Endogenous production of extracellular adenosine by trabecular meshwork cells: potential role in outflow regulation

Development and testing of a metabolic chamber for effluent collection during whole eye perfusions

Ocular hypertension is caused by dysregulated outflow resistance regulation by the conventional outflow (CO) pathway. The physiology of the CO pathway can be directly studied during ex vivo ocular perfusions. In addition to measuring outflow resistance generation by the CO tissues, perfusion media that is conditioned by CO pathway cells can be collected upon exiting the eye as effluent. Thus, contents of effluent include factors contributed by upstream cells that report on the (dys)functionality of the outflow tissues. Two methods have been used in the past to monitor effluent contents from perfused eyes, each with their limitations. To overcome these limitations, we designed and printed a metabolic chamber to accommodate eyes of different sizes during perfusions. To test this new chamber, human eyes were perfused for 4 h at constant flow rate of 2.5 μl/min, while pressure was continuously monitored and effluent was collected every hour. Facility was 0.28 ± 0.16 μl/min/mmHg for OD eyes and 0.33 ± 0.11 μl/min/mmHg for OS eyes (n = 3). Effluent samples were protein rich, with protein concentration ranging from 2700 to 10,000 μg/ml for all eyes and timepoints (N = 3). Effluent samples expressed proteins that were actively secreted by the TM and easily detectible including MYOC and MMP2. Taken together, our model provides a reliable method to collect effluent from ex vivo human eyes, while maintaining whole globe integrity.

Modeling the biomechanics of the conventional aqueous outflow pathway microstructure in the human eye

BACKGROUND AND OBJECTIVE

Intraocular pressure (IOP) is determined by aqueous humor outflow resistance, which is a function of the combined resistance of Schlemm's canal (SC) endothelium and the trabecular meshwork (TM) and their interactions in the juxtacanalicular connective tissue (JCT) region. Aqueous outflow in the conventional outflow pathway results in pressure gradient across the TM, JCT, and SC inner wall, and induces mechanical stresses and strains that influence the geometry and homeostasis of the outflow system. The outflow resistance is affected by alteration in tissues' geometry, so there is potential for active, two-way, fluid-structure interaction (FSI) coupling between the aqueous humor (fluid) and the TM, JCT, and SC inner wall (structure). However, our understanding of the biomechanical interactions of the aqueous humor with the outflow connective tissues and its contribution to the outflow resistance regulation is incomplete.

METHODS

In this study, a microstructural finite element (FE) model of a human eye TM, JCT, and SC inner wall was constructed from a segmented, high-resolution histologic 3D reconstruction of the human outflow system. Three different elastic moduli (0.004, 0.128, and 51.5 MPa based on prior reports) were assigned to the TM/JCT complex while the elastic modulus of the SC inner wall was kept constant at 0.00748 MPa. The hydraulic conductivity was programmed separately for the TM, JCT, and SC inner wall using a custom subroutine. Cable elements were embedded into the TM and JCT extracellular matrix to represent the directional stiffness imparted by anisotropic collagen fibril orientation. The resultant stresses and strains in the outflow system were calculated using fluid-structure interaction method.

RESULTS

The higher TM/JCT stiffness resulted in larger stresses, but smaller strains in the outflow connective tissues, and resulted in a 4- and 5-fold larger pressure drop across the SC inner wall, respectively, compared to the most compliant model. Funneling through µm-sized SC endothelial pores was evident in the models at lower tissue stiffness, but aqueous flow was more turbulent in models with higher TM/JCT stiffness.

CONCLUSIONS

The mechanical properties of the outflow tissues play a crucial role in the hydrodynamics of the aqueous humor in the conventional outflow system.

Variability in CD39 and CD73 protein levels in uveal melanoma patients

Extracellular adenosine is produced from ATP by CD39 and CD73, and can modulate tumor development by acting on cancer cells or immune cells. Adenosine metabolism has been poorly studied in uveal melanoma. We studied the protein levels of CD39 and CD73 in a small, well described cohort of patients with uveal melanoma. Our results show a high variability in the levels of the two proteins, both in positivity and in intensity. Our results suggest that similar studies on larger cohorts could determine the clinical value and the druggability of these enzymes in the given clinical setting.Supplemental data for this article is available online at http://dx.doi.org/10.1080/15257770.2022.2032738.

