Biomedical Engineering Reference
In-Depth Information
1.2 Renewable Cell Sources
Pluripotent stem cells have unlimited self-renewal capacity and can generate all
somatic cell types of the human body. In contrast to pluripotent cells, multipotent
neural stem cells (NSCs) grown as neurospheres are characterized by more limited
proliferative and developmental potentials [
10
,
11
]. Research involving human ES
cells over the last decade has helped to establish and define culture conditions that
maintain human pluripotency ex vivo for extended periods of time under defined
cell culture conditions [
7
,
12
-
14
]. Significant progress has been made in charac-
terizing the molecular circuitry of transcription factors, epigenetic regulators, and
signal transduction pathways that maintain the pluripotent state in human ES cells
[
15
-
17
]. For instance, the transcription factors OCT4, SOX2, and NANOG form
an interconnected auto-regulatory circuitry controlling chromatin structure and
gene expression signatures of pluripotency. In parallel, the field of nuclear
reprogramming made continued progress by demonstrating that pluripotency can
be induced in various somatic cells by cell fusion or improved methods of somatic
cell nuclear transfer [
18
-
21
]. These approaches clearly established that factors
present in the cytoplasm of pluripotent cells or mammalian oocytes can reprogram
the nucleus of fully differentiated cell into an embryonic-like state. Yamanaka and
colleagues were the first to demonstrate in seminal experiments that pluripotency
can be induced in skin fibroblast by transient expression of four transcription
factors: OCT4, SOX2, KLF4, and C-MYC [
8
,
22
]. It was then reported that forced
expression of a different set of transcription factors (OCT4, SOX2, NANOG,
LIN28) can also gives rise to iPS cells [
23
]. Reprogramming by defined tran-
scription factors and production of iPS cells is a robust and straightforward method
and has been reproduced by a number of different laboratories exploiting viruses
and virus-free gene delivery techniques. Understanding the iterative molecular
processes governing successful nuclear reprogramming is currently a major effort
in stem cell biology. From a developmental biology perspective it still remains
extraordinary that the combined action of only four transcription factors can result
in such a dramatic change of cellular fate and identity.
Detailed characterization of iPS cells has firmly established their pluripotent
nature in various available assays for mouse and human cells. Tetraploid com-
plementation is the most stringent assay for pluripotency. While this experiment is
obsolete for human cells due to ethical reasons, mouse iPS cells passed this
pluripotency test [
24
]. Ongoing work is currently revealing similarities and dif-
ferences between ES and iPS cells and it is likely that these findings will reflect the
biological variability among pluripotent cell lines [
25
]. Nevertheless, it is critical
to establish the safety and genomic stability of iPS cells generated from a number
of different cell types and different biological ages. The integration-free delivery of
transcription factors during the reprogramming progress is critical in order to avoid
insertional mutagenesis and oncogene reactivation. Rapid technical progress made
over the last years indicates that virus-free reprogramming approaches will be
widely used in the near future. Similarly, humanized cell culture conditions are
