Priyanka graduated with a MBioSci in Genetics and Microbiology from the University of Sheffield in 2014 after undertaking her final year research project in the Bingle lab, where she worked on aspects of mucociliary differentiation in the airway epithelium. Following her graduation, she undertook an A*STAR PhD project jointly supervised by Professor Bingle and Dr Sudipto Roy at the Institute of Molecular and Cell Biology in Singapore. It is work from her thesis, entitled “Functional analysis of novel protein, PIERCE1, in motile ciliogenesis” that is a key component of this paper.
Priyanka’s supervisor Professor Colin Bingle said ‘This paper combines the work of four labs (in the UK, USA and Singapore) working across scientific specialities. It is an excellent example of the importance of collaboration across the international scientific community and shows how such interactions can produce truly world class science”.
Cilia are microtubule-based hair-like organelles that extend from the surface of almost all cell types. Cilia are primordial and are found from single cell organisms to mammals. They have been adapted for various tissue-specific (motile and sensory) functions during development, morphogenesis and homeostasis. In humans, dysfunction or defects in motile and primary cilia underlie a number of devastating genetic conditions - termed ciliopathies - which carry a heavy economic and health burden. Examples include hydrocephalus, infertility, airway diseases, polycystic diseases of the kidney, liver and pancreas, as well as retinal diseases and defects of hearing and smell. Secondary defects in cilia function also underpin many chronic lung diseases.
Each cilium comprises a microtubular backbone known as the ciliary axoneme, surrounded by plasma membrane. Motile cilia are characterized by a typical ’9+2’ architecture with nine outer microtubule doublets and a central pair of microtubules. The microtubular doublets are associated with multiple small proteins, within their luminal structures, known as microtubular inner protein (MIPs) as well as outer dynein arms (ODAs) critical for the motility of the cilia. Primary non-motile cilia typically exist as single appendages on the apical surface of cells and lack the central pair of microtubules, so are described as “9+0”. These cilia lack ODAs. In early embryonic development, motile, “9+0” cilia, that possess ODSs that enable them to move or spin in a circular direction. The spin causes a flow of extraembryonic fluid to move across the nodal surface, directed to the left and therefore set up the body axis. Defects in the function of these cilia result in abnormalities of sidedness determination and organ symmetry.
Cilia as exceptionally complex organelles and contain many hundreds of proteins. The function of many of these remain unknown. Ciliary proteins are synthesized in the cell body and are transported into the axoneme. Cilia assembly–disassembly is closely linked to cell-cycle regulation, and malfunction of these processes is involved in cancer development. Much is known about the structure and function of motile and primary cilia, but much remains to be discovered about these critically important cellular organelles and their protein components. The identification of the components involved in cilia-specific functions and of the molecular mechanisms underlying the various ciliopathies are likely to facilitate the development of novel therapeutic strategies.
To date much of what we known about cilia has come from candidate gene studies and from genetic analysis, where mutant genes have been associated with cilia abnormalities. This study describes a novel de novo, structural based approach coupled with functional studies to define the components of the microtubular doublet from mammalian motile cilia.
For this study the structure of microtubular doublets isolated from bovine tracheal cilia was solved at high resolution using a technique known as Cryo-Electron Microscopy. For the first time, this allowed the identification of multiple mammalian MIPs within their native context as well as the description of the structure of ODA complexes. Some of these proteins had not previously been associated with ciliary function. The functional roles of two of these MIPs, PIERCE1 and PIERC2 were studied in two vertebrate models, mice and zebrafish. These structurally related proteins exist as a repeating pair in the microtubular doublet. The loss of each gene individually had limited effect on ciliary function by the loss of both genes together resulted in significant ciliary abnormalities.
The work describes the fundamental organisational principals of motile cilia and provide a reference to allow a better understanding of the molecular organisation of cilia that will underpin improvement in diagnosis of ciliopathies. It illustrates the power of de novo structural analysis as a tool for identifying components of complex organelles. It will likely lead to the identification of novel ciliopathy genes.
Further work will be aimed at understanding the functional role of additional MIPs. The group will investigate if any of these genes are associated with the genetics of ciliopathies. It is also important to investigate the organisation of microtubular doublets from other motile cilia to determine if there are differences between cilia from different tissues and species.