Tropomyosin 3.5 protects the F-actin networks required for tissue 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.

Disease-related non-muscle myosin IIA D1424N rod domain mutation, but not R702C motor domain mutation, disrupts mouse ocular lens fiber cell alignment and hexagonal packing

The mouse ocular lens is an excellent vertebrate model system for studying hexagonal cell packing and shape changes during tissue morphogenesis and differentiation. The lens is composed of two types of cells, epithelial and fiber cells. During the initiation of fiber cell differentiation, lens epithelial cells transform from randomly packed cells to hexagonally shaped and packed cells to form meridional row cells. The meridional row cells further differentiate and elongate into newly formed fiber cells that maintain hexagonal cell shape and ordered packing. In other tissues, actomyosin contractility regulates cell hexagonal packing geometry during epithelial tissue morphogenesis. Here, we use the mouse lens as a model to study the effect of two human disease-related non-muscle myosin IIA (NMIIA) mutations on lens cellular organization during fiber cell morphogenesis and differentiation. We studied genetic knock-in heterozygous mice with NMIIA-R702C motor domain or NMIIA-D1424N rod domain mutations. We observed that while one allele of NMIIA-R702C has no impact on lens meridional row epithelial cell shape and packing, one allele of the NMIIA-D1424N mutation can cause localized defects in cell hexagonal packing. Similarly, one allele of NMIIA-R702C motor domain mutation does not affect lens fiber cell organization while the NMIIA-D1424N mutant proteins disrupt fiber cell organization and packing. Our work demonstrates that disease-related NMIIA rod domain mutations (D1424N or E1841K) disrupt mouse lens fiber cell morphogenesis and differentiation.

Preparation and Immunofluorescence Staining of Bundles and Single Fiber Cells from the Cortex and Nucleus of the Eye Lens

The lens is a transparent and ellipsoid organ in the anterior chamber of the eye that changes shape to finely focus light onto the retina to form a clear image. The bulk of this tissue comprises specialized, differentiated fiber cells that have a hexagonal cross section and extend from the anterior to the posterior poles of the lens. These long and skinny cells are tightly opposed to neighboring cells and have complex interdigitations along the length of the cell. The specialized interlocking structures are required for normal biomechanical properties of the lens and have been extensively described using electron microscopy techniques. This protocol demonstrates the first method to preserve and immunostain singular as well as bundles of mouse lens fiber cells to allow the detailed localization of proteins within these complexly shaped cells. The representative data show staining of the peripheral, differentiating, mature, and nuclear fiber cells across all regions of the lens. This method can potentially be used on fiber cells isolated from lenses of other species.

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.

FGF-2 Differentially Regulates Lens Epithelial Cell Behaviour during TGF-β-Induced EMT

Fibroblast growth factor (FGF) and transforming growth factor-beta (TGF-β) can regulate and/or dysregulate lens epithelial cell (LEC) behaviour, including proliferation, fibre differentiation, and epithelial–mesenchymal transition (EMT). Earlier studies have investigated the crosstalk between FGF and TGF-β in dictating lens cell fate, that appears to be dose dependent. Here, we tested the hypothesis that a fibre-differentiating dose of FGF differentially regulates the behaviour of lens epithelial cells undergoing TGF-β-induced EMT. Postnatal 21-day-old rat lens epithelial explants were treated with a fibre-differentiating dose of FGF-2 (200 ng/mL) and/or TGF-β2 (50 pg/mL) over a 7-day culture period. We compared central LECs (CLECs) and peripheral LECs (PLECs) using immunolabelling for changes in markers for EMT (α-SMA), lens fibre differentiation (β-crystallin), epithelial cell adhesion (β-catenin), and the cytoskeleton (alpha-tropomyosin), as well as Smad2/3- and MAPK/ERK1/2-signalling. Lens epithelial explants cotreated with FGF-2 and TGF-β2 exhibited a differential response, with CLECs undergoing EMT while PLECs favoured more of a lens fibre differentiation response, compared to the TGF-β-only-treated explants where all cells in the explants underwent EMT. The CLECs cotreated with FGF and TGF-β immunolabelled for α-SMA, with minimal β-crystallin, whereas the PLECs demonstrated strong β-crystallin reactivity and little α-SMA. Interestingly, compared to the TGF-β-only-treated explants, α-SMA was significantly decreased in the CLECs cotreated with FGF/TGF-β. Smad-dependent and independent signalling was increased in the FGF-2/TGF-β2 co-treated CLECs, that had a heightened number of cells with nuclear localisation of Smad2/3 compared to the PLECs, that in contrast had more pronounced ERK1/2-signalling over Smad2/3 activation. The current study has confirmed that FGF-2 is influential in differentially regulating the behaviour of LECs during TGF-β-induced EMT, leading to a heterogenous cell population, typical of that observed in the development of post-surgical, posterior capsular opacification (PCO). This highlights the cooperative relationship between FGF and TGF-β leading to lens pathology, providing a different perspective when considering preventative measures for controlling PCO.

