Structure and inhibition analysis of the mouse SAD-B C-terminal fragment

The SAD (synapses of amphids defective) kinases, including SAD-A and SAD-B, play important roles in the regulation of neuronal development, cell cycle, and energy metabolism. Our recent study of mouse SAD-A identified a unique autoinhibitory sequence (AIS), which binds at the junction of the kinase domain (KD) and the ubiquitin-associated (UBA) domain and exerts autoregulation in cooperation with UBA. Here, we report the crystal structure of the mouse SAD-B C-terminal fragment including the AIS and the kinase-associated domain 1 (KA1) at 2.8 Å resolution. The KA1 domain is structurally conserved, while the isolated AIS sequence is highly flexible and solvent-accessible. Our biochemical studies indicated that the SAD-B AIS exerts the same autoinhibitory role as that in SAD-A. We believe that the flexible isolated AIS sequence is readily available for interaction with KD-UBA and thus inhibits SAD-B activity.     

Dynamic clamp analysis of synaptic integration in sympathetic ganglia

Advances in modern neuroscience require the identification of principles that connect different levels of experimental analysis, from molecular mechanisms to explanations of cellular functions, then to circuits, and, ultimately, to systems and behavior. Here, we examine how synaptic organization of the sympathetic ganglia may enable them to function as use-dependent amplifiers of preganglionic activity and how the gain of this amplification may be modulated by metabotropic signaling mechanisms. The approach combines a general computational model of ganglionic integration together with experimental tests of the model using the dynamic clamp method. In these experiments, we recorded intracellularly from dissociated bullfrog sympathetic neurons and then mimicked physiological synapses with virtual computer-generated synapses. It, thus, became possible to analyze the synaptic gain by recording cellular responses to complex patterns of synaptic activity that normally arise in vivo from convergent nicotinic and muscarinic synapses. The results of these studies are significant because they illustrate how gain generated through ganglionic integration may contribute to the feedback control of important autonomic behaviors, in particular to the control of the blood pressure. We dedicate this paper to the memory of Professor Vladimir Skok, whose rich legacy in synaptic physiology helped to establish the modern paradigm for connecting multiple levels of analysis in studies of the nervous system. Neirofiziologiya/Neurophysiology, Vol. 39, No. 6, pp. 486–492, November–December, 2007.     

Neuromodulatory Control of Neocortical Microcircuits with Activity-Dependent Short-Term Synaptic Depression

A biophysical model of a neocortical microcircuit system is formulated and employed in studies of neuromodulatory control of dynamics and function. The model is based on recent observations of reciprocal connections between pyramidal cells and inhibitory interneurons and incorporates a new type of activity-dependent short-term depression of synaptic couplings recently observed. The model neurons are of a low-dimensional type also accounting for neuronal adaptation, i.e. the coupling between neuronal activity and excitability, which can be regulated by various neuromodulators in the brain. The results obtained demonstrate a capacity for neuromodulatory control of dynamical mode linked to functional mode. The functional aspects considered refer to the observed resolution of multiple objects in working memory as well as the binding of different features for the perception of an object. The effects of neuromodulators displayed by the model are in accordance with many observations on neuromodulatory influence on cognitive functions and brain disorders.     

Glutamate decarboxylase in developing rat neocortex: Does it correlate with the differentiation of GABAergic neurons and synapses?

Postnatal development of glutamate decarboxylase was studied in the rat cerebral cortex. Two methods were used: estimation of the enzymatic activity of glutamate decarboxylase in homogenates of developing cortical tissue and visualization of structures containing glutamate decarboxylase-like immunoreactivity. Glutamate decarboxylase-like immunoreactivity appeared first in perikarya and dendrites and only later in axons and axon varicosities. The most rapid increase in the glutamate decarboxylase activity took place during the second postnatal week and this coincided with a rapid increase in the density of axon varicosities containing glutamate decarboxylase-like immunoreactivity but preceded the most rapid phase in the formation of GABAergic synapses by several days. However, there was a change in the characteristics of glutamate decarboxylase which correlated with GABA synaptogenesis: two fractions of glutamate decarboxylase with different sensitivities to the activating effects of Triton X-100 could be distinguished as from about the time when most of the GABAergic synapses are formed.     

Neurexins and neuroligins: new partners for GABAA receptors at synapses

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain. As one of several types of endogenous receptors, GABA_A receptors have been shown to be essential in most, if not all, aspects of brain functioning, including neural development and information processing. Mutations in genes encoding GABA_A receptors and alterations in the function of GABA_A receptors are associated with many neurologic diseases, and GABA_A receptors have been clinically targeted by many drugs, such as benzodiazepines and general anesthetics. Extensive studies have revealed a number of intracellular chaperons/interactions for GABA_A receptors, providing a protein-protein network in regulating the trafficking and location of GABA_A receptors in the brain. Recently, neurexins and neuroligins, two families of transmembrane proteins present at neurological synapses, are implicated as new partners to GABA_A receptors. These works shed new light on the synaptic regulation of GABA_A receptor activity. Here, we summarized the proteins that were implicated in the function of GABA_A receptors, including neurexins and neuroligins.     

Dendritic cell ICAM-1 strengthens synapses with CD8 T cells but is not required for their early differentiation

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Building a synapse

L-glutamate is the primary neurotransmitter at excitatory synapses in the vertebrate CNS and at arthropod neuromuscular junctions (NMJs). However, the molecular mechanisms that trigger the recruitment of glutamate receptors at the onset of synaptogenesis and promote their stabilization at postsynaptic densities remain poorly understood. We have reported the discovery of a novel, evolutionary conserved molecule, Neto, essential for clustering of ionotropic glutamate receptors (iGluRs) at Drosophila NMJ. Neto is the first auxiliary subunit described in Drosophila and is the only non-channel subunit absolutely required for functional iGluRs. Here we review the role of Drosophila Neto in synapse assembly, its similarities with other Neto proteins and a new perspective on how glutamatergic synapses are physically assembled and stabilized.     

Oscillatory Plateau Potentials and Bursts in a Simulated Motoneuron during Tonic Coactivation of Somato-Dendritic NMDA and Axon-Hillock GABA Inputs

In a simulated motoneuron, we studied the effects of tonic coactivation of glutamatergic (NMDA-type) synapses covering the somato-dendritic membrane and of GABA-ergic synapses located on the axon hillock. As in the prototypes, NMDA activation caused oscillatory plateau potentials with bursts of action potentials (AP). Plateau depolarizations spreading from the soma inactivated Na⁺ channels and reduced the number of AP in the axon compared with that in the soma. As GABA activation increased, interplateau intervals also increased, while the plateau duration and number of AP per burst decreased.     

Correlation of receptive field position of mechanosensory neurons and the strength of their connections to AP neurons in the CNS of the leech (Whitmania pigra)

An identified neuron of unknown function in the CNS of the leech, the anterior pagoda (AP) cell, receives multiple synaptic inputs from mechanosensory neurons that innervate the skin. Impulses in touch (T), pressure (P) and nociceptive (N) sensory cells on both sides of the ganglion produced electrical coupling potentials on both AP cells. Sensory cells with receptive fields contralateral to the cell body of the AP neuron always gave rise to larger synaptic potentials. In addition sensory cells supplying dorsal skin gave rise to larger synaptic potentials than those with lateral or ventral fields. It is suggested that integration by the AP cell can provide information about the position of mechanical stimuli impinging on the body wall of the animal.