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M C Raff

Publications and source records attributed to M C Raff.

At least 55 records · Page 3Linked to original sources
PubMed1994

A crucial role for neurotrophin-3 in oligodendrocyte development.

The neurotrophins nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 promote the survival of subpopulations of vertebrate neurons in vitro, but so far only nerve growth factor has been demonstrated to be essential for normal neuronal development; no neurotrophin has previously been shown to function in normal glial cell development. We found recently that neurotrophin-3 promotes the survival of pure oligodendrocyte precursor cells in vitro, and, although by itself it induces only a small percentage of these cells to synthesize DNA, in combination with platelet-derived growth factor it induces the majority of them to do so. Neither of these factors, however, has been shown to contribute to oligodendrocyte precursor cell proliferation in vivo or to stimulate pure populations of these cells to proliferate (as opposed to synthesize DNA) in vitro. Here we show that neurotrophin-3 and platelet-derived growth factor collaborate to promote clonal expansion of oligodendrocyte precursor cells in vitro and to drive the intrinsic clock that times oligodendrocyte development. We also show that neurotrophin-3 helps stimulate the proliferation of oligodendrocyte precursor cells in vivo and is thus required for normal oligodendrocyte development.

Animals↗
PubMed1994

Autocrine signals enable chondrocytes to survive in culture.

We recently proposed that most mammalian cells other than blastomeres may be programmed to kill themselves unless continuously signaled by other cells not to. Many observations indicate that some mammalian cells are programmed in this way, but is it the case for most mammalian cells? As it is impractical to test all of the hundreds of types of mammalian cells, we have focused on two tissues--lens and cartilage--which each contain only a single cell type: if there are cells that do not require signals from other cells to avoid programmed cell death (PCD), lens epithelial cells and cartilage cells (chondrocytes) might be expected to be among them. We have previously shown that rat lens epithelial cells can survive in serum-free culture without signals from other cell types but seem to require signals from other lens epithelial cells to survive: without such signals they undergo PCD. We show here that the same is true for rat (and chick) chondrocytes. They can survive for weeks in culture at high cell density in the absence of other cell types, serum, or exogenous proteins or signaling molecules, but they die with the morphological features of apoptosis in these conditions at low cell density. Medium from high density cultures, FCS, or a combination of known growth factors, all support prolonged chondrocyte survival in low density cultures, as long as antioxidants are also present. Moreover, medium from high density chondrocyte cultures promotes the survival of lens epithelial cells in low density cultures and vice versa. Chondrocytes isolated from adult rats behave similarly to those isolated from developing rats. These findings support the hypothesis that most mammalian cells require signals from other cells to avoid PCD, although the signals can sometimes be provided by cells of the same type, at least in tissues that contain only one cell type.

Aging↗
PubMed1994

A novel role for thyroid hormone, glucocorticoids and retinoic acid in timing oligodendrocyte development.

The timing of oligodendrocyte differentiation is thought to depend on an intrinsic clock in oligodendrocyte precursor cells that counts time or cell divisions and limits precursor cell proliferation. We show here that this clock mechanism can be separated into a counting component and an effector component that stops cell proliferation: whereas the counting mechanism is driven by mitogens that activate cell-surface receptors, the effector mechanism depends on hydrophobic signals that activate intracellular receptors, such as thyroid hormones, glucocorticoids and retinoic acid. When purified oligodendrocyte precursor cells are cultured at clonal density in serum-free medium in the presence of mitogens but in the absence of these hydrophobic signals, the cells divide indefinitely and do not differentiate into postmitotic oligodendrocytes. In the absence of mitogens, the precursor cells stop dividing and differentiate prematurely into oligodendrocytes even in the absence of these hydrophobic signals, indicating that these signals are not required for differentiation. The levels of these signals in vivo may normally regulate the timing of oligodendrocyte differentiation, as the maximum number of precursor cell divisions in culture depends on the concentration of such signals and injections of thyroid hormone into newborn rats accelerates oligodendrocyte development. As thyroid hormone, glucocorticoids and retinoic acid have been shown to promote the differentiation of many types of vertebrate cells, it is possible that they help coordinate the timing of differentiation by signalling clocks in precursor cells throughout a developing animal.

Animals↗
PubMed1993

Programmed cell death and the control of cell survival: lessons from the nervous system.

