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= Transdifferentiation =
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Introduction
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Transdifferentiation, also known as lineage reprogramming, is a
process in which one mature somatic cell transforms into another
mature somatic cell without undergoing an intermediate pluripotent
state or progenitor cell type. It is a type of metaplasia, which
includes all cell fate switches, including the interconversion of stem
cells. Current uses of transdifferentiation include disease modeling
and drug discovery and in the future may include gene therapy and
regenerative medicine. The term 'transdifferentiation' was originally
coined by Selman and Kafatos in 1974 to describe a change in cell
properties as cuticle producing cells became salt-secreting cells in
silk moths undergoing metamorphosis.
Discovery
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Davis et al. 1987 reported the first instance of transdifferentiation
where a cell changed from one adult cell type to another. Forcing
mouse embryonic fibroblasts to express MyoD was found to be sufficient
to turn those cells into myoblasts.
Natural examples
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There are no known instances where adult cells change directly from
one lineage to another except 'Turritopsis dohrnii' and in the
'Turritopsis Nutricula', a jellyfish that is theoretically immortal.
Rather, cells dedifferentiate and then redifferentiate into the cell
type of interest. In newts when the eye lens is removed, pigmented
epithelial cells de-differentiate and then redifferentiate into the
lens cells.
In the pancreas, it has been demonstrated that alpha cells can
spontaneously switch fate and transdifferentiate into beta cells in
both healthy and diabetic human and mouse pancreatic islets.
While it was previously believed that oesophageal cells were developed
from the transdifferentiation of smooth muscle cells, that has been
shown to be false.
Induced and therapeutic examples
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The first example of functional transdifferentiation has been provided
by Ferber et al. by inducing a shift in the developmental fate of
cells in the liver and converting them into 'pancreatic
beta-cell-like' cells. The cells induced a wide, functional and
long-lasting transdifferentiation process that reduced the effects of
hyperglycemia in diabetic mice. Moreover, the trans-differentiated
beta-like cells were found to be resistant to the autoimmune attack
that characterizes type 1 diabetes.Shternhall-Ron K et al., Ectopic
PDX-1 expression in liver ameliorates type 1 diabetes, Journal of
Autoimmunity (2007),
doi:10.1016/j.jaut.2007.02.010.
http://www.orgenesis.com/uploads/default/files/shternhall-jai-2007.pdf
The second step was to undergo transdifferentiation in human
specimens. By transducing liver cells with a single gene, Sapir et
al. were able to induce human liver cells to transdifferentiate into
human beta cells.
This approach has been demonstrated in mice, rat, xenopus and human
tissues (Al-Hasani et al., 2013).
Schematic model of the hepatocyte-to-beta cell transdifferentiation
process. Hepatocytes are obtained by liver biopsy from diabetic
patient, cultured and expanded ex vivo, transduced with a PDX1 virus,
transdifferentiated into functional insulin-producing beta cells, and
transplanted back into the patient.
Granulosa and theca cells in the ovaries of adult female mice can
transdifferentiate to Sertoli and Leydig cells via induced knockout of
the FOXL2 gene. Similarly, Sertoli cells in the testes of adult male
mice can transdifferentiate to granulosa cells via induced knockout of
the DMRT1 gene.
Lineage-instructive approach
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In this approach, transcription factors from progenitor cells of the
target cell type are transfected into a somatic cell to induce
transdifferentiation. There exists two different means of determining
which transcription factors to use: by starting with a large pool and
narrowing down factors one by one or by starting with one or two and
adding more. One theory to explain the exact specifics is that ectopic
Transcriptional factors direct the cell to an earlier progenitor state
and then redirects it towards a new cell type. Rearrangement of the
chromatin structure via DNA methylation or histone modification may
play a role as well. Here is a list of in vitro examples and in vivo
examples. In vivo methods of transfecting specific mouse cells utilize
the same kinds of vectors as in vitro experiments, except that the
vector is injected into a specific organ. Zhou et al. (2008) injected
Ngn3, Pdx1 and Mafa into the dorsal splenic lobe (pancreas) of mice to
reprogram pancreatic exocrine cells into β-cells in order to
ameliorate hyperglycaemia.
Initial epigenetic activation phase approach
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Somatic cells are first transfected with pluripotent reprogramming
factors temporarily (Oct4, Sox2, Nanog, etc.) before being transfected
with the desired inhibitory or activating factors. Here is a list of
examples in vitro.
Pharmacological agents
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The DNA methylation inhibitor, 5-azacytidine is also known to promote
phenotypic transdifferentiation of cardiac cells to skeletal
myoblasts.
In prostate cancer, treatment with androgen receptor targeted
therapies induces neuroendocrine transdifferentiation in a subset of
patients. No standard of care exists for these patients, and those
diagnosed with treatment induced neruoendocrine carcinoma are
typically treated palliatively.
Mechanism of action
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The transcription factors serve as a short term trigger to an
irreversible process. The transdifferentiation liver cells observed 8
months after one single injection of pdx1.
