Yoko Yamaguchi

Understanding the growth of organisms is of fundamental importance in achieving the sustainable use of bioresources and/or the conservation of ecosystems. Hagfishes are oceanic scavengers that typically inhabit deep water and play an indispensable role in marine nutrient cycling. However, as the conventional methods for age determination are not applicable, the growth and lifespan of hagfishes are completely unknown. In the present study, the growth and feeding of the inshore hagfish (Eptatretus burgeri) in the laboratory were continuously monitored for an unprecedentedly long period (more than 1.4 years), with feeding once every 4–5 weeks. The animals displayed either positive/negative/zero growth across the experimental period. Eptatretus burgeri typically takes 8–10 days to digest food, and excretes all the gut contents at once as huge feces. Based on the growth, feeding and excretion data, it seems plausible that the wild populations feed at less than 2-week intervals. In addition, by adopting a recently developed technique to estimate fish age from the relative growth data, the age-size relationship in E. burgeri was explored. The generated growth curve predicted that the gonads start to develop only 4 years after hatching at the earliest. Eptatretus burgeri can live nearly 50 years, and the animals targeted in commercial hagfishery are mainly 6–9 years old. Moreover, additional insights into the oviposition cycle were obtained. The present study provides essential information for understanding the ecology of hagfishes and consequently highlights the putative vulnerability of wild hagfish populations to fishing and environmental stresses.

Hagfishes are characterized by omo- and iono-conforming nature similar to marine invertebrates. Conventionally, hagfishes had been recognized as the most primitive living vertebrate that retains plesiomorphic features. However, some of the “ancestral” features of hagfishes, such as rudimentary eyes and the lack of vertebrae, have been proven to be deceptive. Similarly, by the principle of maximum parsimony, the unique body fluid regulatory strategy of hagfishes seems to be apomorphic, since the lamprey, another cyclostome, adopts osmo- and iono-regulatory mechanisms as in jawed vertebrates. Although hagfishes are unequivocally important in discussing the origin and evolution of the vertebrate osmoregulatory system, the molecular basis for the body fluid homeostasis in hagfishes has been poorly understood. In the present study, we explored this matter in the inshore hagfish, Eptatretus burgeri, by analyzing the transcriptomes obtained from the gill, kidney, and muscle of the animals acclimated to distinct environmental salinities. Together with the measurement of parameters in the muscular fluid compartment, our data indicate that the hagfish possesses an ability to conduct free amino acid (FAA)-based osmoregulation at a cellular level, which is in coordination with the renal and branchial FAA absorption. We also revealed that the hagfish does possess the orthologs of the known osmoregulatory genes and that the transepithelial movement of inorganic ions in the hagfish gill and kidney is more complex than previously thought. These observations pose a challenge to the conventional view that the physiological features of hagfishes have been inherited from the last common ancestor of the extant vertebrates.

Prolactin (PRL) cells within the rostral pars distalis (RPD) of euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, rapidly respond to a hyposmotic stimulus by releasing two distinct PRL isoforms, PRL₁₈₈ and PRL₁₇₇. Here, we describe how environmentally relevant temperature changes affected mRNA levels of prl₁₈₈ and prl₁₇₇ and the release of immunoreactive prolactins from RPDs and dispersed PRL cells. When applied under isosmotic conditions (330 mosmol/kgH₂O), a 6°C rise in temperature stimulated the release of PRL₁₈₈ and PRL₁₇₇ from both RPDs and dispersed PRL cells under perifusion. When exposed to this same change in temperature, ∼50% of dispersed PRL cells gradually increased in volume by ∼8%, a response partially inhibited by the water channel blocker, mercuric chloride. Following their response to increased temperature, PRL cells remained responsive to a hyposmotic stimulus (280 mosmol/kgH₂O). The mRNA expression of transient potential vanilloid 4, a Ca²⁺-channel involved in hyposmotically induced PRL release, was elevated in response to a rise in temperature in dispersed PRL cells and RPDs at 6 and 24 h, respectively; prl₁₈₈ and prl₁₇₇ mRNAs were unaffected. Our findings indicate that thermosensitive PRL release is mediated, at least partially, through a cell-volume-dependent pathway similar to how osmoreceptive PRL release is achieved.

Diffusion-weighted magnetic resonance imaging (dMRI) allows non-invasive investigation of whole-brain connectivity, which can potentially help to reveal the brain's global network architecture and abnormalities involved in neurological and mental disorders. However, the reliability of connection inferences from dMRI-based fiber tracking is still debated, due to low sensitivity, dominance of false positives, and inaccurate and incomplete reconstruction of long-range connections. Furthermore, parameters of tracking algorithms are typically tuned in a heuristic way, which leaves room for manipulation of an intended result. Here we propose a data-driven framework to optimize and validate parameters of dMRI-based fiber-tracking algorithms using neural tracer data as a reference. Japan's Brain/MINDS Project provides invaluable datasets containing both dMRI and neural tracer data from the same primates. We considered four criteria for goodness of fiber tracking: distance-weighted coverage, true/false positive ratio, projection coincidence, and commissural passage, applied using a multi-objective optimization algorithm. We implemented a variant of non-dominated sorting genetic algorithm II (NSGA-II) to optimize five major parameters of a global fiber-tracking algorithm over multiple brain samples in parallel. Using optimized parameters compared to the default parameters, dMRI-based fiber tracking performance was significantly improved, while minimizing false positives and impossible cross-hemisphere connections. Parameters optimized for 10 tracer injection sites showed good generalization capability for other brain samples. These results demonstrate the importance of data-driven adjustment of fiber-tracking algorithms and support the validity of dMRI-based tractography, if appropriate adjustments are employed.