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cover story
Understanding
Trabecular
Meshwork Outflow
Its role in modulating aqueous humor outflow and IOP.
By Swarup S. Swaminathan; Dong-Jin Oh, P h D; Min Hyung Kang, P h D;
Ramez I. Haddadin, MD; Guadalupe Villarreal J r , MD;
Marc Töteberg-Harms, MD; Ayan Chatterjee; and Douglas J. Rhee, MD
A
queous humor outflow occurs through two
routes, the conventional and uveoscleral pathways. The conventional pathway is responsible
for approximately 85% of aqueous outflow.1 The
primary constituent is the trabecular meshwork (TM),
which consists of seven to eight layers of cellular beams
surrounded by extracellular matrix (ECM) in human eyes.
The aqueous humor traverses the superficial portions
of the TM, known as the corneoscleral TM, to reach the
juxtacanalicular connective tissue (JCT) TM, which is an
amorphous layer of cells interspersed amongst ECM. The
aqueous humor subsequently passes into Schlemm canal
and enters the episcleral venous system.
A
The TM overall and especially the JCT TM are the
anatomic location of the highest amount of outflow
resistance in the conventional pathway.2,3 TM outflow is
primarily mediated by two forces: (1) alterations in the
ECM-surrounding cells and (2) alterations within the
cells of TM and Schlemm canal’s inner wall that modulate cellular contractility and tension.
PARACELLULAR OUTFLOW
Numerous studies have demonstrated the impact
of increased cellular stiffness on outflow resistance—
specifically, an elevation in the number of actin stress
fibers, actomyosin fiber bundles, and cross-linked actin
B
Figure 1. The effect of a Rho kinase inhibitor on actin filaments. Normal TM cells (A) versus TM cells treated with inhibitor (B).
Reprinted with permission from Inoue T, Tanihara H. Rho-associated kinase inhibitors: a novel glaucoma therapy [published
online ahead of print June 12, 2013]. Prog Retin Eye Res. doi:10.1016/j.preteyeres.2013.05.002.
September/October 2013 glaucoma today 27
cover story
A
“The molecular pathways
responsible for the pathologic
changes leading to ocular
hypertension and subsequent
glaucoma remain elusive.”
B
Figure 2. The eyes of a wild-type control mouse (A) and a
SPARC knockout mouse (B). The distribution of the fluorescent tracer demonstrates that outflow through the mouse
TM is segmental but is more uniform in the secreted protein
acidic and rich in cysteine (SPARC) knockout eye.16
networks within the cells of the TM and inner wall of
Schlemm canal.4,5 Multiple compounds inhibit or hinder such changes. Latrunculins, molecules that disrupt
actin filaments, cause TM cells to shrink and retract,
thereby decreasing IOP.6 Ethacrynic acid, a compound
that inhibits the formation of cytoskeleton, alters cellular shape by disrupting actin networks, leading to greater aqueous flow around cells and reducing IOP.7 The
intracellular enzyme Rho regulates these intracellular
28 glaucoma today September/October 2013
changes in addition to alterations in cell-ECM interactions.5,8,9 If either Rho or a related enzyme, Rho kinase,
is inhibited, TM cells relax significantly, increasing outflow and decreasing IOP (Figure 1).9,10 This discovery led
to the development of Rho kinase inhibitors, a novel
class of glaucoma medications currently being evaluated in clinical trials. If approved, these drugs will be
the first to act directly on the TM to increase aqueous
outflow since the release of pilocarpine in the 1870s.
ALTERING ECM
Changes in the ECM may also alter aqueous outflow.
According to multiple research groups, ECM or ECMmodulating proteins appear to be crucial to mediating
outflow. For example, gremlin mediates the effects
of transforming growth factor-ß2 (TGF-ß2) on ECM
deposition,11 whereas cochlin is an ECM protein that
appears to sense shear stress in the TM.12 Myocilin
influences interactions between TM cells and surrounding ECM,13 and sFRP-1 indirectly modulates ECM
protein synthesis.14 Matricellular proteins are secreted
and modulate ECM organization and the interaction
between TM cells and the ECM. This family of proteins
includes SPARC and thrombospondin-1 and 2, which
have essential roles in the regulation of IOP and aqueous outflow.15,16 Both proteins appear to affect collagen
fiber formation, which may alter the nature of the ECM
around TM cells and change outflow.