The ectopic transcription factors turn off the host repertoire of gene
expression in each of the cells. However, the alternate desired
repertoire is being turned on only in a subpopulation of predisposed
cells. Despite the massive dedifferentiation - lineage tracing
approach indeed demonstrates that transdifferentiation originates in
adult cells.
Mogrify algorithm
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Determining the unique set of cellular factors that is needed to be
manipulated for each cell conversion is a long and costly process that
involved much trial and error. As a result, this first step of
identifying the key set of cellular factors for cell conversion is the
major obstacle researchers face in the field of cell reprogramming. An
international team of researchers have developed an algorithm, called
Mogrify(1), that can predict the optimal set of cellular factors
required to convert one human cell type to another.
When tested, Mogrify was able to accurately predict the set of
cellular factors required for previously published cell conversions
correctly. To further validate Mogrify's predictive ability, the team
conducted two novel cell conversions in the laboratory using human
cells, and these were successful in both attempts solely using the
predictions of Mogrify. Mogrify has been made available online for
other researchers and scientists.
Evaluation
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When examining transdifferentiated cells, it is important to look for
markers of the target cell type and the absence of donor cell markers
which can be accomplished using green fluorescent protein or
immunodetection. It is also important to examine the cell function,
epigenome, transcriptome, and proteome profiles. Cells can also be
evaluated based upon their ability to integrate into the corresponding
tissue in vivo and functionally replace its natural counterpart. In
one study, transdifferentiating tail-tip fibroblasts into
hepatocyte-like cells using transcription factors Gata4, Hnf1α and
Foxa3, and inactivation of p19(Arf) restored hepatocyte-like liver
functions in only half of the mice using survival as a means of
evaluation.
Transition from mouse to human cells
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Generally transdifferentiation that occurs in mouse cells does not
translate in effectiveness or speediness in human cells. Pang et al.
found that while transcription factors Ascl1, Brn2 and Myt1l turned
mouse cells into mature neurons, the same set of factors only turned
human cells into immature neurons. However, the addition of NeuroD1
was able to increase efficiency and help cells reach maturity.
Order of transcription factor expression
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The order of expression of transcription factors can direct the fate
of the cell. Iwasaki et al. (2006) showed that in hematopoietic
lineages, the expression timing of Gata-2 and (C/EBPalpha) can change
whether or not a lymphoid-committed progenitors can differentiate into
granulocyte/monocyte progenitor, eosinophil, basophil or bipotent
basophil/mast cell progenitor lineages.
Immunogenicity
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It has been found for induced pluripotent stem cells that when
injected into mice, the immune system of the synergeic mouse rejected
the teratomas forming. Part of this may be because the immune system
recognized epigenetic markers of specific sequences of the injected
cells. However, when embryonic stem cells were injected, the immune
response was much lower. Whether or not this will occur within
transdifferentiated cells remains to be researched.
Method of transfection
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In order to accomplish transfection, one may use integrating viral
vectors such as lentiviruses or retroviruses, non-integrating vectors
such as Sendai viruses or adenoviruses, microRNAs and a variety of
other methods including using proteins and plasmids; one example is
the non-viral delivery of transcription factor-encoding plasmids with
a polymeric carrier to elicit neuronal transdifferentiation of
fibroblasts. When foreign molecules enter cells, one must take into
account the possible drawbacks and potential to cause tumorous growth.
Integrating viral vectors have the chance to cause mutations when
inserted into the genome. One method of going around this is to excise
the viral vector once reprogramming has occurred, an example being
Cre-Lox recombination Non-integrating vectors have other issues
concerning efficiency of reprogramming and also the removal of the
vector. Other methods are relatively new fields and much remains to be
discovered.
Pluripotent reprogramming
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*Almost all factors that reprogram cells into pluripotency have been
discovered and can turn a wide variety of cells back into induced
pluripotent stem cells (iPSCs). However, many of the reprogramming
factors that can change a cell's lineage have not been discovered and
these factors apply only for that specific lineage.
*The final products of transdifferentiated cells are capable of being
used for clinical studies, but iPSCs must be differentiated.
*It may become possible in the future to use transdifferentiation in
vivo, whereas pluripotent reprogramming may cause teratomas in vivo.
*Transdifferentiated cells will require less epigenetic marks to be
reset, whereas pluripotent reprogramming requires nearly all to be
removed, which may become an issue during redifferentiation.
*Transdifferentiation is geared towards moving between similar
lineages, whereas pluripotent reprogramming has unlimited potential.
*Pluripotent cells are capable of self-renewal and often go through
many cell passages, which increases the chance of accumulating
mutations. Cell culture may also favor cells that are adapted for
surviving under those conditions, as opposed to inside an organism.
Transdifferentiation requires fewer cell passages and would reduce the
chance of mutations.
*Transdifferentiation can also be much more efficient than
pluripotency reprogramming due to the extra step involved in the
latter process.
*Both pluripotent and transdifferentiated cells use adult cells, thus
starting cells are very accessible, whereas human embryonic stem cells
require that one navigate legal loopholes and delve into the morality
of stem cell research debate.
See also
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* Epigenetics
* Induced pluripotent stem cell
* Induced stem cells
* Reprogramming
License
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License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Transdifferentiation
