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Retinitis pigmentosa (RP) is a heterogeneous group of genetic disorders leading to blindness, which remain untreatable at present. Rd1 mice represent a recognized model of RP, and so far only GDNF treatment provided a slight delay in the retinal degeneration in these mice. Bmi1, a transcriptional repressor, has recently been shown to be essential for neural stem cell (NSC) renewal in the brain, with an increased appearance of glial cells in vivo in Bmi1 knockout (Bmi1-/-) mice. One of the roles of glial cells is to sustain neuronal function and survival. In the view of a role of the retinal Müller glia as a source of neural protection in the retina, the increased astrocytic population in the Bmi1-/- brain led us to investigate the effect of Bmi1 loss in Rd1 mice. We observed an increase of Müller glial cells in Rd1-Bmi1-/- retinas compared to Rd1. Moreover, Rd1-Bmi1-/- mice showed 7-8 rows of photoreceptors at 30 days of age (P30), while in Rd1 littermates there was a complete disruption of the outer nuclear layer (ONL). Preliminary ERG results showed a responsiveness of Rd1-Bmi1-/- mice in scotopic vision at P35. In conclusion, Bmi1 loss prevented, or rescued, photoreceptors from degeneration to an unanticipated extent in Rd1 mice. In this chapter, we will first provide a brief review of our work on the cortical NSCs and introduce the Bmi1 oncogene, thus offering a rational to our observations on the retina. [ABSTRACT FROM AUTHOR]

Age related macular degeneration (AMD) is the leading cause of blindness among the elderly in the United States (Klein et al., 1992; Klein et al., 2002), and choroidal neovascularization (CNV) accounts for the majority of severe vision loss (Ferris et al., 1984). The current standard treatment for CNV is verteporfin photodynamic therapy (PDT) (Landy and Brown, 2003), which uses a laser to activate a photosensitizing dye accumulated within the CNV. Although PDT causes less damage to the retina overlying CNV than thermal laser, in normal primate (Husain et al., 1996; Kramer et al., 1996; Reinke et al., 1999; Peyman et al., 2000), rabbit (Peyman et al., 2000) and rat (Zacks et al., 2002) models, PDT damages photoreceptors and retinal pigment epithelial (RPE) cells. Although there has been no histologic evidence of damage to normal human retinal cells after PDT (Schlotzer-Schrehardt et al., 2002), patients treated with PDT experience visual disturbances and acute severe vision loss significantly more often than patients receiving placebo (Arnold et al., 2004; Azab et al., 2004). Because neurotrophic agents, such as brain-derived neurotrophic factor (BDNF) have been proven effective in reducing retinal damage in rodents after exposure to constant light (LaVail et al., 1992; Okoye et al., 2003), we hypothesized that BDNF treatment prior to PDT might reduce collateral damage to retinal and RPE cells in normal rats. [ABSTRACT FROM AUTHOR]

To elucidate the molecular involvement of transglutaminase (TG) in central nervous system (CNS) regeneration, we cloned a full-length cDNA for neural TG (TGN) from axotomized goldfish retinas and produced a recombinant TGN protein from this cDNA. The levels of TGN mRNA and protein were increased at 10-30 days after optic nerve transection, and this increase in TGN was only localized in the ganglion cells in goldfish retinas. In retinal explant cultures, the recombinant TGN protein induced a drastic enhancement of neurite outgrowth, while TGN-specific RNAi significantly suppressed this neurite outgrowth. Taken together, these data strongly indicate that TGN is a key regulatory molecule for CNS regeneration. [ABSTRACT FROM AUTHOR]

Photoreceptor survival depends on the integrity of retinal pigment epithelial (RPE) cells. The pathophysiology of several retinal degenerations involves oxidative stress-mediated injury and RPE cell death; in some instances it has been shown that this event is mediated by A2E and its epoxides. Photoreceptor outer segments display the highest DHA content of any cell type. RPE cells are active in DHA uptake, conservation, and delivery. Delivery of DHA to photoreceptor inner segments is mediated by the interphotoreceptor matrix. DHA is necessary for photoreceptor function and at the same time is a target of oxidative stress-mediated lipid peroxidation. It has not been clear whether specific mediators generated from DHA contribute to its biological properties. Using ARPE-19 cells, we demonstrated the synthesis of 10,17S-docosatriene [neuroprotectin D1 (NPD1)]. This synthesis was enhanced by the calcium ionophore A-23187, by IL-1β, or by supplying DHA. Added NPD1 (50nM) potently counteracted H2O2/tumor necrosis factor-α oxidative stress-triggered apoptotic DNA damage in RPE. NPD1 also up-regulated the anti-apoptotic proteins Bcl-2 and Bcl-xL and decreased pro-apoptotic Bax and Bad expression. Moreover, NPD1 (50nM) inhibited oxidative stress-induced caspase-3 activation. NPD1 also inhibited IL-1β-stimulated expression of COX-2. Furthermore, A2E-triggered oxidative stress induction of RPE cell apoptosis was also attenuated by NPD1. Overall, NPD1 protected RPE cells from oxidative stress-induced apoptosis. In conclusion, we have demonstrated an additional function of the RPE: its capacity to synthesize NPD1. This new survival signaling is potentially of interest in the understanding of the pathophysiology of retinal degenerations and in exploration of new therapeutic modalities. [ABSTRACT FROM AUTHOR]

The RPE performs highly specialized, unique functions essential for homeostasis of the neural retina. These include phagocytosis of photoreceptors shed outer segments, directional transport of nutrients into and removal of waste products from photoreceptor cells and visual pigment transport and regeneration. All of these functions involve the RPE apical microvilli.1-4 [ABSTRACT FROM AUTHOR]

The human retinal pigment epithelium (hRPE) is a monolayer of cells that is located between the photoreceptors and Bruch's membrane. Normally, it is mitotically inactive in adult eyes but sometimes it undergoes mitosis and cell division in pathologic states. Growth factors have been implicated in inducing proliferation and migration of hRPE (Kothary and Del Monte, 2003). [ABSTRACT FROM AUTHOR]

Lipid rafts are specialized membrane domains enriched in certain lipids, cholesterol and proteins. The existence of lipid rafts was first hypothesized in 1988 (Simons and van Meer, 1988; Simons and Ikonen, 1997), but what we know as "caveolae", flask-shaped types of lipid rafts, were observed earlier (Yamada, 1955). Three general types of rafts- caveolae, glycospingolipid enriched membranes (GEM), and polyphosphoinositide-rich rafts- have been described (Jacobson and Dietrich, 1999) and may be oriented on the "inner leaflet" (PIP2 rich rafts and caveolae) or the "outer leaflet" (GEM). The fatty acid chains of lipids within the raft tend to be more saturated and these are more tightly packed, creating domains with higher order. It is therefore thought that rafts exist in a separate ordered phase that floats in a sea of poorly ordered lipids. [ABSTRACT FROM AUTHOR]

