Synaptogenesis and outer segment formation are perturbed in the neural retina of Crx mutant mice
© Morrow et al; licensee BioMed Central Ltd. 2005
Received: 16 November 2004
Accepted: 27 January 2005
Published: 27 January 2005
In Leber's congenital amaurosis (LCA), affected individuals are blind, or nearly so, from birth. This early onset suggests abnormal development of the neural retina. Mutations in genes that affect the development and/or function of photoreceptor cells have been found to be responsible in some families. These examples include mutations in the photoreceptor transcription factor, Crx.
A Crx mutant strain of mice was created to serve as a model for LCA and to provide more insight into Crx's function. In this study, an ultrastructural analysis of the developing retina in Crx mutant mice was performed. Outer segment morphogenesis was found to be blocked at the elongation stage, leading to a failure in production of the phototransduction apparatus. Further, Crx-/- photoreceptors demonstrated severely abnormal synaptic endings in the outer plexiform layer.
This is the first report of a synaptogenesis defect in an animal model for LCA. These data confirm the essential role this gene plays in multiple aspects of photoreceptor development and extend our understanding of the basic pathology of LCA.
Photoreceptor cells play a primary role in vision by capturing light energy and converting it into neural stimuli. These sensory neurons are a shared element in all organisms capable of sensing light. In humans, genetic diseases of the eye are common and the primary site of disease is most frequently photoreceptors (for review see [1–3]).
Photoreceptors have been well studied, particularly with respect to the biochemistry and physiology of phototransduction. Insight into the development of vertebrate photoreceptors, however, has lagged behind our understanding of function. Only recently have the first molecular mechanisms regulating photoreceptor development been identified (for review see, [2, 4]). Crx (cone-rod homeobox) is an otx-family homeobox gene expressed predominantly in photoreceptors, from early in their development through to the adult ages [5–7]. Crx gene expression is critically dependent upon Otx2, another member of the same homeobox family which is expressed in early photoreceptor cells . In rod photoreceptors, Crx appears to work in concert with Nrl, a leucine zipper protein that is also restricted in its expression in the retina to rod photoreceptors . Many photoreceptor-specific genes have putative Crx-binding elements in their regulatory regions , including rhodopsin  and arrestin . Mutations in Crx have been associated with several human diseases that lead to blindness, including cone-rod dystrophy 2 [6, 13, 14], retinitis pigmentosa , and LCA [14–16]. Based on these data, Crx was hypothesized to play a critical role in the differentiation and maintainance of photoreceptor cells [5, 7].
LCA is the most severe genetic disease of photoreceptors (see , for recent review). Affected infants exhibit a complete or near complete absence of vision from birth. Mutations in retinal specific genes, such as Crx, have been associated with LCA [14, 15], as well as GUCY2D , RPE65 , AIPL-1 , CRB-1 , and RPGRIP-1 . There also may be as many as three additional genetically linked loci where genes have not been identified . Crx mutations in LCA are varied, and include a putative dominant mutation that is proposed to encode a dominant-negative form of Crx [14, 15]. Recessive mutations also have been reported and at least one allele encodes a protein with decreased DNA-binding activity . Histopathological and ultrastructural studies of LCA should enable a better understanding of the disease process, and the design of suitable therapies. Few such studies exist for human LCA (reviewed in ) and the majority of such studies examine the globes of adults with LCA, after the tissue has undergone secondary changes. Only a single study exists where the developing eye of an infant was examined . Animal models for LCA have recently been reported and have already served to broaden our understanding of the pathology of this disease [25–28]. Since LCA is a clinically and genetically heterogeneous disorder, additional mouse models are in order to allow a full understanding of the many ways in which photoreceptor development can go awry.
In addition to their importance as a locus of disease, photoreceptor cells serve as an excellent model for studies in neuronal differentiation. Photoreceptor cells are highly polarized. At their apex, these neurons have a membranous outer segment, which contains proteins involved in the phototransduction cascade. Loss of function mutations in rhodopsin , or the structural protein, peripherin , result in an inability to form outer segments. At the other extremity, photoreceptors terminate with synaptic endings that make contact with the processes of horizontal and bipolar cells [31, 32]. Rod spherules establish an invaginating synapse with rod bipolar dendrites and axonal endings of horizontal cells. This synapse is characterized by the presence of a ribbon in the presynaptic cytoplasm. Cone pedicles make invaginating synapses with the dendrites of on-cone bipolar cells and horizontal cells and basal junctions with the dendrites of off-cone bipolar cells. The factors regulating the formation of the photoreceptor synapses are completely unknown. At least one photoreceptor synaptic protein, HRG4, contains a potential Crx target sequence in its transcriptional regulatory sequence .
