Influence of adult Schwann cells and olfactory ensheathing glia on axon--target cell interactions in the CNS: a comparative analysis using a retinotectal co-graft model

We used an in vivo transplant approach to examine how adult Schwann cells and olfactory ensheathing glia OEG influence the specificity of axon-target cell interactions when they are introduced into the CNS. Populations of either Schwann cells or OEG were mixed with dissociated fetal tectal cells presumptive superior colliculus and, after reaggregation, pieces were grafted onto newborn rat superior colliculus. Both glial types were prelabeled with lentiviral vectors encoding green fluorescent protein. Grafts rapidly established fiber connections with the host and retinal projections into co-grafts were assessed 656 days posttransplantation by injecting cholera toxin B into host eyes. In control rats that received pure dissociated-reaggregated tectal grafts, retinal ganglion cell RGC axons selectively innervated defined target areas, corresponding to the retinorecipient layer in normal superior colliculus. The pattern of RGC axon ingrowth into OEG containing co-grafts was similar to that in control grafts. However, in Schwann cell co-grafts there was reduced host retinal input into presumptive target areas and many RGC axons were scattered throughout the neuropil. Given that OEG in co-grafts had minimal impact on axon-target cell recognition, OEG might be an appropriate cell type for direct transplantation into injured neuropil when attempting to stimulate specific pathway reconstruction.     

Establishment of murine cell lines by constitutive and conditional immortalization

Mouse cell lines were immortalized by introduction of specific immortalizing genes. Embryonic and adult animals and an embryonal stem cell line were used as a source of primary cells. The immortalizing genes were either introduced by DNA transfection or by ecotropic retrovirus transduction. Fibroblasts were obtained by expression of SV40 virus large T antigen (TAg). The properties of the resulting fibroblast cell lines were reproducible, independent of the donor mouse strains employed and the cells showed no transformed properties in vitro and did not form tumors in vivo. Endothelial cell lines were generated by Polyoma virus middle T antigen expression in primary embryonal cells. These cell lines consistently expressed relevant endothelial cell surface markers. Since the expression of the immortalizing genes was expected to strongly influence the cellular characteristics fibroblastoid cells were reversibly immortalized by using a vector that allows conditional expression of the TAg. Under inducing conditions, these cells exhibited properties that were highly similar to the properties of constitutively immortalized cells. In the absence of TAg expression, cell proliferation stops. Cell growth is resumed when TAg expression is restored. Gene expression profiling indicates that TAg influences the expression levels of more than 1000 genes that are involved in diverse cellular processes. The data show that conditionally immortalized cell lines have several advantageous properties over constitutively immortalized cells.     

Activity-dependent gene regulation in conditionally-immortalized muscle precursor cell lines

Skeletal muscle contractile activity has been implicated in many aspects of muscle cell differentiation and maturation. Much of the research in this area has depended upon costly and labor-intensive cultures of isolated primary muscle cells because widely available immortalized muscle cell lines often do not display a high level of either spontaneous or stimulated contractile activity. We sought to develop conditionally-immortalized skeletal muscle cell lines that would provide a source of myofibers that exhibit robust spontaneous contractile activity similar to primary muscle cultures. Using a tetracycline-regulated retroviral vector expressing a temperature-sensitive T-antigen to infect primary myoblasts, we isolated individual clonal muscle precursor cell lines that have characteristics of activated satellite cells during growth and rapidly differentiate into mature myotubes with spontaneous contractile activity after culture in non-transformation-permissive conditions. Comparison of these cell lines (known as rat myoblast-like tetracycline (RMT) cell lines) to primary cell cultures revealed that they share a wide variety of morphological, physiological, and biochemical characteristics. Most importantly, the time-course and extent of activity-dependent gene regulation observed in primary cell culture for all genes tested, including subunits of the nicotinic acetylcholine receptor (nAChR), muscle specific kinase (MuSK), and myogenin, is reproduced in RMT lines. These immortalized cell lines are a useful alternative to primary cultures for studying muscle differentiation and molecular and physiological aspects of electrical activity in muscle fibers.     

