The cell surface of embryonic peripheral neurons provides a mitogenic stimulus for Schwann cells. We report (a) the solubilization of this mitogenic activity from rat dorsal root ganglion neurons grown in tissue culture and (u) the solubilization and partial purification of mitogenic activity from neonatal rat brains. Extracted mitogenic activity is peripheral rather than intrinsic to the membrane, stable after extraction, and active as a mitogen in the absence of serum (the most tringent criterion defning the neuronal mitogen). We have previously provided evidence suggesting that a neuronal cellsurface heparan sulfate proteoglycan is required for expression of the neurons' mitogenic activity. We now show that mitogenic activity can be extracted from the membrane diociated from proteoglycan as assayed by its ability to bind to immobilized heparin. After dissociation, low concentrations of heparin (1 Fg/ml) inhibit the ability of the mitogen to stimulate Schwann cell division. Basic fibroblast growth factor (FGF) is weakly mitogenic for Schwann cells, but it is not present in mitogenic brain extracts (based on immunoblottig. Immunodepletion experiments with specific antibodies to FGF indicate that the mitogenic activity extracted from neurons is not a form of this heparin-binding mitogen. Acidic FGF is not mitogenic for Schwann cells and is not present in mitogenic brain extracts. We suggest that these and previous data indicate the neurite mitogen is a proteoglycan-growth factor complex that limits mitogenic activity to the axonal surface, protects mitogen against inactivation by other proteoglycans, and provides for effective presentation of mitogen to the Schwann cell.A complex series of cell-cell interactions among neurons, Schwann cells, and fibroblasts lead to the formation of the peripheral nerve. One of these interactions is the neuronstimulated proliferation of Schwann cells, first suggested by experiments in which the ratio of Schwann cells to neurons remained constant when neuronal numbers were altered (1). These in vivo observations have been extended in an in vitro system is which neurons and Schwann cells can be separated and recombined; in vitro studies demonstrate that contact with the neuronal surface causes Schwann cells to undergo a dramatic increase in both DNA synthesis and cell division (2-4). Several lines of evidence indicate that the cell surface of the neuron (rather than a released factor) carries the mitogenic stimulus. Membrane fragments from dorsal root ganglion neurons grown in tissue culture mimic the mitogenicity of whole cells, and trypsin treatment of intact neurons in culture prevents the mitogenic response of Schwann cells added to these neurons (5). Furthermore, a semipermeable collagen membrane interposed between Schwann cells and neurons in the culture dish inhibits the mitogenic response (6), and conditioned medium from neurons is not mitogenic for Schwann cells (2, 4).Recent tissue culture studies indicate that the neuronal cell-surface mitogen is associated coval...

The neuronal cell surface is believed to carry a mitogenic signal for peripheral glial cells. We have purified a mitogen from fetal bovine brain membranes that, in common with the PNS neuronal mitogen, stimulates the proliferation of Schwann cells in vitro and binds heparin. The purified mitogen has an apparent molecular weight of 50,000 daltons as estimated by elution of activity from non-reducing polyacrylamide gels. Since the developing central nervous system is a rich source of mitogen, we tested whether the protein is mitogenic for one or more cell types isolated from the developing brain. Purified mitogen was added to enriched cultures of astrocytes or developing oligodendrocytes, or to microglial cells. The analyses demonstrated that the protein is mitogenic for developing oligodendrocytes but not astrocytes or microglial cells. These results suggest that during development a membrane-associated mitogen present in the brain might regulate the proliferation of developing oligodendrocytes, and consequently, the population size of oligodendrocytes in the brain.

Most, but not all, patients with von Recklinghausen neurofibromatosis develop tumors (neurofibromas) that contain large numbers of Schwann cells and fibroblasts. To begin to understand the molecular events that contribute to cell proliferation in these benign tumors, we have analyzed extracts of neurofibromas to determine whether they contain mitogens for Schwann cells or fibroblasts, or both. Schwann cell and fibroblast mitogens are present in neurofibroma extracts. All the neurofibromas analyzed contain a Schwann cell mitogen similar to a neuronal cell surface molecule known to stimulate Schwann cell proliferation during normal development; this mitogen also stimulates fibroblast proliferation. Basic fibroblast growth factor is present in 60% of tumors evaluated. Accumulation of mitogenic substances may contribute to the growth of neurofibromas.