Isolation and structural studies on synaptic complexes from rat brain.

A fraction enriched in synaptic complexes has been isolated from rat brain. The major structural elements of synaptic complexes after isolation are a sector of pre- and postsynaptic plasma membranes joined together by a synaptic cleft and a postsynaptic density (PSD) located on the inner surface of the postsynaptic membrane. On its outer surface, the postsynaptic membrane has a series of projections which extend about halfway into the cleft and which occur along the entire length of the PSD. Proteolytic enzymes at high concentrations remove the PSD and open the synaptic cleft; at low concentrations the PSD is selectively destroyed. By contrast, the structural integrity of the PSD is resistant to treatment with NaCl, EGTA, and low concentrations of urea. Pre- and postsynaptic membranes also remain joined by the synaptic cleft after NaCl, EGTA, or mild urea treatment. High concentrations of urea cause the partial dissociation of the PSD. We conclude that polypeptides are probably one of the major components of the PSD and that the structural integrity of the PSD depends on polypeptides because disruption of the covalent or hydrophobic bonding of these polypeptides leads to a progressive loss of PSD structure.

Proteins of the postsynaptic density.

An analysis was made of the protein composition of a fraction of postsynaptic densities (PSDs) prepared from rat brain. Protein makes up 90% of the material in the PSD fraction. Two major polypeptide fractions are present, based on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The major polypeptide fraction has a molecular weight of 53,000, makes up about 45% of the PSD protein, and comigrates on gels with a major polypeptide of the synaptic plasma membrane. The other polypeptide band has a molecular weight of 97,000, accounts for 17% of the PSD protein, and is not a prominent constituent of other fractions. Six other polypeptides of higher molecular weight (100,000-180,000) are consistently present in small amounts (3-9% each). The PSD fraction contains slightly greater amounts of polar amino acids and proline than the synaptic plasma membrane fraction, but no amino acid is usually prominent. The PSD apparently consists of a structural matrix formed primarily by a single polypeptide or class of polypeptides of 53,000 molecular weight. Small amounts of other specialized proteins are contained within this matrix.

Synaptic proteins. Characterization of tubulin and actin and identification of a distinct postsynaptic density polypeptide.

The major proteins in isolated synaptic junctions (SJs) and postsynaptic densities (PSDs) have been compared to actin, tubulin, and the major neurofilament (NF) protein by two-dimensional gel electrophoresis and tryptic peptide map analysis. These studies show: (a) tubulin is present in SJ and PSD fractions and is identical to cytoplasmic tubulin, (b) actin in these fractions is very similar to the gamma- and beta-actin found predominantly in nonmuscle cells, and (c) the major PSD protein is distinct from all other known fibrous proteins.

Distribution and mobility of lectin receptors on synaptic membranes of identified neurons in the central nervous system.

The distribution and mobility of concanavalin A (Con A) and Ricinus communis agglutinin (RCA) receptors (binding sites) on the external surfaces of Purkinje, hippocampal pyramidal, and granule cells and their attached boutons were studied using ferritin-lectin conjugates. Dendritic fields of these cells were isolated by microdissection and gently homogenized. Cell fragments and pre- and postsynaptic membranes were labeled with the ferritin-lectin conjugates at a variety of temperatures, and the distribution of lectin receptors was determined by electron microscopy. Both classes of these lectin receptors were concentrated at nearly all open and partially open postsynaptic junctional membranes of asymmetric-type synapses on all three neuron types. Con A receptors were most concentrated at the junctional membrane region, indicating that the mature neuron has a specialized nonrandom organization of carbohydrates on its outer surface. Lectin receptors located on postsynaptic junctional membranes appeared to be restricted in their mobility compared to similar classes of receptors on extrajunctional membrane regions. Labeling with ferritin-RCA and -Con A at 37 degrees C produced clustering of lectin receptors on nonjunctional surfaces; however, Con A and RCA receptors retained their nonrandom topographic distribution on the postsynaptic junctional surface. The restricted mobility of lectin receptors was an inherent property of the postsynaptic membrane since the presynaptic membrane was absent. It is proposed that structures in the postsynaptic density may be transmembrane-linked to postsynaptic receptors and thereby determine topographic distribution and limit diffusion of specialized synaptic molecules. Speicalized receptor displays may play an important role in the formation and maintenance of specific synaptic contacts.

