Periaxin is required for hexagonal geometry and membrane organization of mature lens fibers

Ankyrin-B is required for the establishment and maintenance of lens cytoarchitecture, mechanics and clarity

The transparent ocular lens is essential for vision because it focuses light onto the retina. Despite recognition of the importance of its unique cellular architecture and mechanical properties, the molecular mechanisms governing these attributes remain elusive. This study aims to elucidate the role of ankyrin-B (AnkB, encoded by ANK2), a membrane scaffolding protein, in lens cytoarchitecture, growth and function using a conditional knockout (cKO) mouse model. The AnkB cKO mouse has no defects in lens morphogenesis but exhibited changes that supported a global role for AnkB in maintenance of lens clarity, size, cytoarchitecture, membrane organization and stiffness. Notably, absence of AnkB led to nuclear cataract formation, which was evident from postnatal day 16. AnkB cKO lens fibers exhibit progressive disruption in membrane organization of the spectrin-actin cytoskeleton, cell adhesion proteins and channel proteins; loss and degradation of several membrane proteins [such as NrCAM. N-cadherin (CDH2) and aquaporin-0 (also known as MIP)]; along with a disorganized plasma membrane and impaired membrane interdigitations. Furthermore, absence of AnkB led to decreased lens stiffness. Collectively, these results illustrate the essential role for AnkB in lens architecture, growth and function through its involvement in membrane skeletal and protein organization and stability.

Tissue, cellular, and molecular level determinants for eye lens stiffness and elasticity

The eye lens is a transparent, ellipsoid tissue in the anterior chamber that is required for the fine focusing of light onto the retina to transmit a clear image. The focusing function of the lens is tied to tissue transparency, refractive index, and biomechanical properties. The stiffness and elasticity or resilience of the human lens allows for shape changes during accommodation to focus light from objects near and far. It has long been hypothesized that changes in lens biomechanical properties with age lead to the loss of accommodative ability and the need for reading glasses with age. However, the cellular and molecular mechanisms that influence lens biomechanical properties and/or change with age remain unclear. Studies of lens stiffness and resilience in mouse models with genetic defects or at advanced age inform us of the cytoskeletal, structural, and morphometric parameters that are important for biomechanical stability. In this review, we will explore whether: 1) tissue level changes, including the capsule, lens volume, and nucleus volume, 2) cellular level alterations, including cell packing, suture organization, and complex membrane interdigitations, and 3) molecular scale modifications, including the F-actin and intermediate filament networks, protein modifications, lipids in the cell membrane, and hydrostatic pressure, influence overall lens biomechanical properties.

Ankyrin-B is required for the establishment and maintenance of lens cytoarchitecture, mechanics, and clarity

This study illustrates a vital role for ankyrin-B in lens architecture, growth and function through its involvement in membrane protein and spectrin-actin cytoskeletal organization and stability The transparent ocular lens is essential for vision by focusing light onto the retina. Despite recognizing the importance of its unique cellular architecture and mechanical properties, the molecular mechanisms governing these attributes remain elusive. This study aims to elucidate the role of ankyrin-B (AnkB), a membrane scaffolding protein, in lens cytoarchitecture, growth and function using a conditional knockout (cKO) mouse model. AnkB cKO mouse has no defects in lens morphogenesis, but exhibited changes that supported a global role for AnkB in maintenance of lens clarity, size, cytoarchitecture, and stiffness. Notably, absence of AnkB led to nuclear cataract formation, evident from P16. AnkB cKO lens fibers exhibit progressive disruption in membrane organization of the spectrin-actin cytoskeleton, channel proteins, cell-cell adhesion, shape change, loss and degradation of several membrane proteins (e.g., NrCAM. N-cadherin and aquaporin-0) along with a disorganized plasma membrane and impaired ball-and-socket membrane interdigitations. Furthermore, absence of AnkB led to decreased lens stiffness. Collectively, these results illustrate the essential role for AnkB in lens architecture, growth and function through its involvement in membrane protein and cytoskeletal organization.

Screening for PRX mutations in a large Chinese Charcot-Marie-Tooth disease cohort and literature review

Background

Periaxins (encoded by PRX) play an important role in the stabilization of peripheral nerve myelin. Mutations in PRX can lead to Charcot-Marie-Tooth disease type 4F (CMT4F).

Methods

In this study, we screened for PRX mutations using next-generation sequencing and whole-exome sequencing in a large Chinese CMT cohort consisting of 465 unrelated index patients and 650 healthy controls. Sanger sequencing was used for the validation of all identified variants. We also reviewed all previously reported PRX-related CMT cases and summarized the clinical manifestations and genetic features of PRX-related CMTs.

Results

The hit rate for biallelic PRX variants in our cohort of Chinese CMT patients was 0.43% (2/465). One patient carried a previously unreported splice-site mutation (c.25_27 + 9del) compound heterozygous with a known nonsense variant. Compiling data on CMT4F cases and PRX variants from the medical literature confirmed that early-onset (95.2%), distal amyotrophy or weakness (94.0%), feet deformity (75.0%), sensory impairment or sensory ataxia (65.5%), delayed motor milestones (60.7%), and spinal deformity (59.5%) are typical features for CMT4F. Less frequent features were auditory impairments, respiratory symptoms, late onset, dysarthria or hoarseness, ophthalmic problems, and central nervous system involvement. The two cases with biallelic missense mutations have later onset age than those with nonsense or frameshift mutations. We did not note clear correlations between the type and site of mutations and clinical severity or distinct constellations of symptoms.

Conclusion

Consistent with observations in other countries and ethnic groups, PRX-related CMT is rare in China. The clinical spectrum is wider than previously anticipated.

Whole-exome sequencing prioritizes candidate genes for hereditary cataract in the Emory mouse mutant

The Emory cataract (Em) mouse mutant has long been proposed as an animal model for age-related or senile cataract in humans-a leading cause of visual impairment. However, the genetic defect(s) underlying the autosomal dominant Em phenotype remains elusive. Here, we confirmed development of the cataract phenotype in commercially available Em/J mice [but not ancestral Carworth Farms White (CFW) mice] at 6-8 months of age and undertook whole-exome sequencing of candidate genes for Em. Analysis of coding and splice-site variants did not identify any disease-causing/associated mutations in over 450 genes known to underlie inherited and age-related forms of cataract and other lens disorders in humans and mice, including genes for lens crystallins, membrane/cytoskeleton proteins, DNA/RNA-binding proteins, and those associated with syndromic/systemic forms of cataract. However, we identified three cataract/lens-associated genes each with one novel homozygous variant including predicted missense substitutions in Prx (p.R167C) and Adamts10 (p.P761L) and a disruptive in-frame deletion variant (predicted missense) in Abhd12 (p.L30_A32delinsS) that were absent in CFW and over 35 other mouse strains. In silico analysis predicted that the missense substitutions in Prx and Adamts10 were borderline neutral/damaging and neutral, respectively, at the protein function level, whereas, that in Abhd12 was functionally damaging. Both the human counterparts of Adamts10 and Abhd12 are clinically associated with syndromic forms of cataract known as Weil-Marchesani syndrome 1 and polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract syndrome, respectively. Overall, while we cannot exclude Prx and Adamts10, our data suggest that Abhd12 is a promising candidate gene for cataract in the Em/J mouse.

Nonmuscle Myosin IIA Regulates the Precise Alignment of Hexagonal Eye Lens Epithelial Cells During Fiber Cell Formation and Differentiation

Purpose

Epithelial cells in the equatorial region of the ocular lens undergo a remarkable transition from randomly packed cells into precisely aligned and hexagon-shaped cells organized into meridional rows. We investigated the function of nonmuscle myosin IIA (encoded by Myh9) in regulating equatorial epithelial cell alignment to form meridional rows during secondary fiber cell morphogenesis.

Methods

We used genetic knock-in mice to study a common human Myh9 mutation, E1841K, in the rod domain. The E1841K mutation disrupts bipolar filament assembly. Lens shape, clarity, and stiffness were evaluated, and Western blots were used to determine the level of normal and mutant myosins. Cryosections and lens whole mounts were stained and imaged by confocal microscopy to investigate cell shape and organization.

Results

We observed no obvious changes in lens size, shape, and biomechanical properties (stiffness and resilience) between the control and nonmuscle myosin IIA-E1841K mutant mice at 2 months of age. Surprisingly, we found misalignment and disorder of fiber cells in heterozygous and homozygous mutant lenses. Further analysis revealed misshapen equatorial epithelial cells that cause disorientation of the meridional rows before fiber cell differentiation in homozygous mutant lenses.

