Transcriptional regionalization of the fruit fly's airway epithelium

Abstract

Although airway epithelia are primarily devoted to gas exchange, they have to fulfil a number of different tasks including organ maintenance and the epithelial immune response to fight airborne pathogens. These different tasks are at least partially accomplished by specialized cell types in the epithelium. In addition, a proximal to distal gradient mirroring the transition from airflow conduction to real gas exchange, is also operative. We analysed the airway system of larval Drosophila melanogaster with respect to region-specific expression in the proximal to distal axis. The larval airway system is made of epithelial cells only. We found differential expression between major trunks of the airways and more distal ones comprising primary, secondary and terminal ones. A more detailed analysis was performed using DNA-microarray analysis to identify cohorts of genes that are either predominantly expressed in the dorsal trunks or in the primary/secondary/terminal branches of the airways. Among these differentially expressed genes are especially those involved in signal transduction. Wnt-signalling associated genes for example are predominantly found in secondary/terminal airways. In addition, some G-protein coupled receptors are differentially expressed between both regions of the airways, exemplified by those activated by octopamine or tyramine, the invertebrate counterparts of epinephrine and norepinephrine. Whereas the OAMB is predominantly found in terminal airway regions, the oct3βR has higher expression levels in dorsal trunks. In addition, we observed a significant association of both, genes predominantly expressed in dorsal trunks or in primary to terminal branches branches with those regulated by hypoxia. Taken together, this observed differential expression is indicative for a proximal to distal transcriptional regionalization presumably reflecting functional differences in these parts of the fly's airway system.

Figure 1. The airway epithelium of Drosophila larvae is characterized by different expression domains.

The larval airway system is composed of interconnected tubes organized in a hierarchic structure (adapted after , A). Dorsal trunks are connected to the outside world and primary branches originate from them. The blind endings of the tracheal system are made from terminal branches (A). Different promoter Gal4 lines direct expression into different parts of the tracheal system. Driver lines such as ppk4-Gal4 label the entire airway system (B). In contrast, c929-Gal4 is specific for structures in the dorsal trunks (C), ppk10-Gal4 is surprisingly specific for primary branches (D) and DSRF-Gal4 labels exclusively terminal cells (E fluorescent channel, F transmitted light). Scale bars in B, C, D is 50 µm, in E, F 20 µm.

Figure 2. Expression pattern of selected G-protein coupled receptors in the airway system of Drosophila larvae.

Promotor-Gal4 lines of different G-protein coupled receptors were used to study their expression pattern in the airway system. The promoter of the OAMB receptor drives expression to terminal branches or cells (A overview, B intestine with terminal cell). The TyrR show higher expression in primary branches (C), the frizzled receptor is also predominantly found in these structures (D) octopamine/tyramine receptors drive expression in different parts of the airway system. The Oct2βR-Gal4 is primarily present in the dorsal trunks (A). The OAMB receptor can be observed in the primary airways (B). Especially the terminal cells located directly adjacent to different organ systems show a strong expression of this receptor (C). The OA2 receptor is also found predominantly in the primary airways (D). In contrast, the TyrRII (E), the DopR2 (F) and the OAR2 (G) show a broader expression pattern. Scale bars in A, C, D, E is 50 µm, in B, F 20 µm and in G 10 µm.

Figure 3. qRT-PCR analyses of randomly selected genes with predicted differential expression between dorsal trunks and primary/secondary/terminal branches.

Differential expression of the listed genes was evaluated by qRT-PCR with cDNA derived from dorsal trunks or primary, secondary and terminal airways. Predominant expression was predicted from DNA-microarray studies for 4 transcripts in the dorsal trunks (A) or in the primary, secondary and terminal ones (B). Values are mean results from three different experiments performed in triplicate (±S.D.). Asterisks show statistically significant differences.

Figure 4. Venn diagram analysis of genes predominantly expressed in dorsal trunks or primary, secondary and terminal branches with those involved in defence responses (A) or regulated by hypoxia (B).

The sets of genes predominantly expressed in dorsal trunks or primary, secondary and terminal branches were compared with those linked to immunity (A) or responses to hypoxia (B). The numbers of genes found in both sets is listed in the intersecting regions. Statistical analyses were performed with Fisher’s exact test, the corresponding p-values are listed close to the intersections.