Humphrey Hung-Chang Yao
Assistant Professor,
Veterinary Biosciences
Assistant Professor, Post-Genomics Institute
Professional Interests: How sex
is determined has fascinated scientists and philosophers alike for thousands
of years. The great philosopher, Aristotle, claimed that sex of the human
embryo was determined by the heat of the male partner during intercourse.
It was the constant battle between the heat of man's semen and the coldness
of the woman's womb that decided the sex of the offspring. This "environment"-dictated
scheme of sex determination has been proven to be nothing more than a
romantic twist of "when-boy-meets-girl". With the advances
in genetic engineering and developmental biology, we can declare that
it is the genetic makeup of the embryo and not the environment or the
moment of the passion that determines the sex of the offspring in mammals.
The presence or absence of the Y chromosome, instead of the dosage of
X chromosomes, determines the sex in most mammals. Before the onset of
sex determination, XX and XY embryos develop a "unisex" gonad,
which is functionally and morphologically indistinguishable in either
sex. The "unisex" gonad undergoes dramatic changes at the onset
of sex determination (11.5 days and 6 weeks of pregnancy in mouse and
human, respectively). In the XY embryo, a single gene on the Y chromosome,
the Sex-determining Region of the Y chromosome or Sry, is expressed in
the unisex gonad. Sry triggers a cascade of molecular and cellular events
that direct the unisex gonad to differentiate into a testis. However,
in the XX embryo where the Sry gene is absent, the unisex gonad follows
the "default" pathway to become an ovary. The focus of my research
is to understand the molecular and cellular mechanisms underlying the
sex determination process. I am particularly interested in answering
the following specific questions:
1. What Determines the Maleness?
Maleness, or external male characteristics such as the external sex organ, muscular
tone, hair pattern, and male behavior, are the product of androgen action. Androgens
are synthesized mainly by a male-specific cell type in the testis, called Leydig
cells. In the XY embryo, androgens from Leydig cells masculinize the reproductive
tract resulting in formation of the vas deferens, male accessory glands, penis,
and scrotum. Proper development of Leydig cells and the subsequent production
of androgens are essential for the establishment of maleness. If androgen production
is absent or insufficient, the reproductive tract will develop into a female
system instead of a male system. This lack of androgen production leads to the
incidence of male pseudohermaphroditism, an individual with an XY genetic makeup
and normal testis but who has the external genitalia of a female! We have started
to understand how the XY-specific differentiation of Leydig cells is regulated.
Apparently, Leydig cell differentiation is downstream of Sry action because this
cell type is not found in the XX individual. We found that Sry induces the expression
of desert hedgehog (Dhh), a secreted signaling molecule. Dhh acts on the interstitial
cells in the embryonic testis and induces the expression of specific cell lineage
markers for Leydig cells. In the absence of Dhh signaling, such as in Dhh knockout
mice, fetal Leydig cell differentiation is compromised and the majority of Dhh
knockout XY mice become male pseudohermaphrodites. A human case with Dhh mutation
was also reported. Interestingly, this human patient developed the same pseudohermaphrodite
phenotype as the Dhh knockout mice indicating a conserved role of Dhh in establishing
maleness in mammals. A current project in my lab is to identify the up-stream
regulator of the expression the Dhh and downstream targets of Dhh signaling in
fetal Leydig cells.
2. What Controls the Development of Femaleness?
We are also interested in understanding how femaleness is established. At present,
the female counterpart of the Sry gene remains elusive. There is no genetic evidence
to demonstrate that a loss or inactivation of a specific gene transforms an ovary
to a testis in the XX individual. This lack of candidates for the ovary-determining
gene promotes the idea that the development of the ovary follows a default pathway,
which arises in the absence of Sry. However, emerging evidence from my lab and
others points out that the development of the ovary is a dynamic process involving
complex cell-cell interactions and cell fate decisions. We have identified a
novel signaling pathway, the Wnt4 and follistatin-signaling cascade, which is
essential for early ovary development. Wnt4 and follistatin are both secreted
molecules expressed exclusively in the XX gonad at the time of sex determination.
