Characterization of the mouse Myoc/Tigr gene

Interleukin-8 enhances myocilin expression, Akt-FoxO3 signaling and myogenic differentiation in rat skeletal muscle cells

Interleukin (IL)-8 is released both in visceral adipose tissue and in contracting skeletal muscles. In this study, we examined cellular pathways associated with muscle hypertrophy, chosen on the basis of microRNA profiling, in differentiating rat primary skeletal muscle cells (RSkMC) treated with IL-8 (1 ng/ml) for 11 days. IL-8 increased myocilin expression, Akt phosphorylation, FoxO3 dispersion throughout the cytoplasm, and reduced FoxO3 level. IL-8 decreased the expression of atrogin and MuRF1 and increased myotube length and diameter. We concluded that IL-8 present in extracellular environment of myoblasts induced to differentiation stimulates expression of myocilin, a protein important for skeletal muscle hypertrophy. This phenomenon was associated with: (a) activation of myogenic transcription, (b) increased phosphorylation and activation of PKB/Akt, leading to (c) cytoplasm distribution and degradation of a transcription factor FoxO3, (d) decreased expression of gene markers of proteolysis, atrogin and Murf1, and (e) increased myotube length and diameter. In this regard, IL-8 affects skeletal muscle cells similarly to IGF-I and can be considered as a potent anticatabolic factor for skeletal muscle.

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.

Myocilin regulates cell proliferation and survival

Myocilin, a causative gene for open angle glaucoma, encodes a secreted glycoprotein with poorly understood functions. To gain insight into its functions, we produced a stably transfected HEK293 cell line expressing myocilin under an inducible promoter and compared gene expression profiles between myocilin-expressing and vector control cell lines by a microarray analysis. A significant fraction of differentially expressed genes in myocilin-expressing cells was associated with cell growth and cell death, suggesting that myocilin may have a role in the regulation of cell growth and survival. Increased proliferation of myocilin-expressing cells was demonstrated by the WST-1 proliferation assay, direct cell counting, and immunostaining with antibodies against Ki-67, a cellular proliferation marker. Myocilin-containing conditioned medium also increased proliferation of unmodified HEK293 cells. Myocilin-expressing cells were more resistant to serum starvation-induced apoptosis than control cells. TUNEL-positive apoptotic cells were dramatically decreased, and two apoptotic marker proteins, cleaved caspase 7 and cleaved poly(ADP-ribose) polymerase, were significantly reduced in myocilin-expressing cells as compared with control cells under apoptotic conditions. In addition, myocilin-deficient mesenchymal stem cells exhibited reduced proliferation and enhanced susceptibility to serum starvation-induced apoptosis as compared with wild-type mesenchymal stem cells. Phosphorylation of ERK1/2 and its upstream kinases, c-Raf and MEK, was increased in myocilin-expressing cells compared with control cells. Elevated phosphorylation of ERK1/2 was also observed in the trabecular meshwork of transgenic mice expressing 6-fold higher levels of myocilin when compared with their wild-type littermates. These results suggest that myocilin promotes cell proliferation and resistance to apoptosis via the ERK1/2 MAPK signaling pathway.

Myocilin mediates myelination in the peripheral nervous system through ErbB2/3 signaling

The glaucoma-associated gene, myocilin, is expressed in ocular and non-ocular tissues including the peripheral nervous system, but its functions in these tissues remain poorly understood. We demonstrate that in sciatic nerve, myocilin is expressed in Schwann cells with high concentrations at the nodes of Ranvier. There, myocilin interacts with gliomedin, neurofascin, and NrCAM, which are essential for node formation and function. Treatment of isolated dorsal root ganglion cultures with myocilin stimulates clustering of the nodal proteins neurofascin and sodium channel Nav1.2. Sciatic nerves of myocilin null mice express reduced levels of several myelin-associated and basal membrane proteins compared with those of wild-type littermates. They also demonstrate reduced myelin sheath thickness and partial disorganization of the nodes. Myocilin signaling through ErbB2/3 receptors may contribute to these observed effects. Myocilin binds to ErbB2/ErbB3, activates these receptors, and affects the downstream PI3K-AKT signaling pathway. These data implicate a role for myocilin in the development and/or maintenance of myelination and nodes of Ranvier in sciatic nerve.

