Endocytosis mediates the cellular uptake of micronutrients and cell surface proteins. Fast Endophilin-mediated endocytosis, FEME, is not constitutively active but triggered upon receptor activation. ...High levels of growth factors induce spontaneous FEME, which can be suppressed upon serum starvation. This suggested a role for protein kinases in this growth factor receptor-mediated regulation. Using chemical and genetic inhibition, we find that Cdk5 and GSK3β are negative regulators of FEME. They antagonize the binding of Endophilin to Dynamin-1 and to CRMP4, a Plexin A1 adaptor. This control is required for proper axon elongation, branching and growth cone formation in hippocampal neurons. The kinases also block the recruitment of Dynein onto FEME carriers by Bin1. As GSK3β binds to Endophilin, it imposes a local regulation of FEME. Thus, Cdk5 and GSK3β are key regulators of FEME, licensing cells for rapid uptake by the pathway only when their activity is low.
Mitochondrial dynamics in astrocytes Stephen, Terri-Leigh; Gupta-Agarwal, Swati; Kittler, Josef T
Biochemical Society transactions
42, Issue:
5
Journal Article
Peer reviewed
Open access
Astrocytes exhibit cellular excitability through variations in their intracellular calcium (Ca²⁺) levels in response to synaptic activity. Astrocyte Ca²⁺ elevations can trigger the release of ...neuroactive substances that can modulate synaptic transmission and plasticity, hence promoting bidirectional communication with neurons. Intracellular Ca²⁺ dynamics can be regulated by several proteins located in the plasma membrane, within the cytosol and by intracellular organelles such as mitochondria. Spatial dynamics and strategic positioning of mitochondria are important for matching local energy provision and Ca²⁺ buffering requirements to the demands of neuronal signalling. Although relatively unresolved in astrocytes, further understanding the role of mitochondria in astrocytes may reveal more about the complex bidirectional relationship between astrocytes and neurons in health and disease. In the present review, we discuss some recent insights regarding mitochondrial function, transport and turnover in astrocytes and highlight some important questions that remain to be answered.
GABAA receptors mediate fast inhibitory transmission in the brain, and their number can be rapidly up- or downregulated to alter synaptic strength. Neuroligin-2 plays a critical role in the ...stabilization of synaptic GABAA receptors and the development and maintenance of inhibitory synapses. To date, little is known about how the amount of neuroligin-2 at the synapse is regulated and whether neuroligin-2 trafficking affects inhibitory signaling. Here, we show that neuroligin-2, when internalized to endosomes, co-localizes with SNX27, a brain-enriched cargo-adaptor protein that facilitates membrane protein recycling. Direct interaction between the PDZ domain of SNX27 and PDZ-binding motif in neuroligin-2 enables membrane retrieval of neuroligin-2, thus enhancing synaptic neuroligin-2 clusters. Furthermore, SNX27 knockdown has the opposite effect. SNX27-mediated up- and downregulation of neuroligin-2 surface levels affects inhibitory synapse composition and signaling strength. Taken together, we show a role for SNX27-mediated recycling of neuroligin-2 in maintenance and signaling of the GABAergic synapse.
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•Neuroligin-2 internalizes to endosomes and interacts with SNX27 and retromer•Direct interaction between neuroligin-2 and SNX27 mediates neuroligin-2 recycling•Modulation of synaptic neuroligin-2 levels affects inhibitory synapse composition•SNX27-mediated recycling of neuroligin-2 regulates inhibitory signaling strength
To maintain healthy brain function, neurons tightly regulate the balance between excitatory and inhibitory synaptic signaling. Neuroligin-2 is a postsynaptic adhesion protein important for inhibitory synapse function. In this article, Halff et al. describe how the protein SNX27 modulates synaptic levels of neuroligin-2, thereby regulating the strength of inhibitory signaling.
Growth factors of the TGFβ superfamily play key roles in regulating neuronal and muscle function. Myostatin (or GDF8) and GDF11 are potent negative regulators of skeletal muscle mass. However, ...expression of myostatin and its cognate receptors in other tissues, including brain and peripheral nerves, suggests a potential wider biological role. Here, we show that Myoglianin (MYO), the
homolog of myostatin and GDF11, regulates not only body weight and muscle size, but also inhibits neuromuscular synapse strength and composition in a Smad2-dependent manner. Both myostatin and GDF11 affected synapse formation in isolated rat cortical neuron cultures, suggesting an effect on synaptogenesis beyond neuromuscular junctions. We also show that MYO acts
to inhibit synaptic transmission between neurons in the escape response neural circuit of adult flies. Thus, these anti-myogenic proteins act as important inhibitors of synapse function and neuronal growth.