Immortalized Human Schwann Cell Lines Derived From Tumors of Schwannomatosis Patients

Schwannomatosis, a rare form of neurofibromatosis, is characterized predominantly by multiple, often painful, schwannomas throughout the peripheral nervous system. The current standard of care for schwannomatosis is surgical resection. A major obstacle to schwannomatosis research is the lack of robust tumor cell lines. There is a great need for mechanistic and drug discovery studies of schwannomatosis, yet appropriate tools are not currently available. Schwannomatosis tumors are difficult to grow in culture as they survive only a few passages before senescence. Our lab has extensive experience in establishing primary and immortalized human Schwann cell cultures from normal tissue that retain their phenotypes after immortalization. Therefore we took on the challenge of creating immortalized human Schwann cell lines derived from tumors from schwannomatosis patients. We have established and fully characterized 2 schwannomatosis cell lines from 2 separate patients using SV40 virus large T antigen. One patient reported pain and the other did not. The schwannomatosis cell lines were stained with S100B antibodies to confirm Schwann cell identity. The schwannomatosis cells also expressed the Schwann cell markers, p75NTR, S100B, and NGF after multiple passages. Cell morphology was retained following multiple passaging and freeze/ thaw cycles. Gene expression microarray analysis was used to compare the cell lines with their respective parent tumors. No differences in key genes were detected, with the exception that several cell cycle regulators were upregulated in the schwannomatosis cell lines when compared to their parent tumors. This upregulation was apparently a product of cell culturing, as the schwannomatosis cells exhibited the same expression pattern of cell cycle regulatory genes as normal primary human Schwann cells. Cell growth was also similar between normal primary and immortalized tumor cells in culture. Accurate cell lines derived directly from human tumors will serve as invaluable tools for advancing schwannomatosis research, including drug screening.     

Schwann cell proliferation in vitro is under negative autocrine control

In healthy adult peripheral nerve, Schwann cells are believed to be generally quiescent. Similarly, cultures of isolated rat sciatic nerve Schwann cells hardly proliferate in serum-supplemented medium. The possibility that Schwann cells negatively regulate their own proliferation was supported by the demonstration that conditioned media from Schwann cell cultures inhibited the proliferation of mitogen- stimulated test cultures. The inhibition could be complete, was dose dependent, and was exhibited when the test Schwann cells were under the influence of different types of mitogens such as cholera toxin, laminin, and living neurons. The inhibition of proliferation was completely reversible and a rapid doubling of cell number resulted when treatment with conditioned medium was withdrawn from mitogen-stimulated Schwann cells. Conditioned medium from cholera toxin-stimulated and immortalized Schwann cell cultures contained less antiproliferative activity than that found in medium from quiescent Schwann cell cultures. However, media conditioned by two actively proliferating rat Schwannoma cell lines were rich sources of antiproliferative activity for Schwann cells. Unlike the mitogen-stimulated Schwann cells, whose proliferation could be inhibited completely, the immortalized and transformed Schwann cell types were nearly unresponsive to the antiproliferative activity. The antiproliferative activity in Schwann and Schwannoma cell conditioned media was submitted to gel filtration and SDS-PAGE. The activity exists in at least two distinct forms: (a) a high molecular weight complex with an apparent molecular mass greater than 1,000 kD, and (b) a lower molecular weight form having a molecular mass of 55 kD. The active 55-kD form could be derived from the high molecular weight form by gel filtration performed under dissociating conditions. The 55-kD form was further purified to electrophoretic homogeneity. These results suggest that Schwann cells produce an autocrine factor, which we designate as a "neural antiproliferative protein," which completely inhibits the in vitro proliferation of Schwann cells but not that of immortalized Schwann cells or Schwannoma lines.     

