Hyperreflective dots in the avascular outer retina in relapsing-remitting multiple sclerosis

BACKGROUND

Hyperreflective granular elements with a transient presence in the retina can be detected non-invasively by optical coherence tomography (OCT). Such foci or dots may represent aggregates of activated microglia. However, in multiple sclerosis an increased number of hyperreflective foci has so far not been demonstrated in the intrinsically hyporeflective and avascular outer nuclear layer of the retina where there are no fixed elements in healthy eyes. Therefore, the present study intended to investigate the presence of hyperreflective foci in the outer nuclear layer in patients with relapsing- remitting multiple sclerosis (RRMS) by using a high-resolution OCT scanning protocol.

METHODS

This cross-sectional exploratory study examined 88 eyes in 44 RRMS patients and 106 eyes in 53 age- and sex-matched healthy subjects. None of the patients had any sign of retinal disease. All patients and healthy subjects each underwent one session of spectral domain OCT imaging. A total of 23,200 B-scans extracted from 8 × 8 mm blocks of linear B-scans at 60 μm intervals were analysed for hyperreflective foci in the outer nuclear layer of the retina. Analyses were made of the total block scan and a circular 6-mm diameter fovea-centered field in each eye. Multivariate logistic regression analysis was used to assess associations between parameters.

RESULTS

Hyperreflective foci were observed in 31 out of 44 (70.5 %) multiple sclerosis patients compared to 1 out of 53 (1.8%) healthy subjects (p < 0.0001). From analyses of the total block scans, the median number of hyperreflective foci in the outer nuclear layer was 1 (range 0-13) in patients and 0 (range 0-2) in healthy subjects (p < 0.0001). In total, 66.2% of all hyperreflective foci were located within 6 mm of the center of the macula. There was no detectable association between the presence of hyperreflective foci and retinal nerve fiber layer or ganglion cell layer thickness.

CONCLUSION

Hyperreflective granular foci in the avascular outer nuclear layer of the retina seen by OCT were almost completely absent in healthy subjects, whereas they were found, albeit at low density, in the majority of patients with RRMS. Hyperreflective foci can be repeatedly examined by non-invasive means and without pupil dilation, which opens a new field of investigation of infiltrating elements in an unmyelinated part of the central nervous system.

Automated detection of hyperreflective foci in the outer nuclear layer of the retina

PURPOSE

Hyperreflective foci are poorly understood transient elements seen on optical coherence tomography (OCT) of the retina in both healthy and diseased eyes. Systematic studies may benefit from the development of automated tools that can map and track such foci. The outer nuclear layer (ONL) of the retina is an attractive layer in which to study hyperreflective foci as it has no fixed hyperreflective elements in healthy eyes. In this study, we intended to evaluate whether automated image analysis can identify, quantify and visualize hyperreflective foci in the ONL of the retina.

METHODS

This longitudinal exploratory study investigated 14 eyes of seven patients including six patients with optic neuropathy and one with mild non-proliferative diabetic retinopathy. In total, 2596 OCT B-scan were obtained. An image analysis blob detector algorithm was used to detect candidate foci, and a convolutional neural network (CNN) trained on a manually labelled subset of data was then used to select those candidate foci in the ONL that fitted the characteristics of the reference foci best.

RESULTS

In the manually labelled data set, the blob detector found 2548 candidate foci, correctly detecting 350 (89%) out of 391 manually labelled reference foci. The accuracy of CNN classifier was assessed by manually splitting the 2548 candidate foci into a training and validation set. On the validation set, the classifier obtained an accuracy of 96.3%, a sensitivity of 88.4% and a specificity of 97.5% (AUC 0.989).

CONCLUSION

This study demonstrated that automated image analysis and machine learning methods can be used to successfully identify, quantify and visualize hyperreflective foci in the ONL of the retina on OCT scans.

Anti-myelin oligodendrocyte glycoprotein antibodies in a girl with good recovery after five episodes of prior idiopathic optic neuritis

Purpose

To describe the clinical, radiological, immunological and electrophysiological features of a myelin oligodendrocyte glycoprotein (MOG)-IgG positive girl with five prior episodes of idiopathic bilateral optic neuritis (ON).

Observations

We report a Danish girl who has been followed by pediatricians and pediatric neurologists since the age of 10 with recurrent episodes of idiopathic bilateral ON. Since the age of 15 there has been no recurrence of ON, and the patient has been thoroughly investigated for Multiple Sclerosis (MS) several times, but with negative findings. At the age of 19 the patient was referred to the Clinic of Optic Neuritis where she was tested seropositive for antibodies against MOG (MOG- IgG) on a conventionally cell-based assay. Despite 5 previous episodes of ON, the latency and amplitude signals of pattern-reversal visual evoked potentials (pVEP) including multifocal VEP were detected within the normal range.

Conclusion

The clinical implications of MOG- IgG are not yet clear, but in cases where the diagnosis of MS is less likely and where ON is the main symptom, testing for both IgG antibodies against AQP4 and MOG while having atypical optic neuropathies in mind is important. MOG-IgG positive patients may have a good prognosis with regards to visual function.

Multifocal visual evoked potentials in optic neuritis and multiple sclerosis: A review

Multifocal visual evoked potential (mf-VEP) represents a new approach to the classical full field (ff-)VEP with separate responses from up to 60 sectors of the visual field. A thorough literature survey of the use of mf-VEP in optic neuritis (ON) and multiple sclerosis (MS) is presented (38 published studies were retrieved). Mf-VEP provides direct topographical information of specific lesions and facilitates investigations on structural-functional correlations thus providing new methods for exploring the interplay between demyelination, atrophy and remyelination in MS. Good correlation was shown between mf-VEP and OCT, ff-VEP, MRI (MTR, DTI), 30-2 standard automated perimetry and low-contrast-visual acuity. All but one study showed superior sensitivity and specificity compared to ff-VEP, especially with regards to small, peripheral lesions or lesions of the upper visual field. Mf-VEP has shown superior sensitivity and specificity than established methods in diagnosing optic nerve lesions and tracking functional recovery following lesions. Abnormal mf-VEP responses in the fellow, non-ON afflicted eye may predict MS risk in ON patients. No standardization currently exists and no direct comparisons in ON and MS between at least 5 different commercially available mf-VEP systems have so far been published. Despite these limitations, mf-VEP is a promising new tool of diagnostic and prognostic value of mf-VEP in ON and MS.

