Expression pattern and in situ localization of the mouse homologue of the human MYOC (GLC1A) gene in adult brain

Durable 3D murine ex vivo retina glaucoma models for optical coherence tomography

Durable and standardized phantoms with optical properties similar to native healthy and disease-like biological tissues are essential tools for the development, performance testing, calibration and comparison of label-free high-resolution optical coherence tomography (HR-OCT) systems. Available phantoms are based on artificial materials and reflect thus only partially ocular properties. To address this limitation, we have performed investigations on the establishment of durable tissue phantoms from ex vivo mouse retina for enhanced reproduction of in vivo structure and complexity. In a proof-of-concept study, we explored the establishment of durable 3D models from dissected mouse eyes that reproduce the properties of normal retina structures and tissue with glaucoma-like layer thickness alterations. We explored different sectioning and preparation procedures for embedding normal and N-methyl-D-aspartate (NMDA)-treated mouse retina in transparent gel matrices and epoxy resins, to generate durable three-dimensional tissue models. Sample quality and reproducibility were quantified by thickness determination of the generated layered structures utilizing computer-assisted segmentation of OCT B-scans that were acquired with a commercial HR-OCT system at a central wavelength of 905 nm and analyzed with custom build software. Our results show that the generated 3D models feature thin biological layers close to current OCT resolution limits and glaucoma-like tissue alterations that are suitable for reliable HR-OCT performance characterization. The comparison of data from resin-embedded tissue with native murine retina in gels demonstrates that by utilization of appropriate preparation protocols, highly stable samples with layered structures equivalent to native tissues can be fabricated. The experimental data demonstrate our concept as a promising approach toward the fabrication of durable biological 3D models suitable for high-resolution OCT system performance characterization supporting the development of optimized instruments for ophthalmology applications.

MYOC Promotes the Differentiation of C2C12 Cells by Regulation of the TGF-β Signaling Pathways via CAV1

Simple Summary

MYOC is a secreted glycoprotein and it expresses at high levels in skeletal muscle cells. However, the function of MYOC in muscle is still unclear. Accordingly, in this study, we examined that MYOC expression increased gradually during C2C12 differentiation and it could promote the differentiation of C2C12. Furthermore, we demonstrated that MYOC could bind to CAV1. We further confirmed that CAV1 could positively regulate C2C12 differentiation through the TGF-β pathway. At last, we determined the relationship among MYOC, CAV1 and TGF-β. We found that MYOC promoted the differentiation of C2C12 cells by regulation of the TGF-β signaling pathways via CAV1. The present study is the first to demonstrate the mechanism of action of MYOC in C2C12 cells. It provides a novel method of exploring the mechanism of muscle differentiation and represents a potential novel method for the treatment of muscle diseases.

Abstract

Myocilin (MYOC) is a glycoprotein encoded by a gene associated with glaucoma pathology. In addition to the eyes, it also expresses at high transcription levels in the heart and skeletal muscle. MYOC affects the formation of the murine gastrocnemius muscle and is associated with the differentiation of mouse osteoblasts, but its role in the differentiation of C2C12 cells has not yet been reported. Here, MYOC expression was found to increase gradually during the differentiation of C2C12 cells. Overexpression of MYOC resulted in enhanced differentiation of C2C12 cells while its inhibition caused reduced differentiation. Furthermore, immunoprecipitation indicated that MYOC binds to Caveolin-1 (CAV1), a protein that influences the TGF-β pathway. Laser confocal microscopy also revealed the common sites of action of the two during the differentiation of C2C12 cells. Additionally, CAV1 was upregulated significantly as C2C12 cells differentiated, with CAV1 able to influence the differentiation of the cells. Furthermore, the Western blotting analysis demonstrated that the expression of MYOC affected the TGF-β pathway. Finally, MYOC was overexpressed while CAV1 was inhibited. The results indicate that reduced CAV1 expression blocked the promotion of C2C12 cell differentiation by MYOC. In conclusion, the results demonstrated that MYOC regulates TGF-β by influencing CAV1 to promote the differentiation of C2C12 cells.

CRISPR-Cas9-based treatment of myocilin-associated glaucoma

Primary open-angle glaucoma (POAG) is a leading cause of irreversible vision loss worldwide, with elevated intraocular pressure (IOP) a major risk factor. Myocilin (MYOC) dominant gain-of-function mutations have been reported in ∼4% of POAG cases. MYOC mutations result in protein misfolding, leading to endoplasmic reticulum (ER) stress in the trabecular meshwork (TM), the tissue that regulates IOP. We use CRISPR-Cas9-mediated genome editing in cultured human TM cells and in a MYOC mouse model of POAG to knock down expression of mutant MYOC, resulting in relief of ER stress. In vivo genome editing results in lower IOP and prevents further glaucomatous damage. Importantly, using an ex vivo human organ culture system, we demonstrate the feasibility of human genome editing in the eye for this important disease.

