alpha6 Integrin is regulated with lens cell differentiation by linkage to the cytoskeleton and isoform switching

Examining PI3K-signaling-dependent regulation of lens organelle free zone formation via immunolocalization and immunoblotting in chick embryos

The elimination of lens organelles during development, required for mature lens function, is an autophagy-dependent mechanism induced through suppression of PI3K signaling. Here, we present a protocol for investigating the signaling pathways responsible for induction of the formation of this lens organelle free zone. We describe steps for preparation of lens organ culture and use of signaling pathway inhibitors. We then detail procedures for analyzing their impact using both confocal microscopy imaging of immunolabeled lens cryosections and immunoblot approaches. For complete details on the use and execution of this protocol, please refer to Gheyas et al. (2022).1.

The involvement of caspases in the process of nuclear removal during lens fiber cell differentiation

The terminal differentiation of lens fiber cells involves elimination of their organelles, which must occur while still maintaining their functionality throughout a lifetime. Removal of non-nuclear organelles is accomplished through induction of autophagy following the spatiotemporal suppression of the PI3K/Akt signaling axis. However, blocking this pathway is not alone sufficient to induce removal of fiber cell nuclei. While the final steps in fiber cell nuclear elimination are highlighted by the appearance of TUNEL-positive nuclei, which are associated with activation of the lens-specific DNaseIIβ, there are many steps in the process that precede the appearance of double stranded DNA breaks. We showed that this carefully regulated process, including the early changes in nuclear morphology resulting in nuclear condensation, cleavage of lamin B, and labeling by pH2AX, is reminiscent of the apoptotic process associated with caspase activation. Multiple caspases are known to be expressed and activated during lens cell differentiation. In this study, we investigated the link between two caspase downstream targets associated with apoptosis, ICAD, whose cleavage by caspase-3 leads to activation of CAD, a DNase that can create both single- and double-stranded DNA cleavages, and lamin B, a primary component of the nuclear lamina. We discovered that the specific inhibition of caspase-3 activation prevents both lamin B and DNA cleavage. Inhibiting caspase-3 did not prevent nuclear condensation or removal of the nuclear membrane. In contrast, a pan-caspase inhibitor effectively suppressed condensation of fiber cell nuclei during differentiation. These studies provide evidence that caspases play an important role in the process of removing fiber cell nuclei during lens differentiation.

Patterns of Crystallin Gene Expression in Differentiation State Specific Regions of the Embryonic Chicken Lens

Purpose

Transition from lens epithelial cells to lens fiber cell is accompanied by numerous changes in gene expression critical for lens transparency. We identify expression patterns of highly prevalent genes including ubiquitous and enzyme crystallins in the embryonic day 13 chicken lens.

Methods

Embryonic day 13 chicken lenses were dissected into central epithelial cell (EC), equatorial epithelial cell (EQ), cortical fiber cell (FP), and nuclear fiber cell (FC) compartments. Total RNA was prepared, subjected to high-throughput unidirectional mRNA sequencing, analyzed, mapped to the chicken genome, and functionally grouped.

Results

A total of 77,097 gene-specific transcripts covering 17,450 genes were expressed, of which 10,345 differed between two or more lens subregions. Ubiquitous crystallin gene expression increased from EC to EQ and was similar in FP and FC. Highly expressed crystallin genes fell into three coordinately expressed groups with R² ≥ 0.93: CRYAA, CRYBB2, CRYAB, and CRYBA2; CRYBB1, CRYBA4, CRYGN, ASL1, and ASL; and CRYBB3 and CRYBA1. The highly expressed transcription factors YBX1, YBX3, PNRC1, and BASP1 were coordinately expressed with the second group of crystallins (r² > 0.88).

Conclusions

Although it is well known that lens crystallin gene expression changes during the epithelial to fiber cell transition, these data identify for the first time three distinct patterns of expression for specific subsets of crystallin genes, each highly correlated with expression of specific transcription factors. The results provide a quantitative basis for designing functional experiments pinpointing the mechanisms governing the landscape of crystallin expression during fiber cell differentiation to attain lens transparency.

Suppression of PI3K signaling is linked to autophagy activation and the spatiotemporal induction of the lens organelle free zone

The terminal steps of lens cell differentiation require elimination of all organelles to create a central Organelle Free Zone (OFZ) that is required for lens function of focusing images on the retina. Previous studies show that the spatiotemporal elimination of these organelles during development is autophagy-dependent. We now show that the inhibition of PI3K signaling in lens organ culture results in the premature induction of autophagy within 24 h, including a significant increase in LAMP1+ lysosomes, and the removal of lens organelles from the center of the lens. Specific inhibition of just the PI3K/Akt signaling axis was directly linked to the elimination of mitochondria and ER, while pan-PI3K inhibitors that block all PI3K downstream signaling removed all organelles, including nuclei. Therefore, blocking the PI3K/Akt pathway was alone insufficient to remove nuclei. RNAseq analysis revealed increased mRNA levels of the endogenous inhibitor of PI3K activation, PIK3IP1, in differentiating lens fiber cells preceding the induction of OFZ formation. Co-immunoprecipitation confirmed that PIK3IP1 associates with multiple PI3K p110 isoforms just prior to formation of the OFZ, providing a likely endogenous mechanism for blocking all PI3K signaling and activating the autophagy pathway required to form the OFZ during lens development.

Hypoxia regulates the degradation of non-nuclear organelles during lens differentiation through activation of HIF1a

Formation of the eye lens depends on the continuous differentiation of lens epithelial cells into lens fiber cells. To attain their mature structure and transparent function, nascent lens fiber cells must complete a precise cellular remodeling program hallmarked by the complete elimination of organelles to form the core lens organelle-free zone (OFZ). Lacking a blood supply, the lens resides in a hypoxic environment that results in a decreasing oxygen concentration from the lens surface to the lens core. This oxygen gradient results in a hypoxic microenvironment in the region of the lens where immature lens fiber cells initiate loss of organelles to form the core OFZ. These features of the lens suggest a potential role for low lens oxygen levels in the regulation of organelle degradation and other events critical for mature lens fiber cell formation. Hypoxia activates the master regulator of the hypoxic response, hypoxia-inducible factor 1a (HIF1a) that regulates hypoxia-responsive genes. To identify a potential role for hypoxia and HIF1a in the elimination of organelles during lens fiber cell maturation, we tested the requirement for hypoxia in the degradation of non-nuclear organelles in ex vivo cultured embryonic chick lenses by monitoring the degradation of mitochondria (MT), Golgi apparatus (GA) and endoplasmic reticulum (ER) under conditions of low (1% O₂) and high (21% O₂) oxygen. We also examined the requirement for HIF1a activation for elimination of these organelles under the same conditions using a specific HIF1a activator (DMOG) and a specific HIF1a inhibitor (chetomin) and examined the requirements for hypoxia and HIF1a for regulating transcription of BNIP3L that we previously showed to be required for elimination of non-nuclear lens organelles. We used ChIP-qPCR to confirm direct binding of HIF1a to the 5' untranslated region of the BNIP3L gene. Finally, we examined the effects of expressing an oxygen insensitive mutant form of HIF1a (P402A/P565A) and BNIP3L on non-nuclear organelle degradation. Our data demonstrate that hypoxia and HIF1a are required for the degradation of non-nuclear organelles during lens fiber cell formation and that they regulate this process by governing BNIP3L transcription. Our results also provide evidence that hypoxia and HIF1a are essential for achieving mature lens structure.

