Fluid-fluid level in an intraosseous ganglion

A case of an intraosseous ganglion with a fluid-fluid level visualised on MR imaging is presented. We discuss its possible pathogenesis and review other lesions that may exhibit this sign.

Studies of neurotrophin biology in the developing trigeminal system

The accessibility of the primary sensory neurons of the trigeminal system at stages throughout their development in avian and mammalian embryos and the ease with which these neurons can be studied in vivo has facilitated investigation of several fundamental aspects of neurotrophin biology. Studies of the timing and sequence of action of neurotrophins and the expression of neurotrophins and their receptors in this well characterised neuronal system have led to a detailed understanding of the functions of neurotrophins in neuronal development. The concepts of neurotrophin independent survival, neurotrophin switching and neurotrophin cooperativity have largely arisen from work on the trigeminal system. Moreover, in vitro studies of trigeminal neurons provided some of the first evidence that the neurotrophin requirements of sensory neurons are related to sensory modality. The developing trigeminal system has been studied most extensively in mice and chickens, each of which has particular advantages for understanding different aspects of neurotrophin biology. In this review, I will outline these advantages and describe some of the main findings that have arisen from this work.

Bcl-2 is required for cranial sensory neuron survival at defined stages of embryonic development

To ascertain the role of endogenous Bcl-2 in maintaining the survival of developing neurons and modulating their responses to neurotrophins, we compared the in vitro and in vivo survival of cranial sensory neurons of wild-type and bcl-2 null mouse embryos. At the peak of naturally occurring neuronal death in the trigeminal ganglion at E14, trigeminal neurons from bcl-2(−/−) embryos initially survived in culture in response to NGF but were not sustained as well as neurons from wild-type embryos. At the end of the period of naturally occurring neuronal death at E18, Bcl-2-deficient trigeminal neurons survived with NGF as well as wild-type neurons. At E14 in vivo, the number of trigeminal neurons undergoing apoptosis was significantly greater in bcl-2(−/−) embryos, and there were significantly fewer neurons in the trigeminal ganglia of bcl-2(−/−) embryos at E16 and E18. Similar age-related changes in the responses of nodose ganglion neurons to BDNF were observed in cultures established from bcl-2(−/−) and wild-type embryos between E14 and E18. These results suggest that endogenous Bcl-2 is required for the sustained survival response of a subset of cranial sensory neurons to neurotrophins at particular stages of embryonic development and show that its absence leads to reduced numbers of these neurons in vivo.

Bcl-2 influences axonal growth rate in embryonic sensory neurons

Bcl-2 plays a key role in regulating cell survival in the immune and nervous systems. Mice lacking the bcl-2 gene have markedly reduced numbers of B and T cells as a result of increased apoptosis, whereas mice with a transgene causing high levels of Bcl-2 expression in the immune system show extended survival of B and T cells. Overexpression of Bcl-2 in cultured neurons prevents their death following neurotrophin deprivation, and mice with a bcl-2 transgene under the control of a neuron-specific enolase promoter have increased numbers of neurons in several regions. Cultured neurons expressing antisense bcl-2 RNA have an attenuated survival response to neurotrophins, and neurons of postnatal bcl-2-deficient mice die more rapidly following NGF deprivation in vitro and are present in reduced numbers in vivo. Here, we show that Bcl-2 also plays a role in regulating axonal growth rates in embryonic neurons. Sensory neurons from the trigeminal ganglia of bcl-2-deficient mouse embryos, removed from the embryo on embryonic day 11 or 12, extend axons more slowly in vitro than do neurons from wild-type embryos of the same age. Serial measurements of axonal length in the same neurons revealed that there were marked differences in axonal growth rate between bcl-2-deficient and wild-type neurons, irrespective of whether the neurons were grown with nerve growth factor, brain-derived neurotrophic factor or neurotrophin-3. Because there was no significant difference in the numbers of wild-type and bcl-2-deficient neurons surviving with each neurotrophin at this early stage of development, the effect of Bcl-2 on axonal growth rate is not a consequence of its well documented role in preventing apoptosis.

Neurturin responsiveness requires a GPI-linked receptor and the Ret receptor tyrosine kinase

Neurturin (NTN) is a recently identified homologue of glial-cell-line-derived neurotrophic factor (GDNF). Both factors promote the survival of a variety of neurons, and GDNF is required for the development of the enteric nervous system and kidney. GDNF signals through a receptor complex consisting of the receptor tyrosine kinase Ret and a glycosyl-phosphatidylinositol (GPI)-linked receptor termed GDNFR-alpha. Here we report the cloning of a new GPI-linked receptor termed NTNR-alpha that is homologous with GDNFR-alpha and is widely expressed in the nervous system and other tissues. By using microinjection to introduce expression plasmids into neurons, we show that coexpression of NTNR-alpha with Ret confers a survival response to neurturin but not GDNF, and that coexpression of GDNFR-alpha with Ret confers a survival response to GDNF but not neurturin. Our findings indicate that GDNF and neurturin promote neuronal survival by signalling through similar multicomponent receptors that consist of a common receptor tyrosine kinase and a member of a GPI-linked family of receptors that determines ligand specificity.