A Novel Luciferase Assay For Sensitively Monitoring Myocilin Variants in Cell Culture

Purpose

Primary open angle glaucoma–associated mutations in myocilin (MYOC) cause protein “nonsecretion,” rendering secreted MYOC difficult to detect using conventional techniques. This study focused on developing and using an assay that can quickly and easily detect mutant MYOC secretion.

Methods

We fused Gaussia luciferase (eGLuc2) to MYOC variants and expressed the constructs in HEK-293T and NTM-5 cells. Secreted and intracellular levels of MYOC eGLuc2 variants were evaluated by Western blotting and compared to untagged and FLAG-tagged MYOC constructs. Secreted and soluble intracellular MYOC eGLuc2 were measured by a GLuc assay. The secretion of nine additional MYOC mutants was assayed in conditioned media from transfected cells to test the applicability of the assay for monitoring other MYOC variants.

Results

Myocilin eGLuc2 behaved similarly to untagged and FLAG-tagged MYOC with respect to secretion, soluble intracellular levels, and in response to drug treatment. The GLuc assay could sensitively detect Y437H MYOC secretion 30 minutes after media change. Gaussia luciferase fused variants followed anticipated trends; nonpathogenic variants (D208E, G244V) were secreted at wild-type (WT) levels, whereas predicted disease-causing variants (C245Y, G246R, E300K, Y437H, I477N) demonstrated substantial secretion defects. Secretion defects caused by the C245Y, G246R, and Y437H mutations were partially rescued by permissive growth temperature. Interestingly, however, this increase in secretion was independent of newly synthesized protein.

Conclusions

Fusion of eGLuc2 to MYOC does not significantly change the behavior of MYOC. This newly developed MYOC reporter system can be used to study engineered MYOC variants and potentially to identify modulators of MYOC secretion and function.

Novel candidate genes for alcoholism--transcriptomic analysis of prefrontal medial cortex, hippocampus and nucleus accumbens of Warsaw alcohol-preferring and non-preferring rats

OBJECTIVE

Animal models provide opportunity to study neurobiological aspects of human alcoholism. Changes in gene expression have been implicated in mediating brain functions, including reward system and addiction. The current study aimed to identify genes that may underlie differential ethanol preference in Warsaw High Preferring (WHP) and Warsaw Low Preferring (WLP) rats.

METHODS

Microarray analysis comparing gene expression in nucleus accumbens (NAc), hippocampus (HP) and medial prefrontal cortex (mPFC) was performed in male WHP and WLP rats bred for differences in ethanol preference.

RESULTS

Differential and stable between biological repeats expression of 345, 254 and 129 transcripts in NAc, HP and mPFC was detected. Identified genes and processes included known mediators of ethanol response (Mx2, Fam111a, Itpr1, Gabra4, Agtr1a, LTP/LTD, renin-angiotensin signaling pathway), toxicity (Sult1c2a, Ces1, inflammatory response), as well as genes involved in regulation of important addiction-related brain systems such as dopamine, tachykinin or acetylcholine (Gng7, Tac4, Slc5a7).

CONCLUSIONS

The identified candidate genes may underlie differential ethanol preference in an animal model of alcoholism.

COMMENT

Names of genes are written in italics, while names of proteins are written in standard font. Names of human genes/proteins are written in all capital letters. Names of rodent genes/proteins are written in capital letter followed by small letters.

Topical ocular sodium 4-phenylbutyrate rescues glaucoma in a myocilin mouse model of primary open-angle glaucoma

PURPOSE

Mutations in the myocilin gene (MYOC) are the most common known genetic cause of primary open-angle glaucoma (POAG). The purpose of this study was to determine whether topical ocular sodium 4-phenylbutyrate (PBA) treatment rescues glaucoma phenotypes in a mouse model of myocilin-associated glaucoma (Tg-MYOC(Y437H) mice).

METHODS

Tg-MYOC(Y437H) mice were treated with PBA eye drops (n = 10) or sterile PBS (n = 8) twice daily for 5 months. Long-term safety and effectiveness of topical PBA (0.2%) on glaucoma phenotypes were examined by measuring intraocular pressure (IOP) and pattern ERG (PERG), performing slit lamp evaluation of the anterior chamber, analyzing histologic sections of the anterior segment, and comparing myocilin levels in the aqueous humor and trabecular meshwork of Tg-MYOC(Y437H) mice.

RESULTS

Tg-MYOC(Y437H) mice developed elevated IOP at 3 months of age when compared with wild-type (WT) littermates (n = 24; P < 0.0001). Topical PBA did not alter IOP in WT mice. However, it significantly reduced elevated IOP in Tg-MYOC(Y437H) mice to the level of WT mice. Topical PBA-treated Tg-MYOC(Y437H) mice also preserved PERG amplitudes compared with vehicle-treated Tg-MYOC(Y437H) mice. No structural abnormalities were observed in the anterior chamber of PBA-treated WT and Tg-MYOC(Y437H) mice. Analysis of the myocilin in the aqueous humor and TM revealed that PBA significantly improved the secretion of myocilin and reduced myocilin accumulation as well as endoplasmic reticulum (ER) stress in the TM of Tg-MYOC(Y437H) mice. Furthermore, topical PBA reduced IOP elevated by induction of ER stress via tunicamycin injections in WT mice.

