Daniel J Hall
Jessica L. Norstog
Andre Barany
Amy M. Regish
Diogo Ferreira-Martins
Jason P. Breves
Lisa M. Komoroske
Stephen D. McCormick
Ciaran A. Shaughnessy
2025
<p><span>All ionoregulating marine fishes examined to date utilize seawater-type ionocytes expressing the apical Cl- channel, cystic fibrosis transmembrane conductance regulator (Cftr) to secrete Cl</span><sup>−</sup><span>. We performed transcriptomic, molecular, and functional studies to identify Cl</span><sup>−</sup><span> transporters in the seawater-type ionocytes of sea lamprey (</span><i>Petromyzon marinus</i><span>). Gill </span><i>cftr</i><span> expression was minimal or undetectable in larvae and post-metamorphic juveniles. We identified other Cl</span><sup>−</sup><span> transporters highly expressed in the gills and/or upregulated following metamorphosis and further investigated two candidates that stood out in our analysis, a Ca</span><sup>2+</sup><span>-activated Cl</span><sup>−</sup><span> channel, </span><i>anoctamin 1</i><span> (</span><i>ano1</i><span>), and the </span><i>Clc chloride channel family member 2</i><span> (</span><i>clcn2</i><span>). Of these, </span><i>ano1</i><span> was expressed 10-100 times more than </span><i>clcn2</i><span> in the gills; moreover, </span><i>ano1</i><span> was upregulated during seawater acclimation, while </span><i>clcn2</i><span> was not. Using an antibody raised against sea lamprey Ano1, we did not detect Ano1 in the gills of larvae, found elevated levels in juveniles and observed a 4-fold increase in juveniles after seawater acclimation. Ano1 was localized to seawater-type branchial ionocytes but, surprisingly, was localized to the basolateral membrane. </span><i>In vivo</i><span> pharmacological inhibition experiments demonstrated that a DIDS-sensitive mechanism was critical to the maintenance of osmoregulatory homeostasis in seawater- but not freshwater-acclimated sea lamprey. Taken together, our results provide evidence of a Cftr-independent mechanism for branchial Cl</span><sup>−</sup><span> secretion in sea lamprey that leverages Ano1-expressing ionocytes. Once further characterized, the Cftr-independent, Ano1-rich ionocytes of sea lamprey could reveal novel strategies for branchial Cl</span><sup>−</sup><span> secretion, whether by Ano1 or some other Cl</span><sup>−</sup><span> transporter, not previously known in ionoregulating marine organisms.</span></p>
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10.1242/jeb.250110
en
The Company of Biologists
A Cftr-independent, Ano1-rich seawater-adaptive ionocyte in sea lamprey gills
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