Proliferation in the posterior region of the lens of c-maf-/- mice

The tumor suppressor merlin is required for cell cycle exit, terminal differentiation, and cell polarity in the developing murine lens

PURPOSE. Neurofibromatosis type 2 (NF2) is an autosomal-dominant CNS tumor syndrome that affects 1:25,000 children and young adults. More than 50% of NF2 patients also develop posterior subcapsular cataracts (PSCs). The authors deleted Nf2 from the lens to determine its role in fiber cell differentiation. METHODS. Nf2 was conditionally deleted from murine lenses using the LeCre transgene. Standard histology and immunohistochemical and immunofluorescent methods were used to analyze lens morphology and markers of cell cycle progression, differentiation, and cell junctions in wild-type and knockout lenses from embryonic day 10.5 through postnatal day 3. RESULTS. Fiber cells lacking Nf2 did not fully exit the cell cycle and continued to express epithelial cell markers, such as FoxE3 and E-cadherin, despite expressing the fiber cell marker Prox1. Many fiber cells lost their elongated morphology. Markers of apical-basal polarity, such as ZO-1, were mislocalized along the lateral and basal membranes of fiber cells. The lens vesicle failed to separate from the surface ectoderm, and prospective lens and corneal epithelial cells formed a multilayered mass of cells at the surface of the eye. Herniation of this membrane caused the fiber mass to erupt through the cornea. CONCLUSIONS. Nf2 is required for complete fiber cell terminal differentiation, maintenance of cell polarity, and separation of lens vesicle from corneal epithelium. Defects identified in fiber cell differentiation may explain the formation of PSCs in patients with NF2. The lens provides an assay system to identify pathways critical for fiber cell differentiation and to test therapies for the tumors that occur in patients with NF2.

Pax6 is essential for lens fiber cell differentiation

The developing ocular lens provides an excellent model system with which to study the intrinsic and extrinsic cues governing cell differentiation. Although the transcription factors Pax6 and Sox2 have been shown to be essential for lens induction, their later roles during lens fiber differentiation remain largely unknown. Using Cre/loxP mutagenesis, we somatically inactivated Pax6 and Sox2 in the developing mouse lens during differentiation of the secondary lens fibers and explored the regulatory interactions of these two intrinsic factors with the canonical Wnt pathway. Analysis of the Pax6-deficient lenses revealed a requirement for Pax6 in cell cycle exit and differentiation into lens fiber cells. In addition, Pax6 disruption led to apoptosis of lens epithelial cells. We show that Pax6 regulates the Wnt antagonist Sfrp2 in the lens, and that Sox2 expression is upregulated in the Pax6-deficient lenses. However, our study demonstrates that the failure of differentiation following loss of Pax6 is independent of beta-catenin signaling or Sox2 activity. This study reveals that Pax6 is pivotal for initiation of the lens fiber differentiation program in the mammalian eye.

A new MAFia in cancer

Like JUN and FOS, the Maf transcription factors belong to the AP1 family. Besides their established role in human cancer--overexpression of the large Maf genes promotes the development of multiple myeloma--they can display tumour suppressor-like activity in specific cellular contexts, which is compatible with their physiological role in terminal differentiation. However, their oncogenic activity relies mostly on the acquisition of new biological functions relevant to cell transformation, the most striking characteristic of Maf oncoproteins being their ability to enhance pathological interactions between tumour cells and the stroma.

L-Maf regulates p27kip1 expression during chick lens fiber differentiation

Organ formation requires spatio-temporal proliferation and differentiation of precursor cells. During lens development, placodal cells in the posterior lens vesicle exit from the cell cycle and enter into the process of differentiation. Cyclin-dependent kinase inhibitors play critical roles in cell cycle exit and promote differentiation in several tissues. We have found that p27kip1 is expressed in the posterior lens cells that undergo differentiation to form the differentiated fiber cells. The transcription factor L-Maf is expressed in these cells earlier than p27kip1. From in ovo gain- or loss-of-function experiments, we have found that L-Maf can, respectively, induce or inhibit the expression of p27kip1 in lens cells. Promoter assays using the 5' upstream sequences of the human p27kip1 gene indicate that L-Maf can activate p27kip1 transcription through the basal regulatory region. We suggest that L-Maf regulates cell cycle exit of the posterior lens cells by activating p27kip1, and thus directs fiber cell differentiation during lens formation in chick.

