Biomechanics of Schlemm's canal endothelium and intraocular pressure reduction

https://doi.org/10.1016/j.preteyeres.2014.08.002Get rights and content

Abstract

Ocular hypertension in glaucoma develops due to age-related cellular dysfunction in the conventional outflow tract, resulting in increased resistance to aqueous humor outflow. Two cell types, trabecular meshwork (TM) and Schlemm's canal (SC) endothelia, interact in the juxtacanalicular tissue (JCT) region of the conventional outflow tract to regulate outflow resistance. Unlike endothelial cells lining the systemic vasculature, endothelial cells lining the inner wall of SC support a transcellular pressure gradient in the basal to apical direction, thus acting to push the cells off their basal lamina. The resulting biomechanical strain in SC cells is quite large and is likely to be an important determinant of endothelial barrier function, outflow resistance and intraocular pressure. This review summarizes recent work demonstrating how biomechanical properties of SC cells impact glaucoma. SC cells are highly contractile, and such contraction greatly increases cell stiffness. Elevated cell stiffness in glaucoma may reduce the strain experienced by SC cells, decrease the propensity of SC cells to form pores, and thus impair the egress of aqueous humor from the eye. Furthermore, SC cells are sensitive to the stiffness of their local mechanical microenvironment, altering their own cell stiffness and modulating gene expression in response. Significantly, glaucomatous SC cells appear to be hyper-responsive to substrate stiffness. Thus, evidence suggests that targeting the material properties of SC cells will have therapeutic benefits for lowering intraocular pressure in glaucoma.

Introduction

The elevated intraocular pressure (IOP) that is associated with primary open-angle glaucoma (POAG) is caused by an increased resistance to the outflow of aqueous humor from the eye through the conventional2 outflow pathway (Grant, 1951). In spite of over 140 years of investigation (Leber, 1873), the precise cause of this increased outflow resistance remains elusive. Interestingly, most treatments for glaucoma focus on diminishing the rate of aqueous humor formation or altering the outflow path. These treatments lower IOP and thereby slow the progression of ganglion cell damage and associated vision loss, but in most cases do not stop it (Hattenhauer et al., 2000, Hattenhauer et al., 1998, Leske et al., 2003, Nouri-Mahdavi et al., 2004). Remarkably, there is currently no drug treatment in clinical use that directly targets the increased flow resistance that is a central characteristic of ocular hypertension in glaucoma, mainly because the mechanism(s) of increased flow resistance remain obscure.

Logically, the primary pathology underlying increased outflow resistance might be cellular, extracellular, or some combination of the two. Cellular contributions might include altered hydraulic conductivity of the endothelial lining of Schlemm's canal (SC), and extracellular contributions might include increased extracellular matrix in the juxtacanalicular tissue (JCT) or altered basement membrane beneath SC cells; however, comparisons of the outflow pathways of glaucomatous and age-matched normal eyes have found only subtle structural differences. Specifically, in glaucoma, there is an accelerated loss of trabecular meshwork (TM) cells that is limited to the inner region of the conventional outflow pathway (Alvarado et al., 1981, Alvarado et al., 1984) and cytoskeletal changes in the actin architecture of JCT-TM3 and SC cells (Read et al., 2007). Additionally, there is an accumulation of a “sheath derived plaque material” in the JCT of glaucomatous eyes (Alvarado et al., 1986, Lutjen-Drecoll et al., 1981), but this accumulation has been shown to have negligible hydrodynamic consequence (Alvarado et al., 1986) (Murphy et al., 1992). There is little other morphological evidence of increased extracellular matrix or altered basement membrane composition in glaucomatous eyes compared to age-matched controls.

At the level of SC, however, several changes in glaucomatous eyes have been observed with the potential to be a significant contributor to the increased outflow resistance. The dimensions of the lumen of SC are smaller in glaucomatous eyes and these changes correlate with outflow resistance (Allingham et al., 1996). Herniations of the inner wall and JCT tissue into collector channels are more frequently observed in glaucomatous eyes than age-matched non-glaucomatous eyes (Gong et al., 2007, Hann et al., 2014). There is also a reduced pore density in the inner wall endothelium of SC comparing normal to glaucomatous eyes (Allingham et al., 1992, Johnson et al., 2002) that is potentially quite important. Collectively, these data point to dysfunction at the level of the inner wall of SC in glaucoma.