During the development of the vertebrate nervous system, up to 50 percent or more of many types of neurons normally die soon after they form synaptic connections with their target cells. This massive cell death is thought to reflect the failure of these neurons to obtain adequate amounts of specific neurotrophic factors that are produced by the target cells and that are required for the neurons to survive. This neurotrophic strategy for the regulation of neuronal numbers may be only one example of a general mechanism that helps to regulate the numbers of many other vertebrate cell types, which also require signals from other cells to survive. These survival signals seem to act by suppressing an intrinsic cell suicide program, the protein components of which are apparently expressed constitutively in most cell types.

Animals↗
PubMed1993

Does oligodendrocyte survival depend on axons?

BACKGROUND: We have shown previously that oligodendrocytes and their precursors require signals from other cells in order to survive in culture. In addition, we have shown that about 50% of the oligodendrocytes produced in the developing rat optic nerve normally die, apparently in a competition for the limiting amounts of survival factors. We have hypothesized that axons may control the levels of such oligodendrocyte survival factors and that the competition-dependent death of oligodendrocytes serves to match their numbers to the number of axons that they myelinate. Here we test one prediction of this hypothesis - that the survival of developing oligodendrocytes depends on axons. RESULTS: We show that oligodendrocyte death occurs selectively in transected nerves in which the axons degenerate. This cell death is prevented by the delivery of exogenous ciliary neurotrophic factor (CNTF) or insulin-like growth factor I (IGF-1), both of which have been shown to promote oligodendrocyte survival in vitro. We also show that purified neurons promote the survival of purified oligodendrocytes in vitro. CONCLUSION: These results strongly suggest that oligodendrocyte survival depends upon the presence of axons; they also support the hypothesis that a competition for axon-dependent survival signals normally helps adjust the number of oligodendrocytes to the number of axons that require myelination. The identities of these signals remain to be determined.

Journal Article↗
PubMed1993

Bcl-2 blocks apoptosis in cells lacking mitochondrial DNA.

When the mammalian proto-oncogene bcl-2 is overexpressed it can protect various types of cells both from normal and from experimentally induced apoptosis, but the molecular mechanisms involved are unknown. Although the Bcl-2 protein is membrane-associated, its subcellular location is controversial: two studies have suggested that it is mainly associated with the nuclear envelope and endoplasmic reticulum, whereas another study has suggested that it is mainly located in the inner mitochondrial membrane. The latter study has suggested that Bcl-2 might protect cells from apoptosis by altering mitochondrial function and that mitochondria may be involved in apoptosis. Here we report that human mutant cell lines that lack mitochondrial DNA (mtDNA), and therefore do not have a functional respiratory chain, can still be induced to die by apoptosis, and that they can be protected from apoptosis by the overexpression of bcl-2, suggesting that neither apoptosis nor the protective effect of bcl-2 depends on mitochondrial respiration. We also show that the Bcl-2 protein in overexpressing cells is associated with the nuclear envelope and endoplasmic reticulum, as well as with mitochondria.

Alkaloids↗
PubMed1993

Proliferation of oligodendrocyte precursor cells depends on electrical activity in axons.

Oligodendrocytes myelinate axons in the vertebrate central nervous system. It would, therefore, make sense if axons played a part in controlling the number of oligodendrocytes that develop in a myelinated tract. Although oligodendrocytes themselves normally do not divide, the precursor cells that give rise to them do. Here we show that the proliferation of oligodendrocyte precursor cells in the developing rat optic nerve depends on electrical activity in neighbouring axons, and that this activity-dependence can be circumvented by experimentally increasing the concentration of platelet-derived growth factor, which is present in the optic nerve and stimulates these cells to proliferate in culture. These findings suggest that axonal electrical activity normally controls the production and/or release of the growth factors that are responsible for proliferation of oligodendrocyte precursor cells and thereby helps to control the number of oligodendrocytes that develop in the region.

Animals↗
PubMed1993

Control of lens epithelial cell survival.

