Bone tissue specific transcriptional control: options for targeting gene therapy to the skeleton

In vivo association of CReMM/CHD9 with promoters in osteogenic cells

Molecular mechanisms that control cell differentiation involve with chromatin remodeling activities. We recently identified Chromatin Related Mesenchymal Modulator (CReMM), a CHD protein expressed by mesenchymal cells. In this study, we analyzed CReMM expression on RNA and protein levels during embryonic development in mouse skeletal tissues. CReMM appears transiently during mesenchymal cell differentiation, being detected first in osteoprogenitors and declining in mature cells. A novel aspect of the study elaborates on in vivo association of CReMM with promoters in cells obtained by laser capture micro-dissection (LCM) technique from periosteum and endochondreal ossification regions. Using chromatin immunoprecipitation (ChIP), we proved that CReMM binds to skeletal tissue-specific promoters: CBFA1, biglycan, osteocalcin (OC), collagen-II, and myosin in a differential manner. The results imply that CReMM selectively interacts with analyzed promoters activated in the tissue at the appropriate time of development. The identification of CReMM and its tissue distribution and function provides an attractive clue for the study of transcriptional regulation of osteogenic cells' maturation.

Determination of osteoprogenitor-specific promoter activity in mouse mesenchymal stem cells by recombinant adeno-associated virus transduction

Towards utilizing gene-targeted, repopulating mesenchymal stem cells (MSC) to increase osteogenesis, we evaluated the expression of bone-specific promoters during MSC differentiation. Multi-lineage potential of cultured MSC was confirmed by osteogenic, adipogenic and chondrogenic differentiation under controlled conditions. Recombinant adeno-associated virus (rAAV) encoding luciferase under the human cytomegalovirus (CMV), mouse alkaline phosphatase (ALP), Runx-2/cbfa1 (RUNX), osteopontin (OPN), collagen type 1a (COL), and osteocalcin (OCN) promoters was used to transduce mouse MSC. Replicate cultures were maintained undifferentiated or differentiated to osteoblast lineage. Luciferase expression was determined on days 1, 2, 3, 7, 14, or 21 as a measure of promoter activity. Expression of osteogenic markers and mineralization was determined as correlates of osteopoiesis. Results indicated expression from CMV promoter in undifferentiated and differentiated cultures at early stage. However, expression from COL and RUNX promoters was abundant only in differentiating cultures as early as 24 h but declined gradually. Expression from OPN and ALP promoters was evident 24 h following osteogenic differentiation and peaked gradually until 2 weeks before declining. Expression from OC promoter was evident only after 7 days of differentiation but remained until final analysis on day 21. That rAAV transduction of MSC does not induce differentiation was also confirmed by quantitative reverse-transcription polymerase chain reaction (QRT-PCR). The observed stage-specific expression of analyzed promoters was not significant when the MSC were differentiated to adipocytes. Thus, the use of RUNX2 or COL promoter to stably express osteoinductive factors in MSC may allow both self-renewal of modified MSC and enrichment of osteoblast commitment.

Directing the expression of a green fluorescent protein transgene in differentiated osteoblasts: comparison between rat type I collagen and rat osteocalcin promoters

The osteocalcin (OC) and a 2.3 kb fragment of the collagen promoter (Col2.3) have been used to restrict transgenic expression of a variety of proteins to bone. Transgenic mice carrying a green fluorescent protein (GFP) gene driven by each promoter were generated. Strong GFP expression was detected in OC-GFP mice in a few osteoblastic cells lining the endosteal bone surface and in scattered osteocytes within the bone matrix in long bones from 1-day-old to 6-month-old transgenic animals. Similar findings were noted in the forming tooth in which only individual odontoblasts expressed GFP without detectable expression from the dental pulp. This limited pattern of OC-GFP-positive cells contrasts with the uniform expression in the Col2.3GFP mice in which large proportion of osteoblasts, odontoblasts, and osteocytes strongly expressed the transgene. To assess transgene expression during in vitro differentiation, marrow stromal cell and neonatal calvarial osteoblast cultures were analyzed. The activity of both transgenes was restricted to mineralized nodules but the number of positive cells was lower in the OC-GFP-derived cultures. The different temporal and spatial pattern of each transgene in vivo and in vitro reveals potential advantages and disadvantages of these two transgene models.

