Human ASPM participates in spindle organisation, spindle orientation and cytokinesis
- Equal contributors
1 Section of Ophthalmology and Neuroscience, Wellcome Trust Brenner Building, Leeds Institute of Molecular Medicine, University of Leeds, St. James's University Hospital, Leeds LS9 7TF, UK
2 Department of Life Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
3 Cancer Research UK Cell Cycle Genetics Research Group, University of Cambridge, Department of Genetics, Downing Street, Cambridge CB2 3EH, UK
4 CRUK Clinical Centre at Leeds, Division of Cancer Medicine Research, Leeds Institute of Molecular Medicine, St James's University Hospital, Leeds LS9 7TF, UK
5 Yorkshire Regional Genetics Service, Yorkshire Regional Genetics Service, Ashley Wing, St James's University Hospital, Leeds LS9 7TF, UK
6 Cambridge Institute for Medical Research, Wellcome Trust/MRC Building, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 2XY, UK
BMC Cell Biology 2010, 11:85 doi:10.1186/1471-2121-11-85Published: 2 November 2010
Mutations in the Abnormal Spindle Microcephaly related gene (ASPM) are the commonest cause of autosomal recessive primary microcephaly (MCPH) a disorder characterised by a small brain and associated mental retardation. ASPM encodes a mitotic spindle pole associated protein. It is suggested that the MCPH phenotype arises from proliferation defects in neural progenitor cells (NPC).
We show that ASPM is a microtubule minus end-associated protein that is recruited in a microtubule-dependent manner to the pericentriolar matrix (PCM) at the spindle poles during mitosis. ASPM siRNA reduces ASPM protein at the spindle poles in cultured U2OS cells and severely perturbs a number of aspects of mitosis, including the orientation of the mitotic spindle, the main determinant of developmental asymmetrical cell division. The majority of ASPM depleted mitotic cells fail to complete cytokinesis. In MCPH patient fibroblasts we show that a pathogenic ASPM splice site mutation results in the expression of a novel variant protein lacking a tripeptide motif, a minimal alteration that correlates with a dramatic decrease in ASPM spindle pole localisation. Moreover, expression of dominant-negative ASPM C-terminal fragments cause severe spindle assembly defects and cytokinesis failure in cultured cells.
These observations indicate that ASPM participates in spindle organisation, spindle positioning and cytokinesis in all dividing cells and that the extreme C-terminus of the protein is required for ASPM localisation and function. Our data supports the hypothesis that the MCPH phenotype caused by ASPM mutation is a consequence of mitotic aberrations during neurogenesis. We propose the effects of ASPM mutation are tolerated in somatic cells but have profound consequences for the symmetrical division of NPCs, due to the unusual morphology of these cells. This antagonises the early expansion of the progenitor pool that underpins cortical neurogenesis, causing the MCPH phenotype.