Biomedical Engineering Reference
In-Depth Information
Classical or canonical Wnt signaling is regulated through control of the cellular
localization of b-catenin. In the absence of Wnt ligand, b-catenin is continu-
ously degraded in the cytoplasm by a complex of proteins consisting of Axin,
GSK3b, and APC. This complex phosphorylates b-catenin, marking it for
proteasomal degradation. Upon binding of Wnt ligands to their receptors,
Axin is recruited to the membrane, resulting in inactivation of the b-catenin
degradation complex, accumulation of b-catenin in the cytoplasm, and even-
tually translocation to the nucleus (Clevers, 2006). There b-catenin associates
with members of the TCF/LEF family of transcription factors and induces
transcription of a variety of target genes. Wnts and their antagonists are
expressed in a complex pattern by both epithelial and stromal cells along the
entire length of the crypt (Gregorieff et al., 2005). Wnt signaling through b-
catenin normally occurs only in the cells at the base of the crypt as these cells
show both nuclear localization of b-catenin and expression of b-catenin-TCF
target genes. In the small intestine these cells include mature Paneth cells. Most
Wnt target genes (approximately 80%) are expressed in the transit-amplifying
precursor cells (Van der Flier et al., 2007) and it has been shown that this b-
catenin-TCF induced transcriptional program is required for the specification
of most aspects of the precursor cell phenotype (Van deWetering et al., 2002). It
is believed that upon migration of the cells toward the top of the crypt the level
of canonical Wnt signaling decreases resulting in a loss of the precursor cell
phenotype and induction of cellular differentiation. It has been shown that Tcf4
is required for stem cell maintenance in the developing small intestine (Korinek
et al., 1998), and mice that express the Wnt antagonist Dkk1 behind an intest-
inal epithelial-specific promoter fail to maintain their precursor cells during
development of the intestine. It has been established that systemic expression of
Dkk1 using a viral delivery system (Kuhnert et al., 2004) or conditional loss of
b-catenin in the small intestinal epithelium of adult mice (Ireland et al., 2004)
resulted in crypt loss. In conclusion, Wnt signaling seems to be the driving force
of stem and precursor cell fate in the intestine although most of the experiments
that have demonstrated this have been performed in the small intestine, an
organ in which carcinogenesis is extremely rare in humans.
2.2.2 Notch Signaling
The Notch signaling pathway is a critical regulator of stem cell/precursor cell
fate regulation and cell lineage specification. Notch interacts with its ligands of
the Delta-like/Jagged family. As these are transmembrane proteins, Notch
signaling is active in direct cell-to-cell contact. The classical example from
Drosophila is the role of Notch signaling in lateral inhibition, allowing asym-
metrical development of similar cells (such as a stem cell and its daughter cell).
In the fly, Notch signaling suppresses Delta expression. Cell fate is therefore
determined through feedback amplification of relatively small differences in
Notch expression between both cells and one cell will end up as 'Notch high'
whereas the other will be 'Delta high.' Activation of Notch receptors by