In this article, we review recent evidence that increased stiffness of SC endothelial cells is responsible for the elevated outflow resistance and IOP characteristic of glaucoma; we also present data showing that drugs that change cell stiffness also alter outflow resistance. By understanding the coupling between biomechanics and flow through the inner wall endothelium, we outline opportunities to exploit cell biomechanics as a targeted approach to reduce IOP at the site of outflow resistance regulation.

Section snippets

The inner wall endothelium of Schlemm's canal experiences a unique biomechanical environment

The typical pressure loading on vascular endothelia generates a pressure gradient in the apical to basal direction. The basement membrane and other tissues underlying vascular endothelia amply support the transcellular pressure drop generated by this gradient, and thus the vascular endothelial cells themselves do not have to support the associated radial and circumferential stresses. This is not the case for the endothelium of SC, where the SC cells themselves must support a “backwards” basal

Conclusions

SC cells are surprisingly contractile, capable of changing their contractile state to employ forces that are comparable to those exerted by smooth muscle cells in the lung (Zhou et al., 2012). While the source of resistance to the flow of aqueous humor through its primary outflow pathway from the eye is still a topic of active research (Keller and Acott, 2013, Overby et al., 2009, Swaminathan et al., 2013), a number of laboratories have now converged on the hypothesis that the contractile state

Future directions

To develop new therapeutic strategies that specifically target SC cells, progress in several areas is needed. First, better mechanistic understanding of the pore forming machinery in SC cells will provide novel targets for intervention. For example, it may be helpful to apply knowledge from mechanistic studies of fenestrae formation or transcellular diapedesis to intracellular pore formation in SC cells (Michel and Neal, 1999). Second, a better understanding of the molecular changes in

Funding support

NIH grants (EY019696, EY022359, HL120839 and HL107561), BrightFocus Foundation, Research to Prevent Blindness Foundation, Georgia Research Alliance and Royal Society Wolfson Research Merit Award.

References (127)

  • M. Johnson

    What controls aqueous humour outflow resistance?

    Exp. Eye Res.

    (2006)
  • M.A. Johnstone et al.

    Pressure dependent changes in the structures of the aqueous outflow system of human and monkey eyes

    Am. J. Ophthalmol.

    (1973)
  • B. Junglas et al.

    Connective tissue growth factor causes glaucoma by modifying the actin cytoskeleton of the trabecular meshwork

    Am. J. Pathol.

    (2012)
  • A.H. Krauss et al.

    Ocular hypotensive activity of BOL-303259-X, a nitric oxide donating prostaglandin F2alpha agonist, in preclinical models

    Exp. Eye Res.

    (2011)
  • E. Lutjen-Drecoll et al.

    Quantitative analysis of 'plaque material' in the inner- and outer wall of Schlemm's canal in normal- and glaucomatous eyes

    Exp. Eye Res.

    (1986)
  • C.T. McKee et al.

    The effect of biophysical attributes of the ocular trabecular meshwork associated with glaucoma on the cell response to therapeutic agents

    Biomaterials

    (2011)
  • V.C. Mow et al.

    Fluid transport and mechanical properties of articular cartilage: a review

    J. Biomech.

    (1984)
  • M. Nagayama et al.

    Contribution of cellular contractility to spatial and temporal variations in cellular stiffness

    Exp. Cell. Res.

    (2004)
  • D. Overby et al.

    The changing paradigm of outflow resistance generation: towards synergistics models of the JCT and inner wall endothelium

    Exp. Eye Res.

    (2009)
  • D.R. Overby et al.

    Novel dynamic rheological behavior of individual focal adhesions measured within single cells using electromagnetic pulling cytometry

    Acta Biomater.

    (2005)
  • R.M. Pedrigi et al.

    A model of giant vacuole dynamics in human Schlemm's canal endothelial cells

    Exp. Eye Res.

    (2011)
  • K.M. Perkumas et al.

    Protein markers and differentiation in culture for Schlemm's canal endothelial cells

    Exp. Eye Res.

    (2012)
  • A.T. Read et al.

    Actin structure in the outflow tract of normal and glaucomatous eyes

    Exp. Eye Res.

    (2007)
  • J. Solon et al.

    Fibroblast adaptation and stiffness matching to soft elastic substrates

    Biophys. J.

    (2007)
  • R.R. Allingham et al.

    Schlemm's canal and primary open angle glaucoma: correlation between Schlemm's canal dimensions and outflow facility

    Exp. Eye Res.

    (1996)
  • R.R. Allingham et al.