Myocilin stimulates osteogenic differentiation of mesenchymal stem cells through mitogen-activated protein kinase signaling

Myocilin is a secreted glycoprotein that is expressed in ocular and non-ocular tissues. Mutations in the MYOCILIN gene may lead to juvenile- and adult-onset primary open-angle glaucoma. Here we report that myocilin is expressed in bone marrow-derived mesenchymal stem cells (MSCs) and plays a role in their differentiation into osteoblasts in vitro and in osteogenesis in vivo. Expression of myocilin was detected in MSCs derived from mouse, rat, and human bone marrow, with human MSCs exhibiting the highest level of myocilin expression. Expression of myocilin rose during the course of human MSC differentiation into osteoblasts but not into adipocytes, and treatment with exogenous myocilin further enhanced osteogenesis. MSCs derived from Myoc-null mice had a reduced ability to differentiate into the osteoblastic lineage, which was partially rescued by exogenous extracellular myocilin treatment. Myocilin also stimulated osteogenic differentiation of wild-type MSCs, which was associated with activation of the p38, Erk1/2, and JNK MAP kinase signaling pathways as well as up-regulated expression of the osteogenic transcription factors Runx2 and Dlx5. Finally, cortical bone thickness and trabecular volume, as well as the expression level of osteopontin, a known factor of bone remodeling and osteoblast differentiation, were reduced dramatically in the femurs of Myoc-null mice compared with wild-type mice. These data suggest that myocilin should be considered as a target for improving the bone regenerative potential of MSCs and may identify a new role for myocilin in bone formation and/or maintenance in vivo.

Myocilin interacts with syntrophins and is member of dystrophin-associated protein complex

Genetic studies have linked myocilin to open angle glaucoma, but the functions of the protein in the eye and other tissues have remained elusive. The purpose of this investigation was to elucidate myocilin function(s). We identified α1-syntrophin, a component of the dystrophin-associated protein complex (DAPC), as a myocilin-binding candidate. Myocilin interacted with α1-syntrophin via its N-terminal domain and co-immunoprecipitated with α1-syntrophin from C2C12 myotubes and mouse skeletal muscle. Expression of 15-fold higher levels of myocilin in the muscles of transgenic mice led to the elevated association of α1-syntrophin, neuronal nitric-oxide synthase, and α-dystroglycan with DAPC, which increased the binding of laminin to α-dystroglycan and Akt signaling. Phosphorylation of Akt and Forkhead box O-class 3, key regulators of muscle size, was increased more than 3-fold, whereas the expression of muscle-specific RING finger protein-1 and atrogin-1, muscle atrophy markers, was decreased by 79 and 88%, respectively, in the muscles of transgenic mice. Consequently, the average size of muscle fibers of the transgenic mice was increased by 36% relative to controls. We suggest that intracellular myocilin plays a role as a regulator of muscle hypertrophy pathways, acting through the components of DAPC.

Myocilin, a glaucoma-associated protein, promotes cell migration through activation of integrin-focal adhesion kinase-serine/threonine kinase signaling pathway

The MYOCILIN gene encodes a secreted glycoprotein which is highly expressed in eye drainage structures. Mutations in this gene may lead to juvenile open-angle glaucoma and adult onset primary open-angle glaucoma, one of the leading causes of irreversible blindness in the world. Functions of wild-type myocilin are still unclear. We have recently demonstrated that myocilin is a modulator of Wnt signaling and may affect actin cytoskeleton organization. Here we report that myocilin and its naturally occurring proteolytic fragments, similar to Wnt3a, are able to stimulate trabecular meshwork, NIH3T3, and FHL124 cell migration with the N-terminal proteolytic fragment of myocilin lacking the olfactomedin domain producing the highest stimulatory effect. Stimulation of cell migration occurs through activation of the integrin-focal adhesion kinase (FAK)-serine/threonine kinase (AKT) signaling pathway. Inhibition of FAK by siRNA reduced the stimulatory action of myocilin by threefold. Activation of several components of this signaling pathway was also demonstrated in the eyes of transgenic mice expressing elevated levels of myocilin in the eye drainage structures. These data extend the similarities between actions of myocilin and Wnt proteins acting through a β-catenin-independent mechanism. The modification of the migratory ability of cells by myocilin may play a role in normal functioning of the eye anterior segment and its pathology including glaucoma.

