Lab web-page: http://labs.umassmed.edu/punzoLab/index.html
|Harvard Medical School, Boston, MA, USA
Postdoctoral fellow, Laboratory of Dr. C.L. Cepko
|Biozentrum, University of Basel, Basel, Switzerland
Doctor of Philosophy, Laboratory of Dr. W.J. Gehring
|University of Basel, Basel Switzerland
Undergraduate studies, Mentored by Dr. W.J. Gehring
Neuro-Degeneration in the Retina
The vertebrate retina has highly specialized sensory neurons, the photoreceptors (PR), which serve to initiate the process of vision. Cone PRs are responsible for vision during the brighter light intensities of the day and mediate color vision. Rod PRs are 1000x more sensitive to light, and initiate vision in dim light. The light captured by PRs is converted into an electrical signal that is passed on to bipolar cells and then to ganglion cells, the output neurons of the retina, that project to the brain.
Blindness is the inevitable end stage of neuro-degeneration in the retina. The two cell types in the retina that are associated with loss of vision in humans are either the ganglion cells or the PRs. Loss of ganglion cells results in Glaucoma. Loss PRs is associated with a large number of retinal degeneration (RD) diseases. Since PRs account for ~75% of all cells in the retina loss of PRs results always in sever RD.
The research focus in my lab is on RD diseases that affect photoreceptors. Retinitis Pigmentosa (RP) is a family of inherited RD that is untreatable and leads to blindness. The pathology is characterized by an initial loss of night vision due to the loss of rod PR, followed by a progressive loss of cone PRs. In many cases, the disease-causing allele is a gene exclusively expressed in rods; nonetheless, cones die too. There is no known form of RD in humans or mice where rods die, and cones survive. In contrast, mutations in cone-specific genes result only in cone death. Understanding this non-autonomous cone death is the key in designing therapeutic strategies. While the dependence of cones on rods plays an important role in RP it remains a fundamental question of retinal biology.
Our recent studies on RP have led us to propose that cone death is preceded by metabolic changes in cone PRs and that, cones die due to nutrient deprivation. This hypothesis is based on changes seen in the Insulin/mTOR pathway and the role the individual members play in cones during degeneration (Fig. 1, 2). How could the observations of nutritionally deprived cones explain the dependence of cones on rods? PR outer segments (OS) interact with the Retinal-Pigmented Epithelium (RPE), which a single sheet of cells adjacent to the PR layer. The OS-RPE interactions are vital since the RPE provides nutrition and oxygen to PRs. Roughly 95% of all PRs in mouse and human are rods and approximately 20-30 OSs contact one RPE cell. Thus, only 1-2 of those RPE-OS contacts are via cones. During the collapse of the PR layer, the few remaining cone:RPE interactions are likely perturbed. If these interactions drop below a threshold required for the proper flow of nutrients, the loss of rods might result in a reduced flow of nutrients to cones. By cross-comparing 4 mouse models of RP we found in our studies that cone death starts always at the same density of leftover rods, meaning after 90% of rods have died. This cell density could represent the crucial threshold of remaining cells after which flow of nutrition is perturbed. This mechanism would also explain why the loss of cones does not lead to rod death. Since in humans and mouse, cones are less than 5% of all PRs, the critical threshold that perturbs OS-RPE interactions would not be reached.
Areas of research interest in the laboratory include the role of molecular signaling pathways such as the Insulin/mTOR pathway in photoreceptor homeostasis and diseases. PRs are among the highest energy consuming cells in the human body. However, how PRs metabolize energy and how they respond to insults and stress is largely understudied. The knowledge gained on PR metabolism by studying diseases such as RP could help us in the future to treat more widespread diseases such as diabetic retinopathy or age-related macular degeneration. Another area of interest includes understanding the upstream and downstream signaling events of the apoptotic pathway that leads to cone cell death.
Fig. 1. Insulin levels affect cone survival in the Retinitis Pigmentosa mutant PDE-b. (a-c) Retinal flat mounts of PDE-b mutants at postnatal week 7 stained for lacZ to detect surviving cones (blue color). (a) Example of untreated control. (b) Example of mouse injected with streptozotocin. Streptozotocin kills the b-cells in the pancreas thereby removes endogenous insulin. (c) Example of mouse injected daily with insulin to increase endogenous insulin.
Fig. 2. (A) shows immunofluorescence on retinal flat mounts (photoreceptor side up) and (b, c) show retinal sections. Blue shows the nuclear DAPI stain. (a-c) p*-mTOR levels in wild type retinae. (a) Dorsal (up) enrichment of p*-mTOR. Higher magnification of dorsal and ventral region is shown to the right showing p*-mTOR in red and cone segments in green as detected by PNA. (b, c) Dorsal retinal sections stained for p*-mTOR (red signal) and PNA (b) (green signal) or a-b-galactosidase (c) (green signal). The b-galactosidase is under the control of the human red/green opsin promoter and is expressed in all cones.