Daily phagocytosis of shed photoreceptor outer segment fragments (POS) is a key task of the retinal pigment epithelium (RPE) in the retina. Lack or inefficiency of daily POS clearance causes early onset, rapid, and complete retinal degeneration in experimental animals and likely contributes to human blinding diseases such as retinitis pigmentosa and age-related macular degeneration (Dowling and Sidman, 1962, Gal et al., 2000). The phagocytic mechanism of the RPE belongs to a group of conserved non-inflammatory clearance pathways that mediate recognition and engulfment of apoptotic cells in both non-professional and professional phagocytic cells, such as fibroblasts and macrophages, respectively (Finnemann and Rodriguez-Boulan, 1999). These pathways share the use of phagocyte cell surface receptors such as the lipid scavenger receptor CD36 (Ryeom et al., 1996), the integrin adhesion receptor αv β5 (Finnemann et al., 1997; Miceli et al., 1997; Lin and Clegg, 1998) and the receptor tyrosine kinase Mer (MerTK) (D'Cruz et al., 2000; Nandrot et al., 2000). In vitro phagocytosis assays studying primary or permanent RPE in culture fed with isolated POS suggest that CD36 and MerTK participate in the engulfment step of the phagocytic process (Chaitin and Hall, 1983; Finnemann and Silverstein, 2001), while αv β5 integrin promotes POS recognition/binding and initiates a downstream cytoplasmic signaling cascade in the RPE (Finnemann et al., 1997). However, the precise function of these receptors and their roles in the intact retina are so far only poorly understood. Most recently, we have begun to study phagocytosis and receptor activity in animal models that lack αv β5 integrin or MerTK to determine how these different plasma membrane receptors of the RPE functionally interact to coordinate particle uptake. [ABSTRACT FROM AUTHOR]

Vertebrate vision begins with the absorption of light by visual pigments in photoreceptor cells. Visual pigments, or opsins, are seven membrane spanning, G protein-coupled receptors located in the membrane of the outer segment discs of rods and cones. In the dark, the light sensitive chromophore 11-cis-retinal is covalently attached to opsin through a Schiff base linkage to a specific lysine residue located in the center of the seventh transmembrane alpha helix. Light stimulation results in isomerization of 11-cis-retinal to all-trans-retinal, which causes a change in the conformation of rhodopsin. The resulting photoactivated metarhodopsin II interacts with the G protein transducin and triggers the phototransduction cascade leading to hyperpolarization of photoreceptors and ultimately to inhibition of neurotransmitter release at the synaptic terminus. After isomerization of 11-cis-retinal to the trans configuration, the Schiff base is hydrolyzed and the photolyzed chromophore separates from opsin. Whether all-trans-retinal is released in the lumen of the discs and subsequently transported to the cytosol by the retinal ATP-binding cassette transporter (ABCR)1 or directly released into the cytosol2 is controversial. Cytosolic all-trans-retinal is then reduced to all-trans-retinol by a retinol dehydrogenase (RDH) located in the membrane of the photoreceptor outer segment discs. This or these enzymes have not yet been identified. However, six distinct RDHs expressed in photoreceptors have recently been cloned (Table 70.1). Their functions, in vivo, are unknown, but all of them were shown to reduce all-trans-retinal in vitro. [ABSTRACT FROM AUTHOR]

Cyclic GMP phosphodiesterase 6 (PDE6), a member of a large superfamily of phosphodiesterases (Soderling et al., 1998; Beavo et al., 1994), is a key enzyme in the rod and cone phototransduction cascades (Polans et al., 1996; McBee et al., 2001). PDE6 in rod photoreceptors (henceforth called PDE) is composed of two catalytic subunits-PDEα and PDE β-and two identical inhibitory subunits, PDEγ (Baehr et al., 1979; Fung et al., 1990; Deterre et al., 1988). Photoreceptor PDE is peripherally membrane-associated via the farnesyl and geranylgeranyl chains at C-termini of PDEα and PDE β, respectively (Anant et al., 1992; Qin et al., 1992; Qin and Baehr, 1994). PDEδ was originally copurified with photoreceptor PDE from bovine retina and considered the fourth subunit of PDE (Gillespie et al., 1989). Functional studies indicated that PDEδ could solubilize membrane-associated PDE and decouple the activation of transducin from hydrolysis of cGMP when added to the permeabilized rod outer segments (Cook and Beavo, 2000; Florio et al., 1996). However, first evidence arguing against PDEδ being an authentic PDE subunit came from the expression profile of PDEδ. Multiple tissue northern blots indicated that PDEδ mRNA is present in all tissues examined with relatively higher level in retinas, in contrast to PDE which is only expressed in photoreceptors of the retina (Florio et al., 1996; Marzesco et al., 1998). Furthermore, PDEδ homologues were also identified in organisms such as C. elegans which has no eyes or retina-like structures and does not express PDE6 (Li and Baehr, 1998). Like mammalian PDEd, recombinant PDEd from C. elegans can elute PDE from bovine rod outer segments, suggesting the functional conservation of PDEδ throughout evolution. [ABSTRACT FROM AUTHOR]

The visual cycle is the complex enzymatic retinoid-processing involved in regenerating bleached rod and cone visual pigments.1 Central to visual cycle physiology is the cellular retinaldehyde-binding protein (CRALBP), a 36kDa cytosolic protein with high affinity for 11-cis-retinal and 11-cis-retinol. CRALBP is expressed in retinal pigment epithelium (RPE) and Müller cells, as well as in ciliary epithelium, iris, cornea, pineal gland and a subset of oligodendrocytes of the optic nerve and brain.2 Its function outside the RPE is not known, although a recent behavioral genetic study suggests that CRALBP may contribute to ethanol preference in mice.3 In the RPE, CRALBP serves as an 11-cis-retinol acceptor in the visual cycle isomerization step and as a substrate carrier for 11-cis-retinol dehydrogenase. 4-8 These functions require the rapid association and release of retinoid from the CRALBP ligand-binding pocket and involve critical protein interactions. To better understand the visual cycle, we are characterizing CRALBP ligand and protein interactions and retinoid trafficking within the RPE. [ABSTRACT FROM AUTHOR]

