Astrocytes in glaucomatous optic neuropathy

Introduction

Primary open angle glaucoma (POAG) is a retinal disease characterized by optic neuropathy associated with optic cupping and loss of visual field (Johnson et al., 2000; Weinreb and Khaw, 2004). Typically affecting older adults, the intraocular pressure (IOP) exceeds the level that is tolerated by an individual optic nerve. Although genes for some

Corresponding author. Tel.: 312-503-1064; Fax: 312-503-1062; E-mail: m-hernandez-neufeld@northwestern.edu

types of glaucoma including POAG have been characterized, the underlying cause of POAG remains unknown (Hewitt et al., 2006; Wiggs, 2007). A likely target of the mechanical stress generated by elevated IOP is the lamina cribrosa in the optic nerve head (ONH) (Bellezza et al., 2003). The signals generated by IOP-related stress are sensed by hypothetical ‘‘mechanosensors’’ in the lamina cribrosa of which the astrocyte is the main cellular component (Hernandez, 2000).

Astrocytes are the major glial cell type in the non-myelinated ONH in most mammals and provide cellular support functions to the axons

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while interfacing between connective tissue surfaces and surrounding blood vessels. In addition to astrocytes, a second cell type exists in the lamina cribrosa of humans and non-human primates, the lamina cribrosa cell. Lamina cribrosa cells can be distinguished from astrocytes because they do not express glial fibrillary acidic protein (GFAP) in vivo or in vitro and they do not express vascular specific markers or microglial markers (Hernandez et al., 1988). In the normal ONH, astrocytes are considered to be quiescent (Bachoo et al., 2004). Astrocytes become reactive in response to injury or disease and participate in the formation of a glial scar, which does not support axonal survival or growth (Liu et al., 2006).

Quiescent astrocytes

Quiescent astrocytes are terminally differentiated and in the CNS there are several subpopulations with regional specialization. In the lamina cribrosa, astrocytes form lamellae oriented perpendicular to the axons surrounding a core of fibroelastic extracellular matrix (Hernandez, 2000) (Fig. 1A, B, D). ONH astrocytes have many of the same functions as astrocytes in white matter (Araque et al., 2001; Hansson and Ronnback, 2003). Astrocytes supply energy substrate to axons in the optic nerve and maintain extracellular pH and ion homeostasis in the periaxonal space (Magistretti, 2006; Obara et al., 2008). Sodium channels in astrocytes participate in Na+ homeostasis, providing a path for Na+ entry into the cytoplasm (Bowman et al., 1984). The level of intracellular Na+ regulates the activity of various transporters, particularly Na+/K+ ATPase and the Na+/glutamate transporter (Anderson and Swanson, 2000). Astrocytes regulate water exchange between the brain and vascular space through expression of the water channel AQP4 in membrane domains in their end-feet around vessels (Tait et al., 2008). In astrocytes, voltagegated calcium channels deliver Ca2+ into the cytoplasm and participate in generation of glial Ca2+ signals. Astrocytes maintain the scant periaxonal ECM consisting of glycoproteins, such as laminin and proteoglycans. In the normal CNS,

astrocytes express a wide variety of growth factors and receptors, many of which serve as trophic and survival factors for neurons. ONH astrocytes and lamina cribrosa cells express bone morphogenetic proteins (Wordinger et al., 2002), neurotrophins, and receptors (Lambert et al., 2004a, b).

Reactive astrocytes in glaucoma

Adult, quiescent astrocytes become ‘‘reactive’’ after injury or disease and participate in formation of a glial scar, which does not support axonal survival or growth (Hernandez and Pena, 1997; Liu et al., 2006). The major hallmarks of a reactive astrocyte are an enlarged cell body and a thick network of processes with increased expression of GFAP and vimentin (Yang and Hernandez, 2003). Reactive astrocytes increase expression of various cell surface molecules that play important roles in cell–cell recognition and in cell adhesion to substrates, as well as various growth factors, cytokines, and receptors (Ricard et al., 2000; Hernandez et al., 2002). Reactive astrocytes express many new ECM proteins such as laminin, tenascin C, and proteoglycans (Pena et al., 1999b). The expression of TGF-a (Junier, 2000) and TGF- b (Pena et al., 1999a), ciliary neurotrophic factor (CNTF) (Liu et al., 2007), fibroblast growth factor 2 (Smith et al., 2001), platelet-derived growth factor (PDGF) (Gris et al., 2007), and their receptors has been reported to induce the transition of quiescent astrocytes into the reactive phenotype or to modulate the function of reactive astrocytes. Participating in the different pathogenic mechanisms of Alzheimer’s disease (Abraham, 2001), amyotrophic lateral sclerosis (ALS) (Pehar et al., 2005), Parkinson’s disease (Croisier and Graeber, 2006), and multiple sclerosis (Kielian and Esen, 2004), quiescent astrocytes become reactive in response to a wide variety of stimuli (Ridet et al., 1997; Heales et al., 2004). Reactive astrocytes in the glaucomatous ONH are large rounded cells with many thick processes which expresses increased amounts of GFAP, vimentin, and HSP27, that is motile, and that is located either at the edge of the laminar plates or inside the nerve bundles (Hernandez, 2000) (Fig. 1C).

Astrocyte cell–cell communication in the optic nerve head

The cell processes of astrocytes are connected to each other via gap junctions (Rose and Ransom, 1997) forming a functional syncytium that allows astrocytes to communicate and maintain control

of the ionic and metabolic homeostasis in the ONH. Astrocytic gap junctions are built mainly of connexin-43 (Cx43; GJA1). Connexins form a special class of ion channels that mediate cell-to- cell passage of ions and small molecules, which is thought to help coordinate the activity of astrocytes (Cusato et al., 2003; Nagy et al., 2004). The