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Regenerative Medicine

Regenerative Medicine | 19 Publications
Kanu L, Ciolino J. Nerve Growth Factor as an Ocular Therapy: Applications, Challenges, and Future Directions. Semin Ophthalmol. 2021;36(4):224–231.
Nerve growth factor (NGF), the prototypical neurotrophin first discovered in the 1950s, has recently garnered increased interest as a therapeutic agent promoting neuronal health and regeneration. After gaining orphan drug status within the last decade, NGF-related research and drug development has accelerated. The purpose of this article is to review the preclinical and clinical evidence of NGF in various applications, including central and peripheral nervous system, skin, and ophthalmic disorders. We focus on the ophthalmic applications including not only the FDA-approved indication of neurotrophic keratitis but also retinal disease and glaucoma. NGF represents a promising therapy whose therapeutic profile is evolving. The challenges related to this therapy are reviewed, along with possible solutions and future directions.
Lu Y, Brommer B, Tian X, Krishnan A, Meer M, Wang C, Vera D, Zeng Q, Yu D, Bonkowski M, Yang JH, Zhou S, Hoffmann E, Karg M, Schultz M, Kane A, Davidsohn N, Korobkina E, Chwalek K, Rajman L, Church G, Hochedlinger K, Gladyshev VN, Horvath S, Levine M, Gregory-Ksander M, Ksander B, He Z, Sinclair D. Reprogramming to recover youthful epigenetic information and restore vision. Nature. 2020;588(7836):124–129.
Ageing is a degenerative process that leads to tissue dysfunction and death. A proposed cause of ageing is the accumulation of epigenetic noise that disrupts gene expression patterns, leading to decreases in tissue function and regenerative capacity. Changes to DNA methylation patterns over time form the basis of ageing clocks, but whether older individuals retain the information needed to restore these patterns-and, if so, whether this could improve tissue function-is not known. Over time, the central nervous system (CNS) loses function and regenerative capacity. Using the eye as a model CNS tissue, here we show that ectopic expression of Oct4 (also known as Pou5f1), Sox2 and Klf4 genes (OSK) in mouse retinal ganglion cells restores youthful DNA methylation patterns and transcriptomes, promotes axon regeneration after injury, and reverses vision loss in a mouse model of glaucoma and in aged mice. The beneficial effects of OSK-induced reprogramming in axon regeneration and vision require the DNA demethylases TET1 and TET2. These data indicate that mammalian tissues retain a record of youthful epigenetic information-encoded in part by DNA methylation-that can be accessed to improve tissue function and promote regeneration in vivo.
Akbari A, Jabbari N, Sharifi R, Ahmadi M, Vahhabi A, Seyedzadeh SJ, Nawaz M, Szafert S, Mahmoodi M, Jabbari E, Asghari R, Rezaie J. Free and hydrogel encapsulated exosome-based therapies in regenerative medicine. Life Sci. 2020;249:117447.
Over the last few decades, mesenchymal stem cells-derived exosomes (MSCs-Ex) have attracted a lot of attention as a therapeutic tool in regenerative medicine. Exosomes are extracellular vehicles (EVs) that play important roles in cell-cell communication through various processes such as stress response, senescence, angiogenesis, and cell differentiation. Success in the field of regenerative medicine sparked exploration of the potential use of exosomes as key therapeutic effectors of MSCs to promote tissue regeneration. Various approaches including direct injection, intravenous injection, intraperitoneal injection, oral administration, and hydrogel-based encapsulation have been exploited to deliver exosomes to target tissues in different disease models. Despite significant advances in exosome therapy, it is unclear which approach is more effective for administering exosomes. Herein, we critically review the emerging progress in the applications of exosomes in the form of free or association with hydrogels as therapeutic agents for applications in regenerative medicine.
Thanos S, Böhm M, Meyer Zu Hörste M, Prokosch-Willing V, Hennig M, Bauer D, Heiligenhaus A. Role of crystallins in ocular neuroprotection and axonal regeneration. Prog Retin Eye Res. 2014;42:145–61.
