Preclinical evaluation of ADVM-062, a novel intravitreal gene therapy vector for the treatment of blue cone monochromacy

Blue cone monochromacy (BCM) is a rare X-linked retinal disease characterized by the absence of L- and M-opsin in cone photoreceptors, considered a potential gene therapy candidate. However, most experimental ocular gene therapies utilize subretinal vector injection which would pose a risk to the fragile central retinal structure of BCM patients. Here we describe the use of ADVM-062, a vector optimized for cone-specific expression of human L-opsin and administered using a single intravitreal (IVT) injection. Pharmacological activity of ADVM-062 was established in gerbils, whose cone-rich retina naturally lacks L-opsin. A single IVT administration dose of ADVM-062 effectively transduced gerbil cone photoreceptors and produced a de novo response to long-wavelength stimuli. To identify potential first-in-human doses we evaluated ADVM-062 in non-human primates. Cone-specific expression of ADVM-062 in primates was confirmed using ADVM-062.myc, a vector engineered with the same regulatory elements as ADVM-062. Enumeration of human OPN1LW.myc-positive cones demonstrated that doses ≥3 × 1010 vg/eye resulted in transduction of 18%-85% of foveal cones. A Good Laboratory Practice (GLP) toxicology study established that IVT administration of ADVM-062 was well tolerated at doses that could potentially achieve clinically meaningful effect, thus supporting the potential of ADVM-062 as a one-time IVT gene therapy for BCM.

Long-Term Safety Evaluation of Continuous Intraocular Delivery of Aflibercept by the Intravitreal Gene Therapy Candidate ADVM-022 in Nonhuman Primates

Purpose

To evaluate the long-term safety of vascular endothelial growth factor (VEGF) suppression with sustained aflibercept expression after a single intravitreal injection (IVI) of ADVM-022, an anti-VEGF gene therapy, in non-human primates (NHPs).

Methods

Non-human primates received bilateral IVI of ADVM-022, a gene therapy vector encoding aflibercept, a standard of care for the treatment of VEGF-based retinal disease. Aflibercept levels from ocular fluids and tissues were measured. Ocular inflammation was assessed by slit lamp biomicroscopy and fundoscopy. The integrity of the retinal structure was analyzed by optical coherence tomography and blue light fundus autofluorescence and electroretinography was performed to determine retinal function. Histologic evaluation of the retina was performed at the longest time point measured (2.5 years after injection).

Results

Sustained expression of aflibercept was noted out to the last time point evaluated. Mild to moderate inflammatory responses were observed, which trended toward spontaneous resolution without anti-inflammatory treatment. No abnormalities in retinal structure or function were observed, as measured by optical coherence tomography and electroretinography, respectively. RPE integrity was maintained throughout the study; no histologic abnormalities were observed 2.5 years after ADVM-022 IVI.

Conclusions

In non-human primates, long-term, sustained aflibercept expression and the resulting continuous VEGF suppression by a single IVI of ADVM-022, appears to be safe, with no measurable adverse effects on normal retinal structure and function evaluated out to 2.5 years.

Translational Relevance

Together with the results from previous ADVM-022 preclinical studies, these data support the evaluation of this gene therapy candidate in clinical trials as a potential durable treatment for various VEGF-mediated ophthalmic disorders.

Brain-derived neurotrophic factor and TrkB modulate visual experience-dependent refinement of neuronal pathways in retina

Sensory experience refines neuronal structure and functionality. The visual system has proved to be a productive model system to study this plasticity. In the neonatal retina, the dendritic arbors of a large proportion of ganglion cells are diffuse in the inner plexiform layer. With maturation, many of these arbors become monolaminated. Visual deprivation suppresses this remodeling. Little is known of the molecular mechanisms controlling maturational and experience-dependent refinement. Here, we tested the hypothesis that brain-derived neurotrophic factor (BDNF), which is known to regulate dendritic branching and synaptic function in the brain, modulates the developmental and visual experience-dependent refinement of retinal ganglion cells. We used a transgenic mouse line, in which a small number of ganglion cells were labeled with yellow fluorescence protein, to delineate their dendritic structure in vivo. We found that transgenic overexpression of BDNF accelerated the laminar refinement of ganglion cell dendrites, whereas decreased TrkB expression or retina-specific deletion of TrkB, the cognate receptor for BDNF, retarded it. BDNF-TrkB signaling regulated the maturational formation of new branches in ON but not the bilaminated ON-OFF ganglion cells. Furthermore, BDNF overexpression overrides the requirement for visual inputs to stimulate laminar refinement and dendritic branching of ganglion cells. These experiments reveal a previously unrecognized action of BDNF and TrkB in controlling cell-specific, experience-dependent remodeling of neuronal structures in the visual system.

