Intercellular viral spread and intracellular transposition of Drosophila gypsy

Abstract

It has become increasingly clear that retrotransposons (RTEs) are more widely expressed in somatic tissues than previously appreciated. RTE expression has been implicated in a myriad of biological processes ranging from normal development and aging, to age related diseases such as cancer and neurodegeneration. Long Terminal Repeat (LTR)-RTEs are evolutionary ancestors to, and share many features with, exogenous retroviruses. In fact, many organisms contain endogenous retroviruses (ERVs) derived from exogenous retroviruses that integrated into the germ line. These ERVs are inherited in Mendelian fashion like RTEs, and some retain the ability to transmit between cells like viruses, while others develop the ability to act as RTEs. The process of evolutionary transition between LTR-RTE and retroviruses is thought to involve multiple steps by which the element loses or gains the ability to transmit copies between cells versus the ability to replicate intracellularly. But, typically, these two modes of transmission are incompatible because they require assembly in different sub-cellular compartments. Like murine IAP/IAP-E elements, the gypsy family of retroelements in arthropods appear to sit along this evolutionary transition. Indeed, there is some evidence that gypsy may exhibit retroviral properties. Given that gypsy elements have been found to actively mobilize in neurons and glial cells during normal aging and in models of neurodegeneration, this raises the question of whether gypsy replication in somatic cells occurs via intracellular retrotransposition, intercellular viral spread, or some combination of the two. These modes of replication in somatic tissues would have quite different biological implications. Here, we demonstrate that Drosophila gypsy is capable of both cell-associated and cell-free viral transmission between cultured S2 cells of somatic origin. Further, we demonstrate that the ability of gypsy to move between cells is dependent upon a functional copy of its viral envelope protein. This argues that the gypsy element has transitioned from an RTE into a functional endogenous retrovirus with the acquisition of its envelope gene. On the other hand, we also find that intracellular retrotransposition of the same genomic copy of gypsy can occur in the absence of the Env protein. Thus, gypsy exhibits both intracellular retrotransposition and intercellular viral transmission as modes of replicating its genome.

Endogenous Retroviruses and TDP-43 Proteinopathy Form a Sustaining Feedback to Drive the Intercellular Spread of Neurodegeneration

“…Indeed, we found that a fraction of the hTDP-43-V5 transfected cells exhibited mCherry signal when they were grown in contact with the mdg4-H2B-mCherry expressing cells (Fig.5a). This viral transmission is as we previously reported 38 . In contrast, the H2B-mCherry control was never seen to transfer to the V5 expressing population of cells that are grown in contact (Fig.5a).…”

“…Indeed, both of these ERVs contain an enveloped glycoprotein that is required to mediate such intercellular transmission. We previously established Drosophila S2 cell culture assays to detect transmission of mdg4-ERV between cells grown in contact, or between cells that were separated by a 0.4 µm filter in a transwell system 38 . We used these assays to test whether viral transmission of mdg4-ERV was sufficient to mediate propagation of TDP-43 pathology to recipient cells.…”

“…Both the Drosophila mdg4-ERV 38, 42, 43 and human HERV-K 39, 44 , produce viral-like particles (VLPs) that are capable of transmitting between cells grown in culture. Indeed, both of these ERVs contain an enveloped glycoprotein that is required to mediate such intercellular transmission.…”

Somatic retrotransposition in the developing rhesus macaque brain

“…Single-cell analyses now provide conclusive evidence of L1-mediated somatic mosaicism in the macaque and human brain. Endogenous retrotransposition is also likely encountered in mouse and fly neuronal lineages ( Muotri et al 2005 ; Coufal et al 2009 ; Li et al 2013 ; Hazen et al 2016 ; Keegan et al 2021 ; Siudeja et al 2021 ). A major limitation of scWGS is the generation of MDA and Illumina library preparation false positives ( Faulkner and Garcia-Perez 2017 ; Treiber and Waddell 2017 ; Abascal et al 2021 ).…”

“…Single-cell whole-genome sequencing (scWGS) of postmitotic neurons ( Evrony et al 2015 ; Sanchez-Luque et al 2019 ) has uncovered somatic retrotransposition events traced to unusual donor (source) L1 loci that evade methylation in mature tissues ( Faulkner and Billon 2018 ; Sanchez-Luque et al 2019 ; Ewing et al 2020 ). Despite relevant studies of endogenous retrotransposition in the mouse and fly ( Li et al 2013 ; Hazen et al 2016 ; Richardson et al 2017 ; Keegan et al 2021 ; Siudeja et al 2021 ), scWGS has to date been applied only to human neurons ( Evrony et al 2015 ; Sanchez-Luque et al 2019 ), leaving the capacity of L1 to retrotranspose in nonhuman primate neuronal lineages an important open question. Starting with Haig Kazazian's first report of germline L1 mutagenesis ( Kazazian et al 1988 ), case studies have largely underpinned efforts to define the spatiotemporal extent of L1 mobility ( Miki et al 1992 ; Brouha et al 2002 ; van den Hurk et al 2007 ; de Boer et al 2014 ; Evrony et al 2015 ; Scott et al 2016 ; Sanchez-Luque et al 2019 ), founded on the principle that one robustly verified insertion is sufficient to show L1 retrotransposition can occur in a given context.…”