@misc{oai:ir.soken.ac.jp:00001066,
author = {佐藤, 優子 and サトウ, ユウコ and SATO, Yuko},
month = {2016-02-17},
note = {Mammalian sex is determined by differentiation of the gonad (testis or
ovary), which governs phenotypic sex through the production of hormones. Initially,
embryonic gonads develop as bipotential gonad, and its sexual differentiation is
genetically controlled: depending on the presence or absence of the Y chromosome.
In the developing testis, male-specific patterning of the vasculature is induced by
Sry. Following the expression of Sry, endothelial cells are recruited vigorously to
the testis from mesonephros and a large artery is formed at the coelomic surface at
around embryonic day (E) 12.5 (coelomic vessel). Thereafter, the vessel branches
from the coelomic vessel and extends progressively between testicular cords. In
contrast, no such active cell migration is observed in the developing fetal ovary.
This male-specific vascular system that develops during fetal life is required for
efficient export of testosterone from the testis to masculinize the embryo.
Forkhead (Fox) transcription factors carry a winged helix DNA-binding
domain that share homology with their founding member forkhead protein in
Drosophila. Phylogenic analysis of the forkhead domain consisting of
highly-conserved 100 amino acids led to placement of the family members into 20
subclasses, FoxA to FoxS. Fox proteins bind to consensus sequences, RYMAAYA
(R=A or G; Y=C or T; M=A or C), as a monomer. Regions other than the conserved
domain vary in terms of sequence and function. Some members act as
transcriptional activators while others as repressors. Probably as transcriptional
regulators, Fox genes are thought to play a variety of roles in fetal and adult tissues
and mutations in FOX genes have been linked to human diseases. Fkhl18, a
member of the Fox family, was originally identified by low-stringency screening of
mouse and human genomic libraries. Fkhl18 has low homology to other members of
the Fox family, and is categorized under the FoxS subclass. However, its expression
and function remain to be examined. In the present study, she demonstrated that
Fkhl18 was expressed in periendothelial cells and Sertoli cells of the developing
fetal testis. She then generated the Fkhl18 knockout (KO) mouse to examine the
physiological function of the gene product. Interestingly, the KO fetuses displayed
affected testicular vasculature, suggesting that Fkhl18 was involved in
development of the fetal testis vasculature system.
Fkhl18 KO mice displayed the following testicular abnormalities during
fetal life; 1) accumulation of blood cells in the central part of the fetal testis, 2)
presence of gaps, measuring 100-400 nm in diameter, between endothelial cells,
allowing leakage of injected carbon ink from the testicular vessels, and 3) aberrant
apoptosis of periendothelial cells. These features strongly suggest the importance of
Fkhl18 expression in the periendothelial cells for development of the testicular
vascular system through direct and indirect regulation of the functions of
periendothelial and endothelial cells, respectively. The indirect function of Fkhl18
indicates a functional interaction between endothelial and periendothelial cells.
The importance of interactions between the two cell types for vascular maturation
has been examined by gene knockout studies of signal transductions:
angiopoietin-1/TIE-2 receptor and platelet-derived growth factor (PDGF)-BB/
PDGF receptor β (PDGFRβ). Unlike these KO mice, recruitment of eriendothelial
cells did not seem to be affected in the fetal testes of Fkhl18 KO mice. Interestingly,
however, marked apoptosis of periendothelial cells was observed; with resultant
focal and transient loss of periendothelial cells. Since the expression of Fkhl18 was
not detected in endothelial cells, the structural defect induced in endothelial cells
possibly resulted from weakened interaction with the affected or decreased
periendothelial cells.
To further investigate the molecular mechanisms underlying proapoptotic
effect of Fkhl18, she demonstrated electrophoretic mobility shift assay (EMSA) and
reporter gene assay. Reporter gene assays revealed that Fkhl18 suppressed
transcription mediated by FoxO3a and FoxO4. Since EMSA showed that Fkhl18 had
ability to bind to consensus DNA binding sequence for Fox, it potentially repressed
transcription by competing for binding sites with other Fox proteins. Considering
the suppressive function of Fkhl18, it is interesting to note that FoxOs mediate
proapoptotic gene expression. For example, overexpression of FoxOs resulted in
apoptosis through direct induction of proapoptotic genes: tumor necrosis
factor-related apoptosis-inducing ligand (TRAIL) in prostate cancer, bim (Bcl-2
interacting mediator of cell death), and FasL. Based on the results published so far,
she hypothesized that the marked apoptosis of periendothelial cells in Fkhl18 KO
testes was caused by defective proapoptotic gene transcription, which was normally
attenuated by Fkhl18. As expected, Fkhl18 suppressed transcription from FasL gene
promoter in cultured smooth muscle cells prepared form bovine blood vessels.
In the present study, she focused on the function of Fkhl18 during blood
vessel formation of the fetal testis; blood vessel development in the ovary remains to
be investigated. Likewise, she has not examined whether the blood vessels in tissues
other than the gonads are affected by Fkhl18. Considering that Fkhl18 is expressed
in periendothelial cells of other tissues, the defects seen in the fetal testis could be
also seen in other tissues. However, obvious accumulation of blood cells was not
observed in any tissues other than the testis, strongly arguing against a major defect
of blood vessel development in these tissues. Together with the highest expression of
Fkhl18 in the developing testis, it is conceivable that Fkhl18 plays a unique role in
the development of the testicular vasculature system., 総研大甲第1170号},
title = {Importance of forkhead transcription factor Fkh118 for development of testicular vasculature},
year = {}
}