Conclusions

Our data indicate that nonmuscle myosin IIA bipolar filament assembly is required for the precise alignment of the meridional rows at the lens equator and that the organization of lens fiber cells depends on the proper patterning of meridional row epithelial cells. These data also suggest that lens fiber cell organization and a hexagonal shape are not required for normal lens size, shape transparency, or biomechanical properties.

A Review of Lens Biomechanical Contributions to Presbyopia

Purpose: Presbyopia-the progressive loss of near focus with age-is primarily a result of changes in lens biomechanics. In particular, the shape of the ocular lens in the absence of zonular tension changes significantly throughout adulthood. Contributors to this change in shape are changes in lens biomechanical properties, continuous volumetric growth lens, and possibly remodeling of the lens capsule. Knowledge in this area is growing rapidly, so the purpose of this mini-review was to summarize and synthesize these gains.Methods: We review the recent literature in this field.Results: The mechanisms governing age-related changes in biomechanical properties remains unknown. We have recently shown that lens growth may be driven by zonular tension. The same mechanobiological mechanism driving lens growth may also lead to remodeling of the capsule, though this remains to be demonstrated.Conclusions: This mini-review focuses on identifying mechanisms which cause these age-related changes, suggesting future work which may elucidate these mechanisms, and briefly discusses ongoing efforts to develop a non-surgical approach for therapeutic management of presbyopia. We also propose a simple model linking lens growth and biomechanical properties.

A tamoxifen-inducible Cre knock-in mouse for lens-specific gene manipulation

Mouse models are valuable tools in studying lens biology and biochemistry, and the Cre-loxP system is the most used technology for gene targeting in the lens. However, numerous genes are indispensable in lens development. The conventional knockout method either prevents lens formation or causes simultaneous cataract formation, hindering the studies of their roles in lens structure, growth, metabolism, and cataractogenesis during lens aging. An inducible Cre-loxP mouse line is an excellent way to achieve such a purpose. We established a lens-specific Cre ERT2 knock-in mouse (LCEK), an inducible mouse model for lens-specific gene targeting in a spatiotemporal manner. LCEK mice were created by in-frame infusion of a P2A-CreERT2 at the C-terminus of the last coding exon of the gene alpha A crystallin (Cryaa). LCEK mice express tamoxifen-inducible Cre recombinase uniquely in the lens. Through ROSAmT/mG and two endogenous genes (Gclc and Rbpj) targeting, we found no Cre recombinase leakage in the lens epithelium, but 50-80% leakage was observed in the lens cortex and nucleus. Administration of tamoxifen almost completely abolished target gene expression in both lens epithelium and cortex but only mildly enhanced gene deletion in the lens nucleus. Notably, no overt leakage of Cre activity was detected in developing LCEK lens when bred with mice carrying loxP floxed genes that are essential for lens development. This newly generated LCEK line will be a powerful tool to target genes in the lens for gene functions study in lens aging, posterior capsule opacification (PCO), and other areas requiring precision gene targeting.

Pitfalls of whole exome sequencing in undefined clinical conditions with a suspected genetic etiology

BACKGROUND

Whole-Exome Sequencing (WES) is a valuable tool for the molecular diagnosis of patients with a suspected genetic condition. In complex and heterogeneous diseases, the interpretation of WES variants is more challenging given the absence of diagnostic handles and other reported cases with overlapping clinical presentations.

OBJECTIVE

To describe candidate variants emerging from trio-WES and possibly associated with the clinical phenotype in clinically heterogeneous conditions.

METHODS

We performed WES in ten patients from eight families, selected because of the lack of a clear clinical diagnosis or suspicion, the presence of multiple clinical signs, and the negative results of traditional genetic tests.

RESULTS

Although we identified ten candidate variants, reaching the diagnosis of these cases is challenging, given the complexity and the rarity of these syndromes and because affected genes are already associated with known genetic diseases only partially recapitulating patients' phenotypes. However, the identification of these variants could shed light into the definition of new genotype-phenotype correlations. Here, we describe the clinical and molecular data of these cases with the aim of favoring the match with other similar cases and, hopefully, confirm our diagnostic hypotheses.

CONCLUSION

This study emphasizes the major limitations associated with WES data interpretation, but also highlights its clinical utility in unraveling novel genotype-phenotype correlations in complex and heterogeneous undefined clinical conditions with a suspected genetic etiology.

Insights into the biochemical and biophysical mechanisms mediating the longevity of the transparent optics of the eye lens

In the human eye, a transparent cornea and lens combine to form the "refracton" to focus images on the retina. This requires the refracton to have a high refractive index "n," mediated largely by extracellular collagen fibrils in the corneal stroma and the highly concentrated crystallin proteins in the cytoplasm of the lens fiber cells. Transparency is a result of short-range order in the spatial arrangement of corneal collagen fibrils and lens crystallins, generated in part by post-translational modifications (PTMs). However, while corneal collagen is remodeled continuously and replaced, lens crystallins are very long-lived and are not replaced and so accumulate PTMs over a lifetime. Eventually, a tipping point is reached when protein aggregation results in increased light scatter, inevitably leading to the iconic protein condensation-based disease, age-related cataract (ARC). Cataracts account for 50% of vision impairment worldwide, affecting far more people than other well-known protein aggregation-based diseases. However, because accumulation of crystallin PTMs begins before birth and long before ARC presents, we postulate that the lens protein PTMs contribute to a "cataractogenic load" that not only increases with age but also has protective effects on optical function by stabilizing lens crystallins until a tipping point is reached. In this review, we highlight decades of experimental findings that support the potential for PTMs to be protective during normal development. We hypothesize that ARC is preventable by protecting the biochemical and biophysical properties of lens proteins needed to maintain transparency, refraction, and optical function.

Pathogenic genetic variants identified in Australian families with paediatric cataract

Objective

Paediatric (childhood or congenital) cataract is an opacification of the normally clear lens of the eye and has a genetic basis in at least 18% of cases in Australia. This study aimed to replicate clinical gene screening to identify variants likely to be causative of disease in an Australian patient cohort.

Methods and analysis

Sixty-three reported isolated cataract genes were screened for rare coding variants in 37 Australian families using genome sequencing.

Results

Disease-causing variants were confirmed in eight families with variant classification as ‘likely pathogenic’. This included novel variants PITX3 p.(Ter303LeuextTer100), BFSP1 p.(Glu375GlyfsTer2), and GJA8 p.(Pro189Ser), as well as, previously described variants identified in genes GJA3, GJA8, CRYAA, BFSP1, PITX3, COL4A1 and HSF4. Additionally, eight variants of uncertain significance with evidence towards pathogenicity were identified in genes: GJA3, GJA8, LEMD2, PRX, CRYBB1, BFSP2, and MIP.

Conclusion

These findings expand the genotype–phenotype correlations of both pathogenic and benign variation in cataract-associated genes. They further emphasise the need to develop additional evidence such as functional assays and variant classification criteria specific to paediatric cataract genes to improve interpretation of variants and molecular diagnosis in patients.

Exome sequencing in a child with neurodevelopmental disorder and epilepsy: Variant analysis of the AHNAK2 gene

Background

The AHNAK2 gene encodes a large nucleoprotein expressed in several tissues, including brain, squamous epithelia, smooth muscle, and neuropil. Its role in calcium signaling has been suggested and to date, clear evidence about its involvement in the pathogenesis of clinical disorders is still lacking.

Methods

Here, we report a female 24‐year‐old patient diagnosed with a cardio‐facio‐cutaneous‐like phenotype (CFC‐like), characterized by epilepsy, psychomotor development delay, atopic dermatitis, congenital heart disease, hypotonia, and facial dysmorphism, who is compound heterozygote for two missense mutations in the AHNAK2 gene detected by exome sequencing.

Results

This patient had no detectable variant in any of the genes known to be associated with the cardio‐facio‐cutaneous syndrome. Moreover, the mode of inheritance does not appear to be autosomal dominant, as it is in typical CFC syndrome. We have performed in silico assessment of mutation severity separately for each missense mutation, but this analysis excludes a severe effect on protein function. Protein structure predictions indicate the mutations are located in flexible regions possibly involved in molecular interactions.

Conclusion

We discuss an alternative interpretation on the potential involvement of the two missense mutations in the AHNAK2 gene on the expression of CFC‐like phenotype in this patient based on inter‐allelic complementation.

