BACKGROUND
during neurogenesis the majority of neurons and glia in the mammalian neocortex arise from the division of npc in the neuroepithelial lining of the central cavities of the brain  <cit> .
primary npc have a specific pattern of mitotic activity.
initially each symmetrical division increases precursor cell number by generating two progenitor cells per division.
subsequent asymmetric neurogenic divisions produce one neuron and regenerate one progenitor cell  <cit> .
in the developing mammalian cortex the division fate of a cell appears dependent upon the orientation of the mitotic spindle and hence the position of the cleavage furrow with respect to the apical surface of the neuroepithelium  <cit> .
as a result of the inheritance of cell lineage determinants located at the apical cell membrane, cleavage parallel to the apical surface results in neurogenic division where the apical contents are inherited by one daughter cell and the basal contents by the other, whereas perpendicular cleavage produces two daughter progenitor cells.
the mechanisms regulating spindle orientation and cleavage furrow positioning in the mammalian neuroepithelium are not well understood.
autosomal recessive primary microcephaly  is a rare mendelian disorder characterized by a congenital deficiency of foetal brain growth, particularly affecting the neocortex.
this results in the formation of a small but structurally normal brain and associated mental retardation but no other neurological defects  <cit> .
the concept that mcph is a primary disorder of neurogenic mitosis, the result of which is a reduction of cell number in the developing human brain, is an attractive one.
mutations that cause the condition have been found in five genes: microcephalin , which functions in the dna damage response pathway; and abnormal spindle-like microcephaly associated gene , cdk <dig> regulatory subunit-associated protein  <dig> , centromeric protein j  and scl/tal1-interupting locus  which are all associated with aspects of centrosome function  <cit> .
the most common cause of mcph is mutation of the aspm gene  <cit>  at the mcph <dig> locus on chromosome 1q <dig>  <cit> .
all known pathogenic mutations produce a single clinical phenotype  <cit>  even though they include nonsense, frameshift, translocation and splice site mutations located throughout the  <dig> exon aspm gene  <cit> .
it was originally assumed that mutations result in either protein truncation or mrna degradation via the nonsense mediated decay  pathway  <cit> .
the human aspm gene encodes a protein composed of  <dig> amino acids  <cit>  that is predicted to contain an amino terminal microtubule binding domain  <cit> ; two highly conserved n-terminal short asnp  repeats  <cit> ; two calponin homology domains; up to  <dig> calmodulin binding isoleucine-glutamine  motifs  <cit> ; an armadillo-like sequence and a carboxyl terminal region of unknown function  <cit> .
figure 1a summarizes the domain structure of the aspm protein.
aspm is expressed in all proliferating tissues and is upregulated in many cancers  <cit> .
human aspm is the orthologue of the drosophila abnormal spindle gene .
asp is involved in spindle microtubule organisation in mitosis and meiosis  <cit>  and in cytokinesis  <cit> .
in dividing drosophila neuroblasts asp mutations cause metaphase arrest, resulting in reduced cns development  <cit> .
sirna depletion of asp produces a severe loss of microtubule focus at spindle poles  <cit> .
in mice, elimination of aspm results in a reduction in neural stem cell proliferation and increases the likelihood that npcs will undergo asymmetric cell division, implying a reduction in the total number of progenitor cells formed during brain development  <cit> .
inhibition of human aspm expression by sirna-mediated knockdown inhibits tumour cell proliferation  <cit> .
although it has been hypothesized that aspm is required for cell division in the developing human brain, direct evidence in support of this has been lacking.
in the present study we investigate the role of aspm in human cell division and extend the characterization of the mitotic distribution of human aspm.
we show that aspm knockdown in u2os osteosarcoma cells by sirna alters the positioning of the mitotic spindle from parallel to the substrate to perpendicular to the substrate, effectively altering the division symmetry from symmetrical to asymmetrical.
aspm sirna-mediated depletion also induced cytokinesis failure and apoptosis.
moreover we show that a pathogenic mutation identified in an mcph patient, and located in the 3'region of the aspm gene instigates the activation of an upstream in frame cryptic splice donor site, resulting in the splicing out of nine nucleotides  from the aspm sequence.
the resultant protein exhibits decreased efficiency in localization to spindle poles, suggesting the presence of the c-terminal domain is of critical importance for aspm function.
supporting this, we find that aspm c-terminal fragments expressed in transfected cells induce dominant-negative defects on spindle organisation and cytokinesis.
this study provides further evidence for the function of aspm in the division cycle of mammalian cells.
we discuss these findings in relation to cerebral cortex neurogenesis and microcephaly.
RESULTS
aspm is a nuclear protein that relocates to the spindle pole matrix and central spindle mt minus ends during mitosis
to determine the intracellular distribution of human aspm, rabbit polyclonal antibodies were raised against aspm-specific peptide sequences  and used to screen a panel of cell lines by immunofluorescence.
two antibodies against n-terminal peptides  and one antibody against a c-terminal peptide  produced virtually identical staining patterns  in a variety of human cell types including hela, primary human dermal fibroblasts , u2os osteosarcoma cells, sh-sy5y neuroblastoma cells and in cos- <dig> african green monkey kidney cells.
pre-incubation with the associated aspm peptide negated the aspm signal .
as previously published  <cit> , we found aspm expression to be predominantly concentrated in the nucleus during interphase , however we did not find evidence of centrosomal distribution.
consistent with this finding, the aspm sequence contains a number of potential nuclear localization sequences .
no specific association with microtubules or microtubule-organizing centres was observed prior to nuclear envelope breakdown.
in agreement with previous reports, aspm immunoreactivity shifted away from condensing dna and onto spindle structures immediately after nuclear envelope breakdown, and from prometaphase to anaphase aspm was seen almost exclusively associated with the mitotic spindle   <cit> .
we localized mitotic aspm immunoreactivity to pole-proximal regions of spindle microtubules, where it formed a ring around the spindle poles.
this ring enclosed but did not extensively overlap with γ-tubulin immunoreactivity , though it co-localised with dynactin  <dig>  immunoreactivity in the pericentriolar matrix .
in anaphase cells the mitotic spindle has flattened and aspm maintains its spindle pole localisation, was observed along the microtubules leading to the pole and a fraction of aspm was found at the minus ends of central spindle microtubules .
throughout telophase and cytokinesis aspm was predominantly positioned in a narrow ring at the centre of the midbody .
this localisation was most visible when using the c-terminal antibody 279- <dig>
immunoblot analysis using the 217- <dig> n-terminal anti-aspm antibody identified a  <dig> kda aspm band in each of a panel of human and monkey cell lines .
this band corresponds to the expected size of the  <dig> amino acid wild-type aspm protein.
other investigators have reported that at least three predominant aspm splice variants may exist  <cit> .
in accordance with the reported literature, 217- <dig> would be predicted to recognise wildtype aspm and the largest variant v <dig>  however we saw no evidence for the existence of v <dig> in our whole cell extracts.
we were unable to optimise antibodies 216- <dig> and 279- <dig> for western blotting at endogenous levels of aspm.
