Nature 436, 770-771 (11 August 2005) | doi: 10.1038/436770a

Cell biology: The secret life of sperm
Claire Ainsworth


Claire Ainsworth is a senior news & features editor for Nature
magasine.


Abstract: Far from being mere DNA delivery boys, it's now becoming
clear that sperm also ship a complex cargo of RNA and proteins that
may be crucial for an embryo's early development. Claire Ainsworth
reports.

"What's in sperm?" demands Tim Karr. It's an unusual start to a lab
visit, and I confess that I know little more than the sketchy
textbook picture. I'm not alone, says Karr, and that is the whole
problem. "How can we not know?" he asks. "It probably explains why
we don't understand sex."

Karr, who works at the University of Bath, UK, is convinced that a
more detailed knowledge of the molecular biology of sperm will help
answer some of biology's most fundamental questions. This is what
prompted him to spend the past three years dissecting fruitfly
testes and developing methods to study the protein content — or
proteome — of their sperm.

Sperm are amenable to detailed proteomic analysis because they
contain no more than a few hundred proteins. But this apparent
simplicity is deceptive. Karr's team is one of several to have found
that there is far more to sperm than we thought. In addition to the
DNA instructions that spell out a male's contribution to a new life,
these sleek, whip-powered cells have in the past few years been
shown to carry other pieces of cellular machinery, such as RNA and
proteins. This discovery is changing our understanding of fertility,
development and the evolution of sex. "It really challenges some
basic ideas," says Stephen Krawetz, a biologist at Wayne State
University School of Medicine in Detroit, Michigan.

Because sperm have to swim far and fast, biologists have come to
view them like racing cars: streamlined and stripped down of all
unnecessary bits and pieces. Generally speaking, the DNA in animal
sperm is tightly packed inside a sleek head structure that contains
little of the cytoplasm that fills most other cells. Behind the head
is the midpiece, containing more than 50 power units called
mitochondria that drive the lashing motion of the attached tail.

Egged on
Given the shortage of cytoplasm, and the lack of any detectable
protein synthesis in mature sperm heads, biologists had long assumed
that sperm contribute little to an embryo bar the father's genes. In
contrast, the egg is replete with molecules such as proteins and
RNAs that nourish and direct the development of the embryo. "The
idea was that the egg was supplying everything and Dad was just
tagging along with his DNA," says Krawetz.

Recent discoveries have revealed the error of this view. Studies now
suggest that defects in sperm can disrupt embryo development even if
the genes carried by the cells are perfectly normal1. And there are
hints that faulty sperm could be the cause of a significant number
of miscarriages, says David Miller, a reproductive biologist at the
University of Leeds, UK. "So we know sperm is important."

But what does a sperm deliver? One popular misconception is that
only the head enters the egg, while the tail is discarded. But in
most species, the entire cell enters the egg — midpiece, tail and
all2. And in many mammals, midpiece and tail structures persist in
the embryo for several cell divisions3. This results in a large
number of proteins and other molecules being delivered to the egg.
In mammals other than rodents, these include a piece of cellular
machinery called the centrosome, which coordinates the molecular
ropes cells use to haul chromosomes around during cell division4.

Until 2002, this was thought to be an isolated example. But then a
team headed by Anthony Lai at the University of Wales in Cardiff
discovered that sperm also deliver a molecule called PLC that
triggers the waves of calcium ions that activate a fertilized egg5.
And a bigger surprise came when Krawetz and Miller studied sperm
from 10 fertile men and found that they contained some 3,000
different kinds of messenger RNA6. Some of them coded for proteins
needed for early embryo development; others were previously unknown,
and had no equivalents in the egg.