The vertebrate photoreceptor is the epitome of polarized neurons, containing two specialized compartments—the outer segment and the inner segment, connected by a narrow non-motile cilium. The outer segment of rod and cone photoreceptors is principally dedicated to capturing light and converting the energy of a photon into a change in membrane potential. The primary function of the inner segment is to provide the metabolic and synthetic demands of the photoreceptors. In order to maintain this high degree of specialization, molecules are routinely targeted to their appropriate compartment during protein synthesis. However, in addition to this relatively slow transport process, photoreceptors have a much more rapid process whereby some molecules are rapidly moved between the inner segment and outer segment through the connecting cilium in response to the light adaptational state of the eye. This translocation process has been conclusively demonstrated for two molecules involved in the phototransduction cascade—transducin and arrestin (Broekhuyse et al. 1985; Mangini and Pepperberg 1988; Whelan and McGinnis 1988; Sokolov et al. 2002; Peterson et al. 2003). [ABSTRACT FROM AUTHOR]

Mutations in the aryl hydrocarbon receptor interacting protein-like 1 (AIPL1) cause the devastating blinding disease Leber's congenital amaurosis (LCA) (Sohocki et al., 2000a). Up to 12% of recessive LCA is caused by mutations in AIPL1 (Sohocki et al., 2000b). In addition to AIPL1, LCA-causing mutations have also been identified in RetGC1, RPE65, CRX, LRAT, CRB1 and RPGRIP1, and a further two loci have been identified on 14q24 and 6q11-16 (www.retina-international.org/sci-news/mutation.htm). Although the function of AIPL1 is unknown, AIPL1 shares 49% identity with the human aryl hydrocarbon receptor (AhR)-interacting protein (AIP), also named XAP2 or ARA9 (Sohocki et al., 2000a). AIP in turn shares similarity with members of the immunophilin family of proteins including the co-chaperones FK506-binding protein (FKBP) 51 and 52 (reviewed in Chapple et al., 2001; van der Spuy and Cheetham, 2004a). Both AIP and the FKBP co-chaperones exist in a cytosolic ternary complex with the molecular chaperone Hsp90 and a specific cognate receptor, and have been shown to regulate the nuclear translocation and transactivation of the associated receptor. At the primary structural level, the tetratricopeptide repeat (TPR) motif is conserved in AIPL1, AIP and FKBP51/52. The TPR motif is an evolutionary and functionally conserved but degenerate motif found in a number of structurally unrelated proteins and mediates the binding of specific protein-interaction partners. The TPR motif in both AIP and FKBP51/52 form a TPR carboxylate clamp that mediates their interaction with the C-terminal MEEVD TPR acceptor site of Hsp90. The similarity of AIPL1 to AIP has led to suggestions that AIPL1 could function in a similar manner to AIP in facilitating protein translocation and as a component of chaperone complexes. [ABSTRACT FROM AUTHOR]

ABCA4, also known as ABCR or the rim protein, is a member of the ABCA subfamily of ATP binding cassette transporters (Allikmets et al., 1997b; Azarian and Travis, 1997; Illing et al., 1997). It is organized into two tandem arranged halves with each half consisting of a transmembrane segment followed by a large extracellular domain, a multi-spanning membrane domain and a cytoplasmic nucleotide binding domain (Figure 64.1) (Bungert et al., 2001). ABCA4 is localized along the rims and incisures of rod and cone photoreceptor outer segment discs where it is thought to play a role in the visual cycle (Molday et al., 2000; Sun et al., 1999; Weng et al., 1999). Mutations in the gene encoding ABCA4 are responsible for a variety of autosomal recessive retinal degenerative diseases including Stargardt macular dystrophy, cone-rod dystrophy, and retinitis pigmentosa (Allikmets, 2000; Allikmets et al., 1997b; Cremers et al., 1998). Individuals who are heterozygous for selected Stargardt-causing mutations are at a high risk for developing age-related macular degeneration (Allikmets et al., 1997a). [ABSTRACT FROM AUTHOR]

In this Chapter we shall briefly review our recent studies in neonatal rats regarding the mechanisms involved in retinal ganglion cell death (RGC) after loss of central visual target areas in the brain. We also describe the influence of neurotrophins on RGC survival and consider the apparently symbiotic relationship between these retinal neurons and the cells they innervate in the developing brain. [ABSTRACT FROM AUTHOR]

We showed that a functional complex of CA4 and Na+/bicarbonate co-transporter (NBC1) is specifically expressed in the choriocapillaris and that mutations in CA4 disrupt NBC1-mediated HCO3−- transport leading to acidification of the retina. This finding (Yang et al., 2005) point to the importance of a functional CA4 for the survival of photoreceptors and imply that CA inhibitors may have long-term adverse effects on vision. [ABSTRACT FROM AUTHOR]

Retinal cell loss in diseases such as Retinitis Pigmentosa occurs through an apoptotic process.1 The mechanism of this cell loss is not completely understood. Models that allow for the study of conditions in which the retina is susceptible or resistant to retinal damage help to elucidate the mechanism underlying the cell death. One model that is used to study retinal cell loss is the light induced retinal degeneration (LIRD) model.2 Intense light exposure leads to rhodopsin bleaching2 and is the trigger for the subsequent photoreceptor cell degeneration, as blocking the regeneration of rhodopsin prevents photoreceptor cell death.3 Oxidative stress is also involved in retinal degeneration,2,4-7 and the administration of natural or synthetic antioxidants prior to light exposure prevents the subsequent cell loss.7-13 Many factors can influence the extent of light induced damage including prior light history of the animals, age, genetics, and diet.4,8,14 The extent of LIRD is also dependent on the time of light exposure initiation. Animals exposed to light during the dark phase of the dark-light cycle suffer greater retinal damage than rats exposed to light during the day.15 More recently, it was reported by Organisciak et al that relatively brief intense light exposure commencing at 0100h lead to a 2-4 fold greater loss of photoreceptor cells in rat retina than light exposure beginning at 1700h.16 A fundamental question then is to ask what differences exist in the retina at various times of the day. It was suggested that endogenous factors regulated in a circadian manner might be involved in the observed difference in susceptibility to LIRD.16 Circadian rhythms cue an organism about night/day changes, have a period of approximately 24 hours, and persist in constant darkness or constant light. These rhythms are entrained by light, but can also be entrained by temperature17 and feeding.18 Circadian rhythms underlie many physiological processes that can lead to a stress tolerant or susceptible environment. There are several prominent circadian regulated physiological processes in the retina. Melatonin and dopamine are synthesized in a circadian manner or in response to light-dark cues, and may be involved in entraining the circadian cycle. They are the most obvious candidates for involvement in increased retinal susceptibility to light. Phototransduction protein levels are also important candidates as they are directly involved in modulating the retinas response to light. A fairly new candidate is metabolic activity of the retina. Increased metabolic activity results in decreased pH levels which may also play a key role as it has been shown to be involved in inducing apoptosis.19 Any or all of these factors may be involved in the circadian dependent susceptibility to light damage. [ABSTRACT FROM AUTHOR]

