The Bioartificial Pancreas: Progress and Challenges Review

THE BIOARTIFICIAL PANCREAS 973
cose-responsive insulin-producing cells using
inductive signals involved in normal pathways
of islet development. Compared with other
methods for insulin-producing cell development
from human stem cells, 91,93 the method
developed by this group produced insulin at
the highest levels yet achieved from an expandable,
human stem cell-derived tissue.
However, the method developed in the study
still needs to be improved, perhaps through the
addition of glucagon-like peptide-1, transforming
growth factor- ligands, or other factors
to potentiate -cell maturation, growth,
and insulin secretion. 94–96
Gene therapy has also been suggested as a
treatment of insulin-dependent diabetes mellitus.
This strategy can be applied by preventing
the autoimmune destruction of -cells, 97,98 by
regenerating -cells, 99,100 or by engineering insulin-secreting
surrogate -cells. 99,101–103 Sapir
et al. 104 recently suggested the potential use of
adult human liver as an alternate tissue for autologous
-cell-replacement therapy. Using
pancreatic and duodenal homeobox gene 1
(PDX-1) and soluble factors, they managed to
induce a comprehensive developmental shift of
adult human liver cells into functional insulinproducing
cells. According to the findings of
that study, PDX-1-treated human liver cells expressed
insulin, stored it in defined granules,
and secreted the hormone in a glucose-regulated
manner. When these cells were implanted
under the renal capsule of immunodeficient
mice with diabetes, the cells ameliorated hyperglycemia
for prolonged periods of time. The
technique offers both the potential of a cell-replacement
therapy for patients with diabetes
and allows the patient to be the donor of his or
her own insulin-producing tissue.
Intestinal K cells have also been engineered
to express and secrete insulin in a glucose dependent
manner, generating functional cells.
105,106 With the development of robust and
safe gene therapy, this strategy may lead to effective
therapy in the future.
Preservation of encapsulated cell systems
The last challenge for a bioartificial pancreas
to become a routine clinical application is the
long-term cryopreservation of encapsulated
cells. Validated technologies are required for
long-term preservation of encapsulated cell systems
to maintain a product inventory, and in order
to meet end-user demands. One of the current
strategies for overcoming the problem of
cryopreservation of tissue is the reduction of cryoprotectant
[dimethyl sulfoxide (DMSO)] concentration
or complete replacement by equally
powerful substances. Recent work in Dr. Opara’s
laboratory has demonstrated that cryopreservation
of islet cells cultured overnight in the presence
of 10% polyvinylpyrrolidone yielded higher
intact islet recovery compared with islets frozen
in the presence of DMSO and glycerol. 107 The
lower islet cell integrity and function were explained
on the basis of hypothesis that low-molecular-weight
compounds, such as DMSO and
glycerol, permeate the cell and interact hydrophobically
with intracellular proteins, which
results in perturbed cytoskeletal architecture of
the frozen cells and diminished islet cell integrity
and function.
The other promising technical improvement
of the cryopreservation technology is the reduction
of total sample volume. Miniaturization
of the cryosubstrates from 1 mL to microliters
reduces temperature gradients in cryosubstrates,
which makes it possible to achieve
higher freezing rates and more homogeneous
freezing of the sample. 108 Islets cryopreserved
with this strategy were highly functional even
with the lower DMSO concentrations. 109
Misler et al. 110 studied stimulus–secretion
coupling in whole islets as well as single cells
from carefully selected cryopreserved and
thawed human islets of Langerhans. They found
that without using any of the recent advances in
cryopreservation technique, cryopreserved and
thawed human islets, which were selected based
on their smooth surface and diameter, responded
to glucose in a calcium- and metabolicdependent
fashion. In another study by von
Mach et al., 111 viability of pancreatic islets after
cryopreservation was correlated negatively with
their size, and suppression of insulin release was
not observed for islets 300 m.
DIFFERENT FORMS OF BIOARTIFICIAL
PANCREAS
Various configurations have been studied for
the purpose of immunoisolation of islets. These