Keeping an eye on myocilin: a complex molecule associated with primary open-angle glaucoma susceptibility

MYOC encodes a secretary glycoprotein of 504 amino acids named myocilin. MYOC is the first gene to be linked to juvenile open-angle glaucoma (JOAG) and some forms of adult-onset primary open-angle glaucoma (POAG). The gene was identified as an up-regulated molecule in cultured trabecular meshwork (TM) cells after treatment with dexamethasone and was originally referred to as trabecular meshwork-inducible glucocorticoid response (TIGR). Elevated intraocular pressure (IOP), due to decreased aqueous outflow, is the strongest known risk factor for POAG. Increasing evidence showed that the modulation of the wild-type (wt) myocilin protein expression is not causative of glaucoma while some misfolded and self-assembly aggregates of mutated myocilin may be associated with POAG in related or unrelated populations. The etiology of the disease remains unclear. Consequently, a better understanding of the molecular mechanisms underlyingPOAG is required to obtain early diagnosis, avoid potential disease progression, and develop new therapeutic strategies. In the present study, we review and discuss the most relevant studies regarding structural characterizations, expressions, molecular interactions, putative functions of MYOC gene and/or its corresponding protein in POAG etiology.

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.

Transgenic mice expressing the Tyr437His mutant of human myocilin protein develop glaucoma

PURPOSE

To developed a genetic mouse model of primary open-angle glaucoma induced by expression of mutated human myocilin in transgenic mice and to test whether expression of mutated human myocilin in the eye angle structures produces more significant damage to the eye than does mutated mouse myocilin.

METHODS

Recombineering in Escherichia coli was used to introduce the Tyr437His point mutation into a BAC carrying the full-length human MYOCILIN (MYOC) gene and long flanking regions. This BAC was used to produce transgenic mice. The expression of myocilin in the iridocorneal angle tissues and aqueous humor was studied by immunohistochemistry and Western blot analysis. Intraocular pressure was measured noninvasively with a fiber optic transducer. Retinal ganglion cells were retrograde labeled with fluorescent gold, and counted 5 days after labeling.

RESULTS

BAC transgenic mice expressed elevated levels of myocilin in tissues of the iridocorneal angle. Expression of mutated myocilin induced its intracellular accumulation and prevented secretion of both mutated and wild-type myocilin into the aqueous humor. Transgenic mice demonstrated a moderate elevation of intraocular pressure, which was more pronounced at night than in daytime. In the peripheral retina, transgenic mice lost 20% of the retinal ganglion cells and 55% of large retinal ganglion cells. Axonal degeneration was observed at the periphery of the optic nerve.

CONCLUSIONS

Expression of equivalent levels of mutated human or mouse myocilin in the eyes of transgenic mice produce comparable pathologic changes that are similar to those observed in patients with glaucoma.

Cloning of canine myocilin cDNA and molecular analysis of the myocilin gene in Shiba Inu dogs

OBJECTIVE

To identify canine myocilin cDNA and compare its sequence in glaucomatous and nonglaucomatous Shiba Inu dogs with closed and open iridocorneal angles (ICAs).

PROCEDURES

Total RNA was extracted from the ciliary body of the eyes of a healthy Beagle, and the canine myocilin gene was cloned and sequenced. Of the Shiba Inu dogs tested, five were glaucomatous with closed ICA, three were nonglaucomatous with open ICA, and two were nonglaucomatous with closed ICA. The genomic DNA of these dogs was extracted from peripheral blood leukocytes. The exons of the canine myocilin gene were amplified using the polymerase chain reaction (PCR), and sequenced. The frequency of mutation in canine myocilin DNA was verified in these dogs by using the myocilin cDNA of a Beagle.

RESULTS

The canine myocilin cDNA was 1452 bp long and contained the entire open reading frame encoding 483 amino acids. A leucine zipper-like motif and olfactomedin-like domain were conserved in the amino acid residues. The presence of sequence variants in the genomic DNA of Shiba Inu dogs was independent of the occurrence of glaucoma and ICA grading.

CONCLUSIONS

Myocilin RNA was detected in the ciliary body and trabecular meshwork (TM) of a Beagle. The myocilin sequence of Shiba Inu dogs suggests that myocilin mutations are unlikely to play a significant role in the pathogenesis of primary closed-angle glaucoma in this breed. However, several mutations in the myocilin gene in exon 1 of Shiba Inu dogs may predispose them to an obstruction in the anterior aqueous outflow.

Mutant myocilin nonsecretion in vivo is not sufficient to cause glaucoma

Glaucoma is a leading cause of blindness, affecting over 70 million people worldwide. Vision loss is the result of death of the retinal ganglion cells. The best-known risk factor for glaucoma is an elevated intraocular pressure (IOP); however, factors leading to IOP elevation are poorly understood. Mutations in the MYOC gene are an important cause of open-angle glaucoma. Over 70 MYOC mutations have been identified, and they lead to approximately 5% of all primary open-angle glaucoma cases. Nevertheless, the pathogenic mechanisms by which these mutations elevate IOP are presently unclear. Data suggest that a dominant interfering effect of misfolded mutant MYOC molecules may be pathogenic. To test this hypothesis, we have generated mice carrying a mutant allele of Myoc that is analogous to a human mutation that leads to aggressive glaucoma in patients. We show that mutant MYOC is not secreted into the aqueous humor. Instead of being secreted, mutant MYOC accumulates within the iridocorneal angle of the eye, consistent with the behavior of abnormally folded protein. Surprisingly, the accumulated mutant protein does not activate the unfolded protein response and lead to elevated intraocular pressure or glaucoma in aged mice of different strains. These data suggest that production, apparent misfolding, and nonsecretion of mutant MYOC are not, by themselves, sufficient to cause glaucoma in vivo.