The Obscure Potential of AHNAK2

Simple Summary

AHNAK2 is a relatively newly discovered protein. It can interact with many other proteins. This protein is increased in cells of variety of different cancers. AHNAK2 may play a vital role in cancer formation. AHNAK2 may have a role in early detection of cancer. This obscure potential of AHNAK2 is being studied.

Abstract

AHNAK2 is a protein discovered in 2004, with a strong association with oncogenesis in various epithelial cancers. It has a large 616 kDa tripartite structure and is thought to take part in the formation of large multi-protein complexes. High expression is found in clear cell renal carcinoma, pancreatic ductal adenocarcinoma, uveal melanoma, and lung adenocarcinoma, with a relation to poor prognosis. Little work has been done in exploring the function and relation AHNAK2 has with cancer, with early studies showing promising potential as a future biomarker and therapeutic target.

Ezrin interacts with L-periaxin by the "head to head and tail to tail" mode and influences the location of L-periaxin in Schwann cell RSC96

In the peripheral nervous system (PNS), Schwann cells (SCs) are required for the myelination of axons. Periaxin (PRX), one of the myelination proteins expressed in SCs, is critical for the normal development and maintenance of PNS. As a member of the ERM (ezrin-radxin-moesin) protein family, ezrin holds our attention since their link to the formation of the nodes of Ranvier. Furthermore, PRX and ezrin are co-expressed in cytoskeletal complexes with periplakin and desmoyokin in lens fiber cells. In the present study, we observed that L-periaxin and ezrin interacted in a "head to head and tail to tail" mode in SC RSC96 through NLS3 region of L-periaxin with F3 subdomain of ezrin interaction, and the region of L-periaxin (residues 1368-1461) with ezrin (residues 475-557) interaction. A phosphorylation-mimicking mutation of ezrin resulted in L-periaxin accumulation on SC RSC96 membrane. Ezrin could inhibit the self-association of L-periaxin, and ezrin overexpression in sciatic nerve injury rats could facilitate the repair of impaired myelin sheath. Therefore, the interaction between L-periaxin and ezrin may adopt a close form to complete protein accumulation and to participate in myelin sheath maintenance.

BFSP1 C-terminal domains released by post-translational processing events can alter significantly the calcium regulation of AQP0 water permeability

BFSP1 (beaded filament structural protein 1, filensin) is a cytoskeletal protein expressed in the eye lens. It binds AQP0 in vitro and its C-terminal sequences have been suggested to regulate the water channel activity of AQP0. A myristoylated fragment from the C-terminus of BFSP1 was found in AQP0 enriched fractions. Here we identify BFSP1 as a substrate for caspase-mediated cleavage at several C-terminal sites including D433. Cleavage at D433 exposes a cryptic myristoylation sequence (434–440). We confirm that this sequence is an excellent substrate for both NMT1 and 2 (N-myristoyl transferase). Thus caspase cleavage may promote formation of myristoylated fragments derived from the BFSP1 C-terminus (G434-S665). Myristoylation at G434 is not required for membrane association. Biochemical fractionation and immunogold labeling confirmed that C-terminal BFSP1 fragments containing the myristoylation sequence colocalized with AQP0 in the same plasma membrane compartments of lens fibre cells. To determine the functional significance of the association of BFSP1 G434-S665 sequences with AQP0, we measured AQP0 water permeability in Xenopus oocytes co-transfected with transcripts expressing both AQP0 and various C-terminal domain fragments of BFSP1 generated by caspase cleavage. We found that different fragments dramatically alter the response of AQP0 to different concentrations of Ca²⁺. The complete C-terminal fragment (G434-S665) eliminates calcium regulation altogether. Shorter fragments can enhance regulation by elevated calcium or reverse the response, indicative of the regulatory potential of BFSP1 with respect to AQP0. In particular, elimination of the myristoylation site by the mutation G434A reverses the order of water permeability sensitivity to different Ca²⁺ concentrations.

Proteome-transcriptome analysis and proteome remodeling in mouse lens epithelium and fibers

Epithelial cells and differentiated fiber cells represent distinct compartments in the ocular lens. While previous studies have revealed proteins that are preferentially expressed in epithelial vs. fiber cells, a comprehensive proteomics library comparing the molecular compositions of epithelial vs. fiber cells is essential for understanding lens formation, function, disease and regenerative potential, and for efficient differentiation of pluripotent stem cells for modeling of lens development and pathology in vitro. To compare protein compositions between the lens epithelium and fibers, we employed tandem mass spectrometry (2D-LC/MS) analysis of microdissected mouse P0.5 lenses. Functional classifications of the top 525 identified proteins into gene ontology categories by molecular processes and subcellular localizations, were adapted for the lens. Expression levels of both epithelial and fiber proteomes were compared with whole lens proteome and mRNA levels using E14.5, E16.5, E18.5, and P0.5 RNA-Seq data sets. During this developmental time window, multiple complex biosynthetic and catabolic processes generate the molecular and structural foundation for lens transparency. As expected, crystallins showed a high correlation between their mRNA and protein levels. Comprehensive data analysis confirmed and/or predicted roles for transcription factors (TFs), RNA-binding proteins (e.g. Carhsp1), translational apparatus including ribosomal heterogeneity and initiation factors, microtubules, cytoskeletal [e.g. non-muscle myosin IIA heavy chain (Myh9) and βB2-spectrin (Sptbn2)] and membrane proteins in lens formation and maturation. Our data highlighted many proteins with unknown functions in the lens that were preferentially enriched in epithelium or fibers, setting the stage for future studies to further dissect the roles of these proteins in fiber cell differentiation vs. epithelial cell maintenance. In conclusion, the present proteomic datasets represent the first mouse lens epithelium and fiber cell proteomes, establish comparative analyses of protein and RNA-Seq data, and characterize the major proteome remodeling required to form the mature lens fiber cells.

Expression of periaxin (PRX) specifically in the human cerebrovascular system: PDZ domain-mediated strengthening of endothelial barrier function

Regulation of cerebral endothelial cell function plays an essential role in changes in blood-brain barrier permeability. Proteins that are important for establishment of endothelial tight junctions have emerged as critical molecules, and PDZ domain containing-molecules are among the most important. We have discovered that the PDZ-domain containing protein periaxin (PRX) is expressed in human cerebral endothelial cells. Surprisingly, PRX protein is not detected in brain endothelium in other mammalian species, suggesting that it could confer human-specific vascular properties. In endothelial cells, PRX is predominantly localized to the nucleus and not tight junctions. Transcriptome analysis shows that PRX expression suppresses, by at least 50%, a panel of inflammatory markers, of which 70% are Type I interferon response genes; only four genes were significantly activated by PRX expression. When expressed in mouse endothelial cells, PRX strengthens barrier function, significantly increases transendothelial electrical resistance (~35%; p < 0.05), and reduces the permeability of a wide range of molecules. The PDZ domain of PRX is necessary and sufficient for its barrier enhancing properties, since a splice variant (S-PRX) that contains only the PDZ domain, also increases barrier function. PRX also attenuates the permeability enhancing effects of lipopolysaccharide. Collectively, these studies suggest that PRX could potentially regulate endothelial homeostasis in human cerebral endothelial cells by modulating inflammatory gene programs.

Schwann Cell O-GlcNAc Glycosylation Is Required for Myelin Maintenance and Axon Integrity

Schwann cells (SCs), ensheathing glia of the peripheral nervous system, support axonal survival and function. Abnormalities in SC metabolism affect their ability to provide this support and maintain axon integrity. To further interrogate this metabolic influence on axon-glial interactions, we generated OGT-SCKO mice with SC-specific deletion of the metabolic/nutrient sensing protein O-GlcNAc transferase that mediates the O-linked addition of N-acetylglucosamine (GlcNAc) moieties to Ser and Thr residues. The OGT-SCKO mice develop tomaculous demyelinating neuropathy characterized by focal thickenings of the myelin sheath (tomacula), progressive demyelination, axonal loss, and motor and sensory nerve dysfunction. Proteomic analysis identified more than 100 O-GlcNAcylated proteins in rat sciatic nerve, including Periaxin (PRX), a myelin protein whose mutation causes inherited neuropathy in humans. PRX lacking O-GlcNAcylation is mislocalized within the myelin sheath of these mutant animals. Furthermore, phenotypes of OGT-SCKO and Prx-deficient mice are very similar, suggesting that metabolic control of PRX O-GlcNAcylation is crucial for myelin maintenance and axonal integrity.