however, these antibodies recognise recombinant forms of the aspm protein expressed in and purified from bacteria .
aspm distribution is microtubule dependent
to examine the dependence of aspm localization upon microtubules, we first treated hela cells with  <dig> μg/ml of the microtubule depolymerising drug nocodazole for  <dig> minutes.
under these conditions γ-tubulin immunostaining was maintained at spindle poles but aspm 216- <dig> staining was lost, indicating that the mitotic distribution of aspm is microtubule-dependent .
we next challenged hela cells with  <dig> μm taxol for  <dig> minutes, a treatment that induces the formation of multiple microtubule asters in mitotic cells.
aspm 216- <dig> immunostaining was observed at the centre of these asters , suggesting a specific localization to microtubule minus ends.
aspm was not specifically associated with the anastral γ-tubulin foci also seen in taxol-treated cells .
sirna mediated aspm knockdown leads to repositioning of the mitotic spindle and cytokinesis dysfunction
to determine the effects of loss of aspm function upon cell cycle progression we carried out aspm rna interference experiments.
as a normal human npc culture amenable to rnai studies was not available we chose to perform these tests in the extensively characterised and readily transfectable u2os osteosarcoma cell line using two individual aspm sirnas  and a luciferase  control.
a clear reduction in aspm at the spindle poles of dividing cells was detected after transfection with asp <dig> and asp <dig> for  <dig> or  <dig> hours.
by  <dig> hours post transfection with asp <dig>  aspm  of metaphase cells and completely knocked down in 76% of dividing cells, whereas asp <dig> sirna achieved a partial knockdown in 92%  metaphase cells  and complete knock down in 8% of dividing cells.
comparative immunofluorescence analysis of average integral intensity measurements of immunofluorescently labelled centrosomal aspm was performed for twenty-five metaphase u2os cells treated with either asp <dig>  asp <dig> or gl <dig> sirnas.
in comparison to the average aspm spindle pole signal measured in the gl <dig> treated metaphases, a significant decrease of  <dig> % in the average integral intensity of aspm signal at the spindle pole was calculated after  <dig> hr treatment with asp <dig>  and  <dig> % after treatment with asp <dig> .
immunofluorescence and western blotting could not detect a change in overall expression level of aspm in interphase cells , suggesting that aspm is stabilised when associated with the nuclear matrix and is more open to degradation after nuclear envelope breakdown.
we therefore chose to investigate the phenotype associated with knock down of aspm at the spindle poles of dividing cells.
comparative immunofluorescence analysis, measured as average integral intensity, was performed on the immunofluorescent centrosomal aspm signal of twenty-five metaphase u2os cells treated with asp <dig>  asp <dig> or gl <dig> sirnas.
in comparison to the average integral intensity of aspm spindle pole signal measured in the gl <dig> treated metaphases, a significant decrease in the average integral intensity of aspm signal at the spindle pole was calculated after  <dig> hr treatment with asp <dig>  and with asp <dig>  the knock down is expressed as a percentage of the expression in gl <dig> cells.
in each case t-tests showed the levels of knockdown to be significant.
a substantial reduction in proliferation of u2os cells was observed after sirna treatment in a celltitre blue cell viability assay .
assessment of the proportions of 2n:4n dna content by flow cytometry  <dig> hours and  <dig> hours after aspm sirna treatment distinguished 4n cells in g2/m from tetraploid cells that had failed cytokinesis by confirming the presence or absence of cyclin b <dig>  we identified an increase in the proportion of 2n cells in g <dig>  a reduction of cells in g2/m and an increase in tetraploid cells in aspm sirna treated cells .
these data are consistent with both a decrease in the number of cells undergoing mitosis and a failure of cytokinesis.
this observation was corroborated by microscopic examination of immunofluorescently labelled cells after  <dig> and  <dig> hours of asp <dig> and asp <dig> sirna knockdown, which indicated a decrease in mitotic index and an increase in multinucleate cells and large mononucleate cells with multiple centrosomes consistent with a failure to complete cytokinesis .
the number of cells in cytokinesis, defined in this instance as daughter cells linked by narrow cytoplasmic bridges containing midbodies, was also decreased by aspm sirna.
to gain further insight into the aspm knockdown phenotype, we carried out phase contrast time-lapse imaging of living u2os cells  <dig> hours post-asp <dig> or gl <dig> sirna treatment.
the results of four independent experiments were combined to produce the final data set.
we monitored  <dig> cell divisions of gl <dig> treated u2os cells and  <dig> cell divisions following asp <dig> sirna.
depletion of aspm had a profound effect on u2os cell division.
three major phenotypic anomalies were identified; an increase in the proportion of divisions with the mitotic spindle perpendicular to the plate surface, an increase in cytokinesis failure, and an increase in apoptosis.
normal symmetrical division necessitates positioning of the mitotic spindle parallel to the surface of the imaging dish resulting in cleavage plane orientation perpendicular to the dish .
in 37% of asp <dig> sirna knockdowns  random deviation in the position of the mitotic spindle was observed  with a significant increase in spindle positioning approximately perpendicular to the surface of the dish .
this spindle alignment is reminiscent of that observed in asymmetric divisions in mouse progenitor cells of the neuroepithelium  <cit> .
we therefore termed these divisions asymmetric.
in comparison, only  <dig> %  of gl <dig> treated cells exhibited such asymmetric divisions.
time-lapse imaging of living u2os cells was performed  <dig> hours post-gl <dig> and asp <dig> sirna treatment.
the results of four independent experiments were combined to produce the final data set.
depletion of aspm had a profound effect on u2os cell division, resulting in an increase in the proportion of divisions with the spindle positioned perpendicular to the plate surface  and an increase in cytokinesis failure.
a second consequence of aspm depletion observed by time-lapse imaging was cytokinesis failure , leading to the formation of multinucleated cells.
although we observed this phenomenon in a minor population of asp <dig> sirna-treated cells that underwent symmetrical division , the phenotype was far more prevalent in asp <dig> treated cells that had undergone asymmetric divisions  .
in contrast, cytokinesis failure was observed in only  <dig> %  of symmetric cell divisions and 4%  of asymmetric cell divisions in gl <dig> treated cells.
in this culture system asymmetric cell division initially results in one daughter cell anchored to the culture dish with the second daughter cell positioned above the first  and not attached to the substrate.
we observed that in many instances the second daughter cell was able to subsequently make contact with the dish and successful cytokinesis occurred.
only 38%  of asymmetrical divisions in gl <dig> control experiments failed cytokinesis compared with 75%  of asymmetrical asp <dig> treated cells.
therefore asymmetrical division per se does not fully account for increased cytokinesis failure in asp <dig> sirna treated u2os cells.
thirdly, a visible increase in cell death was observed in cells transfected with asp <dig>   <dig> hours post transfection.
apoptosis was observed in mononucleate cells, binucleate cells  and in cells exiting mitosis but exhibiting cytokinesis failure.