Sympathetic neuron survival and TrkA expression in NT3-deficient mouse embryos

Several in vitro and in vivo studies have led to the widely accepted view that NT3 is required for sympathetic neuroblast survival, induction of TrkA expression and the acquisition of NGF dependence. However, we show that the number of neurons and the levels of trkA and p75 mRNAs in the superior cervical sympathetic ganglion (SCG) of NT3-/- mouse embryos increase normally up to E16, 2 days after SCG neurons start responding to NGF. At E18 and in the postnatal period, there are significant reductions in the number of SCG neurons and in the levels of trkA and p75 mRNAs. These results show that the neurotrophin survival requirements of SCG neurons do not switch from NT3 to NGF during development and that NT3 is not required for the expression of TrkA and p75 and the acquisition of NGF dependence. Rather, some sympathetic neurons have a late requirement for NT3 at the time when they also depend on NGF for survival. The expression of transcripts encoding catalytic TrkC is negligible at this stage, suggesting that NT3 acts mainly via TrkA.

Skip metastases in Ewing's sarcoma: a report of three cases

The accurate pre-operative staging of all potentially malignant tumours of bone is essential. We report three cases of Ewing's sarcoma of bone in which MR imaging identified skip metastases not visualized on either contemporary radiographs or bone scintigraphy. The implications for patient management and possible reasons for the other imaging modalities failing to reveal the skip metastases are discussed.

TrkB variants with deletions in the leucine-rich motifs of the extracellular domain

We have isolated two novel variants involving the extracellular domain of TrkB from developing sensory neurons. These variants are generated by alternative splicing and lack two or all three of the leucine-rich motifs. Each of these variants is expressed as isoforms that possess or lack the intracellular tyrosine kinase domain. Fibroblast cell lines stably expressing these variants do not bind any of the TrkB ligands (brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5) and neither survive nor undergo morphological transformation in response to neurotrophins. These results demonstrate that the leucine-rich motifs in TrkB are essential for ligand binding and signaling and indicate that the extracellular immunoglobulin-like domains alone are insufficient to confer neurotrophin binding to TrkB.

Neurotrophin switching: where does it stand?

In vitro and in vivo studies suggest that certain populations of neurons switch their survival requirements from one neurotrophin to another during an early stage in their development. Although there is good evidence for neurotrophin switching in sensory neurons, the evidence for switching in sympathetic neurons has become more controversial, as has the identity of the factors that regulate their responsiveness to particular neurotrophins.

Neurotrophins: the yin and yang of nerve growth factor

In the fifty years since its discovery, a substantial body of work has established that nerve growth factor plays a key role in promoting the survival of neurons during development; the recent demonstration that it can also promote cell death therefore comes as a surprise.

Peroxidase activation of 4-hydroxytamoxifen to free radicals detected by EPR spectroscopy

4-Hydroxytamoxifen is a major metabolite of the antiestrogenic drug tamoxifen used in the treatment of women with breast cancer. 4-Hydroxytamoxifen is broken down by a horseradish peroxidase/H2O2 system very much more rapidly than tamoxifen and causes much greater DNA damage determined by 32P-postlabelling. EPR spin trapping of 4-hydroxytamoxifen reaction products in the presence of the free radical trap 5,5-dimethyl-1-pyrroline N-oxide, together with glutathione as a hydrogen donor, resulted in the generation of a species with the characteristics of the glutathione thiyl radical (aN approximately 15.3 G, aH approximately 16.2 G). Support for the creation of thiyl radicals comes from the close to stoichiometric time dependent formation of glutathione disulfide concomitant with the loss of glutathione. Similar results were obtained using 4-hydroxytoremifene but no radical formation or glutathione loss could be detected using 3-hydroxytamoxifen (droloxifene). On-line LC-ESI MS analysis of the incubation products from 4-hydroxytamoxifen has identified three products with a protonated molecular mass of 773, consistent with the formation of dimers of 4-hydroxytamoxifen. The role that radical mechanisms have in the carcinogenic effects of tamoxifen in the endometrium or other target organs of women taking this drug remains to be established.

Expression and function of TrkB variants in developing sensory neurons

Mouse trigeminal neurons survive independently of neurotrophins when their axons are growing to their targets, and are then transiently supported by BDNF before becoming NGF dependent. During the stage of neurotrophin independence, transcripts encoding the BDNF receptor, TrkB, were expressed at very low levels. During the stage of BDNF dependence, high levels of a transcript encoding a receptor with the catalytic tyrosine kinase domain were expressed. Although the levels of this transcript fell as the neurons lost responsiveness to BDNF, there were concomitant increases in the expression of transcripts encoding TrkB variants lacking the kinase domain. Analysis of RNA from purified neurons showed that all of these transcripts were present in neurons. BDNF and NGF up-regulated the expression of these transcripts early in development but had little effect later on. To test whether truncated TrkB modulates BDNF signalling via catalytic TrkB, we injected TrkB expression plasmids into NGF-dependent sympathetic neurons. Whereas expression of catalytic TrkB alone conferred a BDNF survival response, co-expression of non-catalytic TrkB substantially reduced this response. Our results suggest that BDNF responsiveness in sensory neurons during development is modulated by the relative levels of catalytic and non-catalytic TrkB.