SIGNIFICANCE STATEMENT

The nutrient sensing protein O-GlcNAc transferase (OGT) mediates post-translational O-linked N-acetylglucosamine (GlcNAc) modification. Here we find that OGT functions in Schwann cells (SCs) to maintain normal myelin and prevent axonal loss. SC-specific deletion of OGT (OGT-SCKO mice) causes a tomaculous demyelinating neuropathy accompanied with progressive axon degeneration and motor and sensory nerve dysfunction. We also found Periaxin (PRX), a myelin protein whose mutation causes inherited neuropathy in humans, is O-GlcNAcylated. Importantly, phenotypes of OGT-SCKO and Prx mutant mice are very similar, implying that compromised PRX function contributes to the neuropathy of OGT-SCKO mice. This study will be useful in understanding how SC metabolism contributes to PNS function and in developing new strategies for treating peripheral neuropathy by targeting SC function.

Switching of α-Catenin From Epithelial to Neuronal Type During Lens Epithelial Cell Differentiation

Purpose

Ocular lens fiber cell elongation, differentiation, and compaction are associated with extensive reorganization of cell adhesive interactions and cytoskeleton; however, our knowledge of proteins critical to these events is still evolving. This study characterizes the distribution pattern of neuronal-specific α-catenin (αN-catenin) and its interaction with the N-cadherin–associated adherens junctions (AJs) and their stability in the mouse lens fibers.

Methods

Expression and distribution of αN-catenin in developing mouse and adult human lenses was determined by RT-PCR, immunoblot, and immunofluorescence analyses. Characterization of αN-catenin and N-cadherin interacting proteins and colocalization analyses were performed using immunoprecipitation, mass spectrometry, and confocal imaging. Effects of periaxin deficiency on the stability of lens fiber cell AJs were evaluated using perixin-null mice.

Results

αN-catenin exhibits discrete distribution to lens fibers in both mouse and human lenses, undergoing a robust up-regulation during fiber cell differentiation and maturation. Epithelial-specific α-catenin (αE-catenin), in contrast, distributes primarily to the lens epithelium. αN-catenin and N-cadherin reciprocally coimmunoprecipitate and colocalize along with β-catenin, actin, spectrin, vinculin, Armadillo repeat protein deleted in velo-cardio-facial syndrome homolog, periaxin, and ankyrin-B in lens fibers. Fiber cells from periaxin-null mouse lenses revealed disrupted N-cadherin/αN-catenin–based AJs.

Conclusions

These results suggest that the discrete shift in α-catenin expression from αE-catenin to αN-catenin subtype that occurs during lens epithelial cell differentiation may play a key role in fiber cell cytoarchitecture by regulating the assembly and stability of N-cadherin–based AJs. This study also provides evidence for the importance of the fiber cell–specific cytoskeletal interacting periaxin, in the stability of N-cadherin/αN-catenin–based AJs in lens fibers.

Connexin 50 Regulates Surface Ball-and-Socket Structures and Fiber Cell Organization

Purpose

The roles of gap junction protein connexin 50 (Cx50) encoded by Gja8, during lens development are not fully understood. Connexin 50 knockout (KO) lenses have decreased proliferation of epithelial cells and altered fiber cell denucleation. We further investigated the mechanism for cellular defects in Cx50 KO (Gja8^−/−) lenses.

Methods

Fiber cell morphology and subcellular distribution of various lens membrane/cytoskeleton proteins from wild-type and Cx50 KO mice were visualized by immunofluorescent staining and confocal microscopy.

Results

We observed multiple morphological defects in the cortical fibers of Cx50 KO lenses, including abnormal fiber cell packing geometry, decreased F-actin enrichment at tricellular vertices, and disrupted ball-and-socket (BS) structures on the long sides of hexagonal fibers. Moreover, only small gap junction plaques consisting of Cx46 (α3 connexin) were detected in cortical fibers and the distributions of the BS-associated beta-dystroglycan and ZO-1 proteins were altered.

Conclusions

Connexin 50 gap junctions are important for BS structure maturation and cortical fiber cell organization. Connexin 50–based gap junction plaques likely form structural domains with an array of membrane/cytoskeletal proteins to stabilize BS. Loss of Cx50-mediated coupling, BS disruption, and altered F-actin in Cx50 KO fibers, thereby contribute to the small lens and mild cataract phenotypes.

Tropomodulin 1 Regulation of Actin Is Required for the Formation of Large Paddle Protrusions Between Mature Lens Fiber Cells

Purpose

To elucidate the proteins required for specialized small interlocking protrusions and large paddle domains at lens fiber cell tricellular junctions (vertices), we developed a novel method to immunostain single lens fibers and studied changes in cell morphology due to loss of tropomodulin 1 (Tmod1), an F-actin pointed end–capping protein.

Methods

We investigated F-actin and F-actin–binding protein localization in interdigitations of Tmod1^+/+ and Tmod1^−/− single mature lens fibers.

Results

F-actin–rich small protrusions and large paddles were present along cell vertices of Tmod1^+/+ mature fibers. In contrast, Tmod1^−/− mature fiber cells lack normal paddle domains, while small protrusions were unaffected. In Tmod1^+/+ mature fibers, Tmod1, β2-spectrin, and α-actinin are localized in large puncta in valleys between paddles; but in Tmod1^−/− mature fibers, β2-spectrin was dispersed while α-actinin was redistributed at the base of small protrusions and rudimentary paddles. Fimbrin and Arp3 (actin-related protein 3) were located in puncta at the base of small protrusions, while N-cadherin and ezrin outlined the cell membrane in both Tmod1^+/+ and Tmod1^−/− mature fibers.

Conclusions

These results suggest that distinct F-actin organizations are present in small protrusions versus large paddles. Formation and/or maintenance of large paddle domains depends on a β2-spectrin–actin network stabilized by Tmod1. α-Actinin–crosslinked F-actin bundles are enhanced in absence of Tmod1, indicating altered cytoskeleton organization. Formation of small protrusions is likely facilitated by Arp3-branched and fimbrin-bundled F-actin networks, which do not depend on Tmod1. This is the first work to reveal the F-actin–associated proteins required for the formation of paddles between lens fibers.

Retracted Article: Self-association of L-periaxin occurs via its acidic domain and NLS2/NLS3, and affects its trafficking in RSC96 cells

Periaxin (PRX) protein was first identified in myelinating Schwann cells through the screening of cytoskeleton-associated proteins in peripheral nerve myelination.

Identification of a PRX variant in a Chinese family with congenital cataract by exome sequencing

BACKGROUND

Congenital cataract is a common cause of childhood vision impairment or blindness with genetic and clinical heterogeneity. The aim of this study was to identify the disease-associated gene in a Chinese family with congenital cataract.

METHODS

A four-generation Chinese family with three enrolled patients suffering from congenital cataract was studied. Detailed family history and clinical data of all the members were collected and recorded. Exome sequencing was applied in the proband to screen potential genetic variants, and then Sanger sequencing was used to verify the variant within the family.

RESULTS

A heterozygous variant, c.3673G > A (p.V1225M), in the periaxin gene (PRX) was identified in three patients and two asymptomatic individuals of the family. The variant was absent in the other three unaffected family members and in 3290 ethnically matched in-house controls from BGI-Shenzhen.

CONCLUSIONS

By utilizing both exome sequencing and Sanger sequencing, we identified a missense variant in the PRX gene that is possibly associated with disease in this family. Our finding may broaden the spectrum of genes associated with congenital cataract, and may provide insights into lens development, pathogenic mechanism, future clinical genetic diagnosis and therapy of congenital cataract.

Lens transcriptome profile during cataract development in Mip-null mice

Major intrinsic protein or aquaporin-0 (MIP/AQP0) functions as a water channel and a cell-junction molecule in the vertebrate eye lens. Loss of MIP function in the lens leads to degraded optical quality and cataract formation by pathogenic mechanisms that are unclear. Here we have used microarray-hybridization analysis to detect lens transcriptome changes during cataract formation in mice that are functionally null for MIP (Mip-/-). In newborn Mip-/- lenses (P1) 11 genes were up-regulated and 18 were down-regulated (>2-fold, p=<0.05) and a similar number of genes was differentially regulated at P7. The most up-regulated genes (>6-fold) in the Mip-/- lens at P1 included those coding for a mitochondrial translocase (Timmdc1), a matrix metallopeptidase (Mmp2), a Rho GTPase-interacting protein (Ubxn11) and a transcription factor (Twist2). Apart from Mip, the most down-regulated genes (>4-fold) in the Mip-/- lens at P1 included those coding for a proteasome sub-unit (Psmd8), a ribonuclease (Pop4), and a heat-shock protein (Hspb1). Lens fiber cell degeneration in the Mip-/- lens was associated with increased numbers of TUNEL-positive cell nuclei and dramatically elevated levels of calpain-mediated proteolysis of αII-spectrin. However red-ox status, measured by glutathione and free-radical levels, was similar to that of wild-type. These data suggest that while relatively few genes (∼1.5% of the transcriptome) were differentially regulated >2-fold in the Mip-/- lens, calpain hyper-activation acts as a terminal pathogenic event during lens fiber cell death and cataract formation.