Few studies of LCA animal models have extended their examination of retinal pathology to the ultrastructural level. Certain features of neuronal differentiation, such as synapse formation, can be detected definitively at this level of analysis. With the hope of understanding the neuropathology of LCA in greater detail, we have analyzed the differentiation of the outer retina in Crx-/- mice at the ultrastructural level. These retinas exhibit several prominent defects. Crx-/- photoreceptors demonstrate a complete block in outer segment formation at the elongation stage. Further, these cells exhibit abnormal synaptic morphology, thereby broadening the function of Crx to photoreceptor synaptogenesis. This represents the first report strongly implicating the process of synapse formation in LCA.
Multiple pathologies in the outer segment layer in Crx-/- mice
In addition to the complete absence of outer segments, Crx-/- retinas exhibited three other notable pathologies in the outer segment layer. First, an abnormal deposition of matrix of unknown identity was noted (Figure 3A, mx). Second, sporadically displaced nuclei were found residing in the space abutting the PE. Occasionally, these nuclei appeared pyknotic (Figure 3A, n); but, more frequently exhibited the heterochromatin pattern typical of photoreceptors (data not shown), strongly suggesting that they belonged to ectopic photoreceptors. The third pathological entity noted in the outer segment layer were numerous small vesicles (Figure 3A arrow) 100 to 200 nm in diameter. They appeared to be emerging from the inner segments, as scanning electron microscopic images showed spherical structures budding from the inner segments (Figure 3C, arrow).
Crx is necessary for the formation of photoreceptor terminals
In this study, an ultrastructural analysis of Crx-/- photoreceptors was carried out. As Crx mutations have been associated with Leber's congenital amaurosis, the findings in this study broaden our understanding of the pathology of this disease. Two prominent pathologies were characterized in the Crx-/- retina: (1) An absolute block in outer segment morphogenesis was noted, with the block occuring at the elongation stage of outer segment formation; (2) Crx-/- photoreceptors exhibited a severe perturbation in synapse formation. This represents the first report of a synaptogenesis defect in an animal model of LCA.
Crx-/- photoreceptors cannot complete outer segment morphogenesis
Mutations in Crx represent one of a collection of gene mutations that lead to an outer segment formation defect. Homozygous null mutations in the peripherin/RDS gene  or in rhodopsin  lead to a failure of outer segment formation. The deficits in peripherin-/- and Crx-/- photoreceptor morphogenesis were found to be very similar. Vesicular structures in Crx-/- photoreceptors were observed that were similar to those previously noted in the rds/peripherin-/- mouse. It was initially proposed that these vesicles were due to the breakdown of outer segment membranes that were not properly recruited to the outer segments in the absence of peripherin, or were from the result of the breakdown of the microvilli of Müller cells . Strong support in favor of the former explanation was provided by Nir and colleagues who demonstrated the presence of rhodopsin protein in these vesicles using immunoelectron microscopy against a rhodopsin epitope . Further, as shown here, the vesicles appear to bud from the inner segments themselves.
In developing photoreceptors, an extraordinary growth process occurs whereby the outer segment is generated from the nascent connecting cilium (see  and references therein). Peripherin/RDS and ROM-1 proteins (localized in disc rims) and the opsin proteins (localized throughout the discs) have important roles in the structural integrity of mature outer segments (see [39, 29]). ROM-1-/- mice produce disorganized outer segments with large disks . Crx, by virtue of being a transcription factor, presumably controls genes that are responsible for the building and perhaps maintenance of the outer segment structure, including rhodopsin and peripherin. Using northern blots , microarrays , and serial analysis of gene expression (SAGE) , we have defined a large number of genes that are altered in their expression level in Crx-/- mice. We found that rhodopsin expression was severely diminished in Crx-/- animals, and peripherin mRNA was reduced by approximately 30%. Transgenic mice with variable levels of expression of wild type rhodopsin exhibit rod degeneration , indicating the importance of the level of rhodopsin expression. In addition, the timing of rhodopsin expression may be very important, as indicated by studies in Drosophila.
In Drosophila, rhodopsin (ninaE) is expressed in photoreceptors R1–R6. In ninaE null mutants, the rhabdomere, a structure analogous to vertebrate outer segments, fails to develop in R1–R6 photoreceptors , reminiscent of the situation in rhodopsin-/- mice . An intriguing experiment by Kumar et al.  demonstrated a temporal requirement for rhodopsin expression during rhabdomere development. In ninaE null flies, a ninaE transgene under the control of a heat shock promoter was subjected to various temperature shifts during development. Heat shock during the normal time of rhodopsin onset resulted in substantial and long-lasting rescue of photoreceptor structure and transient rescue of photoreceptor physiology. However, expression shortly before or after this critical period failed to rescue, suggesting that rhodopsin expression during a discrete window of time in development is essential for proper rhabdomere morphogenesis. This result is consistent with observations in the rat wherein rhodopsin onset occurs with strict timing in the developmental history of most rods in vivo . Thus, through its regulation of rhodopsin levels, or perhaps through control of the kinetics of the up-regulation of rhodopsin beginning at about P6, Crx may be regulating outer segment morphogenesis. The similarty of the two cases may extend further. At present, the closest Crx relative in Drosophila is Otd, the founding member of the class of homeobox genes to which Crx belongs. Interestingly, in one hypomorphic allele of Drosophila otd, otd uvi , photoreceptor morphogenesis is also disrupted .