Comparative approaches to avian song system function: insights into auditory and motor processing

Many fundamental advances in our understanding of basic neural function have been made using bird song learning and performance as a model system. These advances have included a greater understanding of higher-order neural processing, developmental and hormonal influences on behavior, and the realization that neurogenesis plays an important role in normal adult brain function. The great diversity of passerine birds and song-related behaviors they exhibit suggest that oscine songbirds are ideally suited for comparative studies. While the comparative approach has been used successfully in the past to study song-related phenomena at anatomical and behavioral levels, it has been underutilized in addressing questions at the neurophysiological level. Most neurophysiological studies of songbird auditory and motor processing have been performed in one species, the zebra finch (Taeniopygia guttata). We present and compare neurophysiological studies we have performed in zebra finches and song sparrows (Melospiza melodia), species that differ markedly in their singing behavior and song repertoire characteristics. Interspecific similarities, and striking differences, in song neural processing are apparent. While preliminary, these data suggest that comparative neurophysiological studies of species carefully chosen for their vocal repertoire and singing behavior will contribute significantly to our understanding of vertebrate sensory and motor neural processing.

Biomarkers of joint tissue metabolism in canine osteoarthritic and arthritic joint disorders

OBJECTIVE

To explore the levels of matrix metalloprotease-3 (MMP-3), tissue inhibitor of metalloproteases-1 (TIMP-1), 5D4 keratan sulfate, and two 3B3 chondroitin-sulfate epitopes in several canine osteoarthritic and inflammatory arthropathies.

METHODS

Blood and synovial fluid were obtained from 103 dogs with rupture of the anterior cruciate ligament (ACLR), osteochondritis dissecans (OCD), fragmented coronoid process (FPC), patella luxation (PL), hip dysplasia (HD) or infectious arthritis. Dogs with non-musculosceletal disorders were used as controls. The biomarkers were measured by immunoassays.

RESULTS

Median levels of synovial MMP-3, TIMP-1 and molar ratios of MMP/TIMP-1 were significantly higher in the arthritis than in the control group. The release of 5D4 keratan sulfate epitope and serum 3B3 neoepitope was reduced in arthritis patients. Increases in synovial TIMP-1 in OA were less pronounced and the molar ratio of MMP-3/TIMP-1 remained far below 1.0, demonstrating a surplus of the protease inhibitor. In osteoarthritic patients median levels of synovial 5D4 keratan sulfate were up-regulated after ACLR and PL and were inversely correlated with increasing duration of lameness. Serum TIMP-1 levels were significantly reduced in the joint disorder group when compared with the control group.

CONCLUSION

Our observations present the TIMP-1 serum level as a potential marker for the detection of degenerative changes in cartilage and also indicate that in canine OA, the MMP-3 mediated matrix destruction is not of major importance. However MMP-3 seems to be a sensitive marker for the local inflammation in canine arthritis.

Lipid composition of virosomes modulates their fusion efficiency with cryopreserved bull sperm cells

Virosomes derived from different fusogenic enveloped viruses have been generated for potential application in gene targeting to sperm cells. Comparative characterization of reconstitution products revealed that virosomes derived from influenza viruses are superior to those generated from Sendai viruses, with respect to the fusion rates with cryopreserved bull sperm cells and to sperm cell vitality after fusion. Modulation of the lipid composition during virosome reconstitution affects fusion sites on target sperms and allows optimization of the fusion rate and sperm cell vitality. A fluorescence-based microscopic fusion assay combined with a vital cell stain revealed that about 90% of sperm cells fused with influenza virosomes containing exogenous cholesterol, sphingomyelin, phosphatidylcholine, and phosphatidylethanolamine. About 85% of the fused sperm cells remained vital. Such optimized influenza-derived virosomes provide the basis for ongoing experiments, which aim at eventually generating biologically active transgenic sperms.

Molecular cloning, expression and characterization of the Canis familiaris interleukin-4

Interleukin 4 (IL-4) is an important regulatory cytokine produced by activated T lymphocytes and mast cells, and regulates the growth and differentiation of cells such as B and T lymphocytes. The rapid amplification of cDNA ends (RACE) was used to clone the canine IL-4 gene. It was expressed in mammalian cells and Escherichia coli. Monoclonal antibodies were raised to rcIL-4 for use in an enzyme-linked immunosorbent assay (ELISA). This will facilitate studies on the role of cIL-4 in inflammatory diseases, particularly rheumatoid arthritis.

Enzymatic depalmitoylation of viral glycoproteins with acyl-protein thioesterase 1 in vitro

Many glycoproteins of enveloped viruses as well as cellular proteins are covalently modified with fatty acids. Palmitoylation is often reversible, but the enzymology of this hydrophobic protein modification is not understood. Recently a cytosolic enzyme designated acyl-protein thioesterase 1 (APT1) was purified, which depalmitoylates several cellular proteins. Since hitherto no transmembrane proteins have been tested as substrates for APT1 we have investigated whether palmitoylated viral membrane glycoproteins can be deacylated by use of this enzyme. Recombinant APT1 was purified from Escherichia coli, and depalmitoylation of [3H]palmitate-labeled glycoproteins present in virus particles was measured by SDS-PAGE, fluorography, and scanning densitometry. We find that APT1 causes rapid and almost complete cleavage of fatty acids from the G-protein of vesicular stomatitis virus, hemagglutinin proteins of influenza A and C virus, and E2 of Semliki Forest virus (SFV). In contrast, E1 of SFV is largely resistant against APT1 activity. This substrate specificity of APT1 was also observed using microsomes prepared from SFV-infected cells. Our data emphasize the potential of APT1 as a tool for functional analysis of protein-bound fatty acids.

5-Hydroxytryptamine 4(a) receptor expressed in Sf9 cells is palmitoylated in an agonist-dependent manner

The mouse 5-hydroxytryptamine 4(a) receptor [5-HT(4(a))] was expressed with a baculovirus system in insect cells and analysed for acylation. [(3)H]Palmitic acid was effectively incorporated into 5-HT(4(a)) and label was sensitive to the treatment with reducing agents indicating a thioester-type bond. Analysis of protein-bound fatty acids revealed that 5-HT(4(a)) contains predominantly palmitic acid. Treatment of infected Sf9 (Spodoptera frugiperda) cells with BIMU8 [(endo-N-8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dehydro-2-oxo-3-(prop-2-yl)-1H-benzimid-azole-1-carboxamide], a 5-HT(4) receptor-selective agonist, generated a dose-dependent increase in [(3)H]palmitate incorporation into 5-HT(4(a)) with an EC(50) of approx. 10 nM. The change in receptor labelling after stimulation with agonist was receptor-specific and did not result from general metabolic effects. We also used both pulse labelling and pulse-chase labelling to address the dynamics of 5-HT(4(a)) palmitoylation. Incorporation studies revealed that the rate of palmitate incorporation was increased approx. 3-fold after stimulation with agonist. Results of pulse-chase experiments show that activation with BIMU8 promoted the release of radiolabel from 5-HT(4(a)), thereby reducing the levels of receptor-bound palmitate to approximately one-half. Taken together, our results demonstrate that palmitoylation of 5-HT(4(a)) is a reversible process and that stimulation of 5-HT(4(a)) with agonist increases the turnover rate for receptor-bound palmitate. This provides evidence for a regulated cycling of receptor-bound palmitate and suggests a functional role for palmitoylation/depalmitoylation in 5-hydroxytryptamine-mediated signalling.