Other ECM-related alterations in glaucomatous
eyes have been found. TGF-ß2, an essential molecule
promoting tissue growth, is elevated in the aqueous
humor of glaucomatous eyes.17,18 Studies have demonstrated that, when TGF-ß2 is overexpressed in cadaveric
human eyes, ECM deposition increases within the TM
and reduces the amount of aqueous outflow through
the TM.19,20 Various proteins, including connective tissue growth factor and SPARC, are thought to mediate
the TGF-ß2–driven increase in ECM.21,22 In addition,
protein aggregates known as sheath-derived plaques
have been found in the JCT TM of glaucomatous eyes.23
These aggregates contain several ECM proteins, includ-
cover story
ing elastin, collagen, and various proteoglycans. Studies
have also shown that either increasing ECM production
or decreasing ECM degradation increases IOP. When
SPARC is overexpressed in perfused human eyes, the
concentration of certain metalloproteinases (which
catalyze the enzymatic degradation of ECM proteins)
is decreased, whereas their inhibitors are upregulated.24
Metalloproteinases also play a role in reducing IOP via
ECM degradation when activated by the adenosine
receptor.25 Agonist compounds for this receptor are currently in clinical trials as potential therapeutic agents.
Aqueous outflow does not occur consistently
throughout all 360º of the TM. Rather, outflow occurs
only in certain sections of the TM, a concept referred
to as segmental flow.16,26,27 In mice lacking SPARC, IOP
is decreased, and a reduction in IOP is correlated with
an increase in the amount of area utilized for outflow
(Figure 2).16,28 It appears as though the greater the available area for outflow through the TM, the lower the
IOP. Segmental flow may explain why multiple iStent
Trabecular Micro-Bypass Stents (Glaukos Corporation)
are often required to achieve a substantial reduction in
IOP in glaucomatous eyes.
FINAL THOUGHTS
The TM has been the recent focus of surgical innovation such as ab interno trabeculectomy (Trabectome;
NeoMedix Corporation) and the iStent. Results thus
far have been limited. Further elucidating the mechanics of TM outflow such as segmental outflow will be
essential to identifying the pathophysiologic basis of
primary open-angle glaucoma as well as to increasing
the success rates of TM bypass procedures. Although
investigators have begun to explain outflow physiology
in the nonglaucomatous eye, the molecular pathways
responsible for the pathologic changes leading to ocular
hypertension and subsequent glaucoma remain elusive.
Numerous research groups, including the authors’, have
aimed at therapeutically inhibiting this disease process.
There will be more soon! n
Ayan Chatterjee is a medical student at the Perelman
School of Medicine at the University of Pennsylvania in
Philadelphia.
Ramez I. Haddadin, MD, is a cornea fellow at the
Massachusetts Eye and Ear Infirmary in Boston.
Min Hyung Kang, PhD, is a research scientist at Case
Western Reserve University in Cleveland.
Dong-Jin Oh, PhD, is an assistant professor at Case
Western Reserve University in Cleveland.
Douglas J. Rhee, MD, is the chair of the Department of
Ophthalmology and Visual Sciences at Case Western Reserve
30 glaucoma today September/October 2013
University in Cleveland. He is an ad hoc consultant to AqueSys, Inc., and Glaukos Corporation.
Dr. Rhee may be reached at (216) 844-8590; [email protected].
Swarup S. Swaminathan is a medical student
in the Harvard-Massachusetts Institute of Technology
Division of Health Sciences & Technology at Harvard
Medical School in Boston.
Marc Töteberg-Harms, MD, is a clinical and basic
research glaucoma fellow at Case Western Reserve
University in Cleveland.
Guadalupe Villarreal Jr, MD, is a resident at the Wilmer
Eye Institute of Johns Hopkins University in Baltimore.
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