Greenland Ice Sheet Surface Properties and Ice Dynamics from ERS-1 SAR Imagery.

C-band synthetic aperture radar (SAR) imagery from the European Space Agency's ERS-1 satellite reveals the basic zonation of the surface of the Greenland Ice Sheet. The zones have backscatter signatures related to the structure of the snowpack, which varies with the balance of accumulation and melt at various elevations. The boundaries of zones can be accurately located with the use of this high-resolution imagery. The images also reveal a large flow feature in northeast Greenland that is similar to ice streams in Antarctica and may play a major role in the discharge of ice from the ice sheet.

Physical conditions at the base of a fast moving antarctic ice stream.

Boreholes drilled to the bottom of ice stream B in the West Antarctic Ice Sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1.6 bars of the ice overburden pressure. These conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick. The mechanics of ice streaming plays a role in the response of the ice sheet to climatic change.

Thermal stability comparison of purified empty and peptide-filled forms of a class I MHC molecule.

A secreted form of a class I major histocompatibility complex (MHC) molecule was denatured and renatured in vitro in the absence of peptide. The resulting empty class I heterodimer was immunologically reactive and structurally similar to a heterodimer renatured in the presence of an appropriate restricted peptide. Thermal stability profiles indicated that the two forms of heterodimer differed in their resistance to denaturation by heat but that a significant portion of the empty class I heterodimers had a native conformation at physiological temperatures. Free energies calculated from these data gave a direct measure of the stabilization of the class I MHC molecule that resulted from peptide binding.

High geothermal heat flow, Basal melt, and the origin of rapid ice flow in central Greenland.

Age-depth relations from internal layering reveal a large region of rapid basal melting in Greenland. Melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet. Locally, high melt rates indicate geothermal fluxes 15 to 30 times continental background. The southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin.

Long-term changes in neurotrophic factor expression in distal nerve stump following denervation and reinnervation with motor or sensory nerve.

Several factors have been proposed to account for poor motor recovery after prolonged denervation, including motor neuron cell death and incomplete or poor regeneration of motor fibers into the muscle. Both may result from failure of the muscle and the distal motor nerve stump to continue expression of neurotrophic factors following delayed muscle reinnervation. This study investigated whether regenerating motor or sensory axons modulate distal nerve neurotrophic factor expression. We found that transected distal tibial nerve up-regulated brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) mRNA, down-regulated neurotrophin-3 and ciliary neurotrophic factor mRNA, and that although these levels returned to normal with regeneration, the chronically denervated distal nerve stump continued to express these neurotrophic factors for at least 6 months following injury. A sensory nerve (the cutaneous saphenous nerve) sutured to distal tibial nerve lowered injury-induced BDNF and GDNF mRNA levels in distal stump, but repair with a mixed nerve (peroneal, containing muscle and cutaneous axons) was more effective. Repair with sensory or mixed nerves did not affect nerve growth factor or neurotrophin-3 expression. Thus, distal nerve contributed to a neurotrophic environment for nerve regeneration for at least 6 months, and sensory nerve repair helped normalize distal nerve neurotrophic factor mRNA expression following denervation. Furthermore, as BDNF and GDNF levels in distal stump increased following denervation and returned to control levels following reinnervation, their levels serve as markers for the status of regeneration by either motor or sensory nerve.

NT-3 modulates BDNF and proBDNF levels in naïve and kindled rat hippocampus.