Slow synaptic inhibition mediated by metabotropic glutamate receptor activation of GIRK channels

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.

Combining phage display and screening of cDNA expression libraries: a new approach for identifying the target antigen of an scFv preselected by phage display

A potential method for identifying new tumor-specific antibody structures as well as tumor-associated antigens is by selecting scFv phage libraries on tumor cells. This phage display technique involves multiple rounds of phage binding to target cells, washing to remove non-specific phage and elution to retrieve specific binding phage. Although the binding properties of an isolated tumor-specific scFv can be evaluated by ELISA, FACS and immunohistochemistry, it still remains a challenge to define the corresponding antigen. Here, we provide evidence that the target antigen of a given scFv displayed on phages can be detected in an immobilized lambda phage cDNA expression library containing thousands of irrelevant clones. The library contained CD30-negative breast-cancer specific cDNA as well as human CD30 receptor cDNA. The interaction of anti-CD30 scFv phages and their target antigen after blotting onto nitrocellulose filters was documented under defined conditions. Screening of different ratios between CD30 receptor and breast cancer specific clones (1:1 and 1:200) revealed that the CD30 antigen could be detected by anti-CD30 scFv phages using at least 5x10(12) plaque forming units of filamentous phages per blot. These investigations demonstrate that it is possible to detect the target antigen of a preselected scFv displayed on filamentous phages in lambda phage cDNA expression libraries.

Modification of the cytoplasmic domain of influenza virus hemagglutinin affects enlargement of the fusion pore

The fusion activity of chimeras of influenza virus hemagglutinin (HA) (from A/fpv/Rostock/34; subtype H7) with the transmembrane domain (TM) and/or cytoplasmic tail (CT) either from the nonviral, nonfusogenic T-cell surface protein CD4 or from the fusogenic Sendai virus F-protein was studied. Wild-type or chimeric HA was expressed in CV-1 cells by the transient T7-RNA-polymerase vaccinia virus expression system. Subsequently, the fusion activity of the expression products was monitored with red blood cells or ghosts as target cells. To assess the different steps of fusion, target cells were labeled with the fluorescent membrane label octadecyl rhodamine B-chloride (R18) (membrane fusion) and with the cytoplasmic fluorophores calcein (molecular weight [MW], 623; formation of small aqueous fusion pore) and tetramethylrhodamine-dextran (MW, 10,000; enlargement of fusion pore). All chimeric HA/F-proteins, as well as the chimera with the TM of CD4 and the CT of HA, were able to mediate the different steps of fusion very similarly to wild-type HA. Quite differently, chimeric proteins with the CT of CD4 were strongly impaired in mediating pore enlargement. However, membrane fusion and formation of small pores were similar to those of wild-type HA, indicating that the conformational change of the ectodomain and earlier fusion steps were not inhibited. Various properties of the CT which may affect pore enlargement are considered. We surmise that the hydrophobicity of the sequence adjacent to the transmembrane domain is important for pore dilation.

Acylation of Galpha(13) is important for its interaction with thrombin receptor, transforming activity and actin stress fiber formation

Palmitoylation of alpha-subunits in heterotrimeric G proteins has become a research object of growing attention. Following our recent report on the acylation of the mono-palmitoylated Galpha(12) [Ponimaskin et al., FEBS Lett. 429 (1998) 370-374], we report here on the identification of three palmitoylation sites in the second member of the G(12) family, Galpha(13), and on the biological significance of fatty acids on the particular sites. Using mutants of alpha(13) in which the potentially palmitoylated cysteine residues (Cys) were replaced by serine residues, we find that Cys-14, Cys-18 and Cys-37 all serve as palmitoylation sites, and that the mutants lacking fatty acids are functionally defective. The following biological functions of Galpha(13) were found to be inhibited: coupling to the PAR1 thrombin receptor, cell transformation and actin stress fiber formation. Results from established assays for the above functions with a series of mutants, including derivatives of the constitutively active mutant Galpha(13)Q226L, revealed a graded inhibitory response on the above mentioned parameters. As a rule, it appears that palmitoylation of the N-proximal sites (e.g. Cys-14 and Cys-18) contributes more effectively to biological function than of the acylation site located more internally (Cys-37). However, the mutant with Cys-37 replaced by serine is more severely inhibited in stress fiber formation (80%) than in cell transformation (50%), pointing to the possibility of a differential involvement of the three palmitoylation sites in Galpha(13).

Sendai virosomes revisited: reconstitution with exogenous lipids leads to potent vehicles for gene transfer

A reliable new procedure is described for the reconstitution of Sendai viral envelopes suitable for gene transfer. Both fusion and hemagglutinin-neuraminidase glycoproteins were extracted from purified Sendai virus and reconstituted together with DNA in the presence of cholesterol:sphingomyelin:phosphatidylcholine:phosphatidylethanolamin e (Chol:SM:PC:PE) in a molar ratio of 3.5:3.5:2:1. Before reconstitution, the DNA to be transferred was condensed by pretreatment with polylysine. Exogenous lipid addition and the DNA-condensation step were essential for maximal size as well as for fusogenic activity of the resulting virosomes, the analysis of which revealed (1) the absence of any genomic material originating from Sendai virus, (2) the presence of fusogenic spikes in a functional orientation, (3) the encapsulation of reporter genes, and (4) high-transfer activity for plasmids carrying the green fluorescent protein (GFP) gene and double-stranded nucleotides into different mammalian cells. Transfer rates were up to 10-fold higher than those obtained with different cationic lipids. Gene delivery by means of our lipid-enriched Sendai virosomes extends the known gene transfer strategies, including those based on Sendai virus previously published.