Timing and regulation of trkB and BDNF mRNA expression in placode-derived sensory neurons and their targets

The sensory neurons of the vestibular and nodose ganglia of the chicken embryo have nearby and distant targets, respectively. In vitro studies have shown that these neurons survive independently of neurotrophins when their axons are growing to their targets and become dependent on brain-derived neurotrophic factor (BDNF) for survival when their axons reach the vicinity of their targets. Although the timing of BDNF dependence is principally controlled by an intrinsic timing mechanism in the neurons, the onset of dependence can be accelerated by BDNF exposure toward the end of the phase of neurotrophin independence. We have used quantitative reverse transcription/polymerase chain reaction to study the expression of transcripts coding for BDNF and the catalytic isoform of its receptor tyrosine kinase, TrkB, in these neurons and their targets at different stages of development. We show that the peripheral and central target tissues of these neurons express BDNF mRNA prior to the arrival of sensory axons. Vestibular neurons express trkB mRNA before nodose neurons, which accords with the earlier response of vestibular neurons to BDNF. In culture, early nodose neurons start expressing trkB mRNA after 36 h incubation, which is 36 h before these neurons become dependent on BDNF for survival. Although BDNF does not affect the timing and level of trkB mRNA expression during the first 48 h in vitro, it increases the level of trkB mRNA after this time. The timing of BDNF-induced elevation of trkB mRNA correlates with the period during which BDNF exposure accelerates the onset of BDNF dependence in nodose neurons. These results suggest that the timing of BDNF dependence in developing sensory neurons is due in part to expression of catalytic TrkB and demonstrate that a BDNF autocrine loop is not required for the survival of sensory neurons during the earliest stages of their development.

Timing of neuronal death in trkA, trkB and trkC mutant embryos reveals developmental changes in sensory neuron dependence on Trk signalling

The sensory neurons of the embryonic mouse trigeminal ganglion are supported in culture by different neurotrophins at successive stages of development. Initially the neurons survive in response to BDNF and NT3 and later switch to becoming NGF-dependent (Buchman, V. I. and Davies, A. M. (1993), Development 118, 989–1001). To determine if this in vitro switch in neurotrophin responsiveness is physiologically relevant, we studied the timing of neuronal death in the trigeminal ganglia of embryos that are homozygous for null mutations in the trkA, trkB and trkC genes, which encode receptor tyrosine kinases for NGF, BDNF and NT3, respectively. In wild-type embryos, the number of pyknotic nuclei increased from E11 to peak between E13 and E14, and decreased gradually at later ages, becoming negligible by birth. Neuronal death in the trigeminal ganglia of trkA−/− embryos also peaked between E13 and E14, but was almost threefold greater than in wild-type embryos at this stage. Whereas there was no significant difference between the number of pyknotic nuclei in trkA−/− and wild-type embryos at E11 and E12, there was a substantial increase in the number of pyknotic nuclei in the trigeminal ganglia of trkB−/− at these earlier stages. Counts of the total number of neurons in E13 trigeminal ganglia revealed a marked decrease in trkB−/− but not trkA−/− or trkC−/− embryos. Consistent with the later onset of excessive neuronal death in trkA−/− embryos, there was a marked decrease in the neuronal complement of the trigeminal ganglia of trkA−/− embryos at E15. These results demonstrate that TrkB signalling is required for the in vivo survival of many trigeminal neurons during the early stages of target field innervation before they become NGF-dependent.

Characterization of a multicomponent receptor for GDNF

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent survival factor for central and peripheral neurons, and is essential for the development of kidneys and the enteric nervous system. Despite the potential clinical and physiological importance of GDNF, its mechanism of action is unknown. Here we show that physiological responses to GDNF require the presence of a novel glycosyl-phosphatidylinositol (GPI)-linked protein (designated GDNFR-alpha) that is expressed on GDNF-responsive cells and binds GDNF with a high affinity. We further demonstrate that GDNF promotes the formation of a physical complex between GDNFR-alpha and the orphan tyrosin kinase receptor Ret, thereby inducing its tyrosine phosphorylation. These findings support the hypothesis that GDNF uses a multi-subunit receptor system in which GDNFR-alpha and Ret function as the ligand-binding and signalling components, respectively.

Paracrine and autocrine actions of neurotrophic factors

Neurotrophic factors are proteins that promote the survival and growth of neurons in the vertebrate nervous system. Although it is well known that many neurons obtain these factors from the regions to which their axons project, studies of the sites of neurotrophic factor synthesis have raised the possibility that at least some neurons may obtain these factors from other sources. Alternative sources of neurotrophic factors include cells along a neuron's axon shaft and cells or other axons terminals within the vicinity of a neuron's cell body and dendritic arbour. In addition, recent experimental studies have shown that at certain stages of development neurotrophic factor autocrine loops operate in some neurons. The evidence for and the potential physiological significance of these different modes of action of neurotrophic factors will be discussed.

Intracellular compartmentalization of two differentially spliced s-rex/NSP mRNAs in neurons

Using a subtractive hybridization technique directed to cloning transcripts with compartmentalized distributions within cerebral cortex neurons, we have isolated rat s-rex mRNAs that are analogues of the human neuroendocrine-specific NSP gene transcripts. Differential splicing produces two main s-rex mRNA that have different regional distributions in the developing and mature rat nervous system. In certain populations of adult brain neurons, most of s-rexs, mRNA and a substantial amount of s-rexb mRNA are localized to the axonal pole of the cell body. The localization of S-Rex/NSP proteins in these neurons suggests that s-rex mRNA compartmentalization targets the encoded proteins to specific regions of the neuron.

The neurotrophic hypothesis: where does it stand?