to quantitate the extent of asp <dig> sirna induced apoptosis we performed an in situ activated caspase- <dig> assay on unfixed u2os cells after  <dig> hours of sirna treatment .
on average, asp <dig> sirna treated cells demonstrated a  <dig>  fold increase in caspase- <dig> positive apoptotic cells compared to control gl <dig> sirna cells  compared to mean percentage of apoptotic asp <dig> sirna cells =  <dig> %  .
it has previously been reported that aspm is required for precise orientation of the mitotic spindle in specialised mouse npc  <cit> .
our data demonstrates that aspm function is a determinant of spindle position and division symmetry even in non-neuronal derived cells, suggesting that complete absence of aspm function during human development might be expected to affect whole body development.
since this is not the observed phenotype in mcph, the effect of aspm mutation on the function of the aspm protein expressed in mcph patients was a logical progression of our studies.
an aspm c-terminal splice donor site mutation induces novel splicing and reduces mitotic spindle pole association
aspm contains a number of potential functional domains.
following the positional cloning of the aspm gene, the initial genotype-phenotype correlation studies in mcph patients did not reveal a clustered distribution of microcephaly-associated mutations in aspm  <cit>  and there were therefore few clues to which of these domains were most important for aspm function.
we have previously characterized a c-terminal aspm mutation as a homozygous intronic mutation at ivs <dig> +1g > t , identified in individuals of a consanguineous family of northern pakistani origin  <cit> .
this mutation lies downstream of the armadillo repeat-like domain .
splice site prediction programs forecast the outcome of this mutation to be the removal of the intron  <dig> splice donor site, resulting in translational extension of exon  <dig> by  <dig> amino acids before the incorporation of a stop signal.
we predicted this would result in either the synthesis of a truncated aspm protein of  <dig> kda  that would not include the anti-aspm 279- <dig> peptide sequence, or that it could lead to aspm depletion due to mrna degradation via the nmd pathway.
to determine the effect of the ivs25+1g > t mutation on aspm mitotic expression and localisation, comparative immunofluorescence microscopy of dividing aspmwt and aspm ivs <dig> +1g > t fibroblasts was performed using our n-terminal 216- <dig> anti-aspm antibody.
quantitative analysis of spindle pole staining revealed a significant reduction in aspm levels at the pcm in aspm ivs <dig> +1g > t fibroblasts  , average integral intensity aspmivs <dig> +1g > t =  <dig>  μm <dig> , p = <  <dig> ; average area aspmwt=  <dig>  μm <dig>  average area aspmivs <dig> +1g > t t=  <dig>  μm <dig> , p = <  <dig> ).
unexpectedly, analysis of aspm ivs <dig> +1g > t fibroblasts immunofluorescently labelled with the c-terminal anti-aspm 279- <dig> antibody also revealed weak but clearly visible spindle pole staining , suggesting the ivs25+1g > t mutation did not induce a protein truncation.
to investigate the effect of the ivs25+ <dig> g > t aspm mutation on the aspm protein, we carried out immunoblotting of cell lysates from control  and patient  fibroblasts with the 217- <dig> n-terminal anti-aspm antibody.
expression of full length aspm  in aspmwt cell lines was observed and a band of approximately similar size and equivalent intensity was detected in aspm ivs <dig> +1g > t protein extracts .
thus the ivs <dig> + <dig> g > t mutation does not instigate nmd or protein instability in fibroblasts.
due to the large size of both the full length aspm and the predicted ivs25+1g > t truncated product  we were unable to directly determine size differences in the proteins by immunoblotting.
to further explore the effect of the ivs+1g > t mutation, rt-pcr of aspm exons 24- <dig> of aspmwt and aspm ivs <dig> +1g > t fibroblast mrna was performed and the single pcr product obtained from each was sequenced.
wildtype aspm sequence was observed in the aspmwt fibroblast mrna.
as expected the g > t mutation at the exon  <dig> splice donor site invalidated the native splice site sequence in the aspm ivs <dig> +1g > t mrna.
however, transcription did not continue for the expected  <dig> amino acids before the incorporation of a stop codon.
unexpectedly, an in-frame cryptic splice donor site nine base pairs upstream was recognised and utilised .
the use of the upstream cryptic splice donor site would result in the production of an aspm protein from which only a tripeptide motif  is deleted .
our immunofluorescence data therefore indicates that the loss of three amino acids in the c-terminus of aspm is apparently sufficient to drastically reduce the ability of the protein to localise with the microtubules at the pcm at the spindle poles.
the fact that fibroblasts from this mcph patient have a significant decrease in the quantity of microtubule associated aspm suggests the presence of a functional c-terminal domain in aspm that is critical for normal aspm function.
expression of c-terminal fragments of aspm inhibits spindle assembly and induces mitotic delay in hela cells
our immunostaining data in patient cells indicates a function mediated by the extreme c-terminal region of aspm is required for its localization to spindle poles.
we therefore examined the consequences for mitotic cells of expressing c-terminal fragments of aspm upon mitotic progression.
plasmids directing the expression of gfp fused to three different fragments of  <dig> ,  <dig>  and  <dig>  amino-acids from the c-terminal sequence of aspm were created  to determine whether ectopic expression of these fragments would disrupt the function of endogenous aspm in a dominant-negative manner.
hela cells were transfected with the plasmids and observed  <dig> hours post transfection.
each construct led to similar abnormalities that included the generation of a large number of cells with morphological features consistent with an early mitotic arrest  and in many cells membrane blebbing was also evident .
the dna within these cells was highly condensed and occasionally fragmented, while spindle microtubule organisation was severely disrupted.
where visible, microtubule focusing and spindle formation appeared to be fundamentally impaired .
in most cells the density of microtubules was decreased .
γ-tubulin immunostaining invariably revealed the presence of two separate foci in these cells , consistent with the hypothesis that these were cells displaying a disrupted mitosis.
endogenous aspm immunoreactivity was dispersed .
this is consistent with the observation of diminished microtubule density and our earlier finding that the aspm spindle pole association is microtubule-dependent.
it is also consistent with the interpretation that expression of these aspm fragments directly inhibited the localisation of endogenous aspm to spindle poles.
of control cells expressing gfp alone, 3%  displayed an apoptotic or mitotic delay phenotype, in comparison with 33%  of cells expressing aspm-d1-gfp; 26%  of cells expressing aspm-d2-gfp; and 17%  of cells expressing aspm-d3-gfp.
similar observations were obtained in transfected cos- <dig> cells .
we conclude that a high proportion of hela cells expressing aspm c-terminal fragments, particularly those containing the whole of the armadillo repeat-like region , are defective in assembling a functional spindle.
this leads to mitotic arrest and eventual apoptosis.
expression of aspm cooh-terminal gfp-fusion proteins induces cytokinesis failure in cos- <dig> and hela cells
in addition to the severe mitotic defects seen in cos- <dig> and hela cells transfected with c-terminal fragments of aspm, we also observed a subset of cells that appeared to have undergone mitosis but had failed to complete cytokinesis .