Photoreceptors are vulnerable to both a lack and an excess of oxygen. Hypoxia causes a photoreceptor-specific degeneration during the critical period of normal photoreceptor development (Maslim et al., 1997), in the naturally occurring degeneration of the RCS rat (Valter et al., 1998), in direct hypoxia (induced by low inhaled pO2) of the adult retina (Mervin and Stone, 2002b), and in the detached retina (Mervin et al., 1999). Hyperoxia causes photoreceptor degeneration at the edge of the normally developing retina (Mervin and Stone, 2002b, Stone et al., 2004) and brief reports available for the rabbit (Noell, 1955) and mouse (Yamada et al., 2001, Walsh et al., 2004a) indicate that photoreceptors degenerate when oxygen-enriched air is inhaled. [ABSTRACT FROM AUTHOR]

Retinal degenerations can be promoted by many factors including ageing, ischemia, fluctuation in oxygen tension, oxidative stress, and increased intraocular pressure. We present new evidence that the environment encountered in space shuttle flight can also disrupt normal retinal development and mimic stimuli that induce retinal degenerations on earth. There is experimental evidence linking anomalies in visual perception with space flights since the Apollo missions (Phillpot et al., 1978; Newberg and Alavi, 1998). There is also strong evidence that pathological stimuli that disrupt retinal structure and function on earth are encountered in the space shuttle environment as well. Orbital space flights cause physiological disturbances in humans including cephalad fluid shift (Hoffler et al., 1977; Drummer, 2000), increased intraocular pressure (Mader et al., 1990; Draeger, 2000) disruption of cardiovascular function (Wang et al., 1996) and stress on the musculoskeletal system (Lane and Feedback, 2002; LeBlanc, 2000). [ABSTRACT FROM AUTHOR]

The retina has long been assumed to remain in stasis after photoreceptor degeneration effectively deafferents the neural retina (Zrenner, 2002). However, a growing literature reveals the more insidious details of retinal degeneration and evidence of early plasticity. Retinal degenerations typically undergo three phases. Early changes observed in phase one are triggered by photoreceptor stress and include misrouting of rhodopsin to the inner segments of photoreceptors (Milam et al., 1998) followed by rhodopsin delocalization to processes extending down in fascicles projecting into the inner nuclear and ganglion cell layers (Li et al., 1995; Milam et al., 1996). Phase two is characterized by active photoreceptor cell death eventually deafferenting bipolar cell populations and eliminating light mediated signaling to the neural retina. Also observed in phase two is the formation of the Müller cell (MC) seal, entombing or walling off the remnant neural retina from what is left of the retinal pigment epithelium and vascular choroid (Jones et al., 2001; Jones et al., 2003; Marc et al., 2003). Formation of the Müller cell seal is likely due to collapse of distal elements of Müller cells, but is also possibly due to hypertrophic processes. Before completion of phase two, all dendritic elements of bipolar cells have retracted and horizontal cells typically have sent axonal processes into the inner plexiform layer (IPL). (Strettoi and Pignatelli, 2000; Park et al., 2001; Strettoi et al., 2002; Strettoi et al., 2003). The final stage of remodeling, phase three, was originally described in the GHL mouse (Jones et al., 2001), however at the time the extent of remodeling across models and the implications for vision rescue was not appreciated. Subsequent work in naturally occurring and genetic models (Jones et al., 2003) revealed extensive remodeling in response to photoreceptor degeneration. This remodeling involves the evolution of processes from all classes of neurons into fascicles that may run for >100 microns in addition to elaboration of new "tufts" of IPL (microneuromas) that form outside the boundaries of the normal stratification of the IPL. These microneuromas are populated with synaptic contacts corruptive of normal visual processing (Marc et al., 2003). Finally, migration of adult neuronal phenotypes throughout the vertical axis of the retina is observed with all cell classes participating. It is believed that in order to maintain normal gene expression, neurons will sprout processes to seek lost glutamatergic signaling. Failing to achieve synaptic contact may result either in cell death or cellular soma migration to other regions of the retina. Amacrine cells are commonly observed translocating to the ganglion cell layer with ganglion cells also migrating up into the inner nuclear layer. [ABSTRACT FROM AUTHOR]

The vertebrate visual system is a valuable model for examining recovery after injury to the central nervous system (CNS). It is a relatively "simple" part of the CNS having one major class of projection neuron, the retinal ganglion cells (RGCs), which make topographic connections within well defined visual nuclei, thus recreating visual space within the brain. Topographic maps can be readily assessed electrophysiologically and anatomically and are a critical template for useful visually guided behaviour which can be examined behaviourally. Furthermore, the optic nerve is accessible, an extra-foramenal crush injury severing all RGC axons but leaving the meningeal sheath intact as a conduit for regeneration and preventing gross axonal mis-routing. The procedure also leaves the blood supply to the eye patent, avoiding ischaemic-induced RGC death. [ABSTRACT FROM AUTHOR]

Retinal ganglion cell (RGC) death is the key pathological event in glaucoma and the biological basis for the loss of vision. Although significant advances have been made in the medical and surgical treatment of glaucoma, the disease remains the most common cause, worldwide, of irreversible vision loss (Quigley, 1996). Our understanding of the role played by elevated intraocular pressure (IOP) in the initiation of glaucoma has recently been advanced by evidence from clinical trials that IOP levels within the normal range can influence the degree of retinal ganglion cell death (AGIS, 2000). For those patients with advanced glaucoma damage, a reduction in IOP, even within the normal range, can reduce the risk of progressive vision loss. [ABSTRACT FROM AUTHOR]

It is conceivable that in the future, stem cells will be used in the treatment of retinal degenerative conditions. They could be transplanted to replace the photoreceptor victims of retinal degeneration or to function as vehicles for the provision of survival or regenerative factors. Their full potential will, most likely, only be realized with thorough investigations of both embryonic and adult stem cells. Studies into the use of stem cells for the treatment of retinal degenerations have primarily involved retinal and neural stem cells. A few laboratories, including our own, have investigated allografts of stem cells from more distant sites like the bone marrow to the retina. Still other groups have investigated the application of embryonic stem cells for treatment of retinal degenerations. In this mini-review we will compare adult versus embryonic stem cells for use in the retina and summarize the recent investigations using stem cells to repair the degenerating retina. Due to space limitations, we are unable to cite all relevant papers so have chosen to focus on key articles that have made significant contributions to this field. [ABSTRACT FROM AUTHOR]

In western countries, retinitis pigmentosa (RP) affects 1/3,500 individuals and age related macula degeneration (AMD) affects 1% to 3% of the population aged over 60. In vitro generation of retinal cells is thus a promising tool to screen protective drugs and to provide an unlimited cell source for transplantation. However, one main limitation is the amount of cells available. Stem cells, that can generate unlimited quantity of cells, could overcome this hurdle. Indeed, stem cells are defined by three characteristics: the ability to produce a large population of cells (expansion) and the potency, to produce the differentiated cells composing the organ from which the stem cells are originated. They are also able to self-renew indefinitely: for instance haematopoietic stem cells, located in the bone marrow, can expand, divide and generate differentiated cells into the diverse lineages throughout the life, the stem cells conserving its status (Till et al, 1961). Intestinal stem cells also are able to regenerate the intestine all along life (Potten et al, 1975). The other stem cells properties are the ability to produce a large population of cells (expansion) and as well as the differentiated cells composing the organ from which they originated. [ABSTRACT FROM AUTHOR]

Vascular endothelial growth factor (VEGF) is a potent stimulator of angiogenesis and is essential for normal embryonic development and many physiological events that require the growth of new blood vessels. Abnormal expression of endogenous VEGF can lead to ocular diseases including age related macular degeneration (Ohno-Matsui et al., 2001) and diabetic retinopathy (Aiello et al., 1994; Boulton et al., 1998), which are the two leading causes of blindness in the developed world. Regulation of VEGF expression occurs primarily through trans-factor interactions with cis-elements located on the 5′ and 3′ untranslated regions (UTR's) and include stabilizing and destabilizing elements in addition to enhancer regions (Coles et al., 2004; Dibbens et al., 1999; Iida et al., 2002; Levy et al., 1997; Marano et al., 2004). The prime stimuli of VEGF upregulation are hypoxic or ischemic conditions, which indirectly activates VEGF through interactions between hypoxia inducible factor 1 (HIF-1) and the hypoxia response element (HRE) located within the promoter region of the VEGF gene (Forsythe et al., 1996). [ABSTRACT FROM AUTHOR]

Retinal transplantation is one among the various treatment strategies aimed to prevent and restore visual loss. Sheets of fetal retina with or without retinal pigment epithelium (RPE) are transplanted into the subretinal space. Retinal transplants have been shown to substantially improve visual responses in rat retinal degeneration models following retinal transplantation, based on behavior and electrophysiology. The transplantation effects may be influenced by several factors such as the age of the recipient at transplantation and the type of species used. Modified functional evaluation techniques permit better understanding of the physiological mechanisms underlying visual improvement in animal models. [ABSTRACT FROM AUTHOR]

Brain-derived neurotrophic factor (BDNF) has reported to show photoreceptor protection for retinal degeneration either genetically programmed,1 or light induced experimental retinal degeneration.2 Genetically modified BDNF gene expressing cell transplantation in the subretinal space also rescued light induced photoreceptor degeneration.3 When we consider the genetically modified BDNF gene expressing cell transplantation, the fate or the behavior of the cell at the subretinal regions has still unclear, especially at the diseased subretinal lesions. The cells placed at deeper layer of bruch membrane tend to be affected by apoptosis.4 The fate of the transplanted cell may be one of the important factors for success of the transplantation. Cells cultured on poly-(N-isopropylacrylamide (PIPAAm)-grafted plates were easily detached from the culture plates as cell sheet by reducing the temperature from 37° C to 20° C without using enzymes.5 We further cultured the cell and examined the degree of apoptosis by comparing with those of cells collected by trypsin treatment. [ABSTRACT FROM AUTHOR]

The retinal pigment epithelial (RPE) cell shares its origin with the neural retina as an anterior neural plate derivative. Recently RPE cell was reported on its capacity of transdifferentiating into a neuron-like cell in mammals.2,2 Neurotrophic factors have reported to play essential roles. Some of these factors are basic fibroblast growth factor (bFGF) for transdifferentiation of RPE into neural cells2,3 or epithelial growth factor (EGF) for proliferation of neural progenitor cells.4 [ABSTRACT FROM AUTHOR]

Human Usher syndrome (USH) is the most common form of deaf-blindness and also the most frequent case of recessive retinitis pigmentosa. According to the degree of the clinical symptoms, three different types of the Usher syndrome are distinguished: USH1, USH2 and USH3 (Davenport and Omenn, 1977). USH is genetically heterogeneous with eleven chromosomal loci, which can be assigned to the three USH types (USH1A-G, USH2A-C, USH3A) (Petit, 2001). Out of these, USH1 is the most severe form, characterized by profound congenital deafness, constant vestibular dysfunction and prepubertal-onset retinitis pigmentosa. USH2 patients show a milder congenital deafness, a slightly later onset of retinitis pigmentosa and no vestibular dysfunction. The rarest Usher type 3 shows a late onset of retinitis pigmentosa and a progressing hearing loss. So far the different USH subtypes have been grouped into one disease basically on the same phenotype of the patients, although the clinical symptoms of the individual differ noticeably. The protein harmonin, responsible for USH1C, is of special interest, since it contains three PDZ domains, known for protein-protein interactions. We have gathered evidence that the different USH proteins are molecularly linked essentially via the scaffold protein harmonin. Harmonin interacts hereby not only with USH1 proteins, but also with USH2 proteins. Thus, this is the first evidence for a molecular linkage between USH1 and USH2, beyond the shared phenotype. [ABSTRACT FROM AUTHOR]