Neuroprotection is an emerging challenge in ophthalmology due to the particularly exposed location of retinal neurons and to the steadily increasing rate of intraocular surgical and pharmacological treatments applied to various eye diseases. Within few decades neuroprotection has developed from strongly contested approaches to being recognized and introduced as a potentially clinical application. One of the groups of putative substances for neuroprotection comprises αA- and αB-crystallins, which are types of heat-shock proteins and are considered to be molecular chaperones. The β/γ-crystallins form their own superfamily and are characterized as proteins with a distinct structure containing four Greek key motifs. Besides being abundant in the ocular lens, crystallins are also expressed in both the developing and mature retina. Crystallins are dramatically up-regulated in numerous retinal pathologies, including mechanical injury, ischemic insults, age-related macular degeneration, uveoretinitis, and diabetic retinopathy. Crystallins of the α family are thought to play a crucial role in retinal neuron survival and inflammation. Crystallins of the β/γ superfamily are also small proteins with a possible emerging role in retinal tissue remodeling and repair. One of the typical retinal diseases associated with crystallins is the experimental glaucomatous neuropathy that is characterized by their expression. Another typical retinal disease is the atrophy that occurs after mechanical injury to the optic nerve, which is associated with the need to regrow retinal axons. We have shown in regenerative models in vivo and in vitro that βB2-crystallin actively supports the regenerative growth of cut retinal axons, thereby offering targets for neuroprotective and regenerative treatments. In this review we discuss the discovery that βB2-crystallin is clearly up-regulated in the regenerating retina in vitro. βB2-Crystallin is produced and secreted during axon elongation, while β/γ-crystallins promote axon growth both in vivo and in vitro by acting either directly by uptake into cells, or indirectly by enhancing the production of ciliary neurotrophic factor from astrocytes to synergistically promote axon regrowth. We also discuss methods to induce the continuous production of crystallins at the site of injury and repair based on the use of transfected neural progenitor cells. This review ultimately leads to the conclusion that the postinjury fate of neurons cannot be seen merely as inevitable, but instead should be regarded as a challenge to shaping the neuroprotective and regenerative conditions that promote cell survival and axon repair.
Stern J, Tian Y, Funderburgh J, Pellegrini G, Zhang K, Goldberg J, Ali R, Young M, Xie Y, Temple S. Regenerating Eye Tissues to Preserve and Restore Vision. Cell Stem Cell. 2018;22(6):834–849.
Ocular regenerative therapies are on track to revolutionize treatment of numerous blinding disorders, including corneal disease, cataract, glaucoma, retinitis pigmentosa, and age-related macular degeneration. A variety of transplantable products, delivered as cell suspensions or as preformed 3D structures combining cells and natural or artificial substrates, are in the pipeline. Here we review the status of clinical and preclinical studies for stem cell-based repair, covering key eye tissues from front to back, from cornea to retina, and including bioengineering approaches that advance cell product manufacturing. While recognizing the challenges, we look forward to a deep portfolio of sight-restoring, stem cell-based medicine. VIDEO ABSTRACT.
Vavvas D, Dryja T, Wilson E, Olsen T, Shah A, Jurkunas U, Pineda R, Poulaki V, Palioura S, Veldman P, Moreno-Montañés J, Pinazo-Duran M, Pastor JC, Tsilimbaris M, Rhee D, Colby K, Hunter D, Thanos S, Sakamoto T, Pasquale L, Miller J, Vanderveen D, Lambert S. Lens regeneration in children. Nature. 2018;556(7699):E2-E3.
Levin L, Miller J, Zack D, Friedlander M, Smith L. Special Commentary: Early Clinical Development of Cell Replacement Therapy: Considerations for the National Eye Institute Audacious Goals Initiative. Ophthalmology. 2017;124(7):926–934.
The National Eye Institute launched the Audacious Goals Initiative (AGI) in 2013 with the aim "to restore vision through the regeneration of neurons and neural connections in the eye and visual system." An AGI Town Hall held at the Association for Research in Vision and Ophthalmology Annual Meeting in 2016 brought together basic, translational, and clinical scientists to address the clinical implications of the AGI, with a particular emphasis on diseases amenable to regenerative medicine and strategies to deal with barriers to progess. An example of such a barrier is that replacement of lost neurons may be insufficient because damage to other neurons and non-neuronal cells is common in retinal and optic nerve disease. Reparative processes such as gliosis and fibrosis also can make it difficult to replenish and regenerate neurons. Other issues include choice of animal models, selecting appropriate endpoints, ethics of informed consent, and regulatory issues. Another area critical to next steps in the AGI is the choice of target diseases and the stage at which early development studies should be focused. For example, an advantage of doing clinical trials in patients with early disease is that supporting cellular and structural constituents are still likely to be present. However, regenerative studies in patients with late disease make it easier to detect the effects of replacement therapy against the background of severe visual loss, whereas it may be harder to detect incremental improvement in visual function in those with early disease and considerable remaining visual function. Achieving the goals of the AGI also requires preclinical advances, new imaging techniques, and optimizing translational issues. The work of the AGI is expected to take at least 10 years but should eventually result in therapies to restore some degree of vision to the blind.
