Cdon,a cell surface protein,mediates oligodendrocyte differentiation and myelination

During central nervous system development, oligodendrocyte progenitors (OLPs) establish multiple branched processes and axonal contacts to initiate myelination. A complete understanding of the molecular signals implicated in cell surface interaction to initiate myelination/remyelination is currently lacking. The objective of our study was to assess whether Cdon, a cell surface protein that was shown to participate in muscle and neuron cell development, is involved in oligodendrocyte (OLG) differentiation and myelination. Here, we demonstrate that endogenous Cdon protein is expressed in OLPs, increasing in the early differentiation stages and decreasing in mature OLGs. Immunocytochemistry of endogenous Cdon showed localization on both OLG cell membranes and cellular processes exhibiting puncta‐ or varicosity‐like structures. Cdon knockdown with siRNA decreased protein levels by 62% as well as two myelin‐specific proteins, MBP and MAG. Conversely, overexpression of full‐length rat Cdon increased myelin proteins in OLGs. The complexity of OLGs branching and contact point numbers with axons were also increased in Cdon overexpressing cells growing alone or in coculture with dorsal root ganglion neurons (DRGNs). Furthermore, myelination of DRGNs was decreased when OLPs were transfected with Cdon siRNA. Altogether, our results suggest that Cdon participates in OLG differentiation and myelination, most likely in the initial stages of development. GLIA 2016;64:1021–1033     

Calcium receptor expression and function in oligodendrocyte commitment and lineage progression: potential impact on reduced myelin basic protein in CaR-null mice

Oligodendrocytes develop from oligodendrocyte progenitor cells (OPCs), which in turn arise from a subset of neuroepithelial precursor cells during midneurogenesis. Development of the oligodendrocyte lineage involves a plethora of cell-intrinsic and -extrinsic signals. A cell surface calcium-sensing receptor (CaR) has been shown to be functionally expressed in immature oligodendrocytes. Here, we investigated the expression and function of the CaR during oligodendrocyte development. We show that the order of CaR mRNA expression as assessed by quantitative polymerase chain reaction is mature oligodendrocyte > neuron > astrocyte. We next determined the rank order of CaR expression on inducing specification of neural stem cells to the neuronal, oligodendroglial, or astrocytic lineages and found that the relative levels of CaR mRNA expression are OPC > neuron > astrocytes. CaR mRNA expression in cells at various stages of development along the oligodendrocyte lineage revealed that its expression is robustly up-regulated during the OPC stage and remains high until the premyelinating stage, decreasing thereafter by severalfold in the mature oligodendrocyte. In OPCs, high Ca(2+) acting via the CaR promotes cellular proliferation. We further observed that high Ca(2+) stimulates the mRNA levels of myelin basic protein in preoligodendrocytes, which is also CaR mediated. Finally, myelin basic protein levels were significantly reduced in the cerebellum of CaR-null mice during development. Our results show that CaR expression is up-regulated when neural stem cells are specified to the oligodendrocyte lineage and that activation of the receptor results in OPC expansion and differentiation. We conclude that the CaR may be a novel regulator of oligodendroglial development and function.     

Insulin-like growth factor type 1 receptor signaling in the cells of oligodendrocyte lineage is required for normal in vivo oligodendrocyte development and myelination

Insulin-like growth factor-I (IGF-I) has been shown to be a potent agent in promoting the growth and differentiation of oligodendrocyte precursors, and in stimulating myelination during development and following injury. To definitively determine whether IGF-I acts directly on the cells of oligodendrocyte lineage, we generated lines of mice in which the type 1 IGF receptor gene (igf1r) was conditionally ablated either in Olig1 or proteolipid protein expressing cells (termed IGF1R(pre-oligo-ko) and IGF1R(oligo-ko) mice, respectively). Compared with wild type mice, IGF1R(pre-oligo-ko) mice had a decreased volume (by 35-55%) and cell number (by 54-70%) in the corpus callosum (CC) and anterior commissure at 2 and 6 weeks of age, respectively. IGF1R(oligo-ko) mice by 25 weeks of age also showed reductions, albeit less marked, in CC volume and cell number. Unlike astrocytes, the percentage of NG2(+) oligodendrocyte precursors was decreased by approximately 13% in 2-week-old IGF1R(pre-oligo-ko) mice, while the percentage of CC1(+) mature oligodendrocytes was decreased by approximately 24% in 6-week-old IGF1R(pre-oligo-ko) mice and approximately 25% in 25-week-old IGF1R(oligo-ko) mice. The reduction in these cells is apparently a result of decreased proliferation and increased apoptosis. These results indicate that IGF-I directly affects oligodendrocytes and myelination in vivo via IGF1R, and that IGF1R signaling in the cells of oligodendrocyte lineage is required for normal oligodendrocyte development and myelination. These data also provide a fundamental basis for developing strategies with the potential to target IGF-IGF1R signaling pathways in oligodendrocyte lineage cells for the treatment of demyelinating disorders.     

