C-myb controls the initiation of ciliogenesis in the developing mouse airway epithelium [electronic resource]
Keywords: Biochemistry.

In the mammalian lung, multiciliated cells lining the conducting airways play a crucial role in mucociliary clearance, the primary defense mechanism of the lung against foreign particles and bacterial infection. The cilia of the multiciliated cells beat in metachronic waves to propel mucus up from the deep distal branches of the lung and out of the trachea, to be either expectorated or swallowed. Breakdown of the mucociliary escalator, due to ciliary dysfunction as occurs in primary ciliary dyskinesia, or due to changes in osmotic balance, as occurs in cystic fibrosis, result in chronic airway bacterial infections. Viral infection in healthy individuals also causes loss of cilia, and this correlates with high rates of bacterial infections that follow many viral infections. Despite the importance of these cells, we know very little about how they are specified during development, nor about how multiple cilia are generated. Each cilium is a membrane encased microtubule based structure that protrudes from the apical surface of the cell. Most cells bear a solitary, or primary, cilium, while only a few specialized cell types can generate hundreds of cilia each. Each cilia is nucleated by a single centriole, and centriole number is normally tightly regulated. How these multiciliated cells circumvent this regulation to generate hundreds of centrioles is unknown. One the developing multiciliated cell has generated its centrioles, those centrioles migrate to and dock with the apical membrane and generate their ciliary axonemes. While many proteins involved in ciliary function and intraflagellar transport have been identified, little is known about how these process are controlled from a molecular level. Very few transcription factors are known to play a role in ciliogenesis. The most well-studied one is Foxj1, which is required for motility in single cilia, and is also critical for proper centriole migration in multiciliated cells. However, ectopic expression of Foxj1 cannot confer a ciliated fate on a non-ciliated cell, and cells null for foxj1 still generate centrioles, so there must be other transcription factors that initiate ciliogenesis and act in the early steps of the process, such as centriole generation. Here, we have identified the first transcription factor that acts upstream of Foxj1 during ciliogenesis. We screened through a subset of the known and predicted transcription factors in the mouse genome for those expressed in the developing lung in a pattern similar to that of Foxj1. We isolated a single candidate, the myeloblastosis oncogene (c-myb). We showed that c-Myb was expressed in post-mitotic cells in the developing lung epithelium, many of which co-expressed Foxj1, indicating that c-Myb is expressed in ciliating cells. Analysis of the timing of c-Myb expression during development as compared to Foxj1 and to maturing ciliary axonemes revealed that c-Myb was expressed during the early steps of ciliogenesis and was downregulated as the cells matured. Specific removal of c-Myb from the developing airway epithelium abrogated Foxj1 expression and centriole generation at E15.5. c-myb null epithelia did not have mature ciliated cells at E17.5, although Foxj1 expression and centriole generation appear to have recovered by this time point. This is the first identification of a transcription factor that regulates the initiation of ciliogenesis in the developing mouse lung. Finally, we propose a modular model of transcriptional control of cilia, in which cells deploy specific transcriptional modules to build the particular kind of cilia they require.