Structural analysis of the GAP-related domain from neurofibromin and its implications

Neurofibromin is the product of the NF1 gene, whose alteration is responsible for the pathogenesis of neurofibromatosis type 1 (NF1), one of the most frequent genetic disorders in man. It acts as a GTPase activating protein (GAP) on Ras; based on homology to p120GAP, a segment spanning 250-400 aa and termed GAP-related domain (NF1GRD; 25-40 kDa) has been shown to be responsible for GAP activity and represents the only functionally defined segment of neurofibromin. Missense mutations found in NF1 patients map to NF1GRD, underscoring its importance for pathogenesis. X-ray crystallographic analysis of a proteolytically treated catalytic fragment of NF1GRD comprising residues 1198-1530 (NF1-333) of human neurofibromin reveals NF1GRD as a helical protein that resembles the corresponding fragment derived from p120GAP (GAP-334). A central domain (NF1c) containing all residues conserved among RasGAPs is coupled to an extra domain (NF1ex), which despite very limited sequence homology is surprisingly similar to the corresponding part of GAP-334. Numerous point mutations found in NF1 patients or derived from genetic screening protocols can be analysed on the basis of the three-dimensional structural model, which also allows identification of the site where structural changes in a differentially spliced isoform are to be expected. Based on the structure of the complex between Ras and GAP-334 described earlier, a model of the NF1GRD-Ras complex is proposed which is used to discuss the strikingly different properties of the Ras-p120GAP and Ras-neurofibromin interactions.

Selective disactivation of neurofibromin GAP activity in neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a common familial tumour syndrome with multiple clinical features such as neurofibromas, café-au-lait spots (CLS), iris Lisch nodules, axillary freckling, optic glioma, specific bone lesions and an increased risk of malignant tumours. It is caused by a wide spectrum of mutations affecting the NF1 gene. Most mutations result in the loss of one allele at the DNA, mRNA or protein level and thus in the loss of any function of the gene product neurofibromin. The idea of the simultaneous loss of several different neurofibromin functions has been postulated to explain the pleiotropic effects of its loss. However, we have identified a novel missense mutation in a family with a classical multi-symptomatic NF1 phenotype, including a malignant schwannoma, that specifically abolishes the Ras-GTPase-activating function of neurofibromin. In this family, Arg1276 had mutated into proline. Based on complex biochemical studies as well as the analysis of the crystal structure of the GTPase-activating protein (GAP) domain of p120GAP in the presence of Ras, we unequivocally identified this amino acid as the arginine finger of the neurofibromin GAP-related domain (GRD)-the most essential catalytic element for RasGAP activity. Here, we present data demonstrating that the mutation R1276P, unlike previously reported missense mutations of the GRD region, does not impair the secondary and tertiary protein structure. It neither reduces the level of cellular neurofibromin nor influences its binding to Ras substantially, but it does completely disable GAP activity. Our findings provide direct evidence that failure of neurofibromin GAP activity is the critical element of NF1 pathogenesis. Thus, therapeutic approaches aimed at the reduction of Ras.GTP levels in neural crest-derived cells can be expected to relieve most of the NF1 symptoms.

Interaction of finger representation in the human first somatosensory cortex: a neuromagnetic study

Neuromagnetic responses to separate tactile stimulation of digits I, II and V and simultaneous stimulation of digit pairs II and I, and II and V, were recorded in six healthy adult subjects using a 122-channel whole-head neuromagnetometer in order to investigate functional overlap of finger representations in primary somatosensory cortex (SI). Evoked responses to single digit stimulation were explained by time-varying equivalent current dipoles (ECDs) located in SI. These ECDs were then used to explain responses to stimulation of digit pairs. A cortical interaction ratio (IR) was defined as the vector sum of peak source amplitudes to separate stimulations of two fingers divided by the vector sum of source amplitudes to simultaneous stimulation of the two digits. Mean IR was significantly higher (P<0.05; Wilcoxon test) for digit pair II + I (1.69+/- 0.15) compared to digit pair II + V (1.14+/- 0.12). These results indicate that there is an overlap of finger representations in human SI which differs between anatomically adjacent and non-adjacent digit pairs.

