Microcephaly: How do selective trafficking deficits cause neurodevelopmental defects?

• Applicant: Dr. Christoph Kaether
• Subject Area: Molecular and Cellular Neurology and Neuropathology ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566800237

Mutations in YIPF5 and IER3IP1 can cause MEDS (microcephaly, epilepsy and juvenile diabetes syndrome), a fatal disease leading to childhood death. We showed that knock-out (ko) or mutation of the ER-localized membrane proteins IER3IP1 or YIPF5 cause changes in surfaceome and secretome, mainly of neuronal development proteins. Surprisingly, a strong increase in ER-resident chaperones was detected in the secretome. Further analysis showed that IER3IP1 and YIPF5 control trafficking of the ER-Golgi transport receptors ERGIC53, SURF4 and KDELR2, explaining the changes in surfaceome/secretome. Knock-down (kd) of Ier3ip1 or Yipf5 by in utero electroporation of mouse embryos resulted in morphological changes of newborn neurons and in an over-migration phenotype. We propose that proteostatic changes in surface and secretome of neurons caused by mutations in IER3IP1 or YIPF5 lead to delamination/differentiation of neural precursors or over-migration of newborn neurons in the developing cortex, the latter representing a novel cause for microcephaly. To test our hypothesis, microcephalic brains of mice with Ier3ip1 ko or pathogenic mutations will be analyzed for cortical development, migration of neurons, phenotypes of other cells like neural precursor cells, astrocytes and other glia. Brains will also be used for transcriptomic and proteomic characterization. The experiments will provide novel in-vivo insights into the cause of the microcephaly induced by Ier3ip1 mutations. In the second part, we will elucidate the molecular functions of IER3IP1 and YIPF5 and analyze their interactome using BioID/mass spectroscopy. Based on structure predictions, we will investigate transmembrane domains 4 of YIPF5 and 2 of SURF4 by mutagenesis and co-immunoprecipitation experiments. Of special interest is the increased secretion of KDEL-containing ER-proteins and of SURF4 cargoes. Our preliminary data showed that the loss of YIPF5 causes the appearance of long, thin SURF4-positive tubules emanating from ER exit sites (ERES). These tubules are positive for ERGIC53 and rab1 and, together with the enhanced secretion of SURF4-cargoes suggest that YIPF5 is a negative regulator of SURF4-mediated ER export. Using trafficking assays we will further elucidate the molecular function of YIPF5 in SURF4-transport. Finally, we will make use of human iPSC derived glutamatergic neurons to validate our findings in a human neuronal setting. Functional consequences like cell survival, neurite growth and synapse formation will be analyzed in combination with kd or overexpressing proteins of interest like YIPF5, IER3IP1 as well as candidates identified in the described experiments. Taken together, we aim at a better understanding of a novel type of microcephaly and we hope to contribute to a more detailed understanding of molecular mechanisms in the early secretory pathway, with implications for a broad range of diseases.

• DFG Programme: Research Grants