Both mature and precursor forms of neurotrophins regulate nerve development, survival and plasticity. Brain-derived neurotrophic factor (BDNF) synthesis and secretion in turn are regulated by neuronal activity, such as epilepsy. Further, neurotrophins themselves are regulated by neurotrophin levels. Neurotrophin-3 (NT-3) and BDNF in particular can be co-expressed and each can regulate the levels of the other. This regulation is thought to be mediated through receptor tyrosine kinase (Trk) activity. It is not known whether this neurotrophin-neurotrophin interaction occurs in hippocampal tissue in vivo, or how it is influenced by neuronal activation. In this study, we explored the reciprocal influences of intraventricular infusions of NT-3 and BDNF in naïve and kindled hippocampi of rats using Western blotting. We confirm that hippocampal kindling resulted in a significant increase in levels of BDNF both in cytochrome C (control) infused and NT-3 infused kindled rats. However, NT-3 infusion significantly reduced BDNF levels in both kindled and non-kindled hippocampi compared to their cytochrome C infused counterparts. These results are consistent with our earlier studies demonstrating lowered levels of TrkA and TrkC (NGF modulates BDNF levels via TrkA) following chronic NT-3 infusion. Although kindling led to an increase in BDNF, this was not accompanied by any detectable change in the levels of proBDNF. However, there was a significant increase in proBDNF following NT-3 infusions, suggesting NT-3 may reduce proBDNF processing. In contrast, neither NT-3 nor proNT-3 levels were affected by kindling or chronic BDNF infusions, consistent with down-regulation of TrkB by chronic BDNF infusion. Thus, modulation of BDNF by NT-3, likely mediated by Trk receptors, occurs in naïve and kindled adult rat hippocampus.

Structure and biosynthesis of nerve growth factor.

Most of our knowledge about NGF comes from extensive study of the mouse submaxillary gland protein. NGF from this source is isolated as a high molecular weight complex consisting of beta-NGF and two subunits, alpha and gamma, belonging to the kallikrein family of serine proteases. There are few other tissues where NGF is found in sufficient quantities for protein purification and study, although new molecular biological techniques have accelerated the study of NGFs from a variety of species and tissues. Mouse submaxillary gland NGF is synthesized as a large precursor that is cleaved at both N- and C-terminals to produce mature NGF. This biologically active molecule can be further cleaved by submaxillary gland proteases. The roles of the alpha and gamma subunits in the processing of the beta-NGF precursor, the modulation of the biological activity of beta-NGF, and the protection of mature beta-NGF from degradation have been well studied in the mouse. However, the apparent lack of alpha and gamma subunits in most other tissues and species and the existence of a large family of murine kallikreins, many of which are expressed in the submaxillary gland, challenge the relevance of murine high molecular weight NGF as a proper model for NGF biosynthesis and regulation. It is important therefore to identify and characterize other NGF complexes and to study their subunit interactions, biosynthesis, processing, and regulation. This review points out a number of other species and tissues in which the study of NGF has just begun. At this time, there exist many more questions than answers regarding the presence and the functions of NGF processing and regulatory proteins. By studying NGF in other species and tissues and comparing the processing and regulation of NGF from several sources, we will discover the unifying concepts governing the expression of NGF biological activity.

Synergistic effects of diet and exercise on hippocampal function in chronically stressed mice.

Severe chronic stress can have a profoundly negative impact on the brain, affecting plasticity, neurogenesis, memory and mood. On the other hand, there are factors that upregulate neurogenesis, which include dietary antioxidants and physical activity. These factors are associated with biochemical processes that are also altered in age-related cognitive decline and dementia, such as neurotrophin expression, oxidative stress and inflammation. We exposed mice to an unpredictable series of stressors or left them undisturbed (controls). Subsets of stressed and control mice were concurrently given (1) no additional treatment, (2) a complex dietary supplement (CDS) designed to ameliorate inflammation, oxidative stress, mitochondrial dysfunction, insulin resistance and membrane integrity, (3) a running wheel in each of their home cages that permitted them to exercise, or (4) both the CDS and the running wheel for exercise. Four weeks of unpredictable stress reduced the animals' preference for saccharin, increased their adrenal weights and abolished the exercise-induced upregulation of neurogenesis that was observed in non-stressed animals. Unexpectedly, stress did not reduce hippocampal size, brain-derived neurotrophic factor (BDNF), or neurogenesis. The combination of dietary supplementation and exercise had multiple beneficial effects, as reflected in the number of doublecortin (DCX)-positive immature neurons in the dentate gyrus (DG), the sectional area of the DG and hippocampal CA1, as well as increased hippocampal BDNF messenger ribonucleic acid (mRNA) and serum vascular endothelial growth factor (VEGF) levels. In contrast, these benefits were not observed in chronically stressed animals exposed to either dietary supplementation or exercise alone. These findings could have important clinical implications for those suffering from chronic stress-related disorders such as major depression.