Cell context reveals a dual role for Maf in oncogenesis

Maf b-Zip transcription factors are involved in both terminal differentiation and oncogenesis. To investigate this apparent contradiction, we used two different primary cell types and performed an extensive analysis of transformation parameters induced by Maf proteins. We show that MafA and c-Maf are potent oncogenes in chicken embryo fibroblasts, while MafB appears weaker. We also provide the first evidence that MafA can confer growth factor independence and promote cell division at low density. Moreover, using MafA as a model, we identified several parameters that are critical for Maf transforming activities. Indeed, MafA ability to induce anchorage-independent cell growth was sensitive to culture conditions. In addition, the transforming activity of MafA was dependent on its phosphorylation state, since mutation on Ser65 impaired its ability to induce growth at low density and anchorage-independent growth. We next examined transforming activity of large Maf proteins in embryonic neuroretina cells, where they are known to induce differentiation. Unlike v-Jun, MafA, MafB and c-Maf did not show oncogenic activity in these cells. Moreover, they counteracted transformation induced by constitutive activation of the Ras/Raf/MEK pathway. Taken together, our results show that Maf proteins could display antagonistic functions in oncogenesis depending on the cellular context, and support a dual role for Maf as both oncogenes and tumor suppressor-like proteins.

Immunolocalization of cyclin D1 in the developing lens of c-maf -/- mice

The maf gene encodes a transcription factor protein containing a typical basic/leucine zipper domain structure, a motif for protein dimerization and DNA binding. It has been demonstrated that maf family genes have important roles in embryonic development and cellular differentiation. In this study, localization of cyclin D1, one of the cell cycle-related molecules, was examined immunohistochemically in developing lens cells of c-maf knockout (-/-) mice. At embryonic day 14 in wild-type mice, lens cells consisted of round epithelial cells in a single layer and regularly arranged elongated lens cells, indicating primary lens fiber cells. Cyclin D1-positive nuclei were observed in the lens epithelial cells, whereas cyclin D1 was not detected in the primary lens fiber cells. In c-maf -/- mice, a variety of round epithelial cells were located in the anterior and posterior lens. Many cyclin D1-positive nuclei were observed in lens epithelial cells as well as posterior lens cells. These results are consistent with c-maf playing a role in the regulation of cyclin D1 in developing lens cells.

Disappearance of p27(KIP1) and increase in proliferation of the lens cells after extraction of most of the fiber cells of the lens

PURPOSE

Proliferation of the lens epithelial cells is involved in the fibrotic changes of lens capsules after cataract extraction. However, the mechanisms of the proliferation of the lens epithelial cells are largely unknown. The purpose of this study was to examine the correlation between the expression of p27(KIP1) and cell proliferation in the lens cells after the extraction of lens fiber cells.

METHODS

At embryonic days (E) 14 and 18, the C57Bl6 mice were anesthetized and the embryos were surgically removed. The eyes were dissected from these embryos and also from mice 12 weeks after birth. The 12-week-old mice were anesthetized, and then the lens fiber cells were extracted. Normal eyes at E14 and 18 and eyes fixed at 15 min and 48 hr after the extraction of the lens fiber cells were analyzed using immunohistochemistry with anti-p27(KIP1), p57(KIP2), and phosphorylated extracellular signal-regulated kinase (phospho-ERK) 1/2 antibodies, and cells in the S phase of the cell cycle were also examined using anti-bromodeoxyuridine (BrdU) antibody.

