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Glaucoma researchers have been traveling two separate roads in search of a better understanding of the silent thief of vision. One group, focused on the idea that intraocular pressure is glaucoma’s only modifiable risk factor, is trying to unravel the mystery of the eye’s pressure valve—the trabecular meshwork. The other group is looking to the brain, homing in on the neurological issues associated with glaucoma.
Here’s an overview of recent research findings in both areas.
Nothing major has changed in glaucoma care since Neeru Gupta, MD, PhD, began practicing ophthalmology. She still examines her glaucoma patients by looking into their eyes. And she still offers the same treatments: IOP-lowering medications and surgery.
And yet everything has changed. Today, glaucoma is regarded as a neurodegenerative disorder of the central visual pathways. Now, when Dr. Gupta looks into a patient’s eyes, she sees what she calls the tip of the iceberg (Fig. 1). “I know there’s damage beyond the eye,” she said, referring to bundles of optic nerve fibers that lead from the retinal ganglion cells (RGCs) to major vision centers of the brain.
“We evaluate glaucoma damage by looking at the eye,” said Dr. Gupta, professor of ophthalmology and vision sciences at the University of Toronto in Ontario, Canada. “We don’t say, ‘Let’s look at your brain.’” Someday, however, Eye M.D.s may be doing just that. And they may have neuroprotective treatments to offer their patients. “I’m aware, of course, that I don’t have medications to treat neurodegeneration behind the eye,” said Dr. Gupta, who is also director of the Glaucoma and Nerve Protection Unit at St. Michael’s Hospital.
“As we work to save the RGCs in the eye, we need to think about preserving their connections in the brain,” Dr. Gupta said. “When the RGCs come knocking on the neurons in the brain, we need to make sure that someone is home.”
THE NEURODEGENERATION MODEL
The neurodegeneration paradigm was born just over a decade ago, when Dr. Gupta and colleagues reported that a rise in IOP affected not only the RGCs and nerve fibers but also the major vision centers in the brain connected to the nerve fibers,1 particularly the lateral geniculate nucleus (LGN), to which 90 percent of the RGCs project. In a monkey glaucoma model, the researchers found, in addition to RGC loss, a significant loss and shrinkage of LGN neurons that terminate in the primary visual cortex (Fig. 2).
The progressive and irreversible loss of specific neuron populations, as well as the loss of relevant functions, is the hallmark of neurodegenerative diseases. In Alzheimer’s, for example, the loss of hippocampal neurons leads to memory loss. In glaucoma, RGC loss leads to visual impairment.2
According to Dr. Gupta and colleagues, reframing glaucoma as a neurodegenerative disease may help to explain why patients often continue to lose sight even after their IOP is controlled or why patients without any evidence of elevated pressure may have damage.
The brain research may also explain why visual field testing doesn’t always agree with the appearance of the optic nerve head, especially in later stages of the disease. A visual field is typically interpreted relative to damage in the eye, but it’s really “a snapshot of damage all the way from the eye to the brain at any moment in time,” Dr. Gupta said.
The brain research. Neurodegeneration in glaucoma has given investigators new territory to explore. “We have somewhere else to look now. We can look at cell death and molecular mechanisms beyond the eye,” Dr. Gupta said.
In a postmortem study of a glaucoma patient, Dr. Gupta observed shrinkage of neurons in the LGN compared with age-matched controls (Fig. 3).3 She corroborated the findings in vivo in an MRI study, which showed significant reduction of LGN size in glaucoma patients compared with controls.4
And, contrary to expectation, Dr. Gupta has found that damage may occur in the brain before it is apparent in the eye.5 Specifically, dendrites in the brain showed major structural disturbances (shrinkage and loss of complexity) with high IOP, even though RGC axons were alive and intact. Recently, her team observed that, in the adult nonhuman primate brain, these dendritic changes can be modified by the drug memantine, used clinically in Alzheimer disease.6
Another piece of the puzzle. While Dr. Gupta and colleagues have been studying the neurons in the brain, David J. Calkins, PhD, a neuroscientist, has been exploring a different piece of the puzzle: the end of the axon just before it connects to the brain. He has demonstrated that the RGC axons are the first area vulnerable to stressors that are relevant in glaucoma, such as IOP. If the axons break down, it becomes harder for signals to travel from the retina to the brain. In other words, high pressure slows transport.