defects ranged from cells containing large, highly lobular nuclei and peripheral microtubule bundles  to cells containing discrete nuclei separated by aberrant microtubule arrays , or structures resembling residual central spindles .
in neither case was there clear evidence for the presence of a cleavage furrow.
to exclude fixation artefacts we used co-expression of a gfp-tagged microtubule plus-end binding protein, eb <dig>  <cit>  as a means of observing centrosomes and the tips of growing microtubules and midbody structures in living cos- <dig> cells.
expression of low levels of eb3-gfp alone had no significant impact on cell division in transfected cos- <dig> cells .
the phenotypes of cos- <dig> cells co-expressing aspm-d2-gfp and eb3-gfp confirmed our immunostaining observations .
expression of aspm-d2-gfp alone led to the formation of binucleated cells .
co-expression of eb3-gfp revealed that these cells contained central spindle structures or loose parallel microtubule arrays .
in some examples centrosomes were seen in close association with well-separated nuclei , indicating that the primary defect in these cells was a failure in cytokinesis after the completion of chromosomal segregation.
the observations from both fixed and living cos- <dig> cells were therefore consistent with a role for aspm in organizing the central spindle microtubule array in mitotic cells from anaphase onwards.
CONCLUSIONS
our study provides a mechanism by which mutations in the human aspm gene could result in a developmental reduction of brain size, and supports the hypothesis that the mcph phenotype arises due to defective npc division.
by examining the consequences of sirna mediated knockdown of spindle pole associated aspm expression and the phenotypes seen following expression of fusion proteins derived from the aspm c-terminus we have identified critical roles for human aspm in spindle microtubule organisation, spindle positioning and in the regulation of cytokinesis even in a non-neural cell type.
similar phenotypes have been previously reported in drosophila asp mutants  <cit> .
we suggest that aspm is a reasonably stable protein and that nuclear matrix associated aspm is less open to degradation than aspm after nuclear envelope breakdown.
aspm was recently confirmed as a gene involved in the regulation of mitosis in human cells as part of the substantive mitocheck integrated research project .
as an element of this high-throughput whole genome rnai screen, automated live cell image analysis of aspm knockdown identified similar phenotypes to those identified in our study.
although mitotic spindle position was not a phenotype for which analytical parameters were specifically established, other mitotic phenotypes that could lead to a decrease in cell division were identified.
metaphase delay was robustly observed with further phenotypes of mitotic delay, problems with metaphase alignment, cell death and poly-lobed nuclei identified with one of two sirnas utilised.
we have previously shown that aspm is preferentially expressed during cerebral cortical neurogenesis in the mouse brain  <cit> .
the time of maximal aspm expression in the neuroepithelium corresponds to the period of proliferative cell division, and the subsequent down-regulation of aspm expression is concomitant with a switch to asymmetrical cell division  <cit> .
we have now demonstrated that inhibition of aspm function by sirna causes a highly penetrant loss of precision in the placement of the mitotic spindle in dividing u2os cells, resulting in an alteration of division mode from the symmetrical to the asymmetrical plane.
this implies that in u2os cells symmetrical cell division in a plane that is perpendicular to the substrate is an active process that requires functional aspm, rather than a simple default pathway.
our data therefore demonstrates that the requirement for aspm in the maintenance of symmetrical divisions is not limited to specialised npcs as previously assumed, but that it plays a general role in mitotic cells.
a logical consequence of this hypothesis is that the normal development of the whole human body should be affected by aspm loss of function.
indeed in drosophila loss of asp function results in larval lethality  <cit> .
paradoxically however, the only known phenotype in individuals with homozygous mutations in aspm is mcph  <cit> .
mutations in aspm are scattered throughout the gene, yet they result in a single clinical mcph phenotype.
our immunoblot data of aspm expression in fibroblast lysates from an mcph patient carrying the ivs25+1g > t homozygous mutation established that the mcph phenotype in this patient did not result from complete aspm loss and confirmed a previous report that aspm mutation does not instigate the nmd pathway  <cit> .
partly functional aspm proteins may therefore be expressed in the cells of mcph individuals, and we hypothesised that aspm mutations caused protein truncations and that the mcph phenotype resulted from the common loss of a c-terminal functional domain that led to partial loss of aspm function.
surprisingly, our data from patient cells demonstrates that the mcph phenotype can arise from a cryptic splicing event that removes only nine nucleotides of intragenic sequence within the c-terminal region of aspm and results in a reduction in aspm localisation to the spindle poles.
the minimal ivs25+1g > t mutation therefore causes a decrease in the efficiency of aspm spindle pole localisation without an associated decrease in overall aspm protein levels or a major increase in gross mitotic abnormalities in patient fibroblasts.
we propose that expression of this subtly impaired mutant aspm protein is sufficient to induce small deviations in the precision of mitotic spindle positioning in symmetrically dividing cells.
this contrasts with the severe mitotic phenotypes seen in cultured cells following more profound disruption of aspm function by sirna knockdown of aspm spindle pole expression, or expression of dominant negative aspm c-terminal domains.
the existence of individuals homozygous for mutations in aspm who exhibit a small brain but who are otherwise grossly normal leads us to infer that either  a functional compensatory mechanism exists in somatic cells or  neurogenic cell divisions are sufficiently different to the majority of somatic cell divisions to be profoundly and differentially affected by a subtle perturbation of aspm function.
the reason for this may lie in the unique morphology of the cells undergoing division in the neuroepithelium during cortical expansion  <cit> .
in vertebrates, apical npc are apicobasally elongated to a remarkable extent and possess a small apical surface.
to execute accurate symmetrical divisions in such cells would require extremely precise cleavage along the apicobasal axis  <cit> .
small deviations in spindle position leading to cleavage plane reorientation would result in a transition from symmetrical to asymmetrical division.
in the majority of somatic cells such deviations could be tolerated due to their relatively large apical and basal surfaces.
however in npc such deviation may be sufficient to drive a decrease in the number of cells successfully completing symmetrical cell division.
this would impair the expansion of the progenitor pool that normally occurs at early stages of cortical development.
as a consequence, the npc pool would be insufficient to produce the number of neurones required for a normal sized brain.
what then is the function of aspm during the later stages of mitosis?
we observed an aspm localisation at the minus ends of central spindle microtubules during anaphase and at the centre of the midbody during telophase and cytokinesis.
microtubules of the mitotic apparatus are a critical contributor to cleavage furrow positioning.
both astral microtubules and overlapping equatorial mts in the central spindle have been implicated as playing a significant role in this process  <cit> .
we therefore see a number of possibilities for aspm function during cytokinesis.
aspm might directly contribute to the organisation of midzone microtubules with the cytokinesis defects seen after aspm functional inhibition arising secondary to central spindle disorganisation.
alternatively, its presence at the centrosome might influence the function of astral microtubules.
it also seems possible that aspm might participate more directly in the coupling of spindle microtubule function to cortical events during furrowing.
for example, aspm might be able to influence the local activity of myosins through its interactions with ef-hand ca2+ binding factors such as calmodulin, or to influence signalling events during cytokinesis through interactions with binding partners such as citron kinase  <cit> .
in conclusion, a major finding of our study is that aspm plays a role in cell division, not just those in npcs, but in other cell types.
we propose that mcph <dig> patients have enough residual aspm activity to successfully complete functionally symmetrical cell divisions in all tissues except the developing brain, where extremely unusual morphological constraints result in a specific defect in cortical neurogenic mitosis in response to imprecise spindle position.
in this we echo the arguments presented by fish et al., in an authoritative commentary on npc division  <cit> .
we have also identified a three amino acid sequence in the c-terminal domain of aspm in an mcph patient reduces localisation of aspm to the pcm implying that this region of aspm mediates an important function in npcs.
future studies will be aimed at defining whether this region of aspm mediates an important interaction with a novel binding partner.