Molecular cloning of a cDNA encoding the nerve growth factor precursor from Mastomys natalensis.

Mastomys natalensis is an African rat that has high levels of nerve growth factor (NGF) in its submaxillary glands. Like in the mouse, Mastomys NGF is found as a high-molecular-weight complex. However, the Mastomys complex differs from the mouse complex, in that the gamma-subunit is either missing or is less tightly bound in the Mastomys NGF complex. In the mouse, the gamma-subunit has been implicated in the processing of the beta-NGF precursor. The possible lack of gamma-subunits in the Mastomys NGF high-molecular-weight complex suggested that the Mastomys beta-NGF precursor might differ from the mouse beta-NGF precursor in some of its processing sites. In particular, Mastomys beta-NGF might lack the C-terminal dipeptide cleavage site implicated in beta-gamma subunit interactions in mouse NGF. In order to test this hypothesis, we isolated and sequenced a cDNA clone for Mastomys beta-NGF. We report here the cloning and sequencing of a cDNA coding for beta-NGF from Mastomys natalensis. The cDNA library was prepared from Mastomys submaxillary gland mRNA and the beta-NGF clone was isolated using a mouse cDNA as a probe. The nucleotide sequence of Mastomys beta-NGF is 95% homologous to that of mouse beta-NGF. In particular, the Mastomys beta-NGF precursor contains the same three C-terminal residues as the mouse, suggesting that the Mastomys beta-NGF precursor could interact with a gamma-like subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of brain-derived neurotrophic factor (BDNF) administration on kindling induction, Trk expression and seizure-related morphological changes.

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family that mediates synaptic plasticity and excitability in the CNS. Recent evidence has shown that increased BDNF levels can lead to hyperexcitability and epileptiform activities, while suppression of BDNF function in transgenic mice or by antagonist administration retards the development of seizures. However, several groups, including our own, have reported that increasing BDNF levels by continuous intrahippocampal infusion inhibits epileptogenesis. It is possible that the continuous administration of BDNF produces a down-regulation of its high-affinity TrkB receptor, leading to a decrease of neuronal responsiveness to BDNF. If so, then animals should respond differently to bolus injections of BDNF, which presumably do not alter Trk expression, compared with continuous infusion. To test this hypothesis, we compared the effects of intrahippocampal BDNF continuous infusion and bolus injections on kindling induction. We showed that continuous infusion of BDNF inhibited the development of behavioral seizures and decreased the level of phosphorylated Trks or TrkB receptors. In contrast, multiple bolus microinjections of BDNF accelerated kindling development and did not affect the level of phosphorylated Trks or TrkB receptors. Our results indicate that different administration protocols yield opposite effects of BDNF on neuronal excitability, epileptogenesis and Trk expression. Unlike nerve growth factor and neurotrophin-3, which affect mossy fiber sprouting, we found that BDNF administration had no effect on the mossy fiber system in naive or kindled rats. Such results suggest that the effects of BDNF on epileptogenesis are not modulated by its effect on sprouting, but rather by its effects on excitability.

Improved functional recovery of denervated skeletal muscle after temporary sensory nerve innervation.

