IOP and glaucoma damage: The essential role of optic nerve head and retinal mechanosensors

Abstract

There are many unanswered questions on the relation of intraocular pressure to glaucoma development and progression. IOP itself cannot be distilled to a single, unifying value, because IOP level varies over time, differs depending on ocular location, and can be affected by method of measurement. Ultimately, IOP level creates mechanical strain that affects axonal function at the optic nerve head which causes local extracellular matrix remodeling and retinal ganglion cell death - hallmarks of glaucoma and the cause of glaucomatous vision loss. Extracellular tissue strain at the ONH and lamina cribrosa is regionally variable and differs in magnitude and location between healthy and glaucomatous eyes. The ultimate targets of IOP-induced tissue strain in glaucoma are retinal ganglion cell axons at the optic nerve head and the cells that support axonal function (astrocytes, the neurovascular unit, microglia, and fibroblasts). These cells sense tissue strain through a series of signals that originate at the cell membrane and alter cytoskeletal organization, migration, differentiation, gene transcription, and proliferation. The proteins that translate mechanical stimuli into molecular signals act as band-pass filters - sensing some stimuli while ignoring others - and cellular responses to stimuli can differ based on cell type and differentiation state. Therefore, to fully understand the IOP signals that are relevant to glaucoma, it is necessary to understand the ultimate cellular targets of IOP-induced mechanical stimuli and their ability to sense, ignore, and translate these signals into cellular actions.

Fig. 1.. Sources of IOP variation.

Clinical decision making in glaucoma most often uses isolated measurements (A) or measurements separated by months (E) and all IOP measurements are taken at the surface of the cornea. (B) IOP levels within the eye vary regionally due to pressure gradients that drive fluid flow. IOP level is dynamic with variations that can be seen within a span of several seconds (C), over the course of a day (D), and over months during regular clinical visits (E).

Fig. 2.. Connective tissue components of the optic nerve head.

Illustration of the optic disc (A) and the connective tissue components of the optic nerve head (B) showing the circumferentially arrange collagen lamellae of the peripapillary sclera (PPS), the basket weaved pattern of the peripheral sclera, and lamina cribrosa. These structures form the ONH through which RGC axons exit the eye (C).

Fig. 3.. Regionally specific collagen ultrastructure.

Second harmonic generation (SHG) imaging of collagen lamellae from a normal human eye. PPS collagen lamellae are highly aligned and arranged in a circumferential pattern around the optic nerve while peripheral (Per) collagen lamellae are organized in an interdigitated, basket-weave pattern (scale bar = 50 μm).

Fig. 4.. IOP-induced strain is sensed at astrocyte-ECM junctional complexes.

Astrocytes in mouse ONH can span the diameter of the ON and have junctional complexes (green) consisting of integrin-linked transmembrane units connected to basement membrane produced by astrocytes that are adjacent to PPS and to capillaries. Human astrocytes reside on lamina connective tissue beams with similar junctional complexes. In both species, forces of translaminar pressure difference and centrifugal hoop stress are generated by IOP (arrows).

Fig. 5.. Schematic simplification of complex junctional complex participants.

PPS collagen and elastin harbor latent TGFβ released by stress, signaling through α,β integrin in cell membrane attached to laminin. Pathway includes signaling by Src and FAK to talin, vinculin, zyxin and paxillin at the interface with actin filament network. Myosin and α-actinin form cytoskeletal networks altered by ROCK/RhoA stimulated by TGFβ. Astrocyte ONH junctions contain α,β-dystroglycan, typically attached to agrin in PPS and signaling to cytoskeleton through α-syntrophin & dystrophin. AQP channels, where present, require presence of dystroglycan.

Fig. 6.. Astrocyte response to IOP elevation.

(A) Mouse normal ONH with astrocytes labeled for integrin β1 (green) throughout cytoplasm and at end feet contacting PPS (arrow). (B) Normal human longitudinal section of ONH (integrin β1-green). (C): Mouse ONH at PPS, TEM junctional complex density (arrows) adjoins BM with integrin β1 immunogold label (arrowheads). After IOP elevation, astrocytes withdraw from BM at the PPS leaving remnants of astrocytes membrane still attached to their BM (arrow) (D). Mouse glaucoma 1w astrocytes withdraw from BM at PPS, leaving abnormal spaces, and E: withdraw junctional complex densities from membrane (arrow), with free α-dystroglycan identified by immunogold particles (circle) (E,F). Scale Bar-A: 10 μm, B: 50 μm, C,D,E,F: 200 nm.

Fig. 7.. Complex cytoskeletal organization and cellular morphology of scleral stromal fibroblasts.

(A) Cross section of a normal human optic nerve (right), PPS, and peripheral sclera (left) with phalloidin labelling of filamentous actin (red) shows actin organized circumferentially in the PPS (B) and in a basket-weave patter peripherally (C) (scale bar = 500 μm (A), 100 μm (B,C)).