 during neurogenesis the majority of neurons and glia in the mammalian neocortex arise from the division of npc in the neuroepithelial lining of the central cavities of the brain. primary npc have a specific pattern of mitotic activity. initially each symmetrical division increases precursor cell number by generating two progenitor cells per division. subsequent asymmetric neurogenic divisions produce one neuron and regenerate one progenitor cell. in the developing mammalian cortex the division fate of a cell appears dependent upon the orientation of the mitotic spindle and hence the position of the cleavage furrow with respect to the apical surface of the neuroepithelium. as a result of the inheritance of cell lineage determinants located at the apical cell membrane, cleavage parallel to the apical surface results in neurogenic division where the apical contents are inherited by one daughter cell and the basal contents by the other, whereas perpendicular cleavage produces two daughter progenitor cells. the mechanisms regulating spindle orientation and cleavage furrow positioning in the mammalian neuroepithelium are not well understood. autosomal recessive primary microcephaly is a rare mendelian disorder characterized by a congenital deficiency of foetal brain growth, particularly affecting the neocortex. this results in the formation of a small but structurally normal brain and associated mental retardation but no other neurological defects. the concept that mcph is a primary disorder of neurogenic mitosis, the result of which is a reduction of cell number in the developing human brain, is an attractive one. mutations that cause the condition have been found in five genes: microcephalin, which functions in the dna damage response pathway; and abnormal spindle-like microcephaly associated gene, cdk regulatory subunit-associated protein, centromeric protein j and scl/tal1-interupting locus which are all associated with aspects of centrosome function. the most common cause of mcph is mutation of the aspm gene at the mcph locus on chromosome 1q. all known pathogenic mutations produce a single clinical phenotype even though they include nonsense, frameshift, translocation and splice site mutations located throughout the exon aspm gene. it was originally assumed that mutations result in either protein truncation or mrna degradation via the nonsense mediated decay pathway. the human aspm gene encodes a protein composed of amino acids that is predicted to contain an amino terminal microtubule binding domain; two highly conserved n-terminal short asnp repeats; two calponin homology domains; up to calmodulin binding isoleucine-glutamine motifs; an armadillo-like sequence and a carboxyl terminal region of unknown function. figure 1a summarizes the domain structure of the aspm protein. aspm is expressed in all proliferating tissues and is upregulated in many cancers. human aspm is the orthologue of the drosophila abnormal spindle gene. asp is involved in spindle microtubule organisation in mitosis and meiosis and in cytokinesis. in dividing drosophila neuroblasts asp mutations cause metaphase arrest, resulting in reduced cns development. sirna depletion of asp produces a severe loss of microtubule focus at spindle poles. in mice, elimination of aspm results in a reduction in neural stem cell proliferation and increases the likelihood that npcs will undergo asymmetric cell division, implying a reduction in the total number of progenitor cells formed during brain development. inhibition of human aspm expression by sirna-mediated knockdown inhibits tumour cell proliferation. although it has been hypothesized that aspm is required for cell division in the developing human brain, direct evidence in support of this has been lacking. in the present study we investigate the role of aspm in human cell division and extend the characterization of the mitotic distribution of human aspm. we show that aspm knockdown in u2os osteosarcoma cells by sirna alters the positioning of the mitotic spindle from parallel to the substrate to perpendicular to the substrate, effectively altering the division symmetry from symmetrical to asymmetrical. aspm sirna-mediated depletion also induced cytokinesis failure and apoptosis. moreover we show that a pathogenic mutation identified in an mcph patient, and located in the 3'region of the aspm gene instigates the activation of an upstream in frame cryptic splice donor site, resulting in the splicing out of nine nucleotides from the aspm sequence. the resultant protein exhibits decreased efficiency in localization to spindle poles, suggesting the presence of the c-terminal domain is of critical importance for aspm function. supporting this, we find that aspm c-terminal fragments expressed in transfected cells induce dominant-negative defects on spindle organisation and cytokinesis. this study provides further evidence for the function of aspm in the division cycle of mammalian cells. we discuss these findings in relation to cerebral cortex neurogenesis and microcephaly.  aspm is a nuclear protein that relocates to the spindle pole matrix and central spindle mt minus ends during mitosis to determine the intracellular distribution of human aspm, rabbit polyclonal antibodies were raised against aspm-specific peptide sequences and used to screen a panel of cell lines by immunofluorescence. two antibodies against n-terminal peptides and one antibody against a c-terminal peptide produced virtually identical staining patterns in a variety of human cell types including hela, primary human dermal fibroblasts, u2os osteosarcoma cells, sh-sy5y neuroblastoma cells and in cos- african green monkey kidney cells. pre-incubation with the associated aspm peptide negated the aspm signal. as previously published, we found aspm expression to be predominantly concentrated in the nucleus during interphase, however we did not find evidence of centrosomal distribution. consistent with this finding, the aspm sequence contains a number of potential nuclear localization sequences. no specific association with microtubules or microtubule-organizing centres was observed prior to nuclear envelope breakdown. in agreement with previous reports, aspm immunoreactivity shifted away from condensing dna and onto spindle structures immediately after nuclear envelope breakdown, and from prometaphase to anaphase aspm was seen almost exclusively associated with the mitotic spindle. we localized mitotic aspm immunoreactivity to pole-proximal regions of spindle microtubules, where it formed a ring around the spindle poles. this ring enclosed but did not extensively overlap with γ-tubulin immunoreactivity, though it co-localised with dynactin immunoreactivity in the pericentriolar matrix. in anaphase cells the mitotic spindle has flattened and aspm maintains its spindle pole localisation, was observed along the microtubules leading to the pole and a fraction of aspm was found at the minus ends of central spindle microtubules. throughout telophase and cytokinesis aspm was predominantly positioned in a narrow ring at the centre of the midbody. this localisation was most visible when using the c-terminal antibody 279- immunoblot analysis using the 217- n-terminal anti-aspm antibody identified a kda aspm band in each of a panel of human and monkey cell lines. this band corresponds to the expected size of the amino acid wild-type aspm protein. other investigators have reported that at least three predominant aspm splice variants may exist. in accordance with the reported literature, 217- would be predicted to recognise wildtype aspm and the largest variant v however we saw no evidence for the existence of v in our whole cell extracts. we were unable to optimise antibodies 216- and 279- for western blotting at endogenous levels of aspm. however, these antibodies recognise recombinant forms of the aspm protein expressed in and purified from bacteria. aspm distribution is microtubule dependent to examine the dependence of aspm localization upon microtubules, we first treated hela cells with μg/ml of the microtubule depolymerising drug nocodazole for minutes. under these conditions γ-tubulin immunostaining was maintained at spindle poles but aspm 216- staining was lost, indicating that the mitotic distribution of aspm is microtubule-dependent. we next challenged hela cells with μm taxol for minutes, a treatment that induces the formation of multiple