Biomedical Engineering Reference
In-Depth Information
in living cells. Using GFP protein as a model cargo it was shown that MITOporter
liposomes are able to selectively deliver their cargo to mitochondria (Yamada and
Harashima
2008
; Yamada et al.
2008
). It is also possible that conferring upon a
nanocarrier the properties of the membrane of a particular subcellular compartment
might direct transport through the cytosol to the organelle of interest. This concept
has been explored with a liposomal formulation of a crude mitochondrial fraction
(Inoki et al.
2000
). These so calle proteoliposomes were reported to colocalize with
endogenous mitochondria when microinjected into pre implantation embryos.
In addition to particle composition, it has also been observed that changes in
nanoparticle architecture affect sub-cellular disposition (Xu et al.
2008
). Fluorescein
isothiocyanate labeled layered double hydroxide (LDH) nanoparticles were prepared
from Mg
2
Al under conditions that yielded either hexagonal sheets (5-150 nm wide
and 10-20 nm thick) or nanorods (30-60 nm wide and 100-200 nm long). A com-
parison of the sub-cellular distribution of these two types of preparations revealed
that the nanorods trafficked to the nucleus but the hexagonal sheets remained in the
cytoplasm (Xu et al.
2008
). An active microtubule mediated transport process is
hypothesized to be responsible for the observed rapid nuclear accumulation of the
nanorods (Xu et al.
2008
).
2.3
Organelle Entry
As with the previous two, barriers hitchhiking existing biological processes extend
to mediating organellar entry in most delivery approaches. Proteins synthesized in
the cytosol bear short peptide sequences within the protein as 'molecular zip codes'
that determine the preferential transport of a protein into a membrane bound subcel-
lular organelle (Walter et al.
1982
). These leader sequences or localization signals
are typically recognized by specialized import proteins that are associated with the
organelle (Allen et al.
2000
; Stoffler et al.
1999
). Localization sequences for the
nucleus and the mitochondria are well known and have been explored for mediating
the entry of a variety of molecular cargos as well as nanocarriers into these organ-
elles. The potential of leader sequence peptides for overcoming intracellular barriers
to DNA delivery was first demonstrated a decade ago (Zanta et al.
1999
). To facili-
tate the import of exogenous DNA into the nucleus, a capped 3.3-kbp CMVLuciferase-
NLS gene containing a single nuclear localization signal peptide (PKKKRKVEDPYC)
was synthesized. The resulting transfection enhancement due to the nuclear leader
peptide was about 10-fold to 1,000-fold irrespective of the cationic vector or the cell
type used. At that time the authors hypothesized that the 3 nm wide DNA present in
the cytoplasm was initially docked to and translocated through a nuclear pore by the
nuclear import machinery and as DNA enters the nucleus, it was quickly condensed
into a chromatin-like structure, which provided a mechanism for threading the
remaining worm-like molecule through the pore (Zanta et al.
1999
). In this context
an interesting question arises as to whether localization signals should be directly
conjugated to the bioactive molecule or the nano-carrier. Besides protein import
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