Ankyrin-B in lens architecture and biomechanics: Just not tethering but more

The ankyrins are a family of well-characterized metazoan adaptor proteins that play a key role in linking various membrane-spanning proteins to the underlying spectrin-actin cytoskeleton; a mechanistic understanding of their role in tissue architecture and mechanics, however, remains elusive. Here we comment on a recent study demonstrating a key role for ankyrin-B in maintaining the hexagonal shape and radial alignment of ocular lens fiber cells by regulating the membrane organization of periaxin, dystrophins/dystroglycan, NrCAM and spectrin-actin network of proteins, and revealing that ankyrin-B deficiency impairs fiber cell shape and mechanical properties of the ocular lens. These observations indicate that ankyrin-B plays an important role in maintaining tissue cytoarchitecture, cell shape and biomechanical properties via engaging in key protein: protein interactions required for membrane anchoring and organization of the spectrin-actin skeleton, scaffolding proteins and cell adhesive proteins.

Spatially Directed Proteomics of the Human Lens Outer Cortex Reveals an Intermediate Filament Switch Associated With the Remodeling Zone

PURPOSE

To quantify protein changes in the morphologically distinct remodeling zone (RZ) and adjacent regions of the human lens outer cortex using spatially directed quantitative proteomics.

METHODS

Lightly fixed human lens sections were deparaffinized and membranes labeled with fluorescent wheat germ agglutinin (WGA-TRITC). Morphology directed laser capture microdissection (LCM) was used to isolate tissue from four distinct regions of human lens outer cortex: differentiating zone (DF), RZ, transition zone (TZ), and inner cortex (IC). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) of the plasma membrane fraction from three lenses (21-, 22-, and 27-year) revealed changes in major cytoskeletal proteins including vimentin, filensin, and phakinin. Peptides from proteins of interest were quantified using multiple reaction monitoring (MRM) mass spectrometry and isotopically-labeled internal peptide standards.

RESULTS

Results revealed an intermediate filament switch from vimentin to beaded filament proteins filensin and phakinin that occurred at the RZ. Several other cytoskeletal proteins showed significant changes between regions, while most crystallins remained unchanged. Targeted proteomics provided accurate, absolute quantification of these proteins and confirmed vimentin, periplakin, and periaxin decrease from the DF to the IC, while filensin, phakinin, and brain acid soluble protein 1 (BASP1) increase significantly at the RZ.

CONCLUSIONS

Mass spectrometry-compatible fixation and morphology directed laser capture enabled proteomic analysis of narrow regions in the human lens outer cortex. Results reveal dramatic cytoskeletal protein changes associated with the RZ, suggesting that one role of these proteins is in membrane deformation and/or the establishment of ball and socket joints in the human RZ.

Intermolecular disulfide bond in the dimerization of S-periaxin mediated by Cys88 and Cys139

Periaxin is expressed in mammalian Schwann cells and lens fiber cells, and has been identified in a screen for cytoskeleton-associated proteins. Charcot-Marie-Tooth 4F is caused by losses or mutations of theperiaxingene. Theperiaxingene encodes two protein isoforms, namely, L-periaxin and S-periaxin.S-periaxin contains 147 amino acid residues and has an N-terminal PDZ domain. In this paper, S-periaxin was reported to be homodimerized through the formation of intermolecular disulfide bonds with its Cys88 and Cys139 residues under mild oxidation conditions. The covalent dimer of S-periaxin was also observed by western blot analysis and bimolecular fluorescence complementation analyses. S-periaxin dimerization formation could be regulated by cellular redox fluctuations. These results offer a possible mechanism to the formation of periaxin complexes, improvement of complex stability, and establishment of a link between the extracellular matrix and the cytoskeleton.

The lens actin filament cytoskeleton: Diverse structures for complex functions

The eye lens is a transparent and avascular organ in the front of the eye that is responsible for focusing light onto the retina in order to transmit a clear image. A monolayer of epithelial cells covers the anterior hemisphere of the lens, and the bulk of the lens is made up of elongated and differentiated fiber cells. Lens fiber cells are very long and thin cells that are supported by sophisticated cytoskeletal networks, including actin filaments at cell junctions and the spectrin-actin network of the membrane skeleton. In this review, we highlight the proteins that regulate diverse actin filament networks in the lens and discuss how these actin cytoskeletal structures assemble and function in epithelial and fiber cells. We then discuss methods that have been used to study actin in the lens and unanswered questions that can be addressed with novel techniques.

Ankyrin-B directs membrane tethering of periaxin and is required for maintenance of lens fiber cell hexagonal shape and mechanics

Periaxin (Prx), a PDZ domain protein expressed preferentially in myelinating Schwann cells and lens fibers, plays a key role in membrane scaffolding and cytoarchitecture. Little is known, however, about how Prx is anchored to the plasma membrane. Here we report that ankyrin-B (AnkB), a well-characterized adaptor protein involved in linking the spectrin-actin cytoskeleton to integral membrane proteins, is required for membrane association of Prx in lens fibers and colocalizes with Prx in hexagonal fiber cells. Under AnkB haploinsufficiency, Prx accumulates in the soluble fraction with a concomitant loss from the membrane-enriched fraction of mouse lenses. Moreover, AnkB haploinsufficiency induced age-dependent disruptions in fiber cell hexagonal geometry and radial alignment and decreased compressive stiffness in mouse lenses parallel to the changes observed in Prx null mouse lens. Both AnkB- and Prx-deficient mice exhibit disruptions in membrane organization of the spectrin-actin network and the dystrophin-glycoprotein complex in lens fiber cells. Taken together, these observations reveal that AnkB is required for Prx membrane anchoring and for maintenance of lens fiber cell hexagonal geometry, membrane skeleton organization, and biomechanics.

Rap1 GTPase is required for mouse lens epithelial maintenance and morphogenesis

Rap1, a Ras-like small GTPase, plays a crucial role in cell-matrix adhesive interactions, cell-cell junction formation, cell polarity and migration. The role of Rap1 in vertebrate organ development and tissue architecture, however, remains elusive. We addressed this question in a mouse lens model system using a conditional gene targeting approach. While individual germline deficiency of either Rap1a or Rap1b did not cause overt defects in mouse lens, conditional double deficiency (Rap1 cKO) prior to lens placode formation led to an ocular phenotype including microphthalmia and lens opacification in embryonic mice. The embryonic Rap1 cKO mouse lens exhibited striking defects including loss of E-cadherin- and ZO-1-based cell-cell junctions, disruption of paxillin and β1-integrin-based cell adhesive interactions along with abnormalities in cell shape and apical-basal polarity of epithelium. These epithelial changes were accompanied by increased levels of α-smooth muscle actin, vimentin and N-cadherin, and expression of transcriptional suppressors of E-cadherin (Snai1, Slug and Zeb2), and a mesenchymal metabolic protein (Dihydropyrimidine dehydrogenase). Additionally, while lens differentiation was not overtly affected, increased apoptosis and dysregulated cell cycle progression were noted in epithelium and fibers in Rap1 cKO mice. Collectively these observations uncover a requirement for Rap1 in maintenance of lens epithelial phenotype and morphogenesis.

Retracted Article: Spectrin-like domain 2 of DRP2 serves as a novel binding region for the NLS2 and 3 sub-domains of L-periaxin

The NLS1 domain of L-periaxin involved in nucleo-cytoplasmic shuttling, NLS2 and 3 participated in interaction with spectrin-like domain 2 of DRP2. The binding model of DRP2 and L-periaxin is crucial for understanding the role of L-periaxin in PNS.

