Basal synaptic transmission involves the release of neurotransmitters at individual synapses in response to a single action potential. Recent discoveries show that astrocytes modulate the activity of neuronal networks upon sustained and intense synaptic activity. However, their ability to regulate basal synaptic transmission remains ill defined and controversial. Here, we show that astrocytes in the hippocampal CA1 region detect synaptic activity induced by single-synaptic stimulation. Astrocyte activation occurs at functional compartments found along astrocytic processes and involves metabotropic glutamate subtype 5 receptors. In response, astrocytes increase basal synaptic transmission, as revealed by the blockade of their activity with a Ca(2+) chelator. Astrocytic modulation of basal synaptic transmission is mediated by the release of purines and the activation of presynaptic A(2A) receptors by adenosine. Our work uncovers an essential role for astrocytes in the regulation of elementary synaptic communication and provides insight into fundamental aspects of brain function.

Skeletal muscle aging is associated with a progressive decline in muscle mass and strength, a process termed sarcopenia. Evidence suggests that accumulation of mitochondrial dysfunction plays a causal role in sarcopenia, which could be triggered by impaired mitophagy. Mitochondrial function, mitophagy and mitochondrial morphology are interconnected aspects of mitochondrial biology, and may coordinately be altered with aging. However, mitochondrial morphology has remained challenging to characterize in muscle, and whether sarcopenia is associated with abnormal mitochondrial morphology remains unknown. Therefore, we assessed the morphology of SubSarcolemmal (SS) and InterMyoFibrillar (IMF) mitochondria in skeletal muscle of young (8-12wk-old) and old (88-96wk-old) mice using a quantitative 2-dimensional transmission electron microscopy approach. We show that sarcopenia is associated with larger and less circular SS mitochondria. Likewise, aged IMF mitochondria were longer and more branched, suggesting increased fusion and/or decreased fission. Accordingly, although no difference in the content of proteins regulating mitochondrial dynamics (Mfn1, Mfn2, Opa1 and Drp1) was observed, a mitochondrial fusion index (Mfn2-to-Drp1 ratio) was significantly increased in aged muscles. Our results reveal that sarcopenia is associated with complex changes in mitochondrial morphology that could interfere with mitochondrial function and mitophagy, and thus contribute to aging-related accumulation of mitochondrial dysfunction and sarcopenia.

Astrocytes are important regulators of excitatory synaptic networks. However, astrocytes regulation of inhibitory synaptic systems remains ill defined. This is particularly relevant since GABAergic interneurons regulate the activity of excitatory cells and shape network function. To address this issue, we combined optogenetics and pharmacological approaches, two-photon confocal imaging and whole-cell recordings to specifically activate hippocampal somatostatin or paravalbumin-expressing interneurons (SOM-INs or PV-INs), while monitoring inhibitory synaptic currents in pyramidal cells and Ca2+ responses in astrocytes. We found that astrocytes detect SOM-IN synaptic activity via GABABR and GAT-3-dependent Ca2+ signaling mechanisms, the latter triggering the release of ATP. In turn, ATP is converted into adenosine, activating A1Rs and upregulating SOM-IN synaptic inhibition of pyramidal cells, but not PV-IN inhibition. Our findings uncover functional interactions between a specific subpopulation of interneurons, astrocytes and pyramidal cells, involved in positive feedback autoregulation of dendritic inhibition of pyramidal cells.

The presence and the functionality of a glutamatergic regulation was studied at the frog neuromuscular junction (NMJ), a singly innervated cholinergic synapse. Bath application of glutamate reduced transmitter release without affecting nerve-evoked presynaptic Ca2+ entry and handling. (1S,3R)-aminocyclopentanedicarboxylic acid (ACPD), a metabotropic glutamate receptor (mGluR) agonist, mimicked the effects of glutamate while (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), a mGluR antagonist, blocked glutamate effects. MCPG had no effect on transmitter release evoked at low frequency (0.2 Hz) but significantly reduced synaptic depression (10 Hz, 80 s). This suggests that a frequency-dependent endogenous glutamatergic modulation is present at the frog NMJ and is mediated through mGluRs. Immunohistochemical labelling revealed the presence of mGluRs at the end plate area, primarily on muscle fibers. Functional glutamate uptake machinery was also found at the NMJ as blockade of glutamate transport by the inhibitor dl-threo-beta-benzyloxyaspartate (DL-TBOA) increased high frequency-induced depression, suggesting that the transporters system is used to eliminate glutamate from the extracellular space. Moreover, immunohistochemical labelling revealed that glutamate-aspartate transporters (GLASTs) are predominantly present on perisynaptic Schwann cells (PSCs). However, local application of glutamate on PSCs unreliability evoked small Ca2+ responses. Hence, these data suggest that functional glutamatergic interactions at a purely cholinergic synapse, shape synaptic efficacy and short-term plasticity in a frequency-dependent fashion.