CONCLUSIONS

Topical ocular PBA reduces glaucomatous phenotypes in Tg-MYOC(Y437H) mice, most likely by reducing myocilin accumulation and ER stress in the TM. Topical ocular PBA could become a novel treatment for POAG patients with myocilin mutations.

Exome sequencing and analysis of induced pluripotent stem cells identify the cilia-related gene male germ cell-associated kinase (MAK) as a cause of retinitis pigmentosa

Retinitis pigmentosa (RP) is a genetically heterogeneous heritable disease characterized by apoptotic death of photoreceptor cells. We used exome sequencing to identify a homozygous Alu insertion in exon 9 of male germ cell-associated kinase (MAK) as the cause of disease in an isolated individual with RP. Screening of 1,798 unrelated RP patients identified 20 additional probands homozygous for this insertion (1.2%). All 21 affected probands are of Jewish ancestry. MAK encodes a kinase involved in the regulation of photoreceptor-connecting cilium length. Immunohistochemistry of human donor tissue revealed that MAK is expressed in the inner segments, cell bodies, and axons of rod and cone photoreceptors. Several isoforms of MAK that result from alternative splicing were identified. Induced pluripotent stem cells were derived from the skin of the proband and a patient with non-MAK-associated RP (RP control). In the RP control individual, we found that a transcript lacking exon 9 was predominant in undifferentiated cells, whereas a transcript bearing exon 9 and a previously unrecognized exon 12 predominated in cells that were differentiated into retinal precursors. However, in the proband with the Alu insertion, the developmental switch to the MAK transcript bearing exons 9 and 12 did not occur. In addition to showing the use of induced pluripotent stem cells to efficiently evaluate the pathogenicity of specific mutations in relatively inaccessible tissues like retina, this study reveals algorithmic and molecular obstacles to the discovery of pathogenic insertions and suggests specific changes in strategy that can be implemented to more fully harness the power of sequencing technologies.

Efficient in vivo doxycycline and cre recombinase-mediated inducible transgene activation in the murine trabecular meshwork

PURPOSE

To generate new mouse lines that facilitate inducible gene activation in the murine trabecular meshwork in vivo.

METHODS

Two expression cassettes were knocked into the 3'-UTR of the Myocilin (Myoc) locus, an abundantly expressed extracellular matrix protein produced by cells of the trabecular meshwork. The first cassette directs expression of an inducible form of Cre recombinase, CreER(T2), which is activated by tamoxifen administration under the control of endogenous Myoc regulatory elements. The second cassette contains a reverse tetracycline transactivator, rtTA(M2), which directs the expression of tetracycline-operator transgenes on exposure of animals to doxycycline (Dox). These lines were crossed to GFP and lacZ reporter mice to assay for tamoxifen or Dox-induced transgene expression.

RESULTS

Both the Myoc-CreER(T2) and the Myoc-rtTA(M2) lines were capable of directing efficient and inducible expression of transgenes in the murine trabecular meshwork in vivo. In addition, activation of transgenes by Myoc-rtTA(M2) was reversible with loss of transgene expression after Dox withdrawal. Examination of multiple tissues demonstrates efficient transgene activation in the trabecular meshwork, with additional sites of transgene activation including cells in the retina, sclera, lung, kidney, and abundant activation in the neocortex and hippocampus.

CONCLUSIONS

Two new mouse lines have been generated that allow for efficient and inducible transgene activation in the murine trabecular meshwork in vivo.

Glaucoma-associated myocilin: a better understanding but much more to learn

Over a decade has passed since myocilin was identified as the first gene linked to early and late-onset primary open-angle glaucoma. During this time, considerable effort has been put forth to understand the functional role myocilin has in normal and glaucomatous eyes. Myocilin is expressed in many ocular and non-ocular tissues, is found in both intracellular and extracellular spaces, and has been linked to elevations in intraocular pressure. Mutations in the myocilin gene that have been associated with glaucoma appear to confer a gain-of-functional activity rather than loss of function. Unfortunately, what the normal function of myocilin is and how alterations in the function can confer a glaucoma phenotype have yet to be elucidated. We will review the current understanding of myocilin with special emphasis on the structural makeup of the myocilin gene and protein, its possible physiological roles internal and external to ocular cells, the regulation of intraocular pressure as evidenced through the use of perfusion culture systems and animal models, and as a causative agent in some forms of glaucoma.

