COL1A1 transgene expression in stably transfected osteoblastic cells. Relative contributions of first intron, 3'-flanking sequences, and sequences derived from the body of the human COL1A1 minigene

Mutation of the 5'-untranslated region stem-loop structure inhibits α1(I) collagen expression in vivo

Type I collagen is a heterotrimeric extracellular matrix protein consisting of two α1(I) chains and one α2(I) chain. During liver fibrosis, activated hepatic stellate cells (HSCs) are the major source of the type I collagen that accumulates in the damaged tissue. Expression of α1(I) and α2(I) collagen mRNA is increased 60-fold compared with quiescent stellate cells and is due predominantly to post-transcriptional message regulation. Specifically, a stem-loop structure in the 5'-untranslated region of α1(I) collagen mRNA may regulate mRNA expression in activated HSCs through its interaction with stem-loop binding proteins. The stem-loop may also be necessary for efficient production and folding of the type I collagen heterotrimer. To assess the role of the stem-loop in type I collagen expression in vivo, we generated a knock-in mouse harboring a mutation that abolished the stem-loop structure. Heterozygous and homozygous knock-in mice exhibited a normal phenotype. However, steady-state levels of α1(I) collagen mRNA decreased significantly in homozygous mutant MEFs as well as HSCs; intracellular and secreted type I collagen protein levels also decreased. Homozygous mutant mice developed less liver fibrosis. These results confirm an important role of the 5' stem-loop in regulating type I collagen mRNA and protein expression and provide a mouse model for further study of collagen-associated diseases.

Analysis of inhibitory action of modified U1 snRNAs on target gene expression: discrimination of two RNA targets differing by a 1 bp mismatch

The modified U1 snRNA gene can suppress expression of a target transgene. In the present study, its potential utility to inhibit a dominant negative/gain of function mutation is explored. Using a green fluorescent protein (GFP) target gene, inhibition was achieved in all cells transduced with U1antiGFP directed at multiple sites within GFP. Using a chloramphenicol acetyltransferase (CAT) target gene, inhibition was not increased by increasing the hybridization domain from 10 to 16 bp or when a site in an upstream exon or intron was targeted. To determine if a U1 anti-target design could discriminate between two transcripts that differ by a 1-2 bp mismatch, GFPtpz and GFPsaph were chosen as targets because they share sequence homology except for three regions where a 1, 2 or 3 bp mismatch exists. The results demonstrated that U1antiGFP correctly reduced its cognate GFP expression by >90% and therefore U1 anti-target constructs are able to discriminate a 1 or 2 bp mismatch in their target mRNA. Thus, these U1 anti-target constructs may be effective in a strategy of somatic gene therapy for a dominant negative/gain of function mutation due to the discreteness of its discrimination. It may complement other anti-target strategies to reduce the cellular load of a mutant transcript.

Activation of fibroblast procollagen alpha 1(I) transcription by mechanical strain is transforming growth factor-beta-dependent and involves increased binding of CCAAT-binding factor (CBF/NF-Y) at the proximal promoter

During normal developmental tissue growth and in a number of diseases of the cardiopulmonary system, adventitial and interstitial fibroblasts are subjected to increased mechanical strain. This leads to fibroblast activation and enhanced collagen synthesis, but the underlying mechanisms involved remain poorly understood. In this study, we have begun to identify and characterize mechanical strain-responsive elements in the rat procollagen alpha 1(I) (COL1A1) gene and show that the activity of COL1A1 promoter constructs, transiently transfected into cardiac fibroblasts, was increased between 2- and 4-fold by continuous cyclic mechanical strain. This was accompanied by an approximately 3-fold increase in the levels of total active transforming growth factor-beta (TGF-beta) released into the medium. Inclusion of a pan-specific TGF-beta neutralizing antibody inhibited strain-induced COL1A1 promoter activation. Deletion analysis revealed the presence of two potential strain response regions within the proximal promoter, one of which contains an inverted CCAAT-box overlapping a GC-rich element. Both mechanical strain and exogenously added TGF-beta1 enhanced the binding activity of CCAAT-binding factor, CBF/NF-Y, at this site. Moreover, this element was sufficient to confer strain-responsiveness to an otherwise unresponsive SV40 promoter. In summary, this study demonstrates that strain-induced COL1A1 promoter activation in cardiac fibroblasts is TGF-beta-dependent and involves increased binding of CCAAT-binding factor at the proximal promoter. Furthermore, these findings suggest a novel and potentially important TGF-beta response element in the rat COL1A1 gene.