Characterization of the Felix domesticus (cat) glaucoma-associated protein myocilin

The domestic cat (Felix domesticus) is a useful model for understanding the implications of long-term expression and function of normal and mutant myocilin. To better understand the role myocilin has in the cat eye, we isolated and characterized cat myocilin. Oligonucleotides designed against conserved nucleotide regions of myocilin mammalian orthologs were used to PCR amplify a partial cat myocilin cDNA clone. Rapid amplification of cDNA ends (5' and 3' RACE) was used to obtain full-length cat myocilin. The 2125 nucleotide cat myocilin cDNA contains a 490 amino acid open reading frame. Comparison of cat myocilin to human myocilin shows a 87% similarity, including conservation of the N-terminal leucine zipper, N-linked glycosylation site, C-terminal olfactomedin domain, and all five cysteine residues thought to be involved in disulfide bond formation. Expression in a transformed human trabecular cell line or in Crandall feline kidney cells showed cat myocilin was secreted from these cells, similar to human myocilin, suggesting cat myocilin contains a functional signal peptide sequence. In contrast, expression of cat myocilin containing a known human glaucoma-associated mutation (Y423H in cat; Y437H in human) was not secreted. Characterization of cat myocilin will enable long-term studies be performed in Felix domesticus to analyze changes to intraocular pressure and the aqueous outflow pathway following expression of myocilin and glaucoma causing mutations.

Genetically increasing Myoc expression supports a necessary pathologic role of abnormal proteins in glaucoma

Despite the importance of MYOC for glaucoma, the protein's normal function(s) and the pathogenic mechanism(s) of MYOC mutations are not clear. Elevated intraocular pressure (IOP) and glaucoma are sometimes induced by corticosteroids, and corticosteroid use can result in substantially increased MYOC expression. It has been suggested, therefore, that steroid-induced MYOC protein levels cause steroid-induced glaucoma and that protein level-increasing mutations in MYOC contribute to glaucoma not associated with steroid use. A causative role of elevated MYOC levels in steroid-induced glaucoma is controversial, however, and it is not clear if elevated MYOC levels can result in IOP elevation. To directly test if increased levels of MYOC can cause IOP elevation and glaucoma, we generated bacterial artificial chromosome transgenic mice that overexpress Myoc at a level similar to that induced by corticosteroid use. These mice do not develop elevated IOP or glaucoma. Our present findings, along with the absence of glaucoma in mice completely lacking MYOC, show that changing the level of MYOC is not pathogenic (from absent to approximately 15 times normal). These findings suggest that noncoding sequence variants are unlikely to influence glaucoma and that disease pathogenesis in primary open-angle glaucoma patients is dependent upon the expression of abnormal mutant proteins. This work does not support a causative role for increased MYOC levels or the MYOC gene in steroid-induced glaucoma.

Myocilin binding to Hep II domain of fibronectin inhibits cell spreading and incorporation of paxillin into focal adhesions

Myocilin, a novel matricellular protein found in the human eye, can modify signaling events mediated by the Heparin II domain of fibronectin. Using myocilin produced in sf9 insect cells, myocilin inhibited spreading of cycloheximide-treated human skin fibroblasts plated on substrates co-coated with myocilin and either fibronectin or its Heparin II domain. Cell spreading could be rescued by adding back either substrate adsorbed or soluble Heparin II domains. Myocilin did not inhibit cell attachment to fibronectin even in the presence of a 2400 M excess of myocilin. Myocilin impaired focal adhesion formation and specifically blocked the incorporation of paxillin, but not vinculin, into focal adhesions. The Heparin II domain mediated the incorporation of paxillin into focal adhesions, since paxillin was not assembled into focal adhesions unless the Heparin II domain was present. The effect of myocilin on focal adhesions could be overcome by treating cells with either phorbol 12-myristate (PMA) or oleoyl-L-alpha-lysophosphatidic acid (LPA). Myocilin bound to the fibroblast cell surface, but its binding could not be competed with excess fibronectin, suggesting that myocilin does not compete for cell surface binding sites of fibronectin. Myocilin therefore appears to specifically block functions mediated by the Heparin II domain possibly through direct interactions with it.

