Human autoantibodies to diacyl-phosphatidylethanolamine recognize a specific set of discrete cytoplasmic domains

Clinical description of patients with cytoplasmic discrete dots pattern (lysosome) on indirect immunofluorescence on HEp-2 cells

The cytoplasmic discrete dot (CDD) pattern is an unusual finding in indirect immunofluorescence, and its clinical value is unknown. To describe the clinical characteristics of patients with CDD pattern on indirect immunofluorescence (IIF) from our laboratory database and to evaluate possible associations with other autoantibodies and autoimmune diseases. This is a retrospective descriptive study. We included all patients with CDD pattern on IIF in HEp-2 cells with a titer equal or greater than 1/80, using a database of all IIF performed in a reference immunology and rheumatology laboratory between 2007 and 2015. Data on demographics, past medical history, and relevant laboratory findings were recorded and analyzed. We performed 13.056 IIF on HEp-2 cells tests between January 1, 2007 and December 31, 2015, with 6075 positive results. Among them, 5447 had nuclear pattern, 55 had both nuclear and cytoplasmic pattern, and 573 had cytoplasmic pattern. Only 21 showed a CDD pattern. Four patients were excluded since they did not have medical records at the institution. The prevalence of the CDD pattern in our laboratory was 0.35%. The median age was of 62.3 years (SD 9.16) and 100% were female. Fifty-three percent (9/17) had an autoimmune disease, Hashimoto's thyroiditis (4/9) being the most frequent one. In conclusion, cytoplasmic discrete dot pattern is an uncommon finding and its clinical value is uncertain. However, in our study, 53% of the patients had an autoimmune disease.

Relationship of other cytoplasmic ribonucleoprotein bodies (cRNPB) to GW/P bodies

GW/P body components are involved in the post-transcriptional -processing of messenger RNA (mRNA) through the RNA interference and 5' → 3' mRNA degradation pathways, as well as functioning in mRNA transport and stabilization. It is currently thought that the relevant mRNA silencing and degrading factors are partitioned to these cytoplasmic microdomains thus effecting post-transcriptional regulation and the prevention of accidental degradation of functional mRNA. Although much attention has focused on GW/P bodies, a variety of other cytoplasmic RNP bodies (cRNPB) also have highly specialized functions and have been shown to interact or co-localize with components of GW/P bodies. These cRNPB include neuronal transport RNP granules, stress granules, RNP-rich cytoplasmic germline granules or chromatoid bodies, sponge bodies, cytoplasmic prion protein-induced RNP granules, U bodies and TAM bodies. Of clinical relevance, autoantibodies directed against protein and miRNA components of GW/P bodies have been associated with autoimmune diseases, neurological diseases and cancer. Understanding the molecular function of GW/P bodies and their interactions with other cRNPB may provide clues to the etiology or pathogenesis of diseases associated with autoantibodies directed to these structures. This chapter will focus on the similarities and differences of the various cRNPB as an approach to understanding their functional relationships to GW/P bodies.

Autoantibodies to GW bodies and other autoantigens in primary biliary cirrhosis

Autoantibodies to intracellular targets in mitochondria and nuclei are serological hallmarks of primary biliary cirrhosis (PBC). One of the most recently identified cellular targets of PBC autoantibodies is a novel cytoplasmic structure referred to as GW bodies [GWB, G (glycine) W (tryptophan)-containing bodies (GWB)]. GWB are indentified as discrete cytoplasmic domains that are involved in mRNA processing via the RNA interference (RNAi) pathway. Key components of GWB include the proteins GW182, Ago2, RNA-associated protein 55 (RAP55) and Ge-1/Hedls. The primary objective was to study the frequency and clinical association of antibodies directed to GWB components, in 109 PBC patients. Autoantibodies to mitochondrial antigen-pyruvate dehydrogenase complex (M2), branched-chain 2-oxo-acid dehydrogenase complex and 2-oxo glutarate dehydrogenase complex (3E-BPO), gp210, sp100, promyelocytic leukaemia cell antigen (PML) and liver kidney microsomal-1 antigen (LKM-1) were detected by a line immunoassay and antibodies to GWB (GW182, RAP55, Ge-1, GW2, GW3) and glutamate receptor interacting protein (GRIP)-associated protein-1 (GRASP-1), by an addressable laser bead immunoassay (ALBIA). The most common GWB autoantigen targets were: RAP55-28%, GW182-12%, GW2-2% and antibodies to GRASP-1-17%. By comparison, the frequency of reactivity to established PBC autoantigens was: gp210, 27%; sp100, 27% and PML, 17%. None of the autoantibodies were associated with differences in Mayo risk score or liver decompensation. This study is the first study to show that antibodies to RAP55, GW182 and GRASP-1 are the most common GWB targets in PBC.