Interhemispheric coordination of premotor neural activity during singing in adult zebra finches

The song system, a neural network that mediates the learning and production of song by oscine songbirds, is investigated extensively as a model system for understanding the neural basis of complex skill learning. Part of the complexity of birdsong arises from the coordinated recruitment of multiple groups of muscles on both sides of the body. Although the song system is bilaterally organized, little is known about how premotor activities on the two sides are coordinated during singing. We investigated this by unilaterally recording neural activity in the forebrain song nucleus HVc (also known as the high vocal center) during singing and by forcing the premotor activities in the two hemispheres out of synchrony by perturbing neural activity in the contralateral HVc with electrical stimulation. Perturbing the activity in one HVc at any time during a song led to a short-latency readjustment of activity in the contralateral HVc. This readjustment consisted of a true resetting of the temporal pattern of activity in the contralateral HVc rather than merely a transient activity suppression overlaid on an unaltered pattern of premotor activity. These results strongly suggest that the output of song premotor areas in the forebrain is continuously monitored and that an active mechanism exists for resynchronizing the outputs from the two hemispheres whenever their gross temporal patterns differ significantly. The possible anatomical substrates for these coordinating mechanisms and their potential roles in song learning are discussed.

Palmitoylation of rhodopsin with S-protein acyltransferase: enzyme catalyzed reaction versus autocatalytic acylation

Protein palmitoylation in vitro was studied using bovine rhodopsin as the substrate and a partially purified acylating enzymatic activity (PAT) from placental membranes. PAT incorporates fatty acid into rhodopsin with higher efficiency (10 times higher initial rate), as compared to autoacylation. The activity is sensitive to heat and trypsin, indicating a protein-mediated enzymatic process and requires the native conformation of rhodopsin. The presence of deacylated, free cysteine residues in dark-adapted rhodopsin increases palmitoylation via PAT. The sites for non-enzymatic and enzymatic palmitoylation could not be distinguished by peptide mapping. The reversible palmitoylation described here will provide a tool for the study of the role of palmitoylation in photoreceptor function.

Gating of auditory responses in the vocal control system of awake songbirds

Nucleus HVc of the avian song system is a forebrain structure critical in song production, perception and learning. Here we show that most HVc neurons that respond to auditory stimuli under anesthesia show no responses to the same stimulus in the awake, unrestrained bird. This suppression of auditory responses in awake birds does not occur in the forebrain field L complex, which is one of the auditory input stages for HVc. Gating of auditory input at the junction between the auditory and vocal control system may be essential for regulating auditory feedback signals necessary for song learning and maintenance.

Domain-structure of cytoplasmic border region is main determinant for palmitoylation of influenza virus hemagglutinin (H7)

We have shown previously that the length of cytoplasmic tails influences the selection of lipid substrates for palmitoylation of influenza viral hemagglutinin esterase fusion (HEF) and hemagglutinin (HA) glycoproteins [Veit et al. (1996) Biochem. J. 318, 163-172; Reverey et al. (1996) J. Biol. Chem. 271, 23607-23610]. Using a series of new chimeric mutant proteins derived from acylated influenza virus HA (subtype H7) and from nonacylated Sendai virus fusion protein (F, strain Z), we report here that palmitoylation levels depend on the type of transmembrane or cytoplasmic domain, or both, present in the expression products and that cysteine residues placed close to the cytoplasmic membrane border are not sufficient for acylation. By inserting stretches of the HA transmembrane domain into a nonacylated mutant of Sendai F (FCys), we induce palmitoylation after expression in CV.1 cells, and the level of fatty acid transfer increases with the length of the HA-derived insert. A five-amino-acid shift of the HA transmembrane domain severely augments fatty acid transfer. Our data suggest that the influenza virus HA contains complex conformational signals for palmitoylation that are mainly located within the transmembrane domain but also involve the C-tail region, whereas the extracellular (luminal) domain has only marginal influence on palmitoylation.

A cysteine-11 to serine mutant of G alpha12 impairs activation through the thrombin receptor

We have recently reported that G alpha12 is acylated with palmitic acid [Veit et al., FEBS Lett. 339 (1994) 160-164]. Here we identify cysteine 11 as the sole palmitoylation site and assess the function of G alpha12 palmitoylation after expression of wild type and acylation-deficient mutant in insect cells. Our experimental approach yielded the following results. (1) Palmitoylation of G alpha12 has no influence on the subunit interactions. (2) Palmitoylation promotes membrane binding of G alpha12 when this protein is expressed alone. Membrane attachment of the heterotrimer occurs independent of the presence of fatty acids in G alpha12. (3) Assays for agonist-stimulated binding of [35S]GTPgammaS after expression of the human thrombin receptor (PAR1) along with G alpha12 and the betagamma subunits revealed a 70% inhibition with the palmitoyl-deficient mutant.

Slow synaptic inhibition in nucleus HVc of the adult zebra finch

Nervous systems process information over a broad range of time scales and thus need corresponding cellular mechanisms spanning that range. In the avian song system, long integration times are likely necessary to process auditory feedback of the bird's own vocalizations. For example, in nucleus HVc, a center that contains both auditory and premotor neurons and that is thought to act as a gateway for auditory information into the song system, slow inhibitory mechanisms appear to play an important role in the processing of auditory information. These long-lasting processes include inhibitory potentials thought to shape auditory selectivity and a vocalization-induced inhibition of auditory responses lasting several seconds. To investigate the possible cellular mechanisms of these long-lasting inhibitory processes, we have made intracellular recordings from HVc neurons in slices of adult zebra finch brains and have stimulated extracellularly within HVc. A brief, high-frequency train of stimuli (50 pulses at 100 Hz) could elicit a hyperpolarizing response that lasted 2-20 sec. The slow hyperpolarization (SH) could still be elicited in the presence of glutamate receptor blockers, suggesting that it does not require polysynaptic excitation. Three major components contribute to this activity-induced SH: a long-lasting GABAB receptor-mediated IPSP, a slow afterhyperpolarization requiring action potentials but not Ca2+ influx, and a long-lasting IPSP, the neurotransmitter and receptor of which remain unidentified. These three slow hyperpolarizing events are well placed to contribute to the observed inhibition of HVc neurons after singing and could shape auditory feedback during song learning.