CAPS Acts at a Prefusion Step in Dense-Core Vesicle Exocytosis as a PIP2 Binding Protein

CAPS-1 is required for Ca2+-triggered fusion of dense-core vesicles with the plasma membrane, but its site of action and mechanism are unknown. We analyzed the kinetics of Ca2+-triggered exocytosis reconstituted in permeable PC12 cells. CAPS-1 increased the initial rate of Ca2+-triggered vesicle exocytosis by acting at a rate-limiting, Ca2+-dependent prefusion step. CAPS-1 activity depended upon prior ATP-dependent priming during which PIP2 synthesis occurs. CAPS-1 activity and binding to the plasma membrane depended upon PIP2. Ca2+ was ineffective in triggering vesicle fusion in the absence of CAPS-1 but instead promoted desensitization to CAPS-1 resulting from decreased plasma membrane PIP2. We conclude that CAPS-1 functions following ATP-dependent priming as a PIP2 binding protein to enhance Ca2+-dependent DCV exocytosis. Essential prefusion steps in dense-core vesicle exocytosis involve sequential ATP-dependent synthesis of PIP2 and the subsequent PIP2-dependent action of CAPS-1. Regulation of PIP2 levels and CAPS-1 activity would control the secretion of neuropeptides and monoaminergic transmitters.

PC12 cells as a model for studies of regulated secretion in neuronal and endocrine cells

Pheochromocytoma-derived cell lines such as PC12 cells maintain a differentiated neuroendocrine phenotype and have been widely used as a convenient model system for a wide variety of cell biological studies on neurotrophin action, monoamine biogenesis, protein trafficking, and secretory vesicle dynamics. This chapter reviews a number of methods that are useful for studies of the regulated dense core vesicle secretory pathway. This includes protocols for maintaining cells and preserving their phenotype. A variety of assays are discussed for monitoring secretion in intact or permeable cells and in transfected cells. Specific methods for immunocytochemical studies in permeable cells are discussed. Finally, protocols for high-efficiency PC12 cell transfections and the isolation of stably transfected cell lines are provided.

A Family of Ca2+-Dependent Activator Proteins for Secretion

Ca2+-dependent activator protein for secretion (CAPS) 1 is an essential cytosolic component of the protein machinery involved in large dense-core vesicle (LDCV) exocytosis and in the secretion of a subset of neurotransmitters. In the present study, we report the identification, cloning, and comparative characterization of a second mammalian CAPS isoform, CAPS2. The structure of CAPS2 and its function in LDCV exocytosis from PC12 cells are very similar to those of CAPS1. Both isoforms are strongly expressed in neuroendocrine cells and in the brain. In subcellular fractions of the brain, both CAPS isoforms are enriched in synaptic cytosol fractions and also present on vesicular fractions. In contrast to CAPS1, which is expressed almost exclusively in brain and neuroendocrine tissues, CAPS2 is also expressed in lung, liver, and testis. Within the brain, CAPS2 expression seems to be restricted to certain brain regions and cell populations, whereas CAPS1 expression is strong in all neurons. During development, CAPS2 expression is constant between embryonic day 10 and postnatal day 60, whereas CAPS1 expression is very low before birth and increases after postnatal day 0 to reach a plateau at postnatal day 21. Light microscopic data indicate that both CAPS isoforms are specifically enriched in synaptic terminals. Ultrastructural analyses show that CAPS1 is specifically localized to glutamatergic nerve terminals. We conclude that at the functional level, CAPS2 is largely redundant with CAPS1. Differences in the spatial and temporal expression patterns of the two CAPS isoforms most likely reflect as yet unidentified subtle functional differences required in particular cell types or during a particular developmental period. The abundance of CAPS proteins in synaptic terminals indicates that they may also be important for neuronal functions that are not exclusively related to LDCV exocytosis.

Membrane association domains in Ca2+-dependent activator protein for secretion mediate plasma membrane and dense-core vesicle binding required for Ca2+-dependent exocytosis

Ca2+-dependent activator protein for secretion (CAPS) is a cytosolic protein essential for the Ca2+-dependent fusion of dense-core vesicles (DCVs) with the plasma membrane and the regulated secretion of a subset of neurotransmitters. The mechanism by which CAPS functions in exocytosis and the means by which it associates with target membranes are unknown. We identified two domains in CAPS with distinct membrane-binding properties that were each essential for CAPS activity in regulated exocytosis. The first of these, a centrally located pleckstrin homology domain, exhibited three properties: charge-based binding to acidic phospholipids, binding to plasma membrane but not DCV membrane, and stereoselective binding to phosphatidylinositol 4,5-bisphosphate. Mutagenesis studies revealed that the former two properties but not the latter were essential for CAPS function. The central pleckstrin homology domain may mediate transient CAPS interactions with the plasma membrane during Ca2+-triggered exocytosis. The second membrane association domain comprising distal C-terminal sequences mediated CAPS targeting to and association with neuroendocrine DCVs. The CAPS C-terminal domain was also essential for optimal activity in regulated exocytosis. The presence of two membrane association domains with distinct binding specificities may enable CAPS to bind both target membranes to facilitate DCV-plasma membrane fusion.