Embracing oligodendrocyte diversity in the context of perinatal injury

Emerging evidence is fueling a new appreciation of oligodendrocyte diversity that is overturning the tradi-tional view that oligodendrocytes are a homogenous cell population. Oligodendrocytes of distinct origins, maturational stages, and regional locations may differ in their functional capacity or susceptibility to injury. One of the most unique qualities of the oligodendrocyte is its ability to produce myelin. Myelin abnormali-ties have been ascribed to a remarkable array of perinatal brain injuries, with concomitant oligodendrocyte dysregulation. Within this review, we discuss new insights into the diversity of the oligodendrocyte lineage and highlight their relevance in paradigms of perinatal brain injury. Future therapeutic development will be informed by comprehensive knowledge of oligodendrocyte pathophysiology that considers the particular facets of heterogeneity that this lineage exhibits.     

p75NTR independent oligodendrocyte death in cuprizone-induced demyelination in C57BL/6 mice

Feeding C57Bl/6 J mice the copper chelator cuprizone leads to selective apoptosis of mature oligodendrocytes and concomitant demyelination predominantly in the corpus callosum. The process of oligodendrocyte apoptosis in this animal model for multiple sclerosis (MS) involves early microglial activation, but no infiltration of T-lymphocytes. Therefore, this model could mimic early stages of oligodendrocyte degeneration Affected oligodendrocytes express the common neurotrophin receptor, p75(NTR), a 'stress-receptor' which under certain circumstances can induce apoptosis. Only affected oligodendrocytes in MS lesions and MS animal models express this receptor. In order to study the significance of p75(NTR) in the fate of oligodendrocytes, we have exposed wild-type as well as p75(NTR)-knockout mice to a 0.2% (w/w) cuprizone diet and performed a comparative immunohistochemical analysis of the corpus callosum at various time points. Surprisingly, our results show that the absence of p75(NTR) did not alter cuprizone-induced oligodendrocyte death (and subsequent de- or remyelination). Apparently, intracellular apoptosis pathways in adult oligodendrocytes do not require p75(NTR) activated signal transduction in the absence of T-lymphocytes and T-lymphocyte derived cytokines.     

Review: The chemokine receptor CXCR3 and its ligands CXCL9, CXCL10 and CXCL11 in neuroimmunity – a tale of conflict and conundrum

M. Müller, S. Carter, M. J. Hofer and I. L. Campbell (2010) Neuropathology and Applied Neurobiology 36, 368–387
The chemokine receptor CXCR3 and its ligands CXCL9, CXCL10 and CXCL11 in neuroimmunity – a tale of conflict and conundrum The chemokines CXCL9, CXCL10 and CXCL11 (also known as monokine induced by interferon‐γ, interferon‐inducible protein‐10 and interferon‐inducible T cell α‐chemoattractant, respectively) are structurally and functionally related molecules within the non‐ELR CXC chemokine subgroup. These chemokines are generally not detectable in most non‐lymphoid tissues under physiological conditions but are strongly induced by cytokines, particularly interferon‐γ, during infection, injury or immunoinflammatory responses. CXCL9, CXCL10 and CXCL11 each bind to a common primary receptor, CXCR3, and possibly to additional receptors. They are best known for their role in leucocyte trafficking, principally acting on activated CD4+ Th1 cells, CD8+ T cells and NK cells. An abundance of data demonstrates that CXCL9, CXCL10 and CXCL11 are produced in many diverse pathologic conditions of the central nervous system. More recent attention has focussed on the function of these chemokines in the central nervous system inflammation. The results of these studies have proven to be sometimes surprising and other times contradictory. Here we discuss the likely more subtle and perhaps divergent roles for these chemokines in the pathogenesis of neuroinflammatory diseases.     