Lens differentiation is characterized by stage-specific changes in chromatin accessibility correlating with differentiation state-specific gene expression

Changes in chromatin accessibility regulate the expression of multiple genes by controlling transcription factor access to key gene regulatory sequences. Here, we sought to establish a potential function for altered chromatin accessibility in control of key gene expression events during lens cell differentiation by establishing genome-wide chromatin accessibility maps specific for four distinct stages of lens cell differentiation and correlating specific changes in chromatin accessibility with genome-wide changes in gene expression. ATAC sequencing was employed to generate chromatin accessibility profiles that were correlated with the expression profiles of over 10,000 lens genes obtained by high-throughput RNA sequencing at the same stages of lens cell differentiation. Approximately 90,000 regions of the lens genome exhibited distinct changes in chromatin accessibility at one or more stages of lens differentiation. Over 1000 genes exhibited high Pearson correlation coefficients (r ​> ​0.7) between altered expression levels at specific stages of lens cell differentiation and changes in chromatin accessibility in potential promoter (-7.5kbp/+2.5kbp of the transcriptional start site) and/or other potential cis-regulatory regions ( ±10 ​kb of the gene body). Analysis of these regions identified consensus binding sequences for multiple transcription factors including members of the TEAD, FOX, and NFAT families of transcription factors as well as HIF1a, RBPJ and IRF1. Functional mapping of genes with high correlations between altered chromatin accessibility and differentiation state-specific gene expression changes identified multiple families of proteins whose expression could be regulated through changes in chromatin accessibility including those governing lens structure (BFSP1,BFSP2), gene expression (Pax-6, Sox 2), translation (TDRD7), cell-cell communication (GJA1), autophagy (FYCO1), signal transduction (SMAD3, EPHA2), and lens transparency (CRYBB1, CRYBA4). These data provide a novel relationship between altered chromatin accessibility and lens differentiation and they identify a wide-variety of lens genes and functions that could be regulated through altered chromatin accessibility. The data also point to a large number of potential DNA regulatory sequences and transcription factors whose functional analysis is likely to provide insight into novel regulatory mechanisms governing the lens differentiation program.

α6-Integrin alternative splicing: distinct cytoplasmic variants in stem cell fate specification and niche interaction

α6-Integrin subunit (also known as CD49f) is a stemness signature that has been found on the plasma membrane of more than 30 stem cell populations. A growing body of studies have focused on the critical role of α6-containing integrins (α6β1 and α6β4) in the regulation of stem cell properties, lineage-specific differentiation, and niche interaction. α6-Integrin subunit can be alternatively spliced at the post-transcriptional level, giving rise to divergent isoforms which differ in the cytoplasmic and/or extracellular domains. The cytoplasmic domain of integrins is an important functional part of integrin-mediated signals. Structural changes in the cytoplasmic domain of α6 provide an efficient means for the regulation of stem cell responses to biochemical stimuli and/or biophysical cues in the stem cell niche, thus impacting stem cell fate determination. In this review, we summarize the current knowledge on the structural variants of the α6-integrin subunit and spatiotemporal expression of α6 cytoplasmic variants in embryonic and adult stem/progenitor cells. We highlight the roles of α6 cytoplasmic variants in stem cell fate decision and niche interaction, and discuss the potential mechanisms involved. Understanding of the distinct functions of α6 splicing variants in stem cell biology may inform the rational design of novel stem cell-based therapies for a range of human diseases.

In wound repair vimentin mediates the transition of mesenchymal leader cells to a myofibroblast phenotype

Following injury, mesenchymal repair cells are activated to function as leader cells that modulate wound healing. These cells have the potential to differentiate to myofibroblasts, resulting in fibrosis and scarring. The signals underlying these differing pathways are complex and incompletely understood. The ex vivo mock cataract surgery cultures are an attractive model with which to address this question. With this model we study, concurrently, the mechanisms that control mesenchymal leader cell function in injury repair within their native microenvironment and the signals that induce this same cell population to acquire a myofibroblast phenotype when these cells encounter the environment of the adjacent tissue culture platform. Here we show that on injury, the cytoskeletal protein vimentin is released into the extracellular space, binds to the cell surface of the mesenchymal leader cells located at the wound edge in the native matrix environment, and supports wound closure. In profibrotic environments, the extracellular vimentin pool also links specifically to the mesenchymal leader cells and has an essential role in signaling their fate change to a myofibroblast. These findings suggest a novel role for extracellular, cell-surface–associated vimentin in mediating repair-cell function in wound repair and in transitioning these cells to a myofibroblast phenotype.

Priming integrin alpha 5 promotes the osteogenic differentiation of human periodontal ligament stem cells due to cytoskeleton and cell cycle changes

To seek a potential target for periodontal tissue regeneration, this study aimed to explore the role of Integrin alpha 5 (ITGA5) in human periodontal ligament stem cells (PDLSCs). Transwell assay, Cell Counting Kit 8 (CCK8) assay, cell cycle assay, alkaline phosphatase (ALP) activity, alizarin red staining, and western blot were used to investigate the effects of ITGA5 on PDLSC migration, proliferation and osteogenic differentiation. The in vivo effect was investigated by nude mice subcutaneous transplantation with cell and hydroxyapatite/β-tricalcium phosphate (HA/β-TCP) complex. The involved mechanism was explored by the iTRAQ proteomic technique and validated by western blot and immunofluorescence. We found that ITGA5forced expression enhanced the proliferation, migration, and osteogenic capacity of PDLSCs, while inhibited ITGA5 expression had the opposite effects. The phosphorylation of focal adhesion kinase (FAK), phosphatidylinositide 3-kinases/protein kinase B (PI3K/AKT), and mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinases 1 and 2 (MEK1/2/ERK1/2) were crucial in this process. Forced expression of ITGA5 in PDLSCs increased osteoid and PDL-like tissue formation in vivo. Proteomic and bioinformatic analysis revealed that cytoskeleton and cell cycle changes were involved. Keratin, type II cytoskeletal 6B (KRT6B) and desmin (DES) may distinguish this process and serve as new markers of PDLSC differentiation.

SIGNIFICANCE

Periodontitis is highly prevalent and can impair PDL and teeth functioning. One of the most promising therapies to periodontitis therapies is PDL regeneration by utilizing PDLSCs. While many obstacles remain to be resolved, the regulation of PDLSC osteogenic differentiation is a main concern. The present study demonstrated the potential clinical value of an ITGA5 priming peptide, which may be utilized in PDL tissue repair and regeneration. The mechanism elucidated in this study would help to fuel its application.

Functional role for stable microtubules in lens fiber cell elongation

The process of tissue morphogenesis, especially for tissues reliant on the establishment of a specific cytoarchitecture for their functionality, depends a balanced interplay between cytoskeletal elements and their interactions with cell adhesion molecules. The microtubule cytoskeleton, which has many roles in the cell, is a determinant of directional cell migration, a process that underlies many aspects of development. We investigated the role of microtubules in development of the lens, a tissue where cell elongation underlies morphogenesis. Our studies with the microtubule depolymerizing agent nocodazole revealed an essential function for the acetylated population of stable microtubules in the elongation of lens fiber cells, which was linked to their regulation of the activation state of myosin. Suppressing myosin activation with the inhibitor blebbistatin could attenuate the loss of acetylated microtubules by nocodazole and rescue the effect of this microtubule depolymerization agent on both fiber cell elongation and lens integrity. Our results also suggest that acetylated microtubules impact lens morphogenesis through their interaction with N-cadherin junctions, with which they specifically associate in the region where lens fiber cell elongate. Disruption of the stable microtubule network increased N-cadherin junctional organization along lateral borders of differentiating lens fiber cells, which was prevented by suppression of myosin activity. These results reveal a role for the stable microtubule population in lens fiber cell elongation, acting in tandem with N-cadherin cell-cell junctions and the actomyosin network, giving insight into the cooperative role these systems play in tissue morphogenesis.

β1-Integrin Signaling is Essential for Lens Fiber Survival

Integrins have been proposed to play a major role in lens morphogenesis. To determine the role of β1-integrin and its down-stream signaling partner, integrin linked kinase (ILK), in lens morphogenesis, eyes of WT mice and mice with a nestin-linked conditional knockout of β1-integrin or ILK were analyzed for defects in lens development. Mice, lacking the genes encoding the p1-integrin subunit ( Itgb1) or ILK ( Ilk), showed a perinatal degeneration of the lens. Early signs of lens degeneration included vacuolization, random distribution of lens cell nuclei, disrupted fiber morphology and attenuation and separation of the lens capsule. The phenotype became progressively more severe during the first postnatal week eventually leading to the complete loss of the lens. A more severe phenotype was observed in ILK mutants at similar stages. Eyes from embryonic day 13 β1-integrin-mutant embryos showed no obvious signs of lens degeneration, indicating that mutant lens develops normally until peri-recombination. Our findings suggest that β1-integrins and ILK cooperate to control lens cell survival and link lens fibers to the surrounding extracellular matrix. The assembly and integrity of the lens capsule also appears to be reliant on integrin signaling within lens fibers. Extrapolation of these results indicates a novel role of integrins in lens cell-cell adhesions as well as a potential role in the pathogenesis of congenital cataracts.