Molecular characterization of mouse lens epithelial cell lines and their suitability to study RNA granules and cataract associated genes

The discovery of cytosolic RNA granule (RG) component proteins associated with human cataract has initiated investigations on post-transcriptional mechanisms of gene expression control in the lens. Application of established mouse lens epithelial cell lines (LECs) can provide rapid insights on RG function in lens cells, especially because mouse mutants in several RG components are not available. However, although these LECs represent potential reagents for such analyses, they are uncharacterized for lens gene expression or RG formation. Therefore, a detailed molecular and cellular characterization of three permanent mouse LECs 17EM15, 21EM15 and αTN4 is performed in this study. Comparative analysis between microarray gene expression datasets on LEC 21EM15 and iSyTE lens tissue demonstrates that 30% of top 200 iSyTE identified lens-enriched genes are expressed in these cells. Majority of these candidates are independently validated to either have lens expression, function or linkage to cataract. Moreover, analysis of microarray data with genes described in Cat-Map, an online database of cataract associated genes and loci, demonstrates that 131 genes linked to cataract loci are expressed in 21EM15 cells. Furthermore, gene expression in LECs is compared to isolated lens epithelium or fiber cells by qRT-PCR and by comparative analyses with publically available epithelium or fiber-specific microarray and RNA-seq (sequencing) datasets. Expression of select candidate genes was validated by regular and real-time quantitative RT-PCR. Expression of lens epithelium-enriched genes Foxe3, Pax6, Anxa4 and Mcm4 is up-regulated in LEC lines, compared to isolated lens fiber cells. Moreover, similar to isolated lens epithelium, all three LECs exhibit down-regulation of fiber cell-expressed genes Crybb1, Mip and Prox1 when compared to fiber cells. These data indicate that the LEC lines exhibit greater similarity to lens epithelium than to fiber cells. Compared to non-lens cell line NIH3T3, LECs exhibit significantly enriched expression of transcription factors with important function in the lens, namely Pax6, Foxe3 and Prox1. In addition to these genes, all three LECs also express key lens- and cataract-associated genes, namely Dkk3, Epha2, Hsf4, Jag1, Mab21l1, Meis1, Pknox1, Pou2f1, Sfrp1, Sparc, Tdrd7 and Trpm3. Additionally, 21EM15 microarrays indicate expression of Chmp4b, Cryab and Tcfap2a among others important genes. Immunostaining with makers for Processing bodies (P-bodies) and Stress granules (SGs) demonstrates that these classes of RGs are robustly expressed in all three LECs. Moreover, under conditions of stress, 17EM15 and αTN4 exhibit significantly higher numbers of P-bodies and SGs compared to NIH3T3 cells. In sum, these data indicate that mouse LECs 21EM15, 17EM15 and αTN4 express key lens or cataract genes, are similar to lens epithelium than fiber cells, and exhibit high levels of P-bodies and SGs, indicating their suitability for investigating gene expression control and RG function in lens-derived cells.

RhoA GTPase-induced ocular hypertension in a rodent model is associated with increased fibrogenic activity in the trabecular meshwork

Ocular hypertension arising from increased resistance to aqueous humor (AH) outflow through the trabecular meshwork is a primary risk factor for open-angle glaucoma, a leading cause of blindness. Ongoing efforts have found little about the molecular and cellular bases of increased resistance to AH outflow through the trabecular meshwork in ocular hypertension patients. To test the hypothesis that dysregulated Rho GTPase signaling and a resulting fibrotic activity within the trabecular meshwork may result in ocular hypertension, we investigated the effects of expressing a constitutively active RhoA GTPase (RhoAV14) in the AH outflow pathway in Sprague-Dawley rats by using lentiviral vector-based gene delivery. Rats expressing RhoAV14 in the iridocorneal angle exhibited a significantly elevated intraocular pressure. Elevated intraocular pressure in the RhoAV14-expressing rats was associated with fibrotic trabecular meshwork and increased levels of F-actin, phosphorylated myosin light chain, α-smooth muscle actin, collagen-1A, and total collagen in the trabecular AH outflow pathway. Most of these changes were ameliorated by topical application of Rho kinase inhibitor. Human autopsy eyes from patients with glaucoma exhibited significant increases in levels of collagen-1A and total collagen in the trabecular AH outflow pathway. Collectively, these observations indicate that increased fibrogenic activity because of dysregulated RhoA GTPase activity in the trabecular AH outflow pathway increases intraocular pressure in a Rho kinase-dependent manner.

Periaxin and AHNAK nucleoprotein 2 form intertwined homodimers through domain swapping

Periaxin (PRX) is an abundant protein in the peripheral nervous system, with an important role in myelination. PRX participates in large molecular complexes, most likely through the interactions of its N-terminal PSD-95/Discs-large/ZO-1 (PDZ)-like domain. We present the crystal structures of the PDZ-like domains from PRX and its homologue AHNAK nucleoprotein 2 (AHNAK2). The unique intertwined, domain-swapped dimers provide a structural basis for the homodimerization of both proteins. The core of the homodimer is formed by a 6-stranded antiparallel β sheet, with every other strand from a different chain. The AHNAK2 PDZ domain structure contains a bound class III ligand peptide. The binding pocket is preformed, and the peptide-PDZ interactions have unique aspects, including two salt bridges and weak recognition of the peptide C terminus. Tight homodimerization may be central to the scaffolding functions of PRX and AHNAK2 in molecular complexes linking the extracellular matrix to the cytoskeletal network.

Beta-1 integrin is important for the structural maintenance and homeostasis of differentiating fiber cells

β1-Integrin is a heterodimeric transmembrane protein that has roles in both cell-extra-cellular matrix and cell-cell interactions. Conditional deletion of β1-integrin from all lens cells during embryonic development results in profound lens defects, however, it is less clear whether this reflects functions in the lens epithelium alone or whether this protein plays a role in lens fibers. Thus, a conditional approach was used to delete β1-integrin solely from the lens fiber cells. This deletion resulted in two distinct phenotypes with some lenses exhibiting cataracts while others were clear, albeit with refractive defects. Analysis of "clear" conditional knockout lenses revealed that they had profound defects in fiber cell morphology associated with the loss of the F-actin network. Physiological measurements found that the lens fiber cells had a twofold increase in gap junctional coupling, perhaps due to differential localization of connexins 46 and 50, as well as increased water permeability. This would presumably facilitate transport of ions and nutrients through the lens, and may partially explain how lenses with profound structural abnormalities can maintain transparency. In summary, β1-integrin plays a role in maintaining the cellular morphology and homeostasis of the lens fiber cells.

Newborn mouse lens proteome and its alteration by lysine 6 mutant ubiquitin

Ubiquitin is a tag that often initiates degradation of proteins by the proteasome in the ubiquitin proteasome system. Targeted expression of K6W mutant ubiquitin (K6W-Ub) in the lens results in defects in lens development and cataract formation, suggesting critical functions for ubiquitin in lens. To study the developmental processes that require intact ubiquitin, we executed the most extensive characterization of the lens proteome to date. We quantified lens protein expression changes in multiple replicate pools of P1 wild-type and K6W-Ub-expressing mouse lenses. Lens proteins were digested with trypsin, peptides were separated using strong cation exchange and reversed-phase liquid chromatography, and tandem mass (MS/MS) spectra were collected with a linear ion trap. Transgenic mice that expressed low levels of K6W-Ub (low expressers) had normal, clear lenses at birth, whereas the lenses that expressed high levels of K6W-Ub (higher expressers) had abnormal lenses and cataracts at birth. A total of 2052 proteins were identified, of which 996 were reliably quantified and compared between wild-type and K6W-Ub transgenic mice. Consistent with a delayed developmental program, fiber-cell-specific proteins, such as γ-crystallins (γA, γB, γC, and γE), were down-regulated in K6W-Ub higher expressers. Up-regulated proteins were involved in energy metabolism, signal transduction, and proteolysis. The K6W-Ub low expressers exhibited delayed onset and milder cataract consistent with smaller changes in protein expression. Because lens protein expression changes occurred prior to lens morphological abnormalities and cataract formation in K6W-Ub low expressers, it appears that expression of K6W-Ub sets in motion a process of altered protein expression that results in developmental defects and cataract.

