Biomedical Engineering Reference
In-Depth Information
problems and shows promise for gene delivery into the stem cell. Intracellular delivery of
specific proteins/peptides may be used to influence signaling pathways and manipulate
stem-cell fate. Efficient intracellular delivery occurs following the carrier moving to the cell
surface, cellular uptake, endosomal escape, and carrier unpackaging [6]. Nanocarriers, due
to their small size, can efficiently penetrate across the cell membrane barrier and increase the
efficiency of intracellular delivery. These nanocarriers can be synthesized chemically and
they modify the condensation and physicochemical state of the loaded gene/small interfe
ring RNA (siRNA) to protect them against cytoplasmic damage. Functionality of the nano-
carriers can be adjusted for controlled release of genes/siRNA. Factors that affect delivery
efficiency of nanoparticles include cell type, cell-cycle stage, cell-culture conditions, cell
density, and size of passaging nanoparticles, controlled intracellular release of bioactive
agents, cytotoxicity, stability, storage, and shelf-life of nanocarriers [7].
To attain a successful outcome in stem cells, several factors in delivery systems should be
taken into account. First, dependent on the target and type of bioactive agent, the size and
type of nanocarrier can vary. From the standpoint of the nanocarrier characteristics, size,
size distribution, surface charge, and the nature of the systems play critical roles for success-
ful intracellular delivery [8].
Delivered Bioactive Agents into Stem Cells
Molecules
Application of small molecules to regulate stem cell behavior is particularly beneficial as
they provide a high degree of temporal control over protein performance by either rapid
activation or inhibition of single or multiple targets within a protein family.
Retinoic acid (RA) is a good candidate in controlling stem-cell-derived neuronal
differentiation [9], and has been used to tune neuralization and positional specification dur-
ing mouse ESC differentiation. However, RA is rapidly metabolized by cells, has low water
solubility in aqueous solutions, and requires a fine tuning of the concentration window to
obtain results. These problems pose difficulties in the delivery of therapeutic doses. Moreover,
the use of this bioactive small molecule in an
in vivo
setting for the differentiation of stem
cells remains elusive [10].
Biological systems use a complex transportation network to deliver RA at the cell nucleus
so that it can activate RA receptors. Initially, RA is taken by biological systems as retinol,
which is removed from the blood and bound to cellular retinol-binding proteins in the cyto-
plasm. The retinol dehydrogenase enzymes metabolize retinol to retinal, which in turn is
metabolized to RA by the retinaldehyde dehydrogenases. The RA is then bound to cyto-
plasmic RA-binding proteins and this complex finally enters the nucleus and binds to the
RA receptors and the retinoid X receptors, which in turn heterodimerize and bind to a
sequence of DNA known as the RA-response element. Targeted RA delivery is also of
interest to manipulate stem cells. Increasing the specificity in the intracellular delivery of
RA will reduce not only side-effects but also the necessary amount of the molecule and
resultant costs. The chemical structure of RA is shown in Figure 13.1.
Gene/Small Interfering RNA
Introduction of genes/siRNA to regulate specific genes involved in signaling pathways that
control the cell phenotype can induce specific differentiation of stem cells into specific cell
types. Noncoding RNAi molecules known as microRNA (miRNA) post-transcriptionally
