basal body

Cilia and flagella comprise a microtubular backbone (the ciliary axoneme), which is surrounded by plasma membrane and is nucleated and organized by the basal body, which is a cylindrical structure that consists of nine microtubule triplets and is located underneath the cell membrane.

Functions

The centrosome, the basal body and the cilium are interlinked both structurally, through a network of stable microtubules, and functionally, as a number of centrosomal proteins can normally localize to the centrosome, the basal body and the ciliary axoneme during different phases of the cell cycle.

Basal bodies are closely associated with the formation of functional cilia, as they nucleate and organize the microtubules that give rise to the ciliary axoneme.

The architecture of the microtubule backbone distinguishes motile cilia, in which nine doublets of microtubules surround a central pair (9+2), from primary cilia, in which the central pair of microtubules is absent (9+0).

Protein synthesis does not occur inside cilia and so ciliary integrity and functionality depends on the anterograde and retrograde microtubule-based transport of molecules in the cell.

This specialized process of IFT, begins with the assembly of at least 16 individual peptides, forming IFT particles that consist of complexes A (comprising at least 4 proteins) and B (comprising at least 12 proteins) at the transition fibres, which lie immediately distal to the basal body.

Once assembled, IFT particles are transported to the ciliary tip in a kinesin-driven process and back into the cell interior through a dynein-dependent mechanism.

Pathology: basal body diseases

 cystic kidney diseases

• Bardet-Bield syndrome (BBS)
• autosomal dominant polycystic kidney disease (ADPKD)
• autosomal dominant polycystic kidney disease (ARPKD)
• Alstrom syndrome (AMLS1)

 Cohen syndrome (?)
 Meckel-Gruber syndrome (?)

Pathogenesis

 Given the necessity of basal body integrity for IFT, it is not surprising that the phenotype of some basal body disorders, such as BBS and the clinically related Alstrom syndrome, which is caused by mutations in another centrosomal protein AMLS1, share many phenotypic aspects with disorders that are caused by mutations in proteins that localize exclusively within cilia.

 Cumulatively, these data provide strong clues about the clinical consequences of basal body or ciliary dysfunction to the point where various human phenotypes, including Cohen syndrome and Meckel-Gruber syndrome, might be expected to fall within this functional category.

 Loss-of-function mutations that affect the basal body can result in human disorders with a wide phenotypic spectrum, including reversal or randomization in body symmetry, hydrocephaly, retinal degeneration and cystic kidney diseases (CKDs) and liver disease.

 For example, the inversion of embryonic turning (inv, murine homolog of human gene INVS) mutant mice present with situs inversus, as well as pancreatic cysts and renal cysts.

The defect in inv mice is caused by mutations in inversin, a gene that is expressed in cells with 9+0 cilia, and includes a subpopulation of embryonic nodal cells, renal epithelia and the retina.

Within the cell, inversin localizes to centrosomes, basal bodies and cilia, as revealed in immunocytochemistry studies and through the characterization of an Inv-GFP fusion protein in transgenic mice.

Once thought to be vestigial organs, it is now becoming clear that cilia represent a specialized region of the cellular membrane that is involved in the interaction with, and sensing of, the extracellular medium to transduce stimuli, which ultimately affect many fundamental biological processes.

It is plausible that proteins such as inversin (INVS), and possibly other ciliary polypeptides, including some of the Bardet-Biedl syndrome (BBS) proteins, could provide a functional signal relay between the cilia, the basal body and the centrosome.

Consistent with this idea, inversin (INVS) binds to the APC2 subunit of the anaphase-promoting complex (APC) and it has been suggested that it is involved in the cell cycle, thereby providing a potential link between ciliary defects and abnormal cell proliferation as the underlying cause of the cystic kidney phenotype of inv mice.

References

 Badano JL, Teslovich TM, Katsanis N. The centrosome in human genetic disease. Nat Rev Genet. 2005 Mar;6(3):194-205. PMID: 15738963

 Pazour GJ. Comparative genomics: prediction of the ciliary and basal body proteome. Curr Biol. 2004 Jul 27;14(14):R575-7. PMID: 15268880