Adenosine: The common target between cancer immunotherapy and glaucoma in the eye

Adenosine, an endogenous purine nucleoside, is a well-known actor of the immune system and the inflammatory response both in physiologic and pathologic conditions. By acting upon particular, G-protein coupled adenosine receptors, i.e., A1, A2- a & b, and A3 receptors mediate a variety of intracellular and immunomodulatory actions. Several studies have elucidated Adenosine's effect and its up-and downstream molecules and enzymes on the anti-tumor response against several types of cancers. We have also targeted a couple of molecules to manipulate this pathway and get the immune system's desired response in our previous experiences. Besides, the outgrowth of the studies on ocular Adenosine in recent years has significantly enhanced the knowledge about Adenosine and its role in ocular immunology and the inflammatory response of the eye. Glaucoma is the second leading cause of blindness globally, and the recent application of Adenosine and its derivatives has shown the critical role of the adenosine pathway in its pathophysiology. However, despite a very promising background, the phase III clinical trial of Trabodenoson failed to achieve the non-inferiority goals of the study. In this review, we discuss different aspects of the abovementioned pathway in ophthalmology and ocular immunology; following a brief evaluation of the current immunotherapeutic strategies, we try to elucidate the links between cancer immunotherapy and glaucoma in order to introduce novel therapeutic targets for glaucoma.

The vital role for nitric oxide in intraocular pressure homeostasis

Catalyzed by endothelial nitric oxide (NO) synthase (eNOS) activity, NO is a gaseous signaling molecule maintaining endothelial and cardiovascular homeostasis. Principally, NO regulates the contractility of vascular smooth muscle cells and permeability of endothelial cells in response to either biochemical or biomechanical cues. In the conventional outflow pathway of the eye, the smooth muscle-like trabecular meshwork (TM) cells and Schlemm's canal (SC) endothelium control aqueous humor outflow resistance, and therefore intraocular pressure (IOP). The mechanisms by which outflow resistance is regulated are complicated, but NO appears to be a key player as enhancement or inhibition of NO signaling dramatically affects outflow function; and polymorphisms in NOS3, the gene that encodes eNOS modifies the relation between various environmental exposures and glaucoma. Based upon a comprehensive review of past foundational studies, we present a model whereby NO controls a feedback signaling loop in the conventional outflow pathway that is sensitive to changes in IOP and its oscillations. Thus, upon IOP elevation, the outflow pathway tissues distend, and the SC lumen narrows resulting in increased SC endothelial shear stress and stretch. In response, SC cells upregulate the production of NO, relaxing neighboring TM cells and increasing permeability of SC's inner wall. These IOP-dependent changes in the outflow pathway tissues reduce the resistance to aqueous humor drainage and lower IOP, which, in turn, diminishes the biomechanical signaling on SC. Similar to cardiovascular pathogenesis, dysregulation of the eNOS/NO system leads to dysfunctional outflow regulation and ocular hypertension, eventually resulting in primary open-angle glaucoma.

A model of the oscillatory mechanical forces in the conventional outflow pathway

Intraocular pressure is regulated by mechanosensitive cells within the conventional outflow pathway, the primary route of aqueous humour drainage from the eye. However, the characteristics of the forces acting on those cells are poorly understood. We develop a model that describes flow through the conventional outflow pathway, including the trabecular meshwork (TM) and Schlemm’s canal (SC). Accounting for the ocular pulse, we estimate the time-varying shear stress on SC endothelium and strain on the TM. We consider a range of outflow resistances spanning normotensive to hypertensive conditions. Over this range, the SC shear stress increases significantly and becomes highly oscillatory. TM strain also increases, but with negligible oscillations. Interestingly, TM strain responds more to changes in outflow resistance around physiological values, while SC shear stress responds more to elevated levels of resistance. A modest increase in TM stiffness, as observed in glaucoma, suppresses TM strain and practically eliminates the influence of outflow resistance on SC shear stress. As SC and TM cells respond to mechanical stimulation by secreting factors that modulate outflow resistance, our model provides insight regarding the potential role of SC shear and TM strain as mechanosensory cues for homeostatic regulation of outflow resistance and hence intraocular pressure.

Regulation of intraocular pressure by microRNA cluster miR-143/145

Glaucoma is a major cause of irreversible blindness worldwide. Elevated intraocular pressure (IOP), which causes optic nerve damage and retinal ganglion cell death, is the primary risk factor for blindness in glaucoma patients. IOP is controlled by the balance between aqueous humor secretion from the ciliary body (CB) and its drainage through the trabecular meshwork (TM). How microRNAs (miRs) regulate IOP and glaucoma in vivo is largely unknown. Here we show that miR-143 and miR-145 expression is enriched in the smooth muscle and trabecular meshwork in the eye. Targeted deletion of miR-143/145 in mice results in significantly reduced IOP, consistent with an ~2-fold increase in outflow facilities. However, aqueous humor production in the same mice appears to be normal based on a microbeads-induced glaucoma model. Mechanistically, we found that miR-143/145 regulates actin dynamics and the contractility of TM cells, consistent with its regulation of actin-related protein complex (ARPC) subunit 2, 3, and 5, as well as myosin light chain kinase (MLCK) in these cells. Our data establish miR-143/145 as important regulators of IOP, which may have important therapeutic implications in glaucoma.