    The relationship between pore density and outflow facility in human eyes

    Invest. Ophthalmol. Vis. Sci.

    (1992)
  • J. Alvarado et al.

    Endothelia of Schlemm's canal and trabecular meshwork: distinct molecular, functional, and anatomic features

    Am. J. Physiol.

    (2004)
  • J. Alvarado et al.

    Age-related changes in trabecular meshwork cellularity

    Invest. Ophthalmol. Vis. Sci.

    (1981)
  • J.A. Alvarado et al.

    Juxtacanalicular tissue in primary open angle glaucoma and in nonglaucomatous normals

    Arch. Ophthalmol.

    (1986)
  • J. Ando et al.

    Vascular mechanobiology: endothelial cell responses to fluid shear stress

    Circ. J.

    (2009)
  • A.J. Bank et al.

    Contribution of collagen, elastin, and smooth muscle to in vivo human brachial artery wall stress and elastic modulus

    Circulation

    (1996)
  • A. Bill et al.

    Scanning electron microscopic studies of the trabecular meshwork and the canal of Schlemm –– an attempt to localize the main resistance to outflow of aqueous humor in man

    Acta Ophthamol.

    (1972)
  • A. Boussommier-Calleja et al.

    Pharmacologic manipulation of conventional outflow facility in Ex vivo mouse eyes

    Invest. Ophthalmol. Vis. Sci.

    (2012)
  • J.P. Butler et al.

    Traction fields, moments, and strain energy that cells exert on their surroundings

    Am. J. Physiol. Cell. Physiol.

    (2002)
  • L.J. Camras et al.

    Circumferential tensile stiffness of glaucomatous trabecular meshwork

    Invest. Ophthalmol. Vis. Sci.

    (2014)
  • J.Y. Chang et al.

    Multi-scale analysis of segmental outflow patterns in human trabecular meshwork with changing intraocular pressure

    J. Ocular Pharmacol. Ther. : Official J. Assoc. Ocular Pharmacol. Ther.

    (2014)
  • J. Chen et al.

    Novel ocular antihypertensive compounds in clinical trials

    Clin. Ophthalmol.

    (2011)
  • A.M. Collinsworth et al.

    Apparent elastic modulus and hysteresis of skeletal muscle cells throughout differentiation

    Am. J. Physiol. Cell. Physiol.

    (2002)
  • D.L. Epstein et al.

    Morphology of the trabecular meshwork and inner-wall endothelium after cationized ferritin perfusion in the monkey eye

    Invest. Ophthalmol. Vis. Sci.

    (1991)
  • D.L. Epstein et al.

    Acto-myosin drug effects and aqueous outflow function

    Invest. Ophthalmol. Vis. Sci.

    (1999)
  • C.R. Ethier et al.

    Two pore types in the inner-wall endothelium of Schlemm's canal

    Invest. Ophthalmol. Vis. Sci.

    (1998)
  • C.R. Ethier et al.

    Effects of latrunculin-B on outflow facility and trabecular meshwork structure in human eyes

    Invest. Ophthalmol. Vis. Sci.

    (2006)
  • B. Fabry et al.

    Scaling the microrheology of living cells

    Phys. Rev. Lett.

    (2001)
  • B. Fabry et al.

    Selected contribution: time course and heterogeneity of contractile responses in cultured human airway smooth muscle cells

    J. Appl. Physiol. (1985)

    (2001)
  • D.L. Fleenor et al.

    TGFbeta2-induced changes in human trabecular meshwork: implications for intraocular pressure

    Invest. Ophthalmol. Vis. Sci.

    (2006)
  • H. Gong et al.

    Morphology of the aqueous outflow pathway

    Microsc. Res. Tech.

    (1996)
  • H. Gong et al.

    New Morphological Findings in Primary Open Angle Glaucoma

    (2007)
  • J. Gottanka et al.

    Effects of TGF-beta2 in perfused human eyes

    Invest. Ophthalmol. Vis. Sci.

    (2004)
  • W.M. Grant

    Clinical measurements of aqueous outflow

    Arch. Ophthalmol.

    (1951)
  • I. Grierson et al.

    Associations between the cells of the walls of Schlemm's canal

    Albr. Von. Gr. Arch. Klin. Exp. Ophthalmol.

    (1978)
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    Percentage of work contributed by each author in the production of the manuscript is as follows: Stamer: 20%; Braakman: 10%, Zhou: 10%; Ethier: 15%; Fredberg: 10% Overby: 15%; Johnson: 20%.

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