Lens-specific expression of transforming growth factor beta1 in transgenic mice causes anterior subcapsular cataracts

Development of a smooth muscle-targeted cre recombinase mouse reveals novel insights regarding smooth muscle myosin heavy chain promoter regulation

The use of genetically modified mice has been an important model system to study gene function in cardiovascular development and under pathophysiological conditions. Although conventional gene knockout studies have provided important insights into gene function in the cardiovascular system, they may be limited by upregulation of compensatory pathways and the inability to differentiate direct versus indirect functions in vivo. As a first step in developing systems that can target gene activation or inactivation specifically to smooth muscle cells (SMCs), we coupled the smooth muscle myosin heavy chain (SMMHC) promoter to the cre recombinase gene and generated transgenic mice that express cre in SMCs. In addition, we used these mice to address whether the heterogeneous staining observed in SMMHC-LacZ mice was due to subsets of SMCs that required different regulatory cassettes of the promoter or if it reflected episodic expression of the transgene. To address both the feasibility of SMC targeting and the apparent heterogeneous expression, we bred SMMHC-cre mice to indicator mice containing a cre-activated LacZ gene. Results showed high-level expression in SMCs at various embryonic time points and in adult tissues. Because breeding of SMMHC-cre mice to an indicator line provided an integration of cre activity over time, results of this study revealed that expression of the SMMHC promoter fragment more closely resembled the expression of the endogenous gene, both with respect to the onset of activation during development and uniformity of staining among individual cells within tissues. Overall, these mice will provide a powerful tool to researchers to study gene function in vascular development/disease by using cre/lox technology to direct smooth muscle-specific gene activation or inactivation in vivo.

Identification of a 3.2 kb 5'-flanking region of the murine keratocan gene that directs beta-galactosidase expression in the adult corneal stroma of transgenic mice

The mouse keratocan gene (Ktcn) expression tracks the corneal morphogenesis during eye development and becomes restricted to keratocytes of the adult, implicating a cornea-specific gene regulation of the mouse Ktcn [J. Biol. Chem., 273 (1998) 22584-22588]. To examine the functionality of the mouse Ktcn promoter, we have cloned and sequenced a 3.2kb genomic DNA fragment 5' of the mouse Ktcn gene, which was used to prepare a reporter gene construct that contained the 3.2kb 5' flanking sequence, exon 1 and 0.4kb of intron 1 of Ktcn, and beta-geo hybrid reporter gene. The beta-galactosidase (betaGal) activity was assayed in tissues of two of five transgenic mouse lines obtained via microinjection. In adult transgenic mice, betaGal activity was detected only in cornea, not in other tissues (e.g. lens, retina, sclera, lung, heart, liver, diaphragm, kidney, and brain). During ocular development, the spatial-temporal expression patterns of the betaGal recapitulated that of endogenous Ktcn in transgenic mice. Using XGal staining, strong betaGal activity was first detected in periocular tissues of E13.5 embryos, and restricted to corneal keratocytes at E14.5 and thereafter. Interestingly, in addition to cornea, betaGal activity was transiently found in some non-ocular tissues, i.e. ears, snout, and limbs of embryos of E13.5 and E14.5 but was no longer detected in those tissues of E16.5 embryos. The transient expression of endogenous keratocan in non-ocular tissues during embryonic development was confirmed by in situ hybridization. Taken together, our results suggest that the 3.2kb Ktcn promoter contains sufficient cis-regulatory elements to drive heterologous minigene expression in cells expressing keratocan. The identification of keratocyte-specific expression of betaGal reporter gene in the adult transgenic mice is an important first step in characterizing the Ktcn promoter in order to use it to drive a foreign gene expression in corneal stroma.

