Aparna Lakkaraju

Itinerant exosomes: emerging roles in cell and tissue polarity.

Cells use secreted signals (e.g. chemokines and growth factors) and sophisticated vehicles such as argosomes, cytonemes, tunneling nanotubes and exosomes to relay important information to other cells, often over large distances. Exosomes, 30-100-nm intraluminal vesicles of multivesicular bodies (MVB) released upon exocytic fusion of the MVB with the plasma membrane, are increasingly recognized as a novel mode of cell-independent communication. Exosomes have been shown to function in antigen presentation and tumor metastasis, and in transmitting infectious agents. However, little is known about the biogenesis and function of exosomes in polarized cells. In this review, we discuss new evidence suggesting that exosomes participate in the transport of morphogens and RNA, and thus influence cell polarity and developmental patterning of tissues.

The lipofuscin fluorophore A2E perturbs cholesterol metabolism in retinal pigment epithelial cells

Proteins involved in cholesterol trafficking are known to contribute to the pathogenesis of atherosclerosis and Alzheimers disease. Allelic variants in the cholesterol transporters apolipoprotein E and ATP-binding cassette protein A1 (ABCA1) have recently been associated with susceptibility to age-related macular degeneration (AMD). Histopathological analyses of eyes with AMD demonstrate the presence of cholesterol and cholesteryl ester deposits beneath the retinal pigment epithelium (RPE), implicating abnormal cholesterol trafficking in disease progression. Here, we show that A2E, a quaternary amine and retinoid by-product of the visual cycle, causes the accumulation of free and esterified cholesterol in RPE cells. The mechanism involves neither generalized alterations in late endosomal/lysosomal pH nor a direct inhibition of acid lipase activity. Rather, A2E prevents cholesterol efflux from these organelles, which in turn indirectly inhibits acid lipase, leading to a subsequent accumulation of cholesteryl esters. Transcriptional activation of the ABCA1 cholesterol transporter by agonists of the liver X receptor/peroxisome proliferator-activated receptor pathway relieves the A2E-induced block on cholesterol efflux and restores cholesterol homeostasis in RPE cells. Our data also demonstrate that A2E, which is a cone-shaped lipid, increases the chemical activity and displacement of cholesterol from model membranes, providing a biophysical mechanism for cholesterol sequestration in A2E-loaded cells. Although endogenously produced A2E in the RPE has been associated with macular degeneration, the precise mechanisms are unclear. Our results provide direct evidence that A2E causes aberrant cholesterol metabolism in RPE cells which could likely contribute to AMD progression.

In vivo diffusion of lactoferrin in brain extracellular space is regulated by interactions with heparan sulfate

The intercellular spaces between neurons and glia contain an amorphous, negatively charged extracellular matrix (ECM) with the potential to shape and regulate the distribution of many diffusing ions, proteins and drugs. However, little evidence exists for direct regulation of extracellular diffusion by the ECM in living tissue. Here, we demonstrate macromolecule sequestration by an ECM component in vivo, using quantitative diffusion measurements from integrative optical imaging. Diffusion measurements in free solution, supported by confocal imaging and binding assays with cultured cells, were used to characterize the properties of a fluorescently labeled protein, lactoferrin (Lf), and its association with heparin and heparan sulfate in vitro. In vivo diffusion measurements were then performed through an open cranial window over rat somatosensory cortex to measure effective diffusion coefficients (D*) under different conditions, revealing that D* for Lf was reduced ≈60% by binding to heparan sulfate proteoglycans, a prominent component of the ECM and cell surfaces in brain. Finally, we describe a method for quantifying heparan sulfate binding site density from data for Lf and the structurally similar protein transferrin, allowing us to predict a low micromolar concentration of these binding sites in neocortex, the first estimate in living tissue. Our results have significance for many tissues, because heparan sulfate is synthesized by almost every type of cell in the body. Quantifying ECM effects on diffusion will also aid in the modeling and design of drug delivery strategies for growth factors and viral vectors, some of which are likely to interact with heparan sulfate.

Cholesterol-mediated activation of acid sphingomyelinase disrupts autophagy in the retinal pigment epithelium

How autophagy is regulated in the postmitotic retinal pigment epithelium (RPE) is unclear. Visual cycle metabolites and cholesterol that accumulate in the RPE inhibit autophagic flux by activating acid sphingomyelinase (ASMase). Increased ceramide promotes tubulin acetylation, which prevents autophagosome traffic. ASMase inhibition restores RPE autophagy.

