive chromatin loops around a central protein scaffold. Condensin complexes are involved in chromatin compaction, but the contribution of other chromatin proteins, DNA sequence and histone modifications is less understood. A large region of fission yeast DNA inserted into a mouse chromosome was previously observed to adopt a mitotic organisation distinct from that of surrounding mouse DNA. Here we show that a similar distinct structure is common to a large subset of insertion events in both mouse and human cells and is coincident with the presence of high levels of heterochromatic H3 lysine 9 trimethylation (H3K9me3). Hi-C and microscopy indicate that the heterochromatinised fission yeast DNA is organised into smaller chromatin loops than flanking euchromatic mouse chromatin. We conclude that heterochromatin alters chromatin loop size, thus contributing to the distinct appearance of heterochromatin on mitotic chromosomes, such as at centromeres.

cell biology↗

bioRxiv2019

Bax and Bak jointly control survival and dampen the early unfolded protein response in pancreatic β-cells under glucolipotoxic stress

ER stress and apoptosis contribute to the loss of pancreatic {beta}-cells under the pro-diabetic conditions of glucolipotoxicity. Although activation of the canonical pathway of intrinsic apoptosis is known to require Bax and Bak, their individual and combined involvement in glucolipotoxic {beta}-cell death have not been demonstrated. It has also remained an open question if Bax and Bak in {beta}-cells have non-apoptotic roles in mitochondrial function and ER stress signaling, as suggested in other cell types. Using mice with individual or combined {beta}-cell deletion of Bax and Bak, we demonstrated that glucolipotoxic {beta}-cell death in vitro happens in sequential stages; first via non-apoptotic mechanisms and later by apoptosis, which Bax and Bak were redundant in triggering. In contrast, they had non-redundant roles in mediating staurosporine-induced {beta}-cell apoptosis. We further established that Bax and Bak do not affect normal glucose-stimulated {beta}-cell Ca2+ responses, insulin secretion, or in vivo glucose tolerance. Finally, our experiments revealed that Bax and Bak together dampen the unfolded protein response in {beta}-cells during the early stages of chemical- or glucolipotoxicity-induced ER stress. These findings identify novel roles of the canonical apoptosis machinery in modulating stress signals that are important for the pathobiology of {beta}-cells in diabetes.

cell biology↗

bioRxiv2019

Epigenetic gene silencing by heterochromatin primes fungal resistance

Genes embedded in H3 lysine 9 methylation (H3K9me)-dependent heterochromatin are transcriptionally silenced1-3. In fission yeast, Schizosaccharomyces pombe, H3K9me heterochromatin silencing can be transmitted through cell division provided the counteracting demethylase Epe1 is absent4,5. It is possible that under certain conditions wild-type cells might utilize heterochromatin heritability to form epimutations, phenotypes mediated by unstable silencing rather than changes in DNA6,7. Here we show that resistant heterochromatin-mediated epimutants are formed in response to threshold levels of the external insult caffeine. ChIP-seq analyses of unstable resistant isolates revealed new distinct heterochromatin domains, which in some cases reduce the expression of underlying genes that are known to confer resistance when deleted. Targeting synthetic heterochromatin at implicated loci confirmed that resistance results from heterochromatin-mediated silencing. Our analyses reveal that epigenetic processes allow wild-type fission yeast to adapt to non-favorable environments without altering their genotype. In some isolates, subsequent or co-occurring gene amplification events enhance resistance. Thus, heterochromatin-dependent epimutant formation provides a bet-hedging strategy that allows cells to remain genetically wild-type but transiently adapt to external insults. As unstable caffeine-resistant isolates show cross-resistance to the fungicide clotrimazole it is likely that related heterochromatin-dependent processes contribute to anti-fungal resistance in both plant and human pathogenic fungi.

genetics↗

bioRxiv2019

Hap2-Ino80 facilitated transcription promotes de novo establishment of CENP-A chromatin

Centromeres are maintained epigenetically by the presence of CENP-A, an evolutionarily-conserved histone H3 variant, which directs kinetochore assembly and hence, centromere function. To identify factors that promote assembly of CENP-A chromatin, we affinity selected solubilised fission yeast CENP-ACnp1 chromatin. All subunits of the Ino80 complex were enriched, including the auxiliary subunit Hap2. In addition to a role in maintenance of CENP-ACnp1 chromatin integrity at endogenous centromeres, Hap2 is required for de novo assembly of CENP-ACnp1 chromatin on naive centromere DNA and promotes H3 turnover on centromere regions and other loci prone to CENP-ACnp1 deposition. Prior to CENP-ACnp1 chromatin assembly, Hap2 facilitates transcription from centromere DNA. These analyses suggest that Hap2-Ino80 destabilises H3 nucleosomes on centromere DNA through transcription-coupled histone H3 turnover, driving the replacement of resident H3 nucleosomes with CENP-ACnp1 nucleosomes. These inherent properties define centromere DNA by directing a program that mediates CENP-ACnp1 assembly on appropriate sequences.

molecular biology↗

bioRxiv2017

Centromeric DNA destabilizes H3 nucleosomes to promote CENP-A deposition during the cell cycle

Active centromeres are defined by the presence of nucleosomes containing CENP-A, a histone H3 variant, which alone is sufficient to direct kinetochore assembly. Once assembled at a location CENP-A chromatin and kinetochores are maintained at that location though a positive feedback loop where kinetochore proteins recruited by CENP-A itself promote deposition of new CENP-A following replication. Although CENP-A chromatin itself is a heritable entity, it is normally associated with specific sequences. Intrinsic properties of centromeric DNA may favour the assembly of CENP-A rather than H3 nucleosomes. Here we investigate histone dynamics on centromeric DNA. We show that during S-phase histone H3 is deposited as a placeholder at fission yeast centromeres and is subsequently evicted in G2 when we detect deposition of the majority of new CENP-ACnp1. We also find that centromeric DNA has an innate property of driving high rates of turnover of H3 containing nucleosomes resulting in low nucleosome occupancy. When placed at an ectopic chromosomal location in the absence of any CENP-ACnp1 assembly, centromeric DNA retains its ability to impose S-phase deposition and G2 eviction of H3, suggesting that features within this DNA program H3 dynamics. As RNAPII occupancy on this centromere DNA coincides with H3 eviction in G2, we propose a model in which RNAPII-coupled chromatin remodelling promotes replacement of H3 with CENP-ACnp1 nucleosomes.

cell biology↗