Meibomian gland stem/progenitor cells: The hunt for gland renewal

Meibomian glands (MGs) secrete lipid (meibum) onto the ocular surface to form the outermost layer of the tear film. Proper meibum secretion is essential for stabilizing the tear film, reducing aqueous tear evaporation, and maintaining the homeostasis of the ocular surface. Atrophy of MG as occurs with aging, leads to reduction of meibum secretion, loss of ocular surface homeostasis and evaporative dry eye disease (EDED). Since MGs are holocrine glands, secretion of meibum requires continuous self-renewal of lipid-secreting acinar meibocytes by stem/progenitor cells, whose proliferative potential is dramatically reduced with age leading to MG atrophy and an age-related meibomian gland dysfunction (ARMGD). Understanding the cellular and molecular mechanisms regulating meibocyte stem/progenitor cell maintenance and renewal may provide novel approaches to regenerating MG and treating EDED. Towards that end, recent label retaining cell and lineage-tracing experiments as well as knock-out transgenic mouse studies have begun to identify the location and identities of meibocyte progenitor cells and potential growth and transcription factors that may regulate meibocyte renewal. In addition, recent reports have shown that ARMGD may be reversed by novel therapeutics in mice. Herein, we discuss our current understanding of meibocyte stem/progenitor cells and the hunt for gland renewal.

Spontaneous acinar and ductal regrowth after meibomian gland atrophy induced by deletion of FGFR2 in a mouse model

PURPOSE

We have demonstrated that deletion of fibroblast growth factor receptor 2 gene (Fgfr2) leads to Meibomian gland (MG) atrophy in an inducible conditional knockout mouse model, referred as Fgfr2^CKO. Herein, we investigated whether MG spontaneously recovers after atrophy in this model.

METHODS

Two months old Fgfr2^CKO mice were injected peritoneally once or twice of doxycycline (Dox) at 80 μg/gm of body weight to induce MG atrophy of various severities via Fgfr2 deletion. Recovery of acinar and ductal tissues was monitored by meibography, lipid staining and immunofluorescence against keratin-6a in MG whole-mount. Biomarkers for acinar and ductal differentiation and proliferation were also examined by immunostaining.

RESULTS

Single Dox injection in Fgfr2^CKO mice caused severe acinar and moderate ductal atrophy. Severe ductal shortening or loss occurred after second Dox injection, presumably related to the reported slower cycling of the ductal epithelia. Spontaneous acinar regrowth after atrophy was observed over a period of 60 days in both injection regimens. However, less robust acinar recovery was associated with more disrupted ductal structures in twice injected Fgfr2^CKO mice.

CONCLUSIONS

Our current findings further substantiate the role of FGFR2 in MG homeostasis, and suggest that FGFR2-signaling may provide a potential strategy for regenerating acini from age-related MG dysfunction in humans. Our data demonstrated that spontaneous MG recovery depends on the extent of ductal atrophy, suggesting that ductal epithelia may provide the progenitor cells for acinar regeneration. Nonetheless, the role of ductal tissue as the source of acinar progenitors awaits further investigation.

Histopathology and selective biomarker expression in human meibomian glands

Background/aims

Meibomian gland dysfunction (MGD) is the most common form of evaporative dry eye disease, but its pathogenesis is poorly understood. This study examined the histopathological features of meibomian gland (MG) tissue from cadaver donors to identify potential pathogenic processes that underlie MGD in humans.

Methods

Histological analyses was performed on the MGs in the tarsal plates dissected from four cadaver donors, two young and two old adults, including a 36-year-old female (36F) and three males aged 30, 63 and 64 years (30M, 63M and 64M).

Results

The MGs of 36F displayed normal anatomy and structure, whereas the MGs of 30M showed severe ductal obstruction with mild distortion. The obstruction was caused by increased cytokeratin levels in association with hyperproliferation, but not hyperkeratinisation. In two older males, moderate to severe MG atrophy was noted. Cell proliferation was significantly reduced in the MG acini of the two older donors as measured by Ki67 labelling index (6.0%±3.4% and 7.9%±2.8% in 63M and 64M, respectively) when compared with that of the two younger donors (23.2%±5.5% and 16.9%±4.8% in 30M and 36F, respectively) (p<0.001). The expression patterns of meibocyte differentiation biomarkers were similar in the older and younger donors.

Conclusion

Our histopathological study, based on a small sample size, suggests potentially distinct pathogenic mechanisms in MGD. In the young male adult, hyperproliferation and aberrant differentiation of the central ductal epithelia may lead to the obstruction by overproduced cytokeratins. In contrast, in older adults, decreased cell proliferation in acinar basal epithelia could be a contributing factor leading to MG glandular atrophy.

Fibroblast Growth Factor Receptor 2 (FGFR2) Is Required for Meibomian Gland Homeostasis in the Adult Mouse

Purpose

Little is known about the signaling mechanisms controlling meibomian gland (MG) homeostasis and the pathogenic processes leading to MG atrophy and dysfunction in dry eye disease (DED). We investigated the role of fibroblast growth factor receptor 2 (FGFR2) in the MG homeostasis of adult mice.

Methods

A triple transgenic mouse strain (Krt14-rtTA; tetO-Cre; Fgfr2flox/flox), referred to as Fgfr2CKO mice, was generated in which the Fgfr2 gene is ablated by Cre recombinase in keratin 14 (Krt14)-expressing epithelial cells on doxycycline (Dox) induction. FGFR2 expression in normal human and mouse MGs was evaluated by immunohistochemistry. Pathologic MG changes in transgenic mice with conditional deletion of FGFR2 were examined by lipid staining, histology, and immunostaining.