Characterization of monoclonal antibodies against the glaucoma-associated protein myocilin

Although the glaucoma-associated protein myocilin has been the focus of intensive research, its biological function is still unknown. One of the limiting factors has been the lack of well-characterized antibodies, particularly monoclonal antibodies. We describe the development of six monoclonal antibodies specific to myocilin and characterize their suitability in Western blot and immunohistochemical applications. Three of the six monoclonal antibodies recognize the N-terminus of myocilin (amino acids 33-214), two antibodies recognize the middle third of the protein (amino acids 215-368), and one antibody recognizes the C-terminus (amino acids 369-504). Isotyping revealed that all antibodies are of the IgG1 kappa class except one, which is IgG2b kappa. Purified myocilin monoclonal antibodies were able to recognize myocilin in human aqueous humor separated on denatured/reduced and native gels, and human trabecular meshwork lysate by Western blot. Myocilin was also detected by immunohistochemistry in trabecular meshwork, ciliary body, iris, cornea, sclera, choroid, and retinal pigment epithelial cells.

A knockin mouse model of the Bardet-Biedl syndrome 1 M390R mutation has cilia defects, ventriculomegaly, retinopathy, and obesity

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous disorder that results in retinal degeneration, obesity, cognitive impairment, polydactyly, renal abnormalities, and hypogenitalism. Of the 12 known BBS genes, BBS1 is the most commonly mutated, and a single missense mutation (M390R) accounts for approximately 80% of BBS1 cases. To gain insight into the function of BBS1, we generated a Bbs1(M390R/M390R) knockin mouse model. Mice homozygous for the M390R mutation recapitulated aspects of the human phenotype, including retinal degeneration, male infertility, and obesity. The obese mutant mice were hyperphagic and hyperleptinemic and exhibited reduced locomotor activity but no elevation in mean arterial blood pressure. Morphological evaluation of Bbs1 mutant brain neuroanatomy revealed ventriculomegaly of the lateral and third ventricles, thinning of the cerebral cortex, and reduced volume of the corpus striatum and hippocampus. Similar abnormalities were also observed in the brains of Bbs2(-/-), Bbs4(-/-), and Bbs6(-/-) mice, establishing these neuroanatomical defects as a previously undescribed BBS mouse model phenotype. Ultrastructural examination of the ependymal cell cilia that line the enlarged third ventricle of the Bbs1 mutant brains showed that, whereas the 9 + 2 arrangement of axonemal microtubules was intact, elongated cilia and cilia with abnormally swollen distal ends were present. Together with data from transmission electron microscopy analysis of photoreceptor cell connecting cilia, the Bbs1 M390R mutation does not affect axonemal structure, but it may play a role in the regulation of cilia assembly and/or function.

Comparative genomics and gene expression analysis identifies BBS9, a new Bardet-Biedl syndrome gene

Bardet-Biedl syndrome (BBS) is an autosomal recessive, genetically heterogeneous, pleiotropic human disorder characterized by obesity, retinopathy, polydactyly, renal and cardiac malformations, learning disabilities, and hypogenitalism. Eight BBS genes representing all known mapped loci have been identified. Mutation analysis of the known BBS genes in BBS patients indicate that additional BBS genes exist and/or that unidentified mutations exist in the known genes. To identify new BBS genes, we performed homozygosity mapping of small, consanguineous BBS pedigrees, using moderately dense SNP arrays. A bioinformatics approach combining comparative genomic analysis and gene expression studies of a BBS-knockout mouse model was used to prioritize BBS candidate genes within the newly identified loci for mutation screening. By use of this strategy, parathyroid hormone-responsive gene B1 (B1) was found to be a novel BBS gene (BBS9), supported by the identification of homozygous mutations in BBS patients. The identification of BBS9 illustrates the power of using a combination of comparative genomic analysis, gene expression studies, and homozygosity mapping with SNP arrays in small, consanguineous families for the identification of rare autosomal recessive disorders. We also demonstrate that small, consanguineous families are useful in identifying intragenic deletions. This type of mutation is likely to be underreported because of the difficulty of deletion detection in the heterozygous state by the mutation screening methods that are used in many studies.

Interaction of myocilin with gamma-synuclein affects its secretion and aggregation

Mutations in the gene encoding human myocilin are associated with some cases of juvenile and early-onset glaucoma. Glaucomatous mutations prevent myocilin from being secreted. The analysis of the defects associated with mutations point to the existence of factor(s) in addition to mutations that might be implicated in the development of glaucoma. In the present paper, we found that interaction of myocilin with one of the members of the synuclein family alters its properties, including its ability to be secreted. Results of immunoprecipitation show that myocilin is a gamma-synuclein-interacting protein. Further analysis demonstrated that both myocilin and gamma-synuclein are expressed in human TM cells, immortalized rat ganglion (RGC-5) cells, and HT22 hippocampal neurons. According to Western blotting, in addition to monomeric form with molecular weight 17 kDa gamma-synuclein is present as higher molecular weight forms ( approximately 35 and 68 KDa), presumably dimer and tetramer. Myocilin and gamma-synuclein have partially overlapping perinuclear localization. Dexamethasone upregulates myocilin expression in RGC-5 cells and HT22 hippocampal neurons. We found alterations of myocilin properties as a result of its interaction with gamma-synuclein. In cultured cells, gamma-synuclein upregulates myocilin expression, inhibits its secretion and prevents the formation of high molecular weight forms of myocilin. Although both alpha-synuclein and gamma-synuclein are expressed in HTM cells, only gamma-synuclein interacts with myocilin and alters its properties. We conclude that myocilin and gamma-synuclein interact and as a result, myocilin's properties are changed. Since myocilin and gamma-synuclein have partially overlapping intracellular localization in cell types that are implicated in glaucoma development, their interaction may play an important role in glaucoma.