Utheim TP, Islam R, Fostad I, Eidet J, Sehic A, Olstad O, Dartt DA, Messelt E, Griffith M, Pasovic L. Storage Temperature Alters the Expression of Differentiation-Related Genes in Cultured Oral Keratinocytes.. PLoS One. 2016;11(3):e0152526.
PURPOSE: Storage of cultured human oral keratinocytes (HOK) allows for transportation of cultured transplants to eye clinics worldwide. In a previous study, one-week storage of cultured HOK was found to be superior with regard to viability and morphology at 12°C compared to 4°C and 37°C. To understand more of how storage temperature affects cell phenotype, gene expression of HOK before and after storage at 4°C, 12°C, and 37°C was assessed. MATERIALS AND METHODS: Cultured HOK were stored in HEPES- and sodium bicarbonate-buffered Minimum Essential Medium at 4°C, 12°C, and 37°C for one week. Total RNA was isolated and the gene expression profile was determined using DNA microarrays and analyzed with Partek Genomics Suite software and Ingenuity Pathway Analysis. Differentially expressed genes (fold change > 1.5 and P 0.05) were identified by one-way ANOVA. Key genes were validated using qPCR. RESULTS: Gene expression of cultures stored at 4°C and 12°C clustered close to the unstored control cultures. Cultures stored at 37°C displayed substantial change in gene expression compared to the other groups. In comparison with 12°C, 2,981 genes were differentially expressed at 37°C. In contrast, only 67 genes were differentially expressed between the unstored control and the cells stored at 12°C. The 12°C and 37°C culture groups differed most significantly with regard to the expression of differentiation markers. The Hedgehog signaling pathway was significantly downregulated at 37°C compared to 12°C. CONCLUSION: HOK cultures stored at 37°C showed considerably larger changes in gene expression compared to unstored cells than cultured HOK stored at 4°C and 12°C. The changes observed at 37°C consisted of differentiation of the cells towards a squamous epithelium-specific phenotype. Storing cultured ocular surface transplants at 37°C is therefore not recommended. This is particularly interesting as 37°C is the standard incubation temperature used for cell culture.
Boynton G, Raoof D, Niziol L, Hussain M, Mian S. Prospective Randomized Trial Comparing Efficacy of Topical Loteprednol Etabonate 0.5% Versus Cyclosporine-A 0.05% for Treatment of Dry Eye Syndrome Following Hematopoietic Stem Cell Transplantation.. Cornea. 2015;34(7):725–32.

PURPOSE: To evaluate the safety and efficacy of topical loteprednol etabonate (LE) 0.5% compared with cyclosporine A (CsA) 0.05% for the prophylaxis and treatment of dry eye syndrome (DES) after hematopoietic stem cell transplantation (HSCT). METHODS: Seventy-five patients were randomized to LE (n = 76 eyes of 38 patients) or CsA (n = 74 eyes of 37 patients) pre-HSCT. Lissamine green and fluorescein staining, tear break-up time, tear osmolarity (Osm), Schirmer score (Sch), intraocular pressure, visual acuity, and Ocular Surface Disease Index were assessed pre-HSCT, 3, 6, 9, and 12 months post-HSCT. RESULTS: There were no differences in DES incidence (P = 0.22; log-rank test) or progression (P = 0.41; log-rank test) between the 2 treatment arms during the course of the study. Among eyes with no DES at enrollment, the Kaplan-Meier analysis yielded a 90% rate of DES development in cyclosporine-treated eyes and a 79% rate of DES development in LE-treated eyes by 12 months post-HSCT. The Kaplan-Meier analysis of eyes with DES at enrollment demonstrated a 38% rate of disease progression among cyclosporine-treated eyes and a 26% rate of disease progression among loteprednol-treated eyes by 12 months. No patient in either group had an elevation of 10 mm Hg or greater from baseline at any study visit, and no patients had their treatment discontinued for elevation in intraocular pressure. CONCLUSIONS: Pre-HSCT initiation of LE 0.5% appears to be safe and may be as effective as CsA 0.5% for the treatment and prophylaxis of DES following HSCT.