Stress deprivation of tendon explants or Tpm3.1 inhibition in tendon cells reduces F-actin to promote a tendinosis-like phenotype

Actin is a central mediator between mechanical force and cellular phenotype. In tendons, it is speculated that mechanical stress deprivation regulates gene expression by reducing filamentous (F)-actin. However, the mechanisms regulating tenocyte F-actin remain unclear. Tropomyosins (Tpms) are master regulators of F-actin. There are more than 40 Tpm isoforms, each having the unique capability to stabilize F-actin subpopulations. We investigated F-actin polymerization in stress-deprived tendons and tested the hypothesis that stress fiber-associated Tpm(s) stabilize F-actin to regulate cellular phenotype. Stress deprivation of mouse tail tendon down-regulated tenogenic and up-regulated protease (matrix metalloproteinase-3) mRNA levels. Concomitant with mRNA modulation were increases in G/F-actin, confirming reduced F-actin by tendon stress deprivation. To investigate the molecular regulation of F-actin, we identified that tail, Achilles, and plantaris tendons express three isoforms in common: Tpm1.6, 3.1, and 4.2. Tpm3.1 associates with F-actin in native and primary tenocytes. Tpm3.1 inhibition reduces F-actin, leading to decreases in tenogenic expression, increases in chondrogenic expression, and enhancement of protease expression in mouse and human tenocytes. These expression changes by Tpm3.1 inhibition are consistent with tendinosis progression. A further understanding of F-actin regulation in musculoskeletal cells could lead to new therapeutic interventions to prevent alterations in cellular phenotype during disease progression.

Mapping the Universe of Eph Receptor and Ephrin Ligand Transcripts in Epithelial and Fiber Cells of the Eye Lens

The eye lens is a transparent, ellipsoid organ in the anterior chamber of the eye that is required for fine focusing of light onto the retina to transmit a clear image. Cataracts, defined as any opacity in the lens, remains the leading cause of blindness in the world. Recent studies in humans and mice indicate that Eph–ephrin bidirectional signaling is important for maintaining lens transparency. Specifically, mutations and polymorphisms in the EphA2 receptor and the ephrin-A5 ligand have been linked to congenital and age-related cataracts. It is unclear what other variants of Ephs and ephrins are expressed in the lens or whether there is preferential expression in epithelial vs. fiber cells. We performed a detailed analysis of Eph receptor and ephrin ligand mRNA transcripts in whole mouse lenses, epithelial cell fractions, and fiber cell fractions using a new RNA isolation method. We compared control samples with EphA2 knockout (KO) and ephrin-A5 KO samples. Our results revealed the presence of transcripts for 12 out of 14 Eph receptors and 8 out of 8 ephrin ligands in various fractions of lens cells. Using specific primer sets, RT-PCR, and sequencing, we verified the variant of each gene that is expressed, and we found two epithelial-cell-specific genes. Surprisingly, we also identified one Eph receptor variant that is expressed in KO lens fibers but is absent from control lens fibers. We also identified one low expression ephrin variant that is only expressed in ephrin-A5 control samples. These results indicate that the lens expresses almost all Ephs and ephrins, and there may be many receptor–ligand pairs that play a role in lens homeostasis.