We have studied the survival requirements of developing lens epithelial cells to test the hypothesis that most cells are programmed to kill themselves unless they are continuously signaled by other cells not to do so. The lens cells survived for weeks in both explant cultures and high-density dissociated cell cultures in the absence of other cells or added serum or protein, suggesting that they do not require signals from other cell types to survive. When cultured at low density, however, they died by apoptosis, suggesting that they depend on other lens epithelial cells for their survival. Lens epithelial cells cultured at high density in agarose gels also survived for weeks, even though they were not in direct contact with one another, suggesting that they can promote one another's survival in the absence of cell-cell contact. Conditioned medium from high density cultures promoted the survival of cells cultured at low density, suggesting that lens epithelial cells support one another's survival by secreting survival factors. We show for the first time that normal cell death occurs within the anterior epithelium in the mature lens, but this death is strictly confined to the region of the anterior suture.

Alkaloids↗
PubMed1993

Multiple extracellular signals are required for long-term oligodendrocyte survival.

We showed previously that oligodendrocytes and their precursors require continuous signalling by protein trophic factors to avoid programmed cell death in culture. Here we show that three classes of such trophic factors promote oligodendrocyte survival in vitro: (1) insulin and insulin-like growth factors (IGFs), (2) neurotrophins, particularly neurotrophin-3 (NT-3), and (3) ciliary-neurotrophic factor (CNTF), leukemia inhibitory factor (LIF) and interleukin 6 (IL-6). A single factor, or combinations of factors within the same class, promote only short-term survival of oligodendrocytes and their precursors, while combinations of factors from different classes promote survival additively. Long-term survival of oligodendrocytes in vitro requires at least one factor from each class, suggesting that multiple signals may be required for long-term oligodendrocyte survival in vivo. We also show that CNTF promotes oligodendrocyte survival in vivo, that platelet-derived growth factor (PDGF) can promote the survival of oligodendrocyte precursors in vitro by acting on a novel, very high affinity PDGF receptor, and that, in addition to its effect on survival, NT-3 is a potent mitogen for oligodendrocyte precursor cells.

Animals↗
PubMed1993

Large-scale normal cell death in the developing rat kidney and its reduction by epidermal growth factor.

Although normal cell death is known to occur in many developing vertebrate organs, it has not been thought to play an important part in the development of the mammalian kidney. We show here that normal cell death is found in both the nephrogenic region and medullary papilla of the developing rat kidney and, in each of these areas, it follows a distinct developmental time course. As many as 3% of the cells in these areas have a typical apoptotic morphology and the dead cells seem to be cleared rapidly (within 1-2 hours) by phagocytosis by neighbouring parenchymal cells. These values are similar to those in vertebrate neural tissues where 50% or more of the cells die during normal development, suggesting that large-scale death is a normal feature of kidney development. We also show that in vivo treatment with epidermal growth factor inhibits cell death in the developing kidney, consistent with the possibility that the cells normally die because they lack sufficient survival factors. Our findings suggest that the extent of normal cell death in developing animals is still greatly underestimated and they raise the possibility that many of these cell deaths may reflect limiting amounts of survival factors.

Animals↗
PubMed1992

Cell death and control of cell survival in the oligodendrocyte lineage.

Dead cells are observed in many developing animal tissues, but the causes of these normal cell deaths are mostly unknown. We show that about 50% of oligodendrocytes normally die in the developing rat optic nerve, apparently as a result of a competition for limiting amounts of survival signals. Both platelet-derived growth factor and insulin-like growth factors are survival factors for newly formed oligodendrocytes and their precursors in culture. Increasing platelet-derived growth factor in the developing optic nerve decreases normal oligodendrocyte death by up to 90% and doubles the number of oligodendrocytes in 4 days. These results suggest that a requirement for survival signals is more general than previously thought and that some normal cell deaths in nonneural tissues may also reflect competition for survival factors.

Animals↗
PubMed1992

Social controls on cell survival and cell death.

Programmed cell death occurs in most animal tissues at some stage of their development, but the molecular mechanism by which it is executed is unknown. For some mammalian cells, programmed death seems to occur by default unless suppressed by signals from other cells. Such dependence on specific survival signals provides a simple way to eliminate misplaced cells, for regulating cell numbers and, perhaps, for selecting the fittest cells. But how general is this dependence on survival signals?

Animals↗
PubMed1992

PDGF increases the expression of Fos and Jun in newly formed oligodendrocytes that have become resistant to the mitogenic effect of PDGF.