P2, P1, and P0 myelin protein expression in developing rat sixth nerve: a quantitative immunocytochemical study

Myelination and the expression of myelin proteins P2, P1, and P0 were studied quantitatively in the rat sixth cranial nerve during development. The postnatal development and growth of all myelin sheaths in this nerve have been studied morphometrically in a companion paper. Epon-embedded blocks with closely matched topography in the transverse plane were selected from rats perfused at ages 1-4, 8, 15, and 20 days. From each block, serial semithin sections were cut, etched, and immunostained according to the peroxidase-antiperoxidase method with well-characterized polyclonal antisera that reacted specifically with P0 glycoprotein and the basic proteins P1 and P2. The immunoreactivities of individual myelin sheaths were measured by densitometry. Numbers of compact myelin lamellae, myelin spiral lengths, and axon diameters were determined on electronmicrographs of adjacent thin sections. At birth anti-P0 immunoreactivity was found on sheaths with two and more compact lamellae; neither P1 nor P2 immunoreactivity was observed. On day 2, myelin sheaths with five and eight lamellae were stained respectively by anti-P1 and anti-P2. On day 3 the percentages of myelin sheaths stained were substantially higher: P0 95%, P1 78%, P2 15%. By day 4, anti-P0 and anti-P1 immunoreactivity was present in 95% of myelin sheaths; 35% were stained by anti-P2. For P2, staining intensity and percentage of myelin sheaths stained continued to increase and by day 20, 85% were anti-P2-positive. The density of immunoreactivity was not uniform in all myelin sheaths. At young ages staining varied with all three proteins. The variability decreased as myelin sheaths thickened; it persisted longest for anti-P2. We conclude that the density and distribution of immunoreactivities of P0, P1, and P2 reflect their relative concentrations during myelin sheath development and growth. We attribute lack of detectable anti-P2 immunoreactivity in some small sheaths at 20 days to their early stage of myelination and also to limitations of the method. We infer from our observations that all myelin-forming Schwann cells express P2 basic protein.     

Periventricular leukomalacia,inflammation and white matter lesions within the developing nervous system

Periventricular leukomalacia (PVL) occurring in premature infants, represents a major precursor for neurological and intellectual impairment, and cerebral palsy in later life. The disorder is characterized by multifocal areas of necrosis found deep in the cortical white matter, which are often symmetrical and occur adjacent to the lateral ventricles. There is no known cure for PVL. Factors predisposing to PVL include birth trauma, asphyxia and respiratory failure, cardiopulmonary defects, premature birth/low birthweight, associated immature cerebrovascular development and lack of appropriate autoregulation of cerebral blood flow in response to hypoxic‐ischemic insults. The intrinsic vulnerability of oligodendrocyte precursors is considered as central to the pathogenesis of PVL. These cells are susceptible to a variety of injurious stimuli including free radicals and excitotoxicity induced by hypoxic‐ischemic injury (resulting from cerebral hypoperfusion), lack of trophic stimuli, as well as secondary associated events involving microglial and astrocytic activation and the release of pro‐inflammatory cytokines TNF‐α and IL‐6. It is yet unclear whether activated astrocytes and microglia act as principal participants in the development of PVL lesions, or whether they are representatives of an incidental pathological response directed towards repair of tissue injury in PVL. Nevertheless, the accumulated evidence points to a pathological contribution of microglia towards damage. The topography of lesions in PVL most likely reflects a combination of the relatively immature cerebrovasculature together with a failure in perfusion and/or hypoxia during the greatest period of vulnerability occurring around mid‐to‐late gestation. Mechanisms underlying the pathogenesis of PVL have so far been related to prenatal ischemic injury to the brain initiated within the third trimester, which result in global cognitive and developmental delay and motor disturbances. Over the past few years, several epidemiological and experimental studies have implicated intrauterine infection and chorioamnionitis as causative in the pathogenesis of PVL. In particular, recent investigations have shown that inflammatory responses in the fetus and neonate can contribute towards neonatal brain injury and development‐related disabilities including cerebral palsy. This review presents current concepts on the pathogenesis of PVL and emphasizes the increasing evidence for an inflammatory pathogenic component to this disorder, either resulting from hypoxic‐ischemic injury or from infection. These findings provide the basis for clinical approaches targeted at protecting the premature brain from inflammatory damage, which may prove beneficial for treating PVL, if identified early in pathogenesis.     

