Biomedical Engineering Reference
In-Depth Information
3
Mechanisms of Cellular Uptake
3.1
Overview of Endocytosis Pathways
3.1.1
Endocytosis
Endocytosis is the process in which a cell internalizes exogenous substances such
as extracellular fluid, small molecules, proteins, debris, and even entire cells
such as bacteria and apoptotic cells. There are two main categories of endocytosis,
phagocytosis (cell eating) and pinocytosis (cell drinking). Pinocytosis occurs
through four mechansims: clathrin-dependent endocytosis, caveolin-mediated endo-
cytosis, macropinocytosis, and clathrin/dynamin-independent endocytosis (Liu and
Shapiro
2003
). While virtually all cells are capable of pinocytosis, phagocytosis
may only occur in certain cells, including neutrophils, macrophages, monocytes,
and endothelial cells (Serda et al.
2009a
). In the context of nanoparticles, under-
standing which pathways and cellular compartments are involved in the delivery of
a drug helps to determine its bioavailability and pharmacological activity.
During pinocytosis, the cell membrane invaginates and then fuses, nonspecifi-
cally entrapping fluid and particles in a vesicle that pinches off and moves to the
cell interior. Receptor-mediated endocytosis occurs when the solute binds to a
receptor on the cell membrane and is selectively internalized (Swaan
1998
).
Phagocytosis, on the other hand, involves outward extension of membrane processes
known as pseudopodia. Pseudopodia wrap around the target in a process that is
dependent on actin polymerization (Yacobi et al.
2009
). Once surrounded by
these extensions, the target is pulled into the cell into a membrane-bound vesicle
known as a phagosome. During maturation, phagosomes fuse with lysosomes and
become phagolysosomes (Hartwig et al.
1977
). This fusion exposes the phagocy-
tosed contents to proteolytic enzymes and an acidic pH, which aids in digestion
of the contents.
Similar to phagocytosis, macropinocytosis is dependent on actin polymerization.
The process involves cell surface ruffling and leads to the formation of large macro-
pinosomes (Swanson and Watts
1995
). This route is believed to be less selective
that phagocytosis and results in endocytosis of solute macromolecules. Agents such
as cytochalasin D and latrunculin B can be used to study internalization by this
pathway (Yacobi et al.
2009
). Other studies use amiloride to selectively inhibit
macropinocytosis by blocking the Na+/H+ exchange pump (Park et al.
2010
).
Clathrin-mediated endocytosis is an energy-dependent pathway that involves
cargo being recognized by adapter protein complexes. These complexes then
recruit clathrin coats using other effector proteins. Invaginations, known as clath-
rin-coated pits, pinch off into clathrin-coated vesicles holding the selected cargo.
Once internalized, the clathrin is uncoated and the vesicles fuse together to form
early endosomes (Yacobi et al.
2009
). Subsequently, late endosomes are formed.
Cargo in late endosomes may be shipped to various organelles, including the lyso-
some. Lysosomal pathways will be discussed in greater detail, but in the context of
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