Cells activated for wound repair have the potential to direct collective invasion of an epithelium

Mechanisms regulating how groups of cells are signaled to move collectively from their original site and invade surrounding matrix are poorly understood. Here we develop a clinically relevant ex vivo injury invasion model to determine whether cells involved in directing wound healing have invasive function and whether they can act as leader cells to direct movement of a wounded epithelium through a three-dimensional (3D) extracellular matrix (ECM) environment. Similar to cancer invasion, we found that the injured cells invade into the ECM as cords, involving heterotypical cell-cell interactions. Mesenchymal cells with properties of activated repair cells that typically locate to a wound edge are present in leader positions at the front of ZO-1-rich invading cords of cells, where they extend vimentin intermediate filament-enriched protrusions into the 3D ECM. Injury-induced invasion depends on both vimentin cytoskeletal function and MMP-2/9 matrix remodeling, because inhibiting either of these suppressed invasion. Potential push and pull forces at the tips of the invading cords were revealed by time-lapse imaging, which showed cells actively extending and retracting protrusions into the ECM. This 3D injury invasion model can be used to investigate mechanisms of leader cell-directed invasion and understand how mechanisms of wound healing are hijacked to cause disease.

β-Adducin siRNA disruption of the spectrin-based cytoskeleton in differentiating keratinocytes prevented by calcium acting through calmodulin/epidermal growth factor receptor/cadherin pathway

Here, we report that siRNA transfection of β-adducin significantly disrupted the spectrin-based cytoskeleton and cytoskeletal arrangements of both β-adducin and PKCδ by substantially inhibiting the expression of β-adducin, spectrin and PKCδ proteins in differentiating keratinocytes. However, extracellular Ca2+ treatment blocked the inhibitory effects of the β-adducin siRNA. Ca2+ also prevented the significant down-regulation of two differentiation markers involucrin and K1/10 and the distinct up-regulation of proliferation marker K14 in β-adducin siRNA transfected keratinocytes. In addition, β-adducin knockdown resulted in a substantial reduction of epidermal growth factor receptor (EGFR), cadherin and β-catenin and enhanced phosphorylation of EGFR on tyrosine 1173 and Ca2+ prevented these changes. Furthermore, Ca2+ blocked the inhibitory effects of β-adducin siRNA on the expression of calmodulin, phosphorylated-calmodulin (P-CaM((Tyr138))) and myristoylated alanine-rich C-kinase substrate (MARCKS) in keratinocytes. Co-immunoprecipitation studies further revealed that calmodulin, not MARCKS, strongly interacted with EGFR, cadherin and β-catenin. Our data suggest that Ca2+ plays an important role in regulating the expression and function of β-adducin to sustain normal organization of the spectrin-based cytoskeleton and the differentiation properties in keratinocytes through the calmodulin/EGFR/cadherin signaling pathway.

Differentiation state-specific mitochondrial dynamic regulatory networks are revealed by global transcriptional analysis of the developing chicken lens

The mature eye lens contains a surface layer of epithelial cells called the lens epithelium that requires a functional mitochondrial population to maintain the homeostasis and transparency of the entire lens. The lens epithelium overlies a core of terminally differentiated fiber cells that must degrade their mitochondria to achieve lens transparency. These distinct mitochondrial populations make the lens a useful model system to identify those genes that regulate the balance between mitochondrial homeostasis and elimination. Here we used an RNA sequencing and bioinformatics approach to identify the transcript levels of all genes expressed by distinct regions of the lens epithelium and maturing fiber cells of the embryonic Gallus gallus (chicken) lens. Our analysis detected more than 15,000 unique transcripts expressed by the embryonic chicken lens. Of these, more than 3000 transcripts exhibited significant differences in expression between lens epithelial cells and fiber cells. Multiple transcripts coding for separate mitochondrial homeostatic and degradation mechanisms were identified to exhibit preferred patterns of expression in lens epithelial cells that require mitochondria relative to lens fiber cells that require mitochondrial elimination. These included differences in the expression levels of metabolic (DUT, PDK1, SNPH), autophagy (ATG3, ATG4B, BECN1, FYCO1, WIPI1), and mitophagy (BNIP3L/NIX, BNIP3, PARK2, p62/SQSTM1) transcripts between lens epithelial cells and lens fiber cells. These data provide a comprehensive window into all genes transcribed by the lens and those mitochondrial regulatory and degradation pathways that function to maintain mitochondrial populations in the lens epithelium and to eliminate mitochondria in maturing lens fiber cells.

Suppression of MAPK/JNK-MTORC1 signaling leads to premature loss of organelles and nuclei by autophagy during terminal differentiation of lens fiber cells

Although autophagic pathways are essential to developmental processes, many questions still remain regarding the initiation signals that regulate autophagy in the context of differentiation. To address these questions we studied the ocular lens, as the programmed elimination of nuclei and organelles occurs in a precisely regulated spatiotemporal manner to form the organelle-free zone (OFZ), a characteristic essential for vision acuity. Here, we report our discovery that inactivation of MAPK/JNK induces autophagy for formation of the OFZ through its regulation of MTORC1, where MAPK/JNK signaling is required for both MTOR activation and RPTOR/RAPTOR phosphorylation. Autophagy pathway proteins including ULK1, BECN1/Beclin 1, and MAP1LC3B2/LC3B-II were upregulated in the presence of inhibitors to either MAPK/JNK or MTOR, inducing autophagic loss of organelles to form the OFZ. These results reveal that MAPK/JNK is a positive regulator of MTORC1 signaling and its developmentally regulated inactivation provides an inducing signal for the coordinated autophagic removal of nuclei and organelles required for lens function.

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.

A central role for vimentin in regulating repair function during healing of the lens epithelium

A unique ex vivo mock cataract surgery model is used to study the role of vimentin in repair cell function during wound healing within a clinically relevant setting. Vimentin is found to be critical for the function of repair cells in directing the collective migration of the epithelium during wound healing.

Mock cataract surgery provides a unique ex vivo model for studying wound repair in a clinically relevant setting. Here wound healing involves a classical collective migration of the lens epithelium, directed at the leading edge by an innate mesenchymal subpopulation of vimentin-rich repair cells. We report that vimentin is essential to the function of repair cells as the directors of the wound-healing process. Vimentin and not actin filaments are the predominant cytoskeletal elements in the lamellipodial extensions of the repair cells at the wound edge. These vimentin filaments link to paxillin-containing focal adhesions at the lamellipodial tips. Microtubules are involved in the extension of vimentin filaments in repair cells, the elaboration of vimentin-rich protrusions, and wound closure. The requirement for vimentin in repair cell function is revealed by both small interfering RNA vimentin knockdown and exposure to the vimentin-targeted drug withaferin A. Perturbation of vimentin impairs repair cell function and wound closure. Coimmunoprecipitation analysis reveals for the first time that myosin IIB is associated with vimentin, linking vimentin function in cell migration to myosin II motor proteins. These studies reveal a critical role for vimentin in repair cell function in regulating the collective movement of the epithelium in response to wounding.

Lens extrusion from Laminin alpha 1 mutant zebrafish

We report analysis of the ocular lens phenotype of the recessive, larval lethal zebrafish mutant, lama1^a69/a69. Previous work revealed that this mutant has a shortened body axis and eye defects including a defective hyaloid vasculature, focal corneal dysplasia, and loss of the crystalline lens. While these studies highlight the importance of laminin α1 in lens development, a detailed analysis of the lens defects seen in these mutants was not reported. In the present study, we analyze the lenticular anomalies seen in the lama1^a69/a69 mutants and show that the lens defects result from the anterior extrusion of lens material from the eye secondary to structural defects in the lens capsule and developing corneal epithelium associated with basement membrane loss. Our analysis provides further insights into the role of the lens capsule and corneal basement membrane in the structural integrity of the developing eye.

