Biology subjects

Fraser, J. A.

Publications and source records attributed to Fraser, J. A..

3 recordsLinked to original sources

bioRxiv2026

The length and time constants of propagating action potentials

Length and time constants are foundational to the study of conduction in neurons but are defined only for passive membranes. Here we define length and time constants for uniformly propagating action potentials. The derivation exploits transmembrane current transition (TCT) instants during action potential conduction when the net transmembrane ionic current is zero, but axial current remains non-zero. At these instants, we define a rate coefficient, {kappa}, and from it define {lambda}AP=1/{surd}({kappa}racm) and {tau}AP=1/{kappa}. We show that action potential propagation velocity is exactly {lambda}AP/{tau}AP. We establish that {lambda}AP equals the ratio of axial to local capacitive current throughout the waveform, sets the exponential spatial weighting of ionic current, and is the local length scale of the dV/dt field at a TCT. It also approximates the length scale of the leading edge to within a fractional error of {tau}foot/{tau}m. These identities are independent of channel gating formalism. We relate the new constants exactly to the passive DC, AC and AP foot constants and demonstrate the membrane and channel properties that determine {kappa}. Finally, we suggest how {kappa} may in principle be determined from experimental voltage waveforms.

physiology↗

bioRxiv2022

Lineages derived from Cryptococcus neoformans type strain H99 support a link between the capacity to be pleomorphic and virulence

The pathogenic yeast Cryptococcus neoformans causes nearly 200,000 deaths annually in immunocompromised individuals. Cryptococcus cells can undergo substantial morphological change during mammalian infection, including increased capsule and cell size, the release of shed capsule, and the production of titan (> 10 m), micro (< 2 m) and irregular cells. We examined phenotypic variation under conditions designed to simulate in vivo stress in a collection of nine lineages derived from the C. neoformans type strain H99. These lineages are highly genetically similar but have a range of virulence levels. Strains from hypervirulent lineages had a larger average capsule size, greater variation in cell size, and an increased production of micro cells and shed capsule. We tested whether disruption of SGF29, which encodes a component of the SAGA histone acetylation complex that has previously been implicated in the hypervirulence of some lineages, might also have a role in the production of morphological variants. Deletion of SGF29 in a lineage with intermediate virulence substantially increased its production of micro cells and released capsule, consistent with a switch to hypervirulence. We further examined SGF29 in a set of 52 clinical isolates and found loss-of-function mutations were significantly correlated with patient death. Expansion of a TA repeat in the second intron of SGF29 was positively correlated with cell and capsule size, suggesting it may also affect Sgf29 function. This study extends the evidence for a link between pleomorphism and virulence in Cryptococcus, with a likely role for epigenetic mechanisms mediated by SAGA-induced histone acetylation. IMPORTANCECryptococcosis is a devastating cause of death and disease worldwide. During infection, Cryptococcus cells can undergo substantial changes to their size and shape. In this study, we used a collection C. neoformans strains that are highly genetically similar but possess differing levels of virulence to investigate how morphological variation aligns with virulence. We found hypervirulent strains on average had larger capsules and greater variation in cell size, and also produced more micro cells and shed capsule. These hypervirulent strains possessed a mutation in SGF29, which encodes a component of the SAGA complex involved in epigenetic regulation. Analysis of the SGF29 gene in a set of clinical isolates found strains with loss-of-function mutations were associated with higher patient death rates. The capacity to vary appears to be linked with virulence in Cryptococcus, and this can occur in the absence of genetic variation via epigenetic mechanisms.

microbiology↗

bioRxiv2017

Identification of environmental and genetic factors important for Cryptococcus neoformans titan cell formation using new in vitro inducing conditions

The pathogenic fungus Cryptococcus neoformans exhibits morphological changes in cell size during lung infection, producing both typical size 5 to 7 {micro}m cells and large titan cells (> 10 {micro}m and up to 100 {micro}m). We found and optimized in vitro conditions that produce titan cells in order to identify the ancestry of titan cells, the environmental determinants, and the key gene regulators of titan cell formation. Titan cells generated in vitro harbor the main characteristics of titan cells produced in vivo including their large cell size (>10 {micro}m), polyploidy with a single nucleus, large vacuole, dense capsule, and thick cell wall. Here we show titan cells derived from the enlargement of progenitor cells in the population independent of yeast growth rate. Change in the incubation medium, hypoxia, nutrient starvation and low pH were the main factors that trigger titan cell formation, while quorum sensing factors like the initial inoculum concentration, pantothenic acid, and the quorum sensing peptide Qsp1p also impacted titan cell formation. Inhibition of ergosterol, protein and nucleic acid biosynthesis altered titan cell formation, as did serum, phospholipids and anti-capsular antibodies in our settings. We explored genetic factors important for titan cell formation using three approaches. Using H99-derivative strains with natural genetic differences, we showed that titan cell formation was dependent on LMP1 and SGF29 genes. By screening a gene deletion collection, we also confirmed that GPR4/5-RIM101, and CAC1 genes were required to generate titan cells and that the PKR1, TSP2, USV101 genes negatively regulated titan cell formation. Furthermore, analysis of spontaneous Pkr1 loss-of-function clinical isolates confirmed the important role of the Pkr1 protein as a negative regulator of titan cell formation. Through development of a standardized and robust in vitro assay, our results provide new insights into titan cell biogenesis with the identification of multiple important factors/pathways.\n\nAuthor SummaryCryptococcus neoformans is a yeast that is capable of morphological change upon interaction with the host. Particularly, in the lungs of infected mice, a subpopulation of yeast enlarges, producing cells up to 100 {micro}m in cell body diameter - referred to as titan cells. Along with their large size, the titan cells have other unique characteristics such as thickened cell wall, dense capsule, polyploidization, large vacuole with peripheral nucleus and cellular organelles. The generation of a large number of such cells outside the lungs of mice has been described but was not reproducible nor standardized. Here we report standardized, reproducible, robust conditions for generation of titan cells and explored the environmental and genetic factors underlying the genesis of these cells. We showed that titan cells were generated upon stresses such as change in the incubation medium, nutrient deprivation, hypoxia and low pH. Using collections of well characterized reference strains and clinical isolates, we validated with our model that the cAMP/PKA/Rim101 pathway is a major genetic determinant of titan cell formation. This study opens the way for a more comprehensive picture of the ontology of morphological changes in Cryptococcus neoformans and its impact on pathobiology of this deadly pathogen.

microbiology↗