Uncoupling of neuroinflammation from axonal degeneration in mice lacking the myelin protein tetraspanin‐2

Deficiency of the major constituent of central nervous system (CNS) myelin, proteolipid protein (PLP), causes axonal pathology in spastic paraplegia type‐2 patients and in Plp1^null‐mice but is compatible with almost normal myelination. These observations led us to speculate that PLP's role in myelination may be partly compensated for by other tetraspan proteins. Here, we demonstrate that the abundance of the structurally related tetraspanin‐2 (TSPAN2) is highly increased in CNS myelin of Plp1^null‐mice. Unexpectedly, Tspan2^null‐mutant mice generated by homologous recombination in embryonic stem cells displayed low‐grade activation of astrocytes and microglia in white matter tracts while they were fully myelinated and showed no signs of axonal degeneration. To determine overlapping functions of TSPAN2 and PLP, Tspan2^null*Plp1^null double‐mutant mice were generated. Strikingly, the activation of astrocytes and microglia was strongly enhanced in Tspan2^null*Plp1^null double‐mutants compared with either single‐mutant, but the levels of dysmyelination and axonal degeneration were not increased. In this model, glial activation is thus unlikely to be caused by axonal pathology, and vice versa does not potentiate axonal degeneration. Our results support the concept that multiple myelin proteins have distinct roles in the long‐term preservation of a healthy CNS, rather than in myelination per se. GLIA 2013;61:1832–1847     

Electrophysiological abnormalities precede apparent histological demyelination in the central nervous system of mice overexpressing proteolipid protein

Myelin proteolipid protein (plp), a major myelin protein in the CNS, has been proposed to function in myelin assembly. Transgenic mice overexpressing the plp gene by introduction of two extra wild-type (Wt) mouse plp genes (plp(tg/-)) exhibit normal myelination and ion channel clustering at the age of 2 months. However, at the age of 5 months, demyelination becomes observable, accompanied by a reduction in the number of K+ channel clusters at Ranvier's node and a progressive increase in motor deficit. To clarify how these age-dependent changes are related to nerve conduction in the CNS, we analyzed the conduction velocity (CV) and relative refractory period (RRP) of identified spinal ascending or descending tracts, such as the dorsal column pathway, the vestibulospinal and reticulospinal tracts, and the pyramidal tract, in plp(tg/-) mice 2, 5, and 8 months of age. We found that CVs decreased as age increased. Importantly, CVs were significantly reduced and prolonged RRPs were observed in 2-month-old (2M) plp(tg/-) mice that had no apparent demyelination. Immunohistological examination revealed that densities of Na+ and K+ channel clusters decreased as plp(tg/-) and Wt mice aged. However, a clear correlation was not observed between CVs and mean channel cluster densities or between mean channel cluster densities and progress of demyelination. Performance in the rotarod test was normal in 2M plp(tg/-) mice but deteriorated in mice older than age 5 months. These results suggest that electrophysiological analysis can detect the abnormalities of the plp(tg/-) mice earlier than histological or behavioral measures.     

The ubiquitin ligase Mdm2 controls oligodendrocyte maturation by intertwining mTOR with G protein‐coupled receptor kinase 2 in the regulation of GPR17 receptor desensitization

During oligodendrocyte precursor cell (OPC) differentiation, defective control of the membrane receptor GPR17 has been suggested to block cell maturation and impair remyelination under demyelinating conditions. After the immature oligodendrocyte stage, to enable cells to complete maturation, GPR17 is physiologically down‐regulated via phosphorylation/desensitization by G protein‐coupled receptor kinases (GRKs); conversely, GRKs are regulated by the “mammalian target of rapamycin” mTOR. However, how GRKs and mTOR are connected to each other in modulating GPR17 function and oligodendrogenesis has remained elusive. Here we show, for the first time, a role for Murine double minute 2 (Mdm2), a ligase previously involved in ubiquitination/degradation of the onco‐suppressor p53 protein. In maturing OPCs, both rapamycin and Nutlin‐3, a small molecule inhibitor of Mdm2‐p53 interactions, increased GRK2 sequestration by Mdm2, leading to impaired GPR17 down‐regulation and OPC maturation block. Thus, Mdm2 intertwines mTOR with GRK2 in regulating GPR17 and oligodendrogenesis and represents a novel actor in myelination. GLIA 2015;63:2327–2339     

