Biomedical Engineering Reference
In-Depth Information
Table 1 Putative markers of normal colon stem cells
Markers
Function
Publication
Lgr5 (GPR49)
Unknown
Barker et al. (2007)
Musashi-1 (Msi1)
RNA binding protein
possibly involved in
asymmetrical cell division
Potten et al. (2003)
Label retention (BrdU)
-
Potten et al. (1997)
impossible until recently. Although Wnt signaling probably plays an important
role in stem cell homeostasis in the small intestine and colon, stem cells cannot be
marked using nuclear b-catenin staining (a read-out for active Wnt signaling, see
below). Nuclear b-catenin also marks the transit-amplifying cells and in the small
intestine it wouldmark the differentiated Paneth cells that are located at the crypt
base (van Es et al., 2005a). Recently, however, Van der Flier et al. (2007)
examined the expression of a large number of Wnt target genes in the small
intestine by in situ hybridization and noted that Wnt targets could be categorized
in three different expression patterns. The first pattern comprised most Wnt
target genes (
80%) and showed expression in the transit-amplifying cells. The
second pattern involved genes expressed in Paneth cells at the base of the small
intestinal crypts. The third pattern was more unexpected and was characterized
by expression by a few cells at the crypt base that were not Paneth cells. The same
group has subsequently shown that Lgr5 (Gpr49), one of the genes expressed in
this pattern, marks the so-called crypt base columnar cells and may indeed be an
intestinal stem cell marker, as lineage tracing experiments have demonstrated
that Lgr5-expressing cells could generate all cell lineages (Barker et al., 2007).
Interestingly, and in accordance with the life cycle of hematopoietic stem cells, it
was shown that crypt base columnar cells are not slow-cycling cells but in contrast
cycle approximately once per day. However, an important question still remains
to be answered as the lineage tracing experiments showed that marked cells
populated only part of the crypt. The marked Lgr5
+
cell did not therefore
populate the whole crypt in a clonal manner as stem cells are believed to do.
Does it simply take more time for the progeny of the traced Lgr5
+
cell to take up
all the cell positions in a crypt or is the Lgr5
+
cell a cell that is downstream of a
single dominant stem cell that generates a few different Lgr5
+
cells in a clonal
manner? In conclusion, crypt base columnar cells are the most likely stem cells of
the small intestine, and Lgr5 seems to be an intestinal stem cell marker but much
experimental work remains to further characterize these cells.
2.2 Morphogenetic Pathways Regulate Colonic Epithelial Cell Fate
2.2.1 Wnt Signaling
Wnt are small secreted glycoproteins that signal through a complex of a
Frizzled receptor and a lipoprotein receptor-related protein (Clevers, 2006).
