Brain cell biology最新文献

We have assessed the role of synapsins in catecholamine release by comparing the properties of exocytosis in adrenal chromaffin cells from wild-type and synapsin triple knock-out (TKO) mice. Brief depolarizations led to a greater amount of catecholamine release in chromaffin cells from TKO mice in comparison to chromaffin cells from wild-type mice. This increase in catecholamine release was due to an increased number of exocytotic events, while the properties of individual quanta of released catecholamine were unchanged. Barium ions produced similar amounts of catecholamine release from TKO and wild-type chromaffin cells, suggesting that the reserve pool of chromaffin granules is unchanged following loss of synapsins. Because expression of synapsin IIa in TKO chromaffin cells rescued the defect in depolarization-induced exocytosis, the TKO phenotype apparently results from loss of synapsin IIa. We conclude that synapsin IIa serves as a negative regulator of catecholamine release and that this protein influences exocytosis from a readily releasable pool of chromaffin granules. Further, because these defects in catecholamine release are different from those observed for glutamate and GABA release in TKO mice, we conclude that the functions of synapsins differ for vesicles containing different types of neurotransmitters.

It is now well-established that an active cross-talk occurs between neurons and glial cells, in the adult as well as in the developing and regenerating nervous systems. These functional interactions not only actively modulate synaptic transmission, but also support neuronal growth and differentiation. We have investigated the possible existence of a reciprocal interaction between inner ear vestibular neurons and Schwann cells maintained in primary cultures. We show that ATP released by the extending vestibular axons elevates intracellular calcium levels within Schwann cells. Purinergic activation of the Schwann P2X(7) receptor induces the release of neurotrophin BDNF, which occurs via a regulated, tetanus-toxin sensitive, vesicular pathway. BDNF, in turn, is required by the vestibular neuron to support its own survival and growth. Given the massive release of ATP during tissue damage, cross-talk between vestibular neurons and Schwann cells could play a primary role during regeneration.

Synapsins are a multigene family of neuron-specific phosphoproteins and comprise the most abundant synaptic vesicle proteins. They have been proposed to tether synaptic vesicles to each other to maintain a reserve pool in the vicinity of the active zone. Such a role is supported by the observation that disruption of synapsin function leads to a depletion of the reserve pool of vesicles and an increase in synaptic depression. However, other functions for synapsins have been proposed as well, and there currently exists no coherent picture of how these abundant proteins modulate synaptic transmission. Here, we discuss novel insights into how synapsins may regulate neurotransmitter release.

Pyramidal neurons of the hippocampus and cortex have polarized dendritic arbors, but little is known about the cellular mechanisms distinguishing apical and basal dendrites. We used morphometric analysis and time lapse imaging of cultured hippocampal neurons to show that glutamatergic neurons develop progressive dendritic asymmetry in the absence of polarized extrinsic cues. Thus, pyramidal neurons have a cellular program for polarized dendrite growth independent of tissue microenvironment.

Growing evidence suggests that astrocytes are the active partners of neurons in many brain functions. Astrocytic mitochondria are highly motile organelles which regulate the temporal and spatial patterns of Ca( 2+ ) dynamics, in addition to being a major source of ATP and reactive oxygen species. Previous studies have shown that mitochondria translocate to endoplasmic reticulum during Ca( 2+ ) release from internal stores, but whether a similar spatial interaction between mitochondria and plasma membrane occurs is not known. Using total internal reflection fluorescence (TIRF) microscopy we show that a fraction of mitochondria became trapped near the plasma membrane of cultured hippocampal astrocytes during exposure to the transmitters glutamate or ATP, resulting in net translocation of the mitochondria to the plasma membrane. This translocation was dependent on the intracellular Ca( 2+ ) rise because it was blocked by pre-incubation with BAPTA AM and mimicked by application of the Ca( 2+ ) ionophore ionomycin. Transmembrane Ca( 2+ ) influx induced by raising external Ca( 2+ ) also caused mitochondrial trapping, which occurred more rapidly than that produced by glutamate or ATP. In astrocytes treated with the microtubule-disrupting agent nocodazole, intracellular Ca( 2+ ) rises failed to induce trapping of mitochondria near plasma membrane, suggesting a role for microtubules in this phenomenon. Our data reveal the Ca( 2+ )-dependent trapping of mitochondria near the plasma membrane as a novel form of mitochondrial regulation, which is likely to control the perimembrane Ca( 2+ ) dynamics and regulate signaling by mitochondria-derived reactive oxygen species.