Functional organization of the auditory cortex is different in stutterers and fluent speakers

Impaired auditory feedback has been suggested to cause stuttering, and subtle irregularities of audition have been reported in behavioural studies. To characterize processing at the auditory cortical level, we recorded neuromagnetic responses to monaural tones in nine stutterers and 10 fluent speakers while the subjects were reading silently, with mouth movements only, aloud, and in chorus with another person. The basic functional organization of the auditory cortices was found to be different in stutterers and controls. The altered interhemispheric balance in stutterers was affected by speech production, due to changes in the left auditory cortical representation, and more severely by self-paced than accompanied speech. This may lead to transient non-optimal interpretation of the auditory input and affect speech fluency.

Dystrophin in the retina

Dystrophin is a plasma membrane-associated cytoskeletal protein of the spectrin superfamily. The dystrophin cytoskeleton has been first characterized in muscle. Muscular 427 kDa dystrophin binds to subplasmalemmal actin filaments via its amino-terminal domain. The carboxy-terminus of dystrophin binds to a plasma membrane anchor, beta-dystroglycan, which is associated on the external side with the extracellular matrix receptor, alpha-dystroglycan, that binds to the basal lamina proteins laminin-1, laminin-2, and agrin. In the muscle, the dystroglycan complex is associated with the sarcoglycan complex that consists of several glycosylated, integral membrane proteins. The absence or functional deficiency of the dystrophin cytoskeleton is the cause of several types of muscular dystrophies including the lethal Duchenne muscular dystrophy (DMD), one of the most severe and most common genetic disorders of man. The dystrophin complex is believed to stabilize the plasma membrane during cycles of contraction and relaxation. Muscular dystrophin and several types of dystrophin variants are also present in extramuscular tissues, e.g. in distinct regions of the central nervous systems including the retina. Absence of dystrophin from these sites is believed to be responsible for some extramuscular symptoms of DMD, e.g. mental retardation and disturbances in retinal electrophysiology (reduced b-wave in electroretinograms). The reduced b-wave in electroretinograms indicated a disturbance of neurotransmission between photoreceptors and ON-bipolar cells. At least two different dystrophin variants are present in photoreceptor synaptic complexes. One of these dystrophins (Dp260) is virtually exclusively expressed in the retina. In the neuroretina, dystrophin is found in significant amounts in the invaginated photoreceptor synaptic complexes. At this location dystrophin colocalizes with dystroglycan. Agrin, an extracellular ligand of alpha-dystroglycan, is also present at this location whereas the proteins of the sarcoglycan complex appear to be absent in photoreceptor synaptic complexes. Dystrophin and dystroglycan are located distal from the ribbon-containing active synaptic zones where both proteins are restricted to the photoreceptor plasma membrane bordering on the lateral sides of the synaptic invagination. In addition, some neuronal profiles of the postsynaptic complex also contain dystrophin and beta-dystroglycan. These profiles appear to belong at least in part to projections of the photoreceptor terminals into the postsynaptic dendritic complex. In view of the abnormal neurotransmission between photoreceptors and ON-bipolar cells in DMD patients the dystrophin/beta-dystroglycan-containing projections of photoreceptor presynaptic terminals into the postsynaptic dendritic plexus might somehow modify the ON-bipolar pathway. Another retinal site associated with dystrophin/beta-dystropglycan is the plasma membrane of Müller cells where dystrophin/beta-dystroglycan appear to be present at particular high concentrations. At this location the dystrophin/dystroglycan complex may play a role in the attachment of the retina to the vitreous, and, under pathological conditions, in traction-induced retinal detachment.

Ras-binding domains: predicting function versus folding

Ras interacts with a number of effector molecules to achieve its prolific signalling. Based on iterative sequence profile and motif searches of databases a novel family of Ras-binding domains was recently identified (Ponting and Benjamin, Trends Biochem. Sci. 21: 422-425, 1996). Among them the rat unconventional myosin and Rho-GTPase-activating protein myr 5 was predicted to contain a Ras-binding domain at its N-terminus. Here we report that direct binding experiments between the proposed Ras-binding domain of myr 5 and Ras failed to demonstrate any interaction. Molecular modelling suggests that this domain in myr 5 adopts a similar folding topology as the Ras-binding domain of Raf kinase. However, unlike the Ras-binding domain of Raf kinase, the myr 5 domain lacks the positive surface charges necessary for binding the negatively charged Ras contact site. This result exemplifies the functional diversity of similar structures and suggests that the identified Ras-binding motif does not reliably predict Ras-binding domains.