In the developing peripheral nervous system many neurons die shortly after their axons reach their target fields. This loss is thought to match the number of neurons to the size and requirements of their target fields because altering target field size before innervation affects the number of neurons that survive. The neurotrophic hypothesis provides an explanation for how target fields influence the size of the neuronal populations that innervate them. This hypothesis arose from work on nerve growth factor (NGF), the founder member of the neurotrophin family of secreted proteins. Its principal tenet is that the survival of developing neurons depends on the supply of a neurotrophic factor that is synthesized in limiting amounts in their target fields. The neurotrophic hypothesis has, however, been broadened by the demonstration that multiple neurotrophic factors regulate the survival of certain populations of neurons. For example, some neurons depend on several different neurotrophic factors which may act concurrently or sequentially during target field innervation. In addition, there are aspects of neurotrophin action that do not conform with the classic neurotrophic hypothesis. For example, the dependence of some populations of sensory neurons on particular neurotrophins before significant neuronal death takes place raises the possibility that the supply of these neurotrophins is not limiting for survival at this stage of development. There is also evidence that at stages before and after sensory neurons depend on target-derived neurotrophins for survival, neurotrophins act on at least some sensory neurons by an autocrine route. Yet despite the growing wealth of information on the multiple roles and modes of action of neurotrophic factors, the neurotrophic hypothesis has remained the best explanation for how neuronal target fields in the developing peripheral nervous system regulate their innervation density.

Imaging of pelvic insufficiency fractures

Insufficiency fractures of the pelvis are being increasingly recognized as a major cause of low back pain in elderly women with osteoporosis. Fractures in the sacrum are difficult to diagnose, as plain radiographic findings are either unhelpful or misleading. Bone scintigraphy is very sensitive for the detection of fractures in the sacrum, with demonstration of the H-shaped (or butterfly) sacral pattern or the combination of concomitant sacral and parasymphyseal uptake being considered as characteristic of insufficiency fractures. Computed tomography (CT) is helpful for confirming the presence of fractures in cases with atypical scintigraphic patterns, particularly in those with a known primary malignant neoplasm. CT is especially useful in the further evaluation of parasymphyseal and pubic rami lesions. The majority of patients respond well to periods of enforced bed rest and administration of analgesics. Recognition of the spectrum of imaging findings for this entity should lead to its correct identification and the institution of appropriate treatment.

Bax promotes neuronal survival and antagonises the survival effects of neurotrophic factors

Bcl-2, Bcl-x and Bax are members fo a family of cytoplasmic proteins that influence cell survival. Whereas increased expression of Bcl-2 or Bcl-x promotes cell survival following withdrawal of survival factors, increased expression of Bax is thought to suppress survival. To investigate the potential roles of these proteins in regulating the survival of developing neurons, we compared the effects of overexpressing these proteins in embryonic neurons deprived of different neurotrophic factors in vitro. Surprisingly, overexpression of Bax rescued populations of sensory neurons deprived of nerve growth factor, as did overexpression of Bcl-2 and two Bcl-x variants, Bcl-XL and Bcl-Xbeta. Bax also enhanced the survival of ciliary neurons deprived of ciliary neurotrophic factor, although this effect was short-lived. Whereas Bcl-2 overexpression did not affect the survival response of neurons to neurotrophic factors, Bax overexpression partially inhibited the action of neurotrophic factors. Co-injection of Bcl-2 and Bax expression vectors promoted the survival of neurotrophic factor-deprived neurons if either was in excess, but failed to rescue neurons if they injected at a 1:1 ratio. Our findings demonstrate that Bax can promote the survival of neurotrophic factor-deprived neurons and that its effect on survival is dominant to that of neurotrophic factors. Our results also argue that the relative amounts of Bcl-2 and Bax are critical in regulating neuronal survival.

Paracrine interactions of BDNF involving NGF-dependent embryonic sensory neurons

The expression of BDNF mRNA by a proportion of embryonic dorsal root ganglion neurons has led to the proposal that BDNF acts by an autocrine loop on these neurons. To clarify the role of BDNF expression in developing sensory neurons, we measured the level of BDNF mRNA in purified populations of cranial sensory neurons that depend on either NGF or BDNF for survival. When neuronal death is taking place, the highest levels of BDNF mRNA were detected in NGF-dependent cutaneous sensory neurons. BDNF mRNA was expressed at lower levels in BDNF-dependent cutaneous sensory neurons and was undetectable in BDNF-dependent proprioceptive neurons. In coculture, NGF-dependent neurons promoted the survival of BDNF-dependent neurons by the production and release of BDNF. Depolarizing levels of KCl increased the expression of BDNF mRNA in cultured sensory neurons and this effect was partially inhibited by calcium channel antagonists. Our results suggest that during the phase of naturally occurring neuronal death, BDNF acts by a paracrine mechanism in sensory neurons and that BDNF expression is regulated by neural activity.

MRI characteristics of chondroblastoma

We report a retrospective study of MR imaging of 16 patients with histologically proven chondroblastoma, 12 at primary presentation and four clinically suspected recurrences. In all the primary cases MR imaging showed a lobulated low signal intensity (SI) rim. Low SI foci within the tumour were present in 11 out of the 12 cases and corresponded to calcification seen on radiographs or CT. Bone marrow oedema was also present in 11 out of 12 cases and an adjacent joint effusion in eight out of 12. The STIR sequence was of particular value, giving optimal delineation of marrow and soft tissue oedema. Of the suspected recurrences one showed peritumoral oedema and was subsequently proven histologically. Three showed no peritumoral oedema and subsequent histology was negative in these cases. The presence or absence of oedema may be a useful indicator of tumour activity although further study is required.