Results

FGFR2 was highly expressed in normal human MGs and adult mouse MGs. Two-month-old Fgfr2CKO mice fed Dox-containing chow for 2 weeks developed severe MG atrophy. MG acinar atrophy in the Fgfr2CKO mice was associated with reduced lipid (meibum) production and the development of clinical findings similar to those in humans with evaporative DED related to MG dysfunction (MGD). Immunohistochemical analyses showed that FGFR2 deletion severely affected proliferation and differentiation of MG acinar cells but affected MG ductal cells to a lesser extent.

Conclusions

FGFR2 deletion results in significant MG acinar atrophy and clinical manifestations of MGD in Fgfr2CKO mice, suggesting that MG homeostasis is FGFR2 dependent. The Fgfr2CKO mice with inducible MG atrophy can serve as a valuable animal model for investigating the pathogenesis of MGD and developing novel therapeutic strategies for MGD-related DED.

Regulation of c-Maf and αA-Crystallin in Ocular Lens by Fibroblast Growth Factor Signaling

Fibroblast growth factor (FGF) signaling regulates a multitude of cellular processes, including cell proliferation, survival, migration, and differentiation. In the vertebrate lens, FGF signaling regulates fiber cell differentiation characterized by high expression of crystallin proteins. However, a direct link between FGF signaling and crystallin gene transcriptional machinery remains to be established. Previously, we have shown that the bZIP proto-oncogene c-Maf regulates expression of αA-crystallin (Cryaa) through binding to its promoter and distal enhancer, DCR1, both activated by FGF2 in cell culture. Herein, we identified and characterized a novel FGF2-responsive region in the c-Maf promoter (-272/-70, FRE). Both c-Maf and Cryaa regulatory regions contain arrays of AP-1 and Ets-binding sites. Chromatin immunoprecipitation (ChIP) assays established binding of c-Jun (an AP-1 factor) and Etv5/ERM (an Ets factor) to these regions in lens chromatin. Analysis of temporal and spatial expression of c-Jun, phospho-c-Jun, and Etv5/ERM in wild type and ERK1/2 deficient lenses supports their roles as nuclear effectors of FGF signaling in mouse embryonic lens. Collectively, these studies show that FGF signaling up-regulates expression of αA-crystallin both directly and indirectly via up-regulation of c-Maf. These molecular mechanisms are applicable for other crystallins and genes highly expressed in terminally differentiated lens fibers.

Unfolded-protein response-associated stabilization of p27(Cdkn1b) interferes with lens fiber cell denucleation, leading to cataract

Failure of lens fiber cell denucleation (LFCD) is associated with congenital cataracts, but the pathobiology awaits elucidation. Recent work has suggested that mechanisms that direct the unidirectional process of LFCD are analogous to the cyclic processes associated with mitosis. We found that lens-specific mutations that elicit an unfolded-protein response (UPR) in vivo accumulate p27(Cdkn1b), show cyclin-dependent kinase (Cdk)-1 inhibition, retain their LFC nuclei, and are cataractous. Although a UPR was not detected in lenses expressing K6W-Ub, they also accumulated p27 and showed failed LFCD. Induction of a UPR in human lens epithelial cells (HLECs) also induced accumulation of p27 associated with decreased levels of S-phase kinase-associated protein (Skp)-2, a ubiquitin ligase that regulates mitosis. These cells also showed decreased lamin A/C phosphorylation and metaphase arrest. The suppression of lamin A/C phosphorylation and metaphase transition induced by the UPR was rescued by knockdown of p27. Taken together, these data indicate that accumulation of p27, whether related to the UPR or not, prevents the phosphorylation of lamin A/C and LFCD in maturing LFCs in vivo, as well as in dividing HLECs. The former leads to cataract and the latter to metaphase arrest. These results suggest that accumulation of p27 is a common mechanism underlying retention of LFC nuclei.

Fibroblast growth factor receptor 2 (FGFR2) is required for corneal epithelial cell proliferation and differentiation during embryonic development

Fibroblast growth factors (FGFs) play important roles in many aspects of embryonic development. During eye development, the lens and corneal epithelium are derived from the same surface ectodermal tissue. FGF receptor (FGFR)-signaling is essential for lens cell differentiation and survival, but its role in corneal development has not been fully investigated. In this study, we examined the corneal defects in Fgfr2 conditional knockout mice in which Cre expression is activated at lens induction stage by Pax6 P0 promoter. The cornea in LeCre, Fgfr2^loxP/loxP mice (referred as Fgfr2^CKO) was analyzed to assess changes in cell proliferation, differentiation and survival. We found that Fgfr2^CKO cornea was much thinner in epithelial and stromal layer when compared to WT cornea. At embryonic day 12.5–13.5 (E12.5–13.5) shortly after the lens vesicle detaches from the overlying surface ectoderm, cell proliferation (judged by labeling indices of Ki-67, BrdU and phospho-histone H3) was significantly reduced in corneal epithelium in Fgfr2^CKO mice. At later stage, cell differentiation markers for corneal epithelium and underlying stromal mesenchyme, keratin-12 and keratocan respectively, were not expressed in Fgfr2^CKO cornea. Furthermore, Pax6, a transcription factor essential for eye development, was not present in the Fgfr2^CKO mutant corneal epithelial at E16.5 but was expressed normally at E12.5, suggesting that FGFR2-signaling is required for maintaining Pax6 expression in this tissue. Interestingly, the role of FGFR2 in corneal epithelial development is independent of ERK1/2-signaling. In contrast to the lens, FGFR2 is not required for cell survival in cornea. This study demonstrates for the first time that FGFR2 plays an essential role in controlling cell proliferation and differentiation, and maintaining Pax6 levels in corneal epithelium via ERK-independent pathways during embryonic development.