Bupropion use and risk of open-angle glaucoma among enrollees in a large U.S. managed care network

OBJECTIVE

Tumor Necrosis Factor (TNF) mediates retinal ganglion cell death in glaucoma. Anti-TNF drugs are neuroprotective in an animal model of glaucoma. It is unclear whether medications with anti-TNF properties such as bupropion have an impact on the risk of developing open-angle glaucoma (OAG) in humans. The purpose of this study is to determine whether bupropion use alters the risk of developing OAG.

METHODS

Claims data for beneficiaries age ≥35 years with no pre-existing OAG enrolled in a large nationwide U.S. managed care network continuously for ≥4 years between 2001-2011 was analyzed to identify patients who had been newly-diagnosed with OAG. The amount of bupropion use as captured from outpatient pharmacy claims over a four-year period was also quantified for each beneficiary. Multivariable Cox regression modeling assessed the impact of bupropion and other antidepressant medications on the risk of developing OAG with adjustment for sociodemographic characteristics of the enrollees along with medical and ocular comorbidities.

RESULTS

Of 638,481 eligible enrollees, 15,292 (2.4%) developed OAG. After adjustment for confounding factors including use of other antidepressant medication classes, each additional month of bupropion use was associated with a 0.6% reduced risk of OAG (HR = 0.994, (95% CI: 0.989-0.998), p = 0.007). Compared to nonusers, those with 24-48 months of bupropion use had a 21% reduced hazard (HR=0.79, (CI: 0.65-0.94), p = 0.0099) of OAG. This association did not differ among persons taking bupropion for depression or for other reasons (p-interaction = 0.82). There was no significant association between use of tricyclic antidepressants (HR = 1.000, (CI: 0.997-1.004), p = 0.95) or selective serotonin reuptake inhibitors (HR = 0.999, (CI: 0.997-1.001), p = 0.39) and development of OAG.

CONCLUSION

These findings suggest bupropion use may be beneficial in reducing the risk of OAG. If prospective studies confirm the findings of this analysis, this may identify a novel therapeutic target for OAG.

Intraocular pressure homeostasis: maintaining balance in a high-pressure environment

Although glaucoma is a relatively common blinding disease, most people do not develop glaucoma. A robust intraocular pressure (IOP) homeostatic mechanism keeps ocular pressures within relatively narrow acceptable bounds throughout most peoples' lives. The trabecular meshwork and/or Schlemm's canal inner wall cells respond to sustained IOP elevation and adjust the aqueous humor outflow resistance to restore IOP to acceptable levels. It appears that the cells sense IOP elevations as mechanical stretch or distortion of the actual outflow resistance and respond by initiating a complex extracellular matrix (ECM) turnover process that takes several days to complete. Although considerable information pertinent to this process is available, many aspects of the IOP homeostatic process remain to be elucidated. Components and mechanisms beyond ECM turnover could also be relevant to IOP homeostasis, but will not be addressed in detail here. Known aspects of the IOP homeostasis process as well as possible ways that it might function and impact glaucoma are discussed.

Role of microRNAs in the trabecular meshwork

MicroRNAs (miRNAs) are now recognized as important post-transcriptional regulators of gene expression. MiRNAs are known to modulate cellular functions relevant to the normal and pathological physiology of the trabecular meshwork (TM) such as cell contraction and extracellular matrix turnover. There is also increasing evidence supporting the role of miRNAs in the pathogenesis of multiple diseases, and their potential value as both biomarkers of disease and therapeutic targets. However, compared with other tissues, our current knowledge regarding the roles played by miRNAs in the TM is still very limited. Here, we review the information currently available about miRNAs in the TM and discuss the main challenges and opportunities to incorporate the rapid progress in miRNA biology to the understanding of the normal and pathological physiology of the TM, and to develop novel clinical applications for diagnosis and therapy of high intraocular pressure.

Adenosine A1 modulators: a patent update (2008 to present)

INTRODUCTION

In 2008, we published our review titled 'Therapeutic potential of A1 adenosine receptor ligands - a survey of recent patent literature' that reported the compounds active on A1 adenosine receptors (ARs) and the applications of A1 AR ligands patented in the period 2005 - 2008.

AREAS COVERED

This article is a discussion of the patents about the same subjects, issued in the period 2008 to present. It is organized similarly to the first one, with a section about new compounds, subdivided on the basis of their functional activity (agonists, antagonists and allosteric modulators) and a section regarding new therapeutic applications.

EXPERT OPINION

The main novelty is represented by the patenting of A1 AR ligands with dual selectivity which may show, in some conditions, better efficacy and fewer side effects. Moreover, while the way to arrive into the market appears full of obstacles for selective A1 ligands that need systemic administration for long-term therapy, better chances are foreseen in applications requiring topical administration.