Use of type I collagen green fluorescent protein transgenes to identify subpopulations of cells at different stages of the osteoblast lineage

Green fluorescent protein (GFP)-expressing transgenic mice were produced containing a 3.6-kilobase (kb; pOBCol3.6GFPtpz) and a 2.3-kb (pOBCol2.3GFPemd) rat type I collagen (Col1a1) promoter fragment. The 3.6-kb promoter directed strong expression of GFP messenger RNA (mRNA) to bone and isolated tail tendon and lower expression in nonosseous tissues. The 2.3-kb promoter expressed the GFP mRNA in the bone and tail tendon with no detectable mRNA elsewhere. The pattern of fluorescence was evaluated in differentiating calvarial cell (mouse calvarial osteoblast cell [mCOB]) and in marrow stromal cell (MSC) cultures derived from the transgenic mice. The pOBCol3.6GFPtpz-positive cells first appeared in spindle-shaped cells before nodule formation and continued to show a strong signal in cells associated with bone nodules. pOBCol2.3GFPemd fluorescence first appeared in nodules undergoing mineralization. Histological analysis showed weaker pOBCol3.6GFPtpz-positive fibroblastic cells in the periosteal layer and strongly positive osteoblastic cells lining endosteal and trabecular surfaces. In contrast, a pOBCol2.3GFPemd signal was limited to osteoblasts and osteocytes without detectable signal in periosteal fibroblasts. These findings suggest that Col1a1GFP transgenes are marking different subpopulations of cells during differentiation of skeletal osteoprogenitors. With the use of other promoters and color isomers of GFP, it should be possible to develop experimental protocols that can reflect the heterogeneity of cell differentiation in intact bone. In primary culture, this approach will afford isolation of subpopulations of these cells for molecular and cellular analysis.

A member of the Y-box protein family interacts with an upstream element in the alpha1(I) collagen gene

Transforming growth factor beta (TGF-beta) stimulates protein complex formation on a TGF-beta response element (TAE) found in the distal portion (-1624) of the collagen alpha 1(I) promoter. To identify the fibroblast proteins in this complex, an expression library constructed from human embryonic lung fibroblasts mRNA was screened using a tetramer of TAE. Y-box binding protein (YB-1), was identified as a protein in the TAE-protein complex. The protein expressed by phage clones formed a specific complex with labeled TAE but not mutated TAE (mTAE) similar to the complex formed with nuclear protein. Nuclear protein-TAE complexes isolated from native gels contained YB-1 by Western analysis. TGF-beta treatment increased the amount of YB-1 protein in nuclear extracts, decreased its amount in cytoplasm, but did not alter the steady state levels of YB-1 mRNA. A full-length YB-1 protein expressed in human lung fibroblasts was primarily located in the nucleus with punctate staining in cytoplasmic regions. The expression of YB-1 decreased in the cytoplasm after 2 h of TGF-beta treatment. Therefore, the increased binding activity seen in TGF-beta-stimulated nuclear extracts was due primarily to relocalization of YB-1 from the cytoplasm to the nuclear compartment. Co-transfection of YB-1 cDNA with a collagen promoter-reporter construct caused a dose-dependent activation of collagen promoter activity in rat fibroblasts whereas the promoter with a mutation in the TAE element was not sensitive to YB-1 co-expression. In conclusion, we have identified YB-1 as a protein that interacts with a TGF-beta response element in the distal region of the collagen alpha 1(I) gene. YB-1 protein activates the collagen promoter and translocates into the nucleus during TGF-beta addition to fibroblasts, suggesting a role for this protein in TGF-beta signaling.

