INTRACELLULAR ACCUMULATION OF PROTOCOLLAGEN AND EXTRUSION OF COLLAGEN BY EMBRYONIC CARTILAGE CELLS

The synthesis of collagen can be interrupted, after the assembly of proline-rich and lysine-rich polypeptide chains called protocollagen, by incubating connective tissues anaerobically. Under these conditions the proline and lysine residues in protocollagen are not hydroxylated to hydroxyproline and hydroxylysine, and protocollagen molecules accumulate intracellularly. Chemical data and radioautographs at the level of the light and electron microscopes indicated that in tissues labeled with proline-3,4-³H under nitrogen, there appeared to be an accumulation of radioactivity over the ground cytoplasm. When the inhibition of protocollagen hydroxylase was reversed by exposing the tissue to oxygen, the accumulated protocollagen-³H was converted to collagen-³H and there was a rapid transfer of label from the ground cytoplasm to the extracellular matrix. There was no significant change in distribution of label over either the Golgi vacuoles or the cisternae of the endoplasmic reticulum. The failure to find a significant change in distribution of label over the Golgi vacuoles or the cisternae does not completely exclude the possibility that these two compartments are involved in the extrusion, but the data are consistent with the simpler notion that the completed collagen molecules pass directly from the ground cytoplasm to the extracellular matrix.     

Dkk-1-derived synthetic peptides and lithium chloride for the control and recovery of adult stem cells from bone marrow

It is established that human mesenchymal stem cells (hMSCs) from bone marrow are a source of osteoblast progenitors in vivo and under appropriate conditions, differentiate into osteoblasts ex vivo. Because hMSCs are recovered by iliac crest aspirate and enriched by virtue of their adherence to tissue culture plastic, the cells provide a convenient ex vivo model for the study of osteogenic tissue repair in an experimentally accessible system. Recent advances in the field of skeletal development and osteogenesis have demonstrated that signaling through the canonical wingless (Wnt) pathway is critical for the differentiation of progenitor cell lines into osteoblasts. Inhibition of such signals can predispose MSCs to cell cycle entry and inhibit osteogenesis. Here, we report that synthetic peptides derived from the second cysteine-rich domain of the canonical Wnt inhibitor Dickkopf-1 (Dkk-1) have utility in controlling the growth and recovery of hMSCs from bone marrow stroma. Three peptides corresponding to residues 217-269 in Dkk-1 were each found to enhance the proliferation of hMSCs in culture over 2 days. The most active peptide exhibited agonistic characteristics in that it ablated the proliferation lag observed when cultures of hMSCs receive fresh medium. It also reduced the expression of endogenous Dkk-1 (Gregory, C. A., Singh, H., and Prockop, D. J. (2003) J. Biol. Chem. 278, 28067-28078). When the cytosolic level of beta-catenin was elevated by addition of LiCl to cultures of hMSCs, the peptide also accelerated degradation of beta-catenin on withdrawal of lithium. A second peptide, corresponding to residues 184-204 had preferential and high affinity for hMSCs in the log phase of proliferation. Peptide overlay assays on hMSC lysates confirmed that the peptide bound to a 184-kDa protein corresponding to the molecular mass of LRP6. Cells recovered by this peptide had enhanced osteogenic potential but less chondrogenic potential compared with controls. Because Wnt antagonists increase the number of non-committed hMSCs in culture, they may be of use in increasing the rate of osseous wound healing in vivo by increasing the level of systemically migrating hMSCs. Therefore, such molecules could contribute to the development of a novel family of pharmaceutical agents for the improvement of the healing process in humans.     

Non-hematopoietic bone marrow stem cells: molecular control of expansion and differentiation

The first non-hematopoietic mesenchymal stem cells (MSCs) were discovered by Friedenstein in 1976, who described clonal, plastic adherent cells from bone marrow capable of differentiating into osteoblasts, adipocytes, and chondrocytes. More recently, investigators have now demonstrated that multi-potent MSCs can be recovered from a variety of other adult tissues and differentiate into numerous tissue lineages including myoblasts, hepatocytes and possibly even neural tissue. Because MSCs are multipotent and easily expanded in culture, there has been much interest in their clinical potential for tissue repair and gene therapy and as a result, numerous studies have been carried out demonstrating the migration and multi-organ engraftment potential of MSCs in animal models and in human clinical trials. This review describes the recent advances in the understanding of MSC biology.     

In vitro differentiation of human marrow stromal cells into early progenitors of neural cells by conditions that increase intracellular cyclic AMP

Human marrow stromal cells (hMSCs) are multipotential stem cells that can be differentiated into bone, cartilage, fat, and muscle. In the experiments here, we found that undifferentiated cultures of hMSCs express some markers characteristic of neural cells such as microtubule-associated protein 1B (MAP1B), neuron-specific tubulin (TuJ-1), neuron-specific enolase (NSE), and vimentin. By treating hMSCs with 0.5 mM isobutylmethylxanthine (IBMX)/1 mM dibutyryl cyclic AMP (dbcAMP) for 6 days, about 25% of the hMSCs differentiated into cells with a typical neural cell morphology and with increased levels of both NSE and vimentin. The data suggested that the hMSCs may have been differentiated into early progenitors of neural cells in vitro under conditions that increase the intracellular level of cAMP.     

Mutations that alter the primary structure of type I procollagen have long-range effects on its cleavage by procollagen N-proteinase

Type I/II procollagen N-proteinase was partially purified from chick embryos and used to examine the rate of cleavage of a series of purified type I procollagens synthesized by fibroblasts from probands with heritable disorders of connective tissue. The rate of cleavage was normal with procollagen from a proband with osteogenesis imperfecta that was overmodified by posttranslational enzymes. Therefore, posttranslational overmodification of the protein does not in itself alter the rate of cleavage under the conditions of the assay employed. Cleavage of the procollagen, however, was altered in several procollagens with known mutations in primary structure. Two of the procollagens had in-frame deletions of 18 amino acids encoded by exons 11 and 33 of the pro alpha 2(I) gene. In both procollagens, both the pro alpha 1(I) and the pro alpha 2(I) chains were totally resistant to cleavage. With a procollagen in which glycine-907 of the alpha 2(I) chain domain was substituted with aspartate, both pro alpha chains were cleaved but at a markedly decreased rate. The results, therefore, establish that mutations that alter the primary structure of the pro alpha chains of procollagen at sites far removed from the N-proteinase cleavage site can make the protein resistant to cleavage by the enzyme. The long-range effects of in-frame deletions or other changes in amino acid sequence are probably explained by their disruption of the hairpin structure that is formed by each of the three pro alpha chains in the region containing the cleavage site and that is essential for cleavage of the procollagen molecule by N-proteinase.