From Pond Scum to Human Disease: Using evolution to understand ciliary defects

Cilia are complex macromolecular machines that are built using long polymers of α-and β-tubulin and come in two forms, motile and non-motile. Motile cilia provide movement for unicellular organisms, and in multi-cellular organisms move surrounding fluids as well as receive and transmit signals from and to the environment and adjacent cells. It is an organelle that is only 250nm by 5000 nm and contain over 1000 different proteins, which means that 5% of the human genome is needed to build a motile cilium. The membrane surrounding the less complex non-motile cilia, which are present on most mammalian cells, contains a variety of receptors and ion channels. Patients with defects in motile cilia exhibit newborn respiratory distress, chronic respiratory infections, and male infertility. About one-half of these patients have their heart, stomach, and spleen on the right instead of the left side of their chest. Defects in non-motile cilia can cause obesity, retinal degeneration, kidney dysfunction, anosmia, and autism.
The last common ancestor of the eukaryotic lineage had cilia, which were lost during the evolution of land plants and many fungi. We used this observation to identify proteins that are present in organisms with motile cilia but missing in a plant. This comparative genomic approach led to the identification of a large number of human disease genes and clues to their functions. To understand how cilia are assembled, we developed methods to use single particle cryo electron microscopy (SPCEM). We can visualize over 250 proteins with near atomic resolution and see many key protein-protein interactions that allows us to begin to understand their function. Variants in over 50 of the 800 genes result in defects in motile cilia in people. Variant in two genes, CCDC39 and CCDC40, are responsible for the disease symptoms in about 25% of patients and their disease outcomes are much worse than in other patients. SPCEM showed us that these two proteins form a 96 nm heterodimer that interacts with 8 ciliary structures. Using airway epithelial cells from patients and from induced pluripotent stem cells, we found that in addition to having immotile cilia due a failure to dock over one hundred proteins, the patient airway is under extreme stress that leads to an altered “recycling center” for the undocked proteins and to altered cell fate. Many of the ciliated cells become mucus-secreting cells that result in mucus plugging of the small airways of the lung. The study of this small organelle that once was thought to be unimportant is providing insights into many diseases and into promoting human health.