The Y-box binding protein YB-1 suppresses collagen alpha 1(I) gene transcription via an evolutionarily conserved regulatory element in the proximal promoter

Appropriate expression of collagen type I, a major component of connective tissue matrices, is dependent on tight transcriptional control and a number of trans-activating and repressing factors have been characterized. Here we identify the Y-box binding protein-1 (YB-1) as a novel repressor of the collagen type alpha1(I) (COL1A1) gene. Collagen type I mRNA and protein levels decreased upon overexpression of YB-1 by transfection in NRK fibroblasts. The human, rat, and mouse COL1A1 promoter -220/+115 contains three putative Y-boxes, one of these sites, designated collagen Y-box element (CYE), includes a Y-box plus an adjacent 3' inverted repeat. DNase-I footprinting and Southwestern blotting with fibroblast nuclear extract demonstrated binding of several nuclear proteins across the CYE, one of which was identified as YB-1. Recombinant YB-1 bound the CYE sequence in gel shift assays with a preference for single-stranded templates. The entire sequence (-88/-48) was required for high affinity binding. Complex formation of endogenous YB-1 with the CYE was established by supershift studies. COL1A1 promoter-reporter constructs were suppressed up to 80% by cotransfection with YB-1 in a variety of cell types. In addition, CYE conferred YB-1 responsiveness on two heterologous promoters further demonstrating the importance of this repressor region. Mung bean nuclease sensitivity analysis suggested that repression is most likely exerted through changes in DNA conformation.

Col1a1-driven transgenic markers of osteoblast lineage progression

The modular organization of the type I collagen promoter allows creation of promoter-reporter constructs with preferential activity in different type I collagen-producing tissues that might be useful to mark cells at different stages of osteoblastic differentiation. Primary marrow stromal cell (MSC) and mouse calvarial osteoblast (mCOB) cultures were established from transgenic mice harboring different Col1a1 promoter fragments driving chloramphenicol acetyltransferase (CAT). In these models, Col1a1 messenger RNA (mRNA) and alkaline phosphatase (ALP) are the first markers of differentiation appearing soon after the colonies develop. Bone sialoprotein (BSP) is detected 2-3 days later, followed by osteocalcin (OC) expression and nodule mineralization. A 3.6 Col1a1 fragment (ColCAT3.6) initiated activity concomitant with ALP staining and type I collagen mRNA expression. In contrast, a 2.3 Col1a1 fragment (ColCAT2.3) became active coincident with BSP expression. The pattern of transgene expression assessed by immunostaining was distinctly different. ColCAT3.6 was expressed within and at the periphery of developing nodules whereas the ColCAT2.3 expression was restricted to the differentiated nodules. The feasibility of using green fluorescent protein (GFP) as a marker of osteoblast differentiation was evaluated in ROS17/2.8 cells. A 2.3-kilobase (kb) Col1a1 promoter driving GFP (pOB4Col2.3GLP) was stably transfected into the cell line and positive clones were selected. Subcultures lost and then regained GFP expression that was localized in small clusters of cells throughout the culture. This suggests that expression from the 2.3-kb Col1A1 fragment is determined by the state of differentiation of the ROS17/2.8 cells. Col1a1 transgenes should be useful in appreciating the heterogeneity of a primary or immortalized culture undergoing osteoblastic differentiation.

Transgenic mice with a mutated collagen promoter display normal response during bleomycin-induced fibrosis and possess neurological abnormalities

We have previously identified a potential TGF-beta activation element (TAE) in the rat collagen alpha1(I) promoter at -1624 upstream of the transcriptional start site [Ritzenthaler et al., 1991, 1993]. To determine the importance of the TAE in vivo, we produced transgenic mice carrying 3.6 kb of the rat collagen alpha1(I) promoter linked to the reporter gene chloramphenicol acetyl transferase with and without site-directed mutations that eliminate DNA-protein binding at the TAE site. Tissue-specific expression of the reporter gene in transgenic mice with the mutated collagen promoter was similar to that of transgenic mice with the normal promoter in two genetic backgrounds as judged by in situ hybridization, reporter assays, and immunochemistry. Endotracheal instillation of bleomycin induces lung fibrosis, mediated in part by TGF-beta. Earlier studies indicated that expression of wild-type collagen-reporter gene was upregulated in transgenic mice lungs in response to endotracheal instillation of bleomycin. A similar level of reporter gene upregulation was observed in transgenic mice carrying the mutation in the TAE. Two lines of transgenic mice carrying the mutated promoter construct displayed unexpected neurological abnormalities. In the FVB genetic background, there was a higher than normal incidence of mortality, spontaneous seizures, and an inability to nurture offspring. Histological evidence demonstrated clear abnormalities, including disorderly arrangement of neurons in the hippocampus and significant laminar cortical necrosis in the cerebrum in animals after seizures. In the C57Bl/6 background, there was a high incidence of severe communicating hydrocephalus, early runting, and increased mortality similar to that in transgenic animals with astroglial overexpression of TGF-beta. These animals provide an interesting model system to investigate molecular mechanisms responsible for seizures and hydrocephalus.