Recent advances in molecular genetics of glaucoma

Glaucoma represents a heterogeneous group of optic neuropathies, with different genetic bases. It can affect all ages generally with a rise in intra-ocular pressure. Three major types of glaucoma have been reported: primary open angle glaucoma (POAG), primary acute closed angle glaucoma (PACG) and primary congenital glaucoma (PCG), as well as a few others associated with developmental abnormalities. In recent years impressive progress has been made in the molecular genetic studies of POAG and PCG. These include the discovery of three genes--Myocilin, Optineurin and CYP1B1--defects in which results in Mendelian transmission of glaucoma. Identification of single nucleotide polymorphisms in multiple other genes that are associated with glaucoma and alteration of drug sensitivity are enriching our knowledge regarding the complex nature of the disease. This review attempts to present the recent progress made in the molecular genetics of glaucoma.

Tonography demonstrates reduced facility of outflow of aqueous humor in myocilin mutation carriers

PURPOSE

To demonstrate the effect in vivo of the myocilin gene mutation Thr377Met on outflow facility of aqueous humor, as measured by tonography.

MATERIALS AND METHODS

Forty-two members of a pedigree known to carry the Thr377Met mutation were examined for glaucoma, evaluated with tonography, and screened for myocilin mutations. Tonography was used to calculate the coefficient of aqueous outflow facility (C), as well as the ratio of the resting intraocular pressure to C (P(0)/C). Subjects were reexamined for glaucoma 5 years after tonography.

RESULTS

Seven subjects were excluded because of previous treatment known to alter facility of aqueous outflow. The mean outflow facility of the eyes of the 12 subjects carrying the Thr377Met mutation was significantly reduced compared with the 23 non-carriers' eyes using both C (P<0.001) and P(0)/C (P<0.001). Reduced outflow facility was also demonstrated in those mutation carriers who were not yet expressing clinical signs of glaucoma or ocular hypertension when measured using C (P = 0.015) and P(0)/C (P = 0.001). After 5 years, progression towards glaucoma had occurred in 5 of the myocilin mutation-carriers, 2 of whom showed bilateral progression; 3 carriers remained completely normal. Four subjects had bilateral glaucoma at the outset and remained unchanged. The carriers' eyes that progressed towards glaucoma had reduced outflow facility compared with those that remained normal, although the difference was not statistically significant.

CONCLUSIONS

Carriers of the myocilin Thr377Met mutation have reduced outflow facility, which may be detected prior to developing glaucoma. Tonography was not seen to be clinically useful in predicting progression towards glaucoma.

Characterization of rabbit myocilin: Implications for human myocilin glycosylation and signal peptide usage

BACKGROUND

Mutations in the gene encoding human myocilin (MYOC) have been shown to cause juvenile- and adult-onset glaucoma. In addition, myocilin has been associated with glucocorticoid-induced ocular hypertension and steroid-induced glaucoma. To better understand the role myocilin plays in steroid-induced glaucoma and open-angle glaucoma, we examined rabbit myocilin for use in the rabbit animal model of steroid-induced glaucoma.

RESULTS

We have cloned the rabbit ortholog of human MYOC. Rabbit MYOC consists of three exons and an open reading frame encoding a 490 amino acid, 54,882-Da protein, which is 14 amino acids shorter at the N-terminus than human myocilin but 84% identical overall. Rabbit myocilin migrates as a single electrophoretic band, vs. double-banded human myocilin, by SDS-PAGE/immunoblot analysis. We determined that the differential migration exhibited is due to an N-glycosylation site that is present in human (Asn57), monkey and mouse myocilin but absent in rabbit (Ser43), rat and bovine myocilin. Rabbit myocilin is secreted in vitro in trabecular meshwork cell culture and in vivo in aqueous humor. Secretion of human myocilin is shown to be dependent on the signal peptide and independent of the extra 14 amino acids not found in rabbit myocilin. Many of the amino acids in myocilin that are mutated in glaucoma patients are conserved across species.

CONCLUSION

We have cloned the rabbit MYOC cDNA and determined that rabbit myocilin is secreted but not N-linked glycosylated. Knowledge of the rabbit MYOC cDNA sequence will facilitate future studies in the rabbit animal model examining the role of myocilin in steroid-induced glaucoma and the gain-of-function hypothesis in open-angle glaucoma.

