We study the regulation of morphogen activity during neural development and in the adult cortical stem cell niche. Deficits in morphogen signaling lead to severe malformations of the central nervous system (CNS), including holoprosencephaly and neural tube closure defects. Another current project in our lab is addressing the question whether migratory neuronal precursors need mechanical cues for proper differentiation and pathfinding. We hypothesize that besides the well established chemical signaling, migrating neurons need mechanosensitive protein complexes to transduce motion and interaction with their environment into electrical signals.
Sophisticated regulation of morphogen activity in the embryonic and adult brain
in collaboration with AG Prof. Thomas Willnow
LRP2 (low density lipoprotein receptor related protein 2) is a multifunctional endocytic receptor. We previously clarified the molecular mechanism underlying the holoprosencephaly (HPE) phenotype of mouse embryos deficient for this receptor. LRP2 promotes morphogen signaling as a SHH co-receptor to patched 1 in the ventral forebrain neuroepithelium (Christ et al., 2012). The receptor is also important for neuronal progenitor proliferation and neurogenic output in the adult cortical stem cell niche by maintaining balanced morphogen activity (Gajera et al., 2010). In the retinal stem cell niche, the ciliary marginal zone (CMZ), LRP2 antagonizes SHH activity by mediating endocytic clearance of the morphogen to protect quiescent progenitor cells from mitogenic stimuli. Loss of LRP2 in mice leads to ectopic proliferation in the CMZ progenitor niche (Christ et al., 2015). Our findings document the ability of LRP2 to act in a context dependent manner as activator or inhibitor of the SHH pathway. Overall the data substantiate the emerging concept that auxiliary receptors are critical modulators of morphogen delivery and signal reception in target tissues.
New insights into receptor mediated uptake of folic acid during neurulation
Dr. Nora Mecklenburg, Dr. Esther Kur
LRP2 deficient embryos suffer from impaired closure of the cranial neural tube. This defect is unrelated to the etiology of holoprosencephaly and hinted at an additional role for LRP2 in the dorsal domain of the developing brain. Neural tube closure defects (NTDs) have been attributed to impaired folic acid (vitamin B9) metabolism in mice and humans. We therefore asked whether LRP2 might be required for delivery of folic acid to neuroepithelial cells during neurulation. Our results showed that LRP2 mediates endocytic uptake of folic acid and its binding protein FOLR1 (folate receptor 1), a crucial step for proper neural tube closure (Figure 1; Kur et al., 2014).These findings substantiate our hypothesis that LRP2 teams up with other receptors forming co-receptor complexes to ensure not only ligand specificity in a tissue dependent manner but also to ensure precisely timed and titrated uptake of signaling molecules. Moreover, to better understand the molecular mechanisms of LRP2 dependent NTDs, we will further investigate whether other factors such as altered morphogen signaling or disturbed cilia function in addition to impaired folic acid uptake modulate the NTD phenotype in LRP2 deficient mouse embryos.
Figure 1. Immunohistological detection of sFOLR1-A488 and folic acid-Cy3 on coronal head sections of E8.5 whole-embryo cultures. Uptake of these added ligands was only seen in wild-type but not in Lrp2-/- rostral neural folds.
The model depicts LRP2 dependent endocytic uptake of folic acid into neuroepithelial cells. Soluble FOLR1 and GPI-anchored FOLR1 bind folic acid. Internalization of these complexes relies on the interaction with the co-receptor LRP2.
Understanding the genetic causes of neural tube defects (NTDs)
Fabian Paul, Deborah Kohler, Dr. Nora Mecklenburg
Identifying novel pathways contributing to NTDs
LRP2 deficient mice are ideal to study the multifactorial etiology of human CNS anomalies since the severity of the Lrp2-/- NTD phenotype varies strongly depending on the genetic background. This suggests a strong impact of mouse strain associated genetic modifiers. We generated RNAseq data from Lrp2 mutant embryos on C57Bl6/N background suffering from severe CNS anomalies and from mutants on FVB/N background with a mild phenotype. The aim is to identify differentially regulated pathways and new modifier genes modulating the severity of NTDs. The genetic modifiers identified in our studies will be also used to evaluate analogous gene-gene interactions in NTD patient data in collaboration with Dr. Angela Kaindl and Dr. Gregory Wulczyn (Charité, Berlin). The goal is to improve genetic risk assessment and to gain deeper insights in the pathogenesis of human NTDs.
The role of LRPs in WNT-signaling dependent development of the CNS
Another LRP family member, the WNT receptor LRP6, has been implicated in neural tube defects in mice and NTD patients. To test for gene-gene interactions among Lrp gene family members, linked to the WNT pathway, we are currently analyzing Lrp4-/-; Lrp5-/- and Lrp4-/-; Lrp6-/- compound mutant embryos. Initial data show more severe and highly penetrant CNS anomalies in these compound mutant mouse embryos compared to single mutants, suggesting important new functions for LRP4 and LRP5 in the developing brain that may be synergistic with LRP6 activity. We will test the hypothesis that the phenotypes seen in Lrp4; Lrp6 and Lrp4; Lrp5 compound mutants are linked to disturbed canonical WNT/ß-catenin dependent and/or non-canonical WNT planar cell polarity pathways.
The role of mechanotransduction in the motility and migration of neuronal precursor cells
Carina Fürst in collaboration with AG Prof Gary Lewin
Cell migration is a fundamental process in the developing brain and strongly influenced by chemical signals. However there are still many open questions concerning the mechanisms underlying neuronal migration. We hypothesize that neuronal precursors use mechanotransduction to probe their physical environment during migration. First recordings of mechanically gated currents in neuroepithelial cells provide evidence that neuronal precursors are indeed mechanosensitive. We will further characterize mechanical signaling in neuronal development using mouse genetics, electrophysiology, and high-resolution imaging.