Research Report 2010 - MDC

Annette Hammes(Delbrück Fellow)Structure of the GroupGroup LeaderAnnette HammesScientistOleg LioubinskiGraduate StudentsChandresh GajeraAnnabel ChristMolecular Mechanisms in EmbryonicForebrain DevelopmentHoloprosencephaly (HPE) is defined as a failure of the embryonic forebrain to separate intodiscrete hemispheres along the mid-sagittal axis. HPE is the most common forebrainanomaly in human embryos. Several pathways in neural tube patterning have been implicatedin the etiology of HPE, including megalin (LRP2), a low-density lipoprotein receptor-relatedprotein, expressed in the neuroepithelium during embryonic development. We have usedmegalin mutant mouse models to demonstrate that lack of megalin in the neuroepitheliumcauses HPE. Similarly, mutations in the human Megalin gene cause Donnai-Barrow syndrome inpatients, characterized by facial dysmorphology, ocular anomalies, and agenesis of the corpuscallosum – also features of holoprosencephaly. Our group is studying the mechanisms wherebymegalin regulates signaling pathways crucial for early forebrain development. Our aim is tounderstand the pathophysiological pathways underlying common forebrain midline defects inpatients.Identification of defects in the molecularpathways underlying holoprosencephaly(Annabel Christ, Oleg Lioubinski)Megalin -/- mice suffer from forebrain defects with conditionsassociated with holoprosencephaly (HPE). Onsetof the HPE phenotype becomes evident as early as midgestation.Megalin-deficient embryos at E 10.5 showsignificantly smaller telencephalic vesicles and animpaired subdivision of forebrain hemispheres.To reveal the underlying molecular defects of the HPEphenotype in megalin mutant mice we analyzed theexpression of marker genes involved in early forebraindevelopment and found major changes in the expressionand activity of key morphogens. We demonstratedthat bone morphogenetic protein 4 (BMP4) signaling inthe rostral and dorsal neuroepithelium is increased inmegalin -/- embryos as early as E 9.5. Sonic hedgehog(SHH) expression is lost in the ventral telencephalon atE 10.5, while fibroblast growth factor 8 (FGF8) is aberrantlyexpressed in the rostral midline of the forebrainat E 9.5. Other pathways, e.g. the WNT signaling, do notshow any obvious changes in megalin mutant mice.Recent unpublished findings from our lab now shedlight on the temporal onset of megalin function in forebrainpatterning.The mesodermal prechordal plate (PcP) anterior to thenotochord underlying the rostral diencephalic ventralmidline (RDVM) neuroepithelium in the forebrain is theessential organizing center for midline specification ofbrain, facial, and oral structures. SHH is secreted initiallyfrom the prechordal plate, subsequently the morphogenis found in the ventral midline of the forebrainneural plate where it induces its own expression andthat of other factors involved in early forebrain patterning,e.g. the transcription factor Six3.In our studies we detected SHH protein in the prechordalplate and the RDVM at E 8.25 (8 somites) in wildtype mice whereas in megalin mutant embryos SHHwas only found in the prechordal plate but not in theoverlying neuroepithelium (Figure 1). It is only at the 10somite stage (E 8.75) that the morphogen becomesdetectable in the RDVM neuroepithelium of megalin -/-mice. This intriguing observation suggests that megalindeficiency delays secretion and proper distribution of10 Cardiovascular and Metabolic Disease Research