α6 integrin transactivates insulin-like growth factor receptor-1 (IGF-1R) to regulate caspase-3-mediated lens epithelial cell differentiation initiation

The canonical mitochondrial death pathway was first discovered for its role in signaling apoptosis. It has since been found to have a requisite function in differentiation initiation in many cell types including the lens through low level activation of the caspase-3 protease. The ability of this pathway to function as a molecular switch in lens differentiation depends on the concurrent induction of survival molecules in the Bcl-2 and IAP families, induced downstream of an IGF-1R/NFκB coordinate survival signal, to regulate caspase-3 activity. Here we investigated whether α6 integrin signals upstream to this IGF-1R-mediated survival-linked differentiation signal. Our findings show that IGF-1R is recruited to and activated specifically in α6 integrin receptor signaling complexes in the lens equatorial region, where lens epithelial cells initiate their differentiation program. In studies with both α6 integrin knock-out mice lenses and primary lens cell cultures following α6 integrin siRNA knockdown, we show that IGF-1R activation is dependent on α6 integrin and that this transactivation requires Src kinase activity. In addition, without α6 integrin, activation and expression of NFκB was diminished, and expression of Bcl-2 and IAP family members were down-regulated, resulting in high levels of caspase-3 activation. As a result, a number of hallmarks of lens differentiation failed to be induced; including nuclear translocation of Prox1 in the differentiation initiation zone and apoptosis was promoted. We conclude that α6 integrin is an essential upstream regulator of the IGF-1R survival pathway that regulates the activity level of caspase-3 for it to signal differentiation initiation of lens epithelial cells.

Distinct roles for N-Cadherin linked c-Src and fyn kinases in lens development

BACKGROUND

Src family tyrosine kinases (SFKs) are often coincidently expressed but few studies have dissected their individual functions in the same cell during development. Using the classical embryonic lens as our model, we investigated SFK signaling in the regulation of both differentiation initiation and morphogenesis, and the distinct functions of c-Src and Fyn in these processes.

RESULTS

Blocking SFK activity with the highly specific inhibitor PP1 induced initiation of the lens differentiation program but blocked lens fiber cell elongation and organization into mini lens-like structures called lentoids. These dichotomous roles for SFK signaling were discovered to reflect distinct functions of c-Src and Fyn and their differentiation-state-specific recruitment to and action at N-cadherin junctions. c-Src was highly associated with the nascent N-cadherin junctions of undifferentiated lens epithelial cells. Its siRNA knockdown promoted N-cadherin junctional maturation, blocked proliferation, and induced lens cell differentiation. In contrast, Fyn was recruited to mature N-cadherin junctions of differentiating lens cells and siRNA knockdown suppressed differentiation-specific gene expression and blocked morphogenesis.

CONCLUSIONS

Through inhibition of N-cadherin junction maturation, c-Src promotes lens epithelial cell proliferation and the maintenance of the lens epithelial cell undifferentiated state, while Fyn, signaling downstream of mature N-cadherin junctions, promotes lens fiber cell morphogenesis.

Integrin α5/fibronectin1 and focal adhesion kinase are required for lens fiber morphogenesis in zebrafish

Lens fiber formation and morphogenesis requires a precise orchestration of cell- extracellular matrix (ECM) and cell-cell adhesive changes in order for a lens epithelial cell to adopt a lens fiber fate, morphology, and migratory ability. The cell-ECM interactions that mediate these processes are largely unknown, and here we demonstrate that fibronectin1 (Fn1), an ECM component, and integrin α5, its cellular binding partner, are required in the zebrafish lens for fiber morphogenesis. Mutations compromising either of these proteins lead to cataracts, characterized by defects in fiber adhesion, elongation, and packing. Loss of integrin α5/Fn1 does not affect the fate or viability of lens epithelial cells, nor does it affect the expression of differentiation markers expressed in lens fibers, although nucleus degradation is compromised. Analysis of the intracellular mediators of integrin α5/Fn1 activity focal adhesion kinase (FAK) and integrin-linked kinase (ILK) reveals that FAK, but not ILK, is also required for lens fiber morphogenesis. These results support a model in which lens fiber cells use integrin α5 to migrate along a Fn-containing substrate on the apical side of the lens epithelium and on the posterior lens capsule, likely activating an intracellular signaling cascade mediated by FAK in order to orchestrate the cytoskeletal changes in lens fibers that facilitate elongation, migration, and compaction.

Laminin and integrin expression in the ventral ectodermal ridge of the mouse embryo: implications for regulation of BMP signalling

BACKGROUND

The ventral ectodermal ridge (VER) is an important signalling centre in the mouse tail-bud following completion of gastrulation. BMP regulation is essential for VER function, but how these signals are transmitted between adjacent tissues is unclear.

RESULTS

We investigated the idea that extracellular matrix components might be involved, using immunohistochemistry and in situ hybridisation to detect all known α, β, and γ laminin chains and their mRNAs in the early tail bud. We identified an apparently novel laminin variant, comprising α5, β3 and γ2 chains, as a major component of the VER basement membrane at E9.5. Strikingly, only the mRNAs for these chains were co-expressed in VER cells, suggesting that lamin532 may be the sole basement membrane laminin at this stage. Since α6 integrin was also expressed in VER cells, this raises the possibility of cell-matrix interactions regulating BMP signalling at this site of caudal morphogenesis.

CONCLUSIONS

Laminin532 could interact with α6-containing integrin to direct differentiation of the specialised VER cells from surface ectoderm.

Modulation of N-cadherin junctions and their role as epicenters of differentiation-specific actin regulation in the developing lens

Extensive elongation of lens fiber cells is a central feature of lens morphogenesis. Our study investigates the role of N-cadherin junctions in this process in vivo. We investigate both the molecular players involved in N-cadherin junctional maturation and the subsequent function of these junctions as epicenters for the assembly of an actin cytoskeleton that drives morphogenesis. We present the first evidence of nascent cadherin junctions in vivo, and show that they are a prominent feature along lateral interfaces of undifferentiated lens epithelial cells. Maturation of these N-cadherin junctions, required for lens cell differentiation, preceded organization of a cortical actin cytoskeleton along the cells' lateral borders, but was linked to recruitment of α-catenin and dephosphorylation of N-cadherin-linked β-catenin. Biochemical analysis revealed differentiation-specific recruitment of actin regulators cortactin and Arp3 to maturing N-cadherin junctions of differentiating cells, linking N-cadherin junctional maturation with actin cytoskeletal assembly during fiber cell elongation. Blocking formation of mature N-cadherin junctions led to reduced association of α-catenin with N-cadherin, prevented organization of actin along lateral borders of differentiating lens fiber cells and blocked their elongation. These studies provide a molecular link between N-cadherin junctions and the organization of an actin cytoskeleton that governs lens fiber cell morphogenesis in vivo.

αA-Crystallin associates with α6 integrin receptor complexes and regulates cellular signaling

α-Crystallins are small heat-shock proteins important to lens transparency that provide the lens with its refractive properties. In their role as molecular chaperones, these crystallins also prevent protein aggregation, affect cytoskeletal remodeling, enhance resistance to cell stress, and provide lens cells with protection against apoptosis. While many of the functions assigned to αA-crystallin are attributable to its presence in the cytoplasm of lens cells, αA-crystallin also has been detected at the lens plasma membrane. However, how αA-crystallin becomes linked to the plasma membrane or what its functions are at this site has remained unknown. In this study, we examined the mechanisms by which αA-crystallin becomes associated with the lens membrane, focusing specifically on its interaction with membrane receptors, and the differentiation-specificity of these interactions. We also determined how the long-term absence of αA-crystallin alters receptor-linked signaling pathways. αA-crystallin association with membrane receptors was determined by co-immunoprecipitation analysis; its membrane localization was examined by confocal imaging; and the effect of αA-crystallin loss-of-function on the activation state of signaling molecules in pathways linked to membrane receptors was determined by immunoblot analysis. The results show that, in lens epithelial cells, plasma membrane αA-crystallin was primarily localized to apicolateral borders, reflecting the association of αA-crystallin with E-cadherin complexes. These studies also provide the first evidence that αA-crystallin maintained its association with the plasma membrane in lens cortical fiber cells, where it was localized to lateral interfaces, and further show that this association was mediated, in part, by αA-crystallin interaction with α6 integrin receptor complexes. We report that the absence of αA-crystallin led to constitutive activation of the stress kinases p38 and JNK, classical inducers of apoptotic cell death, and the loss of the phospho-Bad pro-survival signal, effects that were greatest in differentiating lens fiber cells. Concurrent with this, activation of FAK and ERK kinases was increased, demonstrating that these receptor-linked pathways also were dysregulated in the absence of αA-crystallin. These data link αA-crystallin plasma membrane association to its differentiation-state-specific interaction with E-cadherin and α6 integrin receptor complexes. The changes in cell signaling in αA-crystallin-null lenses suggest that dysregulation of receptor-linked cell-signaling pathways that accompany the failure of αA-crystallin to associate with membrane receptors may be responsible for the induction of apoptosis. The observed changes in lens cell signaling likely reflect long-term functional adaptations to the absence of the αA-crystallin chaperone/small heat-shock protein.