Isolation of postsynaptic densities from rat brain.

Most synapses in the central nervous system exhibit a prominent electron-opaque specialization of the postsynaptic plasma membrane called the postsynaptic density (PSD). We have developed a procedure for the isolation of PSDs which is based on their buoyant density and their insolubility in N-lauroyl sarcosinate. Treatment of synaptic membranes with this detergent solubilizes most plasma membranes and detaches PSDs from the plasma membrane so that they can be purified on a density gradient. Isolated PSDs appear structurally intact and exhibit those properties which characterize them in tissue. The isolated PSDs are of the size, shape, and electron opacity of those seen in tissue; they stain with both ethanolic phosphotungstic acid and bismuth iodide-uranyl lead and the fraction contains cyclic 3',5'-phosphodiesterase activity. Quantitative electron microscope analysis of the PSD fraction gives an estimated purity of better than 85%. Inasmuch as the PSD is associated primarily with dendritic excitatory synapses, our PSD fraction represents the distinctive plasma membrane specialization of this specific synaptic type in isolation.

Postsynaptic density antigens: preparation and characterization of an antiserum against postsynaptic densities.

Long-term immunization of rabbits with postsynaptic densities (PSD) from bovine brain produced an antiserum specific for PSD as judged by binding to subcellular fractions and immunohistochemical location at the light and electron microscope levels. (a) The major antigens of bovine PSD preparations were three polypeptides of molecular weight 95,000 (PSD-95), 82,000 (PSD-82), and 72,000 (PSD-72), respectively. Antigen PSD-95, also present in mouse and rat PSDs was virtually absent from cytoplasm, myelin, mitochondria, and microsomes from rodent or bovine brain. Antigens PSD-82 and PSD-72 were present in all subcellular fractions from bovine brain, especially in mitochondria, but were almost absent from rodent brain. The antiserum also contained low-affinity antibodies against tubulin. (b)Immunohistochemical studies were performed in mouse and rat brain, where antigen PSD-95 accounted for 90 percent of the antiserum binding after adsorption with purified brain tubulin. At the light microscope level, antibody binding was observed only in those regions of the brain where synapses are known to be present. No reaction was observed in myelinated tracts, in the neuronal cytoplasm, or in nonneuronal cells. Strong reactivity was observed in the molecular layer of the dentate gyrus, stratum oriens and stratum radiatum of the hippocampus, and the molecular layer of the cerebellum. Experimental lesions, such as ablation of the rat entorhinal cortex or intraventricular injection of kainic acid, which led to a major loss of PSD in well- defined areas of the hippocampal formation, caused a correlative decrease in immunoreactivity in these areas. Abnormal patterns of immunohistochemical staining correlated with abnormal synaptic patterns in the cerebella of reeler and staggerer mouse mutants. (c) At the electron microscopic level, immunoreactivity was detectable only in PSD. The antibody did not bind to myelin, mitochondria or plasma membranes. (d) The results indicate that antigen PSD-95 is located predominantly or exclusively in PSD and can be used as a marker during subcellular fractionation. Other potential uses include the study of synaptogenesis, and the detection of changes in synapse number after experimental perturbations of the nervous system.

Synaptic cleft glycoproteins contain homologous amino acid sequences.

The concanavalin A--binding glycoproteins of the rat synaptic junction were isolated by affinity chromatography. These glycoproteins had molecular weights of 160,000, 123,000, 110,000, and 95,000. The tryptic peptide maps of these glycoproteins showed that the three largest glycoproteins contained a high percentage of identical peptides. This indicates that the amino acid sequences of these glycoproteins have a high degree of homology. The 95,000-dalton glycoprotein was unrelated to the other three. These findings suggest that homologous glycoproteins may participate in synapse formation or maintenance, or both.