Low-density lipoprotein receptor-related protein mediates the endocytosis of anionic liposomes in neurons

We have recently demonstrated that anionic liposomes efficiently introduce foreign DNA into postmitotic neurons and other cell types (Lakkaraju, A., Dubinsky, J. M., Low, W. C., and Rahman, Y.-E. (2001) J. Biol. Chem.276, 32000–32007). To investigate the mechanism of liposome uptake, we followed the internalization of anionic liposome-encapsulated Cy3-labeled oligonucleotides (AL-Cy3ONs) by hippocampal neurons using confocal microscopy. Uptake of AL-Cy3ONs was widespread and time- and temperature-dependent, indicative of receptor-mediated endocytosis. The low-density lipoprotein receptor-related protein (LRP) was crucial for anionic liposome endocytosis because the receptor-associated protein or an anti-LRP antibody inhibited internalization, and fibroblasts lacking LRP did not internalize AL-Cy3ONs. Using selective endocytosis inhibitors, we found that liposome endocytosis and intracellular transport required clathrin, dynamin, an intact cytoskeletal network, and phosphatidylinositol 3-kinase activity. Cy3ONs did not significantly colocalize with recycling endosomal/lysosomal markers and entered neuronal nuclei within 1–3 h of incubation. Approximately 50% of the internalized liposomal phospholipids were recycled back to the cell surface, in keeping with the fluidity of their acyl chains. Liposome endocytosis did not require heparan sulfate proteoglycans or cause calcium influx into neurons. Thus, constitutive endocytosis of anionic liposomes by LRP utilizes only one component, in contrast to the more involved heparan sulfate proteoglycan-LRP pathway implicated in the pathogenesis of Alzheimers disease.

A detailed three-step protocol for live imaging of intracellular traffic in polarized primary porcine RPE monolayers

The retinal pigment epithelium (RPE) performs numerous functions that are indispensable for photoreceptor health and vision. This monolayer of cells is also a major site of insult in inherited and age-related macular degenerations. In vitro models of primary RPE such as human fetal and adult RPE cultures have been invaluable for dissecting disease pathways at the cellular and molecular level. However, numerous studies show that it takes over four weeks for human RPE cell monolayers to become fully polarized after plating on semipermeable membrane supports. Poor persistence of transgene expression over this time period critically limits the applicability of human RPE cultures for live imaging studies required to follow dynamic processes like intracellular trafficking and organelle transport that occur over timescales of milliseconds. Here, we provide a detailed three-step protocol for live imaging of polarized primary RPE using high-speed spinning disk confocal microscopy. Step 1: establish porcine RPE monolayers that undergo differentiation within one week after plating on semipermeable membrane supports; step 2: transfect or transduce RPE using either of two different protocols that result in prolonged transgene expression; and step 3: perform multicolor high-speed live imaging of organelle transport in polarized RPE monolayers. Porcine RPE cells and photoreceptor outer segments were isolated from freshly harvested eyes and plated on collagen-coated Transwell® filters to generate polarized monolayers. After seven days, RPE monolayers were highly pigmented, had TER values ≥ 200 Ω.cm2 and cleared outer segments within 5 hours after phagocytosis. These cells expressed RPE65, localized ZO-1 to the tight junction, Na+,K+-ATPase to the apical membrane and acetylated tubulin to the primary cilium. There was an inverse relationship between initial plating density and the time to differentiation. We used nucleofection to express fluorescently tagged genes in RPE cells prior to plating on filters or baculovirus fusion constructs to transfect polarized monolayers. Both these methods resulted in transfection efficiencies over 40% and transgene expression lasted up to 8 days after plating. These filters were imaged by high-speed spinning disk microscopy to follow tubulovesicular trafficking of lysosomes and actin dynamics in the RPE. Four-dimensional image analysis performed using commercially available software was used to analyze live imaging data. In conclusion, this 3-step protocol describes a powerful method to investigate organelle trafficking and function in real time in the RPE that can be used for answering fundamental questions of RPE cell biology and pathobiology.