MAPK1 is required for establishing the pattern of cell proliferation and for cell survival during lens development

The mitogen-activated protein kinases (MAPKs; also known as ERKs) are key intracellular signaling molecules that are ubiquitously expressed in tissues and were assumed to be functionally equivalent. Here, we use the mouse lens as a model system to investigate whether MAPK1 plays a specific role during development. MAPK3 is known to be dispensable for lens development. We demonstrate that, although MAPK1 is uniformly expressed in the lens epithelium, its deletion significantly reduces cell proliferation in the peripheral region, an area referred to as the lens germinative zone in which most active cell division occurs during normal lens development. By contrast, cell proliferation in the central region is minimally affected by MAPK1 deletion. Cell cycle regulators, including cyclin D1 and survivin, are downregulated in the germinative zone of the MAPK1-deficient lens. Interestingly, loss of MAPK1 subsequently induces upregulation of phosphorylated MAPK3 (pMAPK3) levels in the lens epithelium; however, this increase in pMAPK3 is not sufficient to restore cell proliferation in the germinative zone. Additionally, MAPK1 plays an essential role in epithelial cell survival but is dispensable for fiber cell differentiation during lens development. Our data indicate that MAPK1/3 control cell proliferation in the lens epithelium in a spatially defined manner; MAPK1 plays a unique role in establishing the highly mitotic zone in the peripheral region, whereas the two MAPKs share a redundant role in controlling cell proliferation in the central region of the lens epithelium.

Frs2α enhances fibroblast growth factor-mediated survival and differentiation in lens development

Most growth factor receptor tyrosine kinases (RTKs) signal through similar intracellular pathways, but they often have divergent biological effects. Therefore, elucidating the mechanism of channeling the intracellular effect of RTK stimulation to facilitate specific biological responses represents a fundamental biological challenge. Lens epithelial cells express numerous RTKs with the ability to initiate the phosphorylation (activation) of Erk1/2 and PI3-K/Akt signaling. However, only Fgfr stimulation leads to lens fiber cell differentiation in the developing mammalian embryo. Additionally, within the lens, only Fgfrs activate the signal transduction molecule Frs2α. Loss of Frs2α in the lens significantly increases apoptosis and decreases phosphorylation of both Erk1/2 and Akt. Also, Frs2α deficiency decreases the expression of several proteins characteristic of lens fiber cell differentiation, including Prox1, p57(KIP2), aquaporin 0 and β-crystallins. Although not normally expressed in the lens, the RTK TrkC phosphorylates Frs2α in response to binding the ligand NT3. Transgenic lens epithelial cells expressing both TrkC and NT3 exhibit several features characteristic of lens fiber cells. These include elongation, increased Erk1/2 and Akt phosphorylation, and the expression of β-crystallins. All these characteristics of NT3-TrkC transgenic lens epithelial cells depend on Frs2α. Therefore, tyrosine phosphorylation of Frs2α mediates Fgfr-dependent lens cell survival and provides a mechanistic basis for the unique fiber-differentiating capacity of Fgfs on mammalian lens epithelial cells.

Activation of unfolded protein response in transgenic mouse lenses

PURPOSE

Overloading of unfolded or misfolded proteins in the endoplasmic reticulum (ER) can cause ER stress and activate the unfolded protein response (UPR) in the cell. The authors tested whether transgene overexpression in the mouse lens would activate the UPR.

METHODS

Transgenic mice expressing proteins that either enter the ER secretory pathway or are synthesized in cytosol were selected. Activation of the UPR was assessed by determining the expression levels of the ER chaperone protein BiP, the spliced form of X-box binding protein-1 (Xbp-1) mRNA, and the transcription factor CHOP. Changes in the ubiquitin-proteasome system in the mouse lens were detected by ubiquitin immunofluorescence.

RESULTS

BiP expression was upregulated in the fiber cells of transgenic mouse lenses expressing platelet-derived growth factor-A (PDGF-A), dominant-negative fibroblast growth factor receptor (DN-FGFR), or DN-Sprouty2 (DN-Spy2). BiP upregulation occurred around embryonic day 16.5, primarily in the fiber cells adjacent to the organelle free zone. Fiber cell differentiation was disrupted in the PDGF-A and DN-Spry2 lenses, whereas the fiber cells were degenerating in the DN-FGFR lens. High levels of UPR activation and ubiquitin-labeled protein aggregates were found in the DN-FGFR lens, indicating inefficient disposal of unfolded/misfolded proteins in the fiber cells.

CONCLUSIONS

This study implies that overexpression of some transgenes in the lens can induce ER or overall cell stress in fiber cells, resulting in the activation of UPR signaling pathways. Therefore, investigators should assess the levels of UPR activation when they analyze the downstream effects of transgene expression in the lens.

Induction of corneal myofibroblasts by lens-derived transforming growth factor beta1 (TGFbeta1): a transgenic mouse model

PURPOSE

Transforming growth factor beta (TGFbeta) is an important cytokine in corneal development and wound healing. Transgenic mice that express an active form of human TGFbeta1 driven by a lens-specific promoter were used in the current study to determine the biological effects of lens-derived TGFbeta1 on postnatal corneal development and homeostasis.

METHODS

The postnatal corneal changes in the TGFbeta1 transgenic mice were examined by fluorescein labeling and histology. Epithelial/endothelial-to-mesenchymal transition (E/EnMT) in the transgenic mouse cornea was demonstrated by immunostaining for alpha-smooth muscle actin (alpha-SMA) and cadherin-11. Expression of E- and N-cadherin in the corneal epithelial and endothelial cells, respectively, was analyzed by in situ hybridization.

