Research activity
Genetic control of cerebral cortex development
First step of cortical development is the specification of the cortical field. Transcription factor genes belonging to different families have been suggested to play a key role in this process; however, proper selector genes activating corticogenesis have not been yet identified. We have recently found that the two homeobox genes, Emx2 or Pax6, expressed in the alar telencephalic wall since the very beginning, act in parallel as master genes for corticogenesis. In the absence of both Emx2 and Pax6, cortical neuroblasts are respecified as ganglionic neuroblasts; they fail to generate a proper cerebral cortex and conversely give rise to supernumerary basal ganglia.
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Fig.1. Dorso-ventral telencephalic repatterning in Emx2-/-PaxSey/Sey mutants.
The cortex (cx) is respecified as forerunner of the striatum (lge) and the latter as
precursor of the globus pallidus (mge). | |
Subsequent cortical arealization is the process by which the early cortical field, not displaying any obvious regional morpho-functional peculiarity, gives rise, little by little, to the mature cerebral cortex, with its full repertoire of area-specific cytoarchitectural, biochemical and wiring features. Early phases of this process rely on molecular cues intrinsic to the cortical primordium itself; late phases are largely dependent on information born to this primordium by thalamic afferents. We demonstrated that the homeobox genes Emx2, Emx1 and Pax6, gradedly expressed in the alar telencephalon, control early, thalamus-independent steps of this process. In the absence of Emx2, the full spectrum of cortical areal identities is still encoded, but a dramatic reduction of occipital-visual areas and an enlargement of frontal-motor areas take place; this phenotype worsens in the absence of both Emx1 and Emx2; a complementary phenotype occurs in Pax6 null mutants. We addressed mechanisms underlying these phenomena and found that late embryonal areal phenotypes of Emx2 and Pax6 null mutants reflect both a deep alteration of the pre-neuronogenic cortical protomap and a distorted growing profile of the cortical primordium after its areal committment.
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Fig.2. Emx2, Pax6 and cerebral cortex arealisation.
(A) Emx2 and Pax6 expression gradients in the wild type cortical primordium;
(B) changes of areal profile in Emx2-/- and PaxSey/Sey mutants. | |
At the moment, our analysis of Emx/Pax6 functions in cortical development is fully in progress. We are currently providing us with more advanced genetic tools, including mice conditional knock-out for Emx2 and transgene electroporation into organotypic cultures. Using these new tools and old Emx/Pax6 knock-outs, we have in mind to achieve these goals:
1) assessing in greater detail kinetic and molecular mechanisms by which Emx genes regulate proliferation and differentiation of cortical neuroblasts;
2) reconstructing molecular cascades mastered by Emx genes, relevant for cortical specification and arealisation;
3) exploring late, post-natal effects of Emx2 mutations on cerebral cortex architecture and function.
Modelling the human Williams-Beuren syndrome in the mouse
Williams-Beuren syndrome (WBS) is a complex developmental disorder affecting about 1 out of 20,000 people. It includes supravalvular aortic stenosis (SVAS), transient hypercalcemia, elfin-like facies, cerebral cortex dysplasias, mental retardation, and a peculiar cognitive-behaviourial profile, with relative strenght in auditory memory and weakness in visual-spatial cognition. WBS generally arises from de novo mutations; in a few familiar cases, autosomal dominant inheritance is documented. Most WBS patients are hemizygous for a common deletion in 7q11.23, encompassing about twenty genes, among which ELN, FZD9, LIMK1, STX1A. Whereas it is commonly accepted that ELN haploinsufficiency leads to SVAS, origin of WBS cognitive-behaviourial profile is still object of hot scientific debate.
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| Fig.3. Graded expression of Fzd9 in the mid-gestation murine CNS. | |
We are currently generating mice bearing specific mutations in the genomic region synthenic to human 7q11.23, to be used as models for studying genesis of the human WBS neurological profile and of cortical dysplasias suggested to underly it. Patterned expression of the WNT receptor gene Fzd9, the mouse homolog of FZD9, in the cortical anlage of normal embryos and changes in its expression pattern in mutant embyros with obvious defects of early cortical regionalization prompted us to consider this gene as a key actor in this scenario; however haploinsufficiency for other genes and regulatory elements in the WBS region could be also important in WBS neuropathogenesis. Therefore, our strategy proceeds along two ways. We are conditionally knocking Fzd9 out in the murine brain; in parallel, we are generating a set of hemitargeted nested deletions around the middle of the murine genomic region synthenic to human 7q11.23, in order to score possible contributions of other genes in the 7q11.23 canonical deletion to the origin of the disease. Mutant mice will be analyzed by a variety of approaches, paying attention to the development of their cerebral cortex and scoring specific traits of their cognitive-behaviourial profile.
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