Control of RNA silencing and localization by endolysosomes

Recent advances in the cell biology of RNA silencing have unraveled an intriguing association of post-transcriptionally regulated RNA with endolysosomal membranes in several circumstances of mRNA localization, microRNA activity and viral RNA transport and packaging. Endolysosomal membranes are a nexus of communication and transport between cells and their exterior environment for signaling receptors, pathogens and nutrients. Here, we discuss recent data that support a view that endolysosomal positioning of RNA might facilitate intercellular transmission of RNA and host defence against viruses and retrotransposons. Positioning of RNA regulatory mechanisms on endolysosomal membranes might permit rapid and localized control of microRNA (miRNA) gene regulatory programs and mRNA translation in response to environmental signals, such as activated plasma membrane receptors transported on endosomes. Finally, we suggest that the pathology of several conditions, including Huntington's disease, might be a consequence of the disruption of the control of RNA via endolysosomal membranes.

Multivesicular bodies associate with components of miRNA effector complexes and modulate miRNA activity

In animals, P-bodies or GW-bodies appear to cause the congregation of proteins involved in microRNA (miRNA)-mediated post-transcriptional silencing. The localization of P-bodies does not overlap with that of known organelles and are thus considered independent of lipid bilayers. Nonetheless, an miRNA effector protein, argonaute 2 (AGO2), was initially identified as membrane-associated, and some miRNAs have been found in secreted vesicles (exosomes) that derive from endo-lysosomal compartments called multivesicular bodies (MVBs). Proteins can be sorted in a ubiquitin-dependent manner into MVBs by three heteromeric subcomplexes, collectively termed ESCRT (endosomal sorting complex required for transport), to be further secreted in exosomes and/or degraded by the lysosome. Here we show that GW-bodies containing GW182 and AGO2, two main components of the RNA-induced silencing complex (RISC), are distinct from P-bodies due to their congregation with endosomes and MVBs. Moreover, miRNAs and miRNA-repressible mRNAs are enriched at these cellular membranes, suggesting that endosomes and/or MVBs are sites of miRNA-loaded RISC (miRISC) accumulation and, possibly, action. We further show that purified exosome-like vesicles secreted by MVBs are considerably enriched in GW182, but not P-body components, AGO2 or miRNA-repressible mRNA. Moreover, cells depleted of some ESCRT components show compromised miRNA-mediated gene silencing and over-accumulate GW182, which associates with ubiquitylated proteins. Therefore, GW182, possibly in association with a fraction of miRNA-loaded AGO2, is sorted into MVBs for secretion and/or lysosomal degradation. We propose that this process promotes continuous assembly or disassembly of membrane-associated miRISCs, which is possibly required for miRNA loading or target recognition and subsequent silencing.

Clinical and serological features of patients with autoantibodies to GW/P bodies

GW bodies (GWBs) are unique cytoplasmic structures involved in messenger RNA (mRNA) processing and RNA interference (RNAi). GWBs contain mRNA, components of the RNA-induced silencing complex (RISC), microRNA (miRNA), Argonaute proteins, the Ge-1/Hedls protein and other enzymes involving mRNA degradation. The objective of this study was to identify the target GWB autoantigens reactive with 55 sera from patients with anti-GWB autoantibodies and to identify clinical features associated with these antibodies. Analysis by addressable laser bead immunoassay (ALBIA) and immunoprecipitation of recombinant proteins indicated that autoantibodies in this cohort of anti-GWB sera were directed against Ge-1/Hedls (58%), GW182 (40%) and Ago2 (16%). GWB autoantibodies targeted epitopes that included the N-terminus of Ago2 and the nuclear localization signal (NLS) containing region of Ge-1/Hedls. Clinical data were available on 42 patients of which 39 were female and the mean age was 61 years. The most common clinical presentations were neurological symptoms (i.e. ataxia, motor and sensory neuropathy) (33%), Sjögren's syndrome (SjS) (31%) and the remainder had a variety of other diagnoses that included systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and primary biliary cirrhosis (PBC). Moreover, 44% of patients with anti-GWB antibodies had reactivity to Ro52. These studies indicate that Ge-1 is a common target of anti-GWB sera and the majority of patients in a GWB cohort had SjS and neurological disease.

Autoimmune targeting of key components of RNA interference

RNA interference (RNAi) is an evolutionarily conserved mechanism that is involved in the post-transcriptional silencing of genes. This process elicits the degradation or translational inhibition of mRNAs based on the complementarity with short interfering RNAs (siRNAs) or microRNAs (miRNAs). Recently, differential expression of specific miRNAs and disruption of the miRNA synthetic pathway have been implicated in cancer; however, their role in autoimmune disease remains largely unknown. Here, we report that anti-Su autoantibodies from human patients with rheumatic diseases and in a mouse model of autoimmunity recognize the human Argonaute (Ago) protein, hAgo2, the catalytic core enzyme in the RNAi pathway. More specifically, 91% (20/22) of the human anti-Su sera were shown to immunoprecipitate the full-length recombinant hAgo2 protein. Indirect immunofluorescence studies in HEp-2 cells demonstrated that anti-Su autoantibodies target cytoplasmic foci identified as GW bodies (GWBs) or mammalian P bodies, structures recently linked to RNAi function. Furthermore, anti-Su sera were also capable of immunoprecipitating additional key components of the RNAi pathway, including hAgo1, -3, -4, and Dicer. Together, these results demonstrate an autoimmune response to components of the RNAi pathway which could potentially implicate the involvement of an innate anti-viral response in the pathogenesis of autoantibody production.