It may not be well recognized that TJs are observed only in vertebrate species (and tunicate) with some exceptions. Instead of TJs, invertebrates have "septate junctions"(SJs) as occluding junctions. To date, electron microscopic observations have identified SJs in various invertebrate species. However, little is known about the architecuture and functional regulation of SJs except Drosophila SJs in ectodermally derived epithelia. We focus on another type of Drosophila SJs observed in endodermally derived epithelia, named smooth SJs, (sSJs), which almost no one has ever studied at a molecular level. We have identified three novel proteins specifically localized at sSJs and found that these proteins are required for sSJ formation and intestinal barrier function in a mutually interdependent manner. We are investigating the mechanism of the formation of sSJs and their role in the gut in adult fly, in which its homeostasis is dynamically maintained by the stem cell system. By comparing Drosphila sSJs and vertebrate TJs, we will see not only the diversity but also the basic principle of occluding junctions in evolution.

1.	Izumi Y, Furuse K, Furuse M. (2021) The novel membrane protein Hoka regulates septate junction organization and stem cell homeostasis in the Drosophila gut. J Cell Sci. 134(6):jcs257022. doi: 10.1242/jcs.257022.
2.	Izumi Y, Furuse K, Furuse M. (2019) Septate junctions regulate gut homeostasis through regulation of stem cell proliferation and enterocyte behavior in Drosophila. J Cell Sci. 132(18). pii: jcs232108. doi: 10.1242/jcs.232108.
3.	Furuse M, Izumi Y. (2017) Molecular dissection of smooth septate junctions: understanding their roles in arthropod physiology. Ann N Y Acad Sci. 1397(1):17-24. doi: 10.1111/nyas.13366. Review
4.	Izumi Y, Motoishi M, Furuse K, Furuse M. (2016) A tetraspanin regulates septate junction formation in Drosophila midgut. J Cell Sci. 129:1155-1164
5.	Izumi Y, Furuse M. (2014) Molecular organization and function of invertebrate occluding junctions. Semin Cell Dev Biol. 36:186-93. Review
6.	Izumi Y, Yanagihashi Y, Furuse M. (2012)　A novel protein complex, Mesh-Ssk, is required for septate junction formation in Drosophila midgut. J. Cell Sci. 125(Pt 20):4923-33.
7.	Yanagihashi Y, Usui T, Izumi Y, Yonemura S, Sumida M, Tsukita S, Uemura T and Furuse M. (2012) Snakeskin, a membrane protein associated with smooth septate junctions, is required for intestinal barrier function in Drosophila. J Cell Sci. 125(Pt 8):1980-90.