Interactions between lens epithelial and fiber cells reveal an intrinsic self-assembly mechanism

How tissues and organs develop and maintain their characteristic three-dimensional cellular architecture is often a poorly understood part of their developmental program; yet, as is clearly the case for the eye lens, precise regulation of these features can be critical for function. During lens morphogenesis cells become organized into a polarized, spheroidal structure with a monolayer of epithelial cells overlying the apical tips of elongated fiber cells. Epithelial cells proliferate and progeny that shift below the lens equator differentiate into new fibers that are progressively added to the fiber mass. It is now known that FGF induces epithelial to fiber differentiation; however, it is not fully understood how these two forms of cells assemble into their characteristic polarized arrangement. Here we show that in FGF-treated epithelial explants, elongating fibers become polarized/oriented towards islands of epithelial cells and mimic their polarized arrangement in vivo. Epithelial explants secrete Wnt5 into the culture medium and we show that Wnt5 can promote directed behavior of lens cells. We also show that these explants replicate aspects of the Notch/Jagged signaling activity that has been shown to regulate proliferation of epithelial cells in vivo. Thus, our in vitro study identifies a novel mechanism, intrinsic to the two forms of lens cells, that facilitates self-assembly into the polarized arrangement characteristic of the lens in vivo. In this way the lens, with its relatively simple cellular composition, serves as a useful model to highlight the importance of such intrinsic self-assembly mechanisms in tissue developmental and regenerative processes.

Ultrastructural analysis of the human lens fiber cell remodeling zone and the initiation of cellular compaction

The purpose is to determine the nature of the cellular rearrangements occurring through the remodeling zone (RZ) in human donor lenses, identified previously by confocal microscopy to be about 100 μm from the capsule. Human donor lenses were fixed with 10% formalin followed by 4% paraformaldehyde prior to processing for transmission electron microscopy. Of 27 fixed lenses, ages 22, 55 and 92 years were examined in detail. Overview electron micrographs confirmed the loss of cellular organization present in the outer cortex (80 μm thick) as the cells transitioned into the RZ. The transition occurred within a few cell layers and fiber cells in the RZ completely lost their classical hexagonal cross-sectional appearance. Cell interfaces became unusually interdigitated and irregular even though the radial cell columns were retained. Gap junctions appeared to be unaffected. After the RZ (40 μm thick), the cells were still irregular but more recognizable as fiber cells with typical interdigitations and the appearance of undulating membranes. Cell thickness was irregular after the RZ with some cells compacted, while others were not, up to the zone of full compaction in the adult nucleus. Similar dramatic cellular changes were observed within the RZ for each lens regardless of age. Because the cytoskeleton controls cell shape, dramatic cellular rearrangements that occur in the RZ most likely are due to alterations in the associations of crystallins to the lens-specific cytoskeletal beaded intermediate filaments. It is also likely that cytoskeletal attachments to membranes are altered to allow undulating membranes to develop.

[Charcot-Marie-Tooth disease associated with periaxin mutations (CMT4F): Clinical, electrophysiological and genetic analysis of 24 patients]

Autosomal recessive Charcot-Marie-Tooth disease (AR-CMT) is often characterized by onset in early childhood and severe phenotype compared to the dominant forms. CMT disease associated with periaxin gene (PRX) is rare and characterized by demyelination limited to the major peripheral nerves. Following the discovery of a high frequency of a specific periaxin gene mutation (E1085fsX4 homozygote) in the Reunion Island, we examined all French patients known as carriers of the periaxin gene mutation. There were 24 patients. Eighteen were from the Reunion Island (6 families and 10 sporadic cases). The six remaining patients were in two families, each with two affected individuals, and two sporadic cases. The series included 17 female and seven male patients. Walking was acquired late, on average at 3.4±1.6 years. One patient never learned to walk. The Charcot Marie Tooth Neuropathy Score (CMTNS) averaged 24.5±8.1. Seven patients had been wheelchair-bound since the age of 24±22. Other symptoms were: scoliosis most often observed after the age of 12 years and sometimes complicated by a restrictive respiratory syndrome; foot deformity in 24 patients; strabismus; glaucoma; myopia. When conduction recordings are available, median nerve motor conduction was slow (<10m/s), associated with a major lengthening of distal latencies. Study of the periaxin gene should be considered in patients with severe demyelinating neuropathy associated with early infantile scoliosis. This disease leads to major disability (29% of patients in this series were wheelchair-bound) and to respiratory insufficiency. Genetic counselling is highly recommended for consanguineous families.

Genetic variations and polymorphisms in the ezrin gene are associated with age-related cataract

PURPOSE

Age-related cataract (ARC) is a complex multifactorial disorder, including genetic and environmental factors. Ezrin (EZR), a member of the ezrin/radixin/moesin (ERM) protein family, plays a crucial role in the development of the lens as a plasma membrane-cytoskeleton linker. We conducted this study to investigate the role of genetic variations of ezrin and the relationship between single nucleotide polymorphisms (SNPs) in EZR and susceptibility to ARC in a Chinese population.

METHODS

A total of 205 sporadic age-related cataract patients and 218 unrelated random healthy controls participated in our study. Genomic DNA was extracted from peripheral blood leukocytes. All exons of EZR were sequenced after being amplified with polymerase chain reaction. The functional consequences of the mutations were analyzed using PolyPhen2. SNP statistical analysis was performed using SNPstats.

RESULTS

We found three novel variations in 205 patients. None presented in the 218 controls, including c.441C>G, c.924G>C, and c.1503G>A. PolyPhen2 predicted that the c.924G>C mutation probably had pathogenicity. Compared with the healthy controls, the rs5881286 -/GT genotype and - allele frequencies (p=0.0012; odds ratio [OR]=3.37; 95% confidence interval [CI]=1.70-6.70; p=3.96e-5; χ(2)=18.98, respectively), rs2242318 T/C genotype and C allele frequencies (p=0.0045; OR=3.40; 95% CI=1.70-6.79; p=8.82e-6; χ(2)=21.86, respectively), and rs144581330 A/G genotype and G allele frequencies (p=0.0472; OR=14.46; 95% CI=1.29-162.43; p=0.0244, χ(2)=6.99, respectively) were higher in the patients with age-related cataract. SNP rs144581330 in exon 2 was also predicted to be probably damaging by PolyPhen2. Haplotype association including the - allele of rs5881286, C allele of rs2242318, and A allele of rs144581330 exhibited significantly higher distribution in the patients with ARC (p=8.0e-4; OR=3.38; 95% CI=1.66-6.87).

CONCLUSIONS

This study suggests that the genetic variations and SNPs in the gene EZR possibly contribute to the development of age-related cataracts in the Chinese population.

L-type calcium channels play a critical role in maintaining lens transparency by regulating phosphorylation of aquaporin-0 and myosin light chain and expression of connexins

Homeostasis of intracellular calcium is crucial for lens cytoarchitecture and transparency, however, the identity of specific channel proteins regulating calcium influx within the lens is not completely understood. Here we examined the expression and distribution profiles of L-type calcium channels (LTCCs) and explored their role in morphological integrity and transparency of the mouse lens, using cDNA microarray, RT-PCR, immunoblot, pharmacological inhibitors and immunofluorescence analyses. The results revealed that Ca (V) 1.2 and 1.3 channels are expressed and distributed in both the epithelium and cortical fiber cells in mouse lens. Inhibition of LTCCs with felodipine or nifedipine induces progressive cortical cataract formation with time, in association with decreased lens weight in ex-vivo mouse lenses. Histological analyses of felodipine treated lenses revealed extensive disorganization and swelling of cortical fiber cells resembling the phenotype reported for altered aquaporin-0 activity without detectable cytotoxic effects. Analysis of both soluble and membrane rich fractions from felodipine treated lenses by SDS-PAGE in conjunction with mass spectrometry and immunoblot analyses revealed decreases in β-B1-crystallin, Hsp-90, spectrin and filensin. Significantly, loss of transparency in the felodipine treated lenses was preceded by an increase in aquaporin-0 serine-235 phosphorylation and levels of connexin-50, together with decreases in myosin light chain phosphorylation and the levels of 14-3-3ε, a phosphoprotein-binding regulatory protein. Felodipine treatment led to a significant increase in gene expression of connexin-50 and 46 in the mouse lens. Additionally, felodipine inhibition of LTCCs in primary cultures of mouse lens epithelial cells resulted in decreased intracellular calcium, and decreased actin stress fibers and myosin light chain phosphorylation, without detectable cytotoxic response. Taken together, these observations reveal a crucial role for LTCCs in regulation of expression, activity and stability of aquaporin-0, connexins, cytoskeletal proteins, and the mechanical properties of lens, all of which have a vital role in maintaining lens function and cytoarchitecture.

