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1. Age-Dependent Decline in Salinity Tolerance in a Euryhaline Fish

   https://doi.org/10.3389/fragi.2021.675395  

   Inokuchi, Mayu ; Yamaguchi, Yoko ; Moorman, Benjamin P. ; Seale, Andre P. ( June 2021 , Frontiers in Aging)

   null (Ed.)

   Euryhaline teleost fish are characterized by their ability to tolerate a wide range of environmental salinities by modifying the function of osmoregulatory cells and tissues. In this study, we experimentally addressed the age-related decline in the sensitivity of osmoregulatory transcripts associated with a transfer from fresh water (FW) to seawater (SW) in the euryhaline teleost, Mozambique tilapia, Oreochromis mossambicus . The survival rates of tilapia transferred from FW to SW were inversely related with age, indicating that older fish require a longer acclimation period during a salinity challenge. The relative expression of Na + /K + /2Cl − cotransporter 1a ( nkcc1a ), which plays an important role in hyposmoregulation, was significantly upregulated in younger fish after SW transfer, indicating a clear effect of age in the sensitivity of branchial ionocytes. Prolactin (Prl), a hyperosmoregulatory hormone in O. mossambicus , is released in direct response to a fall in extracellular osmolality. Prl cells of 4-month-old tilapia were sensitive to hyposmotic stimuli, while those of >24-month-old fish did not respond. Moreover, the responsiveness of branchial ionocytes to Prl was more robust in younger fish. Taken together, multiple aspects of osmotic homeostasis, from osmoreception to hormonal and environmental control of osmoregulation, declined in older fish. This decline appears to undermine the ability of older fish to survive transfer to hyperosmotic environments. 

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2. Divergent genes encoding the putative receptors for growth hormone and prolactin in sea lamprey display distinct patterns of expression

   https://doi.org/10.1038/s41598-020-58344-5  

   Gong, Ningping ; Ferreira-Martins, Diogo ; McCormick, Stephen D. ; Sheridan, Mark A. ( February 2020 , Scientific Reports)

   Growth hormone receptor (GHR) and prolactin receptor (PRLR) in jawed vertebrates were thought to arise after the divergence of gnathostomes from a basal vertebrate. In this study we have identified two genes encoding putative GHR and PRLR in sea lamprey (Petromyzon marinus) and Arctic lamprey (Lethenteron camtschaticum), extant members of one of the oldest vertebrate groups, agnathans. Phylogenetic analysis revealed that lamprey GHR and PRLR cluster at the base of gnathostome GHR and PRLR clades, respectively. This indicates that distinct GHR and PRLR arose prior to the emergence of the lamprey branch of agnathans. In the sea lamprey, GHR and PRLR displayed a differential but overlapping pattern of expression; GHR had high expression in liver and heart tissues, whereas PRLR was expressed highly in the brain and moderately in osmoregulatory tissues. Branchial PRLR mRNA levels were significantly elevated by stage 5 of metamorphosis and remained elevated through stage 7, whereas levels of GHR mRNA were only elevated in the final stage (7). Branchial expression of GHR increased following seawater (SW) exposure of juveniles, but expression of PRLR was not significantly altered. The results indicate that GHR and PRLR may both participate in metamorphosis and that GHR may mediate SW acclimation.

    

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3. Rapid blood acid–base regulation by European sea bass ( Dicentrarchus labrax ) in response to sudden exposure to high environmental CO2

   https://doi.org/10.1242/jeb.242735  

   Montgomery, Daniel W. ; Kwan, Garfield T. ; Davison, William G. ; Finlay, Jennifer ; Berry, Alex ; Simpson, Stephen D. ; Engelhard, Georg H. ; Birchenough, Silvana N. ; Tresguerres, Martin ; Wilson, Rod W. ( January 2022 , Journal of Experimental Biology)

   ABSTRACT Fish in coastal ecosystems can be exposed to acute variations in CO2 of between 0.2 and 1 kPa CO2 (2000–10,000 µatm). Coping with this environmental challenge will depend on the ability to rapidly compensate for the internal acid–base disturbance caused by sudden exposure to high environmental CO2 (blood and tissue acidosis); however, studies about the speed of acid–base regulatory responses in marine fish are scarce. We observed that upon sudden exposure to ∼1 kPa CO2, European sea bass (Dicentrarchus labrax) completely regulate erythrocyte intracellular pH within ∼40 min, thus restoring haemoglobin–O2 affinity to pre-exposure levels. Moreover, blood pH returned to normal levels within ∼2 h, which is one of the fastest acid–base recoveries documented in any fish. This was achieved via a large upregulation of net acid excretion and accumulation of HCO3− in blood, which increased from ∼4 to ∼22 mmol l−1. While the abundance and intracellular localisation of gill Na+/K+-ATPase (NKA) and Na+/H+ exchanger 3 (NHE3) remained unchanged, the apical surface area of acid-excreting gill ionocytes doubled. This constitutes a novel mechanism for rapidly increasing acid excretion during sudden blood acidosis. Rapid acid–base regulation was completely prevented when the same high CO2 exposure occurred in seawater with experimentally reduced HCO3− and pH, probably because reduced environmental pH inhibited gill H+ excretion via NHE3. The rapid and robust acid–base regulatory responses identified will enable European sea bass to maintain physiological performance during large and sudden CO2 fluctuations that naturally occur in coastal environments. 

