Biology subjects

Okamura, A.

Publications and source records attributed to Okamura, A..

4 recordsLinked to original sources

bioRxiv2025

Cdc25-mediated activation of the small GTPase RasB is essential for hyphal fusion and symbiotic infection of Epichloë festucae

Epichloe festucae is a filamentous endophytic fungus that symbiotically colonizes the intercellular spaces of aerial tissues in perennial ryegrass (Lolium perenne) without causing disease symptoms. This mutualistic association enhances host resistance to both biotic and abiotic stresses. Balanced and coordinated growth of E. festucae with its host is essential for the establishment and long-term maintenance of the symbiotic relationship. Various E. festucae mutants defective in symbiosis with host plants have been isolated, and notably, many of these symbiosis-defective mutants also lack hyphal fusion ability under culture conditions, supporting a close functional connection between signal transduction required for hyphal fusion and symbiosis establishment. Using a plasmid insertional mutagenesis approach, we identified Cdc25 as an essential regulator of hyphal fusion in E. festucae. Cdc25 encodes a guanine nucleotide exchange factor (GEF) that activates the small GTPase Ras. The{Delta} cdc25 strain lost both hyphal fusion ability and the capacity to infect host plants. Furthermore, yeast two-hybrid assays revealed that Cdc25 specifically interacts with RasB, one of the five Ras proteins in E. festucae. Expression of constitutively active (CA) RasB in the{Delta} cdc25 strain restored both hyphal fusion and host infection, whereas expression of CA-RasB in the{Delta} mpkB deletion strain failed to rescue its defect in hyphal fusion, suggesting that the Cdc25-RasB signaling module acts upstream of the MAPK cascade. In pathogenic fungi, this signaling module is known to regulate infection-related morphogenesis. These findings indicate that E. festucae has evolutionarily repurposed the conserved Cdc25-RasB module to coordinate hyphal fusion and maintain a stable mutualistic interaction with its host.

microbiology↗

bioRxiv2024

Cardiac Pacemaker Cells Harness Stochastic Resonance to Ensure Fail-Safe Operation at Low Rates Bordering on Sinus Arrest

BACKGROUNDThe sinoatrial node (SAN) is the primary pacemaker of the heart. Recent high-resolution imaging showed that synchronized action potentials (APs) exiting the SAN emerge from heterogeneous signals, including subthreshold signals in non-firing (dormant) cells. This raises a new question in cardiac biology: how do these signals contribute to heartbeat generation? Here, we tested the hypothesis that pacemaker cells harness stochastic resonance to ensure fail-safe operation, especially at low rates bordering on sinus arrest. METHODSMembrane potential and Ca signals were measured using perforated-patch recordings in rabbit SAN cells exposed to sine-wave or white-noise currents. Additionally, we imaged Ca signals in intact mouse SAN tissue and performed multiscale model simulations at the subcellular, cellular, and tissue levels. RESULTSIn addition to classical synchronized Ca transients, SAN tissue exhibited heterogeneous local Ca signals of different kinetics. Noise currents, mimicking the heterogeneous natural cell environment, restored AP firing in dormant cells and substantially improved the rate and rhythm of those firing infrequently and irregularly. The benefit followed a bell-shaped curve: the performance improved but then declined, demonstrating a hallmark of stochastic resonance. Rhythmic AP generation in response to sine-wave currents of different frequencies defined a resonance spectrum in SAN cells, reflecting their ability to respond via stochastic resonance to specific frequency components embedded in noise. Cholinergic stimulation shifted the resonance spectrum and responses to noise toward lower frequencies across all amplitudes tested, rendering cells unresponsive to higher-frequency signals while enabling more effective processing of slower signals. Both the numerical models and simultaneous recordings of membrane potential and Ca dynamics demonstrated that stochastic resonance is amplified by coupled electrical and Ca signaling, enhancing AP generation at low noise levels. Adding noise currents to the cell and tissue models allowed firing under conditions where they otherwise would have stopped. CONCLUSIONSSAN cells harness stochastic resonance amplified by coupled membrane-Ca signaling to ensure rhythmic heartbeat initiation, especially at low rates. This new signaling mechanism could help avoid sinus arrest when heart slows but noise increases, such as during parasympathetic stimulation, bradyarrhythmia, or aging.

biophysics↗

bioRxiv2020

Upper extremity exomuscle for shoulder abduction support

Assistive devices may aid motor recovery after a stroke, but access to such devices is limited. Exosuits can aid human movement and may be more accessible alternatives to current devices, but we know little about how they might assist post-stroke upper extremity movement. Here, we designed an exosuit actuator to support shoulder abduction, which we call an "exomuscle" and is based on a form of growing robot called a pneumatic-reel actuator. We also assembled a ceiling-mounted support for a positive control. We verified that both supports reduce the activity of shoulder abductor muscles and do not impede range of motion in healthy participants (n=4). Then, we measured reachable workspace area in stroke survivors (n=6) with both supports and without support. Our exomuscle increased workspace area in four participants (180{+/-}90 cm2) while the ceiling support increased workspace area in five participants (792{+/-}540 cm2). Design decisions that reduce exomuscle complexity, such as leaving the forearm free, likely contribute to performance differences between the two supports. Heterogeneity amongst stroke survivors abilities likely contribute to high variability in our results. Though both supports performed similarly for healthy participants, performance differences in stroke survivors highlight the need to validate assistive devices in the target population.

bioengineering↗

bioRxiv2018

Development of oocytes in a post-spawning Japanese eel Anguilla japonica Temminck & Schlegel in captivity

It is generally believed that parent freshwater eels (Anguilla sp.) die soon after spawning on the assumption that eels are a semelparous (or monocyclic) fish (spawn once at the last stage of life) like Pacific salmonids. However, we observed for the first time a post-spawning female Anguilla japonica again possessed developing oocytes reaching the final maturation stage in captivity five months after the last spawning even without hormonal treatment. Here we describe information on this female about its biological characteristics including gonadal histology and endocrine profiles. The data suggest that lowering water temperature for a period of time is one of the important factors influencing spontaneous gonadal development in this specimen. We also discuss the possibility of induced multiple spawning of this species in captivity.

zoology↗