RESULTS

Among the established TGFbeta1 transgenic lines, mice from line OVE853 and OVE917 had normal-sized eyeballs but developed a corneal haze after eyelid opening. Histological examination showed that prenatal corneal development appeared to be normal. However, after postnatal day 7 (P7), the corneal endothelial cells in transgenic line OVE853 began to lose normal cell-cell contact and basement membrane structure. The endothelial layer was eventually absent in the inner surface of the transgenic mouse cornea. The morphological changes in the cornea correlated with abnormal expression of alpha-SMA, a molecular marker of EMT, and stress fiber formation in myofibroblast-like cells, which initially appeared in the corneal endothelial layer and subsequently in the corneal epithelial and stromal layers. The E/EnMT in the transgenic mouse cornea was further demonstrated by loss of E- and N-cadherin expression in the corneal epithelial and endothelial cells, respectively, and meanwhile increasing expression of cadherin-11 in both corneal epithelium and stroma.

CONCLUSIONS

Elevated levels of active TGFbeta1 in the anterior chamber can lead to myofibroblast formation in the corneal endothelial layer and subsequently in the corneal epithelial and stromal layers. Our data suggest that the levels of biologically active TGFbeta in the aqueous humor must be under tight control to maintain corneal homeostasis. TGFbeta1 is the major cytokine during wound healing. Therefore, our findings also suggest a potential mechanism to explain the loss of corneal endothelial barrier and corneal opacification after intraocular surgery or trauma.

Rho GDP dissociation inhibitor-mediated disruption of Rho GTPase activity impairs lens fiber cell migration, elongation and survival

To explore the role of the Rho GTPases in lens morphogenesis, we overexpressed bovine Rho GDP dissociation inhibitor (Rho GDI alpha), which serves as a negative regulator of Rho, Rac and Cdc42 GTPase activity, in a lens-specific manner in transgenic mice. This was achieved using a chimeric promoter of delta-crystallin enhancer and alpha A-crystallin, which is active at embryonic day 12. Several individual transgenic (Tg) lines were obtained, and exhibited ocular specific phenotype comprised of microphthalmic eyes with lens opacity. The overexpression of bovine Rho GDI alpha disrupted membrane translocation of Rho, Rac and Cdc42 GTPases in Tg lenses. Transgenic lenses also revealed abnormalities in the migration pattern, elongation and organization of lens fibers. These changes appeared to be associated with impaired organization of the actin cytoskeleton and cell-cell adhesions. At E14.5, the size of the Rho GDI alpha Tg lenses was larger compared to wild type (WT) and the central lens epithelium and differentiating fibers exhibited an abnormal increase of bromo-deoxy-uridine incorporation. Postnatal Tg eyes, however, were much smaller in size compared to WT eyes, revealing increased apoptosis in the disrupted lens fibers. Taken together, these data demonstrate a critical role for Rho GTPase-dependent signaling pathways in processes underlying morphogenesis, fiber cell migration, elongation and survival in the developing lens.

Elevated insulin signaling disrupts the growth and differentiation pattern of the mouse lens

PURPOSE

Insulin and insulin-like growth factors (IGFs) are putative regulators of cell proliferation and differentiation during lens development. Transgenic mice that overexpress IGF-1 in the lens have been previously described. To further understand the ocular functions of this growth factor family, the in vivo effects of insulin expression on lens development were investigated using transgenic mice.

METHODS

Expression of insulin receptor (IR) and IGF-1 receptor (IGF-1R) in mouse lens was examined by reverse-transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. Transgenic mice that overexpress insulin in the lens were generated using two different promoters: a fiber-cell specific alphaA-crystallin (alphaA) promoter and a modified alphaA-promoter linked to the chicken delta1-crystallin enhancer (called the deltaenalphaA promoter). The deltaenalphaA promoter is active in both lens epithelial and fiber cells. The lens phenotypes were analyzed by histology and immunohistochemistry. Protein expression was examined by western blotting.

RESULTS

Normal mouse lenses express both the insulin receptor (IR) and the IGF-1 receptor (IGF-1R), and their expression is highest at the lens periphery where the germinative and transitional zones are located. In transgenic mice, insulin expression in the lens induced cataract formation. The severity of the cataracts reflected the level of transgene expression, independent of the type of promoter used. In severely affected families, the spherical shape of the lens was altered and the lenses were smaller than normal. Histological analysis showed no evidence of premature differentiation of the anterior epithelial cells. In contrast to the IGF-1 mice, insulin transgenic mice exhibited an anterior shift in the location of the germinative and transitional zones, leading to a reduction of the lens epithelial compartment. Additional alterations included expansion of the lens transitional zone, variable nuclear positioning in the lens bow region, and inhibition of fiber cell denucleation and terminal differentiation.

CONCLUSIONS

Elevated intraocular insulin does not enhance proliferation nor induce differentiation of mouse lens epithelial cells. Since an increase in IGF-1 causes a posterior shift of the lens geminative and transitional zones, while an increase in insulin causes an anterior shift of these zones, our results suggest that these two growth factors may work together to control the location of this structural domain during normal lens development. Our data also suggest that increased insulin-signaling activity in the lens can antagonize the endogenous signals that are responsible for fiber cell maturation and terminal differentiation.