Methodologies to unlock the molecular expression and cellular structure of ocular lens epithelial cells

The transparent ocular lens in the anterior chamber of the eye is responsible for fine focusing of light onto the retina. The lens is entirely cellular with bulk of the tissue composed of fiber cells, and the anterior hemisphere of the lens is covered by a monolayer of epithelial cells. Lens epithelial cells are important for maintaining fiber cell homeostasis and for continual growth of the lens tissue throughout life. Cataracts, defined as any opacity in the lens, remain the leading cause of blindness in the world. Following cataract surgery, lens epithelial cells can undergo a process of epithelial-to-mesenchymal transition (EMT), leading to secondary cataracts due to posterior capsular opacification (PCO). Since the epithelial cells make up only a small fraction of the lens, specialized techniques are required to study lens epithelial cell biology and pathology. Studies using native lens epithelial cells often require pooling of samples to obtain enough cells to make sufficient samples for traditional molecular biology techniques. Here, we provide detailed protocols that enable the study of native mouse lens epithelial cells, including immunostaining of the native lens epithelium in flat mounts, extraction of RNA and proteins from pairs of lens epithelial monolayers, and isolation of lens epithelial cells for primary culture. These protocols will enable researchers to gain better insight on representative molecular expression and cellular structure of lens epithelial cells. We also provide comparative data between native, primary culture, and immortalized lens epithelial cells and discuss the advantages and disadvantages of each technique presented.

EphA2 Affects Development of the Eye Lens Nucleus and the Gradient of Refractive Index

Purpose

Our studies in mouse eye lenses demonstrate that ephrin-A5 and EphA2 are needed for normal epithelial cells and lens transparency. We sought to determine whether EphA2 and ephrin-A5 are important for lens morphometrics, nucleus formation, and refractive index.

Methods

We performed tissue morphometric measurements, electron microscopy, Western blots, and interferometric measurements using an X-ray synchrotron beam source to measure the gradient of refractive index (GRIN) to compare mouse lenses with genetic disruption of EphA2 or ephrin-A5.

Results

Morphometric analysis revealed that although there is no change in the overall lens volume, there is a change in lens shape in both EphA2-/- lenses and ephrin-A5-/- lenses. Surprisingly, EphA2-/- lenses had small and soft lens nuclei different from hard lens nuclei of control lenses. SEM images revealed changes in cell morphology of EphA2-/- fiber cells close to the center of the lens. Inner EphA2-/- lens fibers had more pronounced tongue-and-groove interdigitations and formed globular membrane morphology only in the deepest layers of the lens nucleus. We did not observe nuclear defects in ephrin-A5-/- lenses. There was an overall decrease in magnitude of refractive index across EphA2-/- lenses, which is most pronounced in the nucleus.

Conclusions

This work reveals that Eph-ephrin signaling plays a role in fiber cell maturation, nuclear compaction, and lens shape. Loss of EphA2 disrupts the nuclear compaction resulting in a small lens nucleus. Our data suggest that Eph-ephrin signaling may be required for fiber cell membrane reorganization and compaction and for establishing a normal GRIN.

Eph-ephrin Signaling Affects Eye Lens Fiber Cell Intracellular Voltage and Membrane Conductance

The avascular eye lens generates its own microcirculation that is required for maintaining lifelong lens transparency. The microcirculation relies on sodium ion flux, an extensive network of gap junction (GJ) plaques between lens fiber cells and transmembrane water channels. Disruption of connexin proteins, the building blocks of GJs, or aquaporins, which make up water and adhesion channels, lead to lens opacification or cataracts. Recent studies have revealed that disruption of Eph-ephrin signaling, in particular the receptor EphA2 and the ligand ephrin-A5, in humans and mice lead to congenital and age-related cataracts. We investigated whether changes in lens transparency in EphA2 or ephrin-A5 knockout (^–/–) mice is related to changes in GJ coupling and lens fluid and ion homeostasis. Immunostaining revealed changes in connexin 50 (Cx50) subcellular localization in EphA2^–/– peripheral lens fibers and alteration in aquaporin 0 (Aqp0) staining patterns in ephrin-A5^–/– and EphA2^–/– inner mature fiber cells. Surprisingly, there was no obvious change in GJ coupling in knockout lenses. However, there were changes in fiber cell membrane conductance and intracellular voltage in knockout lenses from 3-month-old mice. These knockout lenses displayed decreased conductance of mature fiber membranes and were hyperpolarized compared to control lenses. This is the first demonstration that the membrane conductance of lens fibers can be regulated. Together these data suggest that EphA2 may be needed for normal Cx50 localization to the cell membrane and that conductance of lens fiber cells requires normal Eph-ephrin signaling and water channel localization.

EphA2 and Ephrin-A5 Guide Eye Lens Suture Alignment and Influence Whole Lens Resilience

Purpose

Fine focusing of light by the eye lens onto the retina relies on the ability of the lens to change shape during the process of accommodation. Little is known about the cellular structures that regulate elasticity and resilience. We tested whether Eph–ephrin signaling is involved in lens biomechanical properties.