Fusion activity of transmembrane and cytoplasmic domain chimeras of the influenza virus glycoprotein hemagglutinin

The role of the sequence of transmembrane and cytoplasmic/intraviral domains of influenza virus hemagglutinin (HA, subtype H7) for HA-mediated membrane fusion was explored. To analyze the influence of the two domains on the fusogenic properties of HA, we designed HA-chimeras in which the cytoplasmic tail and/or transmembrane domain of HA was replaced with the corresponding domains of the fusogenic glycoprotein F of Sendai virus. These chimeras, as well as constructs of HA in which the cytoplasmic tail was replaced by peptides of human neurofibromin type 1 (NF1) or c-Raf-1, NF78 (residues 1441 to 1518), and Raf81 (residues 51 to 131), respectively, were expressed in CV-1 cells by using the vaccinia virus-T7 polymerase transient-expression system. Wild-type and chimeric HA were cleaved properly into two subunits and expressed as trimers. Membrane fusion between CV-1 cells and bound human erythrocytes (RBCs) mediated by parental or chimeric HA proteins was studied by a lipid-mixing assay with the lipid-like fluorophore octadecyl rhodamine B chloride (R18). No profound differences in either extent or kinetics could be observed. After the pH was lowered, the above proteins also induced a flow of the aqueous fluorophore calcein from preloaded RBCs into the cytoplasm of the protein-expressing CV-1 cells, indicating that membrane fusion involves both leaflets of the lipid bilayers and leads to formation of an aqueous fusion pore. We conclude that neither HA-specific sequences in the transmembrane and cytoplasmic domains nor their length is crucial for HA-induced membrane fusion activity.

Targeted delivery of human neurofibromin and c-Raf-1 mutants to the cytoplasmic membrane by use of the influenza virus hemagglutinin

Mutants of human neurofibromin and c-Raf-1 genes were fused to the 3' end of the hemagglutinin (HA) gene of influenza A virus by oligonucleotide-directed polymerase chain reaction (PCR). The two resulting chimeric genes, HA (1-534)/NF1 (1441-1518) and HA (1-534)/Raf-1 (51-132) which we designated HN and HR, respectively, were cloned in a vaccinia virus expression vector (pTMI) under the control of a T7 RNA polymerase promoter. The clones were expressed in a monkey cell line (CV-1) and the resulting chimeric proteins analysed. We found that expression levels of the chimeric proteins were similar to that of wild-type HA protein. Comparative endoglycosidase treatment revealed that the expressed chimeric proteins HN and HR were processed as wild-type HA, and FACS-analysis showed that both chimeric expression products localised in the cell membrane as the wild-type control. HN and HR expressing cells showed similar fusogenic activity as CV-1 cells transfected with wild-type HA indicating the correct topology of the fusion inducing portion (HA) of these chimera in the membrane. These findings show that the influenza virus hemagglutinin (HA) is a suitable vehicle to target foreign proteins with therapeutical potential into the cell membrane. In this respect HN and HR could potentially be used to block the abnormal signals generated by particular proteins in the cell membrane that lead to cell transformation.

Transient elevations in intracellular calcium are sufficient to induce sustained responsiveness to the neurotrophic factor bFGF

The present study investigates how a neuron's past history of neural activity may alter its responsiveness to subsequent signals. We demonstrate that a depolarizing pulse of extracellular potassium can prime neurons to become responsive to basic fibroblast growth factor (bFGF), even when the pulse is brief and occurs prior to addition of bFGF. Specifically, we subjected cultured embryonic chick ciliary ganglion neurons (E7) to a short pulse of elevated extracellular potassium followed by addition of bFGF and tested the effect of such treatment on neuronal survival. Neurons treated in this manner produced high levels of survival, whereas neurons exposed to either the pulse alone or the continuous presence of bFGF alone failed to promote any significant levels of survival. This priming effect of depolarization on bFGF-induced survival was blocked by calcium channel antagonists. To test the time dependency of this effect, we increased the time interval between termination of the calcium pulse and addition of bFGF. Our results demonstrate that a brief elevation in intracellular calcium has long lasting effects, up to 8 h after cessation of the depolarizing pulse, on neuronal responsiveness to bFGF. These findings suggest how a developing neuron's history of activity can alter its subsequent ability to respond to neurotrophic factors and has significant implications on the mechanisms by which activity may influence neuronal survival.

Deacylation of influenza virus hemagglutinin does not affect the kinetics of low pH induced membrane fusion

The relevance of palmitoylation of cysteine residues of influenza virus hemagglutinin (HA) for the HA-mediated membrane fusion triggered at low pH is investigated. Either wild-type HA (subtype H7) or mutant HA devoid of fatty acids were expressed in insect cells. The kinetics as well as the extent of fusion of HA-expressing cells with human erythrocyte ghosts were measured by a membrane mixing assay. Fusion was measured continuously at different pH by fluorescence dequenching of the lipid-like fluorophore R18 initially incorporated into the erythrocyte membrane. No significant difference between fusion of wild-type and mutant HA expressing cells with ghosts could be detected showing that deacylation does affect neither the extent nor the kinetics of fusion.

Intracellular compartmentalization of the glycoprotein B of herpesvirus Simian agent 8 expressed with a baculovirus vector in insect cells

The intracellular localization of the glycoprotein B of herpesvirus simian agent 8 expressed with a baculovirus system in insect cells was studied. Cell fractionation and immunoprecipitation revealed that gB is present in microsomal as well as in nuclear membranes. Both fractions contain oligomers, probably dimers, of gB with endoglycosidase-H sensitive, mannose-rich carbohydrates. Nuclear transport of gB was further analysed by immuno electron microscopy of recombinant baculovirus-infected cells. The glycoprotein is present both in the outer and the inner nuclear membrane as well as in cytoplasmic structures and at the cell surface. This study precludes the possibility that glycosylation and/or oligomerisation of SA8 gB are responsible for nuclear targeting.