Axonal signalling and the making of olfactory ensheathing cells: a hypothesis

Olfactory ensheathing cells (OECs) are Schwann cell-like glial cells of the olfactory system that promote neural repair under experimental conditions. It is a matter of debate in how far OECs resemble Schwann cells and whether they possess specific properties. Although OECs have been characterized mainly with respect to their regenerative effects after transplantation, both their cellular identity and the regulating factors involved have remained vague. The aim of this article is to define OEC and Schwann-cell identity in molecular terms, and to discuss crucial factors that are involved in determination in vitro and in vivo. Distinct OEC features such as the down-regulation of the low affinity neurotrophin receptor p75(NTR) by neuronal contact are apparent in vivo under physiological conditions, whereas OECs acquire a Schwann cell-like phenotype and up-regulate p75(NTR) expression in vitro and following transplantation into the lesioned spinal cord. This might indicate that establishment of the OEC phenotype depends on specific axonal stimuli. In this review we hypothesize that OECs and Schwann cells possess malleable cellular phenotypes that acquire distinct features only upon specific interaction with their natural neuronal partner.This concept is consistent with previous findings in vitro and in vivo, and might be relevant for studies that use OECs and Schwann cells for nervous system repair.     

Generation of an immortalized human CD4+ T cell clone inhibiting tumor growth in mice

Tumor antigen-specific T cell clones represent a useful tool in tumor immunology; however, their long-term culture is limited. To generate an immortalized cytotoxic T cell clone against the human tumor antigen mucin, we exposed a previously generated T cell culture to Herpesvirus saimiri. We obtained an immortalized human CD4+ T cell clone, termed SITAM. Clonality of these cells was shown by analysis of the alpha/beta-T cell receptor (TCR) repertoire. Cytolytic activity was demonstrated against several mucin-expressing tumor cell lines and could not be detected against non-mucin-expressing cells. SITAM cells maintained their features stably for 2 years. Furthermore, growth of the tumor cell line Capan-2 in NOD/SCID mice was inhibited when SITAM cells were coinjected subcutaneously with tumor cells. SITAM cells provide an unlimited source of clonal T cells for analysis of tumor recognition and may be of help in TCR-targeted immunotherapy.     

Efficient derivation of new human embryonic stem cell lines

Human embryonic stem (hES) cells, unlike most cells derived from adult or fetal human tissues, represent a potentially unlimited source of various cell types for basic clinical research. To meet the increased demand for characterized hES cell lines, we established and characterized nine new lines obtained from frozen-thawed pronucleus-stage embryos. In addition, we improved the derivation efficiency from inner cell masses (to 47.4%) and optimized culture conditions for undifferentiated hES cells. After these cell lines had been maintained for over a year in vitro, they were characterized comprehensively for expression of markers of undifferentiated hES cells, karyotype, and in vitro/in vivo differentiation capacity. All of the cell lines were pluripotent, and one cell line was trisomic for chromosome 3. Improved culture techniques for hES cells should make them a good source for diverse applications in regenerative medicine, but further investigation is needed of their basic biology.     

Role of A-kinase anchoring proteins in cyclic-AMP-mediated Schwann cell proliferation

Proliferation of Schwann cells during peripheral nerve development is stimulated by the heregulin/neuregulin family of growth factors expressed by neurons. However, for neonatal rat Schwann cells growing in culture, heregulins produce only a weak mitogenic response. Supplementing heregulin with forskolin, an agent that elevates cyclic AMP levels, produces a dramatic increase in the proliferation of cultured Schwann cells. The mechanisms underlying this synergistic effect required for Schwann cell proliferation in vivo is not well established. Characterizing the A-kinase anchoring proteins (AKAPs) in Schwann cells might help identify substrates tethered to and phosphorylated by the cAMP-dependent protein kinase A (PKA). Using an RII overlay assay that detects AKAPs that are bound to the type II regulatory subunits of PKA, we identified AKAP150 in Schwann cells. Western blot analysis revealed that additional AKAPs, specifically AKAP95, and yotiao were also present. Disruption of PKA/AKAP interaction with Ht-31 peptide resulted in an increase in luciferase-conjugated cyclin D3 promoter activity. Transfection with sequence-specific AKAP siRNAs for AKAP150 and AKAP95 produced a marked reduction in cell proliferation. Immunoblot analysis revealed that knock down of AKAP95 protein caused a significant decrease in expression of the cell cycle regulatory proteins cyclin D2, cyclin D3 and the cell survival signal Akt/Protein Kinase B (Akt/PKB). Morphological characterization of Schwann cell AKAPs indicated the presence of nuclear (AKAP95), cytoplasm-associated (AKAP150) and perinuclear (yotiao) A-kinase anchoring proteins. These results indicate a role for AKAP95 and AKAP150 in the synergistic response of Schwann cells to treatment with heregulin and forskolin.     