Localization of dystrophin and beta-dystroglycan in bovine retinal photoreceptor processes extending into the postsynaptic dendritic complex

Dystrophin is an actin-binding protein of the membrane cytoskeleton that binds to dystroglycan, an integral membrane protein of the plasma membrane that is posttranslationally cleaved into a transmembrane dystrophin-binding beta-moiety and an extracellular laminin- and agrin-binding alpha-moiety. Mutations of dystrophin may not only cause Duchenne muscular dystrophy but may also be associated with abnormal electroretinograms assumed to result from disturbed neurotransmission between retinal photoreceptors and bipolar cells. Here we show by confocal laser microscopy and immunogold electron microscopy that dystrophin and beta-dystroglycan are colocalized in bovine rod photoreceptor synaptic complexes distal from the ribbon-containing active synaptic zones. Both proteins are restricted to a microdomain of the photoreceptor plasma membrane that forms the lateral wall of the synaptic cavity and projects with finger-like extensions into the postsynaptic dendritic complex. Within the cavity these processes eventually come into close contact with bipolar cell dendritic endings. We speculate that the dystrophin-dystroglycan complex of the cavital plasma membrane stabilizes the elaborate synaptic morphology or plays a role in the immobilization of still unknown transporters and receptors involved in certain aspects of neurotransmission to bipolar cells. A further outcome of this study is that dystrophin and dystroglycan are located along the vitread membrane surface of Müller cell endfeet where this protein complex may be important for the attachment of the retina to the basal lamina and the vitreous.

Rim is a putative Rab3 effector in regulating synaptic-vesicle fusion

Rab3 is a neuronal GTP-binding protein that regulates fusion of synaptic vesicles and is essential for long-term potentiation of hippocampal mossy fibre synapses. More than thirty Rab GTP-binding proteins are known to function in diverse membrane transport pathways, although their mechanisms of action are unclear. We have now identified a putative Rab3-effector protein called Rim. Rim is composed of an amino-terminal zinc-finger motif and carboxy-terminal PDZ and C2 domains. It binds only to GTP (but not to GDP)-complexed Rab3, and interacts with no other Rab protein tested. There is enrichment of Rab3 and Rim in neurons, where they have complementary distributions. Rab3 is found only on synaptic vesicles, whereas Rim is localized to presynaptic active zones in conventional synapses, and to presynaptic ribbons in ribbon synapses. Transfection of PC12 cells with the amino-terminal domains of Rim greatly enhances regulated exocytosis in a Rab3-dependent manner. We propose that Rim serves as a Rab3-dependent regulator of synaptic-vesicle fusion by forming a GTP-dependent complex between synaptic plasma membranes and docked synaptic vesicles.

The Ras-RasGAP complex: structural basis for GTPase activation and its loss in oncogenic Ras mutants

The three-dimensional structure of the complex between human H-Ras bound to guanosine diphosphate and the guanosine triphosphatase (GTPase)-activating domain of the human GTPase-activating protein p120GAP (GAP-334) in the presence of aluminum fluoride was solved at a resolution of 2.5 angstroms. The structure shows the partly hydrophilic and partly hydrophobic nature of the communication between the two molecules, which explains the sensitivity of the interaction toward both salts and lipids. An arginine side chain (arginine-789) of GAP-334 is supplied into the active site of Ras to neutralize developing charges in the transition state. The switch II region of Ras is stabilized by GAP-334, thus allowing glutamine-61 of Ras, mutation of which activates the oncogenic potential, to participate in catalysis. The structural arrangement in the active site is consistent with a mostly associative mechanism of phosphoryl transfer and provides an explanation for the activation of Ras by glycine-12 and glutamine-61 mutations. Glycine-12 in the transition state mimic is within van der Waals distance of both arginine-789 of GAP-334 and glutamine-61 of Ras, and even its mutation to alanine would disturb the arrangements of residues in the transition state.