Early axonal damage. “We can say that the cell body of the RGC actually sticks around a lot longer than we used to think. It sticks around well after the axons have been severely damaged. That’s fact number one,” said Dr. Calkins, vice-chairman and director for research at the Vanderbilt Eye Institute and associate professor of ophthalmology and visual sciences, neuroscience and psychology at Vanderbilt University. “The other strong thing we can say is that the insult to the axon occurs very, very early. Finally, we’ve demonstrated that this early injury to the axons begins at their most distal projection sites in the brain.”7
Questions remain. “We know the pathology involves loss of axonal communication followed by degeneration,” Dr. Calkins said. But what triggers the insult?
One theory, reported by Dr. Calkins, implicates calcium-dependent cascades as a triggering event.8 But given the complexity of neurons, there will be multiple cascades, Dr. Calkins said. Glial cells, immune components and vascular components also may play a role. “All of these components undoubtably contribute to the early pathogenesis,” he said. “They contribute to a confluence of factors that influence the overall physiological state of the axon and the optic nerve.”
Eye stressors cause brain damage. “Glaucoma is an interesting disease, because the things that cause blindness in glaucoma are primarily neurological and occur outside the eye in the optic projection to the brain,” said Dr. Calkins. “But that occurs because the etiology of glaucoma involves stressors in the eye such as IOP, corneal thickness and scleral pliability. All of the mechanics of the eye influence what happens neurologically.”
TRABECULAR MESHWORK FINDINGS
Douglas J. Rhee, MD, doesn’t dispute the importance of the neurodegenerative aspects of glaucoma. “I have no doubt that there are changes in the brain as a result of what’s happening in the eye,” said Dr. Rhee, associate professor of ophthalmology at Harvard University and associate chief of operations and practice development, Massachusetts Eye and Ear Infirmary.
Pressure is the culprit. But Dr. Rhee questioned whether glaucoma is a primary neurodegenerative process. “POAG is mainly a primary pressure problem that creates neurodegeneration,” he said. “Both aspects are very important. And they’re linked by IOP, which is the only modifiable risk factor.”
For more than three decades, Jorge A. Alvarado, MD, professor of ophthalmology at the University of California, San Francisco, has been studying the molecular and cellular mechanisms that regulate how fluid exits the eye. Agreeing with Dr. Rhee’s assessment, Dr. Alvarado said, “The only thing we can do for glaucoma is to lower the pressure. We’re going to continue to keep pressure under control as the major therapy.”
Drainage disturbances in the TM. To that end, Dr. Rhee, Dr. Alvarado and others have homed in on the trabecular meshwork (TM) to identify the proteins, molecules and cells that regulate the drainage system. “The only thing shown to be impacted in glaucoma is aqueous flow through the TM,” Dr. Rhee said, adding that aqueous hypersecretion is not the problem. In POAG, the only known pathophysiology of aqueous humor dynamics is dysregulated pressure-dependent outflow.
Dr. Rhee explained that most, but not all, of the outflow resistance is located within the juxtacanalicular region (Fig. 4). This region includes endothelial cells in the TM and the inner wall of Schlemm’s canal as well as a considerable amount of supporting extracellular matrix (ECM), including a fenestrated basement membrane. The juxtacanalicular region is a continuously active area where numerous cellular functions and constant synthesis and turnover of ECM occur simultaneously.
Accelerated TM aging and cell loss. Work done by Dr. Alvarado in the 1980s described an accelerated aging process in the TM cells of glaucomatous eyes. He reported that the cellularity, or cells per tissue area, in the TM declines with age. However, over a wide range of ages, the meshwork of eyes with POAG has lower cellularity than normal eyes. In other words, in glaucoma, the curve of the decline is much steeper. “That remains a very, very important theory in the pathophysiology of glaucoma,” Dr. Rhee said.
Subsequently, researchers have described other mechanisms by which pressure is regulated. One of the most popular concepts involves hyperaccelerated aging of the ECM within the juxtacanalicular tissue of the TM, Dr. Rhee said. Although changes in the matrix normally occur with aging, they are more profound in glaucoma, in part due to a buildup of sheath-derived (SD) plaque, which is more abundant in glaucoma. Dr. Rhee refers to these SD plaques, or other alterations of ECM, as “the trolls under the bridge, or what might plug up the physical pathways.”
