Transcription Factors and Rod Photoreceptors
Audrey
Ettinger, Ph.D.
"Cold-blooded"
animals, including fish, grow throughout their lives, unlike mammals which
reach an adult size and maintain it.
This continuous growth poses some challenges to the nervous system: how
can an animal continue to accurately process sensory information while
growing? In particular, I am
interested in how the visual system, primarily the retina, has adapted to
continuous growth. A good model
system for
addressing
this question is the teleost fish Haplochromis burtoni. This species displays a complex set of
social interactions which depend on vision, and thus individuals must maintain
accurate vision during growth.
The retina
has adapted by maintaining a population of "stem cells" into
adulthood. Stem cells have long
been studied in the developing embryo where they divide to yield all of the
different types of cells in an adult animal. Over the course of development, stem cells become
progressively
restricted
in what adult cell types they can become.
In some tissues, such as blood and bone marrow, tissue-specific
restricted stem cells supply a continuous source of new adult cells. In the nervous system, stem cells are
found much more rarely. Mammals
have only recently been determined to have any stem cells, and the poor
recovery from central
nervous
system injury indicates that these stem cells do not provide a complete repair
mechanism. In contrast, we have
known for several decades that fish and other "cold-blooded" animals
maintain a significant population of stem cells into adulthood. These cells continue to divide
and
differentiate, forming the specific cell types that the retina needs to
function correctly while it grows.
A population of stem cells at the margin (edge) of the retina forms all
retinal cell types, while a spatially and possibly functionally distinct
population in the center of the retina gives rise to a single cell type, rod
photoreceptors. The molecules which
control the division and differentiation of these cells
are just
beginning to be understood.
1.
"Pick a molecule". One
approach to understanding how the division and differentiation of retinal stem
cells is controlled is to choose an
individual
molecule. Is the molecule
expressed in the teleost retina?
Do progenitor cells or their nearby neighbors express the molecule? What happens if we change the normal
expression of the molecule, either raising
or lowering
its expression? Are division and
differentiation altered? To begin,
we will select a molecule that is likely to be important in retinal stem cell
development by reviewing the scientific literature for work done
in other
species. We will then obtain
antibodies that recognize the H. burtoni version of the molecule, and perform
immunostaining experiments to
determine
the normal expression of the molecule.
Finally, we can inject
the molecule
directly into the retinas of living fish, and determine
whether
there is a direct effect on cell division or differentiation.
Potential
classes of molecules to be examined include the superfamily of
"growth
factors" known to be important in development and maintenance of many
tissues, and transcription factors, which are expressed at specific
times in a
cellÕs life cycle and control its responsiveness to outside factors.
2. "Do
environmental factors have an effect on cell division and cell
differentiation
in the retina?" Fish
encounter many different molecules
as they swim
in their natural environments.
Haplochromis burtoniÕs
natural
environment, Lake Tanganyika in Africa, has been affected by the
human
population living around it, while other speciesÕ environments are
also being
changed. Do these changed
environments affect developmental
processes in
exposed animals? We can use the H.
burtoni retina as a model
system to
determine whether individual molecules have an effect on cell
division and
differentiation. We will select
one or a few molecules
likely to be
found in Lake Tanganyika, and expose fish to the molecule in
their tank
water.
3.
"What about other fish species?" While H. burtoni is an excellent
model, we
would be better able to understand general concepts regulating
adult cell
division and differentiation by studying additional species for
comparison. We will choose either a different Lake
Tanganyika cichlid (available in fish stores for use as pets) or a local fish
species, such as trout or shad, and begin to characterize its retinal stem
cells. Based on the previous work
in H. burtoni, we will look at differences at cell
division
during the day and at night, movement of differentiating
progenitors
within the retina, and some of the molecules already studied in H. burtoni.
4.
"What about the rest of the fish?" Fish muscles have interesting
similarities
and differences to mammalian muscle at a macroscopic level.
Not much is
known about the molecules used to stabilize fish muscle
fibers. We will begin to ask evolutionary
questions about the
dystrophin-glycoprotein
complex, which is crucial for normal muscle
function in
mammals. When this complex is
disrupted, muscular dystrophy
is the
result. We will begin by analyzing
the gene and protein sequence
databases to
look for fish homologues of the known dystrophin-glycoprotein
complex
components. We will then obtain
antibodies against candidate
molecules
likely to be expressed in H. burtoni and examine their expression.
Suggested
undergraduate research projects: