Abstract:
There are disclosed methods and compositions for recovering or improving visual function in a mammal, by means of adeno-associated viral vectors suitable for gene delivery to mammalian retina.

Description:
The invention provides methods and compositions for recovering or improving visual functions, e.g. by treating ocular conditions and diseases that impair eye functionality. More particularly, the invention provides a method for gene delivery to mammalian retina by means of adeno-associated viral vectors. In a preferred embodiment, the invention provides genetic constructs and adeno-associated viral vectors for the transfer of Oa1 gene to the retina of patients affected by ocular albinism type I. 
     BACKGROUND OF THE INVENTION 
     Congenital hypopigmentary diseases (“albinism”) result from a defect in the synthesis or distribution of melanin pigment [1]. Melanin is responsible for skin, hair, and eye pigmentation. It is synthesized from the amino acid tyrosine in special organelles, the melanosomes. Different forms of albinism are due to mutations in genes involved in melanin production and accumulation [2]. Ocular albinism (OA) affects primarily the eye; oculocutaneous albinism (OCA) affects the skin and hair in addition [1]. 
     Ocular Albinism Type I (OA1; MIM 300500) is the most common OA form [1]. OA1 is transmitted as an X-linked trait, with affected males showing the complete phenotype and heterozygous carrier females showing only minor signs of the disease. Visual abnormalities in OA1 are similar to those present in all forms of albinism [1, 3]. OA1 male patients have reduced visual acuity, which represents a major handicap, nystagmus, strabismus and marked photophobia [1]. This results from a developmental disorder of the retina characterized by foveal hypoplasia and misrouting of the optic fibers at the chiasm [3]. In addition, unlike other forms of albinism, the OA1 retinal pigment epithelium (RPE) and, to a lesser extent, the skin melanocytes present with characteristic large pigment granules, the macromelanosomes, suggesting that abnormal melanosomal biogenesis might occur in OA1 [4-6]. 
     The gene responsible for OA1 (OA1) has been identified by positional cloning [7]. It encodes an orphan G-protein coupled receptor (OA1), which crosses the melanosomal membrane. Oa1 is expressed exclusively in RPE and skin melanocytes [8-12] and its transcript is detectable in murine embryonic RPE since early stages of development [10]. 
     Gene transfer holds great promises for the treatment of inherited retinal diseases [16, 17]. Vectors based on adeno-associated viruses (AAV) are able to stably and efficiently transduce the retina of animal models of retinal diseases and their toxicity and efficacy will be soon evaluated in the human retina [18-20]. It has been shown that AAV vectors with an AAV1 capsid (AAV2/1) efficiently transduce the murine RPE [21-23], thus representing important tools for the treatment of animal models of RPE defects, such as Oa1−/− mouse. 
     Retinal electrophysiological function has been analyzed in albino (OCA) rodents and abnormalities in both light-evoked responses and ability to recover from photoreceptor desensitization following bright light exposure (dark adaptation) have been described [24-27]. 
     DESCRIPTION OF THE INVENTION 
     In one aspect, the invention provides a method for recovering or improving visual function in a mammalian subject, which comprises:
         a) providing a recombinant adeno-associated viral (AAV) vector carrying an expression cassette which contains a nucleic acid molecule encoding Oa1 or a protein substantially homologous to Oa1, wherein said nucleic acid molecule is operably linked to regulatory control elements that direct the transcription and translation thereof;   b) transducing retinal pigment epithelial cells with said recombinant AAV vector, whereby said nucleic acid molecule is expressed by the retinal pigment epithelial cells at a level sufficient to recover or improve visual function in said mammalian subject.       

     As a result of this treatment, the number of melanosomes in retinal pigment epithelial cells is increased and the dark adaptation of the treated subject is improved. 
     In a particularly preferred embodiment of the invention, the above method is applied to human subjects who have been positively diagnosed for the Nettleship-Falls Disease (Ocular Albinism Type I). 
     Recombinant adeno-associated viral (AAV) vectors are known to be safe and efficient vehicles for gene transfer. Several serotypes of AAV have so far been isolated and vectors based on them have been constructed and used for gene-therapy protocols in animal models. Various AAV serotypes may be used according to the invention, depending e.g. on their level of transgene expression and on the presence of pre-existing serotype-specific antibodies in the patient to be treated, which may limit the efficiency of infection. AAV vectors of serotype 1, 2, 4 or 6 are preferred. 
     The OA1 gene (GenBank acc. no.: NM000273), also known as GPR143, is located in Xp22.3 and encodes an orphan G-protein coupled receptor, which crosses the melanosomal membrane. Oa1 is expressed exclusively in the retinal pigment epithelium (RPE) and skin melanocytes, and its transcript is detectable in murine embryonic RPE from early stages of development. For the purpose of this invention, the coding sequence of Oa1 (human and murine sequences deposited at GenBank NM — 00027 and NM — 010951, respectively), or a sequence substantially homologous thereto, is functionally linked to a promoter able to regulate the expression of the transgene in a eukaryotic cell, preferably in a mammalian retinal cell, more preferably in a retinal pigment epithelium cell. Suitable promoters that can be used according to the invention include the RPE65, the Oa1 and the ubiquitous cytomegalovirus (CMV) promoters, as well as inducible promoters. 
     In a particularly preferred embodiment of the invention, the expression cassette contains the human or mouse Oa1 coding sequence (SEQ ID No: 1 and SEQ ID No: 2, respectively) functionally linked to a promoter sequence which is selected from the group consisting of the CMV promoter (SEQ ID No: 3), the human OA1 promoter (SEQ ID No: 4; −668 bp from the atg of OA1 coding sequence, GenBank AC003047) and the human RPE65-specific promoter fragment (SEQ ID No: 5; 806 bp, GenBank NM_000329). 
     The construction of an AAV vector can be carried out following procedures and using techniques which are known to any person skilled in the art. The theory and practice for adeno-associated viral vector construction and use in therapy are illustrated in several scientific and patent publications (the following bibliography is herein incorporated by reference: Flotte T R. Adeno-associated virus-based gene therapy for inherited disorders. Pediatr Res. 2005 December; 58(6):1143-7; Goncalves M A. Adeno-associated virus: from defective virus to effective vector, Virol J. 2005 May 6; 2:43; Surace E M, Auricchio A. Adeno-associated viral vectors for retinal gene transfer. Prog Retin Eye Res. 2003 November; 22(6):705-19; Mandel R J, Manfredsson F P, Foust K D, Rising A, Reimsnider S, Nash K, Burger C. Recombinant adeno-associated viral vectors as therapeutic agents to treat neurological disorders. Mol Ther. 2006 March; 13(3):463-83.). 
     In a further aspect, the invention relates to a pharmaceutical composition containing an AAV vector expressing the Oa1 coding sequence, in a form suitable for ocular administration. Suitable administration forms include, but are not limited to, injectable solutions or suspensions, eye lotions and ophthalmic ointment. In a preferred embodiment, the AAV vector is administered by subretinal injection, e.g. by injection in the vitreous, in the anterior chamber or in the retrobulbar space. Preferably the viral vectors are delivered via a transcleral transchorodial approach (as described in Liang, F. Q. et al., Intraocular delivery of recombinant virus. Methods Mol. Med. 47: 125-139). 
     The doses of virus for use in therapy shall be determined on a case by case basis, depending on the administration route, the severity of the disease, the general conditions of the patients, and other clinical parameters. In general, suitable dosages will vary from 10 9  to 10 12  vg (vector genomes)/ml. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A mouse knock-out (KO) model has been generated which shows some of the OA1 landmarks [13]. The Oa1 −/y male or −/− female mice (herein referred to as Oa1−/− mice) are viable and fertile. Ophthalmologic examination shows hypopigmentation of the ocular fundus in mutant animals compared with wild-type [13]. Microscopic examination of the RPE shows the presence of macromelanosomes already detectable at birth (P1) and comparable with those described in OA1 patients [13]. In addition, Oa1−/− mice (similarly to the OA1 patients) show abnormal crossing of the optic fibers at the chiasm which occurs between embryonic days 12 and 18 in mice [13-15]. Foveal hypoplasia can not be evaluated in rodents who lack this structure. The Oa1 mouse KO represents a unique model for the elucidation of the OA pathophysiology and for testing potential therapies for an otherwise untreatable ocular disorder. 
     To test whether Oa1 gene knock-out results in abnormal retinal function, extensive electophysiological analysis was performed in Oa1−/− and wild type, age-matched C57/BL6 mice. Ganzfeld flash electroretinograms (ERG) were measured after 3 hour dark adaptation in Oa1−/− and wild-type mice (8-9 months old,  FIG. 1 ). The amplitude of ERG a—(FIG.  1 B,C) and b—(FIG.  1 B,D) waves is significantly decreased in Oa1−/− mice when compared to wild type C57/BL6 mice, suggesting abnormal photoreceptor function as a result of absence of the Oa1 gene. Compared to the scotopic responses, photopic ERG was affected to a lesser extent (statistical significance among the Oa1−/− and wild type animals was not reached using the ANOVA test described in Material and Methods) suggesting that absence of Oa1 impairs mainly the rod pathway. A similar shift towards lower intensities has been recently observed in other albino mice [24, 25, 27]. These combined results suggest that the RPE defect in albinism, whether due to absence of melanin or to abnormal melanosomal biogenesis, impacts on photoreceptor function as assessed by flash ERG analysis. 
     The ability of the Oa1−/− mouse retina to recover from a photoreceptor desensitizing light stimulus (dark adapt), which has been reported to be delayed in rodents affected by different types of albinism [25-27], was then analyzed. The mice were exposed to an intense bleaching condition (600 cd*m-2 for 3 min) before monitoring the recovery of b-wave using a flash of 1 cd*m-2 sec-1. Recovery of b-wave amplitude was measured over time; the amplitude of the b-wave relative to that prior to photoreceptor desensitization is represented in  FIG. 1E  while absolute values are depicted in table 1. A significant delay in recovery from photoreceptor desensitization was observed in Oa1−/− mice when compared to wild type mice. The Oa1−/− mice b-wave eventually recovers 100′-120′ after bleaching conditions suggesting that prolonged functional uncoupling between the RPE and the photoreceptors may result as a consequence of the absence of Oa1. 
     An AAV2/1 vector expressing the murine Oa1 coding sequence under the control of the ubiquitous cytomegalovirus (CMV) promoter (AAV2/1-CMV-mOa1) was produced and injected subretinally in 1 month-old Oa1−/− mice. Four weeks later Oa1 expression was analyzed in retinal sections by immunofluorescence: a strong signal was detected in both RPE and photoreceptors. 
     To test whether the Oa1−mouse photoreceptor dysfunction is reversible, Oa1−/− mice (8 months old) were subretinally injected in one eye with 2-3×10 9  genome copies (GC) of a 1:1 mixture of AAV2/1-CMV-mOa1+AAV2/1-CMV-EGFP (expressing the enhanced green fluorescence protein, EGFP) and in the contralateral with the same dose of AAV2/1-CMV-EGFP alone as control. Four weeks after vector administration, indirect ophthalmoscopic evaluation was performed to assess EGFP expression [21] followed by flash ERG analysis ( FIG. 2 ). b-wave amplitude elicited in photopic conditions was higher (albeit not statistically significant) in Oa1-treated than untreated mice suggesting a partial rescue of cone function. In addition, both a- and b-waves amplitude in scotopic conditions were significantly higher (albeit not normal) at the highest light intensities in the retinae injected with both the Oa1 and EGFP vectors than in the contralateral injected with the vector expressing EGFP alone ( FIG. 2 ). 
     These data suggest that a partial rescue of both rod and cone function occurred in the retinae expressing recombinant Oa1. Although AAV-mediated Oa1 expression was detected in both RPE and photoreceptors, the recovery of photoreceptors activity in the Oa1−/− mouse is likely due to robust transduction of the RPE which is the only Oa1 physiological site of expression in the retina (and therefore affected in the KO animal). 
     To verify whether the delayed recovery from photoreceptor desensitization present in the Oa1−/− mice was reversible and whether this could be dependent on the age of the animals treated, 2 cohorts of Oa1−/−mice of different ages (1 and 8 months old) were subretinally injected similarly to those treated in  FIG. 2  and four weeks later their ability to dark adapt tested ( FIG. 3 ). Independently of the age of treatment, the retinae that received the Oa1 vector completely recovered from the delay in dark adaptation in 75′ similarly to wild type retinae. The contralateral EGFP-treated retinae do not recover after 90′, the latest time point of the analysis in some animals. This suggests that Oa1 gene delivery, applied at different time points to the adult retina, can rescue the delayed dark adaptation present in the mouse model. 
     To test whether the modification in electrophysiological activity following gene transfer to the Oa1−/− mouse retina could be related to changes occurring in the Oa1−−/− RPE melanosomes, which are of lower number (Marigo et al, unpublished results) and increased size since birth [13], eyes from some of the animals used for the experiment depicted in  FIG. 3A  were enucleated and the transduced, EGFP-positive areas from eyes injected either with the AAV2/1-CMV-mOa1+AAV2/1-CMV-EGFP mixture or with the AAV2/1-CMV-EGFP vector alone were dissected under a fluorescent microscope. EGFP-positive, transduced areas accounted for 40-50% of the whole retina with minor inter-animal variation. Two different regions of the eye were analyzed, one centrally located in the proximity of the optic nerve and the remaining peripheral region, representing the vast majority of the retina. The RPE from these regions was isolated and analyzed by electron microscopy to assess melanosome number and size. 
     A representative picture from Oa1- or EGFP-injected eyes is shown in  FIG. 4 . In two independent electron microscopy measurements, the melanosome density (melanosome/μm 2 ) in the peripheral RPE injected with the Oa1-expressing vector was higher than that measured in the same area of the contra-lateral eyes injected with the EGFP vector alone ( FIG. 5A ). In one eye, both the transduced peripheral and central retinae were analyzed, confirming that treatment with the Oa1 vector increases melanosome density independently of the area of transduction ( FIG. 5B ). The number of normal sized melanosomes in the peripheral RPE is increased in the 2 retinae treated with the Oa1 vector when compared to the contralateral treated with the EGFP vector alone, while the number of giant size melanosomes (&gt;1.5 μm) remained similar ( FIG. 5C ). This suggests that in the period following gene transfer (4 weeks) biogenesis of melanosomes of normal size occurs in the Oa1−/− RPE treated with the therapeutic vector rather than modification of the pre-existing abnormal organelles. 
     The results show that gene transfer to the adult retina can rescue the electrophysiological abnormalities, as well as the altered melanosome density observed in Oa1−/− mice. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 C57/BL6 
                   
                 C57/BL6 Oa1 −/− 
               
             
          
           
               
                 MEAN “B” 
                   
                   
                 TIME 
                 MEAN “B” 
                   
                   
               
               
                 WAVE(μV) 
                 STD. DEV. 
                 STD. ERR. 
                 (min) 
                 WAVE (μV) 
                 STD. DEV. 
                 STD. ERR. 
               
               
                   
               
             
          
           
               
                 201.3201 
                 72.16918313 
                 22.82189956 
                 −4 
                 119.3180833 
                 58.16576671 
                 16.79101053 
               
               
                 24.7725 
                 16.57714234 
                 5.242152688 
                 0 
                 12.79395 
                 7.750638312 
                 2.450967039 
               
               
                 48.0427 
                 26.40620808 
                 8.350376191 
                 5 
                 27.2946 
                 15.88910123 
                 5.024574985 
               
               
                 70.2866 
                 69.77704883 
                 22.06544027 
                 15 
                 37.4196 
                 27.44337372 
                 8.678356765 
               
               
                 115.6165 
                 95.11824733 
                 30.07903086 
                 30 
                 49.0226 
                 31.59256886 
                 9.990447472 
               
               
                 138.601 
                 83.33688361 
                 26.35343653 
                 45 
                 62.1566 
                 34.21601797 
                 10.82005493 
               
               
                 180.1548 
                 87.90791193 
                 27.7989226 
                 60 
                 75.6743 
                 43.81319146 
                 13.85494766 
               
               
                 191.8519 
                 88.42408257 
                 27.96215009 
                 75 
                 80.1195 
                 31.17113631 
                 12.72556311 
               
               
                   
               
             
          
         
       
     
     The non-progressive nature of the disease and the possibility to ameliorate visual function with treatment to the adult retina open novel therapeutic perspectives for albino patients. 
     Table 1. Values of mean b-wave amplitudes—including standard deviation and error—at different time points before (−4′) and after photoreceptor desensitization. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1 . Electrophysiological abnormalities in Oa1 mice. ERGs recorded after dark adaptation. A) Typical ERG produced by a flash (10 cd*m-2) in a dark adapted wild type C57/BL6 mouse: a- and b-waves are indicated in an expanded scale (horizontal bar=20 msec, vertical bar=150 μV). Dotted lines refer to a- and b-wave amplitudes. B) ERG produced by a flash (1 cd*m −2 ) in a wild type C57/BL6 (upper waveform) and Oa1−/− (lower waveform) mouse; horizontal bar=100 msec, vertical bar=200 μV. 
       ERG components in scotopic and photopic conditions: a- (C) and b-waves (D) in Oa1−/− and C57/BL6 wild type mice. The amplitudes (mean±S.E.M.) evoked by increasing light intensities in scotopic conditions in Oa1−/− mice (empty triangles, n=10 eyes) and age matched controls (black circles, n=10 eyes) are shown. The amplitude of a- and b-wave elicited in photopic conditions is indicated by an arrow. E) Recovery of b-wave amplitude after bleaching condition (600 cd*m −2  for 3 min) in Oa1−/− (empty triangles, n=12 eyes) and wild type C57/BL6 mice (black circles, n=12 eyes). The amplitude of b-wave after bleaching condition was measured for flash of 1 cd*m −2 sec −1  and expressed as relative mean value compared to the amplitude of b-wave measured before bleaching condition. Asterisks depict statistical significance (p&lt;0.05). 
         FIG. 2 . Partial recovery of the rod function following subretinal delivery of AAV2/1-CMV-Oa1 to Oa1−/− mice. Amplitude of a- (A) and b-waves (B) in scotopic and photopic (depicted by the arrows) conditions (mean±S.E.M) from AAV2/1-CMV-Oa1 (empty triangles, n=5) and AAV2/1-CMV-EGFP (black circles, n=3) treated eyes recorded 1 month after vector delivery. Asterisks depict statistical significance (p&lt;0.05). 
         FIG. 3 . Rescue from delayed recovery from photoreceptor desensitization in Oa1−/− mice treated with AAV. Progressive recovery over time of the b-wave amplitude following bleaching conditions in 2-(A) and 9-month (B) old Oa1−/− animals injected subretinally either with AAV2/1-CMV-Oa1 (black circles, n=6 eyes in both A and B) or AAV2/1-CMV-EGFP (empty triangles, n=6 eyes in A and 9 in B). Asterisks depict statistical significance (p&lt;0.05). 
         FIG. 4 . Increased density of normal sized stage IV melanosomes in the Oa1 retinae transduced with AAV. Representative electron micrographs of peripheral RPE in 2-months old treated and control Oa1−/− retinae. Left panel shows the low melanosome density and macromelanosome presence typical of the Oa1−RPE transduced with AAV2/1-CMV-EGFP. Middle and right panels show different areas of the same RPE cell in a retina transduced with AAV2/1-CMV-Oa1. Note the increased number of normal sized melanosomes (middle panel) with persistence of macromelanosomes (right panel). Size bar 1.5 μm. 
         FIG. 5 . Melanosomal modifications in the Oa1−/− RPE following delivery of AAV2/1-CMV-Oa1 (black bar) or AAV2/1-CMV-EGFP (white bar). 
       A) Mature (stage IV) melanosome density in the peripheral RPE (mean±S.E.M.; n=2 eyes/group, 2 sections/eye). B) Mature (stage IV) melanosomal density in the peripheral and central RPE area of AAV-treated and wild type C57/BL6 (grey bar) retinae (n=1 eye/group, mean of 2 sections/area). C) Density of different size melanosomes in the peripheral RPE transduced with AAV (mean±S.E.M.; n=2 eyes/group, 2 sections/eye). Melan, melanosome size. 
         FIG. 6 . Recovery of retinal function following subretinal delivery of AAV2/1 vectors encoding Human or mouse Oa1 gene under the transcriptional control of CMV, Oa1, and RPE65 promoters. Max B-wave amplitude (mean±SEM) produced by a flash (10 cd m-2) in dark adapted wt, Oa1 and treated animals. Treated animals were injected with AAV2/1 viruses encoding human or mouse Oa1 under different promoters. Asterisks depict statistical significance (P&lt;0.05): gray is significance vs Oa1 not treated mice; black is significance vs Oa1 mice injected with AAV2/1 CMV eGFP as control. mOa1, mouse Oa1 coding sequence; hOA1, human OA1 coding sequence; promhOA1, human OA1 promoter fragment; pRPE65, human RPE65 promoter fragment. 
     
    
    
     MATERIAL AND METHODS 
     Generation of the pAAV2.1-CMV-Oa1 construct, AAV vector production, and purification. The murine Oa1 coding sequence in pBS-SK plasmid was mutagenized via PCR using the Advantage cDNA PCR Kit (Clontech, Palo Alto, Calif., USA) in order to eliminate the HindIII restriction site, and to insert a Not and a HindIII sites at the 5′ and 3′ end respectively with the following primers: Oa1-NotI-F: AAGCGGCCGCATGGCCTCCCCGCGCCTGGGAATTTTCTGCTGCC CTACGTGGGACGCAGCCACACAGCTGGTGCTAAGUTCCAAC (SEQ ID NO: 6); Oa1-HindIII-R: TTGACTCCATTTCCCAAGCCCAGGGGGAACTCTGAAAGCTTAA (SEQ ID NO: 7). The PCR product was then digested NotI, HindIII and cloned in pAAV2.1-CMV-EGFP [28] by removing the EGFP coding sequence (NotI-HindIII). AAV2/1-CMV-Oa1 and AAV2/1-CMV-EGFP vectors were produced by triple transfection, purified by CsCl2 ultracentrifugation and titered using a real-time PCR-based assay as previously described [21, 28]. AAV vectors were produced by the AAV TIGEM Vector Core. 
     Subretinal vector administration. All procedures on mice (including their euthanasia) were performed in accordance with institutional guidelines for animal research. Oa1 (kept in a C57/BL6) and wild type C57/BL6 mice were used (Charles River Italia, Lecco, Italy). For subretinal vector administration, mice were anesthetized with an intraperitoneal injection of avertin at 2 ml/100 g body weight (1.25% (w/v) 2,2,2-tribromoethanol and 2.5% (v/v) 2-methyl-2-butanol; Aldrich, St. Louis, Mo., USA) and viral vectors were delivered via a transscleral transchoroidal approach as described [29]. 
     Immunofluorescence. One month after injection treated and control eyes were collected, fixed overnight in 4% paraformaldehyde, incubated in 30% sucrose for 2 hours and then frozen in OCT compound (Kaltech, Padova, Italy). Serial cryosections (12 μm thick) were obtained. To detect Oa1 by immunofluorescence, cryosections were fixed in 4% paraformaldehyde for 20′. To detect Oa1 by immunofluorescence, cryosections were fixed in 4% paraformaldehyde for 20′, washed and incubated at room temperature in 30 mM NH 4 Cl for 30′. Sections were then permeabilized and blocked against non-specific binding in a buffer containing 10% FBS (GIBCO, Invitrogen Life Technology, Carlsbad, Calif.), 0.1% Saponin in PBS over night at 4° C. Samples were then washed with PBS and incubated with rabbit anti-mouse-Oa1 primary antibody (1:50 diluted in 0.01% saponin) for two hours at room temperature. After extensive washing in 0.01% saponin, sections were incubated with secondary Cy2-labeled anti-rabbit antibody (Jackson Immunoresearch, Cambridgeshire, UK, 1:100 diluted in 0.01% saponin) for 1 hour at room temperature, washed with PBS and mounted with Vectashield mounting media (Vector Laboratories, Burlingame, Calif.). Treated slides were then analyzed under the Axioplan 2 Imaging fluorescent microscope (Carl Zeiss, Milano, Italy). 
     Electrophysiological recordings. Flash ERG was evoked by 10 msec flashes of light generated through a Ganzfeld stimulator (Lace, Pisa, Italy). The electrophysiological signals were recorded through gold plate electrodes inserted under the lower eyelids in contact with the cornea previously anaesthetized with ossibuprocaine (Novesine, Novartis Pharma, Switzerland). Electrode in each eye was referred to a needle electrode inserted subcutaneously at the level of corresponding frontal region. The different electrodes were connected to a two channels&#39; amplifier. 
     After 180 min. of dark adaptation, mice were anaesthetized by an intraperitoneal injection of Avertin (1.2% tribromoethanol and 2.4% amylene hydrate in distilled water; 2 ml/100 g body weight) and loosely mounted in a stereotaxic apparatus under dim red light with the body temperature maintained at 37.5. For recordings in dark adapted conditions we adopted the following protocol: after dark adaptation ERG was recorded in response to flash of different light intensities, ranging from 1×10 −4  to 20 cd*m-2 sec-1. The time interval between each stimulus was 4-5 min. Amplitudes of a- and b-waves were plotted as function of increasing light intensities. After completion of responses obtained in dark-adapted conditions the recording session continued with the aim to dissect the cone pathway mediating the light response. To this aim the ERG in response to light of 20 cd*m-2 sec-1 was recorded in the presence of constant light background set at 20 cd*m-2. 
     In a different group of mice scotopic ERG was recorded in response to light of 1 cd*m-2 sec-1. For screening purpose 10 different responses were averaged with an interstimulus interval of 2-4 sec. Mice were then exposed to a constant light whose intensity was set at 600 cd/m 2  for 3 minutes (pre-adapting light, bleaching condition). Recovery of b-wave was monitored at fixed intervals after pre-adapting light (0, 5, 15, 30, 45, 60, 75 min). The amplitude of b-wave in response to a flash of 1 cd*m-2 sec-1 after the pre-adapting light was measured and expressed as relative value respect to that measured before the pre-adapting light. 
     Data were statistically analyzed using the Statistica (Statsoft, USA: two-way ANOVA using least significance difference test for pair-wise comparisons). 
     Ultrastructural analysis of the Oa1−/− retinal pigment epithelium. Oa1−/− eyes injected with AVV2/1-CMV-Oa1+AVV2/1-EGFP (right eyes) and AVV2/1-EGFP (left eyes) were removed and fixed in 2.5% glutaraldehyde (PolyScience Inc., Eppelheim, Germany) in Cacodylate buffer 0.1M (Sigma-Aldrich, St. Louis, Mo.). The injected portion of the retina was identified with a dissecting microscope equipped with epifluorescence illumination. The EGFP positive region was dissected and processed for electron microscopy analysis, as described [30]. Briefly, the dissected tissue was fixed 2 hrs in 2.5% glutaraldehyde in Cacodylate buffer 0.1M, post-fixed 2 hrs in 1% Osmium tetroxide (Electron Microscopy Science, Hatfield, Pa.) in Cacodylate buffer 0.1M, and en bloc stained 2 hr with 1% Uranyl Acetate (Electron Microscopy Science, Hatfield, Pa.), at room temperature. Samples were then dehydrated through graded Ethanol series and Propylene Oxide (TAAB Laboratories Equipment Ltd, Aldermaston, England), and embedded in Poly-Bed (PolyScience Inc., Eppelheim, Germany) epoxy resin. Ultrathin sections were obtained from two different areas of the RPE (peripheral and central retina) in each sample. For each area two series of sections at 60 μm distance from each other were collected. Samples were analyzed with a FEI Tecnai 12-G2 TEM and FEI AnalySYS software (FEI Company, Eindhoven, The Netherlands). We analyzed the RPE of 2-months old mice. The analysis was performed on a total of ˜800 μm 2  for each series of sections, calculated with the “closed polygon” option of the software. We determined the density of total stage IV melanosomes (number of melanosome/μm 2 ), and the diameter of stage IV melanosomes, calculated along the major axis of the organelle. We measured the diameter of melanosomes completely enclosed in each micrograph. 
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