Differential fatty acid selection during biosynthetic S-acylation of a transmembrane protein (HEF) and other proteins in insect cells (Sf9) and in mammalian cells (CV1)

The transmembrane glycoprotein HEF and its acylation deficient mutant M1 were expressed in Sf9 insect cells infected with recombinant baculovirus and in CV1 mammalian cells using the vaccinia T7 system. In insect cells (Sf9), both wild type HEF and HEF(M1) are synthesized in their precursor form HEF0, which appears as a double band in SDS gels. Digestion with glycopeptidase F and endoglycosidase H reveals that the larger 84-kDa form is modified by the attachment of unprocessed carbohydrates of the high mannose type whereas the smaller 76-kDa form is non-glycosylated. As revealed by in vitro labeling experiments with palmitic acid another modification of HEF is the attachment of a long chain fatty acid to cysteine residue Cys-652 which is located at the internal border of the cytoplasmic membrane. After labeling with [3H]palmitic acid in both systems only HEF(WT) is acylated, whereas HEF(M1) is not. High performance liquid chromatography analysis of the fatty acids bound to HEF(WT) expressed in Sf9 insect cells reveals nearly 80% of palmitic acid. In contrast to this finding, the acylation pattern of HEF expressed in CV1 cells shows nearly the same amounts of stearic and palmitic acid (40%). Since the interconversion of the input [3H]palmitic acid to stearic acid is even lower in CV1 cells than in insect cells, it follows that only HEF expressed in mammalian, but not in insect cells selects for stearic acid during its biosynthetic acylation. We extended our study to acylation of endogenous proteins in Sf9 cells. In finding only palmitate linked to protein we present evidence that, in contrast to mammalian cells, insect cells (Sf9) cannot transfer stearic acid to polypeptide. This finding favors the hypothesis of enzymatic acylation over non-enzymatic mechanisms of acyl transfer to protein.

Cytoplasmic tail length influences fatty acid selection for acylation of viral glycoproteins

We report remarkable differences in the fatty acid content of thioester-type acylated glycoproteins of enveloped viruses from mammalian cells. The E2 glycoprotein of Semliki Forest virus contains mainly palmitic acid like most other palmitoylated proteins analysed so far. However, the other glycoprotein (E1) of the same virus, as well as the HEF (haemagglutinin esterase fusion) glycoprotein of influenza C virus, are unique in this respect because they are acylated primarily with stearic acid. Comparative radiolabelling of uninfected cells with different fatty acids suggests that stearate may also be the prevailing fatty acid in some cellular acylproteins. To look for further differences between palmitoylated and stearoylated glycoproteins we characterized stearoylation in more detail. We identified the acylation site of HEF as a cysteine residue located at the boundary between the transmembrane region and the cytoplasmic tail. The attachment of stearate to HEF and E1 occurs post-translationally in a pre-Golgi compartment. Thus, stearoylated and palmitoylated proteins cannot be discriminated on the basis of the fatty acid linkage site or the intracellular compartment, where acylation occurs. However, stearoylated acylproteins contain a very short, positively charged cytoplasmic tail, whereas in palmitoylated proteins this molecular region is longer. Replacing the short cytoplasmic tail of stearoylated HEF with the long influenza A virus haemagglutinin (HA) tail in an HEF-HA chimera, and subsequent vaccinia T7 expression in CV-1 cells, yielded proteins with largely palmitic acid bound. The reverse chimera, HA-HEF with a short cytoplasmic tail was not fatty acylated at all during expression, indicating that conformational or topological constraints control fatty acid transfer.

Palmitoylation of endogenous and viral acceptor proteins by fatty acyltransferase (PAT) present in erythrocyte ghosts and in placental membranes

Human erythrocyte ghosts were shown to have palmitoylating activity which acylates both endogenous ghost polypeptides and exogenous proteins derived from Semliki Forest virus (SFV). Cell-free fatty acid transfer from [3H]palmitoyl-CoA to endogenous protein was greatly enhanced in ghosts when pre-existing fatty acids linked to the endogenous acyl proteins were removed by hydroxylamine treatment prior to the transfer reaction. In contrast to erythrocyte acyl proteins acceptor proteins present in human placental membranes were palmitoylated in vitro to a similar extent with or without prior deacylation by hydroxylamine treatment. This indicates the presence of large pools of non-acylated proteins in placenta and small pools in erythrocytes. In testing for the protein substrate specificity of the palmitoyl transferase (PAT) present in ghosts we found that the SFV acceptor proteins, which are totally unrelated to erythrocytes, competed with the palmitoylation of endogenous ghost protein acceptors. This palmitoylating enzyme is inhibited by Cibacron Blue, SDS, and heat treatment, but stimulated in the presence of low concentrations of mild detergent (TX-100). Since PAT operating at the surface membrane of red blood cells has properties very similar to those of PAT present in human placental microsomes [1], we suggest that only one type of PAT may transfer fatty acids to various acylproteins that occur at multiple locations in different tissues [2].

Isolated pulmonary mucormycosis in an apparently normal host: a case report

Mucormycosis is a rare fungal disease commonly affecting individuals with diabetes mellitus, hematological malignancy, and immune deficiency. Isolated pulmonary mucormycosis is extremely rare. This article reports a case of isolated pulmonary mucormycosis that presented as a solitary cavity infiltrate in a patient with no underlying risk factors.

Assessment of fusogenic properties of influenza virus hemagglutinin deacylated by site-directed mutagenesis and hydroxylamine treatment

Influenza virus hemagglutinin (HA) subtype H7 expressed from a baculovirus vector in insect cells requires cysteine residues for palmitoylation. Mutant HA devoid of fatty acids shows hemagglutinating and hemolytic activities almost identical to those of the acylated wild-type HA (wt). Using a membrane mixing assay (R18), neither the kinetics nor the pH dependence of fusion induced by wt or mutant HA was significantly different from virus-induced fusion. HA-induced fusion of insect cells with human erythrocyte ghosts could also be demonstrated by a cytoplasmic content mixing assay. Both species of recombinant HA induced the flow of lucifer yellow from preloaded ghosts into the cytoplasm of HA-bearing cells. This indicates that membrane fusion mediated by wild-type and fatty-acid-free HA includes both leaflets of the lipid bilayers. Hydroxylamine treatment of wt HA (H7) and fatty-acid-free mutant HA present in lysates of insect cells led to the complete inhibition of hemolytic activity. Deacylation of spike proteins by NH2OH treatment of virus particles resulted in a block of hemolytic activity in influenza virus subtypes H7 and H10 as well as of that in the togaviruses Semliki Forest and Sindbis virus. However, the same treatment did not affect subtypes H2 and H3 or two vesicular stomatitis virus serotypes. With such a differential effect whether or not fatty acids are present in the spike proteins of the different virus particles, hydroxylamine must have other effects than just deacylation, and therefore seems unsuitable for the study of the biological functions of acylproteins.

Rapid changes in thyroid function tests upon treatment of tuberculosis

SETTING

Inpatient service and tuberculosis (TB) clinic of a public hospital.

OBJECTIVES

(1) To test the hypothesis that an hepatic effect of antituberculosis drugs increases serum thyroxine-binding globulin (TBG); (2) to resolve conflicting reports on thyroid function in TB.