Atrial but not ventricular fibrosis in mice expressing a mutant transforming growth factor-beta(1) transgene in the heart

Increased transforming growth factor (TGF)-beta(1) activity has been observed during pathologic cardiac remodeling in a variety of animal models. In an effort to establish a causal role of TGF-beta(1) in this process, transgenic mice with elevated levels of active myocardial TGF-beta(1) were generated. The cardiac-restricted alpha-myosin heavy chain promoter was used to target expression of a mutant TGF-beta(1) cDNA harboring a cysteine-to-serine substitution at amino acid residue 33. This alteration blocks covalent tethering of the TGF-beta(1) latent complex to the extracellular matrix, thereby rendering a large proportion (>60%) of the transgene-encoded TGF-beta(1) constitutively active. Although similar levels of active TGF-beta(1) were present in the transgenic atria and ventricles, overt fibrosis was observed only in the atria. Surprisingly, increased active TGF-beta(1) levels inhibited ventricular fibroblast DNA synthesis in uninjured hearts and delayed wound healing after myocardial injury. These data suggest that increased TGF-beta(1) activity by itself is insufficient to promote ventricular fibrosis in the adult mouse ventricle.

Differential gene expression in male and female rat lenses undergoing cataract induction by transforming growth factor-beta (TGF-beta)

Epidemiologic studies in humans as well as immunohistologic studies in animals have demonstrated significant sex differences in the propensity to develop cataract. Several studies suggest that estrogen may play a protective role against cataractogenesis. Indeed, male and ovariectomized female rat lenses have a greater susceptibility to cataract induced by transforming growth factor-beta (TGF-beta) than do normal female lenses. However, in spite of the current evidence that estrogen may play a pivotal role in cataractogenesis, the molecular mechanisms behind this phenomenon are largely undetermined. Our study utilized the differential display procedure to examine gene up- and down-regulation in male, normal female and ovariectomized female rat lenses exposed to TGF-beta. Male and normal female rat lenses were cultured with or without 0.15 ng ml(-1)TGF-beta. Lenses were then harvested, and total RNA was isolated for analysis by reverse-transcriptase differential display. Differentially expressed mRNAs were subcloned, sequenced and identified through GenBank database searches. The original experiment was repeated with the addition of ovariectomized female TGF-beta(+/-) conditions, and all differential patterns of gene expression were verified using Northern blot and RT-PCR analysis. Screening of approximately 12% of the mRNA population led to the identification of 27 differentially expressed cDNAs. Notably, strong gender differences were found in expression levels of gammaB-crystallin. In addition, proteasome Z subunit was up-regulated in TGF-beta-treated male and ovariectomized female lenses, but was down-regulated in TGF-beta-treated normal female lenses. This pattern of expression is consistent with the increased susceptibility of male and ovariectomized lenses to TGF-beta-induced cataract. We conclude that differential display is a useful and expedient method for analysing changes in gene expression in the lens. Structural and functional studies of the genes identified in this study may further elucidate mechanisms underlying the TGF-beta-induced cataract formation and differential rates of cataractogenesis in males vs females. In particular, our data suggest that the role of proteasome Z subunit in TGF-beta-induced anterior subcapsular cataract warrants further investigation.

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.

TGF-beta elicits fibronectin secretion and proliferation in cultured chick lens epithelial cells

PURPOSE

To determine if the cataract forming influence of TGF-beta on lens cells is due to its effects on the ECM.

METHODS

Primary cultures of chick lens annular pad cells were exposed to TGF-beta and various exogenously supplied components of the lens capsule. Proliferative response were measured through tritiated thymidine incorporation into DNA. Cell spreading accompanying increased matrix interactions and growth was monitored with phase contrast microscopy. ECM proteins were detected in culture media and as deposited matrices with Western blotting and silver staining. TGF-beta receptors were identified with Western blotting.

RESULTS

Chick lens cells were shown to express type I and II TGF-beta receptors. TGF-beta stimulated cell growth and ECM production particularly with regard to fibronectin. Fibronectin was secreted into the culture medium and deposited onto plastic substrates. Plating cells on ECM components found in the lens capsule further increased their growth in response to TGF-beta.

CONCLUSIONS

These results indicate that TGF-beta may have a normal function in the lens regulating capsular protein production. The potent stimulation of lens cell growth by TGF-beta may be due to mis-regulated production of lens capsular proteins not normally found in great abundance.