RESULTS

p27(KIP1) and p57(KIP2)-positive cells were present in the equatorial region of E14 mice. At E18, many lens fiber cells showed nuclear immunoreactivity for p27(KIP1), whereas a small number of cells were positive for p57(KIP2) in the equatorial region. At 12 weeks of age, all nuclei of the lens epithelial cells as well as lens fiber cells showed nuclear immunoreactivity for p27(KIP1). In contrast, p57(KIP2) and phospho-ERK1/2 were not expressed in the lens cells. At 15 min after the extraction of lens fiber cells, phospho-ERK1/2 as well as p27(KIP1) were detected in the lens cells. At 48 hr after the extraction of the lens fiber cells, a few p27(KIP1)-positive nuclei were observed in the equatorial region of the lens capsule. In contrast, many lens cells showed nuclear immunoreactivity for BrdU.

CONCLUSIONS

These findings suggest that degradation of p27(KIP1) mediated by phosphorylation of ERK 1/2 is correlated with proliferation of the epithelial cells after the extraction of the lens fiber cells.

DeltaFosB expression and cataract

DeltaFosB is a truncated form of a FosB transcription factor, which is created by alternative splicing. Previous work has shown that transgenic mice expressing DeltaFosB both in the retina and in the lens developed a posterior subcapsular cataract resulting from the misalignment of the fibres in the suture region. In the previous study, it was not clear whether DeltaFosB expression was required in both tissues to produce the cataract. Therefore, DeltaFosB expression targeted to either the lens or the retina was undertaken in order to clarify the contribution of each tissue to cataract development. For lens expression, the R2betaB1DeltaFosB construct was synthesized (R2, an enhancer; betaB1, a chicken betaB1 crystallin gene promoter fragment). For the retina, RhoDeltaFosB was prepared. As a promoter, the bovine rhodopsin upstream region was used. DeltaFosB expression in heterozygote animals was monitored by Western blotting. Cataract development in heterozygotes of R2betaB1DeltaFosB transgenics and in both heterozygotes and homozygotes of RhoDeltaFosB transgenics was followed by slitlamp examination. The transgenic mice prepared with RhoDeltaFosB expressed DeltaFosB only in the retina and showed no sign of lens opacity. One line of the R2betaB1DeltaFosB transgenic was found to have expression only in the lens and developed posterior subcapsular cataract. We concluded that retinal expression of DeltaFosB is not sufficient to cause cataract while expression exclusively in the lens produces posterior subcapsular cataract.

Expression of maf-B mRNA in the epithelium around the eyelid closure of the mouse eye at embryonic day 18

Maf encodes a transcription factor protein containing a typical basic leucine zipper domain structure, a motif for protein dimerization and DNA binding. We examined the expression of maf-B mRNA in the epithelium around the eyelid closure. Expression of maf-B mRNA was examined in C57Bl6 mice at the embryonic stages in 12.5 days of gestation (E12.5) and E18 using in situ hybridization with 35S-labeled antisense riboprobes. In embryos studied 12.5 days postconception, a message specific for maf-B was not detected around the developing eyelid. In contrast, maf-B was strongly expressed in the epithelium of the eyelid closure at E18. Expression of maf-B was strongly noted in the suprabasal differentiating cells derived from the basal layer of the conjunctiva and epidermis. In contrast, basal cells in the eyelid closure and in the epidermis, as well as keratinizing cells, did not express maf-B. These data indicate that maf-B mRNA is expressed during development of the eyelid closure.

Activation of nuclear factor-kappa B in the conjunctiva with the epithelial scraping of the mouse cornea and human epidemic keratoconjunctivitis

AIM

To examine the expression of p65, one of nuclear factor-kappa B (NF-kappa B), in the conjunctival epithelium of the C57Bl6 mouse and a patient with epidemic keratoconjunctivitis (EKC).

METHODS

Normal and epithelial scraped cornea obtained 6 hours after the injury were processed for paraffin section. Samples of a normal and an EKC conjunctival epithelium were obtained using impression cytology. Both samples were analysed by immunocytochemistry using anti-p65 antibody.

RESULTS

Immunocytochemistry with the anti-NF-kappa B p65 antibody revealed that p65 was localised in the cytoplasm of the conjunctival epithelium in the C57Bl6 mouse without the treatment. Six hours after the scraping of the cornea, p65 protein was expressed in the nuclei of the conjunctival epithelium. p65 was localised in the cytoplasm of the conjunctival epithelium in the human normal eye. p65 protein was expressed in the nuclei of the conjunctival epithelial cells in the EKC patient.