We found that there are many other genes that are dependent upon Crx. Those that are expressed at a lower level in the Crx-/- retina, such as rhodopsin and peripherin, comprise many that are either enriched or specific to photoreceptors in their expression . They include enzymes that are important in lipid metabolism, protein folding and transport, as well as in other processes that one might envision would be important in building a structure such as the outer segment. In situ hybridization using probes from this collection of genes has revealed that some of them have their RNA localized to the inner segment, a finding typical for proteins targeted to the outer segment. Future analyses of the function of these genes might reveal their role in outer segment biogenesis.
Finally, polarization of photoreceptors was found to be largely intact, as was ciliogenesis. Another LCA gene, CRB1, and a related gene CRB3, have been implicated in ciliogenesis in in vitro models . The Drosophila homologue of CRB1, Crumbs, has been implicated in photoreceptor morphogenesis . However, the spontaneously occurring mouse mutant in CRB1, the Rd8 mouse, develops shortened outer segments that subsequently degenerate , suggesting that photoreceptor polarization and synaptogenesis are intact in this mutant. While CRB1 and Crx have been both linked to LCA, further work is necessary to determine if their function is linked in retinal development.
Synaptogenesis is perturbed in Crx-/- photoreceptors
The Crx-/- mouse demonstrates the most severe abnormality of photoreceptor synapses reported to date. The peripherin-/- mouse develops a normal complement of photoreceptor terminals which then degenerate as the photoreceptors are lost . Also, similarly in rhodopsin (Rho) and cyclic nucleotide-gated channel, alpha-3 (CNGA3) double knockout mice (Rho-/-, CNGA3-/-), synapses are reported to form normally . These observations demonstrate that photoreceptor synaptogenesis can occur in the absence of outer segment formation. In keeping with this observation is the fact that some electroretinogram activity is present in peripherin-/- mice, suggesting that minimal phototransduction is present in these mice, enough to drive activity at the photoreceptor synapse. In vitro studies wherein synapse elements are formed in the absence of proper outer segment development and, therefore, possible absence of light-dependent photoreceptor activity, have indicated the independence of phototransduction and synapse formation, at least for the initial stages [50, 51]. These data then suggest that the fact that the Crx-/- photoreceptors do not have proper synaptic endings is not due to a lack of outer segment formation. A more likely explanation is that Crx plays a role in photoreceptor synapse formation, perhaps by regulating directly, or indirectly, important genes in this process. Using immunohistochemistry, we examined the expression of common pre-synaptic terminal proteins, including KIF3a, SV2, and synaptophysin, and were unable to observe qualitative differences between Crx-/- and control tissue at P14 (data not shown). Examination of their RNA levels by SAGE showed no significant difference for all 3 genes, though very few tags were recovered from these genes and thus the analysis of RNA levels may not be significant . However, since other genes expressed in photoreceptors were significantly altered in their expression level in the Crx-/- mouse, there are many candidates that could be important for photoreceptor morphogenesis. Tags from three genes from proteins expressed in photoreceptor terminals were found to be decreased in a statistically significant fashion, namely the HGF-regulated tyrosine kinase substrate, the CRIPT protein, and synaptotagmin 1 (Blackshaw and Cepko, unpublished data). An example of a gene that was increased in the Crx-/- retina is HRG4 (a homologue of the C. elegans Unc119 gene) (Blackshaw and Cepko, unpublished data) which encodes a component of the ribbon synapse . The fact that it is upregulated might indicate a response to the lack of proper terminal structures. Several other genes encoding putative cytoskeletal elements also were increased (e.g. microtubule associated protein 4) or decreased (e.g. cofilin 1) in the Crx-/- retina, with P values of <.005. It is not known whether any of these genes are involved in building or regulating synaptic structures, but they are now genes that might lead to a better understanding of the construction and function of the relatively unique structure of the ribbon synapse.