Somatosensory evoked potentials modified by laser-induced lesions of the rat cortex

The effect of focal application of laser energy on the modification of somatosensory evoked potentials (SEPs) was studied in sensory cortical fields of the rat. This article describes the methodological set-up for recording of SEPs and for determining location and size of the laser-induced lesion. The results show that both the size of the lesion of the somatosensory cortex, and the suppression and time of recovery of cortical SEPs varied depending on the laser energy dose. It remains to be analyzed by further experiments if the recovery of SEPs is due to a transient dysfunction of the somatosensory cortex or if it reflects cortical plasticity.

Purification of synaptic ribbons, structural components of the photoreceptor active zone complex

Synaptic ribbons are plasma membrane-associated structural elements in photoreceptor synaptic terminals. They seem to act as high capacity "docking sites" of synaptic vesicles that provide the fusion sites of the photoreceptor synapse ("active zones"), with a large supply of immobilized synaptic vesicles rapidly available for exocytosis. Synaptic ribbons are regarded as a specialized type of presynaptic densities found in virtually all synapses. The molecular composition of presynaptic densities and synaptic ribbons is unknown. The aim of this study was the isolation of synaptic ribbons from photoreceptor synapses. For this purpose, we first isolated a membrane fraction from the bovine retina that was strongly enriched in photoreceptor synapses. From this fraction, a Triton X-100-resistant subfraction was purified that consisted mainly of synaptic ribbons and their disassembly products. The high enrichment of synaptic ribbons was verified by electron microscopy and immunolabeling using an antibody that specifically binds to synaptic ribbons. SDS-PAGE analysis of this synaptic ribbon fraction displayed several major polypeptide bands migrating at approximately 240, 60, 55, 43, and 30 kDa. The purification procedure described here is a first promising step toward the identification of the yet unknown constituents of synaptic ribbons from photoreceptor synapses and possibly also of presynaptic densities from other synapses.

Insulin-like growth factor-I serum concentrations and patterns of insulin-like growth factor binding proteins in patients with chronic liver disease

BACKGROUND/AIMS

Serum concentrations of insulin-like growth factor-I are decreased in liver cirrhosis. However, this growth factor is bound for the most part to specific binding proteins that are known to modulate biological actions. Plasma insulin-like growth factor binding proteins are predominantly synthesized in the liver.

METHODS

The effect of liver disease on basal and on growth hormone-stimulated serum concentrations of total and "free" insulin-like growth factor-I and on insulin-like growth factor binding protein patterns is reported. Sera were obtained from 20 patients with non-cirrhotic chronic liver diseases and from 20 patients with cirrhosis before and 24 h after a single subcutaneous dose of growth hormone. Samples were analyzed using radioimmunoassays, gel chromatography, ligand blotting and immunoblotting.

RESULTS

In cirrhosis, serum concentrations of total and "free" insulin-like growth factor-I were decreased, the binding protein pattern was changed profoundly showing a reduction in the 150 kD complex and an increase in the 30-40 kD complexes. Concentrations of binding protein-1 and -2 were increased, while that of binding protein-3 was decreased in cirrhosis. The response to growth hormone was blunted. These changes were related to the degree of liver dysfunction as assessed by the Child-Pugh classification.

CONCLUSIONS

A pathogenetic link of altered bio-availability of insulin-like growth factor-I to clinical characteristics of advanced liver disease, e.g. insulin resistance or skeletal muscle wasting, may be suggested by the present data.

Identification of cholecystokinin-B/gastrin receptor domains that confer high gastrin affinity: utilization of a novel Xenopus laevis cholecystokinin receptor