Pax6 heterozygous eyes show defects in chamber angle differentiation that are associated with a wide spectrum of other anterior eye segment abnormalities

The development of the chamber angle was studied in the eyes of heterozygous Pax6(lacZ/+) mutant mice (Nature 387 (1997) 406). Mutations in PAX6 cause aniridia, a condition that is frequently associated with glaucoma, a blinding disease that may be associated with chamber angle defects. Mesenchymal cells were seen in the chamber angle at P1-P5. In wild-type mice, these cells differentiated into typical trabecular meshwork (TM) cells next to Schlemm's canal. In Pax6(lacZ/+) mice, TM cells remained undifferentiated and Schlemm's canal was absent. From P1 to P4, staining for beta-galactosidase and immunoreactivity for Pax6 were observed in chamber angle mesenchyme, but were absent later. Cultured murine TM cells expressed Pax6. The defects in chamber angle and TM differentiation were associated with a wide spectrum of other anterior eye defects, which included various degrees of iris hypoplasia and corneal haze, isolated iridocorneal adhesions and atypical coloboma, and a vascularized cornea in all adult animals. A third of the animals showed Peters' anomaly including corneal opacity and iridocorneal adhesions. The separation of the lens from the cornea was incomplete, and epithelial layers of lens and cornea were continuous. Pax6 activity is directly required for differentiation of the chamber angle. Variations in phenotype of Pax6(lacZ/+) mice appear not to involve direct dominant-negative or dose-dependent effects.

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.

Expression of myocilin/TIGR in normal and glaucomatous primate optic nerves

Myocilin/TIGR was the first molecule discovered to be linked with primary open angle glaucoma (POAG), a blinding disease characterized by progressive loss of retinal ganglion cells. Mutations in myocilin/TIGR have been associated with age of disease onset and severity. The function of myocilin/TIGR and its role in glaucoma is unknown. Myocilin/TIGR has been studied in the trabecular meshwork to determine a role in regulation of intraocular pressure. The site of damage to the axons of the retinal ganglion cells is the optic nerve head (ONH). The myocilin/TIGR expression was examined in fetal through adult human optic nerve as well as in POAG. Myocilin/TIGR was expressed in the myelinated optic nerve of children and normal adults but not in the fetal optic nerve before myelination. Also examined was the expression in monkeys with experimental glaucoma. The results demonstrate that optic nerve head astrocytes constitutively express myocilin/TIGR in vivo in primates. Nevertheless, myocilin/TIGR is apparently reduced in glaucomatous ONH. The colocalization of myocilin/TIGR to the myelin suggests a role of myocilin/TIGR in the myelinated optic nerve.

Intraocular pressure in genetically distinct mice: an update and strain survey

BACKGROUND

Little is known about genetic factors affecting intraocular pressure (IOP) in mice and other mammals. The purpose of this study was to determine the IOPs of genetically distinct mouse strains, assess the effects of factors such as age, sex and time of day on IOP in specific strain backgrounds, and to assess the effects of specific candidate gene mutations on IOP.

RESULTS

Based on over 30 studied mouse strains, average IOP ranges from approximately 10 to 20 mmHg. Gender does not typically affect IOP and aging results in an IOP decrease in some strains. Most tested strains exhibit a diurnal rhythm with IOP being the highest during the dark period of the day. Homozygosity for a null allele of the carbonic anhydrase II gene (Car2n) does not alter IOP while homozygosity for a mutation in the leptin receptor gene (Leprdb) that causes obesity and diabetes results in increased IOP. Albino C57BL/6J mice homozygous for a tyrosinase mutation (Tyrc-2J) have higher IOPs than their pigmented counterparts.

CONCLUSIONS

Genetically distinct mouse strains housed in the same environment have a broad range of IOPs. These IOP differences are likely due to interstrain genetic differences that create a powerful resource for studying the regulation of IOP. Age, time of day, obesity and diabetes have effects on mouse IOP similar to those in humans and other species. Mutations in two of the assessed candidate genes (Lepr and Tyr) result in increased IOP. These studies demonstrate that mice are a practical and powerful experimental system to study the genetics of IOP regulation and disease processes that raise IOP to harmful levels.

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.

Age-dependent prevalence of mutations at the GLC1A locus in primary open-angle glaucoma

PURPOSE

To screen a population with primary open-angle glaucoma for mutations in the gene that encodes the trabecular meshwork inducible glucocorticoid response protein (TIGR), also known as myocilin (MYOC).

METHODS

Ophthalmologic information was collected for study subjects with primary open-angle glaucoma and their relatives. Mutation screening of 74 primary open-angle glaucoma probands was conducted by sequencing TIGR/MYOC coding sequence and splice sites.

