Download Independent Essay * Stem Cell Niches

Running head: β–CATENIN IN THE NEURAL STEM CELL NICHE
Independent essay - Stem Cell Niches:
β–catenin in the Neural Stem Cell Niche
Hina Abdulla
McMaster University
β–CATENIN IN THE NEURAL STEM CELL NICHE
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Abstract
Stem cell niches in collaboration with their specified proteins enable one to understand
the function and importance of stem cells, as undertaken through various roles and mechanisms.
One such protein, β–catenin, is imperative in maintaining stem cell identity within the neural
stem cell niche and allowing stem cell proliferation to occur. β–catenin expressed in embryonic
mice facilitates the growth of neuronal progenitor cells, and acts in stem cell fate decisions
through regulatory processes. Transgenic mice with stabilized levels of β –catenin in neural stem
cells demonstrate an increase in the surface area of the cerebral cortex, rendering an enlarged
brain. In addition, lateral ventricles are expanded, inferring a surge in the population of neuronal
progenitor cells. The ablation of β–catenin results in an apparent reduction in the tissue mass of
the spinal cord, midbrain, and the loss of a well-defined ventricular zone. In contrast, subsequent
activation of β–catenin results in an enlarged spinal cord, ventricular zone, and tissue mass of the
midbrain.
The Neural Stem Cell Niche
A stem cell niche can be defined as the microenvironment in which a stem cell resides,
accordingly controlling stem cell fate. Cell signalling interactions in stem cell niches allow stem
cells to remain in their self-renewing and undifferentiated state (Scadden, 2006).
The neural stem cell niche is a specified location in which stem cells are reserved for
neurogenesis in the adult brain, following embryonic development. There are two primary neural
stem cell niches; the subventricular zone (SVZ), also known as the subependymal zone (SEZ),
and the subgranular zone (SGZ) (Conover, 2008). The SVZ niche is a thin layer of dividing cells
which line up the lateral wall of the lateral ventricle. New neurons are consistently generated in
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the SVZ niche, which migrate to the olfactory bulb, where they become differentiated into
specialized granule and periglomerular neurons. In addition to neurogenesis, the SVZ niche is
also an active site of oligodendrocyte formation, subsequently supporting directional migration.
At the ultrastructural level, the SVZ niche has been identified to consist of four main cell types:
neuroblasts (Type A cells), SVZ astrocytes (Type B cells), transitory amplifying progenitor cells
(Type C cells), and ependymal cells. The smaller counterpart of the SVZ niche is the SGZ niche,
which is located in the dentate gyrus of the hippocampus. Unlike the SVZ niche which gives rise
to neurons that migrate to the olfactory bulb, the neurons in the SGZ niche are generated locally.
This occurs in close association with blood vessels, through the SGZ astrocytes which divide to
form granule neurons, in a similar fashion to the SVZ astrocytes in the SVZ niche. The
morphology and organization of the neural stem cell niche, coupled with cell signalling
interactions effectively support neurogenesis in the mammalian brain (Doetsch, 2003).
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Figure 1: Structures of the adult stem cell niche in the subependymal zone (SEZ) and in the subgranular
zone (SGZ). Referring to Figure 1 from Conover, it can be affirmed that the SEZ niche is a thin layer of
cells consisting of neuroblasts, astrocytes, neural stem cells, and transit amplifying precursors in
comparison to the SGZ niche which is characterized by the abundance of granule neurons, generated
locally and in close proximity to the blood vessels. Adapted from Conover, J. C., & Notti, R. Q. (2008).
The neural stem cell niche. Cell and tissue research, 331(1), 211-224.
Role of β-catenin
The specific protein described for the neural stem cell niche is β -catenin. It is located in
the cytoplasmic region of E-cadherin, an epithelial trans-membrane protein, and is therefore
considered to be an intrinsic factor. β –catenin serves two primary unrelated roles; contributing
to cell-cell adhesion, and signaling as a constituent of the Wnt pathway, while acting
downstream of the Frz and LRP receptors (Zechner et al., 2003). The Wnt signaling pathway
induces the expression of β -catenin which results in transcriptional activation of specific target
genes (Morin, 1999). In the absence of the Wnt-signal, known as the off-state, along with the
phosphorylation of β-catenin, β-catenin undergoes degradation. In contrast, in the presence of the
Wnt-signal, known as the on state, along with the inhibition of the phosphorylation of β-catenin,
β-catenin levels are maintained. Thus, the Wnt signalling pathway is effective in inhibiting the
degradation of β-catenin (Kazanis et al., 2008).
Experiments Conducted
In a study conducted by Chenn and Walsh, embryonic mice were induced to express βcatenin to observe the underlying effects on neural precursors. Chenn and Walsh had predicted
that β -catenin plays a significant role in controlling cell fate decisions during the neural
development of a mammal. To test this prediction, in situ hybridization of β-catenin was
executed on embryonic mouse brain sections at embryonic days 12.5, 14.5, and 16.5. Results
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showed that β-catenin was strongly expressed at high levels in the ventricular zone at all ages
during cortical neurogenesis. The lateral ventricles appeared to have grown in size and lined up
with neural progenitor cells, indicating a substantial increase in the precursor population. In
order to further determine if the activation of β-catenin had the capacity to control neural
mammalian development, transgenic mice were created to overexpress a varied form of βcatenin; NH2- terminally deleted β-catenin, and labelled with green fluorescent protein (GFP)
(ΔN90β-catenin-GFP) in neural progenitor cells. In this experiment, Wnt signaling was not
required as the varied form of β-catenin did not have the required phosphorylation sites that
would typically result in its degradation. Transgenic mice on embryonic day 15.5 were noted to
have expanded brains, with a significant increase of area in the cerebral cortex. As a means of
distinguishing actively dividing cells from terminally differentiated cells, BrdU, a thymidine
analog, was used to label neural precursors. The labelled cells were exposed to BrdU for 30
minutes before their elimination. Comparisons between wild-type (normal) and transgenic
sections revealed that the cells which were lining up the ventricle were the same cells labelled by
BrdU. The cells lining up the ventricle were found to have progenitor markers Hes5 and Hes1.
