The role of the microtubule-associated protein Hmmr in forebrain development and Wnt signaling

• Applicant: Privatdozentin Dr. Kerstin Feistel
• Subject Area: Developmental Biology
• Term: from 2018 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 414981996

Separation of the telencephalon into bilateral hemispheres is a prerequisite for vertebrate forebrain function. This midline separation occurs soon after neural tube closure and is mediated by the floor plate and roof plate, two important brain signaling centers. Functional defects in one of these centers culminate in defective hemisphere separation, termed holoprosencephaly (HPE). The morphogenetic mechanisms underlying HPE development, however, have remained elusive. Our published data show that in Xenopus laevis, the loss of function of Hmmr, a microtubule-associated protein, induces roof plate defects leading to HPE. hmmr mediates the mesenchymal-to-epithelial transition (MET) of neural cells. This function depends on the microtubule binding domain of Hmmr and on cooperation with Wnt / PCP signaling. MET as a morphogenetic mechanism is a prerequisite for anterior neural tube closure as well as roof plate formation and requires switching from canonical to non-canonical Wnt signaling. Our unpublished data show that hmmr attenuates canonical Wnt signaling, while – at the same time – it is required for the non-canonical Wnt pathways. Based on these data, we hypothesize that hmmr functions as a mediator of MET-based morphogenesis by modulating Wnt signaling pathways in a microtubule-dependent manner.The function of hmmr will be investigated in Xenopus laevis as a model organism. Hmmr interaction partners will be identified and functionally tested towards their role in MET and neurulation by gain- and loss-of-function as well as by studying their epistatic relationship with hmmr. Functional domains of Hmmr required for MET will be identified by deletion and mutation analysis. This should result in defective and potentially dominant-negative constructs that will be used as tools for functional analysis in the embryo. A second work package will analyze how hmmr differentially influences canonical vs. non-canonical Wnt signaling in a microtubule-dependent manner and how this mediates MET processes. hmmr-mediated regulation of canonical Wnt signaling will be assessed using the Xenopus double axis assay and luciferase reporter constructs. To identify hmmr’s mode of cooperation with non-canonical signaling, localization and activity of PCP downstream targets will be analyzed. These results will set the ground to establish Xenopus as a model to analyze morphogenetic movements in the emergence of HPE. In vivo imaging using phase contrast X-ray microtomography will be employed to reveal movements of deep tissue contributing to the roof plate. Special focus will be on the analysis of a human de novo mutation in the microtubule-binding region of HMMR, which was identified in a patient with roof plate-derived HPE. This project should reveal a novel role for hmmr in the regulation of Wnt signaling pathways and advance the understanding of hmmr-mediated morphogenetic movements underlying the etiology of HPE.

• DFG Programme: Research Grants