Distribution of Cell Adhesion Molecules (CAM) along the Branching Neurite Surface: Model-Inherited Effects of the Branch Diameters and Mode of CAM Transport

In many cases, an increase in the surface density of cell adhesion molecules (CAM) in the distal parts of a growing neurite is favorable for the neurite elongation. This increase is attained by exocytotic insertion of CAM-containing vesicles into the growth cones with subsequent redistribution of CAM along the cell surface due to lateral diffusion and endocytosis. Using a mathematical model describing these processes, we quantitatively describe conditions providing two qualitatively different profiles in a branching neurite: (i) the CAM surface density increases along both daughter branches, which would be in favor of further outgrowth of both branches, i.e., successful branching, or (ii) the CAM surface density increases along one daughter branch and decreases along another branch, which could lead to the retraction of the latter. The geometric factors and mechanisms underlying the intracellular CAM transport to the daughter growth cones were proved to determine the profile of CAM surface density. A similarity in the diameters of daughter branches, their short lengths, a high value of the lateral transfer constant, and partitioning of CAM transport at the branching point proportionally to the surface areas of daughter branches are in favor of an increase in the CAM surface density along both daughter branches. Asymmetric branching can lead to a decrease in the CAM surface density along the thinner or thicker daughter branch, if CAM trafficking was equally partitioned or was proportional to the branch cross-sectional areas, respectively. The proposed model helps to understand possible relationships between the intracellular CAM trafficking, CAM surface distribution, and geometry of branching of the neurites.     

Morphogenic Signaling in Neurons Via Neurotransmitter Receptors and Small GTPases

Several neurotransmitters including serotonin and glutamate have been shown to be involved in many aspects of neural development, such as neurite outgrowth, regulation of neuronal morphology, growth cone motility and dendritic spine shape and density, in addition to their well-established role in neuronal communication. This review focuses on recent advances in our understanding of the molecular mechanisms underlying neurotransmitter-induced changes in neuronal morphology. In the first part of the review, we introduce the roles of small GTPases of the Rho family in morphogenic signaling in neurons and discuss signaling pathways, which may link serotonin, operating as a soluble guidance factor, and the Rho GTPase machinery, controlling neuronal morphology and motility. In the second part of the review, we focus on glutamate-induced neuroplasticity and discuss the evidence on involvement of Rho and Ras GTPases in functional and structural synaptic plasticity triggered by the activation of glutamate receptors.     

Activity-dependent formation and functions of chondroitin sulfate-rich extracellular matrix of perineuronal nets

Extracellular matrix molecules--including chondroitin sulfate proteoglycans, hyaluronan, and tenascin-R--are enriched in perineuronal nets (PNs) associated with subsets of neurons in the brain and spinal cord. In the present study, we show that similar cell type-dependent extracellular matrix aggregates are formed in dissociated cell cultures prepared from early postnatal mouse hippocampus. Starting from the 5th day in culture, accumulations of lattice-like extracellular structures labeled with Wisteria floribunda agglutinin were detected at the cell surface of parvalbumin-expressing interneurons, which developed after 2-3 weeks into conspicuous PNs localized around synaptic contacts at somata and proximal dendrites, as well as around axon initial segments. Physiological recording and intracellular labeling of PN-expressing neurons revealed that these are large fast-spiking interneurons with morphological characteristics of basket cells. To study mechanisms of activity-dependent formation of PNs, we performed pharmacological analysis and found that blockade of action potentials, transmitter release, Ca2+ permeable AMPA subtype of glutamate receptors or L-type Ca2+ voltage-gated channels strongly decreased the extracellular accumulation of PN components in cultured neurons. Thus, we suggest that Ca2+ influx via AMPA receptors and L-type channels is necessary for activity-dependent formation of PNs. To study functions of chondroitin sulfate-rich PNs, we treated cultures with chondroitinase ABC that resulted in a prominent reduction of several major PN components. Removal of PNs did not affect the number and distribution of perisomatic GABAergic contacts but increased the excitability of interneurons in cultures, implicating the extracellular matrix of PNs in regulation of interneuronal activity.     