Type I collagen glomerulopathy: postnatal collagen deposition follows glomerular maturation

In chronic renal disease, the progressive accumulation of collagen and other extracellular matrix proteins in the mesangium results in fibrosis, glomerulosclerosis, and eventual renal failure. Mice deficient in proalpha2(I) collagen are not only a model of osteogenesis imperfecta but also accumulate fibrillar homotrimeric type I collagen in the mesangium. This accumulation spreads to the subendothelial space in the peripheral capillary loops. Picosirius red staining of kidney sections demonstrates that in comparison to wild-type mice, Col1a2-deficient homozygous and heterozygous mice exhibit abnormal glomerular collagen deposition in a gene dosage-dependent manner. The glomerulopathy initiates during the first postnatal week, appears progressive following the pattern of glomerular maturation and results in albuminuria in severely affected animals. In situ hybridization revealed no gross differences in steady-state proalpha1(I) and proalpha2(I) collagen mRNA levels among the three genotypes. Quantitative reverse transcriptase-polymerase chain reaction, however, using whole kidney sections showed a twofold increase in steady-state proalpha1(I) collagen mRNA in 1-month homozygous Col1a2-deficient animals compared with wild-type and heterozygous animals. We suggest that glomerular collagen deposition seen in the osteogenesis imperfecta model mice is, in part, owing to pretranslational mechanisms and may represent an over compensation of wound healing.

Vitamin C mediates chemical aging of lens crystallins by the Maillard reaction in a humanized mouse model

Senile cataracts are associated with progressive oxidation, fragmentation, cross-linking, insolubilization, and yellow pigmentation of lens crystallins. We hypothesized that the Maillard reaction, which leads browning and aroma development during the baking of foods, would occur between the lens proteins and the highly reactive oxidation products of vitamin C. To test this hypothesis, we engineered a mouse that selectively overexpresses the human vitamin C transporter SVCT2 in the lens. Consequently, lenticular levels of vitamin C and its oxidation products were 5- to 15-fold elevated, resulting in a highly compressed aging process and accelerated formation of several protein-bound advanced Maillard reaction products identical with those of aging human lens proteins. These data strongly implicate vitamin C in lens crystallin aging and may serve as a model for protein aging in other tissues particularly rich in vitamin C, such as the hippocampal neurons and the adrenal gland. The hSVCT2 mouse is expected to facilitate the search for drugs that inhibit damage by vitamin C oxidation products.

Ras signaling is essential for lens cell proliferation and lens growth during development

The vertebrate ocular lens is a simple and continuously growing tissue. Growth factor-mediated receptor tyrosine kinases (RTKs) are believed to be required for lens cell proliferation, differentiation and survival. The signaling pathways downstream of the RTKs remain to be elucidated. Here, we demonstrate the important role of Ras in lens development by expressing a dominant-negative form of Ras (dn-Ras) in the lens of transgenic mice. We show that lens in the transgenic mice was smaller and lens growth was severely inhibited as compared to the wild-type lens. However, the lens shape, polarity and transparency appeared normal in the transgenic mice. Further analysis showed that cell proliferation is inhibited in the dn-Ras lens. For example, the percentage of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in epithelial layer was about 2- to 3-fold lower in the transgenic lens than in the wild-type lens, implying that Ras activity is required for normal cell proliferation during lens development. We also found a small number of apoptotic cells in both epithelial and fiber compartment of the transgenic lens, suggesting that Ras also plays a role in cell survival. Interestingly, although there was a delay in primary fiber cell differentiation, secondary fiber cell differentiation was not significantly affected in the transgenic mice. For example, the expression of beta- and gamma-crystallins, the marker proteins for fiber differentiation, was not changed in the transgenic mice. Biochemical analysis indicated that ERK activity, but not Akt activity, was significantly reduced in the dn-Ras transgenic lenses. Overall, our data imply that the RTK-Ras-ERK signaling pathway is essential for cell proliferation and, to a lesser extent, for cell survival, but not for crystallin gene expression during fiber differentiation. Thus, some of the fiber differentiation processes are likely mediated by RTK-dependent but Ras-independent pathways.

Activated Ras induces lens epithelial cell hyperplasia but not premature differentiation

Growth factor signaling is implicated in the regulation of lens cell proliferation and differentiation during development. Activation of growth factor receptor tyrosine kinases is known to activate Ras proteins, small GTP-binding proteins that function as part of the signal transduction machinery. In the present study, we examined which classical Ras genes are expressed in lens cells during normal development and whether expression of an activated version of Ras is sufficient to induce either lens cell proliferation or fiber cell differentiation in transgenic mice. In situ hybridization showed H-Ras, K-Ras and N-Ras are ubiquitously expressed in all cells of the embryonic (E13.5) eye, with N-Ras showing the highest level of expression. The expression level of N-Ras decreases during later stages of embryonic development, and is nearly undetected in postnatal day 21 lenses. To generate transgenic mice, a constitutively active H-Ras mutant was linked to a chimeric regulatory element containing the mouse alphaA-crystallin promoter fused to the chick delta1-crystallin lens enhancer element. In the lenses of the transgenic mice, the transgene was expressed in both lens epithelial and fiber cells. Expression of activated Ras was sufficient to stimulate lens cell proliferation but not differentiation, implying that alternative or additional signal transduction pathways are required to induce fiber cell differentiation.

Chick delta1-crystallin enhancer influences mouse alphaA-crystallin promoter activity in transgenic mice

PURPOSE

Both the -366/+43 and the -282/+43 mouse alphaA-crystallin (or alphaA) promoters have been effective at driving transgene expression in lens fiber cells, but not in lens epithelium. Because the chick delta1-crystallin gene is expressed in lens epithelial cells, an enhancer was borrowed from this gene and linked to the alphaA promoter. This heterogenic enhancer/promoter construct was tested in transgenic mice to see whether it was active in both lens epithelium and fiber cells while retaining lens specificity.

METHODS

The third intron of the chick delta1-crystallin gene, which contains a lens enhancer element, was added to the 5' end of the mouse alphaA promoter. We refer to this chimeric regulatory element as the deltaenalphaA promoter. To test its activity, we inserted coding sequences for five different genes. Transgenic mice were generated by pronuclear microinjection. Transgene expression patterns were analyzed by either X-gal staining, in situ hybridization or immunohistochemical staining.

