MicroTUB(B3)ules and Brain Development
2010; Cell Press; Volume: 140; Issue: 1 Linguagem: Inglês
10.1016/j.cell.2009.12.038
ISSN1097-4172
Autores Tópico(s)Genomics and Chromatin Dynamics
ResumoThe microtubule network is crucial for the developing nervous system, and mutations in tubulin-encoding genes disrupt neuronal migration. Tischfield et al., 2010Tischfield M.A. Baris H.N. Wu C. Rudolph G. Van Maldergem L. He W. Chan W.-M. Andrews C. Demer J.L. Robertson R.L. Cell. 2010; (this issue)PubMed Google Scholar now report that mutations in the tubulin-encoding gene TUBB3 have a striking impact on microtubule dynamics in neurons, resulting in a diverse set of disease symptoms. The microtubule network is crucial for the developing nervous system, and mutations in tubulin-encoding genes disrupt neuronal migration. Tischfield et al., 2010Tischfield M.A. Baris H.N. Wu C. Rudolph G. Van Maldergem L. He W. Chan W.-M. Andrews C. Demer J.L. Robertson R.L. Cell. 2010; (this issue)PubMed Google Scholar now report that mutations in the tubulin-encoding gene TUBB3 have a striking impact on microtubule dynamics in neurons, resulting in a diverse set of disease symptoms. The formation of the nervous system is a complex process that requires functional microtubules during all stages of development. Events such as neurogenesis, neuronal migration, axon pathfinding, and synapse formation are regulated by intrinsic and extrinsic pathways that ultimately impinge on the microtubule network, which carries out the structural changes that underlie each process. However, it is only recently that mutations have been discovered in human genes encoding tubulin, the monomer that polymerizes into microtubules. Mutations in human genes such as TUB1A1 and TUBB2B, which encode α-tubulin and β-tubulin, respectively, or in genes that regulate microtubule function such as Lis1 and Doublecortin (Dcx) (des Portes et al., 1998des Portes V. Pinard J.M. Billuart P. Vinet M.C. Koulakoff A. Carrie A. Gelot A. Dupuis E. Motte J. Berwald-Netter Y. et al.Cell. 1998; 92: 51-61Abstract Full Text Full Text PDF PubMed Scopus (677) Google Scholar, Gleeson et al., 1998Gleeson J.G. Allen K.M. Fox J.W. Lamperti E.D. Berkovic S. Scheffer I. Cooper E.C. Dobyns W.B. Minnerath S.R. Ross M.E. et al.Cell. 1998; 92: 63-72Abstract Full Text Full Text PDF PubMed Scopus (886) Google Scholar, Jaglin et al., 2009Jaglin X.H. Poirier K. Saillour Y. Buhler E. Tian G. Bahi-Buisson N. Fallet-Bianco C. Phan-Dinh-Tuy F. Kong X.P. Bomont P. et al.Nat. Genet. 2009; 41: 746-752Crossref PubMed Scopus (292) Google Scholar, Keays et al., 2007Keays D.A. Tian G. Poirier K. Huang G.J. Siebold C. Cleak J. Oliver P.L. Fray M. Harvey R.J. Molnar Z. et al.Cell. 2007; 128: 45-57Abstract Full Text Full Text PDF PubMed Scopus (360) Google Scholar, Poirier et al., 2007Poirier K. Keays D.A. Francis F. Saillour Y. Bahi N. Manouvrier S. Fallet-Bianco C. Pasquier L. Toutain A. Tuy F.P. et al.Hum. Mutat. 2007; 28: 1055-1064Crossref PubMed Scopus (191) Google Scholar) give rise to disorders of brain development. Such disorders are characterized by lissencephaly (lack of brain folds) and polymicrogyria (excessive brain convolutions), which are caused principally by dysfunctional neuronal migration. In this issue of Cell, Tischfield et al., 2010Tischfield M.A. Baris H.N. Wu C. Rudolph G. Van Maldergem L. He W. Chan W.-M. Andrews C. Demer J.L. Robertson R.L. Cell. 2010; (this issue)PubMed Google Scholar add to this growing literature with their report of new mutations in the human TUBB3 gene encoding neuronal βIII-tubulin. Using a multidisciplinary approach, they uncover TUBB3 mutations that produce diverse clinical phenotypes including ocular motility disorder (CFEOM3). Surprisingly, this is due primarily to disrupted axon guidance and not dysfunctional neuronal migration. This study elegantly examines the relationship among TUBB3 mutations, their impact on microtubule function, and clinical symptoms. Using a family-based approach, the authors identified eight heterozygous mutations in TUBB3. Clinically, the authors discovered that patients with the R262C mutation (the most commonly mutated residue) or the D417N mutation display hypoplasia of the ocular motor nerve and of several other nerve tracts, indicating defects in axon guidance and maintenance. Equally interesting is their observation that patients harboring different mutations in TUBB3 display a variety of clinical diagnoses. Although most patients suffer from CFEOM3, those with R262H, E410K, or D417H mutations also possess varying degrees of facial paralysis and progressive sensorimotor polyneuropathy. This led the authors to speculate that a genotype-phenotype relationship exists, where a specific mutation is associated with a particular set of clinical syndromes. Although these classifications would be an asset for genetic counseling, they are not absolute as there are varying degrees of a given clinical manifestation. Nonetheless, how do different clinical phenotypes arise from mutations within the same gene? To answer this, the authors turned to mouse genetics and created a knock-in model of the most common mutation, R262C. Analysis of mice with the homozygous R262C mutation in the Tubb3 gene revealed profound disruption of several axon tracts, including oculomotor nerves, commissural axons, branching of cranial nerves, thinning of the anterior commissure, as well as stunted growth of the corpus callosum. These data strongly suggest that the primary defect is disrupted axon growth, which agrees well with the human imaging data and demonstrates the validity of using the Tubb3R262C/R262C mouse as a model. Surprisingly, the authors found that the neuronal layers in the cerebral cortex were normal with no evidence of structural defects. This suggests that the R262C mutation does not affect neuronal migration, which is