Methods

We used confocal microscopy and tissue mechanical testing to examine mouse lenses with genetic disruption of EphA2 or ephrin-A5.

Results

Confocal imaging revealed misalignment of the suture between each shell of newly added fiber cells in knockout lenses. Despite having disordered sutures, loss of EphA2 or ephrin-A5 did not affect lens stiffness. Surprisingly, knockout lenses were more resilient and recovered almost completely after load removal. Confocal microscopy and quantitative image analysis from live lenses before, during, and after compression revealed that knockout lenses had misaligned Y-sutures, leading to a change in force distribution during compression. Knockout lenses displayed decreased separation of fiber cell tips at the anterior suture at high loads and had more complete recovery after load removal, which leads to improved whole-lens resiliency.

Conclusions

EphA2 and ephrin-A5 are needed for normal patterning of fiber cell tips and the formation of a well-aligned Y-suture with fiber tips stacked on top of previous generations of fiber cells. The misalignment of lens sutures leads to increased resilience after compression. The data suggest that alignment of the Y-suture may constrain the overall elasticity and resilience of the lens.

Whole mount staining of lenses for visualization of lens epithelial cell proteins

Whole mount imaging of the lens allows for high spatial resolution visualization of lens epithelial structures by using small molecule fluorescent probes. However, the visualization of specific proteins in lens epithelial cells within whole lenses remains a challenge as the capsule that surrounds the lens does not allow penetration of antibodies. Here we describe a whole mount imaging method that allows us to overcome this challenge by digesting the lens capsules of paraformaldehyde fixed lenses using collagenase. This method enables the penetration of antibodies for effective visualization of proteins in the epithelium of whole lenses.•

A limitation to lens whole mount imaging is the ability to visualize specific proteins as the collagen capsule surrounding the lens impedes the penetration of antibodies

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This protocol helps overcome this limitation by a light collagenase digestion of the capsule of fixed lenses prior to immunostaining

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This method allows for the imaging of specific proteins in the epithelium of the whole lens tissue

Biochemical and biomechanical characteristics of dystrophin-deficient mdx3cv mouse lens

The molecular and cellular basis for cataract development in mice lacking dystrophin, a scaffolding protein that links the cytoskeleton to the extracellular matrix, is poorly understood. In this study, we characterized lenses derived from the dystrophin-deficient mdx3cv mouse model. Expression of Dp71, a predominant isoform of dystrophin in the lens, was induced during lens fiber cell differentiation. Dp71 was found to co-distribute with dystroglycan, connexin-50 and 46, aquaporin-0, and NrCAM as a large cluster at the center of long arms of the hexagonal fibers. Although mdx3cv mouse lenses exhibited dramatically reduced levels of Dp71, only older lenses revealed punctate nuclear opacities compared to littermate wild type (WT) lenses. The levels of dystroglycan, syntrophin, and dystrobrevin which comprise the dystrophin-associated protein complex (DAPC), and NrCAM, connexin-50, and aquaporin-0, were significantly lower in the lens membrane fraction of adult mdx3cv mice compared to WT mice. Additionally, decreases were observed in myosin light chain phosphorylation and lens stiffness together with a significant elevation in the levels of utrophin, a functional homolog of dystrophin in mdx3cv mouse lenses compared to WT lenses. The levels of perlecan and laminin (ligands of α-dystroglycan) remained normal in dystrophin-deficient lens fibers. Taken together, although mdx3cv mouse lenses exhibit only minor defects in lens clarity possibly due to a compensatory increase in utrophin, the noted disruptions of DAPC, stability, and organization of membrane integral proteins of fibers, and stiffness of mdx3cv lenses reveal the importance of dystrophin and DAPC in maintaining lens clarity and function.

Tropomyosin 3.1 Association With Actin Stress Fibers is Required for Lens Epithelial to Mesenchymal Transition

Purpose

Epithelial to mesenchymal transition (EMT) is a cause of anterior and posterior subcapsular cataracts. Central to EMT is the formation of actin stress fibers. Selective targeting of actin stress fiber-associated tropomyosin (Tpm) in epithelial cells may be a means to prevent stress fiber formation and repress lens EMT.

