Morphogenesis of the protein secretory system in PC12 cells infected with Japanese encephalitis virus

Summary

Infection of PC12 cells with Japanese encephalitis (JE) virus caused marked proliferation of the protein secretory system. Accordingly, in this study the morphogenesis of the secretory orgenelles, i.e., rough endoplasmic reticulum (RER) and the Golgi apparatus, in JE virus-infected PC12 cells was analyzed by electron microscopical observation. Starting 24 h postinoculation (p.i.), a structure that represented nascent RER appeared in the cytoplasm in the form of rows of ribosomes which surrounded membrane-unbounded, electron-lucent lacunae in a reticular, honey-comb pattern (reticular RER). Although the reticular RER lacked membrane components, its lacunae contained progeny virions, indicating that the rows of ribosomes synthesized the viral proteins and discharged them into the lacunae for the viral assembly. The reticular RER apparently transformed into the familiar lamellar RER during the RER morphogenesis as the lacunae coalesced to form flat cisternae and RER membrane assembled to border the cisternae. These findings indicated that the proliferating RER was the site of not only active protein synthesis but also active membrane biogenesis. The proliferating RER released a large number of membrane vesicles including virion-carrying vesicles into the cytoplasm. These vesicles congregated in the juxtanuclear region, especially around the centrioles, and fused to existing Golgi complexes for enlargement or fused among themselves to form new Golgi complexes. The present study, therefore, indicated that (a) nascent RER was formed by polysomes that arranged themselves in rows of ribosomes without participation of a preexisting membrane framework of endoplasmic reticulum (ER), (b) membrane components of RER were assembled de novo within the structure during the RER morphogenesis, and (c) RER released membrane vesicles that moved to the Golgi apparatus and contributed to the morphogenesis of the Golgi apparatus. Possible causative mechanisms involved in the proliferation of the secretory system in JE virus-infected PC12 cells are discussed.