Cell biology of synaptic plasticity.

The nervous system of mammals retains throughout the animals' life-span the ability to modify the number, nature, and level of activity of its synapses. Synaptic plasticity is most evident after injury to the nervous system, and the cellular and molecular mechanisms that make it possible are beginning to be understood. Transplantation of brain tissue provides a powerful approach for studying mechanisms of synaptic plasticity. In turn, understanding the response of the central nervous system to injury can be used to optimize transplant survival and integration with the host brain.

Amyloid beta protein enhances the survival of hippocampal neurons in vitro.

The beta-amyloid protein is progressively deposited in Alzheimer's disease as vascular amyloid and as the amyloid cores of neuritic plaques. Contrary to its metabolically inert appearance, this peptide may have biological activity. To evaluate this possibility, a peptide ligand homologous to the first 28 residues of the beta-amyloid protein (beta 1-28) was tested in cultures of hippocampal pyramidal neurons for neurotrophic or neurotoxic effects. The beta 1-28 appeared to have neurotrophic activity because it enhanced neuronal survival under the culture conditions examined. This finding may help elucidate the sequence of events leading to plaque formation and neuronal damage in Alzheimer's disease.

Decrease in adrenergic axon sprouting in the senescent rat.

When the septal area in young adult rats is denervated by a lesion of the fimbria-fornix, adrenergic fibers proliferate within the denervated area. The same operation performed on aged animals gives rise to a qualitatively similar but quantitatively less pronounced response. This reduction in reactive growth may reflect a decreased capacity of the aged brain to remodel its circuitry and restore lost function.

Stimulus-coupled secretion of gamma-aminobutyric acid from rat brain synaptosomes.

Synaptosomes treated with radioactive gamma-aminobutyric acid can be stimulated to release this substance. The release is maximal within 40 seconds after stimulation and is dependent on calcium. Magnesium and manganese ions, known to block stimulus-secretion coupling processes, depress calcium-dependent release. This release is specific to synaptosomes because microsomal or myelin fractions do not release accumulated gamma-aminobutyric acid. The data illustrate a simple in vitro system suitable for analysis of secretion of gamma-aminobutyric acid in brain and in addition describe several new aspects of uptake and secretion of this compound at brain nerve endings.

Changes in protein levels in perfusates of freely moving cats: relation to behavioral state.

Perfusates from the brains of freely moving cats, obtained by means of a push-pull cannula, contain high concentrations of proteins. The levels vary in a cyclic fashion and are higher during rapid eye movement sleep than during the waking state. The proteins represent a distinctive class of tissue protein and their changing levels appear to reflect an alteration in the protein content of the extracellular space of brain related to behavioral state.

Electroshock effects on brain protein synthesis: relation to brain seizures and retrograde amnesia.

The effects of electroshock on brain seizure activity and brain protein synthesis were studied in male mice. A significant but short-lasting inhibition of brain protein synthesis and an increase in the amount of free leucine were produced by electroshock at intensities above the brain seizure threshold. Electroshock at intensities below the brain seizure threshold did not affect brain protein synthesis.

Plasticity of hippocampal circuitry in Alzheimer's disease.

Two markers of neuronal plasticity were used to compare the response of the human central nervous system to neuronal loss resulting from Alzheimer's disease with the response of rats to a similar neuronal loss induced by lesions. In rats that had received lesions of the entorhinal cortex, axon sprouting of commissural and associational fibers into the denervated molecular layer of the dentate gyrus was paralleled by a spread in the distribution of tritiated kainic acid-binding sites. A similar expansion of kainic acid receptor distribution was observed in hippocampal samples obtained postmortem from patients with Alzheimer's disease. An enhancement of acetylcholinesterase activity in the dentate gyrus molecular layer, indicative of septal afferent sprouting, was also observed in those patients with a minimal loss of cholinergic neurons. These results are evidence that the central nervous system is capable of a plastic response in Alzheimer's disease. Adaptive growth responses occur along with the degenerative events.