Bipotential glial (O-2A) progenitor cells give rise to oligodendrocytes on a predictable schedule in the developing rat optic nerve. The loss of mitotic responsiveness to platelet-derived growth factor (PDGF) that is seen when O-2A progenitor cells initially differentiate into oligodendrocytes seems to reflect blocks or deficiencies downstream to some of the early intracellular signalling events induced by PDGF. Here we show that PDGF increases the expression of the Fos and Jun nuclear proteins in newly formed oligodendrocytes in vitro, suggesting that at least one intracellular signalling pathway to the nucleus is activated by PDGF in these cells even though they do not synthesize DNA in response to PDGF.

Animals↗
PubMed1992

Cell death in the oligodendrocyte lineage.

We have recently found that about 50% of newly formed oligodendrocytes normally die in the developing rat optic nerve. When purified oligodendrocytes or their precursors are cultured in the absence of serum or added signalling molecules, they die rapidly with the characteristics of programmed cell death. This death is prevented either by the addition of medium conditioned by cultures of their normal neighboring cells in the developing optic nerve, or by the addition of platelet-derived growth factor (PDGF) or insulin-like growth factors (IGFs). Increasing PDGF in the developing optic nerve decreases normal oligodendrocyte death by up to 90% and doubles the number of oligodendrocytes, suggesting that this normally occurring glial cell death might result from a competition for limiting amounts of survival signals. These results suggest that competition for limiting amounts of survival factors is not confined to developing neurons, and raise the possibility that a similar mechanism may be responsible for some naturally occurring cell deaths in nonneural tissues.

Animals↗
PubMed1992

Diffusible rod-promoting signals in the developing rat retina.

We previously developed a reaggregate cell culture system in which embryonic rat retinal neuroepithelial cells proliferate and give rise to opsin-expressing rod photoreceptor cells (rods) on the same schedule in vitro as they do in vivo. We showed that the proportion of neuroepithelial cells in the embryonic day 15 (E15) retina that differentiated into opsin+ rods after 5-6 days in such cultures increased by approximately 40-fold when the E15 cells were cultured in the presence of an excess of postnatal day 1 (P1) neural retinal cells. In the present study, we have further analyzed this rod-promoting activity of neonatal neural retinal cells. We show that the activity is mediated by a diffusible signal(s) that seems to act over a relatively short distance. Whereas neonatal (P1-P3) neural retina has rod-promoting activity, E15 and adult neural retina, neonatal thymus, cerebrum and cerebellum do not. Finally, we show that neonatal neural retina promotes rod but not amacrine cell development.

Animals↗
PubMed1992

Visualization of O-2A progenitor cells in developing and adult rat optic nerve by quisqualate-stimulated cobalt uptake.

Some macroglial cells of the O-2A lineage express glutamate receptor channels of the quisqualate/kainate type and take up extracellular cobalt when activated by glutamate agonists. These cells can be identified both in vitro and in situ following precipitation and intensification of the intracellular cobalt. We have used this technique to characterize these cells in the developing and adult rat optic nerve. In purified cultures of optic nerve cells, O-2A progenitor cells and type 2 astrocytes took up cobalt in the presence of quisqualate, while oligodendrocytes, type 1 astrocytes, and microglial cells did not. When whole optic nerves of various postnatal ages were exposed to quisqualate and cobalt, a subpopulation of glial cells took up cobalt. Cobalt uptake in vitro and in situ was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The number, morphology, and spatial distribution of cobalt-filled cells in situ varied with age. In perinatal nerves, 9% of glial cells took up cobalt. These cells had a simple unipolar or bipolar morphology and were two to three times more concentrated at the chiasm end than at the eye end of the nerve. During subsequent development, this gradient disappeared and the cobalt-filled cells became progressively more complex in morphology and increased in number and density, reaching a peak toward the end of the second postnatal week. The number subsequently declined to about 16,000 (7%) in the adult nerve. The processes of some cobalt-filled cells appeared to contact nodes of Ranvier. All cobalt-filled cells in 2 1/2-week-old optic nerves had a similar ultrastructural appearance and did not resemble either mature oligodendrocytes or astrocytes. Our results suggest that the cells stimulated by quisqualate to take up cobalt in the optic nerve are the in vivo counterpart of O-2A progenitor cells. We found no evidence that any of these cells are type 2 astrocytes.

Aging↗