When either Wnt4 or follistatin is inactivated, a striking phenotype arises with
formation of a testis-specific blood vessel on the surface of the embryonic ovary.
Most interestingly, none of the key regulators downstream of Sry are expressed,
suggesting that the formation of this testis-specific vessel does not require
the activation of the Sry pathway. This observation creates a paradigm shift
in the field, which is that at least the development of testis-specific vasculature
is "default". The mechanism for the development of this vasculature
is present in both sexes. The Sry gene in the XY embryo probably functions to
maintain this vasculature structure. On the other hand, in the XX gonad, this
testis-specific vasculature is inhibited by the Wnt4/follistatin pathway in order
to establish an environment for normal ovary development. We are trying to identify
the connection between Wnt4 and follistatin and understand how they work together
to inhibit vasculogenesis.
3. Specification of Primordial Germ Cell Fate
The developmental fate of primordial germ cells in the mammalian gonad depends
on their environment. In the XY gonad, Sry induces a cascade of molecular and
cellular events leading to the organization of testis cords. Germ cells are sequestered
inside testis cords by 12.5 dpc where they arrest in mitosis. If the testis pathway
is not initiated, germ cells spontaneously enter meiosis by 13.5 dpc, and the
gonad follows the ovarian fate. How the somatic environment and germ cell components
determine the germ cell fate is an unsolved mystery. We plan to combine micro
array technology and organ culture to identify novel genes in regulating germ
cell meiosis.
4. Is There a Conserved Mechanism in Vertebrate Sex Determination?
We are comparing the molecular and cellular mechanisms among different vertebrate
species such as mouse, chicken, turtle, and alligator. We are also interested
in finding whether the key elements of the sex determination cascade are conserved
in vertebrates using different strategies to determine their sexes (for example,
genetic regulation in mammals versus environmental regulation in reptiles). The
application of this research is to examine the effects of environmental toxicants
on sex determination.
Selected Publications:
Yao HHC, Matzuk MM, Jorges JC, Menke DB, Page DC, Swain A, Capel B. Follistatin Operates Downstream of WNT4 in Mammalian Ovary Organogenesis. Developmental Dynamics. 230:210-215, 2004.
Yao HHC, DiNapli L, Capel B. Gonadogenesis in Red-Eared Slider Turtle (Trachemys scripta): A Search for Conserved Cellular Mechanisms in Vertebrate Species. Mechanism of Development 121: 1393-1201, 2004.
Yao HHC, Capel B. Disruption of Testis Cords by Cyclopamine or Forskolin Reveals Independent Cellular Pathways in Testis Organogenesis. Developmental Biology 246: 356-365, 2002.
Yao HHC, Whoriskey W, Capel B. Desert
Hedgehog/Patched-1 Signaling Specifies Early Leydig Cell Fate in Testis
Organogenesis. Genes & Development 16: 1433-1440, 2002.
Schmahl J, Yao HHC, Pierucci-Alves F, Capel B. Colocalization of WT1 and Cell Proliferation Reveals Conserved Mechanisms in Temperature-Dependent Sex Determination. Genesis 35:193-201, 2003.
Ross AJ, Tilman C, Yao HHC, MacLaughlin D and Capel B. AMH induces mesonephric cell migration in XX gonads, Molecular and Cellular Endocrinology 211:1-7, 2003.
Yao HHC, DeNapoli L, Capel B. Meiotic Germ Cells Antagonize Mesonephric Cell Migration and Testis Cord Formation in Mouse Gonads. Development, published online ahead of print, Oct 15, 2003.
Yao HHC, Matzuk MM, Jorges JC, Menke DB, Page DC, Swain A, Capel B. Follistatin Operates Downstream of WNT4 in Mammalian Ovary Organogenesis. Developmental Dynamics 230:210-215, 2004.
Yao HHC, DiNapli L, Capel B. Gonadogenesis in Red-Eared Slider Turtle (Trachemys scripta): A Search for Conserved Cellular Mechanisms in Vertebrate Species. Mechanism of Development 121: 1393-1201, 2004.