Protective responses to sublytic complement in the retinal pigment epithelium

Significance The complement system regulates immune defense and inflammation. Abnormal complement activation in the retina is associated with blinding macular degenerations, which have limited treatment options. The retinal pigment epithelium (RPE) is an initial site of injury in macular degenerations, but mechanisms that protect the RPE from complement-mediated damage are unclear. Here, we identify two critical responses to complement in the RPE: accelerated recycling of the complement inhibitor CD59 and lysosome-mediated membrane repair. We show that in models of macular degeneration, excess cholesterol impairs both defense mechanisms, resulting in mitochondrial damage. Drugs that decrease RPE cholesterol restore these mechanisms and help the RPE combat complement attack. Our studies identify promising drug targets to preserve RPE health and function in macular degenerations. The retinal pigment epithelium (RPE) is a key site of injury in inherited and age-related macular degenerations. Abnormal activation of the complement system is a feature of these blinding diseases, yet how the RPE combats complement attack is poorly understood. The complement cascade terminates in the cell-surface assembly of membrane attack complexes (MACs), which promote inflammation by causing aberrant signal transduction. Here, we investigated mechanisms crucial for limiting MAC assembly and preserving cellular integrity in the RPE and asked how these are compromised in models of macular degeneration. Using polarized primary RPE and the pigmented Abca4−/− Stargardt disease mouse model, we provide evidence for two protective responses occurring within minutes of complement attack, which are essential for maintaining mitochondrial health in the RPE. First, accelerated recycling of the membrane-bound complement regulator CD59 to the RPE cell surface inhibits MAC formation. Second, fusion of lysosomes with the RPE plasma membrane immediately after complement attack limits sustained elevations in intracellular calcium and prevents mitochondrial injury. Cholesterol accumulation in the RPE, induced by vitamin A dimers or oxidized LDL, inhibits these defense mechanisms by activating acid sphingomyelinase (ASMase), which increases tubulin acetylation and derails organelle traffic. Defective CD59 recycling and lysosome exocytosis after complement attack lead to mitochondrial fragmentation and oxidative stress in the RPE. Drugs that stimulate cholesterol efflux or inhibit ASMase restore both these critical safeguards in the RPE and avert complement-induced mitochondrial injury in vitro and in Abca4−/− mice, indicating that they could be effective therapeutic approaches for macular degenerations.

Should I stay or should I go? Trafficking of sub-lytic MAC in the retinal pigment epithelium.

Assembly of sub-lytic C5b-9 membrane attack complexes (MAC) on the plasma membrane of retinal pigment epithelial cells contributes to the pathogenesis of age-related macular degeneration. C5b-9 pores induce calcium influx, which activates signaling pathways that compromise cell function. Mechanisms that limit sub-lytic MAC activity include: cell surface complement regulatory proteins CD46, CD55, and CD59 that inhibit specific steps of MAC formation; elimination of assembled MAC by exocytosis of membrane vesicles or by endocytosis and subsequent lysosomal degradation; and rapid resealing of pores by the exocytosis of lysosomes. Aging in the post-mitotic retinal pigment epithelium is characterized by the accumulation of cellular debris called lipofuscin, which has also been associated with retinal diseases such as age-related macular degeneration. Lipofuscin has been shown to activate complement components both in vitro and in vivo, suggesting that it could contribute complement-mediated dysfunction in the retinal pigment epithelium. Here, we discuss emerging evidence that vesicular trafficking in the retinal pigment epithelium is critical for efficient removal of MAC from the cell surface and for limiting inflammation in the outer retina.

Cell biology: Caught in the traffic

In mice, deletion of the Rab8 protein disrupts organized molecular distribution to membranes of intestinal epithelial cells. Death by starvation follows, exactly as it does in humans with microvillus inclusion disease.

Apolipoprotein E Isoforms and AMD

The cholesterol transporting protein apolipoprotein E (ApoE) occurs in three allelic variants in humans unlike in other species. The resulting protein isoforms E2, E3 and E4 exhibit differences in lipid binding, integrating into lipoprotein particles and affinity for lipoprotein receptors. ApoE isoforms confer genetic risk for several diseases of aging including atherosclerosis, Alzheimers disease, and age-related macular degeneration (AMD). A single E4 allele increases the risk of developing Alzheimers disease, whereas the E2 allele is protective. Intriguingly, the E4 allele is protective in AMD. Current thinking about different functions of ApoE isoforms comes largely from studies on Alzheimers disease. These data cannot be directly extrapolated to AMD since the primary cells affected in these diseases (neurons vs. retinal pigment epithelium) are so different. Here, we propose that ApoE serves a fundamentally different purpose in regulating cholesterol homeostasis in the retinal pigment epithelium and this could explain why allelic risk factors are flipped for AMD compared to Alzheimers disease.