DESIGN

Measurement of serum thyroid hormones, thyroid hormone binding (T3RU) and binding proteins (TBG, transthyretin [TTR] and albumin) in 38 patients with active TB and in 29 healthy tuberculin-positive controls, before and about 10 days into therapy.

RESULTS

With therapy of TB (with isoniazid [INH], rifampin [RIF], ethambutol and/or pyrazinamide), TBG increased above control values and T3RU decreased (P < 0.001). These changes were weakly correlated with liver enzyme activities but did not predict clinical hepatitis, which developed in only 1 patient. T3 was initially subnormal in 61% of 38 TB patients, while T4, thyrotropin (TSH) and TBG were normal. T3, TTR and albumin, all negative acute phase reactants, increased towards normal by day 10 (P < 0.001). Thyroid function remained unaltered in 14 control patients taking INH, whereas T3RU decreased (binding increased) and T3 increased in 15 taking INH and RIF (P < 0.001).

CONCLUSIONS

TB patients manifest the expected low T3 of non-thyroid illness, but, unlike most sick patients, usually have normal or increased serum binding of thyroid hormones. Chemotherapy further increases binding by increasing TBG, an effect probably due to RIF.

Depolarization and laminin independently enable bFGF to promote neuronal survival through different second messenger pathways

This study presents evidence that cellular responsiveness to the neurotrophic factor basic fibroblast growth factor (bFGF) can be achieved by at least two classes of agents using different initial second messenger pathways. Embryonic chick ciliary ganglion neurons plated on polyornithine normally undergo rapid cell death in culture and could not be rescued by addition of bFGF. Similarly, neither exposure to laminin alone nor low levels of depolarization alone was sufficient to promote neuronal survival. In contrast, if ciliary ganglion neurons were exposed to bFGF in the presence of either laminin or low levels of depolarization, nearly all neurons were rescued from cell death. The observed synergistic effect of bFGF and low levels of depolarization required influx of calcium since addition of the L-type calcium channel antagonists PN200-110 completely prevented neuronal survival. The synergistic effect observed between laminin and bFGF was independent of changes in intracellular calcium since PN200-110 failed to block the synergistic effect and because neither addition of bFGF to neurons plated on laminin nor direct addition of laminin caused any significant changes in intracellular calcium. The ability of laminin to enable bFGF to promote neuronal survival could completely be blocked by the addition of antibodies to beta 1-integrin, suggesting a calcium-independent integrin-mediated response. Taken together, these results suggest that both laminin and depolarization can enable bFGF responsiveness by acting through different second messenger pathways.

Laminin and fibronectin guideposts signal sustained but opposite effects to passing growth cones

Guidepost cells are known to alter the behavior of growth cones in vivo, yet the nature of communication and the type of signals employed are largely undefined. The present study demonstrates that model guideposts, composed of a single molecular species, are sufficient to change the navigation and the behavior of advancing growth cones well beyond the time of contact. Laminin on model guideposts caused a sustained increase in growth cone velocity, whereas fibronectin led to a sustained decrease. A spatially discrete array of multiple laminin-model guideposts maintained increased growth rates on fibronectin, as expected for homogeneous laminin, and also provided unambiguous directional guidance information. Laminin-evoked growth cone responses required activation of protein kinase C-dependent intracellular signalling mechanisms.

Expression of the Sendai virus fusion protein in insect cells and characterization of its post-translational modifications

The fusion (F) protein of Sendai virus was expressed with a baculovirus system in insect cells. The F protein is synthesized in its uncleaved form F0, which appears in SDS-containing gels as two bands with M(r) values of 66K and 64K. Digestion of the F protein with endoglycosidase H revealed that the 66K species contains high mannose-type carbohydrates, whereas partially processed oligosaccharides are attached to the 64K species. Pulse-chase experiments showed that the F protein is initially synthesized as its 66K form. After 1 h of chase this precursor glycoprotein is partially converted to the 64K species and exposed at the cell surface. Other modifications of the F protein in insect cells are the formation of intramolecular disulphide linkages and oligomerization. However, processing and plasma membrane transport of the F protein in insect cells are incomplete and retarded compared to the F protein synthesized in Sendai virus-infected mammalian cells.

Post-translational folding of the influenza C virus glycoprotein HEF: defective processing in cells expressing the cloned gene

The post-translational processing of the influenza C virus glycoprotein HEF was analysed. In cells infected with influenza C virus, HEF protein is synthesized as a glycosylated 80K polypeptide. A post-translational conformational rearrangement involving the formation of intramolecular disulphide bonds results in a decrease in its electrophoretic mobility. Therefore, SDS-PAGE under non-reducing conditions suggests an Mr of about 100K, whereas under reducing conditions an 80K protein is observed which is in accordance with the sequence data. The 100K form was detected 10 min after synthesis of HEF, and transport to the cell surface took about 60 min. This result indicates that the conformational change presumably occurs in the endoplasmic reticulum. A difference in post-translational processing was observed when the HEF gene was expressed in the absence of other influenza C virus genes. In cells infected with recombinant simian virus 40, the 80K precursor was synthesized, but this protein was neither converted to the 100K form nor transported to the cell surface. Deletion of the short cytoplasmic tail of HEF (Arg-Thr-Lys) or replacement of the two basic amino acids by hydrophobic (Ile) or acidic residues (Glu) resulted in HEF protein which was partially converted to the 100K form. Influenza C virus glycoprotein obtained after transient expression of the HEF gene using the vaccinia virus system was completely converted to the 100K form. However, in neither expression system was HEF transported to the cell surface. The possibility is discussed that the interaction of HEF with another viral protein is required for the post-translational folding and transport of this glycoprotein. The M protein of influenza C virus is suggested as a candidate for the chaperone which might interact with the cytoplasmic tail of HEF.

Self-recognition: a constraint on the formation of electrical coupling in neurons

Electrical coupling between specific neurons is important for proper function of many neuronal circuits. Identified cultured neurons from the snail Helisoma show a strong correlation between electrical coupling and presence of gap junction plaques in freeze-fracture replicas. Gap junction plaques, however, were never seen between overlapping neurites from a single neuron, even though those same neurites formed gap junctions with neurites from another essentially identical identified neuron. This observation suggests that a form of self-recognition inhibits reflexive gap junction formation between sibling neurites. When one or both of those growth cones had been physically isolated from the neuronal cell body, both electrical coupling and gap junction plaques, between growth cones from the same neuron, were observed to form rapidly (within 30 min). Thus, inhibition of electrical coupling between sibling neurites apparently depends on cytoplasmic continuity between neurites, and not the molecular composition of neurite membrane. The formation of gap junctions is not likely due to the isolation process; rather, the physical isolation appears to release an inhibition of reflexive gap junction formation. These data demonstrate the existence of a previously unknown constraint on the formation of electrical synapses.