Usher syndrome (USH) describes a group of inherited blindness-deafness disorders, resulting from retinal degeneration and cochlear dysfunction (Usher, 1914). There are three subtypes of Usher syndrome, Usher syndrome type 1 is the most severe of the three. Seven Usher 1 genes have been mapped (Usher 1A-G), and all show the same clinical phenotype in humans. The most common form is Usher 1B, which accounts for at least 50% of Usher 1 cases (Astuto et al., 2000). Usher 1B is caused by mutations in the gene, MYO7A, which encodes an unconventional myosin, myosin VIIa (Weil et al., 1995). Usher 1C has been shown to be caused by defects in the gene, USH1C, which encodes harmonin, a scaffold protein with PDZ domains (Verpy et al., 2000; Bitner-Glindzicz et al., 2000). PDZ proteins, such as harmonin, form multiprotein complexes that are localized in specific subcellular domains, such as the microvilli of epithelial cells, synaptic terminals and the tight junctions (Sheng and Sala, 2001). Alternative splicing of the USH1C gene results in multiple harmonin isoforms, named a, b and c (Verpy et al., 2000). The short isoform a is the most abundant of the three and is present in most tissues. Both harmonin and myosin VIIa have been found in the stereocilia of the hair cells in the inner ear and the microvilli of other epithelial cells (Wolfrum et al., 98; Verpy et al., 2000; Boöda et al., 2002). Recent studies have shown that these two proteins interact to shape the stereocilia bundle in the inner ear (Boöda et al., 2002). [ABSTRACT FROM AUTHOR]

Generally, nerve injury of adult mammalian CNS neurons leads to a retrograde neuronal degeneration and cell death. The retinal ganglion cells (RGCs) of rat fail to regenerate and become apoptotic after optic nerve injury. In contrast, goldfish RGCs can survive and regrow their axons after injury. Focusing on this different response of RGCs in both species to optic nerve injury, we compared cell death and cell survival signals in the rat and goldfish RGCs after optic nerve injury. In goldfish retina, levels of phospho-Akt (p-Akt) and phospho-Bad (p-Bad) first rapidly increased at 3-5 days after optic nerve injury. Subsequently, levels of Bcl-2 increased and caspase-3 activity decreased at 10 days after nerve injury. In rat retina, levels of p-Akt and p-Bad first rapidly decreased at 1-2 days after optic nerve injury. Subsequently, levels of Bax and caspase-3 activity increased 6 days after optic nerve crush. These changes after optic nerve injury were all morphologically localized only in the RGCs. The data suggest that goldfish RGCs are warranted the cell survival by rapid p-Akt and subsequent Bcl-2 activations during the optic nerve regeneration, whereas rat RGCs are made a progress of the cell death by rapid inactivation of p-Akt and subsequent activation of Bax after optic nerve crush. [ABSTRACT FROM AUTHOR]

The regulation of extracellular glutamate levels in the retina, under physiological and pathophysiological conditions, is essential for the prevention of excitotoxic neurodegeneration. Glial and neuronal high-affinity glutamate transporters (excitatory amino acid transporters, EAATs) facilitate the rapid removal of glutamate from the extracellular space, thereby terminating the excitatory signal and reducing the possibility of excitotoxic neuronal damage. Failure or reversal of these transport systems leads to raised levels of extracellular glutamate and contributes to the development of excitotoxic retinal degeneration. [ABSTRACT FROM AUTHOR]

The artificial silicon retina (ASRTM) consists of an array of photodiodes on a silicon disk that responds to incident light in a gradient fashion (Peyman et al., 1998; Chow et al., 2001, 2002). This device is designed to be placed in the subretinal space and serve as a replacement for degenerating photoreceptors. Two possible mechanisms for the ASR device to improve visual function include 1) direct activation of the remaining inner retinal neurons and subsequent activation of visual centers in the brain or 2) a delay in photoreceptor loss due to a neurotrophic effect from subretinal electrical stimulation. Initial results of ongoing FDA trials with the ASR device suggest that subretinal electrical stimulation could elicit a neurotrophic effect (Chow et al., 2004). Ten advanced retinitis pigmentosa (RP) patients implanted with the ASR device have increased central visual fields and improved visual acuity and color vision (Chow et al., 2004). These improvements cannot be easily explained by direct activation since the implant was placed 20° from the macula. To determine whether neuroprotection results from subretinal electrical stimulation, the RCS rat model of RP was implanted with an ASR device. Subretinal implantation of an ASR device into the superior retina of the Royal College of Surgeons (RCS) rat resulted in preservation of photoreceptors (Pardue et al., 2004). However, the RCS rat is known to have delayed photoreceptor degeneration in the superior region of the retina (LaVail and Battelle, 1975). To determine whether the superior retina is a "privileged" site in the RCS rat, ASR devices were subretinally implanted in the superior and inferior retina. [ABSTRACT FROM AUTHOR]

The foremost cause of irreversible blindness in major retinal diseases is photoreceptor degeneration. In animal models as well as in human retinal hereditary dystrophies, the mutations described since 1990 affect mainly coding sequences for structural proteins (peripherine, Rom 1) or components of the phototransduction cascade (rhodopsin, cGMP-dependent phosphodiesterase) found in the rod outer segments.1,2,3 The mechanisms leading to programmed cell death of these cells are still hypothetical.4 In addition to this direct rapid rod loss, delayed cone loss is seen in clinical situations and was described in 1978 in the "retinal degeneration" (rd) mouse model. Their loss is responsible for the major visual handicap because cones are essential for diurnal, colour and central vision.6 This secondary loss of cone photoreceptors does not have any obvious explanation since cones are generally not directly affected by the genetic anomaly found in these diseases. [ABSTRACT FROM AUTHOR]

Macular edema is a swelling of the macula that can result in decreased visual acuity. Because the macula is extensively surrounded by blood vessels, any resulting leakage can lead to macular edema and subsequent visual loss. Such leakage can be secondary to retinitis pigmentosa, and other retinal diseases including diabetic retinopathy, retinal vein occlusion, inflammatory processes such as uveitis, or can be a result of ocular surgery, referred to as Irvine-Gass Syndrome. [ABSTRACT FROM AUTHOR]

Neurons are continuously subjected to fluctuating levels of oxidative stress, neurotransmitters and other compounds that have the potential of damaging the cells. Under physiological conditions the levels of these compounds do not reach pathological levels and the cells survive. Under pathological conditions, however, the compounds reach toxic levels and apoptotic cell death can be triggered. Endogeneous neuroprotective factors are molecules which can prevent the switch from survival to cell death. These factors work in several ways, only a few of which will be considered in this chapter. [ABSTRACT FROM AUTHOR]