Methods

We identified Tpm isoforms in mouse immortalized lens epithelial cells and epithelial and fiber cells from whole lenses by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) followed Sanger sequencing. We focused on the role of one particular tropomyosin isoform, Tpm3.1, in EMT. To induce EMT, we treated cells or native lenses with TGFβ2. To test the function of Tpm3.1, we exposed cells or whole lenses to a Tpm3.1-specific chemical inhibitor, TR100, as well as investigated lenses from Tpm3.1 knockout mice. We examined stress fiber formation by confocal microscopy and assessed EMT progression by analysis of alpha-smooth muscle actin (αSMA) mRNA (real-time RT-PCR), and protein (Western immunoassay [WES]).

Results

Lens epithelial cells express eight Tpm isoforms. Cell culture studies showed that TGFβ2 treatment results in the upregulation of Tpm3.1, which associates with actin in stress fibers. TR100 prevents stress fiber formation and reduces αSMA in TGFβ2-treated cells. Using an ex vivo lens culture model, TGFβ2 treatment results in stress fiber formation at the basal regions of the epithelial cells. Genetic knockout of Tpm3.1 or treatment of lenses with TR100 prevents basal stress fiber formation and reduces epithelial αSMA levels.

Conclusions

Targeting specific stress fiber associated tropomyosin isoform, Tpm3.1, is a means to repress lens EMT.

Age-related changes in eye lens biomechanics, morphology, refractive index and transparency

Life-long eye lens function requires an appropriate gradient refractive index, biomechanical integrity and transparency. We conducted an extensive study of wild-type mouse lenses 1-30 months of age to define common age-related changes. Biomechanical testing and morphometrics revealed an increase in lens volume and stiffness with age. Lens capsule thickness and peripheral fiber cell widths increased between 2 to 4 months of age but not further, and thus, cannot account for significant age-dependent increases in lens stiffness after 4 months. In lenses from mice older than 12 months, we routinely observed cataracts due to changes in cell structure, with anterior cataracts due to incomplete suture closure and a cortical ring cataract corresponding to a zone of compaction in cortical lens fiber cells. Refractive index measurements showed a rapid growth in peak refractive index between 1 to 6 months of age, and the area of highest refractive index is correlated with increases in lens nucleus size with age. These data provide a comprehensive overview of age-related changes in murine lenses, including lens size, stiffness, nuclear fraction, refractive index, transparency, capsule thickness and cell structure. Our results suggest similarities between murine and primate lenses and provide a baseline for future lens aging studies.

Impact of the actin cytoskeleton on cell development and function mediated via tropomyosin isoforms

The physiological function of actin filaments is challenging to dissect because of the pleiotropic impact of global disruption of the actin cytoskeleton. Tropomyosin isoforms have provided a unique opportunity to address this issue. A substantial fraction of actin filaments in animal cells consist of co-polymers of actin with specific tropomyosin isoforms which determine the functional capacity of the filament. Genetic manipulation of the tropomyosins has revealed isoform specific roles and identified the physiological function of the different actin filament types based on their tropomyosin isoform composition. Surprisingly, there is remarkably little redundancy between the tropomyosins resulting in highly penetrant impacts of both ectopic overexpression and knockout of isoforms. The physiological roles of the tropomyosins cover a broad range from development and morphogenesis to cell migration and specialised tissue function and human diseases.

Actin-tropomyosin distribution in non-muscle cells

The interactions of cytoskeletal actin filaments with myosin family motors are essential for the integrity and function of eukaryotic cells. They support a wide range of force-dependent functions. These include mechano-transduction, directed transcellular transport processes, barrier functions, cytokinesis, and cell migration. Despite the indispensable role of tropomyosins in the generation and maintenance of discrete actomyosin-based structures, the contribution of individual cytoskeletal tropomyosin isoforms to the structural and functional diversification of the actin cytoskeleton remains a work in progress. Here, we review processes that contribute to the dynamic sorting and targeted distribution of tropomyosin isoforms in the formation of discrete actomyosin-based structures in animal cells and their effects on actin-based motility and contractility.

First person – Catherine (Cathy) Cheng

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Cathy Cheng is the first author on ‘Tropomyosin 3.5 protects the F-actin networks required for tissue biomechanical properties’, published in Journal of Cell Science. Cathy did her postdoctoral studies in the lab of Dr Velia Fowler at the Scripps Research Institute, La Jolla, USA, and, in September 2018, joined Indiana University, Bloomington, USA, as an Assistant Professor, where she will continue to investigate the mechanisms for establishing and maintaining lifelong homeostasis and transparency in the eye lens.