Increased NG2(+) glial cell proliferation and oligodendrocyte generation in the hypomyelinating mutant shiverer

Glial cells that express the NG2 proteoglycan (NG2(+) cells) are considered to be oligodendrocyte progenitors (OPCs) in the central nervous system (CNS), based on their ability to give rise to mature oligodendrocytes in vitro. To understand how dysmyelinated conditions influence OPC proliferation and differentiation, we studied proliferation and differentiation of NG2(+) OPCs in vivo in the shiverer mutant (shi), which do not form compact myelin due to a deletion in the myelin basic protein gene. Acute bromodeoxyuridine (BrdU) labeling studies revealed a 4- to 6-fold increase in NG2(+) cell proliferation in shi spinal cord between postnatal day18 (P18) and P60, and most BrdU(+) cells were NG2(+) after P18. The increased proliferation was accompanied by a 2-fold increase in the number of OPCs and oligodendrocytes. Survival studies following a single injection of BrdU at P18 revealed a decline in the number of BrdU(+)/NG2(+) cells with a concomitant increase in the number of BrdU(+) oligodendrocytes over time, suggesting that the proliferated NG2(+) cells had differentiated into oligodendrocytes. BrdU(+) oligodendrocytes were generated over a longer period of time in shi spinal cord and persisted longer in shi than in wild type spinal cord. These findings suggest that new oligodendrocytes continue to be generated in the dysmyelinated shi spinal cord by enhanced proliferation and differentiation of NG2(+) oligodendrocyte progenitor cells.     

Phosphodiesterase III inhibition promotes differentiation and survival of oligodendrocyte progenitors and enhances regeneration of ischemic white matter lesions in the adult mammalian brain

Vascular dementia is caused by blockage of blood supply to the brain, which causes ischemia and subsequent lesions primarily in the white matter, a key characteristic of the disease. In this study, we used a chronic cerebral hypoperfusion rat model to show that the regeneration of white matter damaged by hypoperfusion is enhanced by inhibiting phosphodiesterase III. A rat model of chronic cerebral hypoperfusion was prepared by bilateral common carotid artery ligation. Performance at the Morris water-maze task, immunohistochemistry for bromodeoxyuridine, as well as serial neuronal and glial markers were analyzed until 28 days after hypoperfusion. There was a significant increase in the number of oligodendrocyte progenitor cells in the brains of patients with vascular dementia as well as in rats with cerebral hypoperfusion. The oligodendrocyte progenitor cells subsequently underwent cell death and the number of oligodendrocytes decreased. In the rat model, treatment with a phosphodiesterase III inhibitor prevented cell death, markedly increased the mature oligodendrocytes, and promoted restoration of white matter and recovery of cognitive decline. These effects were cancelled by using protein kinase A/C inhibitor in the phosphodiesterase III inhibitor group. The results of our study indicate that the mammalian brain white matter tissue has the capacity to regenerate after ischemic injury.     

Central and peripheral nervous system functions are independently disturbed in HIV-1 infected patients

We examined the peripheral nervous system (PNS) (nerve conduction velocity (NCV)) and the central nervous system (CNS) (basal ganglia-mediated psychomotor speed) in 93 males seropositive for human immunodeficiency virus type 1 (HIV-1) with no prior history of opportunistic brain disease, antiretroviral treatment or intravenous drug use. Patients with different degrees of slowing of peroneal and sural NCV showed no significant differences in psychomotor speed as assessed by tremor peak frequency, most rapid alternating movements, reaction times and contraction times. There was no significant correlation between psychomotor measures and NCV. Psychomotor slowing test findings were independent from peripheral nervous system damage indicating uncorrelated disturbances of CNS and PNS function in HIV-1 infection. Differences in HIV-1 viral quasispecies or host responses may determine the predominance of CNS or PNS injury. Received: 7 September 2001, Received in revised form: 9 November 2001, Accepted: 13 November 2001     

Autoradiographic distribution of [3H]YM-09151-2, a high-affinity and selective antagonist ligand for the dopamine D2 receptor group,in the rat brain and spinal cord