microtubule asters in mitotic cells. aspm 216- immunostaining was observed at the centre of these asters, suggesting a specific localization to microtubule minus ends. aspm was not specifically associated with the anastral γ-tubulin foci also seen in taxol-treated cells. sirna mediated aspm knockdown leads to repositioning of the mitotic spindle and cytokinesis dysfunction to determine the effects of loss of aspm function upon cell cycle progression we carried out aspm rna interference experiments. as a normal human npc culture amenable to rnai studies was not available we chose to perform these tests in the extensively characterised and readily transfectable u2os osteosarcoma cell line using two individual aspm sirnas and a luciferase control. a clear reduction in aspm at the spindle poles of dividing cells was detected after transfection with asp and asp for or hours. by hours post transfection with asp aspm of metaphase cells and completely knocked down in 76% of dividing cells, whereas asp sirna achieved a partial knockdown in 92% metaphase cells and complete knock down in 8% of dividing cells. comparative immunofluorescence analysis of average integral intensity measurements of immunofluorescently labelled centrosomal aspm was performed for twenty-five metaphase u2os cells treated with either asp asp or gl sirnas. in comparison to the average aspm spindle pole signal measured in the gl treated metaphases, a significant decrease of % in the average integral intensity of aspm signal at the spindle pole was calculated after hr treatment with asp and % after treatment with asp. immunofluorescence and western blotting could not detect a change in overall expression level of aspm in interphase cells, suggesting that aspm is stabilised when associated with the nuclear matrix and is more open to degradation after nuclear envelope breakdown. we therefore chose to investigate the phenotype associated with knock down of aspm at the spindle poles of dividing cells. comparative immunofluorescence analysis, measured as average integral intensity, was performed on the immunofluorescent centrosomal aspm signal of twenty-five metaphase u2os cells treated with asp asp or gl sirnas. in comparison to the average integral intensity of aspm spindle pole signal measured in the gl treated metaphases, a significant decrease in the average integral intensity of aspm signal at the spindle pole was calculated after hr treatment with asp and with asp the knock down is expressed as a percentage of the expression in gl cells. in each case t-tests showed the levels of knockdown to be significant. a substantial reduction in proliferation of u2os cells was observed after sirna treatment in a celltitre blue cell viability assay. assessment of the proportions of 2n:4n dna content by flow cytometry hours and hours after aspm sirna treatment distinguished 4n cells in g2/m from tetraploid cells that had failed cytokinesis by confirming the presence or absence of cyclin b we identified an increase in the proportion of 2n cells in g a reduction of cells in g2/m and an increase in tetraploid cells in aspm sirna treated cells. these data are consistent with both a decrease in the number of cells undergoing mitosis and a failure of cytokinesis. this observation was corroborated by microscopic examination of immunofluorescently labelled cells after and hours of asp and asp sirna knockdown, which indicated a decrease in mitotic index and an increase in multinucleate cells and large mononucleate cells with multiple centrosomes consistent with a failure to complete cytokinesis. the number of cells in cytokinesis, defined in this instance as daughter cells linked by narrow cytoplasmic bridges containing midbodies, was also decreased by aspm sirna. to gain further insight into the aspm knockdown phenotype, we carried out phase contrast time-lapse imaging of living u2os cells hours post-asp or gl sirna treatment. the results of four independent experiments were combined to produce the final data set. we monitored cell divisions of gl treated u2os cells and cell divisions following asp sirna. depletion of aspm had a profound effect on u2os cell division. three major phenotypic anomalies were identified; an increase in the proportion of divisions with the mitotic spindle perpendicular to the plate surface, an increase in cytokinesis failure, and an increase in apoptosis. normal symmetrical division necessitates positioning of the mitotic spindle parallel to the surface of the imaging dish resulting in cleavage plane orientation perpendicular to the dish. in 37% of asp sirna knockdowns random deviation in the position of the mitotic spindle was observed with a significant increase in spindle positioning approximately perpendicular to the surface of the dish. this spindle alignment is reminiscent of that observed in asymmetric divisions in mouse progenitor cells of the neuroepithelium. we therefore termed these divisions asymmetric. in comparison, only % of gl treated cells exhibited such asymmetric divisions. time-lapse imaging of living u2os cells was performed hours post-gl and asp sirna treatment. the results of four independent experiments were combined to produce the final data set. depletion of aspm had a profound effect on u2os cell division, resulting in an increase in the proportion of divisions with the spindle positioned perpendicular to the plate surface and an increase in cytokinesis failure. a second consequence of aspm depletion observed by time-lapse imaging was cytokinesis failure, leading to the formation of multinucleated cells. although we observed this phenomenon in a minor population of asp sirna-treated cells that underwent symmetrical division, the phenotype was far more prevalent in asp treated cells that had undergone asymmetric divisions. in contrast, cytokinesis failure was observed in only % of symmetric cell divisions and 4% of asymmetric cell divisions in gl treated cells. in this culture system asymmetric cell division initially results in one daughter cell anchored to the culture dish with the second daughter cell positioned above the first and not attached to the substrate. we observed that in many instances the second daughter cell was able to subsequently make contact with the dish and successful cytokinesis occurred. only 38% of asymmetrical divisions in gl control experiments failed cytokinesis compared with 75% of asymmetrical asp treated cells. therefore asymmetrical division per se does not fully account for increased cytokinesis failure in asp sirna treated u2os cells. thirdly, a visible increase in cell death was observed in cells transfected with asp hours post transfection. apoptosis was observed in mononucleate cells, binucleate cells and in cells exiting mitosis but exhibiting cytokinesis failure. to quantitate the extent of asp sirna induced apoptosis we performed an in situ activated caspase- assay on unfixed u2os cells after hours of sirna treatment. on average, asp sirna treated cells demonstrated a fold increase in caspase- positive apoptotic cells compared to control gl sirna cells compared to mean percentage of apoptotic asp sirna cells = %. it has previously been reported that aspm is required for precise orientation of the mitotic spindle in specialised mouse npc. our data demonstrates that aspm function is a determinant of spindle position and division symmetry even in non-neuronal derived cells, suggesting that complete absence of aspm function during human development might be expected to affect whole body development. since this is not the observed phenotype in mcph, the effect of aspm mutation on the function of the aspm protein expressed in mcph patients was a logical progression of our studies. an aspm c-terminal splice donor site mutation induces novel splicing and reduces mitotic spindle pole association aspm contains a number of potential functional domains. following the positional cloning of the aspm gene, the initial genotype-phenotype correlation studies in mcph patients did not reveal a clustered distribution of microcephaly-associated mutations in aspm and there were therefore few clues to which of these domains were most important for aspm function. we have previously characterized a c-terminal aspm mutation as a homozygous intronic mutation at ivs +1g > t, identified in individuals of a consanguineous family of northern pakistani origin. this mutation lies downstream of the armadillo