Leukemia inhibitory factor and ciliary neurotrophic factor in sensory neuron development

Leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), and related proteins are potentially involved in several aspects of sensory neuron development. There is evidence that LIF promotes the differentiation of sensory neurons from progenitor cells of neural crest origin. Later in development, LIF, CNTF, oncostatin M and interleukin-6 promote the survival of cultured neurons. Some neurons, like those of the nodose ganglion, respond early in their development to these factors, whereas other neurons, like those of the trigeminal ganglion, respond much later. In addition to promoting sensory neuron survival, there is some evidence that LIF is able to influence neurotransmitter and neuropeptide expression in these neurons. These observations suggest that several kinds of sensory neurons may be influenced in various ways by LIF and related factors at different stages of their development.

High specificity of neurotrophins in the embryonic chicken trigeminal system

Studies of cell lines and some cultured neurons have demonstrated potential cross-talk between neurotrophins and their receptors; high concentrations of neurotrophins can exhibit either agonist or antagonistic actions on heterologous neurotrophin receptors. We have studied neurotrophin discrimination among the sensory neurons of the embryonic chicken trigeminal system. We show that nerve growth factor (NGF) at a concentration that is six orders of magnitude greater than that required to promote the survival of NGF-dependent dorsomedial trigeminal ganglion (DMTG) neurons has no effect on the survival of brain-derived neurotrophic factor (BDNF)-dependent trigeminal mesencephalic nucleus (TMN) neurons and does not affect the dose-response relationship of these neurons to BDNF. A similar high level of neurotrophin-3 neither promotes the survival of BDNF-dependent ventrolateral trigeminal ganglion neurons nor affects the dose response of these neurons to BDNF. High levels of BDNF have a negligible effect on the survival of mid-embryonic DMTG neurons. These results show that some neurons are able to discriminate completely between neurotrophins at very high concentrations, indicating that neurotrophin responses can be far more highly specific than previously appreciated.

GDNF is an age-specific survival factor for sensory and autonomic neurons

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of two populations of CNS neurons: motoneurons and midbrain dopaminergic neurons. To see whether GDNF promotes the survival of PNS neurons, we studied embryonic chicken autonomic and sensory neurons in culture. We show that GDNF promotes the survival of sympathetic, parasympathetic, proprioceptive, enteroceptive, and small and large cutaneous sensory neurons. Whereas sympathetic, parasympathetic, and proprioceptive neurons become less responsive to GDNF with age, enteroceptive and cutaneous sensory neurons become more responsive. GDNF mRNA is expressed in the tissues innervated by these neurons, and developmental changes in its expression in several tissues mirror the changing responses of the innervating neurons to GDNF. These results show that GDNF promotes the survival of multiple PNS and CNS neurons and suggest that GDNF may be important for regulating the survival of various populations of neurons at different stages of their development.

NGF receptor expression in sensory neurons develops normally in embryos lacking NGF

Numerous in vivo and in vitro studies have shown that NGF increases the expression of its receptors, p75 and TrkA, in NGF-responsive cell lines and in NGF-responsive neurons of the developing and mature nervous system. To determine if endogenous NGF is required for the normal developmental increase in p75 and TrkA expression that occurs in sensory neurons shortly after they innervate their targets, we used quantitative RT/PCR to compare the levels of p75 and trkA mRNAs in the trigeminal ganglia of normal mouse embryos and embryos that are homozygous for a null mutation in the NGF gene. We show that the marked increase in p75 and trkA mRNA expression that occurs between E11 and E13 in normal embryos takes place on time and to the same extent in NGF-/- embryos. We also show that trigeminal neurons from E13 NGF+/+ and NGF-/- embryos have very similar dose responses for survival induced by NGF. These findings clearly show that the expression of both p75 and TrkA and the sensitivity of developing sensory neurons to NGF do not require and are not modulated by target-derived NGF during the early stages of target field innervation.

Trigeminal sensory neurons require extrinsic signals to switch neurotrophin dependence during the early stages of target field innervation

Sensory neurons of the embryonic mouse trigeminal ganglion are transiently supported by brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), and NT4/5 during the earliest stages of target field innervation before they become dependent on nerve growth factor (NGF) for survival. To determine whether the switch from BDNF/NT3/NT4/5 dependence to NGF dependence occurs autonomously in these neurons or whether extrinsic signals are required, we studied the survival of trigeminal neurons in vitro before, during, and after the switchover period. Trigeminal neurons cultured before they show any response to NGF survived with BDNF, NT3, or NT4/5 well beyond the switchover period. When these early neurons were switched from BDNF, NT3, or NT4/5 to NGF after various times in culture they died as rapidly as neuro-trophin-deprived neurons. Neurons that were switched from BDNF, NT3, or NT4/5 to NGF in cultures set up at stages throughout the switchover period exhibited an NGF survival response that improved with age. Moreover, the ability of NGF to promote long-term survival also increased with embryonic age. These results show that, unlike the onset of BDNF dependence, which is controlled by an intrinsic timing mechanism in early sensory neurons, the switch to NGF dependence relies on extrinsic signals acting on the neurons during the switchover period and that in vivo signals are also required for the maturation of the NGF survival response from a transient to a long-term response. Retinoic acid, which induces NGF dependence in early sympathetic neurons, was ineffective in promoting NGF dependence in early sensory neurons.

