The astrocytic Na+-HCO3- exchanger NBCe1 is dispensable for respiratory chemosensitivity

The interoceptive homeostatic mechanism that controls breathing, blood gases and acid-base balance in response to changes in CO2/H+ is exquisitely sensitive, with convergent roles proposed for chemosensory brainstem neurons in the retrotrapezoid nucleus (RTN) and their supporting glial cells. For astrocytes, a central role for NBCe1, a Na+-HCO3- cotransporter encoded by Slc4a4, has been envisaged in multiple mechanistic models (i.e. underlying enhanced CO2-induced local extracellular acidification or purinergic signalling). We tested these NBCe1-centric models by using conditional knockout mice in which Slc4a4 was deleted from astrocytes. In GFAP-Cre;Slc4a4(fl/fl) mice we found diminished expression of Slc4a4 in RTN astrocytes by comparison to control littermates, and a concomitant reduction in NBCe1-mediated current. Despite disrupted NBCe1 function in RTN-adjacent astrocytes from these conditional knockout mice, CO2-induced activation of RTN neurons or astrocytes in vitro and in vivo, and CO2-stimulated breathing, were indistinguishable from NBCe1-intact littermates; hypoxia-stimulated breathing and sighs were likewise unaffected. We obtained a more widespread deletion of NBCe1 in brainstem astrocytes by using tamoxifen-treated Aldh1l1-Cre/ERT2;Slc4a4(fl/fl) mice. Again, there was no difference in effects of CO2 or hypoxia on breathing or on neuron/astrocyte activation in NBCe1-deleted mice. These data indicate that astrocytic NBCe1 is not required for the respiratory responses to these chemoreceptor stimuli in mice, and that any physiologically relevant astrocytic contributions must involve NBCe1-independent mechanisms.

Automated summary

The role of astrocytic Na+-HCO3- cotransporter NBCe1 in the respiratory response to chemoreceptor stimuli was investigated in mice. Conditional knockout of Slc4a4 in astrocytes did not affect the CO2-induced activation of neurons or astrocytes, CO2-stimulated breathing, or hypoxia-stimulated breathing. These results suggest that astrocytic NBCe1 is not required for the respiratory responses in mice and that other NBCe1-independent mechanisms are involved.