repeat-like domain. splice site prediction programs forecast the outcome of this mutation to be the removal of the intron splice donor site, resulting in translational extension of exon by amino acids before the incorporation of a stop signal. we predicted this would result in either the synthesis of a truncated aspm protein of kda that would not include the anti-aspm 279- peptide sequence, or that it could lead to aspm depletion due to mrna degradation via the nmd pathway. to determine the effect of the ivs25+1g > t mutation on aspm mitotic expression and localisation, comparative immunofluorescence microscopy of dividing aspmwt and aspm ivs +1g > t fibroblasts was performed using our n-terminal 216- anti-aspm antibody. quantitative analysis of spindle pole staining revealed a significant reduction in aspm levels at the pcm in aspm ivs +1g > t fibroblasts, average integral intensity aspmivs +1g > t = μm, p = <; average area aspmwt= μm average area aspmivs +1g > t t= μm, p = < ). unexpectedly, analysis of aspm ivs +1g > t fibroblasts immunofluorescently labelled with the c-terminal anti-aspm 279- antibody also revealed weak but clearly visible spindle pole staining, suggesting the ivs25+1g > t mutation did not induce a protein truncation. to investigate the effect of the ivs25+ g > t aspm mutation on the aspm protein, we carried out immunoblotting of cell lysates from control and patient fibroblasts with the 217- n-terminal anti-aspm antibody. expression of full length aspm in aspmwt cell lines was observed and a band of approximately similar size and equivalent intensity was detected in aspm ivs +1g > t protein extracts. thus the ivs + g > t mutation does not instigate nmd or protein instability in fibroblasts. due to the large size of both the full length aspm and the predicted ivs25+1g > t truncated product we were unable to directly determine size differences in the proteins by immunoblotting. to further explore the effect of the ivs+1g > t mutation, rt-pcr of aspm exons 24- of aspmwt and aspm ivs +1g > t fibroblast mrna was performed and the single pcr product obtained from each was sequenced. wildtype aspm sequence was observed in the aspmwt fibroblast mrna. as expected the g > t mutation at the exon splice donor site invalidated the native splice site sequence in the aspm ivs +1g > t mrna. however, transcription did not continue for the expected amino acids before the incorporation of a stop codon. unexpectedly, an in-frame cryptic splice donor site nine base pairs upstream was recognised and utilised. the use of the upstream cryptic splice donor site would result in the production of an aspm protein from which only a tripeptide motif is deleted. our immunofluorescence data therefore indicates that the loss of three amino acids in the c-terminus of aspm is apparently sufficient to drastically reduce the ability of the protein to localise with the microtubules at the pcm at the spindle poles. the fact that fibroblasts from this mcph patient have a significant decrease in the quantity of microtubule associated aspm suggests the presence of a functional c-terminal domain in aspm that is critical for normal aspm function. expression of c-terminal fragments of aspm inhibits spindle assembly and induces mitotic delay in hela cells our immunostaining data in patient cells indicates a function mediated by the extreme c-terminal region of aspm is required for its localization to spindle poles. we therefore examined the consequences for mitotic cells of expressing c-terminal fragments of aspm upon mitotic progression. plasmids directing the expression of gfp fused to three different fragments of, and amino-acids from the c-terminal sequence of aspm were created to determine whether ectopic expression of these fragments would disrupt the function of endogenous aspm in a dominant-negative manner. hela cells were transfected with the plasmids and observed hours post transfection. each construct led to similar abnormalities that included the generation of a large number of cells with morphological features consistent with an early mitotic arrest and in many cells membrane blebbing was also evident. the dna within these cells was highly condensed and occasionally fragmented, while spindle microtubule organisation was severely disrupted. where visible, microtubule focusing and spindle formation appeared to be fundamentally impaired. in most cells the density of microtubules was decreased. γ-tubulin immunostaining invariably revealed the presence of two separate foci in these cells, consistent with the hypothesis that these were cells displaying a disrupted mitosis. endogenous aspm immunoreactivity was dispersed. this is consistent with the observation of diminished microtubule density and our earlier finding that the aspm spindle pole association is microtubule-dependent. it is also consistent with the interpretation that expression of these aspm fragments directly inhibited the localisation of endogenous aspm to spindle poles. of control cells expressing gfp alone, 3% displayed an apoptotic or mitotic delay phenotype, in comparison with 33% of cells expressing aspm-d1-gfp; 26% of cells expressing aspm-d2-gfp; and 17% of cells expressing aspm-d3-gfp. similar observations were obtained in transfected cos- cells. we conclude that a high proportion of hela cells expressing aspm c-terminal fragments, particularly those containing the whole of the armadillo repeat-like region, are defective in assembling a functional spindle. this leads to mitotic arrest and eventual apoptosis. expression of aspm cooh-terminal gfp-fusion proteins induces cytokinesis failure in cos- and hela cells in addition to the severe mitotic defects seen in cos- and hela cells transfected with c-terminal fragments of aspm, we also observed a subset of cells that appeared to have undergone mitosis but had failed to complete cytokinesis. defects ranged from cells containing large, highly lobular nuclei and peripheral microtubule bundles to cells containing discrete nuclei separated by aberrant microtubule arrays, or structures resembling residual central spindles. in neither case was there clear evidence for the presence of a cleavage furrow. to exclude fixation artefacts we used co-expression of a gfp-tagged microtubule plus-end binding protein, eb as a means of observing centrosomes and the tips of growing microtubules and midbody structures in living cos- cells. expression of low levels of eb3-gfp alone had no significant impact on cell division in transfected cos- cells. the phenotypes of cos- cells co-expressing aspm-d2-gfp and eb3-gfp confirmed our immunostaining observations. expression of aspm-d2-gfp alone led to the formation of binucleated cells. co-expression of eb3-gfp revealed that these cells contained central spindle structures or loose parallel microtubule arrays. in some examples centrosomes were seen in close association with well-separated nuclei, indicating that the primary defect in these cells was a failure in cytokinesis after the completion of chromosomal segregation. the observations from both fixed and living cos- cells were therefore consistent with a role for aspm in organizing the central spindle microtubule array in mitotic cells from anaphase onwards.  