X-linked retinoschisis (RS) is an inherited recessive macular degeneration that affects between 1 in 5000 and 1 in 25,000 males early in life (George et al., 1995; Sieving, 1998; Tantri et al., 2004). It is characterized by a loss in central vision, splitting of the retina with the appearance of spoke-like cystic cavities radiating from the parafoveal region of the retina, a loss in the b-wave of the electroretinogram (ERG), and progressive atrophy of the macula. In about 50% of the cases, bilateral schisis is observed in the peripheral retina with some loss in peripheral vision. During the course of the disease, complications can arise which include retinal detachment, vitreal hemorrhage and choroidal sclerosis. [ABSTRACT FROM AUTHOR]

Gene therapy (GT) can be described as the in vivo transfer of DNA for therapeutic purposes. In the case of congenital retinal dystrophies (RD), GT can only be considered a successful treatment option if the gene transfer results in the restoration of vision at some level. Thus, a critical component of developing GT treatments for RDs is assessing the amount of functioning vision that is produced. In mouse models of RD, visual function after gene therapy is tested using techniques such as electroretinograms (ERGs) and retinoid analysis.1-3 A potentially useful addition to these tests would be a murine behavior-based technique,4 like those used with the RPE65 dog model, to demonstrate effective GT-induced visual recovery. [ABSTRACT FROM AUTHOR]

X-linked retinoschisis (RS) is an inherited recessive macular degeneration that affects between 1 in 5000 and 1 in 25,000 males early in life (George et al., 1995; Sieving, 1998; Tantri et al., 2004). It is characterized by a loss in central vision, splitting of the retina with the appearance of spoke-like cystic cavities radiating from the parafoveal region of the retina, a loss in the b-wave of the electroretinogram (ERG), and progressive atrophy of the macula. In about 50% of the cases, bilateral schisis is observed in the peripheral retina with some loss in peripheral vision. During the course of the disease, complications can arise which include retinal detachment, vitreal hemorrhage and choroidal sclerosis. [ABSTRACT FROM AUTHOR]

Gene therapy has been reported to show potential as an alternative treatment to conventional therapy. In ocular research, recombinant adeno-associated virus (rAAV) has been chosen as a vector of interest due to its low immunogenicity, its broad host range and its ability to result in long-term transduction (Ali et al., 1997; Bennett and Maguire, 2000). [ABSTRACT FROM AUTHOR]

Leber congenital amaurosis (LCA) is a retinitis pigmentosa with early onset, leading to blindness in infants. There is currently no efficient therapy to treat LCA. At the present time, mutations in seven different genes have been associated with the disease (Hanein et al. 2004). In 10 to 15% of the cases LCA originates from a mutation in RPE65 (Gu et al. 1997), a gene specifically expressed in the cells of the retinal pigment epithelium layer (RPE cells). This gene encodes a 65kD protein the function of which has been dissected in a recently published study demonstrating its crucial role as a regulator of the visual cycle and a chaperone for the chromophore of the visual pigment (Xue et al. 2004). The patients affected by a mutation in this gene could benefit from a substitutive gene therapy consisting in the transfer of a fully functional allele of the RPE65 gene in RPE cells. Furthermore, animal models of RPE65 mutations have been identified (Aguirre et al. 1998; Veske et al. 1999) or genetically produced (Redmond et al. 1998) and thus provide the necessary tools to set up the conditions of such a strategy before a clinical trial can be started. The proof of feasibility of this approach has indeed already been established in dogs bearing a spontaneous mutation in the RPE65 gene (Acland et al. 2001; NarfstrÖm et al. 2003), as well as in knock-out mice (Dejneka et al. 2004; Lai et al. 2004). These studies have shown that an adeno-associated virus (AAV)-derived vector is able to deliver the RPE65 gene to RPE cells and thus to restore vision at least partially. Nevertheless, before a clinical trial can take place, a great effort must be provided to assess the bio-safety of the procedure. In particular, trans-gene expression has to be tightly controlled to achieve the following criteria: (i) expression should occur only in RPE cells; (ii) expression should reach the therapeutic level without disturbing the homeostasis of the target cells. [ABSTRACT FROM AUTHOR]

The delivery of foreign DNA to the retina has proven to be a valuable tool for investigations of retinal disease, development, and complex cellular interactions. To achieve efficient and stable retinal gene expression with minimal unwanted side effects, viral vectors derived from AAV (adeno-associated virus) and LV (lentivirus) remain the vehicles of choice. LV vectors have gained recent attention in CNS gene delivery due in part to their large transgene capacity, however contradictory results regarding retinal transduction ability exist in the literature. We sought specifically to characterize the temporal and spatial expression pattern of LV vectors when delivered to the rodent retina. [ABSTRACT FROM AUTHOR]

In eukaryotic cells, gene expression is controlled at multiple levels that could be grouped in two large categories, transcriptional and post-transcriptional regulatory events. Regulation of gene expression at the level of transcription is the major determinant of the initiation of protein synthesis and of the level of gene expression. However, there is increasing evidence of the important contribution of the post-transcriptional control mechanisms in determining and fine-tuning the final amount of the synthesized protein product. Post-transcriptional regulatory mechanisms could be grouped into events that modulate mRNA stability, localization, translation, as well as protein stability and modifications. [ABSTRACT FROM AUTHOR]

RNA interference (RNAi) is an evolutionarily ancient method of genome defense in many organisms. RNAi is triggered when a cell encounters a long double-stranded RNA (dsRNA) and results in the silencing of gene expression (Fire et al., 1998). The dsRNA is processed by the RNAse III-like nuclease, Dicer, into 20-25nt duplex RNA, termed short interfering RNA (siRNA, Zamore et al., 2000; Hammond et al., 2000). The siRNAs assemble into RNA-induced silencing complexes (RISCs) containing endoribonucleases and are unwound. It is believed that the sense strands are degraded, while the anti-sense strands activate the RISC to participate in repeated cycles of cleavage and degradation of complementary cognate mRNA, effectively silencing the gene such that no protein is made. For this reason, RNAi is seen as a useful tool for analyzing gene function. Given that introducing long dsRNA into mammalian cells initiates a potent antiviral response (Stark et al., 1998), most investigators bypass this by introducing or expressing siRNAs, which are the key mediators of RNAi (Elbashir et al., 2001a and b). [ABSTRACT FROM AUTHOR]