Prolonged muscle denervation results in poor functional recovery after nerve repair. The possible protective effect of temporary sensory innervation of denervated muscle, prior to motor nerve repair, has been examined in the rat. Soleus and gastrocnemius muscles were denervated by cutting the tibial nerve, and the peroneal nerve was then sutured to the transected distal tibial nerve stump either immediately or after two, four or six months. In half of the animals with delayed repair, the saphenous (sensory) nerve was temporarily attached to the distal nerve stump. Muscles were evaluated three months after the peroneal-to-tibial union, and were compared with each other, with unoperated control muscles and with untreated denervated muscles. After four to six months of sensory "protection", gastrocnemius muscles weighed significantly more than unprotected muscles, and both gastrocnemius and soleus muscles exhibited better preservation of their structure, with less fiber atrophy and connective tissue hyperplasia. The maximum compound action potentials were significantly larger in gastrocnemius and soleus muscles following sensory protection, irrespective of the delay in motor nerve union. Isometric force, although less than in control animals and in those with immediate nerve repair, remained reasonably constant after sensory protection, while in unprotected muscles there was a progressive and significant decline as the period of denervation lengthened. We interpret these results as showing that, although incapable of forming excitable neuromuscular junctions, sensory nerves can nevertheless exert powerful trophic effects on denervated muscle fibers. We propose that these findings indicate a useful strategy for improving the outcome of peripheral nerve surgery.

A ligand of the p65/p95 receptor suppresses perforant path kindling, kindling-induced mossy fiber sprouting, and hilar area changes in adult rats.

Kindling, an animal model of epilepsy, results in an increased volume of the hilus of the dentate gyrus and sprouting of the mossy fiber pathway in the hippocampus. Our previous studies have revealed that chronic infusion of neurotrophins can regulate not only seizure development, but also these kindling-induced structural changes. Kindling, in turn, can alter the expression of neurotrophins and their receptors. We previously showed that intraventricular administration of a synthetic peptide that interferes with nerve growth factor stability and thus its binding to TrkA and p75(NTR) receptors suppressed kindling and sprouting. However, the precise involvement of TrkA, p75(NTR), and downstream signaling effectors of neurotrophins on kindling, sprouting and hilar changes are unknown. One of these downstream effectors is Ras. In the present study, we find that intraventricular infusion of the synthetic peptide Reo3Y, which binds to p65/p95 receptors and causes a rapid inactivation of Ras protein, impairs development of perforant path kindling, reduces the growth in afterdischarge duration, blocks kindling-induced mossy fiber sprouting in area CA3 of hippocampus and in inner molecular layer of the dentate gyrus, and prevents kindling-induced increases in hilar area. These results are consistent with a mediation of neurotrophin effects on kindling, hilar area, and axonal sprouting via Trk receptors, and suggest important roles for Ras in kindling and in kindling-induced structural changes.

Activity-dependent changes in synaptophysin immunoreactivity in hippocampus, piriform cortex, and entorhinal cortex of the rat.

Synaptophysin, an integral membrane glycoprotein of synaptic vesicles, has been widely used to investigate synaptogenesis in both animal models and human patients. Kindling is an experimental model of complex partial seizures with secondary generalization, and a useful model for studying activation-induced neural growth in adult systems. Many studies using Timm staining have shown that kindling promotes sprouting in the mossy fiber pathway of the dentate gyrus. In the present study, we used synaptophysin immunohistochemistry to demonstrate activation-induced neural sprouting in non-mossy fiber cortical pathways in the adult rat. We found a significant kindling-induced increase in synaptophysin immunoreactivity in the stratum radiatum of CA1 and stratum lucidum/radiatum of CA3, the hilus, the inner molecular layer of the dentate gyrus, and layer II/III of the piriform cortex, but no significant change in layer II/III of the entorhinal cortex, 4 weeks after the last kindling stimulation. We also found that synaptophysin immunoreactivity was lowest in CA3 near the hilus and increased with increasing distance from the hilus, a reverse pattern to that seen with Timm stains in stratum oriens following kindling. Furthermore, synaptophysin immunoreactivity was lowest in dorsal and greatest in ventral sections of both CA3 and dentate gyrus in both kindled and non-kindled animals. This demonstrates that different populations of sprouting axons are labeled by these two techniques, and suggests that activation-induced sprouting extends well beyond the hippocampal mossy fiber system.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats.