Diverse roles of E-cadherin in the morphogenesis of the submandibular gland: insights into the formation of acinar and ductal structures

The formation of acinar and ductal structures during epithelial tissue branching morphogenesis is not well understood. We report that in the mouse submandibular gland (SMG), acinar and ductal cell fates are determined early in embryonic morphogenesis with E-cadherin playing pivotal roles in development. We identified two morphologically distinct cell populations at the single bud stage, destined for different functions. The outer layer of columnar cells with organized E-cadherin junctions expressed the neonatal acinar marker B1 by E13.5, demonstrating their acinar fate. The interior cells initially lacked distinct E-cadherin junctions, but with morphogenesis formed cytokeratin 7 (K7) -positive ductal structures with organized E-cadherin junctions and F-actin filaments. Inhibition of E-cadherin function with either siRNA or function blocking antibody caused extensive apoptosis of ductal cells and aberrantly dilated lumens, providing the first evidence that E-cadherin regulates ductal lumen formation during branching morphogenesis of the salivary gland.

Periplakin interactions with lens intermediate and beaded filaments

PURPOSE

The lens assembles two systems of intermediated filaments-vimentin intermediate filament (IF) and highly divergent, lens-specific beaded filament (BF)-sequentially as epithelial cells differentiate into fiber cells. The goal of this study was to identify linker proteins that integrate the different lens IF into the biology of the lens fiber cells.

METHODS

Antibodies to periplakin were used in coimmunoprecipitation studies to identify proteins that complex with BF and IF in detergent extracts of mouse lens. GST-periplakin fusion proteins were used to confirm coimmunoprecipitation

RESULTS

Yeast two-hybrid analysis was used to establish direct linkage between periplakin and BF/IF proteins and to narrow down binding domains. Immunocytochemistry was used to establish spatial and temporal coexpression of periplakin and BF/IF. results. Periplakin is found complexed to BF and IF in the lens. The COOH terminus of periplakin was shown to have a strong affinity for the CP49 rod 2 domain but not its head or rod 1 domains. Low-level affinity was seen between the filensin rod domain and periplakin. Periplakin localization in lens overlapped with BF and IF.

CONCLUSIONS

Despite divergence in primary sequence, predicted secondary structure, and filament structure, CP49 has conserved the capacity to bind a common IF linker protein, periplakin, and shares that binding capacity with the other major lens IF protein, vimentin. This suggests that mutations in periplakin have the potential to emulate the cataract seen in lenses with defective BF proteins.

Identification of a novel intermediate filament-linked N-cadherin/gamma-catenin complex involved in the establishment of the cytoarchitecture of differentiated lens fiber cells

Tissue morphogenesis and maintenance of complex tissue architecture requires a variety of cell-cell junctions. Typically, cells adhere to one another through cadherin junctions, both adherens and desmosomal junctions, strengthened by association with cytoskeletal networks during development. Both beta- and gamma-catenins are reported to link classical cadherins to the actin cytoskeleton, but only gamma-catenin binds to the desmosomal cadherins, which links them to intermediate filaments through its association with desmoplakin. Here we provide the first biochemical evidence that, in vivo, gamma-catenin also mediates interactions between classical cadherins and the intermediate filament cytoskeleton, linked through desmoplakin. In the developing lens, which has no desmosomes, we discovered that vimentin became linked to N-cadherin complexes in a differentiation-state specific manner. This newly identified junctional complex was tissue specific but not unique to the lens. To determine whether in this junction N-cadherin was linked to vimentin through gamma-catenin or beta-catenin we developed an innovative "double" immunoprecipitation technique. This approach made possible, for the first time, the separation of N-cadherin/gamma-catenin from N-cadherin/beta-catenin complexes and the identification of multiple members of each of these isolated protein complexes. The study revealed that vimentin was associated exclusively with N-cadherin/gamma-catenin junctions. Assembly of this novel class of cadherin junctions was coincident with establishment of the unique cytoarchitecture of lens fiber cells. In addition, gamma-catenin had a distinctive localization to the vertices of these hexagonally shaped differentiating lens fiber cells, a region devoid of actin; while beta-catenin co-localized with actin at lateral cell interfaces. We believe this novel vimentin-linked N-cadherin/gamma-catenin junction provides the tensile strength necessary to establish and maintain structural integrity in tissues that lack desmosomes.

Integrins in lens development and disease

Integrins are the major cell surface receptors for proteins in the extracellular matrix. These receptors form major cell signaling centers that are bidirectional, communicating messages between the cell and its environment. They are a large receptor family, with members well-known to regulate cellular processes essential to both development and disease. In this review we examine the literature regarding integrins in the lens. Here we cover integrin function in lens cell differentiation, in the development of the lens and in protection of the lens epithelial cell phenotype. In addition, we analyze the role of integrins in the progression of lens fibrotic diseases, focusing particularly on integrin regulation of TGFbeta signaling pathways in posterior capsule opacification (PCO) and anterior subcapsular cataract (ASC).

Lens fiber cell elongation and differentiation is associated with a robust increase in myosin light chain phosphorylation in the developing mouse

Myosin II, a molecular motor, plays a critical role in cell migration, cell shape changes, cell adhesion, and cytokinesis. To understand the role of myosin II in lens fiber cell elongation and differentiation, we determined the distribution pattern of nonmuscle myosin IIA, IIB, and phosphorylated regulatory myosin light chain-2 (phospho-MLC) in frozen sections of the developing mouse lens by immunofluorescence analysis. While myosin IIA was distributed uniformly throughout the differentiating lens, including the epithelium and fibers, myosin IIB was localized predominantly to the epithelium and the posterior tips of the lens fibers. In contrast, immunostaining with a di-phospho-MLC antibody localized intensely and precisely to the elongating and differentiating primary and secondary lens fibers, co-localizing with actin filaments. An in situ analysis of Rho GTPase activation revealed that Rho-GTP was distributed uniformly throughout the embryonic lens, including epithelium and fibers. Inhibition of myosin light chain kinase (MLCK) activity by ML-7 in organ cultured mouse lenses led to development of nuclear lens opacity in association with abnormal fiber cell organization. Taken together, these data reveal a distinct spatial distribution pattern of myosin II isoforms in the developing lens and a robust activation of MLC phosphorylation in the differentiating lens fibers. Moreover, the regulation of MLC phosphorylation by MLCK appears to be critical for crystallin organization and for maintenance of lens transparency and lens membrane function.

Conditional deletion of beta1-integrin from the developing lens leads to loss of the lens epithelial phenotype

Beta1-integrins are cell surface receptors that participate in sensing the cell's external environment. We used the Cre-lox system to delete beta1-integrin in all lens cells as the lens vesicle transitions into the lens. Adult mice lacking beta1-integrin in the lens are microphthalmic due to apoptosis of the lens epithelium and neonatal disintegration of the lens fibers. The first morphological alterations in beta1-integrin null lenses are seen at 16.5 dpc when the epithelium becomes disorganized and begins to upregulate the fiber cell markers beta- and gamma-crystallins, the transcription factors cMaf and Prox1 and downregulate Pax6 levels demonstrating that beta1-integrin is essential to maintain the lens epithelial phenotype. Furthermore, beta1-integrin null lens epithelial cells upregulate the expression of alpha-smooth muscle actin and nuclear Smad4 and downregulate Smad6 suggesting that beta1-integrin may brake TGFbeta family signaling leading to epithelial-mesenchymal transitions in the lens. In contrast, beta1-integrin null lens epithelial cells show increased E-cadherin immunoreactivity which supports the proposed role of beta1-integrins in mediating complete EMT in response to TGFbeta family members. Thus, beta1-integrin is required to maintain the lens epithelial phenotype and block inappropriate activation of some aspects of the lens fiber cell differentiation program.