Abnormal photoreceptor terminal formation was noted in a study that examined retinal development in the laminin beta2 chain-deficient mouse . Several pathologies were noted in these mice. First, laminin beta2 chain-deficient mice displayed abnormal outer segment elongation, but a more mild phenotype than that of the Crx-/- mice; the outer segments were reduced by 50% in length. Also photoreceptor terminals were perturbed in laminin beta2 mutants, but again the phenotype was more subtle then that of Crx-/- mice. The outer plexiform layer of the beta2-deficient retinas demonstrated only 7% normal invaginating synapses, while the remainder had various pathologies, including floating synaptic ribbons, as seen here. The mechanistic relationship of these two molecules, if any, in photoreceptor morphogenesis is unknown to date. The mRNA for laminin beta2 was not detected in the SAGE study of the relative RNA levels in Crx-/- and Crx+/+ and thus we cannot comment on whether the levels of RNA for laminin beta2 were altered.
Crx-/- mice are a model for LCA
Crx has been implicated in three photoreceptor diseases that result in human blindness, cone-rod dystrophy2, Leber's congenital amaurosis, and retinitis pigmentosa (for review, see ). The cone-rod dystrophies (CRDs) are characterized by loss of cone-mediated vision in the first decade of life or later, with concomitant or subsequent loss of rod-mediated vision . Conversely, RP is notable for initial loss of rod function, followed by loss of cone-mediated vision . The majority of known genes responsible for human genetic blindness, encode proteins expressed almost exclusively, or exclusively, in photoreceptors, particularly in the outer segment . Many of these proteins are required for phototransduction or outer segment structure. The mechanisms whereby mutations in rod-specific genes, such as rhodopsin, lead eventually to cone degeneration in RP remain obscure. Mutations in Crx were the first, and still one of a very few examples of a transcription factor mutation leading to photoreceptor disease.
LCA is a disease in which there is little or no photoreceptor function in infancy; thereby, likely developmental in etiology ([17, 56] for review). The Crx-/- mouse may be an excellent model for studying the pathology of this disorder, particularly the subtype of the disorder where Crx mutations are involved. The vast majority of histopathological studies of LCA in human tissue have been derived from adult patients with LCA where secondary changes are likely to be present. Indeed in animal models of LCA, secondary reactive and/or degenerative changes occur early after the abnormal formation of retinal tissue . The only study in human tissue derived from a human 33-week retina with proposed RPE65 mutations was reported to have abnormal retinae at this early stage . These authors report cell loss, including thinning of the photoreceptor layer. In addition, they claim in the text to have seen aberrant synaptic and inner retinal organization, although their examination of photoreceptor synapses unfortunately are not presented in the data section of the paper. Given the scarcity of available human tissue, the characterization of the primary pathology of LCA will require animal models. In the current study, we present data that argue that, in addition to outer segment morphogenesis, synaptogenesis may also be critically impaired in at least a subset of LCA.
Transmission electron microscopy
Littermate Crx-/- and wildtype pups were perfused in 1% formaldehyde and 0.5% glutaraldehyde at various postnatal stages. The eyes were then enucleated, and the cornea and lens were removed. The eye cup was immersed in fixative (1% formaldehyde and 2.5% glutaraldehyde) for 3 to 4 hours at 4°C. The sclera was then partially removed and the retinas were sliced into small pieces and fixed (1% paraformaldehyde and 2.5% glutaraldehyde) overnight at 4°C. These procedures were found optimal for maintaining the structural integrity of the photoreceptor outer segments.
After fixing, the tissue was washed 2X in PBS for thirty minutes per wash. The tissue was then postfixed in a 1% osmium tetroxide/1.5% potassium ferrocyanide mixture for 2 hours at 4°C. Staining was carried out for 30 minutes in 1% uranyl acetate in maleate buffer (pH = 6.0) at room temperature followed by 1% tannic acid in 0.1 M cacodylate buffer (pH = 7.4) for thirty minutes. The specimens were then dehydrated and embedded in Epon/Araldite. Thin sections were stained with uranyl acetate and lead citrate, and examined in a Jeol JEM-1200EX electron microscope.
Scanning electron microscopy
Specimens used for SEM required removal of the retinal pigment epithelium (PE), enabling visualization of the outer surface of the neural retina. Retinae from Crx-/-, rhodopsin-/-, RDS, or wildtype eyes were dissected free from PE in a dispase solution and fixed in 1.25% glutaraldehyde and 1.0% formaldehyde overnight at 4°C. Tissue was then washed 5X in cacodylate buffer and dehydrated in ascending grades of ethanol. Tissues were subsequently critical point dried in carbon dioxide. All specimens were mounted and coated with sublimated gold-palladium by the sputtering technique. Micrographs were obtained with a Jeol JSM-35CF scanning electron microscope.
We would like to thank Heather Regan and Dr. Susumu Ito for helping with ultrastructural analyses and Dr. David Papermaster for helpful discussions on the formation of outer segments.
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