Silvia C. Finnemann

Diploma in Biochemistry, Free University of Berlin, Germany, 1991.
Ph.D. in Biochemistry, Department of Chemistry, Free University of Berlin, Germany, 1995.

To Eat Or Not To Eat? Diurnal RPE Phagocytosis is Essential for Vision

Photoreceptor rod and cone neurons in the retina are highly specialized to detect light and promote vision. The machinery for these functions localizes to the light-sensitive outer segment of photoreceptors, which continuously turns over in a process termed outer segment renewal. Production of new outer segment disks is in precise balance with elimination by “shedding” of distal portions of outer segments. Mammalian rod shedding is highly synchronized and occurs at light onset regulated by the circadian rhythm. It is a major task of the adjacent retinal pigment epithelium (RPE) to respond to outer segment shedding with a diurnal burst of phagocytosis that promptly and completely clears outer segment debris from the retina. The rhythmic and continuous nature of phagocytosis of photoreceptor outer segment fragments (POS) renders RPE cells the most active phagocytes in nature.

Both RPE and photoreceptors are subjected to high levels of damaging photo-oxidative stress. Because they do not normally turn over, they are also particularly susceptible to cumulative damage acquired with age. The most profound change of human RPE with age is the enormous accumulation of RPE lipofuscin, a complex mixture of undigested, oxidized POS derivatives trapped in RPE lysosomes. Oxidative damage promotes lipofuscin formation and both exacerbate RPE dysfunction promoting age-related macular degeneration (AMD), the leading cause of blindness among the elderly in the United States. However, cellular-molecular mechanisms linking RPE phagocytosis, oxidative stress, lipofuscin, and vision loss remain obscure to date.

It is the central goal of our research to identify the molecular mechanisms of phagocytosis used by the RPE to clear shed POS in a diurnal rhythm and to understand and combat their changes with age and disease that cause age-related blindness.

We have developed novel in vivo and cell culture models and quantitative functional assays that allow us for the first time to directly measure RPE signaling during POS phagocytosis in intact retina. Our experiments have identified the first receptor-ligand pair essential for synchronized RPE phagocytosis: shed POS require ligation of avb5 integrin adhesion receptors by their ligand, the secreted glycoprotein MFG-E8, to stimulate distinct signaling pathways via p125-focal adhesion kinase towards actin regulation and towards Mer receptor tyrosine kinase, both indispensable for engulfment.

Furthermore, we found that RPE cells actively and specifically regulate the POS binding activity of avb5 receptors using mechanisms hitherto completely unexplored: avb5 activity fluctuates in vivo in a diurnal rhythm. Complex formation of the tetraspanin CD81 directly regulates avb5 binding activity in RPE cells in culture providing the first evidence for tetraspanin requirement in any phagocytic mechanism.

Ongoing projects continue to explore the molecular-cellular mechanisms that regulate RPE phagocytosis. Specific immediate aims are to:

1. Identify cytoplasmic signaling pathways that transduce avb5 integrin receptor activation to the actin cytoskeleton during retinal adhesion and POS phagocytosis.
2. Identify cellular and molecular mechanisms that regulate receptor protein function and receptor cross-talk at the apical, phagocytic surface of the RPE thereby synchronizing RPE activity to fit it to the circadian rhythm of POS shedding.
3. Identify soluble and outer segment-bound ligands that synchronize activation of RPE phagocytosis in the retina.
4. Identify cellular pathways that interconnect digestion of phagocytosed outer segments, mitochondrial function, and energy metabolism and decipher how they deteriorate with age contributing to accumulation of toxic lipofuscin in the RPE

Vives-Bauza, C, M Anand, AK Shirazi, J Magrane, J Gao, H Vollmer-Snarr, G Manfredi & SC Finnemann. The age lipid A2E and mitochondrial dysfunction synergistically impair phagocytosis by retinal pigment epithelial cells. J. Biol. Chem. 283(36):24770-24780 (2008).

Finnemann, SC, & Y Chang. Photoreceptor-RPE Interactions: Physiology and Molecular Mechanisms. Ch. 4 in "Visual Transduction and Non Visual Light Perception" J. Tombran-Tink & C. J. Barnstable, Ed. (2008).

Nandrot, EF, Y Chang & SC Finnemann. avb5 integrin receptors at the apical surface of the RPE: one receptor, two functions. Adv Exp Med Biol. 613:369-75 (2008).

Nandrot, EF, & SC Finnemann. Lack of avb5 integrin receptor or its ligand MFG-E8: distinct effects on retinal function. Ophthalmic Res. 40:120-3 (2008).

Chang, Y, & SC Finnemann. Tetraspanin CD81 is required for the avb5 integrin-dependent particle binding step of retinal phagocytosis. J. Cell Sci. 120(17):3053-3063 (2007).

Nandrot, EF, M Anand, D Almeida, K Atabai, D Sheppard & SC Finnemann. Essential role for MFG-E8 as ligand for avb5 integrin in diurnal retinal phagocytosis. Proc. Natl. Acad. Sci. USA. 104(29):12005-12010 (2007).

Lakkaraju, A, SC Finnemann & E Rodriguez-Boulan. The lipofuscin fluorophore A2E perturbs cholesterol metabolism in retinal pigment epithelial cells. Proc. Natl. Acad. Sci. USA. 104(26):11026-11031 (2007).

Finnemann, SC, & EF Nandrot. MerTK activation during RPE phagocytosis in vivo requires avb5 integrin. Adv. Exp. Med. Biol. 572: 499-504 (2006).

Nandrot, EF, & SC Finnemann. Altered rhythm of photoreceptor outer segment phagocytosis in b5 integrin knockout mice. Adv. Exp. Med. Biol. 572:119-124 (2006).

Sun, M, SC Finnemann, M Febbraio, L Shan, SP Annangudi, EA Podrez, G Hoppe, R Darrow, DT Organisciak, RG Salomon, RL Silverstein & SL Hazen. Light-induced oxidation of photoreceptor outer segment phospholipids generates ligands for CD36-mediated phagocytosis by retinal pigment epithelium. J. Biol. Chem. 281:4222-4230 (2006).

Nandrot, EF, M Anand, M Sircar, & SC Finnemann. Novel function for avb5 integrin in retinal adhesion and its diurnal peak. Am. J. Phys. Cell Phys. 290: C1256 –C1262 (2006).

Nandrot, EF, YH Kim, SE Brodie, X Huang, D Sheppard & SC Finnemann. Loss of synchronized RPE phagocytosis and age-related blindness in mice lacking avb5 integrin. J. Exp. Med. 200:1539-1545 (2004) (cover image).

Finnemann, SC. Focal adhesion kinase signaling promotes phagocytosis of integrin-bound photoreceptors. EMBO J. 16:4143-4154 (2003).