RESULTS

In 23 families we detected 13 nonsynonymous sequence changes, nine of which appear to be mutations likely to cause or contribute to primary open-angle glaucoma. Two mutations, Arg272Gly and Ile499Ser, and one nonsynonymous sequence variant, Asn57Asp, are novel. We found mutations in nine of 25 juvenile glaucoma probands (36%) and two of 49 adult-onset glaucoma probands (4%). Age classification of families rather than individual probands revealed mutations in three of nine families with strictly juvenile primary open-angle glaucoma (33%), and no mutations in 39 families with strictly adult-onset primary open-angle glaucoma (0%). In families with mixed-onset primary open-angle glaucoma containing both juvenile primary open-angle glaucoma and adult-onset primary open-angle glaucoma cases, we found mutations in eight of 26 families (31%).

CONCLUSIONS

Our data suggest that Gly252Arg, Arg272Gly, Glu323Lys, Gln368STOP, Pro370Leu, Thr377Met, Val426Phe, Ile477Asn, and Ile499Ser are likely to play roles that cause or contribute to the etiology of autosomal dominant primary open-angle glaucoma. Our finding of more TIGR/MYOC mutations in families with mixed-onset primary open-angle glaucoma than in the families with strictly adult-onset primary open-angle glaucoma implies that the presence of relatives with juvenile primary open-angle glaucoma in a family could be used as a basis for identifying a subset of the population with adult-onset primary open-angle glaucoma with higher prevalence of TIGR/MYOC mutations. To address this issue, and to refine estimations of mutation prevalence in these age-defined subpopulations, prospective study of a larger population ascertained entirely through adult-onset primary open-angle glaucoma probands will be needed.

Regulation of human myocilin/TIGR gene transcription in trabecular meshwork cells and astrocytes: role of upstream stimulatory factor

BACKGROUND

Mutations in the myocilin (MYOC)/TIGR gene are responsible for autosomal-dominant juvenile primary open-angle glaucoma (POAG). In patients with non-autosomal-dominant POAG, such mutations are rare, but the expression of MYOC/TIGR in the trabecular meshwork (TM) of the eye is considerably higher than in normals. We performed transfection, DNAse I footprinting, mutagenesis and electrophoretic mobility shift assays (EMSA) to identify elements responsible for the basal transcription of MYOC/TIGR in TM cells and astrocytes.

RESULTS

DNAse I footprinting experiments of the human MYOC/TIGR promoter showed a major protected area between nt -106 to -77, which was not conserved in the homologous region of the mouse myoc/tigr promoter. In addition, the TATA-box was protected, as well as at least three downstream sites, including an AP-1-like sequence. Deletion of the -106 to -77 region caused a substantial loss of functional promotor activity in all cell types. Site-directed mutagenesis and EMSA experiments revealed the presence of two regulatory elements in the -106 to -77 region. Each of these cis-elements is essential for minimal promoter activity. The 5'-half of the region contains a sequence with similarities to NF-kappaB-related sites, however, binding of NF-kappaB could not be confirmed by EMSA. The 3'-half contains a canonical E-box sequence. EMSA experiments showed that the upstream regulatory factor (USF) was binding to the E-box sequence and that the binding can be supershifted by specific antibodies.

CONCLUSIONS

Several DNA-protein binding elements contribute to a transcription of MYOC/TIGR, and USF is critically required for its basal transcription in trabecular meshwork cells and astrocytes.

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.

A cellular assay distinguishes normal and mutant TIGR/myocilin protein

Glaucoma is a blinding eye disease that affects approximately 70 000 000 people world-wide. Mutations in the gene TIGR / MYOC have been shown to cause the most common form of the disease, primary open angle glaucoma, in selected families. Amino acid sequence variants of the gene have been found in 2-4% of sporadic primary open angle glaucoma cases. Most variants are rare and it is often difficult to definitively distinguish between a deleterious mutation and a benign variant solely on the basis of relative frequencies in patient and control groups. The function of the TIGR/myocilin protein is unknown and an assay to functionally classify variants is lacking. We sought to develop a biochemical assay to distinguish different forms of TIGR/myocilin. We investigated the Triton X-100 detergent solubility characteristics of mutant and normal forms of the protein, expressed by transfection in cultured cells. We observed a clear difference in the behavior of the two types of TIGR/myocilin; all confirmed mutant proteins tested were substantially Triton insoluble, while normal protein and controls were completely soluble. We also tested seven ambiguous variant proteins and classified them as mutant or normal on the basis of their Triton solubility. The results in some cases validated, and in other cases contradicted, earlier classifications of these variants. To our knowledge, Triton solubility is the first example of a general difference in the properties of mutant and normal forms of TIGR/myocilin. The assay we have developed will be useful for discerning protein functional information from the location of mutations, will aid genetic counseling of individuals with TIGR/myocilin variants and may provide a clue to understanding a mechanism by which mutations in TIGR / MYOC cause glaucoma.