Developmental changes in NT3 signalling via TrkA and TrkB in embryonic neurons

Neurotrophins promote neuronal survival by signalling through Trk receptor tyrosine kinases: nerve growth factor signals through TrkA, brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)4 through TrkB and NT3 through TrkC. Although studies in some, but not all, cell lines indicate that NT3 can also signal through TrkA and TrkB, it is not known if such signalling can occur in neurons. We show that NT3 can promote the in vitro survival of sensory and sympathetic neurons isolated from embryos that are homozygous for a null mutation in the trkC gene. During the mid-embryonic period, NT3 promoted the survival of as many trigeminal and nodose neurons as the preferred neurotrophins, NGF and BDNF. However, later in development, these neurons lost their ability to respond to NT3. NT3 also promoted the survival of almost all sympathetic neurons, but no decrease in effectiveness was observed during development. Trigeminal neurons from trkC-/- trkA-/- embryos did not respond to NT3 and nodose neurons from trkB-/- embryos likewise failed to respond to NT3. These results show that NT3 can signal through TrkA and TrkB in neurons at certain stages of development and may explain why the phenotype of NT3-/- mice is more severe than that of trkC-/- mice.

Regulation of nerve growth factor receptor gene expression in sympathetic neurons during development

We used quantitative reverse transcription (RT)/PCR to study the regulation of p75 mRNA and trkA mRNA expression in the developing sympathetic neurons of the mouse superior cervical sympathetic ganglion (SCG) in vivo and in vitro. At E13, the SCG contains proliferating cells that express many features of differentiated neurons. These immature neurons survived in culture without NGF, and NGF did not induce c-fos expression. Low levels of p75 and trkA mRNAs were expressed at this stage in vivo. There was no significant increase in the level of either trkA mRNA or p75 mRNA in E13 control cultures up to 72 h in vitro, and neither NGF nor depolarizing levels of K+ ions (40 mM KC1) affected the expression of trkA mRNA. In E14 cultures, NGF induced c-fos expression in 10-15% of the neurons and enhanced the survival of a similar percentage of neurons. The proportion of neurons responding to NGF increased with age, reaching 90% in E18 cultures. The in vivo level of trkA mRNA increased markedly from E14 onward, but in contrast to sensory neurons (in which p75 and trkA mRNA levels increase in parallel), the level of trkA mRNA initially increased far more rapidly than that of p75 mRNA. After E17, the level of p75 mRNA increased rapidly and approached that of trkA mRNA postnatally, but at no stage did this exceed the level of trkA mRNA. In E14 cultures, the level of trkA mRNA increased in the absence of neurotrophins or 40 mM KC1. The level of p75 mRNA in E14 cultures was enhanced by NGF but was unaffected by 40 mM KC1. Our findings show that NGF receptor expression during the earliest stages of sympathetic neuron development is not affected by depolarization but indicate that by an early developmental stage (between E13 and E14 in vivo), sympathetic neurons become specified to upregulate trkA mRNA in culture independently of added factors. In addition, our findings reveal several distinctive features of p75 mRNA and trkA mRNA expression in sympathetic neurons compared with sensory neurons and provide a plausible explanation for previously observed differences in the effects of a p75 null mutation on the response of sensory and sympathetic neurons during embryonic and postnatal development.

The Bcl-2 family of proteins, and the regulation of neuronal survival

Bcl-2 is the founder member of a growing family of cytoplasmic proteins that modulate the responses of many cell types to the diverse extracellular signals that affect their survival. Although knowledge of the functions of these proteins has come largely from studying cells of the immune system, increasing evidence implicates these proteins in modulating neuronal survival. Several of these proteins are expressed in the nervous system, and experimental overexpression of Bcl-2 prevents the death of neurones deprived of particular neurotrophic factors in vitro, and rescues developing neurones that would otherwise die in vivo.

Sciatica in degenerative spondylolisthesis of the lumbar spine

Intraspinal synovial cysts are an uncommon but well recognised cause of backache and sciatica, and should be considered in patients, in particular with degenerative spondylolisthesis, who are symptomatic. MRI is the initial investigation of choice. If there is any doubt as to the diagnosis, CT with or without facet joint arthrography is helpful.

Neurotrophic factors. Neurotrophin autocrine loops

Developing neurons depend on neurotrophins supplied by the tissues they innervate. Before and after this period of target-dependent survival, brain-derived neurotrophic factor also has autocrine actions on some neurons.

Role of Bcl-2 in the brain-derived neurotrophic factor survival response

Developing neurons die if they fail to obtain an adequate supply of neurotrophins from their targets but how neurotrophins suppress cell death is not known. Although over-expression of exogenous Bcl-2 can prevent the death of cultured neurons deprived of members of the nerve growth factor family of neurotrophins it is not known if this effect is physiologically relevant. To determine if Bcl-2 participates in the neurotrophin survival response we used antisense bcl-2 RNA to inhibit endogenous Bcl-2 expression. Here we show that brain-derived neurotrophic factor (BDNF)-dependent neurons are killed by antisense bcl-2 RNA in the presence of BDNF. However, when these neurons were supported with ciliary neurotrophic factor (CNTF) their survival was not affected by antisense bcl-2 RNA. Likewise, the survival of CNTF-dependent ciliary neurons was not affected by antisense bcl-2 RNA. Our findings suggest that Bcl-2 is required for the BDNF survival response and that alternative, Bcl-2-independent survival mechanisms operate in sensory and parasympathetic neurons exposed to CNTF.