Mice with a targeted intronic deletion in the Col1a1 gene respond to bleomycin-induced pulmonary fibrosis with increased expression of the mutant allele

Experiments designed to examine the role of the first intron in regulation of the Col1a1 gene by transfection and in transgenic mice have led to conflicting conclusions. Recently, Hormuzdi et al. [Hormuzdi, S.G., Penttinen, R., Jaenisch, R., Bornstein, P., 1998. A gene-targeting approach identifies a function for the first intron in expression of the alpha1(I) collagen. Mol. Cell. Biol. 18, 3368-3375.] created a targeted deletion in this intron in mice and demonstrated an age-dependent reduction in expression of the mutated allele in lung and skeletal muscle. In this study, intratracheal instillation of bleomycin in mice was used to induce pulmonary fibrosis in control and intron-deleted animals. This stimulus for collagen synthesis was associated with a marked upregulation of the intron-deleted allele in mutant mice. Our results establish that the inhibition of expression of the mutant Col1a1 gene is not fixed, since the gene can still respond to physiological signals. We propose that cis-acting elements, elsewhere in the gene, can compensate for the lack of intronic sequences in the mutated Col1a1 allele and account for the conditional nature of the inhibition. This model has the potential to resolve the conflicting results of previous transfection and transgenic experiments in which different fragments of the Col1a1 gene were used.

A gene-targeting approach identifies a function for the first intron in expression of the alpha1(I) collagen gene

The role of the first intron of the Col1A1 gene in the regulation of type I collagen synthesis remains uncertain and controversial despite numerous studies that have made use of transgenic and transfection experiments. To examine the importance of the first intron in regulation of the gene, we have used the double-replacement method of gene targeting to introduce, by homologous recombination in embryonic stem (ES) cells, a mutated Col1A1 allele (Col-IntDelta). The Col-IntDelta allele contains a 1. 3-kb deletion within intron I and is also marked by the introduction of a silent mutation that created an XhoI restriction site in exon 7. Targeted mice were generated from two independently derived ES cell clones. Mice carrying two copies of the mutated gene were born in the expected Mendelian ratio, developed normally, and showed no apparent abnormalities. We used heterozygous mice to determine whether expression of the mutated allele differs from that of the normal allele. For this purpose, we developed a reverse transcription-PCR assay which takes advantage of the XhoI polymorphism in exon 7. Our results indicate that in the skin, and in cultured cells derived from the skin, the intron plays little or no role in constitutive expression of collagen I. However, in the lungs of young mice, the mutated allele was expressed at about 75% of the level of the normal allele, and in the adult lung expression was decreased to less than 50%. These results were confirmed by RNase protection assays which demonstrated a two- to threefold decrease in Col1A1 mRNA in lungs of homozygous mutant mice. Surprisingly, in cultured cells derived from the lung, the mutated allele was expressed at a level similar to that of the wild-type allele. Our results also indicated an age-dependent requirement for the intact intron in expression of the Col1A1 gene in muscle. Since the intron is spliced normally, and since the mutant allele is expressed as well as the wild-type allele in the skin, reduced mRNA stability is unlikely to contribute to the reduction in transcript levels. We conclude that the first intron of the Col1A1 gene plays a tissue-specific and developmentally regulated role in transcriptional regulation of the gene. Our experiments demonstrate the utility of gene-targeting techniques that produce subtle mutations for studies of cis-acting elements in gene regulation.