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.    
Suggested undergraduate research projects:  

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.