William Joyce

The sensing of environmental fluctuations and initiation of appropriate physiological responses is crucial to homeostasis. Neuroepithelial cells (NECs) in fishes are putative chemoreceptors, resembling mammalian Type I (glomus) cells, that respond in vitro to changes in O 2 , CO 2 , NH 3 , and pH. Cytosolic carbonic anhydrase (Ca17a) is thought to be involved in CO 2 sensing owing to its presence in NECs. Zebrafish ( Danio rerio) lacking functional Ca17a were generated via CRISPR/Cas9 technology and used to assess the role of Ca17a in initiating the cardiorespiratory responses to elevated CO 2 (hypercapnia). Unfortunately, the homozygous knockout mutants ( ca17a −/− ) did not survive more than ∼12–14 days postfertilization (dpf), restricting experiments to early developmental stages (4–8 dpf). Changes in ventilation ( f V ) and cardiac ( f H ) frequency in response to hypercapnia (1% CO 2 ) in wild-type ( ca17a +/+ ), heterozygous ( ca17a +/− ) and ca17a −/− fish were used to investigate Ca17a-dependent CO 2 sensing and downstream signaling. Wild-type fish exhibited hyperventilation during hypercapnia as indicated by an increase in f V . In the ca17a −/− fish, the hyperventilatory response was attenuated markedly but only at 8 dpf. Hypercapnic tachycardia was observed for all genotypes and did not appear to be influenced by the absence of Ca17a. Interestingly, ca17a −/− fish exhibited a significantly lower resting f H that became more pronounced as the fish aged. The decrease in resting f H was prevented (“rescued”) when ca17a −/− embryos were injected with ca17a mRNA. Collectively, the results of this study support a role for Ca17a in promoting hyperventilation during hypercapnia in larval zebrafish and suggest a previously unrecognized role for Ca17a in determining resting heart rate.