Mkks-null mice have a phenotype resembling Bardet-Biedl syndrome

McKusick-Kaufman syndrome (MKS) is an autosomal recessive disorder characterized by post-axial polydactyly, congenital heart defects and hydrometrocolpos, a congenital structural abnormality of female genitalia. Mutations in the MKKS gene have also been shown to cause some cases of Bardet-Biedl syndrome (BBS) which is characterized by obesity, pigmentary retinopathy, polydactyly, renal abnormalities and hypogenitalism with secondary features of hypertension and diabetes. Although there is overlap in clinical features between MKS and BBS, MKS patients are not obese and do not develop retinopathy or have learning disabilities. To further explore the pathophysiology of BBS and the related disorder MKS, we have developed an Mkks(-/-) mouse model. This model shows that the absence of Mkks leads to retinal degeneration through apoptosis, failure of spermatozoa flagella formation, elevated blood pressure and obesity. The obesity is associated with hyperphagia and decreased activity. In addition, neurological screening reveals deficits in olfaction and social dominance. The mice do not have polydactyly or vaginal abnormalities. The phenotype of the Mkks(-/-) mice closely resembles the phenotype of other mouse models of BBS (Bbs2(-/-) and Bbs4(-/-)). These observations suggest that the complete absence of MKKS leads to BBS while the MKS phenotype is likely to be due to specific mutations.

Bbs2-null mice have neurosensory deficits, a defect in social dominance, and retinopathy associated with mislocalization of rhodopsin

Bardet-Biedl syndrome (BBS) is a heterogeneous, pleiotropic human disorder characterized by obesity, retinopathy, polydactyly, renal and cardiac malformations, learning disabilities, hypogenitalism, and an increased incidence of diabetes and hypertension. No information is available regarding the specific function of BBS2. We show that mice lacking Bbs2 gene expression have major components of the human phenotype, including obesity and retinopathy. In addition, these mice have phenotypes associated with cilia dysfunction, including retinopathy, renal cysts, male infertility, and a deficit in olfaction. With the exception of male infertility, these phenotypes are not caused by a complete absence of cilia. We demonstrate that BBS2 retinopathy involves normal retina development followed by apoptotic death of photoreceptors, the primary ciliated cells of the retina. Photoreceptor cell death is preceded by mislocalization of rhodopsin, indicating a defect in transport. We also demonstrate that Bbs2(-/-) mice and a second BBS mouse model, Bbs4(-/-), have a defect in social function. The evaluation of Bbs2(-/-) mice indicates additional phenotypes that should be evaluated in human patients, including deficits in social interaction and infertility.

Bardet-Biedl syndrome type 4 (BBS4)-null mice implicate Bbs4 in flagella formation but not global cilia assembly

The functions of the proteins encoded by the Bardet-Biedl syndrome (BBS) genes are unknown. Mutations in these genes lead to the pleiotropic human disorder BBS, which is characterized by obesity, retinopathy, polydactyly, renal and cardiac malformations, learning disabilities, and hypogenitalism. Secondary features include diabetes mellitus and hypertension. Recently, it has been suggested that the BBS phenotypes are the result of a lack of cilia formation or function. In this study, we show that mice lacking the Bbs4 protein have major components of the human phenotype, including obesity and retinal degeneration. We show that Bbs4-null mice develop both motile and primary cilia, demonstrating that Bbs4 is not required for global cilia formation. Interestingly, male Bbs4-null mice do not form spermatozoa flagella, and BBS4 retinopathy involves apoptotic death of photoreceptors, the primary ciliated cells of the retina. These mutation data demonstrate a connection between the function of a BBS protein and cilia. To further evaluate an association between cilia and BBS, we performed homology comparisons of BBS proteins in model organisms and find that BBS proteins are specifically conserved in ciliated organisms.

Optic nerve hypoplasia with intracranial arachnoid cyst

BACKGROUND

The frequent association between optic nerve hypoplasia (ONH) and other central nervous system abnormalities has been widely reported in the literature. Occasional reference has been made to arachnoid cyst as one of the associated findings.

METHOD

The charts and neuroradiologic findings of the 40 patients with ONH seen in our department as well as the Visually Impaired Program during the past 8 years were reviewed.

RESULTS

An intracranial arachnoid cyst was present in 5 of the 40 patients (12.5%) with ONH and visual impairment. These patients' clinical and neuroradiologic findings are reported here.