V-myc immortalization of early rat neural crest cells yields a clonal cell line which generates both glial and adrenergic progenitor cells.

We describe the isolation and characterization of an immortal cell line derived by infection of rat neural crest cells with a v-myc-containing replication-defective retrovirus. This clonal cell line, called NCM-1, contains a majority cell population with antigenic and morphologic properties that suggest it may represent a peripheral glial progenitor. In conditioned or in serum-free medium, these NGF receptor-positive cells differentiate to an elongated, bipolar morphology resembling that of primary Schwann cells. This morphologic differentiation is prevented by TGF-beta 1, which also acts as a mitogen for the cells. The NCM-1 line is also able to generate clonal derivatives which have extinguished expression of most or all glial markers. Once generated, such cells are stable and do not revert to the glial phenotype. At least some of these cells have acquired sympathoadrenal progenitor-like properties, as shown by their capacity to coexpress tyrosine hydroxylase (TH) and neurofilament (NF) in response to basic FGF and dexamethasone. These data imply that the NCM-1 line contains self-renewing cells with the potential to generate precursors in at least two of the sublineages that normally develop from the neural crest. This in turn suggests that the process of immortalization may preserve at least some of the developmental properties characteristic of multipotential neural crest cells. NCM-1 cells may prove useful for the study of neural crest cell lineage segregation, Schwann cell differentiation, and the mechanisms controlling the initial induction of TH and NF gene expression.     

L-3,4-Dihydroxyphenylalanine Synthesis by Genetically Modified Schwann Cells

We have investigated whether Schwann cells can be modified by gene transfer to synthesize L-3,4-dihydroxyphenylalanine (L-DOPA), the immediate precursor in the formation of dopamine. By using a retrovirus containing a rat tyrosine hydroxylase (TH) cDNA, we established an immortalized rodent Schwann cell line that stably expressed high levels of TH and secreted L-DOPA in vitro when supplied with tyrosine and the essential cofactor biopterin. We also infected primary Schwann cells and demonstrated that cells expressing TH secreted L-DOPA while maintaining their capacity to myelinate neurons in vitro. This study indicate that it may be feasible to utilize autotransplantation of genetically modified Schwann cells to alleviate the movement disorders in Parkinson's disease.     

Characterization and differentiation of human embryonic stem cells

Cell replacement therapies have been limited by the availability of sufficient quantities of cells for transplantation. Human ES (hES) cell lines have recently been generated by several laboratories. When maintained for over 1 year in vitro, they remain karyotypically and phenotypically stable and may therefore provide an excellent source material for cell therapies. Currently, data is available for 26 hES cell lines. Although limited characterization has been performed on most of these lines, there are remarkable similarities in expression of markers. hES cell lines derived in different laboratories show similar expression profiles of surface markers, including SSEA-4, Tra-1-60, and Tra-1-81. In addition, markers associated with pluripotent cells such as OCT-4 are expressed at in all cell lines tested. These cells express high levels of telomerase and appear to have indefinite growth potential. The generation of the large quantities of cells necessary for cell replacement therapies will require a cell population which is stable over long term culture. We have characterized the properties of multiple hES cell lines that have been maintained in culture for extended periods. Quantitative analyses demonstrate that all of the cell lines examined show consistent marker expression and retain a normal karyotype after long-term culture. hES cells have been differentiated into the derivatives of all three germ layers. Specifically this includes cardiomyocytes, neural cells, hepatocyte-like cells, endothelial cells and hematopoietic progenitor cells. These data demonstrating the karyotypic and phenotypic stability of hES cells and their extensive differentiative capacity indicate that they may be an appropriate source of cells for multiple regenerative medicine applications.     

Expression of the Myelin-Associated Glycoprotein in Cultures of Immortalized Schwann Cells

Although the myelin-associated glycoprotein (MAG) cannot be detected in primary cultures of rat Schwann cells in the absence of neurons, MAG expression was demonstrated in some lines of cultured Schwann cells that had been immortalized by repetitive passaging. Radioimmunoassay of one such Schwann cell line, S-16, showed a remarkably high MAG concentration of about 1 ng/microgram of total protein, a level that is comparable to the MAG concentration in adult sciatic nerve. The S-16 cells divide very rapidly, are rounder than normal Schwann cells, and elaborate many processes after reaching high density. The cells are galactocerebroside positive, but express little or no P0 glycoprotein or myelin basic protein. As in nerve, the MAG synthesized by the cultured cells is primarily the shorter isoform (S-MAG). Furthermore, the posttranslational processing resembles that occurring in vivo including a similar degree of glycosylation, sulfation of oligosaccharides, and phosphorylation of the polypeptide. The sensitivity of MAG to treatment of the intact cells with trypsin or neuraminidase, as well as surface labeling with [3H]borohydride reduction after periodate oxidation, demonstrated that most of the MAG expressed by the S-16 cells is located on the cell surface. This line of immortalized Schwann cells expressing a remarkably high level of MAG should be useful for investigating the cell biology and function of this glycoprotein.     

Heterogeneity of the tumorigenic phenotype expressed by Madin-Darby canine kidney cells

The mechanisms by which cells spontaneously immortalized in tissue culture develop the capacity to form tumors in vivo likely embody fundamental processes in neoplastic development. The evolution of Madin-Darby canine kidney (MDCK) cells from presumptively normal kidney cells to immortalized cells that become tumorigenic represents an example of neoplastic development in vitro. Studies of the mechanisms by which spontaneously immortalized cells develop the capacity to form tumors would benefit from quantitative in vivo assays. Most mechanistic correlations are evaluated by using single-dose tumor-induction experiments, which indicate only whether cells are or are not tumorigenic. Here we used quantitative tumorigenicity assays to measure dose-and time-dependent tumor development in nude mice of 3 lots of unmodified MDCK cells. The results revealed lot-to-lot variations in the tumorigenicity of MDCK cells, which were reflected by their tumor-inducing efficiency (threshold cell dose represented by mean tumor-producing dose; log(10) 50% endpoints of 5.2 for vial 1 and 4.4 for vial 2, and a tumor-producing dose of 5.8 for vial 3) and mean tumor latency (vial 1,6.6 wk; vial 2,2.9 wk; and vial 3,3.8 wk). These studies provide a reference for further characterization of the MDCK cell neoplastic phenotype and may be useful in delineating aspects of neoplastic development in vitro that determine tumor-forming capacity. Such data also are useful when considering MDCK cells as a reagent for vaccine manufacture.     

Isolation of a stem cell for neurons and glia from the mammalian neural crest.

We have isolated mammalian neural crest cells using a monoclonal antibody to the low affinity NGF receptor, and established conditions for the serial propagation of these cells in clonal culture to assess their developmental potential. This analysis indicates that, first, single mammalian neural crest cells are multipotent, able to generate at least neurons and Schwann cells like their avian counterparts. Second, multipotent neural crest cells generate multipotent progeny, indicating that they are capable of self-renewal and therefore are stem cells. Third, multipotent neural crest cells also generate some clonal progeny that form only neurons or glia, suggesting the production of committed neuroblasts and glioblasts. Manipulation of the substrate alters the fate of the multipotent cells. These findings have implications for models of neural crest development in vivo, and establish a system for studying the generation of cellular diversity by a multipotent stem cell in vitro.     

In vitro characteristics of metastatic variant subclones of restricted genetic origin

We have studied several metastatic variant cell lines derived from a common clonal origin and their transformed and untransformed parental cell lines. A number of in vitro characteristics were examined for each tumor line and these properties were correlated with the ability of the tumor cells to form pulmonary nodules in an experimental metastasis assay. Direct correlations with metastatic behavior in the lung colony assay were found to exist with the amount of cell-bound Concanavalin A and the procoagulant activities of cell lysates. In vitro parameters that did not correlate with the metastatic phenotype were: population doubling times in culture, saturation density achieved in culture, the number of colony-forming cells shed from confluent cultures, rates of cellular attachment to homotypic or heterotypic cell monolayers, plasminogen-activator production and procoagulant activity produced in serum-free conditioned medium.