Evidence for two chemosensory pathways in Rhodobacter sphaeroides

In contrast to enteric bacteria, chemotaxis in Rhodobacter sphaeroides requires transport and partial metabolism of chemoattractants. Although a chemotaxis operon has been identified containing homologues of the enteric cheA, cheW, cheR genes and two homologues of the cheY gene, deletion of the entire chemotaxis operon had only minor effects on chemotactic behaviour under the conditions tested. Responses to sugars were enhanced in tethered cells but in all other chemotaxis assays behaviour of the operon deletion mutant was wild type. The mutant also showed wild-type responses to weak organic acids such as acetate and propionate, the dominant chemoattractants for this organism, under all conditions. This is in direct contrast to the enterics in which CheA, CheW and CheY are absolutely essential for taxis to PTS sugars, oxygen and MCP-dependent chemoeffectors. The operon deletion mutant was subjected to Tn5 transposon mutagenesis and new mutants selected using a chemotaxis and phototaxis screen. One mutant, JPA203, was non-chemotactic on swarm plates and showed inverted responses when tethered or subjected to changes in light intensity. Characterization of the Tn5 insertion in JPA203 identified a second chemotaxis operon in R. sphaeroides that contains homologues of cheY, cheA and cheR, the first homologue of cheB and two homologues of cheW. The new genes were labelled orf10, cheY(III), cheA(II) cheW(II), cheW(III), cheR(II), cheB and tlpC. When introduced into a wild-type background, deletion of cheA(II) produced a chemotaxis minus phenotype in R. sphaeroides, suggesting that cheA(II) forms part of a dominant chemotactic pathway, whereas the earlier identified operon plays only a minor role under laboratory conditions. The data presented here support the existence of two chemosensory pathways in R. sphaeroides, a feature that so far is unique in bacterial chemotaxis.

Antimicrobial action of propolis and some of its components: the effects on growth, membrane potential and motility of bacteria

The effect of the natural bee product propolis on the physiology of microorganisms was investigated using B. subtilis, E. coli and R. sphaeroides. An ethanolic extract of propolis had a bactericidal effect caused by the presence of very active, but labile, ingredients. The exact bactericidal effect of propolis was species dependent: it was effective against gram-positive and some gram-negative bacteria. Propolis and some of its cinnamic and flavonoid components were found to uncouple the energy transducing cytoplasmic membrane and to inhibit bacterial motility. These effects on the bioenergetic status of the membrane may contribute to the antimicrobial action of propolis and its observed synergism with selected antibiotics.

Mechanisms of oxygen taxis in bacteria

Since 1881 when Englemann reported aerotaxis in bacteria, an understanding of the molecular nature of the signal transduction remains a daring goal for microbiologists. This short review discusses known facts and recent advances in the field including the discovery of the flavoprotein receptor which drives Escherichia coli towards oxygen. Possible mechanisms of oxygen sensing in various bacterial species are considered in connection with the existing, often fragmental, data on phototaxis, redox taxis and taxis repellent effect of the reactive oxygen species (ROS).

Photoresponses in Rhodobacter sphaeroides: role of photosynthetic electron transport

Rhodobacter sphaeroides responds to a decrease in light intensity by a transient stop followed by adaptation. There is no measurable response to increases in light intensity. We confirmed that photosynthetic electron transport is essential for a photoresponse, as (i) inhibitors of photosynthetic electron transport inhibit photoresponses, (ii) electron transport to oxidases in the presence of oxygen reduces the photoresponse, and (iii) the magnitude of the response is dependent on the photopigment content of the cells. The photoresponses of cells grown in high light, which have lower concentrations of light-harvesting photopigment and reaction centers, saturated at much higher light intensities than the photoresponses of cells grown in low light, which have high concentrations of light-harvesting pigments and reaction centers. We examined whether the primary sensory signal from the photosynthetic electron transport chain was a change in the electrochemical proton gradient or a change in the rate of electron transport itself (probably reflecting redox sensing). R. sphaeroides showed no response to the addition of the proton ionophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone, which decreased the electrochemical proton gradient, although a behavioral response was seen to a reduction in light intensity that caused an equivalent reduction in proton gradient. These results strongly suggest that (i) the photosynthetic apparatus is the primary photoreceptor, (ii) the primary signal is generated by a change in the rate of electron transport, (iii) the change in the electrochemical proton gradient is not the primary photosensory signal, and (iv) stimuli affecting electron transport rates integrate via the electron transport chain.