CONCLUSION

The presence of an intracranial arachnoid cyst in a patient with hypoplastic optic nerves could occur as a coincidental association between these lesions. Alternatively, a common mechanism could give rise to both abnormalities. Three such possibilities-including damage to the developing visual pathway by the arachnoid cyst, absence of an axonal guidance molecule similar to netrin-1 identified in the mouse, or a common genetic mutation involving the myocillin gene causing both abnormalities-are considered.

The expression of myocilin during murine eye development

PURPOSE

To study the expression and localization of myocilin in the developing mouse eye. Myocilin is a 55- to 57-kDa secreted glycoprotein that is mutated in some forms of primary open-angle glaucoma.

METHODS

The eyes of NMRI mice were studied from embryonic day (E) 14.5 to postnatal day (P) 21, and at 2-3 months of age. Immunohistochemistry was performed with antibodies against myocilin. The specificity of the antibodies was checked by two-dimensional gel electrophoresis. RNA was isolated from eyes at various ages, and the presence of myocilin mRNA was analyzed by northern blot hybridization.

RESULTS

No immunostaining for myocilin was seen before E16.5. At around E17.5, a distinct positive immunoreactivity of optic nerve axons in the developing nerve fiber layer of the retina was observed. At P5-6, immunostaining appeared in perikarya of optic nerve ganglion cells. In the anterior eye, no immunoreactivity was observed until P10. At P12-14, the cells of the epithelial layers of ciliary body and iris, as well as the cells of the trabecular meshwork and iris stroma, became immunoreactive for myocilin. At that time, positive staining for myocilin was also seen in the corneal endothelium and in keratocytes of the corneal stroma. An essentially similar staining pattern was seen in adult eyes. Northern blot analysis for myocilin mRNA in RNA from developing mouse eyes was negative until P9. At P12, a distinct band was observed. A band with similar mobility, but somewhat more intense, was detected in mRNA from adult mouse eyes 2-3 months of age.

CONCLUSIONS

The onset of immunoreactivity for myocilin in the retina occurs in parallel with the maturation of optic nerve ganglion cells. In the anterior eye, the expression of myocilin is associated with the final development of those tissues that are directly involved in aqueous humor dynamics. The presence of myocilin might be important for proper function and structure of mature optic nerve ganglion cells and aqueous humor outflow.

Secreted glycoprotein myocilin is a component of the myelin sheath in peripheral nerves

The structure of the myelin sheath in peripheral nerves requires the expression of a specific set of proteins. In the present study, we report that myocilin, a member of the olfactomedin protein family, is a component of the myelin sheath in peripheral nerves. Myocilin is a secreted glycoprotein that forms multimers and contains a leucine zipper and an olfactomedin domain. Mutations in myocilin are responsible for some forms of glaucoma, a neurodegenerative disease that is characterized by a continuous loss of optic nerve axons. Myocilin mRNA was detected by Northern blotting in RNA from the rat sciatic and ophthalmic nerves. By one- and two-dimensional gel electrophoresis of proteins from the rat and human sciatic nerves, myocilin was found to migrate at an isoelectric point (pI) of 5.2-5.3 and a molecular weight of 55-57 kDa. Immunohistochemistry showed immunoreactivity for myocilin in paranodal terminal loops of the nodes of Ranvier and outer mesaxons and basal/abaxonal regions of the myelin sheath. Double-labeling experiments with antibodies against myelin basic protein showed no overlapping, while overlapping immunoreactivity was observed with antibodies against myelin-associated glycoprotein. The expression of myocilin in the sciatic nerve became detectable at postnatal day (P) 15 and reached adult levels at P20. No or minor expression of myocilin mRNA was found in brain, spinal cord, and optic nerve. mRNA of myocilin was detected in schwannoma cells in situ, but at considerably lower levels than in myelinated nerves. Myocilin might significantly contribute to the structure of the myelin sheath in peripheral nerves.

Myocilin and glaucoma: facts and ideas

Mutations in the MYOC gene that encodes for myocilin are causative for some forms of juvenile and adult-onset primary open-angle glaucoma (POAG). Myocilin is a secreted 55-57kDa glycoprotein that forms dimers and multimers. Characteristic structural motifs include a myosin-like domain, a leucine zipper region and an olfactomedin domain. Most of the mutations that have been identified in patients with POAG are localized in the olfactomedin domain, which is highly conserved among species. In the eye, myocilin is expressed in high amounts in the trabecular meshwork (TM), sclera, ciliary body and iris, and at considerable lower amounts in retina and optic nerve head. Secreted myocilin is present in the aqueous humor. In the TM, myocilin is found within the cytoplasm of TM cells and in the juxtacanalicular region in association with fibrillar extracellular matrix components. Since patients with mutations in myocilin may have high intraocular pressures, the role of myocilin for aqueous humor outflow has been investigated and conflicting results have been obtained. Recombinant myocilin increases outflow resistance in perfused anterior segment organ cultures, while overexpression of myocilin after viral gene transfer appears to reduce outflow resistance. In TM cells, the expression of myocilin is induced upon treatment with dexamethasone at a time course similar to that observed in steroid-induced glaucoma. Other factors that induce myocilin expression are transforming growth factor-beta and mechanical stretch. Promoter elements that are important for the glucocorticoid induction have not been identified, but it has been shown that upstream stimulatory factor is critical for the basal promoter activity of MYOC. Mice with a targeted disruption of the myocilin gene do not express a phenotype, indicating that the glaucomatous phenotype in humans is not because of a loss-of-function effect. Experimental studies show that mutated myocilin is not secreted, but appears to accumulate in the cells. Such an accumulation might interfere with TM function and lead to impaired outflow resistance, but, so far, experimental evidence for such a scenario is lacking. In addition, the normal function(s) of myocilin is (are) still elusive.