Signal changes in the intervertebral discs on MRI of the thoracolumbar spine in ankylosing spondylitis

Contrary to standard teaching in magnetic resonance imaging (MRI), recent reports have documented calcification appearing as areas of increased signal intensity (SI) on T1-weighted images. Intervertebral disc calcification is a frequent finding on radiographs in chronic ankylosing spondylitis (AS). This study was performed to investigate the appearance of variable degrees of disc calcification in MRI. Thirty-six patients with AS of variable duration underwent an MRI scan of the thoraco-lumbar spine and the MR appearances, particularly of the discs, were compared with plain radiographs. Increased SI of the discs on T1-weighted images were found in 17 of 36 patients, occurring over a range of disc levels, and correlating with disc calcification on the radiographs in 78% of cases. This group tended to be older and have a longer duration of disease than those with normal appearing discs. Four different patterns of increased signal within the discs termed Type A (marginal), Type B (annular), Type C (central) and Type D (solid) were identified. In those with less than six discs involved Type A was the most common pattern. In those with more than six discs involved Type D was the most common pattern, suggesting a progression of disc involvement with more advanced disease. Although these findings will not affect the management of the disease, they do highlight the recently described phenomenon of calcification appearing as increased SI on T1-weighted images, likely to be related to the surface area of the calcium crystal. This should lead to the consideration of calcium in the differential diagnosis of increased SI on T1-weighted images. End-plate marrow changes were a relatively frequent finding in this study but did not correlate with the signal changes seen within the discs; in a number of cases they related to variable degrees of bony bridging.

Survival of newly postmitotic motoneurons is transiently independent of exogenous trophic support

We compared the survival requirements of early- and late-born motoneurons from E5 chicken spinal cord. Density gradient centrifugation followed by immunopanning using SC1 antibody allowed us to purify two size classes of motoneuron. Large motoneurons retained by 6.8% metrizamide were shown by BrdU labeling in ovo to be born on average 1.5 d earlier than the small motoneurons recovered from the metrizamide pellet. Large motoneurons were both biochemically and functionally more mature: they expressed higher levels of choline acetyltransferase and low-affinity neurotrophin receptor, and had an acute requirement for trophic support from muscle-derived factors. After 24 hr in culture in basal medium, all early-born motoneurons died, whereas 60% of late-born motoneurons survived. Small motoneurons can develop into large motoneurons in ovo, suggesting that they represent a general transitional stage in motoneuron development. Our results suggest that a defined period elapses between birth of a motoneuron and its acquisition of trophic dependence, possibly corresponding to the time required for target innervation. This property may have important consequences for the timing and regulation of developmental motoneuron death.

Peroxidase activation of tamoxifen and toremifene resulting in DNA damage and covalently bound protein adducts

When [14C]tamoxifen was incubated with horseradish peroxidase and H2O2, two major metabolites, separated and identified by HPLC, were N-desmethyltamoxifen and tamoxifen N-oxide. Toremifene incubated in a similar system yielded N-desmethyltoremifene and toremifene N-oxide. No 4-hydroxylated metabolites were detected with either drug. When calf thymus DNA was included in peroxidase incubation mixtures, DNA damage, as assessed by 32P-postlabelling, could also be detected. The extent of damage caused by tamoxifen and toremifene was similar. The major adducts formed following incubation of DNA with tamoxifen had similar Rf values to two of the 32P-postlabelled adducts seen following dosing of rats with tamoxifen. Peroxidase was able to activate both drugs to derivatives which covalently bound to bovine serum albumin. The pH optimum for covalent binding and N-demethylation was near to pH 6.0. Results from liquid chromatography-electrospray secondary ion mass spectrometry suggest that tamoxifen and toremifene are metabolized by peroxidase to putative reactive epoxide intermediates responsible for the genotoxic effects. It is proposed that peroxidase oxidizes tamoxifen to a carbon-centred free radical which reacts with oxygen to form peroxy radicals capable of inserting an oxygen atom into tamoxifen. Lactoperoxidase and prostaglandin synthase are also able to catalyse tamoxifen N-demethylation and binding to protein. These data show that peroxidase can activate both tamoxifen and toremifene to an intermediate(s) that can damage DNA and covalently react with protein. Since it is known that women treated with tamoxifen can develop endometrial tumours, it may be relevant to determine whether activation of tamoxifen by peroxidases may contribute to its carcinogenic action at extrahepatic sites.

Organized chaos? Computed tomographic evaluation of the neuropathic diabetic foot

Accurate radiographic evaluation of diabetic neuroarthropathy is increasingly difficult as the disease becomes more florid. 22 patients with a known diabetic neuroarthropathy of one or both feet were prospectively examined by computed tomography (CT) in the axial and coronal planes. Bilateral changes of a neuroarthropathy were present in 75% of cases. Distinct patterns of disease were seen and categorized into five types in order of increasing severity. Changes at the medial tarsometatarsal joints and adjacent intercuneiform joints were seen in all affected feet. More extensive disease involved the medial arch more commonly than the lateral. Fractures of the tarsal bones were found in 32% of cases and were associated with neuroarthropathic changes in adjacent joints. Calcaneal fractures were seen in four feet. A Lisfranc fracture-dislocation was present in 41% of cases and a bilateral in only 21%. A single CT examination of the foot, while an accurate method of demonstrating the extent of the disease, is an insensitive indicator of disease activity.

