RNAi Therapeutic Delivery by Exosomes - RNA Interference from Biology to Therapeutics - page 189

9.3.3.1 Exosome Biogenesis and Biological Role

Exosome release is part of the endosomal trafficking system, a mechanism by which
the cell sorts incoming molecules for recycling, storing, or degradation by transpor-
tation to the lysosome. Endocytic vesicles form at the plasma membrane via differ-
ent endocytic mechanisms and fuse to form the early endosome. This structure
receives the endocytosed cargo from the plasma membrane and sorts it. Early endo-
somes survey the peripheral cytoplasm of the cell via salutatory movement along
microtubules [ 45, 46 ] .
Late endosomes are situated close to the nucleus and develop from early endo-
somes by acidification, altered protein content, and fusion of vesicles [ 47 ] . The
limiting membrane, which is rich in LAMP1 to protect the membrane from acid
hydrolases, continuously buds into the lumen trapping cytosol to enrich the popula-
tion of ILVs [ 48 ]. Upon fusion with a lysosome, the endocytosed cargo is hydro-
lyzed, or on fusion with the plasma membrane, the ILVs are released as exosomes.
A mechanism for selective sorting of membrane-associated cargo for degradation in
lysosomes versus exportation from the cell in an exosome is regulated by the endo-
somal sorting complex required for transport (ESCRT) complex which recognizes
and sequesters mono-ubiquitinated proteins in the endosomal membranes [ 49 ] .
There is also an ESCRT-independent sorting mechanism involving lipid raft micro-
domains laterally segregating cargo with the involvement of ceramide that promotes
vesicle budding [ 50 ] .
The fate of exosomes after binding to their target cells remains to be fully eluci-
dated, but they can activate cell surface receptors on a target cell or fuse with the
target cell to transfer bioactive exosomal proteins and RNA or undergo internaliza-
tion by endocytosis [ 51 ]. MVs and exosomes can signal at a distance and transfer
proteins and mRNA horizontally, allowing them to coordinate processes such as
inflammation and coagulation whereby their phosphatidylserine-enriched mem-
branes act as a surface for the assembly of clotting factors [ 52 ] and contribute to
antigen presentation as exosomes from antigen-presenting cells have functional
MHC complexes on their surface [ 53 ]. They can regulate immune responses by
stimulating or inhibiting T cells [ 54 ]. They are also implicated in the pathogenesis
of different diseases. For example, exosomes derived from cancer cells can them-
selves be tumorigenic and are rich in metalloproteinases which degrade the extra-
cellular matrix to facilitate the growth and spread of a tumor [ 55, 56 ] . The release of
membrane vesicles from tumor cells correlates with poor prognosis [ 57 ] . MVs and
exosomes are also involved in the spread of infectious agents including viruses such
as HIV and prions [ 58, 59 ] .

9.3.3.2 Exosomes as Biomarkers and Therapeutic Tools

Exosomes are being increasingly recognized as potential biomarkers for different
disorders. They are easily obtainable from biological fluids and contain a unique
mRNA and miRNA fingerprint reflective of the cell of origin [ 60 ] . Exosome
numbers are elevated in different disease states, and the RNA expression profiles in