Cellular tone in drainage channels. Another mechanism involves the regulation of cellular tone by altering the actin cytoskeleton of the inner wall of Schlemm’s canal and TM cells, Dr. Rhee said. Actin is the component of the cytoskeletal system that allows movement of cells and cellular processes. It can make the TM looser, thus facilitating better drainage, or stiffer, inhibiting drainage. Many potential classes of new drugs, including Rho-kinase inhibitors (ROCK inhibitors), latrunculins and activated sulfhydryl compounds, primarily affect this cellular mechanism.
Pathologic alterations of the actin network, called cross-linked actin networks (CLANs), within the cells of the TM or the endothelial cells of the inner wall of Schlemm’s canal, may play a role in the pathophysiology of POAG. Although CLAN formations are found in normal eyes, they are more abundant in glaucoma patients. Dr. Rhee said there is some thought that CLAN formation could be a primary event in POAG. And there is strong evidence that CLAN formation, along with deposits of ECM (but not SD plaques), alters TM cell phagocytosis and is relevant to the pathophysiology of steroid-induced glaucoma.
Dr. Rhee’s work involves a family of matricellular proteins that are highly expressed in TM cells, and certain members appear to contribute to the regulation of IOP.9 In other human tissues, matricellular proteins are known to help cells control their surrounding ECM. “One specific matricellular protein, SPARC, appears to affect IOP by concurrently increasing the synthesis of certain ECM components and inhibiting enzymes that normally degrade the ECM matrix metalloproteinases,” he said.
A related finding involves the cytokine transforming growth factor ß2 (TGF-ß2), a secreted protein that is found in the ECM. TGF-ß2 is highly increased in the aqueous of POAG patients; and when it is introduced into normal tissue, it increases IOP and causes pathological changes in ECM. Dr. Rhee’s group has found that SPARC is regulated by TGF-ß2.
Immune system effects. Dr. Alvarado’s research has shown that IOP may be regulated by a systemic mechanism involving the innate immune system and monocytes. This suggests that glaucoma could even be related to a derangement of the innate immune mechanism, he said.
Dr. Alvarado discovered that the POAG phenotype is characterized by a decreased number of endothelial cells lining aqueous channels within the TM. To understand how this difference in TM cell content could adversely affect regulation of aqueous outflow, he grew endothelial cells from the TM and the lumen of Schlemm’s canal in cell culture. He then discovered the involvement of a third cell type consisting of monocytes, a major cell in the innate immune system.
Building on these findings, Dr. Alvarado discovered a fivefold increase in monocytes circulating through the aqueous outflow pathway in both humans and monkeys after selective laser trabeculoplasty (SLT). When he introduced autologous cultured monocytes intracamerally in rabbits, IOP decreased nearly immediately, and the reduction lasted for the four-day duration of his experiments.10
The brain and “drain” research suggests that the etiology of glaucoma is multifactorial. Two independent fields of investigation appear to be inextricably linked.
Dr. Gupta looks forward to the day when their paths will cross in the clinic. “Wouldn’t it be great,” she said, “to figure out that the best cocktail is a pressure- lowering drug in the eye and, who knows, some molecule that addresses the neurodegenerative process beyond the eye?”
1 Yücel, Y. H. et al. Arch Ophthalmol 2000;118(3):378–384.
2 Gupta, N. et al. Curr Opin Ophthalmol 2007;18(2):110–114.
3 Gupta, N. et al. Br J Ophthalmol 2006;90(6):674–678.
4 Gupta, N. et al. Br J Ophthalmol 2009;93(1):56–60.
5 Gupta, N. et al. Exp Eye Res 2007;84(1):176–184.
6 Ly, T. et al. Vision Res 2011;51(2):243–250.
7 Crish, S. D. et al. Proc Natl Acad Sci U S A 2010;107(11):5196–5201.
8 Crish, S. D. and Calkins, D. J. Neuroscience 2011;176:1–11.
9 Haddadin, R. I. et al. Invest Ophthalmol Vis Sci 2009;50(8):3771–3777.
10 Alvarado, J. A. et al. Arch Ophthalmol 2010;128(6):731–737.
FROM THE LAB TO THE CLINIC
What does this flurry of research mean for glaucoma patients?
So far, no approved drugs have emerged from this work, although some are in clinical testing. But one day, the glaucoma armamentarium might include neuroprotective agents that halt degeneration and novel therapies that target the mechanisms that regulate aqueous outflow.