RESULTS

When deltaenalphaA-lacZ transgenic embryos were stained with X-gal at embryonic day (E)11.5, beta-galactosidase activity was detected only in the eye. Histologic sections of the stained embryos revealed that lacZ was expressed exclusively in the lens, in both epithelial and fiber cells. Transgenic mice were also generated using either the original alphaA- or the new deltaenalphaA promoter linked to an insulin cDNA. In situ hybridizations confirmed that the short alphaA promoter targeted prenatal insulin expression specifically to the lens fiber cells, whereas the deltaenalphaA promoter was active in both lens epithelial and fiber cells. Developmental studies of the deltaenalphaA-insulin mice showed that the deltaenalphaA promoter became active at the lens pit stage and remained active in all lens cells, even at postnatal ages. The deltaenalphaA promoter also successfully directed expression of SV40 T-antigen (TAg), human E2F2, and dominant negative Sprouty2 (dn-Spry2) genes to lens epithelial and fiber cells. The lens specificity of the deltaenalphaA promoter was maintained in minigenes with different types of introns and polyadenylation signals.

CONCLUSIONS

A new lens-specific regulatory element was generated-the deltaenalphaA promoter, which can drive high levels of transgene expression in both lens epithelium and fiber cells throughout development. This modified promoter can be used for future transgenic studies of signal transduction and cell cycle regulation in lens epithelial cells.

Ectopic Pax6 Expression Disturbs Lens Fiber Cell Differentiation

PURPOSE

Pax6 is a transcription factor necessary for the specification and subsequent formation of the ocular lens. It is expressed in all lens cells at early stages of development. After lens formation, Pax6 expression is maintained in the lens epithelium, whereas its level abruptly decreases in differentiated fiber cells. This study is to test the hypothesis that normal fiber cell differentiation would be perturbed by sustained Pax6 expression.

METHODS

Transgenic mice expressing the canonical form of mouse Pax6 were created under the control of a modified mouse alphaA-crystallin promoter. The phenotypic changes in the transgenic lens were analyzed by light and electron microscopy. The effect of ectopic Pax6 expression on the lens fiber cells was investigated by in situ hybridization, immunohistochemical staining, real-time reverse transcriptase-polymerase chain reaction (RT-PCR), and two-dimensional (2-D) gel electrophoresis.

RESULTS

Transgenic mice from seven different lines all had cataracts with severity that correlated with the transgene expression level in lens fiber cells. In severely affected lines, a lumen was present between the apical surfaces of the epithelial and fiber cells, suggesting that secondary fiber cell elongation is incomplete. Electron microscopy analysis showed that the ball-and-socket interdigitations between neighboring fiber cells were underdeveloped or attenuated in the transgenic lens. Most interesting, elevated levels of Pax6 in fiber cells reduced the protein levels of transcription factor cMaf, which is known to be essential in fiber cell differentiation. Furthermore, the total amount of lens proteins was 60% less than normal in the Pax6 transgenic lens. Among the crystallins examined, the relative ratio of intact betaB1-crystallin protein to total lens protein was significantly reduced. Real-time reverse transcriptase PCR showed that the ratio of betaB1-crystallin transcript levels to total mRNA levels were reduced by 87%.

CONCLUSIONS

The data demonstrate that high levels of Pax6 expression disrupt normal fiber cell differentiation and maturation.

Expression patterns of cytochrome P4501B1 (Cyp1b1) in FVB/N mouse eyes

Homozygous and compound heterozygous mutations in cytochrome P4501B1 (CYP1B1) cause primary congenital glaucoma (PCG) in humans. It is hypothesized that developmental anomalies of the trabecular meshwork prevent appropriate drainage of the aqueous humor and cause PCG in human patients. In this report, we studied the expression patterns of Cyp1b1 in the eye of albino FVB/N mouse at different developmental stages. We isolated a cDNA fragment corresponding to the 3'untranslated region (3'UTR) of the Cyp1b1 gene by PCR and used it to make an (35)S-labelled riboprobe for in situ hybridization. We found that Cyp1b1 is expressed in both anterior and posterior segments of the eye. Anteriorly, the expression is confined to the ciliary body, most likely in the outer/pigmented ciliary epithelial cells. Cyp1b1 mRNA can be detected in these cells at postnatal day 4 (P4) and the expression continues into adulthood. Surprisingly, no above-background levels of Cyp1b1 mRNA were found at or around the trabecular meshwork at all the stages we examined. In the posterior region of the embryonic day 15 (E15) eye, Cyp1b1 is expressed in the retinal neuroepithelium and in the tissues surrounding the optic nerve, but not in the optic nerve itself. In the P7 retina, Cyp1b1 mRNA is found in the inner nuclear layer. Based on our finding that Cyp1b1 is expressed in the developing and mature ciliary body of the mouse eye, we speculate that mutation in this gene can directly contribute to the abnormal elevation of the intraocular pressure (IOP) in the PCG patients or indirectly affect the aqueous outflow by disrupting the proper development of the trabecular meshwork in these patients.