A hallmark of the mammalian brain cholecystokinin (CCK) receptor, CCK-B/gastrin (CCK-BR), is its high affinity for two structurally related peptides, CCK and gastrin. Previous radioligand binding experiments suggested that the predominant CCK receptor from Xenopus laevis brain shares high affinity for sulfated cholecystokinin octapeptide but has > or = 1000-fold lower affinity for gastrin. To determine the molecular basis for this pharmacological divergence between mammalian and lower vertebrate receptors, we isolated a cDNA encoding the X. laevis brain CCK receptor (CCK-XLR). CCK-XLR shares approximately 50% homology at the amino acid level with both the human CCK-BR and the peripheral CCK-A receptor subtypes. The recombinant X. laevis receptor has a distinct pharmacological profile of agonist and antagonist affinities and as such offers a useful tool for structure-function studies. We used CCK-XLR to map the human CCK-BR domains that confer high affinity for gastrin. A series of chimeric CCK-BR/CCK-XLR constructs was generated and pharmacologically characterized. While maintaining wild-type affinity for sulfated cholecystokinin octapeptide, receptors with increasing amino-terminal contributions from CCK-BR demonstrated a stepwise increase in gastrin affinity. Further dissection of the amino-terminal third of the human receptor, a domain that confers a > 250-fold increase in gastrin affinity, revealed the importance of interactions among at least three subdomains. Additional structural requirements for gastrin affinity mapped to a segment spanning transmembrane domains IV and V.

Dystrophin and the dystrophin-associated glycoprotein, beta-dystroglycan, co-localize in photoreceptor synaptic complexes of the human retina

Mutations in the gene encoding for dystrophin, a membrane-associated cytoskeletal protein of muscle and several non-muscle cells, are the cause of Duchenne muscular dystrophy and Becker muscular dystrophy. Patients suffering from Duchenne muscular dystrophy have recently been shown to display an abnormal b-wave of the electroretinogram, suggesting that dystrophin is important for normal retinal transmission. In the retina, dystrophin has been localized in the outer plexiform layer where dystrophin co-localizes with postsynaptic markers of photoreceptor synaptic complexes. In the present study we addressed the question of whether two major dystrophin-associated integral membrane proteins of the muscular plasma membrane, beta-dystroglycan and adhalin, are also present in photoreceptor synaptic complexes. By double immunostaining and immunoblotting we show here that beta-dystroglycan is expressed in the human retina where it co-localizes with dystrophin in photoreceptor synaptic complexes most likely on the postsynaptic side. Adhalin was not detected in the retina. Since beta-dystroglycan is a member of a transmembrane supramolecular complex thought to be important for differentiation of the neuromuscular junction, it is an attractive hypothesis that dystroglycan (linked to dystrophin) might also play a similar role in differentiation of the photoreceptor synapse. A further outcome of this study is that beta-dystroglycan is not only present in the neuromuscular junction but also associated with a well-defined synaptic complex of the central nervous system. These findings indicate a more general role of this dystrophin-associated membrane protein in synaptic functions.

Somatosensory evoked potentials elicited by intraneural microstimulation of afferent nerve fibers

Cortical somatosensory evoked potentials (SEPs) after intraneural microstimulation (IMS) of cutaneous and afferent muscle nerve fibers in the median nerve were recorded to study the contribution of different afferent fiber groups to the SEP. Thirty-seven cutaneous fiber bundles, 10 afferent muscle nerve fiber bundles, and 45 single mechanoreceptive afferents of FA I- (n = 12), FA II- (n = 8), SA I- (n = 13), and SA II-type (n = 12) were studied in 29 healthy subjects. IMS of cutaneous fiber bundles evoked cortical responses corresponding to the N20 component after median nerve stimulation in 86% of the fascicles studied, whereas IMS of muscle nerve fiber bundles elicited responses only in 20%. After IMS of single mechanoreceptive afferents of FA I-, FA II-, and SA I-type cortical responses were obtained in all groups in approximately 80% of the stimulated units. The latencies of the SEPs evoked by IMS were comparable to that after compound median nerve stimulation (mean difference 0.58 ms). The N20 amplitudes of SEPs after IMS of cutaneous fiber bundles were on average 28% (n = 32) and for single afferents 22% (n = 30) of that after compound nerve stimulation. It is concluded that the median nerve SEPs evoked by compound median nerve stimulation of the resting hand are dominated by cutaneous rather than muscle afferent input. Furthermore, the fact that selective stimulation of only a few cutaneous afferents produces cortical SEPs of rather high amplitude is compatible with the view that sensory information produced by selective afferent stimulation is differentially gated into the somatosensory cortex.