Missense mutations (P56S) in Vapb are associated with autosomal dominant motor neuron diseases: amyotrophic lateral sclerosis and lower motor neuron disease. Although transgenic mice overexpressing the mutant vesicle-associated membrane protein-associated protein B (VAPB) protein with neuron-specific promoters have provided some insight into the toxic properties of the mutant proteins, their role in pathogenesis remains unclear. To identify pathological defects in animals expressing the P56S mutant VAPB protein at physiological levels in the appropriate tissues, we have generated Vapb knock-in mice replacing wild-type Vapb gene with P56S mutant Vapb gene and analyzed the resulting pathological phenotypes. Heterozygous P56S Vapb knock-in mice show mild age-dependent defects in motor behaviors as characteristic features of the disease. The homozygous P56S Vapb knock-in mice show more severe defects compared with heterozygous mice reflecting the dominant and dose-dependent effects of P56S mutation. Significantly, the knock-in mice demonstrate accumulation of P56S VAPB protein and ubiquitinated proteins in cytoplasmic inclusions, selectively in motor neurons. The mutant mice demonstrate induction of ER stress and autophagic response in motor neurons before obvious onset of behavioral defects, suggesting that these cellular biological defects might contribute to the initiation of the disease. The P56S Vapb knock-in mice could be a valuable tool to gain a better understanding of the mechanisms by which the disease arises.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motoneurons in a motor-unit (MU) dependent manner. Glial dysfunction contributes to numerous aspects of the disease. At the neuromuscular junction (NMJ), early alterations in perisynaptic Schwann cell (PSC), glial cells at this synapse, may impact their ability to regulate NMJ stability and repair. Indeed, muscarinic receptors (mAChR) regulate the repair phenotype of PSCs and are overactivated at disease-resistant NMJs (Soleus muscle) in SOD1 G37R mice. However, it remains unknown whether this is the case at disease-vulnerable NMJs and whether it translates into an impairment of PSC-dependent repair mechanisms. We used Soleus and Sternomastoid muscles from SOD1 G37R mice and performed Ca 2+ -imaging to monitor PSC activity and used immunohistochemistry to analyze their repair and phagocytic properties. We show that PSC mAChR-dependent activity was transiently increased at disease-vulnerable NMJs (Sternomastoid muscle). Furthermore, PSCs from both muscles extended disorganized processes from denervated NMJs and failed to initiate or guide nerve terminal sprouts at disease-vulnerable NMJs, a phenomenon essential for compensatory reinnervation. This was accompanied by a failure of numerous PSCs to upregulate Galectin-3 (MAC-2), a marker of glial axonal debris phagocytosis, upon NMJ denervation in SOD1 mice. Finally, differences in these PSC-dependent NMJ repair mechanisms were MU-type dependent, thus reflecting MU vulnerability in ALS.Together, these results reveal that neuron-glia communication is ubiquitously altered at the NMJ

Evidence showing the ability of glial cells to detect, respond to and modulate synaptic transmission and plasticity has contributed to the notion of glial cells as active synaptic partners. However, synaptically induced plasticity of glia themselves remains ill defined. Here we used the amphibian neuromuscular junction (NMJ) to study plasticity of perisynaptic Schwann cells (PSCs), glial cells at this synapse, following long-term in vivo modifications of synaptic activity. We used two models that altered synaptic activity in different manners. First, chronic blockade of postsynaptic nicotinic receptors using α-bungarotoxin (α-BTx) decreased facilitation, increased synaptic depression and decreased post-tetanic potentiation (PTP). Second, chronic nerve stimulation increased facilitation and resistance to synaptic depression, while leaving PTP unaltered. Our results indicate that there is no direct relationship between transmitter release and PSC calcium responses. Indeed, despite changes in transmitter release and plasticity in stimulated NMJs, nerve-evoked PSC calcium responses were similar to control. Similarly, PSC calcium responses in α-BTx treated NMJs were delayed and smaller in amplitude, even though basal level of transmitter release was increased. Also, when isolating purinergic and muscarinic components of PSC calcium responses, we found an increased sensitivity to ATP and a decreased sensitivity to muscarine in chronically stimulated NMJs. Conversely, in α-BTx treated NMJs, PSC sensitivity remained unaffected, but ATP-and muscarine-induced calcium responses were prolonged. Thus, our results reveal complex modifications of PSC properties, with differential modulation of signalling pathways that might underlie receptor regulation or changes in Ca 2+ handling. Importantly, similar to neurons, perisynaptic glial cells undergo plastic changes induced by altered synaptic activity. IntroductionThere is now a growing body of evidence indicating that glial cells play an active role in synaptic communication throughout the nervous system. They are indeed involved in the regulation of synaptic transmission and plasticity (reviewed by Auld & Robitaille, 2003;Halassa et al. 2007), believed to be the functional basis of learning and memory. Hence, glial cells are full participants at synapses and contribute actively to neuronal processing and integration. Complementary to their active role in regulating synaptic plasticity, glial cells themselves exhibit activity-dependent changes in their properties during development (Pasti et al. 1997). However, there is no direct evidence that glial cell modifications are specific and adapted to the level of synaptic activity and that they can occur in mature systems. Such adaptation, concomitant with pre-and postsynaptic plasticity, would be essential to maintain appropriate communication between all components of the tripartite synapse. Hence, the goal of this work was to test whether the properties and activation of perisynaptic glial cells are differentially regulated in re...

During neuromuscular junction maturation and repair, ephrin-A3 is expressed by all slow (Type I) fibers and only by slow fibers, and inhibits their innervation by fast motor axons via repulsive interactions with the ephrin-A3 receptor EphA8.