Ma J, Guo C, Guo C, Sun Y, Liao T, Beattie U, López F, Chen DF, Lashkari K. Transplantation of Human Neural Progenitor Cells Expressing IGF-1 Enhances Retinal Ganglion Cell Survival.. PLoS One. 2015;10(4):e0125695.

We have previously characterized human neuronal progenitor cells (hNP) that can adopt a retinal ganglion cell (RGC)-like morphology within the RGC and nerve fiber layers of the retina. In an effort to determine whether hNPs could be used a candidate cells for targeted delivery of neurotrophic factors (NTFs), we evaluated whether hNPs transfected with an vector that expresses IGF-1 in the form of a fusion protein with tdTomato (TD), would increase RGC survival in vitro and confer neuroprotective effects in a mouse model of glaucoma. RGCs co-cultured with hNPIGF-TD cells displayed enhanced survival, and increased neurite extension and branching as compared to hNPTD or untransfected hNP cells. Application of various IGF-1 signaling blockers or IGF-1 receptor antagonists abrogated these effects. In vivo, using a model of glaucoma we showed that IOP elevation led to reductions in retinal RGC count. In this model, evaluation of retinal flatmounts and optic nerve cross sections indicated that only hNPIGF-TD cells effectively reduced RGC death and showed a trend to improve optic nerve axonal loss. RT-PCR analysis of retina lysates over time showed that the neurotrophic effects of IGF-1 were also attributed to down-regulation of inflammatory and to some extent, angiogenic pathways. This study shows that neuronal progenitor cells that hone into the RGC and nerve fiber layers may be used as vehicles for local production and delivery of a desired NTF. Transplantation of hNPIGF-TD cells improves RGC survival in vitro and protects against RGC loss in a rodent model of glaucoma. Our findings have provided experimental evidence and form the basis for applying cell-based strategies for local delivery of NTFs into the retina. Application of cell-based delivery may be extended to other disease conditions beyond glaucoma.

Katikireddy KR, Jurkunas U. Limbal Stromal Tissue Specific Stem Cells and Their Differentiation Potential to Corneal Epithelial Cells.. Methods Mol Biol. 2016;1341:437–44.

From the derivation of the first human embryonic stem (hES) cell line to the development of induced pluripotent stem (iPS) cells; it has become evident that tissue specific stem cells are able to differentiate into a specific somatic cell types. The understanding of key processes such as the signaling pathways and the role of the microenvironment in epidermal/epithelial development has provided important clues for the derivation of specific epithelial cell types.Various differentiation protocols/methods were used to attain specific epithelial cell types. Here, we describe in detail the procedure to follow for isolation of tissue specific stem cells, mimicking their microenvironment to attain stem cell characteristics, and their potential differentiation to corneal epithelial cells.

Shikari H, Amparo F, Saboo U, Dana R. Onset of ocular graft-versus-host disease symptoms after allogeneic hematopoietic stem cell transplantation.. Cornea. 2015;34(3):243–7.
OBJECTIVE: To study the factors affecting the time to onset of ocular graft-versus-host disease (GVHD) in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). METHODS: A retrospective chart review of 200 patients with ocular GVHD was performed to evaluate the association between various donor-recipient characteristics and the time to onset of ocular GVHD after allo-HSCT. RESULTS: The median time to onset of chronic ocular GVHD after allo-HSCT was 293 days (range, 26-2308 days). Patients receiving fully human leukocyte antigen (HLA)-matched transplants had a delayed onset of ocular GVHD (median, 294 days) compared with mismatched transplants (219 days; P = 0.029). HLA-matched transplants from related donors had delayed onset of ocular GVHD (307 days) compared with HLA-matched (286 days; P = 0.168) and HLA-mismatched (231 days; P = 0.015) transplants from unrelated donors. Ocular GVHD followed systemic GVHD in 76% of patients but preceded systemic disease in 7%, occurred concurrently in 15%, and was not associated with systemic GVHD in 2% of patients. The time elapsed between the occurrence of systemic and ocular GVHD was significantly longer in matched-related transplants (250 days) than in matched-unrelated transplants (120 days; P = 0.004). CONCLUSIONS: The onset of ocular GVHD after allo-HSCT is variable and is influenced by donor-recipient matching characteristics. In the majority of patients with GVHD, ocular involvement follows the occurrence of systemic manifestations; however, importantly, it can also precede or develop independently of systemic disease in a minority of patients. Regular ophthalmic follow-up is recommended after allo-HSCT regardless of concurrent systemic GVHD status.