We determined the regional distribution of the dopamine D₂ receptor group in the rat central nervous system by quantitative receptor autoradiography with a high-affinity and selective antagonist, [³H]YM-09151-2. Saturation and competition experiments demonstrated that the binding of [³H]YM-09151-2 to striatal sections was saturable (Bmax=37.3 fmol/section), of high affinity (Kd=0.315 nM), and was inhibited selectively by prototypic D₂ ligands. The anatomical localization of binding sites was determined by comparison of autoradiograms and the original ³H-ligand-exposed sections stained with cresyl violet. Very high levels of [³H] YM-09151-2 binding were found in the caudate-putamen, nucleus accumbens, tuberculum olfactorium and the insula of Calleja, to each of which midbrain dopaminergic neurons project densely. High levels of binding were also observed in other regiions rich in dopaminergic neurons and fibers including the glomerular layer of the olfactory bulb, the intermediate lobe of the pituitary, lateral septum, substantia nigra pars compacta, interfascicular nucleus, dorsal raphe nucleus, locus coeruleus, and nucleus of the solitary tract. Some regions poor in dopaminergi innervation, however, had high levels of [³H]YM-09151-2 binding including the molecular layer of gyrus dentatus, all layers of CA1 and the nonpyramidal layer of CA4 of hippocampus, and the deeper layer of medial entorhinal cortex. Motor neurons present in brainstem motor nuclei and spinal ventral horn were also strongly labeled. Neocortical, cerebellar, and thalamic regions had low levels of binding, except lobules 9–10 of the cerebellum, the olivary pretectal nucleus, zona incerta and lateral mammillary nucleus, in which moderate to high levels of binding were detected. Our findings concerning the widespread but region-specific localization of [³]YM-09151-2 binding sites in the brain and spinal cord may prove useful for analyzing varoius dopaminergic functions in the central nervous system. © 1994 Wiley-Liss, Inc.     

In vitro myelin basic protein synthesis in the PNS and CNS of myelin deficient (mld) mutant mice

The CNS of the mutant mld has a severe myelin deficit. In contrast, myelination is normal in the PNS. Previous studies showed that myelin basic proteins (MBP) are practically missing in mld CNS and PNS tissues. Using an in vitro system and immunoprecipitation, we present evidence that MBP synthesis is repressed in the PNS and in the CNS of mld mutants.     

Immunocytochemical localization of enkephalins in the brain of the African lungfish,Protopterus annectens,provides evidence for differential distribution of Met-enkephalin and Leu-enkephalin

The distribution of various opioid peptides derived from proenkephalin A and B was studied in the brain of the African lungfish Protopterus annectens by using a series of antibodies directed against mammalian opioid peptides. The results show that both Met-enkephalin- and Leu-enkephalin-immunoreactive peptides are present in the lungfish brain. In contrast, enkephalin forms similar to Met-enkephalin-Arg-Phe, or Met-enkephalin-Arg-Gly-Leu, as well as mammalian α-neoendorphin, dynorphin A (1–8), dynorphin A (1–13), or dynorphin A (1–17) were not detected. In all major subdivisions of the brain, the overwhelming majority of Met-enkephalin- and Leu-enkephalin-immunoreactive cells were distinct. In particular, cell bodies reacting only with Leu-enkephalin antibodies were detected in the medial subpallium of the telencephalon, the griseum centrale, the reticular formation, the nucleus of the solitary tract, and the visceral sensory area of the rhombencephalon. Cell bodies reacting only with Met-enkephalin antibodies were found in the lateral subpallium of the telencephalon, the caudal hypothalamus, and the tegmentum of the mesencephalon. The preoptic periventricular nucleus of the hypothalamus exhibited a high density of Met-enkephalin-immunoreactive neurons and only a few Leu-enkephalin-immunoreactive neurons. The distribution of Met-enkephalin- and Leu-enkephalin-immunoreactive cell bodies and fibers in the lungfish brain showed similarities to the distribution of proenkephalin A-derived peptides described previously in the brain of land vertebrates. The presence of Met-enkephalin- and Leu-enkephalin-like peptides in distinct regions, together with the absence of dynorphin-related peptides, suggests that, in the lungfish, Met-enkephalin and Leu-enkephalin may originate from distinct precursors. J. Comp. Neurol. 396:275–287, 1998. © 1998 Wiley-Liss, Inc.