Tubular nuclear envelope invaginations in early vertebrate embryos – A transcription regulation mechanism?

The nuclear envelope (NE) is a structure comprised of a double membrane, pores and functional proteins, surrounding the nuclei of most eukaryotic cells. Via electron microscopy and super-resolution imaging, it was found that the NE can contain tubular NE invaginations (NEIs). NEIs are formed by foldings of the nuclear membrane and underlying lamin-rich layers, giving rise to tubular structures open to the cytosol. They were observed in a variety of cell types and are poised to facilitate signaling between the cytoplasm and the nucleus as they bring cytoplasmic molecules close to genomic regions located deep within the nucleus. Moreover, NEIs were shown to be abundant in early embryonic development when gene transcription programs undergo dramatic changes. Nevertheless, their function remains unknown.

Using Expansion Microscopy, a cutting-edge super-resolution microscopy technology where specimens are physically magnified via hydrogel embedding, we recently revealed that NEIs are abundant in early stage zebrafish embryos. Building on this discovery, we are now characterizing these structures throughout embryonic development, and are characterizing their association with transcription activity. Thus, we found that NEIs are highly abundant and complex at the earliest stages of development, including before and during the maternal to zygotic transition when embryonic genome transcription is activated. NEIs become more simply structured, yet wider, during gastrulation. In addition, we found evidence for transcriptional repression at NEI boundaries and we are examining the relation between the nuclear locations of NEIs and of highly active transcription hubs.