Tmod1 and CP49 synergize to control the fiber cell geometry, transparency, and mechanical stiffness of the mouse lens

The basis for mammalian lens fiber cell organization, transparency, and biomechanical properties has contributions from two specialized cytoskeletal systems: the spectrin-actin membrane skeleton and beaded filament cytoskeleton. The spectrin-actin membrane skeleton predominantly consists of α₂β₂-spectrin strands interconnecting short, tropomyosin-coated actin filaments, which are stabilized by pointed-end capping by tropomodulin 1 (Tmod1) and structurally disrupted in the absence of Tmod1. The beaded filament cytoskeleton consists of the intermediate filament proteins CP49 and filensin, which require CP49 for assembly and contribute to lens transparency and biomechanics. To assess the simultaneous physiological contributions of these cytoskeletal networks and uncover potential functional synergy between them, we subjected lenses from mice lacking Tmod1, CP49, or both to a battery of structural and physiological assays to analyze fiber cell disorder, light scattering, and compressive biomechanical properties. Findings show that deletion of Tmod1 and/or CP49 increases lens fiber cell disorder and light scattering while impairing compressive load-bearing, with the double mutant exhibiting a distinct phenotype compared to either single mutant. Moreover, Tmod1 is in a protein complex with CP49 and filensin, indicating that the spectrin-actin network and beaded filament cytoskeleton are biochemically linked. These experiments reveal that the spectrin-actin membrane skeleton and beaded filament cytoskeleton establish a novel functional synergy critical for regulating lens fiber cell geometry, transparency, and mechanical stiffness.

Autotaxin-lysophosphatidic acid axis is a novel molecular target for lowering intraocular pressure

Primary open-angle glaucoma is the second leading cause of blindness in the United States and is commonly associated with elevated intraocular pressure (IOP) resulting from diminished aqueous humor (AH) drainage through the trabecular pathway. Developing effective therapies for increased IOP in glaucoma patients requires identification and characterization of molecular mechanisms that regulate IOP and AH outflow. This study describes the identification and role of autotaxin (ATX), a secretory protein and a major source for extracellular lysophosphatidic acid (LPA), in regulation of IOP in a rabbit model. Quantitative proteomics analysis identified ATX as an abundant protein in both human AH derived from non-glaucoma subjects and in AH from different animal species. The lysophospholipase D (LysoPLD) activity of ATX was found to be significantly elevated (by ∼1.8 fold; n=20) in AH derived from human primary open angle glaucoma patients as compared to AH derived from age-matched cataract control patients. Immunoblotting analysis of conditioned media derived from primary cultures of human trabecular meshwork (HTM) cells has confirmed secretion of ATX and the ability of cyclic mechanical stretch of TM cells to increase the levels of secreted ATX. Topical application of a small molecular chemical inhibitor of ATX (S32826), which inhibited AH LysoPLD activity in vitro (by >90%), led to a dose-dependent and significant decrease of IOP in Dutch-Belted rabbits. Single intracameral injection of S32826 (∼2 µM) led to significant reduction of IOP in rabbits, with the ocular hypotensive response lasting for more than 48 hrs. Suppression of ATX expression in HTM cells using small-interfering RNA (siRNA) caused a decrease in actin stress fibers and myosin light chain phosphorylation. Collectively, these observations indicate that the ATX-LPA axis represents a potential therapeutic target for lowering IOP in glaucoma patients.

Tropomodulin 1 constrains fiber cell geometry during elongation and maturation in the lens cortex

Lens fiber cells exhibit a high degree of hexagonal packing geometry, determined partly by tropomodulin 1 (Tmod1), which stabilizes the spectrin-actin network on lens fiber cell membranes. To ascertain whether Tmod1 is required during epithelial cell differentiation to fiber cells or during fiber cell elongation and maturation, the authors quantified the extent of fiber cell disorder in the Tmod1-null lens and determined locations of disorder by confocal microscopy and computational image analysis. First, nearest neighbor analysis of fiber cell geometry in Tmod1-null lenses showed that disorder is confined to focal patches. Second, differentiating epithelial cells at the equator aligned into ordered meridional rows in Tmod1-null lenses, with disordered patches first observed in elongating fiber cells. Third, as fiber cells were displaced inward in Tmod1-null lenses, total disordered area increased due to increased sizes (but not numbers) of individual disordered patches. The authors conclude that Tmod1 is required first to coordinate fiber cell shapes and interactions during tip migration and elongation and second to stabilize ordered fiber cell geometry during maturation in the lens cortex. An unstable spectrin-actin network without Tmod1 may result in imbalanced forces along membranes, leading to fiber cell rearrangements during elongation, followed by propagation of disorder as fiber cells mature.

The role of calcium-independent phospholipase A2γ in modulation of aqueous humor drainage and Ca2+ sensitization of trabecular meshwork contraction

The contractile and relaxation characteristics of trabecular meshwork (TM) are presumed to influence aqueous humor (AH) drainage and intraocular pressure. The mechanisms underlying regulation of TM cell contractile properties, however, are not well understood. This study investigates the role of calcium-independent phospholipase A(2) (iPLA(2)), which controls eicosanoid synthesis, in regulation of TM cell contraction and AH outflow using mechanism-based isoform specific inhibitors (R)-bromoenol lactone (R-BEL, iPLA(2)γ specific) and (S)-bromoenol lactone (S-BEL, iPLA(2)β specific). Immunohistochemical analysis revealed intense staining for both iPLA(2)β and γ isoforms throughout the TM, juxtacanalicular tissue, and Schlemm's canal of human eye. Inhibition of iPLA(2)γ by R-BEL or small interfering RNA-mediated silencing of iPLA(2)γ expression induced dramatic changes in TM cell morphology, and decreased actin stress fibers, focal adhesions, and myosin light-chain (MLC) phosphorylation. AH outflow facility increased progressively and significantly in enucleated porcine eyes perfused with R-BEL. This response was associated with a significant decrease in TM tissue MLC phosphorylation and alterations in the morphology of aqueous plexi in R-BEL-perfused eyes. In contrast, S-BEL did not affect either of these parameters. Additionally, R-BEL-induced cellular relaxation of the TM was associated with a significant decrease in the levels of active Rho GTPase, phospho-MLC phosphatase, phospho-CPI-17, and arachidonic acid. Taken together, these observations demonstrate that iPLA(2)γ plays a significant and isoform-specific role in regulation of AH outflow facility by altering the contractile characteristics of the TM. The effects of iPLA(2)γ on TM contractile status appear to involve arachidonic acid and Rho GTPase signaling pathways.

The role of NrCAM in neural development and disorders--beyond a simple glue in the brain

NrCAM is a neuronal cell adhesion molecule of the L1 family of immunoglobulin super family. It plays a wide variety of roles in neural development, including cell proliferation and differentiation, axon growth and guidance, synapse formation, and the formation of the myelinated nerve structure. NrCAM functions in cell adhesion and modulates signaling pathways in neural development through multiple molecular interactions with guidance and other factors. Alterations in NrCAM structure/expression are associated with psychiatric disorders such as autism and drug addiction and with tumor progression. The mechanisms of NrCAM participation in development and how these might be perturbed in disorders are reviewed.

Rac1 GTPase-deficient mouse lens exhibits defects in shape, suture formation, fiber cell migration and survival

Morphogenesis and shape of the ocular lens depend on epithelial cell elongation and differentiation into fiber cells, followed by the symmetric and compact organization of fiber cells within an enclosed extracellular matrix-enriched elastic capsule. The cellular mechanisms orchestrating these different events however, remain obscure. We investigated the role of the Rac1 GTPase in these processes by targeted deletion of expression using the conditional gene knockout (cKO) approach. Rac1 cKO mice were derived from two different Cre (Le-Cre and MLR-10) transgenic mice in which lens-specific Cre expression starts at embryonic day 8.75 and 10.5, respectively, in both the lens epithelium and fiber cells. The Le-Cre/Rac1 cKO mice exhibited an early-onset (E12.5) and severe lens phenotype compared to the MLR-10/Rac1 cKO (E15.5) mice. While the Le-Cre/Rac1 cKO lenses displayed delayed primary fiber cell elongation, lenses from both Rac1 cKO strains were characterized by abnormal shape, impaired secondary fiber cell migration, sutural defects and thinning of the posterior capsule which often led to rupture. Lens fiber cell N-cadherin/β-catenin/Rap1/Nectin-based cell-cell junction formation and WAVE-2/Abi-2/Nap1-regulated actin polymerization were impaired in the Rac1 deficient mice. Additionally, the Rac1 cKO lenses were characterized by a shortened epithelial sheet, reduced levels of extracellular matrix (ECM) proteins and increased apoptosis. Taken together, these data uncover the essential role of Rac1 GTPase activity in establishment and maintenance of lens shape, suture formation and capsule integrity, and in fiber cell migration, adhesion and survival, via regulation of actin cytoskeletal dynamics, cell adhesive interactions and ECM turnover.