“As our understanding of the pathophysiology of the trabecular meshwork improves, we may be able to develop treatments that directly interrupt a pathophysiologic process and halt the downstream consequences of IOP,” Dr. Rhee said. “I also have great hope for my neuroprotective colleagues. Halting the disease on multiple fronts would be great.”
Research Supports Current Treatments
Notably, research in both the neurodegeneration and TM areas has found support for existing glaucoma therapies.
Brimonidine may be neuroprotective. Dr. Calkins, for example, reported a neuroprotective effect with the IOP-lowering drug brimonidine. In an animal model, systemically delivered brimonidine prevented axonal and somatic degeneration of retinal ganglion cell neurons independent of lowering IOP.1
Now, he said, the question is whether other glaucoma drugs might have secondary effects that are directly neuroprotective. Many experimental drugs appear to have effects similar to brimonidine in investigational models, but substantial work remains to be done to identify those with the most neuroprotective promise and design clinical trials to test their efficacy and safety.
New light on prostaglandin analogues. These drugs were originally thought to enhance only the uveoscleral outflow channel. However, building on the work of Jamie D. Lindsey, PhD, and Robert N. Weinreb, MD, Dr. Rhee found that prostaglandin analogues affect the matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) not only in ciliary body smooth muscle cells but also in TM endothelial cells.
At the same time, Jorge A. Alvarado, MD, has found that prostaglandin analogues mimic the effect of cytokines by increasing the permeability of the barrier in Schlemm’s canal, thus facilitating egress of aqueous from the eye. Like cytokines, the medications open up the zipperlike junctions that hold the barrier closed, explained Dr. Alvarado, who is now investigating cytokines for the treatment of glaucoma.
Laser therapy. He also found that argon laser trabeculoplasty and selective laser trabeculoplasty activate the same mechanism. “The final pathway is common to both the laser procedure [which induced secretion of cytokine factors] and the application of prostaglandin analogues.”
Borrowing from other fields. Insights from other fields could accelerate the search for pathways of neurodegeneration in the central visual system. “Many of the cascades are relevant in all of the neurodegenerative disorders,” Dr. Calkins said. “There are experimental therapies in Alzheimer’s and Parkinson’s that may translate to glaucoma.”
The Alzheimer’s drug memantine is an example. But a clinical trial of memantine for glaucoma did not pan out. Allergan reported that the phase 3 results showed no significant benefit in glaucoma patients compared with the placebo group.
However, Dr. Gupta and colleagues have found memantine to be protective in animal models. It helped prevent cell shrinkage in the LGN, and in a primate model it helped protect the dendrites of neurons in the brain.
How close are we to having new treatments for our patients?
“I think we’re very, very close,” Dr. Calkins said. “The progress in the last 10 years has been amazing. I suspect that, within the next decade, we will have multiple neurocascades to target, especially as we appreciate that the pathology is similar to other neurodegenerative disorders.”
1 Lambert, W. S. et al. Mol Neurodegener 2011;6(1):4.
MEET THE EXPERTS
JORGE A. ALVARADO, MD Professor of ophthalmology at the University of California, San Francisco. Financial disclosure: Receives speaking honoraria from Lumenis.
DAVID J. CALKINS, PHD Vice-chairman and director for research at the Vanderbilt Eye Institute and associate professor of ophthalmology and visual sciences, neuroscience and psychology at Vanderbilt University in Nashville, Tenn. Financial disclosure: Receives research funding from Acucela, Allergan and QLT.
NEERU GUPTA, MD, PHD Professor of ophthalmology and vision sciences as well as laboratory medicine and pathobiology at the University of Toronto and director, Glaucoma and Nerve Protection Unit, Keenan Research Centre, St. Michael’s Hospital, Toronto, Ontario, Canada. Financial disclosure: Consults for Merck, Alcon, Allergan and Pfizer and was an investigator for the Allergan memantine clinical trial. Experimental studies with memantine were partially funded by a peer-reviewed operating grant of the Canadian Institutes of Health Research/Small Medium Enterprise Allergan.
DOUGLAS J. RHEE, MD Associate professor at Harvard University and associate chief of operations and practice development, Massachusetts Eye and Ear Infirmary, Boston. Financial disclosure: Is an ad hoc consultant for Allergan, Alcon, Merck and Santen and receives research funding from Alcon.