Effect of insulin-like growth factor II on uptake of arylsulfatase A by cultured rat hepatocytes and Kupffer cells

Mannose 6-phosphate/insulin-like growth factor II receptors have been characterized in hepatocytes and Kupffer cells isolated from adult rat liver. Affinity labeling with [125I]insulin-like growth factor II revealed a protein of Mr 250,000 in both cell types. Labeling was inhibited by an antiserum against the mannose 6-phosphate/insulin-like growth factor II receptor. In Kupffer cells, [125I]insulin-like growth factor II was also cross-linked to a second protein of Mr 130,000. In both cell types, insulin-like growth factor II was 10 times more potent than insulin-like growth factor I in displacing [125I]insulin-like growth factor II from its receptor. The mannose 6-phosphate-specific uptake of [125I]arylsulfatase A via the mannose 6-phosphate/insulin-like growth factor II receptor was inhibited by insulin-like growth factor II and antibodies against the receptor, but was not affected by insulin-like growth factor I, insulin or transforming growth factor beta 1. Cell surface iodination followed by immunoprecipitation of the mannose 6-phosphate/insulin-like growth factor II receptor showed that expression of the mannose 6-phosphate/insulin-like growth factor II receptors at the plasma membrane was increased two-fold by insulin-like growth factor II. These results suggest that binding of insulin-like growth factor II to the mannose 6-phosphate/insulin-like growth factor II receptor blocks the binding and uptake of mannose 6-phosphate-containing lysosomal enzymes and may be directly involved in a co-ordinate regulation of ligand uptake from plasma into hepatocytes and Kupffer cells.

Distribution of neurofilament-positive nerve fibres and sensory endings in the human anterior cruciate ligament

The present immunocytochemical study describes the distribution of neurofilament-containing nerve fibres and corpuscular-like endings in the human anterior cruciate ligament. The entire anterior cruciate ligament of a non-injured knee joint from a child was cut into serial 15-microns-thick frozen sections, which were processed for immunofluorescence staining with a monoclonal antibody against the 68-kDa neurofilament subunit. Numerous neurofilament-positive fibres were found in bundles. These bundles were mostly located near blood vessels in the subsynovial layer and in interfascicular gaps. Only a few single nerve fibres were found independent of blood vessels in interfascicular gaps and between collagen bundles. Neurofilament-containing nerve fibres were preferentially located near the bony attachments of the anterior cruciate ligament. Two types of corpuscular-like endings were found, i.e. "spiral-like" (type I) and "spray-like" (type II) endings. Similarly to nerve fibres, both types of corpuscular-like endings were found mainly near the tibial and femoral attachment sites (15 of 17), whereas only two were found in the middle third of the ligament. Most likely, the type I and type II corpuscular-like endings serve a mechanoreceptive function involved in the sensory control of normal movements and in stress protection.

[Frontal and temporal onset of brain atrophy. Clinical and instrumental findings]

We report the cases of a 70-year old man with left temporal brain atrophy and of a 39-year-old man with neuropathologically verified frontal lobe degeneration (FLD) of Non-Alzheimer type. 10 patients with FLD collected during a prospective study on degenerative dementia had more severe volumetric brain changes and less severe quantitative band power changes than a group of matched patients with clinically diagnosed Alzheimer's disease.

Intracellular distribution of kinesin in chromaffin cells

In this paper we examined the association of the microtubule motor protein kinesin with organelles in chromaffin cells. Approximately 15% of kinesin was associated with membranes as determined by differential and equilibrium centrifugation on sucrose gradients. Kinesin was not enriched in a particular organelle fraction but cofractionated with a variety of organelle markers including markers for early and late endosomes, smooth and rough endoplasmic reticulum (ER) and the Golgi apparatus. Surprisingly, low amounts of kinesin were present in fractions of purified chromaffin granules. The absence of kinesin from the bulk of chromaffin granules was also indicated by immunostaining of tissue sections. A polyclonal antibody that specifically recognized the 120 kDa kinesin heavy chain labeled predominantly a perinuclear region that is typical for most of the kinesin-binding organelles identified by cell fractionation (endosomes, Golgi, ER). Since these organelles are compartments with high membrane turnover, we speculate that kinesin might be involved in certain aspects of trafficking of these membrane systems.