Mutually regulated expression of Pax6 and Six3 and its implications for the Pax6 haploinsufficient lens phenotype

Pax6 is a key regulator of eye development in vertebrates and invertebrates, and heterozygous loss-of-function mutations of the mouse Pax6 gene result in the Small eye phenotype, in which a small lens is a constant feature. To provide an understanding of the mechanisms underlying this haploinsufficient phenotype, we evaluated in Pax6 heterozygous mice the effects of reduced Pax6 gene dosage on the activity of other transcription factors regulating eye formation. We found that Six3 expression was specifically reduced in lenses of Pax6 heterozygous mouse embryos. Interactions between orthologous genes from the Pax and Six families have been identified in Drosophila and vertebrate species, and we examined the control of Pax6 and Six3 gene expression in the developing mouse lens. Using in vitro and transgenic approaches, we found that either transcription factor binds regulatory sequences from the counterpart gene and that both genes mutually activate their expression. These studies define a functional relationship in the lens in which Six3 expression is dosage-dependent on Pax6 and where, conversely, Six3 activates Pax6. Accordingly, we show a rescue of the Pax6 haploinsufficient lens phenotype after lens-specific expression of Six3 in transgenic mice. This phenotypic rescue was accompanied by cell proliferation and activation of the platelet-derived growth factor alpha-R/cyclin D1 signaling pathway. Our findings thus provide a mechanism implicating gene regulatory interactions between Pax6 and Six3 in the tissue-specific defects found in Pax6 heterozygous mice.

Systematic analysis of E-, N- and P-cadherin expression in mouse eye development

Cadherins are a family of Ca(2+)-dependent cell adhesion molecules. Through their homophilic binding interactions, cadherins play important roles in tissue formation and maintenance during development. Here the authors compare the expression patterns of the three classical cadherins, E-, N- and P-cadherin, during mouse eye development from embryonic day 9.5 (E9.5) to adult. It was found that: (1) The expression patterns of N- and P-cadherin are mutually exclusive in most ocular tissues during development. N-cadherin mRNA is detected specifically in the lens placode during lens induction at E9.5, and is absent in the rest of the surface ectodermal tissues. In contrast, P-cadherin is expressed in the surface ectoderm but not in the lens vesicle. N-cadherin is expressed continuously in the lens pit, lens vesicle, and in the epithelial cells and newly differentiating fiber cells of the mature lens. P-cadherin is expressed in the epithelial cells of the cornea, eyelids and Harderian gland. Reciprocal expression patterns of N-and P-cadherins are also seen during retinal development. N-cadherin is initially expressed in both the inner and outer layers of the optic cup at E9.5. N-cadherin expression persists in the inner layer as it develops into neural retina, but is turned off in the outer layer where the cells differentiate into retinal pigment epithelial (RPE) cells and express P-cadherin. Reciprocal patterns of expression are also seen in the ciliary epithelium. N-cadherin is expressed in the inner layer and P-cadherin in the outer layer of the ciliary epithelium. (2) E- and P-cadherins are epithelial cadherins. Their expression patterns in the eye are not identical. Both cadherins are found in the epithelia of the cornea, eyelid and Harderian gland. In contrast, lens epithelial cells express E- but not P-cadherin, and RPE cells express P- but not E-cadherin. (3) In addition to its high expression in surface ectoderm-derived tissues, E-cadherin mRNA was also detected in some of the retinal ganglion neurons at postnatal day 14 (P14). E-cadherin expression in the neural retina has not been reported before. This study shows that cell fate determination in the eye occurs in conjunction with distinct changes in the patterns of cadherin gene expression.

Formation of corneal endothelium is essential for anterior segment development - a transgenic mouse model of anterior segment dysgenesis

The anterior segment of the vertebrate eye is constructed by proper spatial development of cells derived from the surface ectoderm, which become corneal epithelium and lens, neuroectoderm (posterior iris and ciliary body) and cranial neural crest (corneal stroma, corneal endothelium and anterior iris). Although coordinated interactions between these different cell types are presumed to be essential for proper spatial positioning and differentiation, the requisite intercellular signals remain undefined. We have generated transgenic mice that express either transforming growth factor (alpha) (TGF(alpha)) or epidermal growth factor (EGF) in the ocular lens using the mouse (alpha)A-crystallin promoter. Expression of either growth factor alters the normal developmental fate of the innermost corneal mesenchymal cells so that these cells often fail to differentiate into corneal endothelial cells. Both sets of transgenic mice subsequently manifest multiple anterior segment defects, including attachment of the iris and lens to the cornea, a reduction in the thickness of the corneal epithelium, corneal opacity, and modest disorganization in the corneal stroma. Our data suggest that formation of a corneal endothelium during early ocular morphogenesis is required to prevent attachment of the lens and iris to the corneal stroma, therefore permitting the normal formation of the anterior segment.

Lens-specific expression of PDGF-A alters lens growth and development

The vertebrate lens provides an in vivo model to study the molecular mechanisms by which growth factors influence development decisions. In this study, we have investigated the expression patterns of platelet-derived growth factor (PDGF) and PDGF receptors during murine eye development by in situ hybridization. Postnatally, PDGF-A is highly expressed in the iris and ciliary body, the ocular tissues closest to the germinative zone of the lens, a region where most proliferation of lens epithelial cells occurs. PDGF-A is also present in the corneal endothelium anterior to the lens epithelium in embryonic and early postnatal eyes. PDGF-B is expressed in the iris and ciliary body as well as in the vascular cells which surround the lens during early eye development. In the lens, expression of PDGF-alpha receptor (PDGF-alphaR), a receptor that can bind both PDGF-A and PDGF-B, is restricted to the lens epithelium throughout life. The expression of PDGF-alphaR in the lens epithelial cells and PDGF (A- and B-chains) in the ocular tissues adjacent to the lens suggests that PDGF signaling may play a key role in regulating lens development. To further examine how PDGF affects lens development in vivo, we generated transgenic mice that express human PDGF-A in the lens under the control of the alphaA-crystallin promoter. The transgenic mice exhibit lenticular defects that result in cataracts. The percentage of surface epithelial cells in S-phase is increased in transgenic lenses compared to their nontransgenic littermates. Higher than normal levels of cyclin A and cyclin D2 expression were also detected in transgenic lens epithelium. These results together suggest that PDGF-A can induce a proliferative response in lens epithelial cells. The lens epithelial cells in the transgenic mice also exhibit characteristics of differentiating fiber cells. For example, the transgenic lens epithelial cells are slightly elongated, contain larger and less condensed nuclei, and express fiber-cell-specific beta-crystallins. Our results suggest that PDGF-A normally acts as a proliferative factor for the lens epithelial cells in vivo. Elevated levels of PDGF-A enhance proliferation, but also appear to induce some aspects of the fiber cell differentiation pathway.