Brain injury causes a time-dependent increase in neuronotrophic activity at the lesion site.

A cavity was made in the brain (entorhinal cortex) of developing or adult rats, and a small piece of Gelfoam was emplaced to collect fluid secreted into the wound. The neuronotrophic activity of the fluid was assayed with sympathetic and parasympathetic neurons in culture. The results show that wounds in the brain of developing or adult rats stimulate the accumulation of neuronotrophic factors and that the activity of these factors increases over the first few days after infliction of the damage.

Short-term exercise in aged Tg2576 mice alters neuroinflammation and improves cognition.

Exercise is a treatment paradigm that can ameliorate cognitive dysfunction in Alzheimer disease (AD) and AD mouse models. Since exercise is also known to alter the peripheral immune response, one potential mechanism for the cognitive improvement following exercise may be by modulating the inflammatory repertoire in the central nervous system. We investigated the effects of voluntary exercise in the Tg2576 mouse model of AD at a time-point at which pathology has already developed. Inflammatory mRNA markers are increased in sedentary Tg2576 mice versus non-transgenic controls. We demonstrate that short-term voluntary wheel running improved spatial learning in aged transgenic mice as compared to sedentary Tg2576 controls. Inflammatory profiles of the Tg2576 and non-transgenic mice were different following exercise with the non-transgenic mice showing a broader response as compared to the Tg2576. Notably, exercising Tg2576 exhibited increases in a few markers including CXCL1 and CXCL12, two chemokines that may affect cognition.

Melatonin treatment in old mice enables a more youthful response to LPS in the brain.

Melatonin modulates the expression of a number of genes related to inflammation and immunity. Declining levels of melatonin with age may thus relate to some of the changes in immune function that occur with age. mRNA expression levels in murine CNS were measured using oligonucleotide microarrays in order to determine whether a dietary melatonin supplement may modify age-related changes in the response to an inflammatory challenge. CB6F1 male mice were fed 40-ppm melatonin for 9 weeks prior to sacrifice at 26.5 months of age, and compared with age-matched untreated controls and 4.5-month-old controls. A subset of both young and old animals was injected i.p. with lipopolysaccharide (LPS). After 3 h, total RNA was extracted from whole brain (excluding brain stem and cerebellum), and individual samples were hybridized to Affymetrix Mouse 430-2.0 arrays. Data were analyzed in Dchip and GeneSpring. Melatonin treatment markedly altered the response in gene expression of older animals subjected to an LPS challenge. These changes in general, caused the response to more closely resemble that of young animals subjected to the same LPS challenge. Thus melatonin treatment effects a major shift in the response of the CNS to an inflammatory challenge, causing a transition to a more youthful mRNA expression profile.

Wheel running and fluoxetine antidepressant treatment have differential effects in the hippocampus and the spinal cord.