Colocalization of retinal dystrophin and actin in postsynaptic dendrites of rod and cone photoreceptor synapses

In this paper we demonstrate immunostaining specific for dystrophin in photoreceptor synapses of human, bovine and rat retinas. Cryosections of retinas incubated with dystrophin-specific monoclonal antibodies displayed a punctuate staining pattern in the outer plexiform layer. This pattern resulted from binding of the antibodies to synaptic complexes of both rods and cones, shown by double-labelling with antibodies to either synaptophysin or actin. Confocal laser fluorescence microscopy demonstrated that dystrophin staining colocalized predominantly with actin, which is concentrated in the postsynaptic portions of the synaptic complex. No significant dystrophin immunolabel was seen in the presynaptic terminals labelled with antibodies to synaptophysin, a marker of synaptic vesicles. Immunoblot analysis confirmed the presence of approximately 420 kDa and approximately 360 kDa dystrophin-like polypeptide bands associated with membranes of the bovine retina. We speculate that retinal dystrophin is involved in the linkage of actin filaments to the postsynaptic plasma membrane. Such a linkage may be important for the generation of synaptic microdomains and for certain phenomena of synaptic plasticity. The absence of dystrophin in patients suffering from Duchenne's muscular dystrophy is accompanied by visual problems and abnormalities of the electroretinogram. Therefore it is likely that retinal dystrophin plays a role in certain stages of synaptic transmission between photoreceptors and the postsynaptic dendritic complex formed by horizontal and bipolar cells.

Distribution of actin in cone photoreceptor synapses

The invaginated photoreceptor terminal harbours a great number of postsynaptic dendrites that contact the photoreceptor terminal at morphologically distinct synapses. In this paper we studied the subcellular distribution of actin in cone photoreceptor synapses of light-adapted goldfish and guppy retinas and its relation to ribbon and spinule synapses. In the outer plexiform layer, fluorescent staining for actin by antibodies and also by fluorescently labelled phalloidin was discontinuous and showed large dot-like immunoreactive structures (3-5 microns in diameter). Immunogold electron microscopy of light-adapted retinas isolated 3 h after the onset of light demonstrated that these dot-like structures corresponded to dendritic profiles of bipolar and horizontal cells within the synaptic cavity of cones. A much less intense staining was observed within the presynaptic cone photoreceptor terminal itself. Immunogold particles were also found overlying the tips of horizontal and bipolar dendrites that form the triadic postsynaptic complex of the ribbon synapse. In contrast to the triads light-induced terminal outgrowings of horizontal cells, also known as spinules, remained largely unlabelled. These observations suggest that actin plays a role in maintenance and/or certain steps of the formation of the postsynaptic dendritic complex.

Li(+)-induced structural changes of synaptic ribbons are related to the phosphoinositide metabolism in photoreceptor synapses

Synaptic ribbons are specialized cytoskeletal components of the presynaptic exocytotic machinery in photoreceptors. In cone photoreceptors, these structures are highly dynamic disappearing during darkness and reforming in the light phase. In this study we wanted to characterize the second messengers involved in the regulation of these cyclic changes. We show that synaptic ribbons in cone photoreceptors are very sensitive to both Li+ and inositol 1,4,5-trisphosphate suggesting that inositol polyphosphates might play a physiological role in the disassembly of synaptic ribbons. The presence of the phosphoinositide pathway was demonstrated in isolated photoreceptor synapses. The phosphoinositide metabolism in photoreceptor synapses was dark-activated and correlated with the disappearance of synaptic ribbons during dark-adaptation. Since Li+ and inositol 1,4,5-trisphosphate only influenced synaptic ribbons in cones but not in rods the dark-activated phosphoinositide metabolism should be largely ascribed to cone synapses.

Influence of Ca2+ on synaptic morphology of fish cone photoreceptors

Isolated fish retinas were incubated with the Ca(2+)-ionophore A23187 at various Ca2+ concentrations. Computer reconstructions of the ultrastructure of cone synapses revealed pronounced reduction of the presynaptic surface (complete loss of the synaptic invaginations) in response to low intracellular Ca2+ (less than 10(-8) M). No obvious effect on synaptic morphology was noticed at Ca2+ greater than 10(-8) M. Ca(2+)-dependent exocytotic activity appears to be a major factor controlling the invaginated morphology of cone synapses.