Targeted Disruption of the Myocilin Gene (Myoc) Suggests that Human Glaucoma-Causing Mutations Are Gain of Function

Glaucoma is a heterogeneous eye disease and a major cause of blindness worldwide. Recently, primary open angle glaucoma (POAG)-associated mutations have been found in the trabecular meshwork inducible glucocorticoid response gene (TIGR), also known as the myocilin gene (MYOC), at the GLC1A locus on chromosome 1q21-q31. These mutations occurred in a subset of patients with juvenile- and adult-onset POAG and exhibited autosomal dominant inheritance. Ocular expression and its involvement in POAG suggest that TIGR/MYOC may have a role(s) in regulating intraocular pressure (IOP). Here, we report the generation and analysis of mice heterozygous and homozygous for a targeted null mutation in Myoc. Our study shows that Myoc mutant mice are both viable and fertile. Our in vivo findings further demonstrate that Myoc is not required for normal IOP or normal ocular morphology. The lack of a discernable phenotype in both Myoc-heterozygous and Myoc-null mice suggests that haploinsufficiency is not a critical mechanism for POAG in individuals with mutations in MYOC. Instead, disease-causing mutations in humans likely act by gain of function.

The role of myocilin/TIGR in glaucoma: results of the Glaucoma Research Foundation catalyst meeting in Berkeley, California, March 2000

Approximately 3 years ago, the first major (biochemical, molecular biologic, and biologic) insight into primary open-angle glaucoma was the finding that mutations in the myocilin (MYOC/TIGR) gene were related to certain forms of juvenile onset of this disease. Since then, a great deal of work has been done to determine the possible mechanisms by which MYOC/TIGR might cause not only juvenile but also adult-onset primary open-angle glaucoma. To assess the current knowledge and those areas in which more research is necessary, a meeting of scientists was held by the Glaucoma Research Foundation of San Francisco, California in the spring of 2000. This meeting attempted to concentrate on the MYOC/TIGR protein rather than the genetics of this gene. Possible functions and roles of this protein intracellularly and extracellularly were critically examined and discussed. Normal transcriptional and translational events and the effect of mutations on these events were explored. The discussions yielded insight not only in those areas in which important information is known but also in vital areas in which little is currently understood. This review attempts to summarize the current knowledge regarding MYOC/TIGR and to elucidate the points that the people attending the meeting thought needed further study.

Evidence supporting WNT2 as an autism susceptibility gene

We examined WNT2 as a candidate disease gene for autism for the following reasons. First, the WNT family of genes influences the development of numerous organs and systems, including the central nervous system. Second, WNT2 is located in the region of chromosome 7q31-33 linked to autism and is adjacent to a chromosomal breakpoint in an individual with autism. Third, a mouse knockout of Dvl1, a member of a gene family essential for the function of the WNT pathway, exhibits a behavioral phenotype characterized primarily by diminished social interaction. We screened the WNT2 coding sequence for mutations in a large number of autistic probands and found two families containing nonconservative coding sequence variants that segregated with autism in those families. We also identified linkage disequilibrium (LD) between a WNT2 3'UTR SNP and our sample of autism-affected sibling pair (ASP) families and trios. The LD arose almost exclusively from a subgroup of our ASP families defined by the presence of severe language abnormalities and was also found to be associated with the evidence for linkage to 7q from our previously published genomewide linkage screen. Furthermore, expression analysis demonstrated WNT2 expression in the human thalamus. Based on these findings, we hypothesize that rare mutations occur in the WNT2 gene that significantly increase susceptibility to autism even when present in single copies, while a more common WNT2 allele (or alleles) not yet identified may exist that contributes to the disorder to a lesser degree.

