Phagocytosis by the Retinal Pigment Epithelium: Recognition, Resolution, Recycling.

Tissue-resident phagocytes are responsible for the routine binding, engulfment, and resolution of their meals. Such populations of cells express appropriate surface receptors that are tailored to recognize the phagocytic targets of their niche and initiate the actin polymerization that drives internalization. Tissue-resident phagocytes also harbor enzymes and transporters along the endocytic pathway that orchestrate the resolution of ingested macromolecules from the phagolysosome. Solutes fluxed from the endocytic pathway and into the cytosol can then be reutilized by the phagocyte or exported for their use by neighboring cells. Such a fundamental metabolic coupling between resident phagocytes and the tissue in which they reside is well-emphasized in the case of retinal pigment epithelial (RPE) cells; specialized phagocytes that are responsible for the turnover of photoreceptor outer segments (POS). Photoreceptors are prone to photo-oxidative damage and their long-term health depends enormously on the disposal of aged portions of the outer segment. The phagocytosis of the POS by the RPE is the sole means of this turnover and clearance. RPE are themselves mitotically quiescent and therefore must resolve the ingested material to prevent their toxic accumulation in the lysosome that otherwise leads to retinal disorders. Here we describe the sequence of events underlying the healthy turnover of photoreceptors by the RPE with an emphasis on the signaling that ensures the phagocytosis of the distal POS and on the transport of solutes from the phagosome that supersedes its resolution. While other systems may utilize different receptors and transporters, the biophysical and metabolic manifestations of such events are expected to apply to all tissue-resident phagocytes that perform regular phagocytic programs.

Independent roles of methionine sulfoxide reductase A in mitochondrial ATP synthesis and as antioxidant in retinal pigment epithelial cells.

The antioxidant enzyme methionine sulfoxide reductase A (MsrA) is highly expressed in the retinal pigment epithelium (RPE), a support tissue for neighboring photoreceptors. MsrA protein levels correlate with sensitivity of RPE in culture to experimental oxidative stress. To investigate whether and how MsrA affects RPE functionality regardless of oxidative stress, we tested the effects of acute silencing or overexpression of MsrA on the phagocytosis of photoreceptor outer segment fragments (POS), a demanding, daily function of the RPE that is essential for vision. Endogenous MsrA localized to mitochondria and cytosol of rat RPE in culture. RPE cells manipulated to express higher or lower levels of MsrA than control cells showed no signs of cell death but increased or decreased, respectively, POS binding as well as engulfment. These effects of altered MsrA protein concentration on phagocytosis were independent of the levels of oxidative stress. However, altering MsrA expression had no effect on phagocytosis when mitochondrial respiration was inhibited. Furthermore, ATP content directly correlated with MsrA protein levels in RPE cells that used mitochondrial oxidative phosphorylation for ATP synthesis but not in RPE cells that relied on glycolysis alone. Overexpressing MsrA was sufficient to increase specifically the activity of complex IV of the respiratory chain, whereas activity of complex II and mitochondrial content were unaffected. Thus, MsrA probably enhances ATP synthesis by increasing complex IV activity. Such contribution of MsrA to energy metabolism is independent of its function in protection from elevated oxidative stress but contributes to routine but vital photoreceptor support by RPE cells.

Pharmacokinetics of pars plana intravitreal injections versus microcannula suprachoroidal injections of bevacizumab in a porcine model.

PURPOSE: To compare the pharmacokinetics and tissue response between intravitreal and microcannulation injections into the suprachoroidal space using bevacizumab. METHODS: Sixty-two pigs were studied. Either a pars plana intravitreal bevacizumab or a viscoelastic-enhanced microcannula suprachoroidal injection was performed with either 1.25 mg (group 1) or 3 mg (group 2). In group 1, six animals were euthanatized at 0.5, 7, 30, 60, 90, and 120 days after injection (n = 36). In group 2, six animals were euthanatized at 0.5, 7, 14, and 32 days (n = 24). Eyes were enucleated, dissected, and snap-frozen, or they were fixed for histology. Analysis of drug tissue levels was performed at two separate laboratories using masked specimens. RESULTS: Both laboratories were confirmatory. Intravitreal bevacizumab pharmacokinetics demonstrated a gradual decline in tissue levels over 30 to 60 days in both groups 1 and 2. In addition, suprachoroidal bevacizumab tissue levels declined rapidly and were not measurable at or beyond 7 days. Vitreitis and granulomatous vasculitis were noted in 7 of 30 intravitreal injection eyes. Immunohistology suggested a distinctive drug distribution. CONCLUSIONS: Direct intravitreal injection of bevacizumab has a more sustained pharmacologic profile than does a similar dose delivered to the suprachoroidal space. Intravitreal injections distributed more to the inner retina, whereas suprachoroidal delivery occurred primarily at the choroid, retinal pigment epithelium, and photoreceptor outer segments. Sustained release formulation of larger biological molecules should be considered to optimize suprachoroidal delivery. Inflammation from injections is granulomatous, seen only with intravitreal injections, and may result from either an altered immune response or a dose-related effect.