Neural development. Chemoattractants for navigating axons

Newly identified proteins that seem to act as diffusible attractants for circumferentially growing axons in the vertebrate embryonic spinal cord are related to a protein that directs circumferential axon growth in the nematode.

Neurons from mouse embryos with a null mutation in the tumour suppressor gene p53 undergo normal cell death in the absence of neurotrophins

Cell death plays an important role in regulating cell numbers in a wide variety of tissues during development and throughout life. Cell death can be triggered by changes in the levels of hormones and growth factors and is regulated by the expression of the tumour suppressor gene p53 in many cells. To determine if p53 plays a role in neuronal death resulting from neurotrophin deprivation, we studied the survival of neurons obtained from normal mouse embryos and embryos with a null mutation in the p53 gene. Embryonic sensory and sympathetic neurons from mutant embryos survived in response to the appropriate neurotrophin and died normally in the absence of neurotrophins. These results indicate that neurotrophin-deprived neurons die by a p53-independent pathway.

GDNF: a potent survival factor for motoneurons present in peripheral nerve and muscle

For survival, embryonic motoneurons in vertebrates depend on as yet undefined neurotrophic factors present in the limb bud. Members of the neurotrophin family are currently the best candidates for such neurotrophic factors, but inactivation of their receptor genes leads to only partial loss of motoneurons, which suggests that other factors are involved. Glial cell line-derived neurotrophic factor (GDNF), originally identified as a trophic factor specific for dopaminergic neurons, was found to be 75-fold more potent than the neurotrophins in supporting the survival of purified embryonic rat motoneurons in culture. GDNF messenger RNA was found in the immediate vicinity of motoneurons during the period of cell death in development. In vivo, GDNF rescues and prevents the atrophy of facial motoneurons that have been deprived of target-derived survival factors by axotomy. GDNF may therefore be a physiological trophic factor for spinal motoneurons. Its potency and specificity in vitro and in vivo also make it a good candidate for treatment of motoneuron disease.

Neurological disturbances in ankylosing spondylitis

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The role of neurotrophins in the developing nervous system

Neurotrophins were originally identified by their ability to promote the survival of developing neurons. However, recent work on these proteins indicates that they may also influence the proliferation and differentiation of neuron progenitor cells and regulate several differentiated traits of neurons throughout life. Moreover, the effects of neurotrophins on survival have turned out to be more complex than originally thought. Some neurons switch their survival requirements from one set of neurotrophins to another during development, and several neurotrophins may be involved in regulating the survival of a population of neurons at any one time. Much of our understanding of the developmental physiology of neurotrophins has come from studying neurons of the peripheral nervous system. Because these neurons and their progenitors are segregated into anatomically discrete sites, it has been possible to obtain these cells for in vitro experimental studies from the earliest stage of their development. The recent generation of mice having null mutations in the neurotrophin and neurotrophin receptor genes has opened up an unparalleled opportunity to assess the physiological relevance of the wealth of data obtained from these in vitro studies. Here I provide a chronological account of the effects of members of the NGF family of neurotrophins on cells of the neural lineage with special reference to the peripheral nervous system.

A comparison of a conventional non-ionic contrast medium (iohexol) alone and with adrenaline and an iso-osmolar non-ionic contrast medium (iotrolan) in computed tomographic arthrography of the shoulder

The physico-chemical properties of the iso-osmolar dimeric contrast medium iotrolan offers potential advantages in computed tomographic arthrography (CTA). A trial was undertaken comparing iotrolan with iohexol to assess if these theoretical benefits produced an increase in the measured densities in a series of shoulder CTAs. The results showed that iotrolan did produce clinically useful increases in density when compared to a monomeric non-ionic contrast medium. The addition of adrenaline to the monomeric contrast medium produced a significant improvement in the computed tomography density which surpassed that offered by the dimeric contrast medium alone.

Fenfluramine-induced modification of palatability: analysis by the taste reactivity test

The ability of various doses (0, 1.25, 2.5, and 5.0 mg/kg) of fenfluramine to modify the palatability of sucrose and quinine solutions was assessed by means of the taste reactivity test. Although fenfluramine did not modify the positive hedonic ingestive reactions elicited by sucrose solution, it consistently enhanced the negatively hedonic aversive reactions elicited by unpalatable 0.05% quinine solution and moderately palatable 2% sucrose solution. The results suggest that fenfluramine enhances the aversive properties of tastants without suppressing the positive hedonic properties of tastants. The results support a two-dimensional model of palatability.

MRI of eosinophilic granuloma

The imaging studies of nine histologically proven eosinophilic granulomas were reviewed. Radiographs and MRI studies were performed on all patients, with eight patients being examined by bone scintigraphy and six by CT. Matrix calcification, not evident on radiography, was demonstrated in two cases by CT. MRI proved superior to both the radiographs and CT in defining the medullary extent of the lesion and the surrounding soft tissue changes. In eight of nine cases, on STIR sequences, an endosteal rim of low signal intensity surrounding the main lesion was present and may be an early feature of healing. The degree of peritumoral oedema accompanying eosinophilic granuloma was less extensive than that seen in either Ewing's sarcoma or osteomyelitis. The presence of both a low signal endosteal rim and limited peritumoural oedema on STIR sequences may be a useful indicator to the diagnosis of underlying eosinophilic granuloma.