our study provides a mechanism by which mutations in the human aspm gene could result in a developmental reduction of brain size, and supports the hypothesis that the mcph phenotype arises due to defective npc division. by examining the consequences of sirna mediated knockdown of spindle pole associated aspm expression and the phenotypes seen following expression of fusion proteins derived from the aspm c-terminus we have identified critical roles for human aspm in spindle microtubule organisation, spindle positioning and in the regulation of cytokinesis even in a non-neural cell type. similar phenotypes have been previously reported in drosophila asp mutants. we suggest that aspm is a reasonably stable protein and that nuclear matrix associated aspm is less open to degradation than aspm after nuclear envelope breakdown. aspm was recently confirmed as a gene involved in the regulation of mitosis in human cells as part of the substantive mitocheck integrated research project. as an element of this high-throughput whole genome rnai screen, automated live cell image analysis of aspm knockdown identified similar phenotypes to those identified in our study. although mitotic spindle position was not a phenotype for which analytical parameters were specifically established, other mitotic phenotypes that could lead to a decrease in cell division were identified. metaphase delay was robustly observed with further phenotypes of mitotic delay, problems with metaphase alignment, cell death and poly-lobed nuclei identified with one of two sirnas utilised. we have previously shown that aspm is preferentially expressed during cerebral cortical neurogenesis in the mouse brain. the time of maximal aspm expression in the neuroepithelium corresponds to the period of proliferative cell division, and the subsequent down-regulation of aspm expression is concomitant with a switch to asymmetrical cell division. we have now demonstrated that inhibition of aspm function by sirna causes a highly penetrant loss of precision in the placement of the mitotic spindle in dividing u2os cells, resulting in an alteration of division mode from the symmetrical to the asymmetrical plane. this implies that in u2os cells symmetrical cell division in a plane that is perpendicular to the substrate is an active process that requires functional aspm, rather than a simple default pathway. our data therefore demonstrates that the requirement for aspm in the maintenance of symmetrical divisions is not limited to specialised npcs as previously assumed, but that it plays a general role in mitotic cells. a logical consequence of this hypothesis is that the normal development of the whole human body should be affected by aspm loss of function. indeed in drosophila loss of asp function results in larval lethality. paradoxically however, the only known phenotype in individuals with homozygous mutations in aspm is mcph. mutations in aspm are scattered throughout the gene, yet they result in a single clinical mcph phenotype. our immunoblot data of aspm expression in fibroblast lysates from an mcph patient carrying the ivs25+1g > t homozygous mutation established that the mcph phenotype in this patient did not result from complete aspm loss and confirmed a previous report that aspm mutation does not instigate the nmd pathway. partly functional aspm proteins may therefore be expressed in the cells of mcph individuals, and we hypothesised that aspm mutations caused protein truncations and that the mcph phenotype resulted from the common loss of a c-terminal functional domain that led to partial loss of aspm function. surprisingly, our data from patient cells demonstrates that the mcph phenotype can arise from a cryptic splicing event that removes only nine nucleotides of intragenic sequence within the c-terminal region of aspm and results in a reduction in aspm localisation to the spindle poles. the minimal ivs25+1g > t mutation therefore causes a decrease in the efficiency of aspm spindle pole localisation without an associated decrease in overall aspm protein levels or a major increase in gross mitotic abnormalities in patient fibroblasts. we propose that expression of this subtly impaired mutant aspm protein is sufficient to induce small deviations in the precision of mitotic spindle positioning in symmetrically dividing cells. this contrasts with the severe mitotic phenotypes seen in cultured cells following more profound disruption of aspm function by sirna knockdown of aspm spindle pole expression, or expression of dominant negative aspm c-terminal domains. the existence of individuals homozygous for mutations in aspm who exhibit a small brain but who are otherwise grossly normal leads us to infer that either a functional compensatory mechanism exists in somatic cells or neurogenic cell divisions are sufficiently different to the majority of somatic cell divisions to be profoundly and differentially affected by a subtle perturbation of aspm function. the reason for this may lie in the unique morphology of the cells undergoing division in the neuroepithelium during cortical expansion. in vertebrates, apical npc are apicobasally elongated to a remarkable extent and possess a small apical surface. to execute accurate symmetrical divisions in such cells would require extremely precise cleavage along the apicobasal axis. small deviations in spindle position leading to cleavage plane reorientation would result in a transition from symmetrical to asymmetrical division. in the majority of somatic cells such deviations could be tolerated due to their relatively large apical and basal surfaces. however in npc such deviation may be sufficient to drive a decrease in the number of cells successfully completing symmetrical cell division. this would impair the expansion of the progenitor pool that normally occurs at early stages of cortical development. as a consequence, the npc pool would be insufficient to produce the number of neurones required for a normal sized brain. what then is the function of aspm during the later stages of mitosis? we observed an aspm localisation at the minus ends of central spindle microtubules during anaphase and at the centre of the midbody during telophase and cytokinesis. microtubules of the mitotic apparatus are a critical contributor to cleavage furrow positioning. both astral microtubules and overlapping equatorial mts in the central spindle have been implicated as playing a significant role in this process. we therefore see a number of possibilities for aspm function during cytokinesis. aspm might directly contribute to the organisation of midzone microtubules with the cytokinesis defects seen after aspm functional inhibition arising secondary to central spindle disorganisation. alternatively, its presence at the centrosome might influence the function of astral microtubules. it also seems possible that aspm might participate more directly in the coupling of spindle microtubule function to cortical events during furrowing. for example, aspm might be able to influence the local activity of myosins through its interactions with ef-hand ca2+ binding factors such as calmodulin, or to influence signalling events during cytokinesis through interactions with binding partners such as citron kinase. in conclusion, a major finding of our study is that aspm plays a role in cell division, not just those in npcs, but in other cell types. we propose that mcph patients have enough residual aspm activity to successfully complete functionally symmetrical cell divisions in all tissues except the developing brain, where extremely unusual morphological constraints result in a specific defect in cortical neurogenic mitosis in response to imprecise spindle position. in this we echo the arguments presented by fish et al., in an authoritative commentary on npc division. we have also identified a three amino acid sequence in the c-terminal domain of aspm in an mcph patient reduces localisation of aspm to the pcm implying that this region of aspm mediates an important function in npcs. future studies will be aimed at defining whether this region of aspm mediates an important interaction with a novel binding partner. 
the majority of aspm depleted mitotic cells fail to complete cytokinesis. mutations in the abnormal spindle microcephaly related gene are the commonest cause of autosomal recessive primary microcephaly a disorder characterised by a small brain and associated mental retardation. we show that aspm is a microtubule minus end-associated protein that is recruited in a microtubule-dependent manner to the pericentriolar matrix 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. 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. aspm encodes a mitotic spindle pole associated protein. it is suggested that the mcph phenotype arises from proliferation defects in neural progenitor cells. our data supports the hypothesis that the mcph phenotype caused by aspm mutation is a consequence of mitotic aberrations during neurogenesis. this antagonises the early expansion of the progenitor pool that underpins cortical neurogenesis, causing the mcph phenotype. 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. 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.
 mutations in the abnormal spindle microcephaly related gene are the commonest cause of autosomal recessive primary microcephaly 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.  we show that aspm is a microtubule minus end-associated protein that is recruited in a microtubule-dependent manner to the pericentriolar matrix 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. 