Assembly of adenovirus-specific nuclear inclusions in lytically infected HeLa cells: an ultrastructural and cytochemical study

Abstract

Adenoviruses (Ads) are nuclear DNA viruses that remodel host nuclear structure and function and induce formation of a variety of nuclear inclusions within which Ad DNA is replicated and transcribed. In this study, we have examined inclusion assembly by electron microscopy of samples stained conventionally or with bismuth to detect phosphoproteins. Small dense fibrillar bodies (DFBs) appeared very early associated with interchromatin granule (ICG) clusters. Somewhat later, similar DFBs lay near amorphous, loosely fibrillar structures that were moderately electron dense and showed little bismuth deposition. These clear fibrillar bodies (CFBs) enlarged and DFBs became embedded in their surface. At later stages, CFBs and DFBs were again dissociated. DFBs seen very early were poor in phosphoproteins, but later DFBs, whether embedded in the CFBs or lying near them, were intensely bismuth stained. DFBs and CFBs were less prominent once assembled virions were seen. At this late stage, virions were generally associated with moderately dense, slightly bismuth positive, irregularly shaped fibrillar inclusions that have previously been identified as viral genome storage sites. In addition, very dense fibrillar bodies, consisting usually of an electron-dense fibrillar shell and a less dense fibrogranular core, were observed at all but the earliest stages of infection, often at some distance from CFBs. There was also a major reorganization of host components during infection, including chromatin condensation, reduction of nucleolar volume and aggregation of the fibrillar regions at the nucleolar surface, and increased prominence of ICG clusters. A model is proposed for the assembly of Ad replication factories.Key words: adenovirus, lytic infection, replication factories, nuclear ultrastructure, cytochemistry.