Molecular signals of plasticity at the tetrapartite synapse

The emergence of astroglia as an important participant of the synaptic machinery has led to the 'tripartite synapse' hypothesis. Recent findings suggest that synaptic signaling also involves the surrounding extracellular matrix (ECM). The ECM can incorporate and store molecular traces of both neuronal and glial activities. It can also modulate function of local receptors or ion channels and send diffuse molecular signals using products of its use-dependent proteolytic cleavage. Recent experimental findings implicate the ECM in mechanisms of synaptic plasticity and glial remodeling, thus lending support to the 'tetrapartite synapse' concept. This inclusive view might help to understand better the mechanisms underlying signal integration and novel forms of long-term homeostatic regulation in the brain.     

Neural cell adhesion molecule-associated polysialic acid potentiates alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor currents

The highly negatively charged polysialic acid (PSA) is a carbohydrate predominantly carried by the neural cell adhesion molecule (NCAM) in mammals. NCAM and, in particular, PSA play important roles in cellular and synaptic plasticity. Here we investigated whether PSA modulates the activity of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors (AMPA-Rs). Single channel recordings of affinity-purified AMPA-Rs reconstituted in lipid bilayers revealed that bacterially derived PSA, called colominic acid, prolonged the open channel time of AMPA-R-mediated currents by severalfold and altered the bursting pattern of the receptor channels but did not modify AMPA-R single channel conductance. This effect was reversible, concentration-dependent, and specific, since monomers of sialic acid and another negatively charged carbohydrate, chondroitin sulfate, did not potentiate single channel AMPA-R currents. Recombinant PSA-NCAM also potentiated currents mediated by reconstituted AMPA-Rs. In pyramidal neurons acutely isolated from the CA1 region of the early postnatal hippocampus, l-glutamate or AMPA (applied in the presence of antagonists blocking voltage-gated Na(+) and K(+) currents and N-methyl-d-aspartate and metabotropic glutamate receptors) induced inward currents, which were significantly increased by co-application of colominic acid. Chondroitin sulfate did not affect AMPA-R-mediated currents in CA1 neurons. The effect of colominic acid was age-dependent, since in pyramidal neurons from adult hippocampus, colominic acid failed to potentiate glutamate responses. Thus, our study demonstrates age-dependent potentiation of AMPA receptors by PSA via a mechanism probably involving direct PSA-AMPA-R interactions. This mechanism might amplify AMPA-R-mediated signaling in immature cells, thereby affecting their development.     

Functional heterogeneity at the frog sensory-motor synapse

This article compares four models of amplitude fluctuations in postsynaptic potentials. The convolution of two binomial distributions and the beta model proved the best fit with experimentally obtained data (as compared with the binomial model). The beta model is based on the assumption that the probability of quantal transmitter release is a random variable with a beta distribution. Numbers of quantal generators as estimated by the beta model were found to resemble numbers of morphological identifiable synaptic boutons. Findings from research using this model showed that the binomial parameter n may be interpreted as the number of transmitter release sites functioning with a probability in excess of 0.2. The findings obtained confirm the postulated functional diversity of release sites at interneuronal synapses.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 21, No. 6, pp. 780–788, November–December, 1989.     

PSA-NCAM: Synaptic functions mediated by its interactions with proteoglycans and glutamate receptors

Dynamic regulation of glycosylation of the neural cell adhesion molecule (NCAM) by an unusual large negatively charged polysialic acid (PSA) is the major prerequisite for correct formation of brain circuitries during development and for normal synaptic plasticity, learning and memory in the adult. Traditionally, PSA is viewed as a de-adhesive highly hydrated molecule, which interferes with cell adhesion and promotes cellular/synaptic dynamics by steric hindrance. Analysis of synaptic functions of PSA-NCAM highlighted additional features of this molecule. First, PSA promotes interaction of NCAM with heparan sulfate proteoglycans and thus stimulates synaptogenesis. Second, PSA-NCAM modulates glutamate receptors: it restrains activity of extrasynaptic GluN2B-containing NMDA receptors and facilitates activity of a subset of AMPA receptors. Perturbation in polysialylation and/or NCAM expression in mouse models recapitulates many symptoms of human brain disorders such as schizophrenia, depression, anxiety and Alzheimer's disease.     