[Waldheim--the difficult task of investigation]

This is a report on a mental hospital--or, rather, a department of a mental hospital--called "Waldheim", (literal meaning: "hostal, or home in the woods") which is a section of the mental hospital at Hochweitzschen in Saxony in the former German Democratic Republic (GDR). Public opinion had been aroused some time ago by an article published in the German weekly "Stern" in which it was stated that psychotic patients were being subjected to leukotomy or stereotactic surgery, and that castration by x-ray treatment was being performed in mentally handicapped women, in that hospital. The article prompted the then Ministery of Health of the GDR to constitute an "expert committee" to investigate into the matter. The magazine's allegations were partly confirmed by the committee. The GDR parliament then constituted a "special parliamentary commission" for another inquiry. Again several allegations were confirmed. It was stated that the events and actions in that hospital had been unsatisfactorily assessed and digested. The authors undertake to make the procedures followed by the medical and nursing staff, accessible to a wider public and to render them more lucid. Important structural characteristics of the patterns that developed there, were the governmentally prescribed segregation of severely diseased persons to "treat" them far away from their residential community, the absence of a critical general public, and the amalgamation of supervisory institutions with those that require to be supervised.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo metabolic action of insulin-like growth factor I in adult rats

The acute metabolic actions of insulin-like growth factor I were studied in anaesthetized adult rats and its potency was compared to that of insulin. Following an i.v. bolus injection of insulin-like growth factor I a dose-dependent decrease of blood glucose and serum non-esterified fatty acid concentrations was noted with a potency of about 2% that of insulin. Stimulation of total body glucose disposal during euglycaemic clamping required approximately 50 times higher insulin-like growth factor I serum concentrations to achieve an identical half-maximal response. A similar difference in potency was observed for the stimulatory action on 2-deoxyglucose uptake and on glycogen formation in skeletal muscle. Lipogenesis in epididymal fat pads was increased dose-dependently by both hormones requiring approximately 30 times higher half-maximally effective serum concentrations of insulin-like growth factor I. These data demonstrate that insulin-like growth factor I exerted acute insulin-like metabolic actions in vivo with low potency. These effects were probably mediated via insulin receptors. A preferential stimulation of glucose metabolism in skeletal muscle was not observed.

Metabolic actions of insulin-like growth factor-I in cultured hepatocytes from adult rats

Short-term and long-term regulation of hepatic carbohydrate metabolism by insulinlike growth factor-I was studied in primary cultures of adult rat hepatocytes and compared with the metabolic potency of insulin. Insulinlike growth factor-I stimulated the formation of [14C]lactate from [14C]glucose up to three-fold with a half-maximally effective concentration of approximately 50 nmol/L. Basal glycogenolysis was inhibited by about 20%, and glucagon-activated glycogenolysis was blocked completely by insulinlike growth factor-I with half-maximally effective concentrations of about 1.5 to 2 nmol/L. The activity of the key glycolytic enzymes glucokinase and pyruvate kinase were induced twofold. The glucagon-dependent induction of phosphoenolpyruvate carboxykinase--the key gluconeogenic enzyme--was antagonized with a half-maximally effective concentration of about 5 nmol/L. This inhibition of the glucagon-dependent induction of the enzyme was accompanied by a similar reduction of the increase in phosphoenolpyruvate carboxykinase-mRNA level as assessed by Northern blot analysis. The potency of insulinlike growth factor-I at half-maximally effective concentrations was approximately 2% to 4% that of insulin. Because binding studies demonstrated a comparably low affinity of insulinlike growth factor-I to the insulin receptor, it is suggested that in adult liver--in contrast to fetal and regenerating liver--insulinlike growth factor-I could exert short-term and long-term metabolic effects on parenchymal cells only through interaction with the insulin receptor.

Diffusion of Pu in U02

The diffusion of 239Pu in sintered pellets of UO2 (density 10.6) has been measured with the methods of α radiation absorption and sectioning. The temperature dependence of the diffusion coefficient is represented by the equation: D = 0.34 exp (-97300 ± 4100/RT) cm2 sec-1.

Comparison has been made with the self-diffusion of Uranium by a new technique of differential diffusion measurements. It results that the activation energy for Plutonium diffusion is 6.0 kcals less than that of Uranium diffusion.