Kinesins are motor proteins responsible for the anterograde transport on microtubules. Kinesin-1 is the first characterized kinesin, and it consists of two heavy chains and two light chains. KIF5B is a form of Kinesin-1 heavy chains that is ubiquitously expressed in mammals. The head domain of KIF5B is responsible for ATP-dependent mechanical movement along microtubules, while the tail region is well-known for its interaction with cell specific cargos. Recent studies reveal a second microtubule binding site in the tail, suggesting special functions of KIF5B in microtubule sliding and bundling.
To understand the role of KIF5B in long bone development, a conditional knockout mouse model was generated, in which Kif5b is deleted in early limb mesenchyme using Prx1-cre/LoxP mediated recombination. Unlike Col2a1-cre directed Kif5b knockout in chondrocytes, the expression of Prx1-cre in limb mesenchyme results in Kif5b knockout in both chondrocyte and osteoblast lineages. The Prx1-cre mediated Kif5b conditional knockout mice develop malformed long bones characterized by their bowed shape, shortened length and multiple fractures, which reflects a combination of defects in bone matrix and growth plate. The mutant mice demonstrate impaired bone matrix formation, as indicated by both collagen density reduction and collagen matrix disorganization. Also, the growth plate does not retain its normal organization, and the hypertrophic zone is absent. The KIF5B deficient chondrocytes not only lose planar cell polarity, but also undergo early apoptosis and fail in terminal differentiation. Interestingly, the binucleation rate is significantly increased in these chondrocytes, suggesting a severe cytokinesis defect. Besides, the intracellular retention of extracellular matrix (ECM) molecules and the uneven distribution of ECM in the cartilage imply both blockage and inappropriate direction of secretion.
Cytokinetic defect in chondrocytes is closely associated with growth plate abnormality and growth retardation. In Kif5b knockout chondrocytes, cytokinetic defect is also one of the earliest and principal phenotypes. Therefore the underlying mechanism of cytokinetic defect was further investigated at cellular level. Since Kif5b knockout chondrocytes cannot survive in primary culture, RNA interference approach was adopted to generate a Kif5b-knockdown chondrogenic cell line. As expected, the Kif5b knockdown cells demonstrate cytokinetic defects characterized by increased binucleation rate and prolonged cytokinesis phase. In control cells, KIF5B becomes concentrated in the midbody during cytokinesis, and the midbody organization is disrupted in Kif5b knockdown cells. Furthermore, transient expression of full-length KIF5B significantly reduces the binucleation rate of these KIF5B deficient cells, whereas over-expression of a truncated KIF5B (without microtubule binding sites in tail region) cannot rescue the defect. Additionally, KIF5B is found to interact with midbody components PRC1 and Aurora B kinase by GST pull-down assay.
This study demonstrates the multiple functions of KIF5B in long bone development and emphasizes its significant role as a key modulator in chondrocyte cytokinesis. More importantly, the study also brings new insights into the mechanisms of cytokinesis: we propose that KIF5B may participate in cytokinesis by regulating the midbody organization and stability via microtubule bundling and transporting or anchoring important components to the midbody. / published_or_final_version / Biochemistry / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/211554 |
| Date | January 2012 |
| Creators | Gan, Huiyan, 甘慧妍 |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | Creative Commons: Attribution 3.0 Hong Kong License, The author retains all proprietary rights, (such as patent rights) and the right to use in future works. |
| Relation | HKU Theses Online (HKUTO) |
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