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secreted upon fusion of the MVBs with the plasma membrane for release into the
extracellular space and body fluids.
miRNAs in complex with AGO2 have been found to represent the predominant
form of freely circulating miRNAs in plasma and cell culture media and is stable
over a 2-month period at room temperature [
157,
158
]. It has been hypothesized that
these complexes represent by-products of dead cells secreted into the extracellular
space, and that they are unlikely to possess paracrine cell signaling functions. This
is in contrast to studies of miRNA-containing exosomes, for which a biological
function involving paracrine delivery of miRNA, mRNA, DNA, and protein is well
documented.
13.4.3
Exosome Biogenesis and Secretion
The mechanism for sorting of exosomal cargo into the exocytic MVB is largely
unknown. It is speculated to involve ubiquitination and the endosomal sorting com-
plex required for transport (ESCRT) sorting system, since ubiquitinated proteins
and ESCRT components are enriched in purified exosomes from some cell types
[
161,
162
]. Furthermore, phosphatidylinositol-(4,5)-bisphosphate (PIP2) and phos-
phatidylinositol-(3,4,5)-trisphosphate (PIP3)-binding domains can induce targeting
of highly oligomeric cytoplasmic proteins to exosomes [
163,
164
] . Studies have
shown that the miRNA content of exosomes resembles overall that of the donor cell
[
165,
166
]. The proportion, however, of the most and least abundant miRNAs are
often shifted, suggesting a specific enrichment or depletion of certain miRNAs in
the vesicles [
166,
167
]. The mechanism of this differential packing into exosomes
is presently unknown. Notably, miRNA target transcripts appear to be underrepre-
sented in exosomes, while housekeeping mRNAs (less prone to miRNA-mediated
repression) seem overrepresented [
165
]. To explain this, it was hypothesized that
miRNAs and their corresponding target mRNAs are enriched in GW182 (glycine-
tryptophan protein of 182-kDa) and AGO2-associated membrane fractions and
thereby excluded from exosome-like vesicles.
The events leading to fusion of the MVBs to the plasma membrane and exosome
release are not fully understood, but involves the action of RAB (member RAS
oncogene family) proteins, specifically RAB27A, RAB27B, and RAB35 [
168,
169
] .
The inhibition of ceramide production, via neutral sphingomyelinase (nSMAse) by
small molecule antagonist GW4869, reduces exosome-mediated secretion of miR-
NAs [
170
]. In addition, intracellular rise in calcium levels and induction of TP53
upon DNA-damage have been shown to enhance exosome secretion [
171,
172
] . In
contrast, release is inhibited by the induction of autophagy, a normal self-catabolic
process involving degradation and recycling of cellular components through the
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