Cell cycle regulation in the developing lens

Regulation of cell proliferation is a critical aspect of the development of multicellular organisms. The ocular lens is an excellent model system in which to unravel the mechanisms controlling cell proliferation during development. In recent years, several cell cycle regulators have been shown to be essential for maintaining normal patterns of lens cell proliferation. Additionally, many growth factor signaling pathways and cell adhesion factors have been shown to have the capacity to regulate lens cell proliferation. Given this complexity, understanding the cross talk between these many signaling pathways and how they are coordinated are important directions for the future.

The role of the lens actin cytoskeleton in fiber cell elongation and differentiation

The vertebrate ocular lens is a fascinating and unique transparent tissue that grows continuously throughout life. During the process of differentiation into fiber cells, lens epithelial cells undergo dramatic morphological changes, membrane remodeling, polarization, transcriptional activation and elimination of cellular organelles including nuclei, concomitant with migration towards the lens interior. Most of these events are presumed to be influenced in large part, by dynamic reorganization of the cellular actin cytoskeleton and by intercellular and cell: extracellular matrix interactions. In light of recent and unprecedented advancement in our understanding of the mechanistic bases underlying regulation of actin cytoskeletal dynamics and the role of the actin cytoskeleton in cell function, this review attempts to summarize current knowledge regarding the role of the cellular actin cytoskeleton, in lens fiber cell elongation and differentiation, and regulation of actin cytoskeletal organization in the lens.

Extracellular matrix and integrin signaling in lens development and cataract

During development of the vertebrate lens there are dynamic interactions between the extracellular matrix (ECM) of the lens capsule and lens cells. Disruption of the ECM causes perturbation of lens development and cataract. Similarly, changes in cell signaling can result in abnormal ECM and cataract. Integrins are key mediators of ECM signals and recent studies have documented distinct repertoires of integrin expression during lens development, and in anterior subcapsular cataract (ASC) and posterior caspsule opacification (PCO). Increasingly, studies are being directed to investigating the signaling pathways that integrins modulate and have identified Src, focal adhesion kinase (FAK) and integrin-linked kinase (ILK) as downstream kinases that mediate proliferation, differentiation and morphological changes in the lens during development and cataract formation.

N-glycosylation affects the molecular organization and stability of E-cadherin junctions

Epithelial cell-cell adhesion is mediated by E-cadherin, an intercellular N-glycoprotein adhesion receptor that functions in the assembly of multiprotein complexes anchored to the actin cytoskeleton named adherens junctions (AJs). E-cadherin ectodomains 4 and 5 contain three potential N-glycan addition sites, although their significance in AJ stability is unclear. Here we show that sparse cells lacking stable AJs produced E-cadherin that was extensively modified with complex N-glycans. In contrast, dense cultures with more stable AJs had scarcely N-glycosylated E-cadherin modified with high mannose/hybrid and limited complex N-glycans. This suggested that variations in AJ stability were accompanied by quantitative and qualitative changes in E-cadherin N-glycosylation. To further examine the role of N-glycans in AJ function, we generated E-cadherin N-glycosylation variants lacking selected N-glycan addition sites. Characterization of these variants in CHO cells, lacking endogenous E-cadherin, revealed that site 1 on ectodomain 4 was modified with a prominent complex N-glycan, site 2 on ectodomain 5 did not have a substantial oligosaccharide, and site 3 on ectodomain 5 was decorated with a high mannose/hybrid N-glycan. Removal of complex N-glycan from ectodomain 4 led to a dramatically increased interaction of E-cadherin-catenin complexes with vinculin and the actin cytoskeleton. The latter effect was further enhanced by the deletion of the high mannose/hybrid N-glycan from site 3. In MDCK cells, which produce E-cadherin, a variant lacking both complex and high mannose/hybrid N-glycans functioned like a dominant positive displaying increased interaction with gamma-catenin and vinculin compared with the endogenous E-cadherin. Collectively, our studies show that N-glycans, and complex oligosaccharides in particular, destabilize AJs by affecting their molecular organization.

Integrin α6β1-laminin interactions regulate early myotome formation in the mouse embryo

We addressed the potential role of cell-laminin interactions during epaxial myotome formation in the mouse embryo. Assembly of the myotomal laminin matrix occurs as epaxial myogenic precursor cells enter the myotome. Most Myf5-positive and myogenin-negative myogenic precursor cells localise near assembled laminin, while myogenin-expressing cells are located either away from this matrix or in areas where it is being assembled. In Myf5nlacZ/nlacZ (Myf5-null) embryos, laminin,collagen type IV and perlecan are present extracellularly near myogenic precursor cells, but do not form a basement membrane and cells are not contained in the myotomal compartment. Unlike wild-type myogenic precursor cells, Myf5-null cells do not express the α6β1 integrin, a laminin receptor, suggesting that integrin α6β1-laminin interactions are required for myotomal laminin matrix assembly. Blockingα6β1-laminin binding in cultured wild-type mouse embryo explants resulted in dispersion of Myf5-positive cells, a phenotype also seen in Myf5nlacZ/nlacZ embryos. Furthermore, inhibition ofα6β1 resulted in an increase in Myf5 protein and ectopic myogenin expression in dermomyotomal cells, suggesting that α6β1-laminin interactions normally repress myogenesis in the dermomyotome. We conclude that Myf5 is required for maintaining α6β1 expression on myogenic precursor cells, and that α6β1 is necessary for myotomal laminin matrix assembly and cell guidance into the myotome. Engagement of laminin byα6β1 also plays a role in maintaining the undifferentiated state of cells in the dermomyotome prior to their entry into the myotome.

Actin filament organization regulates the induction of lens cell differentiation and survival

The actin cytoskeleton has the unique capability of integrating signaling and structural elements to regulate cell function. We have examined the ability of actin stress fiber disassembly to induce lens cell differentiation and the role of actin filaments in promoting lens cell survival. Three-dimensional mapping of basal actin filaments in the intact lens revealed that stress fibers were disassembled just as lens epithelial cells initiated their differentiation in vivo. Experimental disassembly of actin stress fibers in cultured lens epithelial cells with either the ROCK inhibitor Y-27632, which destabilizes stress fibers, or the actin depolymerizing drug cytochalasin D induced expression of lens cell differentiation markers. Significantly, short-term disassembly of actin stress fibers in lens epithelial cells by cytochalasin D was sufficient to signal lens cell differentiation. As differentiation proceeds, lens fiber cells assemble actin into cortical filaments. Both the actin stress fibers in lens epithelial cells and the cortical actin filaments in lens fiber cells were found to be necessary for cell survival. Sustained cytochalasin D treatment of undifferentiated lens epithelial cells suppressed Bcl-2 expression and the cells ultimately succumbed to apoptotic cell death. Inhibition of Rac-dependent cortical actin organization induced apoptosis of differentiating lens fiber cells. Our results demonstrate that disassembly of actin stress fibers induced lens cell differentiation, and that actin filaments provide an essential survival signal to both lens epithelial cells and differentiating lens fiber cells.

laminin alpha 1 gene is essential for normal lens development in zebrafish

Background

Laminins represent major components of basement membranes and play various roles in embryonic and adult tissues. The functional laminin molecule consists of three chains, alpha, beta and gamma, encoded by separate genes. There are twelve different laminin genes identified in mammals to date that are highly homologous in their sequence but different in their tissue distribution. The laminin alpha -1 gene was shown to have the most restricted expression pattern with strong expression in ocular structures, particularly in the developing and mature lens.

Results

We identified the zebrafish lama1 gene encoding a 3075-amino acid protein (lama1) that possesses strong identity with the human LAMA1. Zebrafish lama1 transcripts were detected at all stages of embryo development with the highest levels of expression in the developing lens, somites, nervous and urogenital systems. Translation of the lama1 gene was inhibited using two non-overlapping morpholino oligomers that were complementary to sequences surrounding translation initiation. Morphant embryos exhibited an arrest in lens development and abnormalities in the body axis length and curvature.

Conclusion

These results underline the importance of the laminin alpha 1 for normal ocular development and provide a basis for further analysis of its developmental roles.

