The discovery that perturbation of primary cilia function in the ift88TGN737Rpw
mouse (hereafter called ORPK) caused cyst formation in the kidney was a seminal finding that revealed the cilium as a clinically relevant organelle [15
]. Subsequent data showed that primary cilia on renal epithelium function as mechanosensors and that deflection of these cilia in response to fluid movement initiated an intracellular calcium signal [17
]. The connection between the cilium and renal cystic disease was initially shown by Barr et al
] who demonstrated that LOV-1 and PKD-2 function in the cilium of the C elegans
male sensory neurons [19
]. These proteins are homologs of human polycystin-1 (PC1) and polycystin-2 (PC2), mutations in which cause human autosomal dominant polycystic kidney disease [21
]. PC1 and PC2 are transmembrane proteins present in the cilium. Additionally, PKHD1 polyductin/fibrocystin (PD) is disrupted in autosomal recessive polycystic kidney disease and also localizes to the cilium. While the functions of PC1 and PD are unknown, PC2 is a transient receptor potential (TRP) like cation channel. PC1, PC2, and PD appear to function in the cilium as part of a mechanosensory system. This is supported by the loss of a calcium (Ca2+
) signal induced by cilia deflection in the absence of these proteins [23
], despite normal cilia structure (). Similar roles for cilia have been reported in tissues such as the endothelium and the biliary duct [24
The association of cilia dysfunction and cyst formation is further established through genetic mutations in mouse models and from the identification of genes involved in human cystic kidney disorders, many of whose corresponding proteins localize to primary cilia or basal bodies. How loss of cilia-mediated mechanosensation within the tubule leads to cyst formation remains to be determined. One model proposes that cilia dysfunction alters the orientation of mitotic spindles[26
]. In tubules where cilia function has been perturbed, the orientation of division is randomized and results in expansion (cyst formation) of the tubule. This could represent defects in mechanotransduction, a role for ciliary proteins in proper centriole migration, or an unrecognized connection between the cilium and centrioles during spindle formation.
In addition to mediating mechanosenitive calcium flux, PC1 and PD undergo post-translational processing that result in cleavage of their carboxy-terminal regions in the absence of flow (). In the case of PC1, the carboxy-terminal domain translocates to the nucleus and associates with the transcription factor STAT6 and coactivator p100 to stimulate gene expression () [27
]. PC1 cleavage is important, as mice expressing a non-cleavable form of PC1 develop cysts [28
]. PD goes through a complicated cleavage process to produce a large amino-terminal extracellular domain that remains tethered to its carboxyl-terminus by di-sulfide bridges (). Intriguingly, the amino-terminus of PD can be detected in cultured renal cell media, indicating that it may undergo ectodomain shedding. The carboxyl-terminus of PD is present in the nucleus; however, the consequences of its localization are unknown.
Fluid movement through the tubules and mechanosensory activities of the cilium may have an important impact on cellular responses. The activity of the amino-terminal domain of PC1 and the effect of ectodomain shedding of PD is unknown; however, these findings raise the possibility that the release of PC1 and PD signaling peptides into the renal lumen may affect communication between cells. One area of cilia biology that is largely unexplored is the potential for cilia to not only receive but also transmit signals. For example both secretion of vesicular particles from ciliated cells of the embryonic node [29
] as well as the presence of PKD-protein containing exosomes that may be derived from the cilium have been reported [30
]. However, the relevance of these observations remains to be determined.
All of the above data led to the ‘ciliary hypothesis of polycystic kidney disease’ which views a mechanosensitive cilium as crucial for the etiology of cyst formation [31
]. However, recent findings suggest that the renal cilium and its role in kidney development and homeostasis may be more complex. Several groups have disrupted cilia or PC1 function in mice at different postnatal time points and found that cyst formation is dependent on when cilia function is impaired. If induced prior to postnatal day 12 (P12), a severe cystic phenotype ensues within three weeks. In contrast, if cilia function is disrupted after P14, cysts are not evident for six months and progression occurs slowly [32
]. This suggests a critical time period for cilia function that is needed to prevent rapid cyst formation and challenges the idea that loss of mechanosensory input in itself results in cysts. Thus there is likely a combination of factors needed for cystogenesis. One of additional factor may be that it requires a proliferative environment, such as the perinatal kidney. Under these conditions, the randomized orientation of cell divisions associated with the cilia defects could contribute to tubule expansion and cysts. This model of cyst formation is supported by studies that show rapid cyst formation in adult cilia mutants after proliferation and repair is induced by renal injury [34
Another putative mechanosensory role for the cilium is in the establishment of left-right asymmetry during development. In the mouse, left-right asymmetry is established by the node, a cup-like structure found at the ventral tip of the embryo, each cell of which has a cilium. Some of the nodal cilia rotate counterclockwise creating a directed fluid flow. The immotile cilia on the node detect this fluid flow and, in a PC2 dependent manner, initiate an asymmetric Ca2+ influx on the future left side of the embryo.
In addition to responses induced by fluid shear, cilia have important functions in pressure, touch and vibration sensation. This is exemplified in invertebrates such as Drosophila melanogaster
and C. elegans
. In Drosophila,
neurons of a sensory organ known as the ‘chordotonal organ’ [35
] extend a cilium into a cavity, the scolopale, generated by support cells. The tip cilium is attached to the cap of the scolopale. The sensory neurons respond to vibrations when the cap is displaced stretching the cilium. The deformation of the cilium is thought to initiate a rapid electrical response via an ion channel in the axoneme. Mutations in IFT result in short cilia altering the organs structure and result in loss of sensory reception [35
]. Similar to Drosophila
, cilia in C. elegans
extend from dendrites of mechanosensory neurons beneath the cuticle. It is unknown if these cilia connect to the cuticle but the enclosed space does contain extracellular matrix (ECM) material and mutations disrupting this ECM result in impaired mechanosensory functions [36
]. Analogous to the chordotonal organ of insects, mechanosensation in C. elegans
involves movement of a mechanosensory complex relative to the ECM and the underlying cytoskeleton. This stimulus regulates the activity of sodium channels mec-4 and mec-10 (ENaC superfamily) and converts the stimuli into a rapid electrical response.
These studies of the chordotonal organ in fly and mechanosensation in C. elegans
raise intriguing ideas for how cilia could be involved in sensory reception in mammals. Although cilia have not been reported in association with mechanosensitive structures such as Meissner and Pacinian corpuscles in mammalian skin, it would not be too surprising if a similar mechanism is operative as Bardet-Biedl Syndrome (BBS) patients have defects in mechanosensation and thermosensation [37
]. Furthermore, this may be a paradigm of how cilia function on cells tightly embedded in the ECM. For example, the axonemes of cilia that are present on cells in cartilage or tendons are thought to make direct connections with the surrounding extracellular matrix [38
]. The functional importance of these embedded cilia is unexplored. Based on the above model, it is possible that responses to stress placed on joints or tendons through muscle activity, or pressure placed on the skin could be initiated by deformation of the cilium as a consequence of its association with the ECM. In the context of chondrocytes and cartilage, studies have revealed that cilia defects alter cortical actin and microtubule architecture which may be associated with osteoarthritis [39