Cerebral cortex development is a complicated process which is not completely
understood. There are a lot of signaling interactions during morphogenesis of the cortex.
One of the signaling pathways is Wnt. Adenomatous polyposis coli (Apc) is a key
regulator of the canonical Wnt pathway. Also Ape is important to transducer signals
from the planar cell polarity (PCP) Wnt pathway. This thesis is devoted to the role of
Ape protein in the development of the mouse dorsal telencephalon. Apc⁻/⁻ embryos die at
gastrulation, therefore Emxlᶜʳᵉ conditional knock-out approach was used to overcome
this problem. Cre expression driven by Emxl promoter allows knock-out Apc in the
cortex. Current work presents description and possible explanations of Apc functions in
the developing cerebral cortex.Conditional knock-out of Apc in the cortex using Emxlᶜʳᵉ leads to severe
developmental defects in this region. This shows that Apc is required for normal
development of the cerebral cortex. The earliest found defect is nuclear translocation of
beta-catenin which is demonstrating that Apc is important to regulate translocation of
beta-catenin to the nucleus. This finding supports results of other researchers. This
abnormality was found at embryonic day 10.5 (El0.5), which shows that Apc has a
controlling function from the beginning of cortical development. Later in development
nuclear beta-catenin accumulation becomes more pronounced. Experiments with BatGal
reporter mice show that stabilized beta-catenin is able to stimulate the canonical Wnt
pathway. Wnt target genes (C-myc, Cyclin Dl) are activated also. However Axin 2
expression is highly up-regulated which reflects negative feedback to the canonical Wnt
signaling activation. Polarity of cells is lost from El2.5, which suggests that the
cytoskeletal functions of Ape are affected by Ape deletion. Adhesion defect and defect
in neuronal processes elongation provide additional evidence of the cytoskeleton
disregulation. Therefore, the deletion of Apc leads to over-activation of the canonical
Wnt pathway and disregulation of cytoskeleton functioning.Identity of the dorsal telencephalon is unclear in conditional Ape mutant
embryos. Expression of Foxgl shows that the mutant dorsal telencephalon loses
telencephalic identity from El2.5. There are signs that the mutant dorsal telencephalon
expresses markers (Pax3, Wntl) of more caudal regions of the brain. However markers
normally expressed in the cerebral cortex (Pax6, Tbr1) are still present. Pax6 is downregulated in the mutant but there are no signs that the pallial subpal 1 ial boundary is
compromised. Apical progenitor population is decreased. Decreased Tbr2 expression
shows that intermediate progenitor pool is reduced also. Medial regions of the mutant
dorsal telencephalon are more affected than lateral possibly due to a gradient in Emxl
expression in the cortex. My data show that Ape is important for proper patterning of the
cortex probably mostly by antagonizing the canonical Wnt pathway. However a precise
mechanism is yet to be elucidated.Cell-cycle investigation revealed that in the absence of Apc, S-phase and cell
cycle length remains more or less similar to the control. However the proliferative pool
is decreased and most cells of the mutant are blocked in G1 phase. This defect
progresses with the age. Aneuploidy was not detected in the mutant cells. The G1
blockade is related to p21 up-regulation. Also apoptosis is increased in the mutant but
level of p53 is not changed. My data suggests that apoptosis and p21 expression is
stimulated by Wtl expression, which reflects tumour suppression. Thus, Ape deletion
leads to defects in maintenance of the size of progenitor pool and regulation of
apoptosis