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

The MYOC (GLC1A) gene has recently been associated with both juvenile-onset primary open angle glaucoma (JOAG) and typical late-onset primary open angle glaucoma (POAG). As a result, much scrutiny has been focused on the pathology of these diseases. In order to better understand the pathophysiology of POAG, we have been developing a mouse model of the disease. As a step in this development, we have investigated the expression pattern of Myoc transcripts in embryonic and adult mouse tissue using Northern blot and in situ hybridization analyses. Myoc transcripts were found in high levels in adult eye, heart, brain, skeletal muscle and testis and to a lesser extent in lung and kidney. They were also present, albeit in very low amounts, during mouse embryogenesis. We present new evidence using in situ hybridization analysis that Myoc transcripts were present in widespread regions of the adult brain including the ependymal lining of the third and fourth ventricles, in the choroid plexus, the zonal layer of the junction of the inferior and superior colliculi, the neurons of the habenula, the piriform cortex, the median pre-optic nucleus of the hypothalamus, the olfactory tubercle, and in the inferior olive. In a functional sense, Myoc expression in the ependyma and choroid plexus, two regions of the brain involved in cerebrospinal fluid synthesis and resorption, parallels Myoc expression in the ciliary body and trabecular meshwork of the anterior segment of the eye where aqueous humor synthesis and outflow occur.

Glaucoma genetics: where are we? Where will we go?

The understanding of the genetic basis of the glaucomas has advanced rapidly. Mutations in the myocilin gene (previously known as TIGR) at the GLC1A locus on chromosome 1q21-q31 occur in a subset of patients with juvenile- and adult-onset primary open-angle glaucoma. Five other genetic localizations for primary open-angle glaucoma have now been reported. In patients with primary congenital glaucoma, mutations have been found in the CYP1B1 gene on chromosome 2p21. At least one other locus for primary congenital glaucoma is mapped. In the developmental glaucomas, mutations in the PITX2 gene on chromosome 4q25 have been associated with Rieger syndrome, iris hypoplasia, and iridogoniodysgenesis. A second locus for Rieger syndrome resides on chromosome 13q14. Mutations in the FKHL7 gene on chromosome 6p25 have been described in patients with Axenfeld-Rieger anomaly. A new ocular finding of glaucoma in pedigrees with the nailpatella syndrome has been described, and mutations in the LMX1B gene on chromosome 9q34 are now known to underlie nail-patella syndrome. Two loci for the pigment dispersion syndrome have been mapped. This paper provides an overview of recent literature, summarizes developments in glaucoma genetics, and addresses their potential relevance to the clinical management of glaucoma.

Mouse myocilin (Myoc) gene expression in ocular tissues

Human myocilin is identical to TIGR (trabecular meshwork inducible glucocorticoid response) which is responsible for the pathogenesis of juvenile-onset primary open angle glaucoma (GLCIA). We have isolated cDNA for mouse myocilin (Myoc) and investigated mouse myocilin gene expression in ocular tissues with in situ RNA hybridization. Hybridization signals were observed in the iris, ciliary body, trabecular meshwork, sclera, and retina in the mouse eye. The marked signals were seen in trabecular meshwork cells and the anterior portion of sclera. These findings suggest that myocilin mutation could affect the capacity of aqueous outflow and cause elevation of the intraocular pressure which is involved in the pathogenesis of glaucoma.

In-vitro study of the activity of ciprofloxacin alone and in combination against strains of Pseudomonas aeruginosa with multiple antibiotic resistance

Ciprofloxacin appears to have useful activity against Pseudomonas aeruginosa. We have studied its in-vitro activity against ten strains of Ps. aeruginosa with multiple antibiotic resistance. We have confirmed that ciprofloxacin is very active against Ps. aeruginosa with minimal inhibitory concentrations ranging from 0.07 to 0.7 mg/l. Killing curves show ciprofloxacin to be rapidly bactericidal with no regrowth after 24 h. Checkerboard studies with ciprofloxacin in combination with gentamicin, azlocillin and ceftazidime show no consistent interaction. These studies suggest that ciprofloxacin should prove a useful antibiotic in treating infections caused by multiresistant Ps. aeruginosa.