Hunting for disease genes in multi-functional diseases

Disease genes may be identified through functional, positional, and candidate gene approaches. Although extensive and often labor-intensive studies such as family linkage analysis, functional investigation of gene products and genome database searches are usually involved, thousands of human disease genes, especially for monogenic diseases with Mendelian transmission, have been identified. However, in diseases caused by more than one gene, or by a combination of genetic and environmental factors, identification of the genes is even more difficult. Common examples include atherosclerosis, cancer, Alzheimer's disease, asthma, diabetes, glaucoma, and age-related macular degeneration. There have been conflicting reports on the roles of associated genes. Even with population-based case-control studies and new statistical methods such as the sib-ship disequilibrium test and the discordant alleles test, there is no agreement on whether alpha2-macroglobulin (A2M) is a gene for Alzheimer's disease. Another example is the inconsistent association between age-related macular degeneration and ATP-binding cassette transporter (ABCR). Ethnic variation causes further complications. In our investigation of LDL-receptor variants in familial hypercholesterolemia, and the trabecular meshwork inducible glucocorticoid response protein, or myocillin (TIGR-MYOC) mutation pattern in primary open angle glaucoma, we did find dissimilar results in Chinese compared to Caucasians. New information from the Human Genome Project and advancements in technologies will aid the search for and confirm identification of disease genes despite such challenges.

The genetics of open-angle glaucoma: the story of GLC1A and myocilin

A linkage analysis study was performed on a single large family with juvenile-onset primary open-angle glaucoma (POAG). This led to the recognition that there was a region of chromosome 1q that harboured a gene for juvenile-onset POAG. This chromosomal site was called GLC1A. It was discovered that a gene that produces the protein myocilin resides within this interval and that mutations in myocilin caused most cases of autosomal dominant juvenile-onset POAG. More importantly myocilin mutations also cause up to 4.6% of cases of adult-onset POAG. The prevalence of myocilin mutations is similar regardless of race or geographic location. There are widely variable glaucoma phenotypes depending on the specific mutation in myocilin. Myocilin is expressed in multiple tissues throughout the eye and in many other organs. In the trabecular meshwork the production of myocilin can be induced by the application of topical corticosteroids. The exact function of myocilin in health and disease remains a mystery.

Molecular cloning and expression profile of rat myocilin

Myocilin is known to be associated with the pathogenesis of juvenile-onset primary open angle glaucoma. The tissue distribution of myocilin transcripts has been analyzed in both humans and mice, and a high level of expression in the retina and skeletal muscle has been reported. The functions of myocilin in these tissues are unknown. We isolated rat myocilin cDNA and examined the expression pattern of myocilin, including its expression in endocrine organs, using Northern blot analysis. The rat myocilin cDNA sequence has two in-frame initiation codons, the upstream and downstream ATGs corresponding to the initiation codon of human and murine myocilin, respectively. It is most likely that the first ATG is a translational initiation codon, since 8 of 13 amino acid residues deduced from the rat cDNA sequence between the first and the second ATGs are the same as those in human myocilin. The open reading frame encodes 502 amino acids. Rat myocilin also has both a myosin-like domain and an olfactmedin-like domain, which have been identified in human and murine myocilin. Northern analysis of rat myocilin mRNA revealed substantial expression in the thyroid gland, as well as in the retina and muscle. No transcripts were detected in other endocrine glands, including the adrenal gland, pituitary, and testis. Myocilin may play an important role in thyroid function. Further study of the expression and role of myocilin in the thyroid is required.

Heredity in primary open-angle glaucoma

The past years have seen considerable progress in the characterization of hereditary factors in primary open-angle glaucoma. Epidemiologic studies strengthened our knowledge of the hereditary factors in this multifactorial disease. Several loci in the human genome have been described, which segregate with different glaucoma phenotypes. Mutations of the MYOC/TIGR (myocilin/trabecular meshwork inducible glucocorticoid response) gene on chromosome 1q account for most, but probably not all, cases of glaucoma linked to chromosome 1q, and other additional pathologic factors may be implicated. The properties of the normal myocilin protein point to a crucial role in the regulation of intraocular pressure. However, in spite of the knowledge obtained so far, routinely performed genetic screening of patients at risk for primary open-angle glaucoma is not yet clinically useful.

Mutation of a nuclear receptor gene, NR2E3, causes enhanced S cone syndrome, a disorder of retinal cell fate

Hereditary human retinal degenerative diseases usually affect the mature photoreceptor topography by reducing the number of cells through apoptosis, resulting in loss of visual function. Only one inherited retinal disease, the enhanced S-cone syndrome (ESCS), manifests a gain in function of photoreceptors. ESCS is an autosomal recessive retinopathy in which patients have an increased sensitivity to blue light; perception of blue light is mediated by what is normally the least populous cone photoreceptor subtype, the S (short wavelength, blue) cones. People with ESCS also suffer visual loss, with night blindness occurring from early in life, varying degrees of L (long, red)- and M (middle, green)-cone vision, and retinal degeneration. The altered ratio of S- to L/M-cone photoreceptor sensitivity in ESCS may be due to abnormal cone cell fate determination during retinal development. In 94% of a cohort of ESCS probands we found mutations in NR2E3 (also known as PNR), which encodes a retinal nuclear receptor recently discovered to be a ligand-dependent transcription factor. Expression of NR2E3 was limited to the outer nuclear layer of the human retina. Our results suggest that NR2E3 has a role in determining photoreceptor phenotype during human retinogenesis.