Retinal neovascularization is a feature shared by many disease processes including diabetic retinopathy, retinopathy of prematurity, branch retinal vein occlusion and central retinal vein occlusion, which are collectively referred to as ischemic retinopathy (Campochiaro, 2000). Retinal neovascularization is the most common cause of blindness in young diabetic patients. Investigations of the pathogenic mechanisms and therapeutic interventions for retinal neovascularization require reproducible and clinically related animal models. Currently, all diabetic models exhibit only early retinal vasculopathy after 1 or 2 years of the disease (Kondo and Kahn, 2004). The lack of retinal neovascularization in diabetic models is probably due to the natural short life span of rodents (2-3 years). In humans, DR is detected only after at least 3 years of diabetes (Dorchy et al., 2002). As angiogenesis is tightly controlled by the relative balance of stimulators and inhibitors, a shift in their balance, such as increased expression of vascular endothelial growth factor (VEGF) or decreased production of pigment epithelium-derived factor, would initiate angiogenesis (Okamoto et al., 1997; Ruberte et al., 2004; Renno et al., 2002). It is clear from literature that ischemia-induced upregulation of VEGF is a potent mediator of retinal neovascularization (Campochiaro, 2000; Miller, 1997). Animal models of retinal neovascularization have been established by oxygen-induced retinal ischemia, photodynamically-induced retinal branch vein occlusion and intravitreal implantation of VEGF sustained-release pellets (Campochiaro, 2000; Saito et al., 1997; Ozaki et al., 1997; Tolentino et al., 2002). However, retinal neovascularization in these animal models is either transient or occurs with a delayed onset. [ABSTRACT FROM AUTHOR]

During the past two decades, zebrafish (Danio rerio) have become established as a prolific model for biological research (for review, see Udvadia and Linney, 2003). Zebrafish exhibit many features of an ideal model organism, including small size, rapid development, and high fecundity. Since zebrafish are vertebrates, there is a conservation of physiology with higher vertebrates, including humans. Juvenile zebrafish develop rapidly and oviparously, facilitating developmental studies without invasive procedures. Zebrafish are amenable to drug discovery, having similar responses to mammals in pharmacological tests using cardiovascular, anti-angiogenic and anti-cancer drugs (for review, see Langheinrich et al., 2002). To date, zebrafish have been used in genetic screens to identify genes involved in development and function of organs including the eye, brain, ear and heart (Haffter et al., 1996). More recently, transgenic technologies have been developed for zebrafish. In this review, we will describe how this transgenic technology can be applied to study retinal biology. [ABSTRACT FROM AUTHOR]

Diabetic retinopathy is a severe complication of diabetes leading to some degree of vision impairment in long-term diabetes sufferers. Currently, the most successful treatment available for diabetic retinopathy is laser photocoagulation, a therapy that destroys part of the retina to save central vision. The principal aim of laser photocoagulation in the treatment of diabetic retinopathy is to effect regression of abnormal vessels, reduce oxygen tension and reverse angiogenesis in the retina. Although laser photocoagulation has been employed for more than 30 years, its underlying molecular mechanisms remain unknown. Research is now focused on identifying and understanding these factors, to ultimately develop therapies to protect against the initiation and progression of neovascularisation. [ABSTRACT FROM AUTHOR]

Tight junctions (TJ) are specialized multiprotein complexes which act to seal the intercellular space and thereby generate a permeability barrier required for transport processes (Matter and Balda, 1999). In addition, by regulation of the TJs, the paracellular pathway may be opened for selective transport of molecules, ions (Madara et al., 1992) and neutrophils (Huber et al., 2000). Thus far several interacting constituents comprising of transmembrane proteins including occludins (OCLN-TM4+/TM4− and 1B), claudins with >20 members and a series of cytoplasmic plaque proteins including ZO-1, -2, or -3, cingulin, 7H6 and symplekin have been identified in several tissue types of different species (Citi and Cordenonsi, 1998; Furuse et al., 1993; Ghassemifar et al., 2002; Matter and Balda, 1999; Mitic and Anderson, 1998). While the inner blood-retinal barrier, the retinal vascular endothelial cells, maintain a restricted and regulated trans-/paracellular transport from the blood to the surrounding tissue, the outer blood-retinal barrier, the retinal pigment epithelium (RPE), provides a permeability barrier between the retina and the choroid allowing vectorial exchange of solutes between these layers (Thumann, 2001). The RPE plays an important role in the proper function and maintenance of the photoreceptors by releasing differentiation and survival promoting factors. Despite the identification of TJ proteins in retinal tissue, little is known about the effect of pathological insult, such as hypoxia, on the expression of TJ proteins (Mark and Davis, 2002). It has also been reported that factors present in embryonic or whole eye-extract can influence the neurons of chick sympathetic ganglia to express choline acetyltransferase (Iacovitti et al., 1987) and also support full survival ganglion neurons for up to 6 days in culture (Margiotta and Howard, 1994). Retinal hypoxia has been reported to increase production of vascular endothelial growth factor (VEGF), a known stimulant of retinal neovascularization (Okamoto et al., 1997). The aim of this study was to investigate the regulation of tight junction (TJ) gene expression in cultured retinal pigment epithelial (RPE) cells and in the retina of normal and vascular endothelial growth factor (VEGF) transgenic mice. [ABSTRACT FROM AUTHOR]

The retina is an important model for studies of neurodegeneration. More than 100 gene mutations are known to cause retinal degeneration (Chader, 2002; Pacione et al., 2003; Rattner et al., 1999). Studies of retinal degeneration in mammals have focused on mouse because of the knowledge of its genome together with the ease of its breeding and husbandry (Naash et al., 2004; Olsson et al., 1992; Pinto et al., 2004). Assessment of the progression of retinal degeneration generally involves measurements of retinal sensitivity using the electroretinogram (ERG) and analysis of retinal anatomy using histological and histochemical techniques. In addition to knowledge of the anatomical and physiological consequences of retinal degeneration, it is important to assess its affect on visual function, that is, to answer the critical question: "How well can a mouse see?" [ABSTRACT FROM AUTHOR]

The generation of gene knockout mice by injection of homologous gene targeted embryonic stem (ES) cells into blastocyst has rapidly advanced our knowledge of gene function in mammals. However, knockout of essential genes often causes embryonic or neonatal lethality and thus obscures the particular role of genes in a target tissue or in adults. To circumvent this problem and disrupt genes in a temporal and spatial fashion, the Cre/lox system based gene targeting strategy has become a method of choice (for review, see Le and Sauer (2000)). This method can also be used in dissection of the roles of multifunctional genes in a particular tissue/cell-type. To study the function of widely expressed essential genes in the retinas, we are systematically establishing Cre/lox conditional knockout systems in retinal cells. This report is a review of our recent work on the generation and characterization of the cone-specific cre mice (Le et al., 2004). [ABSTRACT FROM AUTHOR]