The alpha-subunits of G-proteins G12 and G13 are palmitoylated, but not amidically myristoylated

The alpha-subunits of the G-proteins G12 and G13 were expressed with a baculovirus system in insect cells and analysed for acylation. Both proteins incorporated tritiated palmitic and to a lesser extent also tritiated myristic acid. Radiolabel from both fatty acids was sensitive to treatment with neutral hydroxylamine. This result supports a thioester-type fatty acid bond and argues against amidical N-myristoylation. Fatty acid analysis after labeling with [3H]palmitic acid showed that palmitate represents the predominant fatty acid linked to G alpha 12 and G alpha 13. Separation of cells into cytosolic and membranous fractions revealed that palmitoylated alpha-subunits of G12 were exclusively membrane-bound, whereas [35S]methionine-labeled proteins were detected in soluble and particulate fractions. Inhibition of protein synthesis with cycloheximide did not block palmitoylation of the alpha-subunits, which indicates that palmitoylation occurs independently of protein synthesis.

Timing of palmitoylation of influenza virus hemagglutinin

The timing of the attachment of fatty acids to the hemagglutinin (HA) of influenza A virus was studied. Treatment of virus infected cells with brefeldin A (BFA), a drug which blocks intracellular transport along the exocytic pathway at a pre-Golgi site, does not prevent palmitoylation of HA. The relationship of HA-palmitoylation to the oligomerisation and to the proteolytical cleavage of the protein revealed that the uncleaved trimer of HA is the substrate for the acylating enzyme in virus infected cells. The results are discussed with regard to the intracellular site of palmitoylation.

Structure, function, and intracellular localization of glycoprotein B of herpesvirus simian agent 8 expressed in insect and mammalian cells

The cloned gene of glycoprotein B (gB) of herpesvirus simian agent 8 (SA 8) was expressed with a baculovirus system in insect cells. Expression of gB was easily detectable over the cellular background by Coomassie staining of electrophoretically separated proteins. Endoglycosidase digestion of immunoprecipitated gB revealed that the gene product is N-glycosylated, but only with unprocessed, endoglycosidase-H sensitive carbohydrates. The lack of terminal glycosylation of gB is consistent with the observation that gB expressed in insect cells has a molecular weight slightly lower than gB synthesized during an SA 8 infection in mammalian cells. The truncated carbohydrates of gB from insect cells have no measurable effect on the tertiary structure of gB. Immunofluorescence studies on mammalian cells expressing gB from a simian virus 40 based vector revealed that the glycoprotein is localized to cytoplasmic membranes, to the plasma membrane and to the nuclear envelope. Cells expressing gB were fused to polykaryons, which shows that gB has cell fusing activity in the absence of any other SA 8 gene product.

Fibroblast growth factors, depolarization, and substratum interact in a combinatorial way to promote neuronal survival

Neuronal survival in vivo may be determined by the combined effects of multiple agents rather than simply by the effect of an individual agent. This idea is supported by experimental evidence showing that neuronal survival can independently be influenced by target-derived factors as well as afferent inputs. To test this idea directly, cultured chick ciliary ganglion neurons were used to study the potentially interactive and combinatorial effects of trophic factors (acidic and basic fibroblast growth factor; FGF), depolarization (as would be expected from afferent activity), and substrate (laminin and collagen IV). Our results were consistent with the idea that combinatorial interactions between multiple agents may be critical in the regulation of cell survival. Exposure to either basic FGF (bFGF) or depolarization on a laminin substrate promoted neuron survival. However, bFGF did not promote survival and depolarization-mediated survival was significantly reduced when neurons were plated on collagen. The simultaneous addition of FGF and depolarization affected survival synergistically when plated on both laminin and collagen. Surprisingly, while survival by FGF or depolarization alone was greatly dependent on substrate, the simultaneous addition of FGF and depolarization appeared to greatly reduce this dependency on substrate. Taken together, these data demonstrate the potential importance of synergistic interactions between trophic factors and depolarizing stimuli.

Role of intracellular calcium in NI-35-evoked collapse of neuronal growth cones

A myelin-associated protein from the central nervous system, the neurite growth inhibitor NI-35, inhibits regeneration of lesioned neuronal fiber tracts in vivo and growth of neurites in vitro. Growth cones of cultured rat dorsal root ganglion neurons arrested their growth and collapsed when exposed to liposomes containing NI-35. Before morphological changes, the concentration of free intracellular calcium ([Ca2+]i) showed a rapid and large increase in growth cones exposed to liposomes containing NI-35. Neither an increase in [Ca2+]i nor collapse of growth cones was detected in the presence of antibodies to NI-35. Dantrolene, an inhibitor of calcium release from caffeine-sensitive intracellular calcium stores, protected growth cones from collapse evoked by NI-35. Depletion of these caffeine-sensitive intracellular calcium stores prevented the increase in [Ca2+]i evoked by NI-35. The NI-35-evoked cascade of intracellular messengers that mediates collapse of growth cones includes the crucial step of calcium release from intracellular stores.

Sustained increase in intracellular calcium promotes neuronal survival

Ciliary ganglion neurons, half of which normally suffer developmental death in the embryo, will survive in culture in medium supplemented with depolarizing concentrations of potassium. It is not known how elevated potassium acts inside the cell to promote survival. We report here that depolarizing concentrations of extracellular potassium promote neuronal survival by causing a sustained increase in intracellular calcium. Raising extracellular potassium from 5 to 40 mM, an optimal concentration for survival, caused a sustained increase in intracellular calcium from 250 nM to greater than 600 nM. By 26 hr, at which time greater than 90% of neurons in 5 mM potassium had died, the calcium concentration of neurons in 40 mM potassium was still above 400 nM. Reduction of extracellular potassium from 40 to 5 nM, which prevents the increase in survival, also reduced intracellular calcium back to rest levels. PN200-110, a dihydropyridine calcium channel blocker that inhibits the survival-promoting effect of elevated potassium, also prevented and reversed the potassium,-mediated increase in intracellular calcium. In addition, there was a strong, quantitative correlation between the percentage of neuronal survival and the intracellular calcium concentration over a wide range of extracellular potassium concentrations. These results suggest that elevated potassium opens dihydropyridine-sensitive calcium channels, causing a sustained increase in intracellular calcium that quantitatively determines the number of surviving neurons.