Conditional gene targeting for cancer gene therapy

Current treatment of solid tumors is limited by severe adverse effects, resulting in a narrow therapeutic index. Therefore, cancer gene therapy has emerged as a targeted approach that would significantly reduce undesired side effects in normal tissues. This approach requires a clear understanding of the molecular biology of both the malignant clone and the biological vectors that serve as vehicles to target cancer cells. In this review we discuss novel approaches for conditional gene expression in cancer cells. Targeting transgene expression to malignant tissues requires the use of specific regulatory elements including promoters based on tumor biology, tissue-specific promoters and inducible regulatory elements. We also discuss the regulation of both replication and transgene expression by conditionally-replicative viruses. These approaches have the potential to restrict the expression of transgenes exclusively to tissues of interest and thereby to increase the therapeutic index of future vectors for cancer gene therapy.

Differential expression patterns of Runx2 isoforms in cranial suture morphogenesis

Runx2 (previously known as Cbfal/Pebp2alphaA/AML3), a key transcription factor in osteoblast differentiation, has at least two different isoforms using alternative promoters, which suggests that the isoforms might be expressed differentially. Haploinsufficiency of the Runx2 gene is associated with cleidocranial dysplasia (CCD), the main phenotype of which is inadequate development of calvaria. In spite of the biological relevance, Runx2 gene expression patterns in developing calvaria has not been explored previously, and toward this aim we developed three probes: pRunx2, which comprises the common coding sequence of Runx2 and hybridizes with all isoforms; pPebp2alphaA, which specifically hybridizes with the isoform transcribed with the proximal promoter; and pOsf2, which hybridizes with the isoform transcribed with the distal promoter. These probes were hybridized with tissue sections of mouse calvaria taken at various time points in development. Runx2 expression was localized to the critical area of cranial suture closure, being found in parietal bones, osteogenic fronts, and sutural mesenchyme. Pebp2alphaA and Osf2 showed tissue-specific expression patterns. The sites of Pebp2alphaA expression were almost identical to that of pRunx2 hybridization but expression was most intense in the sutural mesenchyme, where undifferentiated mesenchymal cells reside. The Osf2 isoform was strongly expressed in the osteogenic fronts, as well as in developing parietal bones, where osteopontin (OP) and osteocalcin (OC) also were expressed. However, in contrast to Pebp2alphaA, Osf2 expression did not occur in sutural mesenchyme. Pebp2alphaA also was expressed prominently in primordial cartilage that is found under the sutural mesenchyme and is not destined to be mineralized. Thus, Osf2 isoforms contribute to events later in osteoblast differentiation whereas the Pebp2alphaA isoform participates in a wide variety of cellular activities ranging from early stages of osteoblast differentiation to the final differentiation of osteoblasts.

Insulin-like growth factor binding proteins in femoral and vertebral bone marrow stromal cells: expression and regulation by thyroid hormone and dexamethasone

Insulin-like growth factor (IGF)-I is an important regulator of bone metabolism. Clinical observations suggest that different anatomic sites within the adult skeleton respond differently to hormonal and therapeutic treatment, and recent studies on bone marrow stromal cells in culture show that there are skeletal site-dependent differences in the gene expression of IGF-I. The actions of IGF-I and -II on bone cells are known to be modulated by the IGF binding proteins (IGFBP)-1 through -6 and the Type I and Type II IGF receptors. Therefore, we compared the expression of IGFBP-1 through -6 in adult female rat bone marrow stromal cell cultures derived from two separate skeletal sites: vertebrae and femurs. The cultures were maintained simultaneously under conditions that support osteoblast differentiation from osteoprogenitors present in the femoral and vertebral marrow cell populations. We also addressed whether IGFBP messenger RNA levels are regulated by thyroid hormone (T(3)) and dexamethasone (dex) treatment in femoral vs. vertebral marrow stromal cells in vitro, since steroid hormones play an important role in skeletal function. Northern blot analyses revealed that there are distinct skeletal site differences in the gene expression of IGFBPs. The vertebral marrow cultures express IGFBP-2 through -6 mRNAs, with IGFBP-2, IGFBP-4, and IGFBP-6 mRNAs predominating. The femoral marrow stromal cell cultures express only IGFBP-4 and IGFBP-6. Importantly, vertebral marrow cultures have much higher IGFBP mRNA steady-state levels than femoral cultures for all the detected IGFBP transcripts. IGFBP-1 is not detected in either femoral or vertebral cultures. In addition to a skeletal site difference, we show that T(3) and dex regulate the expression of specific IGFBP mRNAs. T(3) treatment also upregulates IGF-I protein secretion by vertebral marrow stromal cell cultures. Interestingly, the type I receptor for IGF-I was expressed equivalently in cultures from the two skeletal sites. These findings have important implications for the anatomical site specificities of hormonal responses that are noted in the skeleton.