An important question in biology is what makes cells die. We are studying the degeneration and premature cell death of terminally differentiated photoreceptor neurons (PRNs) in the adult retina. In flies and humans, many point mutations in the photoreceptor protein rhodopsin dominantly trigger PRN apoptosis and loss of vision. In humans, the disease is called retinitis pigmentosa (RP). The phenotype in Drosophila is remarkably similar to that in humans. Among other things, not only do PRNs expressing the mutant opsins die, but also PRNs that express different opsins. The mechanism is thus at least partly non-cell-autonomous. Our goal is to determine the mechanism of neuronal cell death and degeneration. Projects: Current research and future plans are focused on three major questions:1. The relationship between neurodegeneration and apoptosisQuantitation of neurodegeneration in vivo has historically been challenging. Yet it is essential for genetic modifier studies (see below). We have taken advantage of unique attributes of the fly compound eye to quantitate degeneration in vivo. But in the process of validating the assay, we discovered early, pre-apoptotic stages of degeneration whose relationship to classic apoptosis is unclear. We are particularly interested in the activation of cell death, so this is fortuitious. However, these degenerated morphologies often persist for weeks before apoptotic morphologies appear, whereas classic apoptosis takes only hours from start to finish. Amazingly, lowered dosages of proapoptotic genes such as hid suppress even these early changes. This raises the question of whether neurodegeneration in RP is a slowed-down form of apoptosis, or a separate process coincidentally controlled by hid and followed by apoptosis. Having determined specific methods of detection of these stages, we are engaged in identifying the genes involved using microarray, molecular biological and genetic approaches. 2. The trigger for degeneration provided by the dominant rhodopsin mutationsIn cases where RP mutations cause opsin misfolding, the leading theory has been that apoptosis is triggered by overstimulation of the unfolded protein response (UPR), constituting a toxic-gain-of-function mechanism. However, our research now shows that the mutant proteins are in fact partial loss-of-function mutants. This implies that wild type rhodopsin actually plays a role in PRN maintenance and survival, not just visual signaling. While the UPR undoubtedly occurs, the evidence does not support an essential role for it in cell killing. The mechanism we propose unifies all the previously reported data on PRN homeostasis, including studies of the effects of photoactivation in other phototransduction -signaling-mutant backgrounds, and the requirement for rhodopsin in PRN development. Most significantly, the mechanism makes us shift focus from """"""""How do mutant opsins kill PRNs?"""""""" to """"""""How does wild type opsin promote PRN maintenance and survival?"""""""" Given the evolutionary conservation of heterotrimeric G protein signaling, we anticipate that our results will be relevant to the control of apoptosis in a variety of cells.3. The genetic pathway of degeneration and cell death activated by rhodopsin mutations.By genetic deficiency screening, we have identified suppressor and enhancer loci affecting opsin mutant induced degeneration. Identification of the responsible genes is a future focus. In addition, candidate gene screening indicates both similarities and dissimilarities in how cell death is controlled in adult PRNs vs. developing PRNs and mitotic cells. These factors include light, cell-death-regulatory genes such as hid, TRAF, caspases and dIAP1, and the ras mitogen-activated kinase pathway genes. However, the activating or suppressing activities of these genes are sometimes opposite to those in mitotic cells, indicating different or additional control mechanisms in the adult, terminally differentiated neurons.