also consistent with the lack of cortical layering abnormalities seen in patients with TUBB3 mutations. The authors further characterized the mutant mice and speculated that the R262C mutation disrupts the biochemical properties of βIII-tubulin resulting in abnormal microtubule function. Indeed, the microtubules in neurons from these mutant mice show increased tyrosination, a posttranslational modification that increases microtubule stability. However, the microtubules also show decreased binding to Kif21a, a kinesin motor protein. This is very interesting given that humans with heterozygous mutations in Kif21a display isolated ocular motor dysfunction with signs of axon deficits in the cranial nerves, similar to patients with TUBB3 mutations (Yamada et al., 2003Yamada K. Andrews C. Chan W.M. McKeown C.A. Magli A. de Berardinis T. Loewenstein A. Lazar M. O'Keefe M. Letson R. et al.Nat. Genet. 2003; 35: 318-321Crossref PubMed Scopus (214) Google Scholar). Given that motor proteins such as the kinesins are important for transport of cargo along microtubules (De Vos et al., 2008De Vos K.J. Grierson A.J. Ackerley S. Miller C.C. Annu. Rev. Neurosci. 2008; 31: 151-173Crossref PubMed Scopus (593) Google Scholar), decreased interaction between Kif21a and microtubules might lead to defective axonal transport of growth materials and signals necessary for axon outgrowth. So how do the different mutations produce the different clinical phenotypes given that they are all located within βIII-tubulin? To answer this, the authors performed a series of in-depth biochemical and cell-imaging assays to determine the effects of the different mutations on microtubule function. First, a cell-free system was used to demonstrate that some of the mutations disrupted the ability of tubulin to form heterodimers, which is a prerequisite for tubulin polymerization into microtubules. Second, the authors used a yeast model system to test the effects of single TUBB3 mutations and found that all of the mutant yeast strains displayed some degree of resistance to pharmacologically induced microtubule destabilization. This supports previous experiments suggesting that TUBB3 mutations increase microtubule stability. Finally, the authors turned to cell imaging using α-tubulin tagged with yellow fluorescent protein (YFP) or the YFP-labeled kinesins Kip3p and Kip2p. These experiments revealed that some of the mutations caused microtubule depolymerization rates to be less dynamic, resulting in increased microtubule stability. Interestingly, imaging of YFP-Kip3p and YFP-Kip2p revealed that many of the common TUBB3 mutations (E410, D417, and R262) reduced the localization of kinesins at microtubule tips. This suggested that the mutations not only disrupt microtubule dynamics but also perturb the interaction of microtubules with proteins that use the microtubule system, similar to the observations in the mutant mice. The Tischfield et al. study answers many questions stemming from the impact of TUBB3 mutations, but many more remain. First, why do mutations in TUBB3 produce axon growth defects, whereas mutations in TUBA1A and TUBB2B primarily produce neuronal migration disorders? Based on the disease phenotype, it is likely that the functions of these genes differ during brain development; TUBB3 is possibly more important during axon guidance but dispensable for neuronal migration. As loss of TUBB3 leads to increased microtubule stability, this implies that neuronal migration may require increased microtubule stability whereas axonal growth may require more dynamic microtubules in the growth cone. However, although the mutations produce different phenotypes, they all have the disruption of tubulin heterodimerization in common resulting in aberrant microtubule polymerization (Figure 1). A second question is whether TUBB3 mutations disrupt the interaction of microtubules with tubulin-interacting proteins. The authors' data suggest that TUBB3 mutations inhibit the interaction of microtubules with kinesin motor proteins. However, as tubulin subunits must interact with molecular chaperones to fold appropriately before microtubule polymerization (Lewis et al., 1997Lewis S.A. Tian G. Cowan N.J. Trends Cell Biol. 1997; 7: 479-484Abstract Full Text PDF PubMed Scopus (131) Google Scholar), it is possible that TUBB3 mutations also perturb the interaction of tubulin with chaperones or other tubulin-modifying proteins. The answer to this question may also provide insight into why different TUBB3 mutations give rise to distinct clinical conditions. For example, it is possible that different genetic mutations may hinder the interaction of tubulin with individual tubulin-interacting proteins (including chaperones), which are expressed and utilized at different neurodevelopmental time points, leading to different clinical outcomes. Although the current study warrants further investigation, TUBB3 mutations, together with the recent discovery of human TUBB2B and TUBA1A mutations, clearly demonstrate that microtubule dynamics play a key role in brain development. Human TUBB3 Mutations Perturb Microtubule Dynamics, Kinesin Interactions, and Axon GuidanceTischfield et al.CellJanuary 08, 2010In BriefWe report that eight heterozygous missense mutations in TUBB3, encoding the neuron-specific β-tubulin isotype III, result in a spectrum of human nervous system disorders that we now call the TUBB3 syndromes. Each mutation causes the ocular motility disorder CFEOM3, whereas some also result in intellectual and behavioral impairments, facial paralysis, and/or later-onset axonal sensorimotor polyneuropathy. Neuroimaging reveals a spectrum of abnormalities including hypoplasia of oculomotor nerves and dysgenesis of the corpus callosum, anterior commissure, and corticospinal tracts. Full-Text PDF Open Archive
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