Cyclin-dependent kinase 5 activators p35 and p39 facilitate formation of functional synapses

Cyclin-dependent kinase 5 (Cdk5) has emerged as a key coordinator of cell signaling in neurite outgrowth. Cdk5 needs to associate with one of the regulatory proteins p35 or p39 to be an active enzyme. To investigate if Cdk5 plays a role in the establishment of functional synapses, we have characterized the expression of Cdk5, p35, and p39 in the neuroblastoma-glioma cell line NG108-15, and recorded postsynaptic activity in myotubes in response to presynaptic overexpression of Cdk5, p35, and p39. Endogenous Cdk5 and p35 protein levels increased with cellular differentiation and preferentially distributed to soluble pools, whereas the level of p39 protein remained low and primarily was present in membrane and cytoskeletal fractions. Transient transfection of a dominant-negative mutant of Cdk5 in NG108-15 cells and subsequent culturing on differentiating muscle cells resulted in a significant reduction in synaptic activity, as measured by postsynaptic miniature endplate potentials (mEPPs). Overexpression of either Cdk5/p35 or Cdk5/p39 resulted in a substantial increase in synaptic structures that displayed postsynaptic activities, as well as mEPP frequency. These findings demonstrate that Cdk5, p35, and p39 are endogenously expressed in NG108-15 cells, exhibit distinct subcellular localizations, and that both Cdk5/p35 and Cdk5/p39 are central in formation of functional synapses.

Laminin-binding integrins in rat lens morphogenesis and their regulation during fibre differentiation

Mammalian lens development involves cell-cell and cell-ECM interactions. As integrins are a major family of cell adhesion molecules, we examined the expression patterns of several integrin subunits (alpha3A, alpha3B, alpha6A, alpha6B, beta1 and beta4) during rat lens development. RT-PCR, in situ hybridisation, immunofluorescence and immunoblotting were used to investigate expression of integrin subunits during lens development and differentiation. RT-PCR showed expression of alpha3A, alpha6A, alpha6B and beta1A but not alpha3B or beta4 subunits in postnatal rat lenses. Each subunit displayed distinct spatio-temporal expression patterns. beta1 integrin was expressed in both epithelium and fibres. alpha3A subunit expression was restricted to the epithelium; expression ceased abruptly at the lens equator. Expression of the alpha6A subunit increased during fibre differentiation, whereas alpha6B expression was predominantly associated with epithelial cells during lens development. In lens epithelial explants, FGF induced some of the changes in integrin expression that are characteristic of fibre differentiation in vivo. One notable exception was the inability of FGF to reproduce the distinctive down-regulation of the alpha3 isoform that is associated with initiation of elongation in vivo. Interestingly, vitreous treatment was able to reproduce this shift in alpha3 expression indicating that another factor(s), in addition to FGF, may be required for full and complete transition from an epithelial cell to a fibre cell. Integrin subunit expression therefore appears to be highly regulated during lens development and fibre differentiation with evidence of major changes in alpha3 and alpha6 isoform expression. These results indicate that integrins may play important roles in development and growth of the lens. How specific integrin subunits influence the behaviour of cells in different developmental compartments of the lens remains to be determined.

The Canonical Intrinsic Mitochondrial Death Pathway Has a Non-apoptotic Role in Signaling Lens Cell Differentiation

The mitochondrial cell death pathway is known for its role in signaling apoptosis. Here, we describe a novel function for the mitochondrial cell death pathway in signaling initiation of differentiation in the developing lens. Most remarkably, we induced lens cell differentiation by short-term exposure of lens epithelial cells to the apoptogen staurosporine. Activation of apoptosis-related pathways induced lens epithelial cells to express differentiation-specific markers and to undergo morphogenetic changes that led to formation of the lens-like structures known as lentoids. The fact that multiple stages of differentiation are expressed at a single stage of development in the embryonic lens made it possible to precisely determine the timing of expression of proteins associated with the apoptotic pathway. We discovered that there was high expression in the lens equatorial epithelium (the region of the lens in which differentiation is initiated) of pro-apoptotic molecules such as Bax and Bcl-x(S) and release of cytochrome c from mitochondria. Furthermore, we found significant caspase-3-like activity in the equatorial epithelium, yet this activity was far lower than that associated with lens cell apoptosis. These apoptotic pathways are likely regulated by the concurrent expression of prosurvival molecules, including Bcl-2 and Bcl-x(L); phosphorylation of Bad; and high expression of inhibitor of apoptosis proteins chicken IAP1, IAP3, and survivin. This finding suggests that prosurvival pathways allow pro-apoptotic molecules to function as molecular switches in the differentiation process without tipping the balance toward apoptosis. We call this process apoptosis-related Bcl-2- and caspase-dependent (ABC) differentiation.

Expression patterns of beta1-related alpha integrin subunits in murine lens during embryonic development and wound healing

PURPOSE

To study the expression patterns of b1-related alpha integrin subunits in murine lens epithelial cells, comparing embryonic fiber differentiation with injury-induced epithelial mesenchymal transition (EMT).

METHODS

Adult mice type C57BL/6, pregnant as well as with an eye injured, were sacrificed at different time-course intervals. The embryonic and the injured eyes were obtained and deparaffinized sections of these eyes were processed for immunohistochemistry staining for detection of integrin a subunits.

RESULTS

Embryonic lens epithelial cells expressed primarily a3 and a5 subunits, whereas embryonic fiber cells expressed a2, a5, and a6 subunits. Adult lens epithelial cells expressed a3, and a6 subunits,whereas injured lens cells expressed a2, a3, and a6 integrin subunits.

CONCLUSIONS

The phenotypic changes of lens epithelial cells during embryonic fiber differentiation and EMT are characterized by different expression of integrin subunits as a result both of the altered extracellular matrix conditions and of the altered cell signaling pathways recruited in each process.

Retinoic acid disintegrated desmosomes and hemidesmosomes in stratified oral keratinocytes

BACKGROUND

Although it is known that retinoic acid (RA) regulates the cellular differentiation of skin keratinocytes, the effects of RA on the anchoring junction have not been clarified. The effects of all-trans RA on cell-cell and cell-matrix connections of gingival epithelial (GE)1 cells in a multilayered culture were investigated.

METHODS

Ultrastructures of GE1 cells were observed and immunohistochemistry was used to detect keratin 4, keratin 13, and desmoglein expression. Reverse transcription-polymerase chain reaction was performed to detect expression of desmosome and hemidesmosome-associating adhesion molecules, keratin 13, and keratin14.

RESULTS

Retinoic acid caused immunohistochemical diminution of keratin 4, keratin 13, and desmoglein. Ultrastructurally, RA induced drastic loss of typical desmosomes and complete loss of hemidesmosomes. RA significantly decreased the transcript levels of keratin 13, keratin 14, desmoglein 1, and desmocollin 1 in a dose-dependent manner. The 230-kD bullous pemphigoid antigen (BPAG1) gene expression was also reduced by RA, whereas transcript levels of integrin alpha6, integrin beta4, the 180-kD bullous pemphigoid antigen (BPAG2), and laminin 5 were not affected.

CONCLUSION

These results indicated that RA disintegrated not only desmosomes by depriving the cells of desmoglein 1, desmocollin 1, keratin 13, and keratin 4, but also hemidesmosomes by reducing the expression of BPAG1 and keratin 14 in basal keratinocytes.

A signaling role for the uncleaved form of alpha 6 integrin in differentiating lens fiber cells

Many alpha integrin subunits are cleaved during their processing to yield heavy and light chains, which remain associated by disulfide bonds. While uncleaved alpha integrin subunits can form functional receptors that sometimes have distinct signaling roles from their better-characterized endoproteolytically cleaved counterparts, their expression at the cell surface and their association with signaling complexes have yet to be determined in vivo. In this study, we demonstrate that, in differentiating lens fiber cells, the uncleaved form of alpha 6 integrin was expressed at the cell surface. This form of alpha 6 integrin coimmunoprecipitated with both the signaling adaptor molecule Shc and its downstream effector Grb2, suggesting that, in lens fiber cells, uncleaved alpha 6 integrin was associated with a Shc-mediated signaling complex. We show that expression of the cleaved form of alpha 6 integrin progressively decreased relative to uncleaved alpha 6 integrin as the state of lens cell differentiation increased, resulting in the predominance of uncleaved alpha 6 integrin in the lens fiber cell zones. Interestingly, we previously have shown that alpha 6 integrin is localized principally along the extensive cell-cell interfaces of these lens fiber cells, in the absence of its extracellular matrix ligand laminin. While we found that the cleaved form of alpha 6 integrin contained both high mannose and complex sugars, the uncleaved form of alpha 6 integrin contained only high mannose sugars. These properties suggest that the uncleaved form of alpha 6 integrin may have a unique role in the embryonic lens. Its high association with Shc and Grb2 in the differentiating cortical fiber cell zone indicates that alpha 6 integrin may provide a cell survival signal in the presence of the apoptotic-like processes that are initiated in this region of the embryonic lens to clear the lens cells of their organelles.