Schwartz S, Regillo C, Lam B, Eliott D, Rosenfeld P, Gregori N, Hubschman JP, Davis J, Heilwell G, Spirn M, Maguire J, Gay R, Bateman J, Ostrick R, Morris D, Vincent M, Anglade E, Del Priore L, Lanza R. Human embryonic stem cell-derived retinal pigment epithelium in patients with age-related macular degeneration and Stargardt’s macular dystrophy: follow-up of two open-label phase 1/2 studies.. Lancet. 2015;385(9967):509–16.

BACKGROUND: Since they were first derived more than three decades ago, embryonic stem cells have been proposed as a source of replacement cells in regenerative medicine, but their plasticity and unlimited capacity for self-renewal raises concerns about their safety, including tumour formation ability, potential immune rejection, and the risk of differentiating into unwanted cell types. We report the medium-term to long-term safety of cells derived from human embryonic stem cells (hESC) transplanted into patients. METHODS: In the USA, two prospective phase 1/2 studies were done to assess the primary endpoints safety and tolerability of subretinal transplantation of hESC-derived retinal pigment epithelium in nine patients with Stargardt's macular dystrophy (age >18 years) and nine with atrophic age-related macular degeneration (age >55 years). Three dose cohorts (50,000, 100,000, and 150,000 cells) were treated for each eye disorder. Transplanted patients were followed up for a median of 22 months by use of serial systemic, ophthalmic, and imaging examinations. The studies are registered with ClinicalTrials.gov, numbers NCT01345006 (Stargardt's macular dystrophy) and NCT01344993 (age-related macular degeneration). FINDINGS: There was no evidence of adverse proliferation, rejection, or serious ocular or systemic safety issues related to the transplanted tissue. Adverse events were associated with vitreoretinal surgery and immunosuppression. 13 (72%) of 18 patients had patches of increasing subretinal pigmentation consistent with transplanted retinal pigment epithelium. Best-corrected visual acuity, monitored as part of the safety protocol, improved in ten eyes, improved or remained the same in seven eyes, and decreased by more than ten letters in one eye, whereas the untreated fellow eyes did not show similar improvements in visual acuity. Vision-related quality-of-life measures increased for general and peripheral vision, and near and distance activities, improving by 16-25 points 3-12 months after transplantation in patients with atrophic age-related macular degeneration and 8-20 points in patients with Stargardt's macular dystrophy. INTERPRETATION: The results of this study provide the first evidence of the medium-term to long-term safety, graft survival, and possible biological activity of pluripotent stem cell progeny in individuals with any disease. Our results suggest that hESC-derived cells could provide a potentially safe new source of cells for the treatment of various unmet medical disorders requiring tissue repair or replacement. FUNDING: Advanced Cell Technology.

Sümbül U, Zlateski A, Vishwanathan A, Masland R, Seung S. Automated computation of arbor densities: a step toward identifying neuronal cell types. Front Neuroanat. 2014;8:139.
The shape and position of a neuron convey information regarding its molecular and functional identity. The identification of cell types from structure, a classic method, relies on the time-consuming step of arbor tracing. However, as genetic tools and imaging methods make data-driven approaches to neuronal circuit analysis feasible, the need for automated processing increases. Here, we first establish that mouse retinal ganglion cell types can be as precise about distributing their arbor volumes across the inner plexiform layer as they are about distributing the skeletons of the arbors. Then, we describe an automated approach to computing the spatial distribution of the dendritic arbors, or arbor density, with respect to a global depth coordinate based on this observation. Our method involves three-dimensional reconstruction of neuronal arbors by a supervised machine learning algorithm, post-processing of the enhanced stacks to remove somata and isolate the neuron of interest, and registration of neurons to each other using automatically detected arbors of the starburst amacrine interneurons as fiducial markers. In principle, this method could be generalizable to other structures of the CNS, provided that they allow sparse labeling of the cells and contain a reliable axis of spatial reference.