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

Brain-derived neurotrophic factor infusion delays amygdala and perforant path kindling without affecting paired-pulse measures of neuronal inhibition in adult rats.

Kindling is an animal model of human temporal lobe epilepsy in which excitability in limbic structures is permanently enhanced by repeated stimulations. Kindling also increases the expression of nerve growth factor, brain-derived neurotrophic factor, and brain-derived neurotrophic factor receptor messenger RNAs in both the hippocampus and cerebral cortex and causes structural changes in the hippocampus including hilar hypertrophy. We have recently shown that intraventricular nerve growth factor infusion enhances the development of kindling, whereas blocking nerve growth factor activity retards amygdaloid kindling. Furthermore, we have shown that nerve growth factor protects against kindling-induced hilar hypertrophy. The physiological role of brain-derived neurotrophic factor in kindling is not as clear. Acute injection of brain-derived neurotrophic factor increases neuronal excitability and causes seizures, whereas chronic brain-derived neurotrophic factor infusion in rats slows hippocampal kindling. In agreement with the latter, we show here that intrahilar brain-derived neurotrophic factor infusion delays amygdala and perforant path kindling. In addition, we show that brain-derived neurotrophic factor, unlike nerve growth factor, does not protect against kindling-induced increases in hilar area. To test the hypothesis that brain-derived neurotrophic factor suppresses kindling by increasing inhibition above normal levels, we performed paired-pulse measures in the perforant path-dentate gyrus pathway. Brain-derived neurotrophic factor infused into the hippocampus had no effect on the stimulus intensity function (input/output curves); there was also no significant effect on paired-pulse inhibition. We then kindled the perforant path 10 days after the end of brain-derived neurotrophic factor treatment. Once again, kindling was retarded, showing that the brain-derived neurotrophic factor effect is long-lasting. These results indicate that prolonged in vivo infusion of brain-derived neurotrophic factor reduces, rather than increases, excitability without increasing inhibitory neuron function, at least as assessed by paired-pulse protocols. This effect may be mediated by long-lasting effects on brain-derived neurotrophic factor receptor regulation.

NGF mRNA is not decreased in frontal cortex from Alzheimer's disease patients.

Alzheimer's disease (AD) is characterized by neuronal dysfunction and degeneration in certain brain regions such as cortex, hippocampus and basal forebrain. Specific neurochemical defects such as decreases in cholinergic enzymes and in the amounts of mRNA in AD brain have also been reported. Nerve growth factor (NGF), a protein necessary for the development, regulation and survival of basal forebrain cholinergic neurons (BFCN), is synthesized in target areas of BFCN (cortex, hippocampus) and is supplied to BFCN by retrograde transport. Thus, NGF is under investigation both as a potential therapeutic agent and for its possible involvement in the pathogenesis of AD. In this study, postmortem brain tissues from both control and AD cases were investigated for amounts of poly (A)+ mRNA and NGF mRNA in the frontal cortex, a region rich in cholinergic afferents. Yields of poly(A)+ mRNA were similar from normal and AD tissues. Human NGF mRNA comigrated with murine NGF mRNA on Northern blots. Additionally, dot blot quantitation demonstrated that NGF mRNA levels do not differ in the inferior frontal gyrus of normal and AD patients. Thus, we conclude that levels of mRNA in general, and of NGF mRNA in particular, are unchanged in the frontal cortex of individuals affected by AD.

Mouse NGF promoter upstream sequences do not affect gene expression in mouse fibroblasts.