Integrins alpha(6A)beta 1 and alpha(6B)beta 1 promote different stages of chondrogenic cell differentiation

The differentiation of chondrocytes and of several other cell types is associated with a switch from the alpha(6B) to the alpha(6A) isoform of the laminin alpha(6)beta(1) integrin receptor. To define whether this event plays a functional role in cell differentiation, we used an in vitro model system that allows chick chondrogenic cells to remain undifferentiated when cultured in monolayer and to differentiate into chondrocytes when grown in suspension culture. We report that: (i) upon over-expression of the human alpha(6B), adherent chondrogenic cells differentiate to stage I chondrocytes (i.e. increased type II collagen, reduced type I collagen, fibronectin, alpha(5)beta(1) and growth rate, loss of fibroblast morphology); (ii) the expression of type II collagen requires the activation of p38 MAP kinase; (iii) the over-expression of alpha(6A) induces an incomplete differentiation to stage I chondrocytes, whereas no differentiation was observed in alpha(5) and mock-transfected control cells; (iv) a prevalence of the alpha(6A) subunit is necessary to stabilize the differentiated phenotype when cells are transferred to suspension culture. Altogether, these results indicate a functional role for the alpha(6B) to alpha(6A) switch in chondrocyte differentiation; the former promotes chondrocyte differentiation, and the latter is necessary in stabilizing the differentiated phenotype.

Transition between proliferation and differentiation for lens epithelial cells is regulated by Src family kinases

As in many cell types, lens cells must withdraw from the cell cycle before they initiate their differentiation. The involvement of Src family kinases (SFKs) in this key initiating event in cell differentiation was examined in lens epithelial cell cultures. SFK activity was suppressed with the specific inhibitor PP1. This induced expression of the cyclin-dependent kinase (CDK) inhibitors p27 and p57 and suppressed lens epithelial cell proliferation. Therefore, inhibition of SFK activity created conditions permissive for undifferentiated lens epithelial cells to withdraw from the cell cycle. Growth of the lens epithelial cell cultures in the presence of PP1 induced expression of filensin and CP49, lens differentiation-specific intermediate filament proteins, providing evidence that suppression of SFK activity also promoted the initiation of lens cell differentiation. The mechanism by which PP1 signaled cell cycle withdrawal and commitment to differentiation was shown to involve induction of N-cadherin cell-cell junction assembly and reorganization of the actin cytoskeleton from stress fibers to cortical filaments. This result was supported by the compaction of the epithelial monolayer in response to PP1, a morphogenetic change that we have previously shown to be dependent on N-cadherin function and a hallmark of the commencement of the lens differentiation program in culture. The results presented in this study suggest that the decision of lens epithelial cells to withdraw from the cell cycle and initiate differentiation requires inhibition of SFKs and the formation of N-cadherin cell-cell junctions.

The role of mitochondria, cytochrome c and caspase-9 in embryonic lens fibre cell denucleation

During the differentiation of secondary lens fibre cells from the lens epithelium, the fibre cells lose all of their cytoplasmic organelles as well as their nuclei. The fibre cells, containing crystallins, which confer optical clarity, then persist in the adult lens. The process of denucleation of these cells has been likened to an apoptotic event which is not followed by the plasma membrane changes that are characteristic of apoptosis. We have examined the expression and subcellular translocation of molecules of the apoptotic cascade in differentiating lens epithelial cells in culture. In this culture system, the epithelial cells differentiate into lentoids composed of lens fibre cells. We find that caspase-9, which is expressed and activated before embryonic day 12 in intact lenses, is localized in the cytosol outside mitochondria in non-differentiating cultured cells. In lentoid cells, caspase-9 migrates into mitochondria after the latter undergo a membrane permeability transition that is characteristic of apoptotic cells. At the same time, caspase-9 co-localizes with cytochrome c in the cytosol. The cytochrome c is apparently released from the mitochondria in lentoid cells after the mitochondrial membrane permeability transition and during the period of nuclear shrinkage. Also during this time, the mitochondria aggregate around the degenerating nuclei. Cytochrome c disappears rapidly, while mitochondrial breakdown occurs approximately coincident with the disappearance of the nuclei, but mitochondrial remnants persist together with cytochrome c oxidase, which is a mitochondrial marker protein. Apaf-1, another cytosolic protein of the apoptotic cascade, also migrates to the permeabilized mitochondria and also co-localizes with caspase-9 and cytochrome c in the cytosol or mitochondria of denucleating cells, thus providing evidence for the formation of an 'apoptosome' in these cells, as in apoptotic cells. At no time did we observe the translocation of molecules between cytoplasmic compartments and the nucleus in differentiating lentoid cells. We suggest that the uncoupling of nuclear and membrane apoptotic events in these cells may be due to the early permeability changes in the mitochondria, resulting in the loss of mitochondrial signalling molecules, or to the failure of molecules to migrate to the nucleus in these cells, thus failing to activate nuclear-plasma membrane signalling pathways.

Inhibitory effects of salmosin, a disintegrin, on posterior capsular opacification in vitro and in vivo

The proliferation, migration and transdifferentiation of the remaining lens epithelial cells (LECs) after cataract surgery are a major cause of posterior capsular opacification (PCO). It has previously been reported that salmosin, a novel disintegrin, significantly inhibits solid tumor growth in mice by perturbation of tumor-specific angiogenesis via blocking alpha v beta 3 integrin expressed on vascular endothelial cells. In this study, the inhibitory function of salmosin in PCO was investigated and was found that salmosin inhibits the attachment of bovine LECs and rabbit lens cells (N/N1003A) to extracellular matrix-coated plates. The anti-adhesive activity of salmosin was approximately 1000 times higher than that of synthetic Arg-Gly-Asp peptide. In addition, the cell proliferation and migration of bovine LECs and N/N1003A were strongly inhibited by salmosin, whereas the proliferation of corneal endothelial cells was less affected. LEC migration and proliferation were also decreased by salmosin treatment in rabbit eyes without any toxic effect in the cornea, iris and retina. In this study, salmosin was shown to specifically inhibit LEC migration and proliferation in an animal model. Therefore, the authors suggest that further investigation may show salmosin to be a good candidate for inhibiting PCO development.

Role for alpha 6 integrin during lens development: Evidence for signaling through IGF-1R and ERK

We show that alpha 6 integrin function was required for normal lens cell differentiation by using an antisense construct to suppress alpha 6 integrin expression. To elucidate the mechanism by which this integrin functions in the regulation of the lens cell differentiation process, we determined the molecular composition of alpha 6 integrin signaling complexes at distinct stages of differentiation in vivo. Because both alpha 6 integrin and insulin-like growth factor-1 (IGF-1) have been implicated in signaling lens cell differentiation, we examined the possibility that they formed a signaling complex in the embryonic lens. Coprecipitation analysis revealed that alpha 6 integrin/IGF-1 receptor complexes were present and that their association was greatest in the equatorial zone, the region of the embryonic lens in which lens cells proliferate and then initiate their differentiation. These results provide in vivo support for the formation of integrin/growth factor receptor signaling complexes. We also found that extracellular signal-regulated kinase (ERK), a downstream effector of both integrin and growth factor receptor signaling pathways, was associated with the alpha 6 integrin signaling complexes in the embryonic lens. This result was supported by our findings that activated ERK, in addition to its nuclear location, localized to lens cell membranes in specific regions of cell-matrix and cell-cell contact. A connection between integrin ligand engagement and ERK activation was shown in vitro after lens cell attachment to laminin. These results demonstrate that alpha 6 integrin function is required for the early stages of lens cell differentiation most likely through its association with the IGF-1 receptor and the activation of ERK.