Biomedical Engineering Reference
In-Depth Information
endosomal compartments, further distinctions between early endosomes and late
endosomes exist (
Park et al.
). For some nanoparticles, it has been determined that
clathrin-mediated internalization is the predominant pathway by using chlorprom-
azine to inhibit internalization (
Park et al.
). Chlorpromazine causes dissociation of
clathrin from the cell membrane, effectively blocking clathrin-mediated endocy-
tosis. Dansylcadaverine has also been used as an effective inhibitor of clathrin-
mediated endocytosis, but the exact mechanism of this drug is not known (Yacobi
et al.
2009
). Clathrin-mediated endocytic pathways can lead to the lysosomal
degradation, reducing the effectiveness of a drug (
Park et al.
).
Similar to clathrin-mediated endocytosis, caveolae-mediated endocytosis cre-
ates vesicles 50-70 nm in diameter, called caveolae, through invaginations of the
cell membrane (Brown and London
1998
). Caveolae have been demonstrated to
be localized to lipid rafts, areas of plasma membrane highly concentrated in
sphingolipids and cholesterol (Brown and London
1998
). It is this property of
caveolae that allows their function to be disrupted in the presence of sterol-
binding drugs that sequester cholesterol, such as methyl-beta-cyclodextrin
(Yacobi et al.
2009
) and filipin. Park et al. (
2010b
) used filipin to demonstrate
the involvement of the caveolae-mediated pathway in the endocytosis of hydro-
phobically modified glycol chitosan (HGC). Confirmation that the caveolae path-
way is the mechanism for cellular uptake has also been demonstrated through
colocalization of labeled substrates, such as nanoparticles, with caveolin-1, the
major protein in caveolae (Yacobi et al.
2009
). Caveolin-1 is also found in trans-
Golgi and post-Golgi vesicles.
Pinocytic pathways that do not fall under the above categories are referred to as
clathrin- and caveolae-independent endocytosis (Park et al.
2010b
). These pathways
are a challenge to study and the extent of involvement of these pathways is hard to
quantify due to the lack of know inhibitors. Thus, inhibition studies of defined path-
ways are performed, and this mechanism will often be named by default.
3.1.2
Targeting Specific Endocytic Pathways
A cell uses various pathways in order to internalize molecules and particles. What
remains an unanswered question is what characteristics of a substance render it
more favorable for one pathway over another. However, this is obviously not the
case for cargo known to bind to specific cell surface receptors, and for molecules
that enter cells by diffusion, whose characteristics are well defined. For novel thera-
peutics, such as nanoparticle drug carriers and exogenous drugs, future studies will
be required to determine the physiological pathways involved during their endocy-
tosis by target cells. Understanding which properties of nanoparticles dictate which
pathways are utilized will benefit the development of particles and will aid in
designing drug carriers that traffic along desired pathways to reach the targeted
intracellular locations.
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