exosome

Definition: The exosome complex is a multi-protein complex, capable of degrading various types of RNAs. Exosomes are extracellular cell-derived phospholipid nanovesicles that function as signalosomes, transmitting prodigious amounts of bioactive molecules to specific recipient tissues.

Exosomes (versatile, cell-derived nanovesicles naturally endowed with exquisite target-homing specificity and the ability to surmount in vivo biological barriers) hold substantial promise for developing exciting approaches in drug delivery and cancer immunotherapy.

Specifically, bioengineered exosomes are being successfully deployed to deliver potent tumoricidal drugs (siRNAs and chemotherapeutic compounds) preferentially to cancer cells, while a new generation of exosome-based therapeutic cancer vaccines has produced enticing results in early-phase clinical trials. Here, we review the state-of-the-art technologies and protocols, and discuss the prospects and challenges for the clinical development of this emerging class of therapeutics.

Exosomes may offer a tractable, bioinspired system for targeted drug delivery, which could improve the therapeutic indices of conventional cytotoxic chemotherapeutic agents, and help to realize the enormous potential of gene therapy in oncology.

Owing to their immunomodulatory potential, exosomes may also be deployed in innovative immunological approaches to activate adaptive and innate effector cell-mediated anticancer immunosurveillance.

Exosome complexes can be found in both eukaryotic cells and archaea, while in bacteria a simpler complex called the degradosome carries out similar functions. The core of the complex has a six-membered ring structure, to which other proteins are attached.

In eukaryotic cells, it is present in the cytoplasm, nucleus and especially the nucleolus, although different proteins interact with the complex in these compartments, in order to regulate the RNA degradation activity of the complex to substrates specific for these cell compartments. Substrates of the exosome include messenger RNA, ribosomal RNA, and many species of small RNAs.

The exosome has an exoribonucleolytic function, meaning it degrades RNA starting at one side (the so-called 3’ end in this case), rather than cleaving the RNA at specific sites.

Although no causative relation between the complex and any disease is known, several proteins in the complex are the target of autoantibodies in patients with specific autoimmune diseases (especially the PM/Scl overlap syndrome) and some antimetabolitic chemotherapies for cancer function by blocking the activity of the complex.

In the eukaryotic ribosomal RNA processing pathway, a complex of ten riboexonucleases called the exosome that plays a central role in the precise formation of the 3’ ends of several types of RNAs.

The exosome also destroys excess ribosomal RNA precursors and unused intermediates and degrades poly(A)-mRNAs in the cytoplasm.

In the nucleus, the complex appears to function in a regulated mRNA surveillance system that degrades transcripts in response to defects in the mRNA processing and export pathways.

The exosome complex of 3’—>5’ exonucleases is an important component of the RNA-processing machinery in eukaryotes. This complex functions in the accurate processing of nuclear RNA precursors and in the degradation of RNAs in both the nucleus and the cytoplasm.

Pathology

 Syndromic diarrhea (or trichohepatoenteric syndrome)

• Syndromic diarrhea (or trichohepatoenteric syndrome) is a rare congenital bowel disorder characterized by intractable diarrhea and woolly hair.
• It has been associated with mutations in TTC37.
• Mutations in genes encoding cofactors of the human Ski complex cause syndromic diarrhea, establishing a link between defects of the human exosome complex and a Mendelian disease.

 Autoimmunity

• The exosome complex is the target of autoantibodies in patients that suffer from various autoimmune diseases. These autoantibodies are mainly found in people that suffer from the PM/Scl overlap syndrome, an autoimmune disease in which patients have symptoms from both scleroderma and either polymyositis or dermatomyositis.
• Autoantibodies can be detected in the serum of patients by a variety of assays. In the past, the most commonly used methods were double immunodiffusion using calf thymus extracts, immunofluorescence on HEp-2 cells or immunoprecipitation from human cell extracts.
• In immunoprecipitation assays with sera from anti-exosome positive sera, a distinctive set of proteins is precipitated. Already years before the exosome complex was identified, this pattern was termed the PM/Scl complex.
• Immunofluorescence using sera from these patients usually shows a typical staining of the nucleolus of cells, which sparked the suggestion that the antigen recognized by autoantibodies might be important in ribosome synthesis.
• More recently, recombinant exosome proteins have become available and these have been used to develop line immunoassays (LIAs) and enzyme linked immunosorbent assays (ELISAs) for detecting these antibodies.
• In these diseases, antibodies are mainly directed against two of the proteins of the complex, called PM/Scl-100 (the RNase D like protein) and PM/Scl-75 (one of the RNase PH like proteins from the ring) and antibodies recognizing these proteins are found in approximately 30% of patients with the PM/Scl overlap syndrome.
• Although these two proteins are the main target of the autoantibodies, other exosome subunits and associated proteins (like C1D) can be targeted in these patients.
• Currently, the most sensitive way to detect these antibodies is by using a peptide, derived from the PM/Scl-100 protein, as the antigen in an ELISA, instead of complete proteins. By this method, autoantibodies are found in up to 55% of patients with the PM/Scl overlap syndrome, but they can also be detected in patients suffering from either scleroderma, polymyositis or dermatomyositis alone.
• As the autobodies are mainly found in patients that have characteristics of several different autoimmune diseases, the clinical symptoms of these patients can vary widely. The symptoms that are seen most often are the typical symptoms of the individual autoimmune diseases and include Raynaud’s phenomenon, arthritis, myositis and scleroderma.
• Treatment of these patients is symptomatic and is similar to treatment for the individual autoimmune disease, often involving either immunosuppressive or immunomodulating drugs.

 Cancer

• The exosome has been shown to be inhibited by the antimetabolite drug fluorouracil, which is a drug used in chemotherapy treatment of cancer. It is one of the most successful drugs for treating solid tumors.
• In yeast cells treated with fluorouracil, defects were seen in the processing of ribosomal RNA identical to those seen when the activity of the exosome was blocked by molecular biological strategies.
• Lack of correct ribosomal RNA processing is lethal to cells, explaining the antimetabolic effect of the drug.

Oncology biomarkers

It has been developed working procedures to evaluate exosomal RNA signature as novel biomarkers for cancer prognosis, prediction and patient stratification.

In particular, it has refreshed current practice and demonstrated a new approach for studying RNA signature in patient blood samples.

See also

 proteasome
 spliceosome

Paywall References

 Exosomes in Cancer Nanomedicine and Immunotherapy: Prospects and Challenges. Cell, 2017. doi : 10.1016/j.tibtech.2017.03.004

 Houseley J, LaCava J, Tollervey D. RNA-quality control by the exosome. Nat Rev Mol Cell Biol. 2006 Jul;7(7):529-39. PMID: 16829983

 Butler JS. The yin and yang of the exosome. Trends Cell Biol. 2002 Feb;12(2):90-6. PMID: 11849973