CONCLUSION

These findings suggest that NF-kappa B was activated in the conjunctiva in the epithelial scraping of the mouse cornea and in human EKC.

Differential regulation of components of the ubiquitin-proteasome pathway during lens cell differentiation

PURPOSE

To investigate the role for the ubiquitin-proteasome pathway in controlling lens cell proliferation and differentiation and the regulation of the ubiquitin conjugation machinery during the differentiation process.

METHODS

bFGF-induced lens cell proliferation and differentiation was monitored in rat lens epithelial explants by bromodeoxyuridine (BrdU) incorporation and expression of crystallins and other differentiation markers. Levels of typical substrates for the ubiquitin-proteasome pathway, p21(WAF) and p27(Kip), were monitored during the differentiation process, as were levels and activities of the enzymes involved in ubiquitin conjugation.

RESULTS

Explants treated with bFGF initially underwent enhanced proliferation as indicated by BrdU incorporation. Then they withdrew from the cell cycle as indicated by diminished BrdU incorporation and accumulation of p21(WAF) and p27(Kip). bFGF-induced cell proliferation was prohibited or delayed by proteasome inhibitors. Lens epithelial explants treated with bFGF for 7 days displayed characteristics of lens fibers, including expression of large quantities of crystallins. Whereas levels of E1 remained constant during the differentiation process, the levels of ubiquitin-conjugating enzyme (Ubc)-1 increased approximately twofold, and the thiol ester form of Ubc1 increased approximately threefold on 7 days of bFGF treatment. Levels of Ubc2 increased moderately on bFGF treatment, and most of the Ubc2 was found in the thiol ester form. Although levels of total Ubc3 and -7 remained unchanged, the proportions of Ubc3 and -7 in the thiol ester form were significantly higher in the bFGF-treated explants. Levels of Ubc4/5 and -9 also increased significantly on treatment with bFGF, and more than 90% of Ubc9 was found in the thiol ester form in the bFGF-treated explants. In contrast, levels of Cul1, the backbone of the SCF type of E3s, decreased 50% to 70% in bFGF-treated explants.

CONCLUSIONS

The data show that proteolysis through the ubiquitin-proteasome pathway is required for bFGF-induced lens cell proliferation and differentiation. Various components of the ubiquitin-proteasome pathway are differentially regulated during lens cell differentiation. The downregulation of Cul1 appears to contribute to the accumulation of p21(WAF) and p27(Kip), which play an important role in establishing a differentiated phenotype.

Absence of transcription factor c-maf causes abnormal terminal differentiation of hypertrophic chondrocytes during endochondral bone development

In this study, we report that the transcription factor c-Maf is required for normal chondrocyte differentiation during endochondral bone development. c-maf is expressed in hypertrophic chondrocytes during fetal development (E14.5-E18.5), with maximal expression in the tibia occurring at E15.5 and E16.5, in terminally differentiated chondrocytes. In c-maf-null mice, fetal bone length is decreased approximately 10%, and hypertrophic chondrocyte differentiation is perturbed. There is an initial decrease in the number of mature hypertrophic chondrocytes at E15.5 in c-maf-null tibiae, with decreased expression domains of collagen X and osteopontin, markers of hypertrophic and terminal hypertrophic chondrocytes, respectively. By E16.5, there is an expanded domain of late hypertrophic, osteopontin-positive chondrocytes in the c-maf-/-. This accumulation of hypertrophic chondrocytes persists and is still observed at 4 weeks of age. These data suggest that c-Maf facilitates the initial chondrocyte terminal differentiation and influences the disappearance of hypertrophic chondrocytes. BrdU and TUNEL analyses show normal proliferation rate and apoptosis in the c-maf-null. There is a specific decrease in MMP-13 expression at E15.5 in the c-maf-null. MMP-13 is known to be regulated by AP-1 and may also be a target of c-Maf. Thus, cartilage is a novel system in which c-Maf acts during development, where c-Maf is required for normal chondrocyte differentiation.