A homeostatic model of neuronal firing governed by feedback signals from the extracellular matrix

Molecules of the extracellular matrix (ECM) can modulate the efficacy of synaptic transmission and neuronal excitability. These mechanisms are crucial for the homeostatic regulation of neuronal firing over extended timescales. In this study, we introduce a simple mathematical model of neuronal spiking balanced by the influence of the ECM. We consider a neuron receiving random synaptic input in the form of Poisson spike trains and the ECM, which is modeled by a phenomenological variable involved in two feedback mechanisms. One feedback mechanism scales the values of the input synaptic conductance to compensate for changes in firing rate. The second feedback accounts for slow fluctuations of the excitation threshold and depends on the ECM concentration. We show that the ECM-mediated feedback acts as a robust mechanism to provide a homeostatic adjustment of the average firing rate. Interestingly, the activation of feedback mechanisms may lead to a bistability in which two different stable levels of average firing rates can coexist in a spiking network. We discuss the mechanisms of the bistability and how they may be related to memory function.     

Importance of Shank3 protein in regulating metabotropic glutamate receptor 5 (mGluR5) expression and signaling at synapses

Shank3/PROSAP2 gene mutations are associated with cognitive impairment ranging from mental retardation to autism. Shank3 is a large scaffold postsynaptic density protein implicated in dendritic spines and synapse formation; however, its specific functions have not been clearly demonstrated. We have used RNAi to knockdown Shank3 expression in neuronal cultures and showed that this treatment specifically reduced the synaptic expression of the metabotropic glutamate receptor 5 (mGluR5), but did not affect the expression of other major synaptic proteins. The functional consequence of Shank3 RNAi knockdown was impaired signaling via mGluR5, as shown by reduction in ERK1/2 and CREB phosphorylation induced by stimulation with (S)-3,5-dihydroxyphenylglycine (DHPG) as the agonist of mGluR5 receptors, impaired mGluR5-dependent synaptic plasticity (DHPG-induced long-term depression), and impaired mGluR5-dependent modulation of neural network activity. We also found morphological abnormalities in the structure of synapses (spine number, width, and length) and impaired glutamatergic synaptic transmission, as shown by reduction in the frequency of miniature excitatory postsynaptic currents (mEPSC). Notably, pharmacological augmentation of mGluR5 activity using 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)-benzamide as the positive allosteric modulator of these receptors restored mGluR5-dependent signaling (DHPG-induced phosphorylation of ERK1/2) and normalized the frequency of mEPSCs in Shank3-knocked down neurons. These data demonstrate that a deficit in mGluR5-mediated intracellular signaling in Shank3 knockdown neurons can be compensated by 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)-benzamide; this raises the possibility that pharmacological augmentation of mGluR5 activity represents a possible new therapeutic approach for patients with Shank3 mutations.     

Neural cell adhesion molecule promotes accumulation of TGN organelles at sites of neuron-to-neuron contacts

Transformation of a contact between axon and dendrite into a synapse is accompanied by accumulation of the synaptic machinery at this site, being delivered in intracellular organelles mainly of TGN origin. Here, we report that in cultured hippocampal neurons, TGN organelles are linked via spectrin to clusters of the neural cell adhesion molecule (NCAM) in the plasma membrane. These complexes are translocated along neurites and trapped at sites of initial neurite-to-neurite contacts within several minutes after initial contact formation. The accumulation of TGN organelles at contacts with NCAM-deficient neurons is reduced when compared with wild-type cells, suggesting that NCAM mediates the anchoring of intracellular organelles in nascent synapses.