delta psi-mediated signalling in the bacteriorhodopsin-dependent photoresponse

It has been shown previously that the proton-pumping activity of bacteriorhodopsin from Halobacterium salinarium can transmit an attractant signal to the bacterial flagella upon an increase in light intensity over a wide range of wavelengths. Here, we studied the effect of blue light on phototactic responses by the mutant strain Pho8l-B4, which lacks both sensory rhodopsins but has the ability to synthesize bacteriorhodopsin. Under conditions in which bacteriorhodopsin was largely accumulated as the M412 bacteriorhodopsin photocycle intermediate, halobacterial cells responded to blue light as a repellent. This response was pronounced when the membrane electric potential level was high in the presence of arginine, active oxygen consumption, or high-background long-wavelength light intensity but was inhibited by an uncoupler of oxidative phosphorylation (carbonyl cyanide 3-chlorophenylhydrazone) and was inverted in a background of low long-wavelength light intensity. The response to changes in the intensity of blue light under high background light was asymmetric, since removal of blue light did not produce an expected suppression of reversals. Addition of ammonium acetate, which is known to reduce the pH gradient changes across the membrane, did not inhibit the repellent effect of blue light, while the discharge of the membrane electric potential by tetraphenylphosphonium ions inhibited this sensory reaction. We conclude that the primary signal from bacteriorhodopsin to the sensory pathway involves changes in membrane potential.

Bacteriorhodopsin is involved in halobacterial photoreception

The bacterio-opsin gene was introduced into a "blind" Halobacterium salinarium mutant that (i) lacked all the four retinal proteins [bacteriorhodopsin (BR), halorhodopsin, and sensory rhodopsins (SRs) I and II] and the transducer protein for SRI and (ii) showed neither attractant response to long wavelength light nor repellent response to short wavelength light. The resulting transformed cells acquired the capability to sense light stimuli. The cells accumulated in a light spot, demonstrating the BR-mediated orientation in spatial light gradients. As in wild-type cells, a decrease in the intensity of long wavelength light caused a repellent response by inducing reversals of swimming direction, but, in contrast to wild-type cells, a decrease in the intensity of short wavelength light also repelled the cells. An increase in light intensity evoked an attractant response (i.e., a transient suppression of spontaneous reversals). Signal processing times and adaptation kinetics were similar to the SRI-mediated reactions. However, compared to SR-mediated photoresponses, higher light intensities were necessary to induce the BR-mediated responses. The light sensitivity of the transformant was increased by adding 1 mM cyanide and decreased by the addition of arginine, agents that respectively reduce and increase the light-independent generation of the electrochemical potential difference of H+ ions (delta mu H+). A decrease in irradiance to an intensity that was still high enough to saturate BR-initiated delta mu H+ changes failed to induce the repellent effect, but the addition of a protonophorous uncoupler sensitized the cell to these light stimuli. The BR D96N mutant (Asp-96 is replaced by Asn) with decreased proton pump activity showed strongly reduced BR-mediated responses. Azide, which increases this mutant's H+ pump efficiency, increased the photosensitivity of the mutant cells. Moreover, azide diminished (i) the membrane potential decreasing and (ii) repellent effects of blue light added to the orange background illumination in this mutant. We conclude that the BR-mediated photoreception is due to the light-dependent generation of delta mu H+. Our data are consistent with the assumption that the H. salinarium cell monitors the membrane energization level with a "protometer" system measuring total delta mu H+ changes or its electric potential difference component.

The proton pump bacteriorhodopsin is a photoreceptor for signal transduction in Halobacterium halobium

Halobacterium halobium swims by rotating its polarly inserted flagellar bundle. The cells are attracted by green-to-orange light which they can use for photophosphorylation but flee damaging blue or ultraviolet light. It is generally believed that this kind of 'colour vision' is achieved by the combined action of two photoreceptor proteins, sensory rhodopsins-I and -II, that switch in the light the rotational sense of the bundle and in consequence the swimming direction of a cell. By expressing the bacteriorhodopsin gene in a photoreceptor-negative background we have now demonstrated the existence of a proton-motive force sensor (protometer) and the function of bacteriorhodopsin as an additional photoreceptor covering the high intensity range. When the bacteriorhodopsin-generated proton-motive force drops caused by a sudden decrease in light intensity, the cells respond by reversing their swimming direction. This response does not occur when the proton-motive force is saturated by respiration or fermentation.