Exercise and antidepressants used independently have been shown to increase hippocampal brain-derived neurotrophic factor (BDNF) and neurogenesis. Despite the fact that patients with depression are often prescribed both, the effects of the exercise and fluoxetine antidepressant treatment used in combination are unknown. Using C57Bl/10 female mice, BDNF protein, insulin-like growth factor 1 (IGF-1) protein and neurogenesis were measured in the hippocampus after 21 days of wheel running, 21 days of fluoxetine antidepressant therapy (daily i.p. injections of 5 mg/kg, 10 mg/kg or 25 mg/kg) and the combination of the two. BDNF protein and cytogenesis/neurogenesis increased in the hippocampus with fluoxetine (high dose), but not wheel running. Hippocampal IGF-1 protein did not change with either treatment. There were no synergistic effects of combining exercise and fluoxetine treatment. Recent reports have also shown that exercise induces molecular mechanisms that benefit the spinal cord and can improve recovery after spinal cord injury (SCI); therefore, we repeated the assays in the spinal cord. Results showed that BDNF, IGF-1 and neurogenesis behave independently in the hippocampus and spinal cord. BDNF protein did not change in the spinal cord with either wheel running or fluoxetine treatment. Spinal cord IGF-1 protein did not change with wheel running, but it decreased with fluoxetine (high dose). Furthermore, spinal cord cytogenesis decreased with fluoxetine treatment. The combined wheel running and fluoxetine groups did not show synergistic results. Thus, the hippocampus and the spinal cord respond in distinct ways to wheel running and fluoxetine, and a prior induction of BDNF, IGF-1 or cytogenesis is unlikely to be the mechanism for wheel running providing a margin of protection against SCI.

Mechanisms of trafficking in axons and dendrites: implications for development and neurodegeneration.

In the area of routing and sorting of dendritic traffic, the current phenomenological data beg questions about the cellular mechanisms utilized not only to transport material but also to modulate activity in a process, even apoptosis. To aid in formulating testable hypotheses, many plausible models are developed here and linked with some of the preliminary data that supports them. We first assume that in long dendrites the sorting of membranous proteins into transport vesicles also involves the linkage of motor proteins to the vesicles. Second, we assume that the cytoskeleton in dendrites is altered from the cytoskeleton in axons and the cell body. Viral glycoproteins, MAP2 and specific mRNA sorting into dendrites provide the simplest models for analyzing vesicular, cytoskeletal and RNA sorting. In the case of viral glycoproteins, initial sorting appears to occur at the Golgi but additional routing steps involve further complexities that could best be served by an additional sorting step at the junction of the cell body and the process. Transport of the specialized cytoskeletal proteins and specific mRNAs as well as vesicular material could be controlled by a similar gatekeeper at the mouth of a process. Studies of the microtubule-organelle motor complex, regulation of microtubule-based motility by microtubule-associated proteins, and slow axonal transport all provide insights into important aspects of the routing and sorting. These processes are in turn controlled by extracellular signals such as those generated by matrix molecules or their hydrolysis products in the case of amyloid precursor protein (APP). Routing and sorting mechanisms may be central to the development of Alzheimer's disease in view of evidence that APP processing is affected, transport is disturbed, and intracellular vesicles (early endosomes) hypertrophied. Further it is possible that routing mechanisms play a role in cell-cell interactions as, for example, the possibility that pathogenic/cellular stress signals may be passed along circuits transsynaptically.

Cell adhesion molecules in neural plasticity and pathology: similar mechanisms, distinct organizations?

Brain plasticity and the mechanisms controlling plasticity are central to learning and memory as well as the recovery of function after brain injury. While it is clear that neurotrophic factors are one of the molecular classes that continue to regulate brain plasticity in the adult central nervous system (CNS), less appreciated but equally profound is the role of cell adhesion molecules (CAMs) in plasticity mechanisms such as long term potentiation, preservation of neurons and regeneration. Ironically, however, CAMs can also reorganize the extra-cellular space and cause disturbances that drive the development of brain pathology in conditions such as Alzheimer's disease and multiple sclerosis. Candidate molecules include the amyloid precursor protein which shares many properties of a classical CAM and beta-amyloid which can masquerade as a pseudo CAM. Beta-Amyloid serves as a nidus for the formation of senile plaques in Alzheimer's disease and like CAMs provides an environment for organizing neurotrophic factors and other CAMs. Inflammatory responses evolve in this environment and can initiate a vicious cycle of perpetuated neuronal damage that is medicated by microglia, complement and other factors. Certain CAMs may converge on common signal transduction pathways involving focal adhesion kinases. Thus a breakdown in the organization of key CAMs and activation of their signal transduction mechanisms may serve as a new principle for the generation of brain pathology.