Respiratory consequences of targeted losses ofHoxa5gene function in mice

Fetal development of the respiratory tract and diaphragm requires strict coordination between genetically controlled signals and mechanical forces produced by the neural network that generates breathing. Being expressed in the mesenchyme of trachea, lung and diaphragm, and in phrenic motoneurons HOXA5 is a key transcription factor regulating lung development and function. Consequently, most Hoxa5−/− mutants die at birth from respiratory failure. However, the extensive effect of the null mutation makes it difficult to identify the origins of respiratory dysfunction in newborns. To address the physiological impact of Hoxa5 tissue-specific roles, we used conditional gene targeting with the Dermo1Cre and Olig2Cre mouse lines to produce specific Hoxa5 deletions in mesenchyme and motoneurons, respectively. Hoxa5 expression in mesenchyme is critical for trachea development, whereas its expression in phrenic motoneurons is essential for diaphragm formation. Breathing measurements in adult mice with whole body plethysmography demonstrated that at rest, only the motoneuron deletion affects respiration due to higher breathing frequency and decreased tidal volume. But subsequent exposure to a moderate hypoxic challenge (FiO2=0.12; 10 min) revealed that both mutant mice hyperventilate more than controls. Hoxa5flox/flox;Dermo1+/cre mice showed augmented tidal volume while Hoxa5flox/flox;Olig2+/cre mice had the largest increase in breathing frequency. No significant differences were observed between medulla-spinal cord preparations from E18.5 control and Hoxa5flox/flox;Olig2+/Cre mouse embryos that could support a role for Hoxa5 in fetal inspiratory motor command. According to our data, Hoxa5 expression in mesenchyme and phrenic motor neurons controls distinct aspects of respiratory development.