These findings validate that the neural precursors labelled with BrdU and the progenitor cells
identified with the markers Hes5 and Hes1, are in fact identical. The aforementioned
experiments provide evidence that β-catenin plays an important role in the regulation of
proliferation and cell growth, by controlling the neural precursor population. Therefore, β catenin signaling can effectively give rise to an expanded progenitor population which induces
an increase in cerebral cortical size (Chenn and Walsh, 2002).
In a separate study, Zechner et al administered two different mutant β-catenin alleles to
mice as a means of modifying the β-catenin signaling pathway in the central nervous system to
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test the potential of β-catenin in cellular growth. One allele was used to ablate β-catenin and the
other was used to induce expression of an active β-catenin. Due to the fact that β-catenin is an
integral component of the Wnt signaling pathway, Zechner et al hypothesized that β-catenin
signals would have the ability to regulate the proliferation of neural precursors. Through the
activation or ablation of β-catenin, Wnt signaling could be effectively analyzed and its impacts
evaluated. The mutations were introduced into the central nervous system with the aid of a
transgenic mouse strain expressing cre-recombinase, under the regulation of the Brn4 promoter.
For the purpose of identifying β-catenin in the tissue of the experimented mice, embryos were
dissected and placed in 4% paraformaldehyde in phosphate buffered saline (PBS). Neural tissue
sections were stained with toluidine blue for enhanced viewing. Immunohistology and Western
blot analysis assessments revealed a reduced spinal cord, absence of a ventricular zone, and
decreased tissue mass of the midbrain for mice that had a cre-induced loss-of-function mutation.
In contrast, mice with a cre-induced gain-of-function mutation showed an enlarged spinal cord,
ventricular zone, and tissue mass of the midbrain. These important findings suggest that a lack of
the β-catenin protein, as observed through mice with ablated β-catenin, leads to a subsequent loss
of progenitors in the developing neural system. Conversely, the activation of β-catenin, as
observed through mice with active β-catenin, leads to an eventual increase of progenitors in the
developing neural system (Zechner et al., 2003).
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Figure 2: Sections of neural tissue expressing either a loss-of-function or gain-of-function mutation of
β-catenin. (a-c) sections of the spinal cord (a) control, (b) loss-of-function, and (c) gain-of-function at
embryonic day 12. (d-f) sections of the midbrain of (d) control, (e) loss-of-function, and (f) gain-offunction at embryonic day 11.5. Referring to Figure 2 from Zechner et al., it can be noted that the spinal
cord with ablated β-catenin shows a reduced ventricular zone in (b) in comparison to (a), the control.
Similarly, there is an apparent reduction in tissue mass of the midbrain when comparing (d) the control
to (e) loss-of-function and an increase in tissue mass when comparing (e) to (f) gain-of-function. As
modified from Zechner, D., Fujita, Y., Hülsken, J., Müller, T., Walther, I., Taketo, M. M., ... &
Birchmeier, C. (2003). β-Catenin signals regulate cell growth and the balance between progenitor cell
expansion and differentiation in the nervous system. Developmental biology, 258(2), 406-418.
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Conclusion
In conclusion, β-catenin serves as an essential protein during mammalian neuronal
development. As identified through the conducted experiments and supporting evidence, the
activation of β-catenin is vital in regulating neuronal progenitor populations and cerebral cortical
size. β-catenin signals are required for the maintenance of neuronal stem cell identity and
inducing stem cell proliferation. Thus, β-catenin, being a critical component of the Wnt signaling
pathway, has the ability to influence and impact cell fate decisions in the mammalian developing
nervous system.
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References
Chenn, A., & Walsh, C. A. (2002). Regulation of cerebral cortical size by control of cell cycle
exit in neural precursors. Science Signalling, 297(5580), 365.
Conover, J. C., & Notti, R. Q. (2008). The neural stem cell niche. Cell and tissue
research, 331(1), 211-224.
Doetsch, F. (2003). A niche for adult neural stem cells. Current opinion in genetics &
development, 13(5), 543-550.
Kazanis, I., Lathia, J., & Moss, L. (2008). The neural stem cell microenvironment.
Morin, P. J. (1999), β-catenin signaling and cancer. Bioessays, 21: 1021–1030.
doi: 10.1002/(SICI)1521-1878(199912)22:1<1021::AID-BIES6>3.0.CO;2-P
Scadden, D. T. (2006). The stem-cell niche as an entity of action. Nature,441(7097), 1075-1079.
Zechner, D., Fujita, Y., Hülsken, J., Müller, T., Walther, I., Taketo, M. M., ... & Birchmeier, C.
(2003). β-Catenin signals regulate cell growth and the balance between progenitor cell expansion
and differentiation in the nervous system. Developmental biology, 258(2), 406-418.