Recently, intense interest has focussed on electrical coupling between interneurones in cortical regions and their contributions towards oscillatory network activity. Despite mounting circumstantial evidence that pyramidal cells are also coupled, the paucity of direct evidence has made this controversial. Dual intracellular recordings from pairs of cortical and hippocampal pyramids demonstrated strong, but sparse coupling. Approximately 70% of CA1 pyramids close to the stratum radiatum border were coupled to another pyramid, but only to one or two of their very closest neighbours. On average 25% of the steady state and 10% of the peak action potential voltage change in one cell transferred to the other, supporting synchrony and promoting burst firing. The very high incidence of convergent inputs from coupled pyramids onto single targets provided additional evidence that 'spikelets' reflected full action potentials in a coupled cell, since the EPSPs activated by APs and by 'spikelets' had significantly different amplitude distributions.

Varicosities are ubiquitous neuronal structures that appear as local swellings along neurites of invertebrate and vertebrate neurons. Surprisingly little is known about their cell biology. We use here cultured Aplysia neurons and demonstrate that varicosities are motile compartments that contain large clusters of organelles. The content of varicosities propagate along neurites within the plasma membrane "sleeve", split and merge, or wobble in place. Confocal imaging, retrospective immunolabeling, electron microscopy and pharmacological perturbations reveal that the motility of the varicosities' organelle content occurs in concert with an actin scaffold and is generated by actomyosin motors. Despite the motility of these organelle clusters within the cytoplasm along the neurites, elevation of the free intracellular calcium concentration within varicosities by trains of action potentials induces exocytosis followed by membrane retrieval. Our observations demonstrate that varicosities formed in the absence of postsynaptic cells behave as "ready to go" prefabricated presynaptic terminals. We suggest that the varicosities' motility serves to increase the probability of encountering a postsynaptic cell and to rapidly form a functional synapse.

The receptor tyrosine kinase TrkB and its ligands, brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5), are critically important for growth, survival and activity-dependent synaptic strengthening in the central nervous system. These TrkB-mediated actions occur in a highly cell-type specific manner. Here we report that cerebellar Purkinje cells, which are richly endowed with TrkB receptors, develop a normal morphology in trkB-deficient mice. Thus, in contrast to other types of neurons, Purkinje cells do not need TrkB for dendritic growth and spine formation. Instead, we find a moderate delay in the maturation of GABAergic synapses and, more importantly, an abnormal multiple climbing fiber innervation in Purkinje cells in trkB-deficient mice. Thus, our results demonstrate an involvement of TrkB receptors in synapse elimination and reveal a new role for receptor tyrosine kinases in the brain.

A key event in neurite initiation is the accumulation of microtubule bundles at the neuron periphery. We hypothesized that such bundled microtubules may generate a force at the plasma membrane that facilitates neurite initiation. To test this idea we observed the behavior of microtubule bundles that were induced by the microtubule-associated protein MAP2c. Endogenous MAP2c contributes to neurite initiation in primary neurons, and exogeneous MAP2c is sufficient to induce neurites in Neuro-2a cells. We performed nocodazol washout experiments in primary neurons, Neuro-2a cells and COS-7 cells to investigate the underlying mechanism. During nocodazol washout, small microtubule bundles formed rapidly in the cytoplasm and immediately began to move toward the cell periphery in a unidirectional manner. In neurons and Neuro-2a cells, neurite-like processes extended within minutes and concurrently accumulated bundles of repolymerized microtubules. Speckle microscopy in COS-7 cells indicated that bundle movement was due to transport, not treadmilling. At the periphery bundles remained under a unidirectional force and induced local cell protrusions that were further enhanced by suppression of Rho kinase activity. Surprisingly, this bundle motility was independent of classical actin- or microtubule-based tracks. It was, however, reversed by function-blocking antibodies against dynein. Suppression of dynein expression in primary neurons by RNA interference severely inhibited the generation of new neurites, but not the elongation of existing neurites formed prior to dynein knockdown. Together, these cell biological data suggest that neuronal microtubule-associated proteins induce microtubule bundles that are pushed outward by dynein and locally override inward contraction to initiate neurite-like cell protrusions. A similar force-generating mechanism might participate in spontaneous initiation of neurites in developing neurons.

Electrical transmission in the mammalian brain is now well established. A new study by Thomson and colleagues elegantly demonstrates coupling between CA1 hippocampal pyramidal cells, which is far more common than previously supposed. Although the history of coupling is extensive, doubt, predjudice, and technical issues long kept it from wide acceptance. Here "spikelets" or "fast prepotentials" are found when two cells are coupled and in this situation result from electrical transmission of impulses from one coupled cell to the other. Interesting questions remain as to whether connexin or pannexin gap junctions serve as the molecular substrate of transmission, and the role of electrical transmission in hippocampal physiology is uncertain. Increased coupling could well contribute to the known tendency of the hippocampus to exhibit seizure activity.