The expression of nerve growth factor (NGF) is tightly controlled in a tissue-specific manner during development and in response to injury. In fibroblasts and in other cell types, expression of NGF is regulated at the transcriptional level. In order to elucidate the mechanism of this regulation, we have undertaken the analysis of the mouse NGF promoter in a mouse fibroblast cell line (LTA), using transient transfection of NGF promoter-human growth hormone (hGH) reporter gene plasmids. We find that sequences between +8bp and +120bp, containing an AP-1 site, confer increased levels of expression from the full length and truncated NGF promoters. When this region is deleted, a significant decrease in expression is observed from both the full length promoter and truncated versions thereof. A gradual increase in expression is observed with successive 5' deletions of both the AP-1 containing and AP-1 deleted promoters; this effect results from the juxtapositioning of adjacent plasmid sequences closer to the transcription initiation site and not from deletion of promoter sequences as was previously reported. When the NGF promoter is analyzed using a luciferase reporter plasmid, these 5' promoter deletions have no significant effect on reporter gene expression in fibroblasts. Thus, sequences downstream of the transcription start site influence NGF promoter activity in fibroblasts, but sequences upstream of the TATA box fail to affect promoter activity in these cells.

Stimulatory G-protein alpha-subunit mRNA levels are not increased in autopsied cerebral cortex from patients with bipolar disorder.

Increased alpha-subunit (alpha s) levels of both the 45- and 52-kDa isoforms of the stimulatory guanine nucleotide binding protein (G-protein), have been found in postmortem brain and mononuclear leukocytes from patients with bipolar disorder (BD). The pathophysiological mechanism responsible for increased alpha s protein levels is unknown, however, it may involve increased expression of the gene encoding this protein. To assess this possibility, alpha s mRNA levels were determined by RT-PCR in postmortem brain from 10 subjects with an antemortem diagnosis of BD and age- and sex-matched control subjects in whom we had previously reported increased alpha s protein levels. There were no significant differences in alpha s mRNA levels in frontal, temporal, or occipital cortex between BD and control subjects. Cerebral cortex alpha s mRNA levels did not correlate with age or postmortem interval. These findings do not support the notion that higher alpha s levels found in BD postmortem brain are a result of increased gene expression.

Nerve growth factor mRNA and protein levels measured in the same tissue from normal and Alzheimer's disease parietal cortex.

Nerve growth factor (NGF) mRNA and protein levels were determined in parietal cortex samples from both normal and Alzheimer's disease (AD) patients. NGF protein levels were slightly elevated in AD patients compared to controls, but NGF mRNA levels were unchanged in the same tissue samples. Thus, small but reproducible increases in NGF protein reported in AD cortex do not result from increases in NGF mRNA.

Quantitation of BDNF mRNA in human parietal cortex by competitive reverse transcription-polymerase chain reaction: decreased levels in Alzheimer's disease.

Alzheimer's disease is a progressive neurodegenerative disorder of the central nervous system. One pathological characteristic is excessive neuronal loss in specific regions of the brain. Among the areas most severely affected are the basal forebrain cholinergic neurons and their projection regions, the hippocampus and cortex. Neurotrophic factors, particularly the neurotrophins nerve growth factor and brain-derived neurotrophic factor, play an important role in the development, regulation and survival of basal forebrain cholinergic neurons. Furthermore, brain-derived neurotrophic factor regulates the function of hippocampal and cortical neurons. Neurotrophins are synthesized in hippocampus and cortex and retrogradely transported to the basal forebrain. Decreased levels of neurotrophic factors are suspected to be involved in the neurodegenerative changes observed in Alzheimer's disease. We examined autopsied parietal cortex samples from age- and gender-matched Alzheimer's diseased and neurologically non-impaired individuals using the quantitative technique of competitive RT-PCR. We demonstrate a 3.4-fold decrease in brain-derived neurotrophic factor mRNA levels in the parietal cortex of patients with Alzheimer's disease compared to controls (p<0.004). A decrease in brain-derived neurotrophic factor synthesis could have detrimental effects on hippocampal, cortical and basal forebrain cholinergic neurons and may account for their selective vulnerability in Alzheimer's disease.