Signal molecule changes in the gills and lungs of the African lungfish Protopterus annectens, during the maintenance and arousal phases of aestivation.

African lungfishes are obligate air breathers, with reduced gills and pulmonary breathing throughout their life. During the dry season they aestivate on land, with the collapse of secondary lamellae of their gills and the establishment of an exclusive aerial ventilation through the vascularization and expansion of their lungs. To date, the mechanisms underlining the respiratory organ remodeling in aestivating lungfishes are unknown. This study aimed to identify key switch components of the stress-induced signal transduction networks implicated in both rapid and medium-long term remodeling of the gills and lungs of the African lungfish Protopterus annectens during aestivation. Through immunofluorescence microscopy and Western blotting, the localization and the expression of nitric oxide synthase (NOS), Akt, Hsp-90 and HIF-1α were evaluated in both gills and lungs exposed to three experimental conditions: freshwater (FW), 6 months of experimentally induced aestivation (6mAe), and 6 days after arousal from 6 months of aestivation (6mAe6d). After 6mAe, the expression of NOS (p-eNOS antibody), Akt (p-Akt antibody), and Hsp-90 decreased in the gills, while NOS and Hsp-90 expression increased with Akt remained unchanged in the lungs. Upon 6mAe6d, NOS, Akt and Hsp-90 expression in the gills returned to the respective FW values. In the lungs of the aroused fish, NOS and Akt decreased to their respective FW levels, while Hsp-90 expression was enhanced with respect to aestivation. In both respiratory organs, the qualitative and quantitative patterns of HIF-1α expression correlated inversely to those of NOS. Overall, our findings suggest that the molecular components of the NOS/NO system changed in a tissue-specific manner in parallel with organ readjustment in the gills and lungs of P. annectens during aestivation and arousal.