Tong, X., Xia, Z., Zu, Y., Telfer, H., Hu, J., Yu, J., Liu, H., Zhang, Q., Sodmergen, D., Lin, S., and Zhang, B. (2013) Ngs (notochord granular surface) encodes a novel type of intermediate filament family protein essential for notochord maintenance in zebrafish. The Journal of biological chemistry. 288(4):2711-2720.
Notochord is an important organ involved in embryonic patterning and locomotion. In zebrafish, mature notochord consists of
a single stack of fully differentiated, large vacuolated cells called chordocytes, surrounded by a single layer of less differentiated
notochordal epithelium cells called chordoblasts. Through genetic analysis of zebrafish lines carrying pseudo-typed retroviral
insertions, a mutant exhibiting defective notochord with a granular appearance was isolated and the corresponding gene was
identified as ngs (notochord granular surface), which was specifically expressed in notochord. In the mutants, the notochord
started to degenerate from 32 hours post fertilization (hpf), and the chordocytes were then gradually replaced by smaller
cells derived from chordoblasts. The granular notochord phenotype could be alleviated through anesthetizing the mutant embryos
with tricaine to prevent their muscle contraction and locomotion. Phylogenetic analysis showed that ngs encodes a new type
of intermediate filament (IF) family protein, thereafter named as chordostatin according to its function. Through transmission
electron microcopy, bundles of 10-nm-thick IF like filaments were shown to be enriched in the chordocytes of the wild-type
zebrafish embryos, while the chordocytes in ngs mutants lack IF like structures. Furthermore, chordostatin-EGFP fusion protein
could assemble into filamentous network specifically in the chordocytes. Taken together, our work demonstrates that ngs encodes
a novel type of IF protein and functions to maintain the notochord integrity for larval development and locomotion. Our work
should shed light on the in-depth understanding of the mechanism of notochord structural maintenance as well as the evolution
and biological function of IF family proteins.