Zhu R, Cho KS, Chen DF, Yang L. Ephrin-A2 and -A3 are negative regulators of the regenerative potential of Möller cells. Chin Med J (Engl). 2014;127(19):3438–42.
BACKGROUND: In a previous study, we demonstrated that ephrin-A2 and -A3 negatively regulate the growth of neural progenitor cells in the central nervous system. Adult mice deficient in ephrin-A2 and -A3 (A2(-/-)A3(-/-)) displayed active ongoing neurogenesis throughout the brain, and mice deficient in ephrin-A3 alone showed increased proliferation of ciliary epithelium derived retinal stem cells. This study aimed to detect that the increase in proliferation and neurogenic potential of Müller cells is influenced by the absence of ephrin-A2 and -A3. METHODS: We assessed the retinal and Müller cell expression of ephrin-As and their receptor and neural progenitor cell markers by immunostaining and real-time PCR. We cultured purified primary Müller cells derived from wild-type and A2(-/-)A3(-/-) mice in a defined culture medium that enables trans-differentiation of Müller cells into retinal neurons. To evaluate proliferating Müller cells in vivo, we injected 5'-ethylnyl-2'-deoxiuridine (EdU) intraperitoneally to adult mice. RESULTS: Expression of ephrin-A2/A3 and their receptor EphA4 were detected in the retinas of adult mice, with EphA4 expression particularly enriched in Müller cells. Müller cells of A2(-/-)A3(-/-) mice exhibited significantly elevated expression of retinal progenitor cell markers, Pax6 and Chx10, when compared with those from wild-type mice. Moreover, a higher percentage of Müller cells of A2(-/-)A3(-/-) mice trans-differentiated and became recoverin+ and β-III-tublin+ in the culture than those from wild type mice. Strikingly, an increased number of EdU+ retinal cells was detected in the retinas of adult A2(-/-)A3(-/-) mice as compared with wild-type mice. CONCLUSIONS: Ephrin-A2 and -A3 are negative regulators of the proliferative and neurogenic potentials of Müller cells. Manipulating ephrin-A signaling may thus represent a novel strategy for stimulating neuroregeneration from endogenous progenitors to participate in retinal repair in case of disease or damage.
Sanderson J, Dartt DA, Trinkaus-Randall V, Pintor J, Civan M, Delamere N, Fletcher E, Salt T, Grosche A, Mitchell C. Purines in the eye: recent evidence for the physiological and pathological role of purines in the RPE, retinal neurons, astrocytes, Müller cells, lens, trabecular meshwork, cornea and lacrimal gland. Exp Eye Res. 2014;127:270–9.
This review highlights recent findings that describ how purines modulate the physiological and pathophysiological responses of ocular tissues. For example, in lacrimal glands the cross-talk between P2X7 receptors and both M3 muscarinic receptors and α1D-adrenergic receptors can influence tear secretion. In the cornea, purines lead to post-translational modification of EGFR and structural proteins that participate in wound repair in the epithelium and influence the expression of matrix proteins in the stroma. Purines act at receptors on both the trabecular meshwork and ciliary epithelium to modulate intraocular pressure (IOP); ATP-release pathways of inflow and outflow cells differ, possibly permitting differential modulation of adenosine delivery. Modulators of trabecular meshwork cell ATP release include cell volume, stretch, extracellular Ca(2+) concentration, oxidation state, actin remodeling and possibly endogenous cardiotonic steroids. In the lens, osmotic stress leads to ATP release following TRPV4 activation upstream of hemichannel opening. In the anterior eye, diadenosine polyphosphates such as Ap4A act at P2 receptors to modulate the rate and composition of tear secretion, impact corneal wound healing and lower IOP. The Gq11-coupled P2Y1-receptor contributes to volume control in Müller cells and thus the retina. P2X receptors are expressed in neurons in the inner and outer retina and contribute to visual processing as well as the demise of retinal ganglion cells. In RPE cells, the balance between extracellular ATP and adenosine may modulate lysosomal pH and the rate of lipofuscin formation. In optic nerve head astrocytes, mechanosensitive ATP release via pannexin hemichannels, coupled with stretch-dependent upregulation of pannexins, provides a mechanism for ATP signaling in chronic glaucoma. With so many receptors linked to divergent functions throughout the eye, ensuring the transmitters remain local and stimulation is restricted to the intended target may be a key issue in understanding how physiological signaling becomes pathological in ocular disease.