Male delivery
This suggested that sperm could deliver RNAs that help direct an
embryo's early development. Some biologists were sceptical, arguing
that the RNAs were simply non-functional leftovers from the process
of sperm development. But Krawetz, Miller and others have since
gathered more evidence, and last year showed that a specific package
of RNAs are indeed transferred from sperm to egg7. Earlier this
year, Krawetz and his colleagues found that these include micro-
RNAs, which don't code for proteins but are known to play a role in
controlling gene activity8.

It remains unclear what the transferred RNA does. The fact that
cloning works, and the creation last year of a mouse by combining
the nuclei of two eggs9, both suggest it is not absolutely essential
for embryo development. But these processes are grossly inefficient
and often result in birth defects or abnormal gene activity, which
hints that the paternal RNA may be important.

Miller suggests that messenger RNAs help protect paternal genes that
are needed soon after fertilization from being shut down as sperm
mature. Normally, most of a sperm's DNA is tightly wrapped up and
gagged by proteins called protamines. The RNAs could stick to the
genes that code for them and stop this, he argues. Another
possibility is that paternal RNAs, particularly micro-RNAs, might be
involved in controlling imprinting — the differential activation of
genes according to whether they are inherited from the mother or the
father. Certainly, the idea that the RNAs have no function is
becoming a minority view. "Sperm are so sleek and have such powerful
methods for eliminating everything that causes drag that I don't
believe they are vestigial," says Gerald Schatten, a reproductive
biologist at the University of Pittsburgh, Pennsylvania.


The new view of sperm as carriers of molecules crucial for early
embryo development has thought-provoking implications for
reproductive medicine. Comparing the RNA profiles of fertile and
infertile men might reveal causes of unexplained infertility, says
Miller.

Such studies may also raise questions about the wisdom of an in
vitro fertilization technique called intracytoplasmic sperm
injection, or ICSI, used to help men whose sperm do not fertilize
their partner's eggs. ICSI involves injecting faulty or immature
sperm — which might lack the normal complement of RNAs — directly
into eggs. So far, there are no clear signs of problems among
children conceived by ICSI, although long-term follow-up is needed
to confirm the safety of the technique.

Now the action in sperm biology is moving from RNA to proteins. Last
year, Christopher Barratt, a reproductive biologist at the
University of Birmingham, UK, published the first proteomic study of
male infertility. His aim was not to produce a complete proteome for
sperm; instead, his team looked for differences in the protein
profiles in the sperm of an infertile and a fertile man. The
researchers found at least 20 proteins present in significantly
different quantities10, giving them a starting point to study cases
of unexplained infertility and suggesting targets for new
contraceptives.

Origins of life
The coming months could see the first publications from groups,
including Karr's, that are conducting more comprehensive proteomic
studies. Being able to compare the structure and content of the
proteomes of sperm from different species should help researchers
understand the evolution and origin of sperm. In particular, having
a comprehensive catalogue of proteins to compare between different
species may reveal how natural selection is operating on them, says
Steve Dorus, a postdoc in Karr's lab. "It should give us some pretty
powerful information about what our ancestors' core sperm attributes
were."

The view of sperm as carriers of molecules crucial for early embryo
development has thought-provoking implications for reproductive
medicine.
While Karr's group works on fruitfly sperm, Victor Vacquier, a
reproductive biologist at the Scripps Institution of Oceanography in
La Jolla, California, is producing a catalogue of the proteins found
in the outer membrane of sperm of the purple sea urchin
(Strongylocentrotus purpuratus). His particular interest lies in
understanding how sperm and egg interact and recognize each other.
Surprisingly, scientists know comparatively little about the
molecules that interact when a sperm comes into contact with an
egg's surface. But they do know that, in some species, such proteins
can evolve extraordinarily rapidly11. Vacquier believes that these
fast-evolving proteins may help explain why some animal populations
become reproductively isolated, leading to the formation of new
species.

The origin of species, the evolution of sex, the mysteries of
infertility — these are some of the biggest and most intriguing
questions in biology. Long dismissed as mere delivery boys, it seems
that sperm are about to be put on the promotion fast track.