Fertility, Kinases, and Cancer
By Jason Socrates Bardi
"The
one without born of the one within."
From
Paradiso: Canto XII by Dante Alighieri.
Last week a Manchester newspaper reported that Louise Joy
Brown, 24, is engaged. Though of note in this British industrial
city, where Ms. Brown is something of a minor celebrity, the
story was not widely reported worldwide.
Twenty-five years ago, however, the news that Brown, the
world's first "test-tube" baby, had been born was widely and
exhaustively reported. She was the Dolly of the decade, the
Clonaid baby of the late 70s, and her birth heralded doom
in the view of some and a new age in assisted reproduction
for others. Today, the fact that Brown's news does not make
the wires is a measure how far in vitro fertilization
and fertility science has come in just one generation.
In the world of fertility science, the last 25 years have
been most productive. In the year 2000, the most recent for
which the U.S. Centers for Disease Control and Prevention
makes data available, more than 400 fertility clinics were
operating in the United States alone, and nearly one out of
every hundred children born in this country was born thanks
to assisted reproductive technologies like in vitro
fertilization.
Yet infertility in both men and women still poses myriad
problems, and some of its basic causes remain unexplained.
Now a team of scientists led by Professor Steven Reed and
Research Associate Charles Spruck, both members of the Department
of Molecular Biology at The Scripps Research Institute (TSRI),
has identified a mammalian protein that seems to play a crucial
role in "meiosis," a biological process critical for fertility.
Certain female fertility problems arise from an inability
in a woman's ovum, or egg, to complete the complicated process
known as oogenesis, whereby it divides and matures through
sequential "mitotic" and "meiotic" divisions in order to prepare
the egg for fertilization by a man's sperm. Male fertility
problems can occur because the early sperm cells, called "spermatocytes,"
are not able to undergo the same sort of meiotic division
to produce viable sperm.
It seems strange that a single protein would have such a
profound biological effect on the fertility of both men and
women. After all, the ovaries and the testes are two very
different organs, and the process of meiosis that occurs in
both spermatocytes and oocytes is different in these two types
of cells.
Nevertheless, a protein called "Cks2" that Reed, Spruck,
and their colleagues describe in the April 25, 2003 issue
of Science seems to be critical for both male and female
fertility. Knocking out the protein in vivo blocked both spermatocyte
meiosis in males and oocyte meiosis in females.
"Any increase in our understanding of how meiotic divisions
occur may help in addressing these fertility problems," says
Reed, though also cautioning, "it's [too] early to say how."
Also, in knocking out Cks2 from mammals, the team has created
in vivo models that can potentially serve for studying
the process of human fertility disorders and meiosis.
Sexual Reproduction Starts With Cell Division
Meiosis is a special kind of cell division that prepares
germ cells—the male sperm cells and the female eggs—for
sexual reproduction. The process of meiosis is not completely
understood, largely due to the difficulty of extracting germ
cells to study.
What is special about meiosis is that it reduces the number
of chromosomes a cell normally has by half. Germ cells are
"haploid," which means that they have only one set of chromosomes
(a total of 23). This allows the germ cell to fuse with another
haploid germ cell to form a "zygote" in which the two sets
of chromosomes from the two parents are combined (giving a
total of 46 chromosomes). Thus, a haploid sperm can combine
with a haploid egg and result in a fertilized embryonic cell.
Meiosis contrasts to mitosis, the normal process of cell
division where one cell makes two copies of its DNA and splits
into two "diploid" daughter cells, which each contain two
copies of the parent cell's chromosomes.
The paper that Reed and his colleagues published in Science
last week describes a knockout model of the Cks2 protein.
The Cks2 knockouts are normal in every way except that they
are sterile. "Without Cks2," says Reed, "the cells become
hung up [in meiosis] and cannot proceed any further."
When members of the team analyzed the model's gonads, they
found a severe defect in the male spermatogenesis, the process
where sperm is generated in the testes. Without the Cks2 protein,
the process became stalled in one particular stage of meiosis.
Similarly, they found a defect in the female counterparts
during oogenesis, the meiotic formation and maturation of
the female ovum or egg.
"What we found is that oogenesis and spermatogenesis are
blocked at the exact same stage in meiosis," says Reed.
If the sperm cell cannot divide, it dies before it has a
chance to fuse with an egg. An egg cell without the Cks2 protein
similarly never reaches the stage where it can be fertilized
by a sperm.
"We don't really know why this happens," says Reed, "but
what this [study] suggests is that the Cks2 protein is [functionally]
required for some early aspect of meiosis."
In order to demonstrate the dependence of meiosis on the
Cks2 protein, Reed turned to two collaborating groups, led
by Peter Donovan of the Kimmel Cancer Center at Thomas Jefferson
University, who specializes in sperm cell and oocyte development,
and Richard Schultz of the University of Pennsylvania, who
specializes in oogenesis.
The team isolated oocytes that would have been stuck in
meiosis (because they were derived from Cks2 knockout females)
and injected them with an RNA that produces the Cks2 protein.
"That rescued them and allowed them to go through normal meiosis,"
says Reed.
A Cousin to the Rescue
Interestingly, the protein "Cks1" (a close cousin of Cks2)
was able to rescue meiosis in the knockout oocytes as well.
And both of these proteins, Cks1 and Cks2 were able to rescue
a division problem in yeast cells caused by loss of the yeast
Cks1 protein. The replacement was not perfect, but the cells
were viable.
Mammalian and yeast Cks proteins are "homologs." Homologs
are genes in different species that originated in a common
ancestral gene long ago. In this case, the Cks genes in the
different species have retained about 50 percent amino-acid
identity and much of the same function. Mouse and human Cks1
and Cks2 are 80 percent identical.
"The [Cks family of genes] are structurally and functionally
conserved through about a billion years of evolution," says
Reed.
The Cks proteins were originally discovered in yeast—a
useful organism to study given that both yeast and mammalian
cells are eukaryotic and have many of the same basic proteins
and mechanisms. Discoveries in yeast also often drive discoveries
in mammalian cells.
After the first Cks protein was discovered in yeast, the
two orthologs Cks1 and Cks2 were discovered in insects and
mammals, and Reed has been studying them—and the original
yeast Cks protein—ever since.
The observation that either Cks1 and Cks2 are able to rescue
meiosis in the Cks2 knockouts is consistent with the finding
that Cks1 is not present in meiosis. If it were, it would
already have come to the rescue.
Why Cks1 is not there is not so clear. However, one possible
reason is that Cks1 (but not Cks2) is normally involved in
proteolysis, the process whereby proteins are chewed up inside
the cell and recycled for their amino acids. During meiosis,
it may be more important to maintain certain proteins and
not turn them over, which may be why the Cks1 protein is not
widely expressed at this stage.
Interestingly, a study Reed and his colleagues published
about a year ago reported the results of knocking out the
Cks1 protein. Mice without the Cks1 protein were fertile and
appeared normal except that they were slightly smaller than
normal (by 15 to 20 percent).
"They just didn't grow as well when they were developing,"
says Reed, "because the Cks1 protein is required for the turnover
of an inhibitor of growth." Without the Cks1 protein, he explains,
the growth inhibitor stays around and the organisms end up
slightly smaller than they should be.
Interestingly, even though both the Cks1 knockout and the
Cks2 knockout are viable, the double knockout is not. These
models do not survive past early embryonic development.
"This suggests that even though [Cks1 and Cks2] have evolved
enough to have separate functions, there is a redundant, shared
function," says Reed.
Reed has reason to think that this shared function somehow
involves the regulation of transcription.
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