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4. Tissue and salinity specific Na+/Cl− cotransporter (NCC) orthologues involved in the adaptive osmoregulation of sea lamprey (Petromyzon marinus)

   https://doi.org/10.1038/s41598-021-02125-1  

   Barany, A. ; Shaughnessy, C. A. ; Pelis, R. M. ; Fuentes, J. ; Mancera, J. M. ; McCormick, S. D. ( November 2021 , Scientific Reports)

   Two orthologues of the gene encoding the Na⁺-Cl⁻cotransporter (NCC), termednccaandnccb, were found in the sea lamprey genome. No gene encoding the Na⁺-K⁺-2Cl⁻cotransporter 2 (nkcc2) was identified. In a phylogenetic comparison among other vertebrate NCC and NKCC sequences, the sea lamprey NCCs occupied basal positions within the NCC clades. In freshwater,nccamRNA was found only in the gill andnccbonly in the intestine, whereas both were found in the kidney. IntestinalnccbmRNA levels increased during late metamorphosis coincident with salinity tolerance. Acclimation to seawater increasednccbmRNA levels in the intestine and kidney. Electrophysiological analysis of intestinal tissue ex vivo showed this tissue was anion absorptive. After seawater acclimation, the proximal intestine became less anion absorptive, whereas the distal intestine remained unchanged. Luminal application of indapamide (an NCC inhibitor) resulted in 73% and 30% inhibition of short-circuit current (I_sc) in the proximal and distal intestine, respectively. Luminal application of bumetanide (an NKCC inhibitor) did not affect intestinal I_sc. Indapamide also inhibited intestinal water absorption. Our results indicate that NCCb is likely the key ion cotransport protein for ion uptake by the lamprey intestine that facilitates water absorption in seawater. As such, the preparatory increases in intestinalnccbmRNA levels during metamorphosis of sea lamprey are likely critical to development of whole animal salinity tolerance.

    

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5. Observation of Reduced Homeostatic Metabolic Activity and/or Coupling in White Matter Aging

   https://doi.org/10.1111/jon.12744  

   Anderson, Valerie C. ; Tagge, Ian J. ; Li, Xin ; Quinn, Joseph F. ; Kaye, Jeffrey A. ; Bourdette, Dennis N. ; Spain, Rebecca I. ; Riccelli, Louis P. ; Sammi, Manoj K. ; Springer, Jr, Charles S. ; et al ( June 2020 , Journal of Neuroimaging)

   Transvascular water exchange plays a key role in the functional integrity of the blood–brain barrier (BBB). In white matter (WM), a variety of imaging modalities have demonstrated age‐related changes in structure and metabolism, but the extent to which water exchange is altered remains unclear. Here, we investigated the cumulative effects of healthy aging on WM capillary water exchange.

   A total of 38 healthy adults (aged 36‐80 years) were studied using 7T dynamic contrast enhanced MRI. Blood volume fraction (v_b) and capillary water efflux rate constant (k_po) were determined by fitting changes in the¹H₂O longitudinal relaxation rate constant (R₁) during contrast agent bolus passage to a two‐compartment exchange model. WM volume was determined by morphometric analysis of structural images.

   R₁values and WM volume showed similar trajectories of age‐related decline. Among all subjects,v_bandk_poaveraged 1.7 (±0.5) mL/100 g of tissue and 2.1 (±1.1) s⁻¹, respectively. Whilev_bshowed minimal changes over the 40‐year‐age span of participants,k_podeclined 0.06 s⁻¹(ca. 3%) per year (r= −.66;P < .0005), from near 4 s⁻¹at age 30 to ca. 2 s⁻¹at age 70. The association remained significant after controlling for WM volume.

   Previous studies have shown thatk_potracks Na⁺, K⁺‐ATPase activity‐dependent water exchange at the BBB and likely reflects neurogliovascular unit (NGVU) coupled metabolic activity. The age‐related decline ink_poobserved here is consistent with compromised NGVU metabolism in older individuals and the dysregulated cellular bioenergetics that accompany normal brain aging.

    

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