(DBA/2J) has keratopathy at young ages and many mice develop corneal calcification as they age. These changes often render noninvasive measurements inaccurate. Therefore, we strongly recommend the use of invasive methods in the DBA/2J model. Currently, invasive methods are the only way to obtain accurate and reproducible IOP measurements in the majority of DBA/2J mice.

The future of IOP assessment

To truly understand the relationship between high IOP (duration, fluctuation, and magnitude) and glaucoma, new technologies are needed. An ideal system for measuring IOP would be capable of automated IOP assessment around the clock. A small implantable device that measures IOP over an extended period of time (ideally months or years) would revolutionize glaucoma research in mice and other animal models, and possibly even human patients.

Assessment of RGC function

Electrophysiological assessment of retinal function has been used to gain a better understanding of the function of many molecules in the retina, to understand disease pathogenesis, and to measure the efficacy of treatments for retinal disease in mice (Peachey and Ball, 2003; Pinto et al., 2007). Pang and Clark have reviewed the different electrophysiological tests and measures that have been used to assess retinal function in rodent models of glaucoma (Pang and Clark, 2007). Electroretinography (ERG) has been used extensively to assess retinal function after artificial elevation of IOP in rodents (Pang and Clark, 2007). The pattern electroretinogram (PERG) is a specialized kind of ERG that isolates inner retinal activity, including a large component that is generated by RGCs (Holder, 2004). In humans, the PERG appears to be an effective tool for assessing glaucomatous damage (Bach, 2001; Ventura et al., 2006). Recently, Porciatti and colleagues have refined the PERG for use in mice (Nagaraju et al., 2007; Porciatti, 2007; Porciatti et al., 2007; Saleh et al., 2007). The mouse retinal activity detected by PERG is greatly

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reduced early in glaucoma and thus PERG is a useful tool for assessing the efficacy of neuroprotection (Howell et al., 2007). PERG and other ERG approaches will likely prove valuable tools for assessing neuroprotective therapies as well as for helping to understand early pathophysiological events in glaucomatous neurodegeneration.

Mouse models of glaucoma

This section does not provide an exhaustive list of all mouse glaucoma models, as other reviews are available (John et al., 1999; Goldblum and Mittag, 2002; Gould et al., 2004b; John, 2005; Libby et al., 2005a; Lindsey and Weinreb, 2005; Weinreb and Lindsey, 2005; Pang and Clark, 2007). Mouse models relevant to high IOP and glaucoma can be divided into two classes, those that are inherited and those that are experimentally induced. Although experimentally induced models are valuable, inherited models are likely to more accurately model human glaucomas. We now discuss available inherited mouse models in the context of different types of human glaucoma.

Primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is the most common clinically defined subtype of glaucoma. In POAG, the angle and drainage routes are clinically observed to be unimpeded. There are at least 20 loci implicated in initiating POAG and only a few genes have been identified to date (Fan et al., 2006). The pathological mechanisms of POAG remain poorly defined. Studies in mice are helping to unravel mechanisms of POAG pathogenesis, but the challenge of developing mouse models of POAG remains.

MYOC

Approximately 3–5% of late-onset POAG patients and up to 30% of juvenile open-angle glaucoma patients (an earlier and more severe form of POAG) are caused by mutations in the myocilin gene (MYOC) (Wiggs et al., 1998; Fingert et al., 1999). Interestingly, patients with null or early truncation

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alleles of MYOC do not develop high IOP and POAG (Lam et al., 2000; Wiggs and Vollrath, 2001), and Myoc null mice of different genetic backgrounds do not develop high IOP (Kim et al., 2001). This suggests that mutant MYOC causes glaucoma through a gain of function mechanism. Supporting this, overexpressing Myoc in mice did not alter IOP (Gould et al., 2004a; Kim et al., 2001). Surprisingly, introducing a Y423H mutation (analogous to a severe human mutation, Y437H) into the mouse Myoc gene did not cause elevated IOP or glaucomatous neurodegeneration (Gould et al., 2006). Similarly, although open-angle glaucoma is reported in a separate study that introduced the same mutation into mice, IOP was elevated by only 2 mmHg in the mutant mice (Senatorov et al., 2006). This magnitude of IOP elevation seems highly unlikely to cause glaucomatous degeneration as the mean value remained well below that which normally causes glaucoma in mice. The inability of the mutant mouse Myoc gene to substantially elevate IOP may suggest that the response of mouse drainage structure cells to mutant MYOC is different to that of humans. Alternatively, a key difference between the mouse and human genes and encoded proteins may determine whether or not mutant MYOC is pathogenic.

In support of an important difference between the mouse and human proteins, virally expressing the human MYOC gene with the analogous Y437H mutation in mice resulted in substantial IOP elevation (Shepard et al., 2007). In the same study, viral expression of the same mutation in a version of the human gene that did not contain the peroxisomal targeting signal type 1 receptor (PTS1R) binding sequence did not elevate IOP (Shepard et al., 2007) (Fig. 1). The authors suggest that mutations in human MYOC gene expose a cryptic PTS1R binding site that is ordinarily buried within the normal protein. Exposing this site causes mislocalization of mutant MYOC protein to peroxisomes. Importantly, the cryptic PTS1R binding site does not exist in the mouse MYOC protein and so mutations in mouse Myoc gene cannot induce targeting of mutant proteins to peroxisomes. The inability of human and mouse proteins that lack the cryptic PTS1R sequence to elevate IOP, despite intracellular accumulation of

mutant proteins (Gould et al., 2006), suggests that peroxisomal targeting is a necessary component of pathogenesis.

Although not yet assessed, the model of virally expressing mutant MYOC to elevate IOP may provide a valuable and convenient model of glaucomatous neurodegeneration. If further studies show that this viral model does induce neurodegeneration, then it would be relatively easy to induce high IOP in mice with different genetic backgrounds. This approach has great potential for identifying genes that modify susceptibility to neurodegeneration. In addition, it will be important to develop a transgenic model expressing the mutant human MYOC gene. This will allow assessment of disease pathology without the complications and potential variability of viral infection. Transgenically humanizing the mouse would be likely to more closely model the human condition and may allow more convenient mechanistic studies.

OPTN

The optineurin (OPTN) gene is implicated in POAG, including patients without significant IOP elevation (normal-tension glaucoma, NTG). NTG may result from direct insults within the optic nerve and retina that are not pressure-induced. Although it is not clear if all reported OPTN mutations cause POAG (Alward et al., 2003; Libby et al., 2005a), the E50K mutation is clearly important (Rezaie et al., 2002; Sarfarazi and Rezaie, 2003). OPTN is expressed in RGCs (De Marco et al., 2006) and in astrocytes of the optic nerve (Obazawa et al., 2004). Overexpression of OPTN appears to have a protective effect on RGCs that are exposed to noxious stimuli (De Marco et al., 2006). In response to apoptotic stimuli, OPTN ordinarily translocates from the Golgi to the nucleus. This suggests OPTN translocation to the nucleus protects from apoptosis. The E50K mutant OPTN no longer relocates to the nucleus (De Marco et al., 2006) and this may render RGCs more susceptible to apoptotic death and glaucoma. Although some mouse strains have Optn mutations (Libby et al., 2005a), mice with the