Lens-specific expression of PDGF-A in transgenic mice results in retinal astrocytic hamartomas

PURPOSE

To investigate the possibility that platelet-derived growth factor (PDGF) might regulate aspects of mouse retinal development in vivo.

METHODS

In situ hybridization was used to study the expression patterns of PDGF-A and PDGF-B and their receptors during normal mouse eye development. Transgenic mice that express human PDGF-A in the lens under the control of alpha A-crystallin promoter were generated by pronuclear microinjection. The effects of PDGF overexpression on eye development were analyzed by ocular histology, immunohistochemistry and in situ hybridizations.

RESULTS

The PDGF genes are expressed by cells in close contact with retinal astrocytes. The PDGF-A messenger RNA is upregulated in the retinal ganglion neurons after birth, and PDGF-B is expressed by the blood vessel cells in the hyaloid vasculature. The authors found that lens-specific expression of PDGF-A in the eye can induce hyperplasia of retinal astrocytes, which express PDGF-alpha receptor (PDGF-alpha R) during development. The retinal alterations in the PDGF-A transgenic mice closely resemble the retinal astrocytic hamartomas found in human tuberous sclerosis (TSC) disease.

CONCLUSIONS

These findings suggest that proliferation of retinal astrocytes is regulated by PDGF during normal eye development. The authors speculate that proliferation of retinal astrocytes is mediated through a PDGF signaling pathway, which may involve the TSC gene product.

TGF alpha can act as a chemoattractant to perioptic mesenchymal cells in developing mouse eyes

Growth factors are believed to play an important role in regulating cell fate and cell behavior during embryonic development. Transforming growth factor alpha (TGF alpha), a member of the epidermal growth factor (EGF) superfamily, is a small polypeptide growth factor. Upon binding to its receptor, the EGF receptor (EGFR), TGF alpha can exert diverse biological activities, such as induction of cell proliferation or differentiation. To explore the possibility that TGF alpha might regulate cell fate during murine eye development, we generated transgenic mice that express human TGF alpha in the lens under the control of the mouse alpha A-crystallin promoter. The transgenic mice displayed multiple eye defects, including corneal opacities, cataracts and microphthalmia. At early embryonic stages TGF alpha induced the perioptic mesenchymal cells to migrate abnormally into the eye and accumulate around the lens. In situ hybridization revealed that the EGFR mRNA is highly expressed in the perioptic mesenchyme, suggesting that the migratory response is mediated by receptor activation. In order to test this model, the TGF alpha transgenic mice were bred to EGFR mutant waved-2 (wa-2) mice. We found that the eye defects of the TGF alpha transgenic mice are significantly abated in the wa-2 homozygote background. Because the EGFR mutation in the wa-2 mice is located in the receptor kinase domain, this result indicates that the receptor tyrosine kinase activity is critical for signaling the migratory response. Taken together, our studies demonstrate that TGF alpha is capable of altering the migratory decisions and behavior of perioptic mesenchyme during eye development.

Transgenic mice expressing full-length human apolipoprotein B-100. Full-length human apolipoprotein B mRNA is essentially not edited in mouse intestine or liver

Apolipoprotein (apo) B-100 mRNA is edited in the small intestine (in all mammals examined) and the liver (in mice and rats only) to produce apoB-48 mRNA. ApoB mRNA editing involves a C-->U conversion of the first base of the codon CAA for Gln-2153 in apoB-100, changing it to an in-frame stop codon (UAA). The edited mRNA encodes apoB-48, which is colinear with the N-terminal 48% of apoB-100. ApoB mRNA editing can be reproduced in vitro using cellular extracts from one species to edit synthetic apoB mRNA sequences from a different species. Editing of transcripts from transfected genes also appears not to be species-specific. We have produced transgenic mice that express full-length human apoB-100 mRNA at high levels in the liver and small intestine. Human apoB-100 (a 550-kDa protein) but not apoB-48 (a 260-kDa protein) is detected in total plasma (at approximately 22 mg/dl) and in very low density and low density lipoproteins. The endogenous mouse plasma apoB concentration is reduced by approximately 45% in the transgenic animals. Thus, the transgenic mice form an animal model for familial hyperapolipoprotein B, an inherited form of hyperlipidemia. To our surprise, we found that the full-length human apoB mRNA consists of > 99% apoB-100 mRNA in both the liver and small intestine; < 1% of edited (apoB-48) mRNA was detected. The proportions of endogenous mouse apoB-48 (edited) mRNA (60 and 90% in the liver and small intestine, respectively) were identical in transgenic mice and their nontransgenic littermates. Therefore, full-length human apoB mRNA is resistant to editing by the mouse editing enzyme in vivo; the unchanged proportion of endogenous mouse apoB-48 mRNA in the transgenic mice suggests that the human mRNA competes poorly with the mouse sequence for interacting with the editing enzyme. This observation has implications for the sequence specificity and mechanism of RNA editing. Furthermore, we should exercise caution in the interpretation of in vitro RNA-editing experiments.