Fast inhibitory signalling in the mammalian brain is mediated by gamma-aminobutyric acid type A receptors (GABAARs), which are targets for anti-epileptic therapy such as benzodiazepines. GABAARs ...undergo tightly regulated trafficking processes that are essential for maintenance and physiological modulation of inhibitory strength. The trafficking of GABAARs to and from the membrane is altered during prolonged seizures such as in Status Epilepticus (SE) and has been suggested to contribute to benzodiazepine pharmacoresistance in patients with SE. However, the intracellular signalling mechanisms that cause this modification in GABAAR trafficking remain poorly understood. In this study, we investigate the surface stability of GABAARs during SE utilising the low Mg2+ model in hippocampal rat neurons. Live-cell imaging of super ecliptic pHluorin (SEP)-tagged α2 subunit containing GABAARs during low Mg2+ conditions reveals that the somatic surface receptor pool undergoes down-regulation dependent on N-methyl-d-aspartate receptor (NMDAR) activity. Analysis of the intracellular Ca2+ signal during low Mg2+ using the Ca2+-indicator Fluo4 shows that this reduction of surface GABAARs correlates well with the timeline of intracellular Ca2+ changes. Furthermore, we show that the activation of the phosphatase calcineurin was required for the decrease in surface GABAARs in neurons undergoing epileptiform activity. These results indicate that somatic modulation of GABAAR trafficking during epileptiform activity in vitro is mediated by calcineurin activation which is linked to changes in intracellular Ca2+ concentrations. These mechanisms could account for benzodiazepine pharmacoresistance and the maintenance of recurrent seizure activity, and reveal potential novel targets for the treatment of SE.
This article is part of the Special Issue entitled ‘GABAergic Signaling in Health and Disease’.
•We investigate surface stability of GABAARs in an in vitro model of Status Epilepticus.•Live-cell imaging shows a decrease in surface GABAARs in the cell soma.•Decrease of somatic GABAARs from the surface is mediated by NMDAR activity.•In vitro Status Epilepticus induces intracellular Ca2+ changes.•Calcineurin mediates the decrease of somatic GABAARs from the surface.
Modification of the number of GABA A receptors (GABA A Rs) clustered at inhibitory synapses can regulate inhibitory synapse strength with important implications for information processing and nervous ...system plasticity and pathology. Currently, however, the mechanisms that regulate the number of GABA A Rs at synapses remain poorly understood. By imaging superecliptic pHluorin tagged GABA A R subunits we show that synaptic GABA A R clusters are normally stable, but that increased neuronal activity upon glutamate receptor (GluR) activation results in their rapid and reversible dispersal. This dispersal correlates with increases in the mobility of single GABA A Rs within the clusters as determined using single-particle tracking of GABA A Rs labeled with quantum dots. GluR-dependent dispersal of GABA A R clusters requires Ca 2+ influx via NMDA receptors (NMDARs) and activation of the phosphatase calcineurin. Moreover, the dispersal of GABA A R clusters and increased mobility of individual GABA A Rs are dependent on serine 327 within the intracellular loop of the GABA A R γ2 subunit. Thus, NMDAR signaling, via calcineurin and a key GABA A R phosphorylation site, controls the stability of synaptic GABA A Rs, with important implications for activity-dependent control of synaptic inhibition and neuronal plasticity.
Fast synaptic inhibition in the brain is largely mediated by GABA(A) receptors. These ligand-gated ion channels are crucial in the control of cell and network activity. Therefore, modulating their ...function or cell surface stability will have major consequences for neuronal excitation. It has become clear that the stability and activity of GABA(A) receptors at synapses can be dynamically modulated by receptor trafficking and phosphorylation. Here, we discuss these regulatory mechanisms, and their consequences for the efficacy of GABA(A) receptor mediated synaptic inhibition.
The disrupted in schizophrenia 1 (DISC1) protein is implicated in major mental illnesses including schizophrenia and bipolar disorder. A key feature of psychiatric disease is aberrant synaptic ...communication. Correct synaptic transmission is dependent on spatiotemporally regulated energy provision and calcium buffering. This can be achieved by precise distribution of mitochondria throughout the elaborate architecture of the neuron. Central to this process is the calcium sensor and GTPase Miro1, which allows mitochondrial trafficking by molecular motors. While the role of Miro1-calcium binding in mitochondrial transport is well described, far less is known regarding the functions of the two GTPase domains. Here, we investigate the effects of a psychiatric disease-associated mutation in DISC1 on mitochondrial trafficking. We show that this DISC1 mutation impairs Miro1’s ability to transport mitochondria. We also demonstrate the necessity of the first Miro1 GTPase domain in determining direction of mitochondrial transport and the involvement of DISC1 in this process. Finally, we describe the effects of mutant DISC1 on positioning of mitochondria at synapses.
The transport of mitochondria to specific neuronal locations is critical to meet local cellular energy demands and for buffering intracellular calcium. A critical role for kinesin motor proteins in ...mitochondrial transport in neurons has been demonstrated. Currently however the molecular mechanisms that underlie the recruitment of motor proteins to mitochondria, and how this recruitment is regulated remain unclear. Here we show that a protein trafficking complex comprising the adaptor protein Grif-1 and the atypical GTPase Miro1 can be detected in mammalian brain where it is localised to neuronal mitochondria. Increasing Miro1 expression levels recruits Grif-1 to mitochondria. This results in an enhanced transport of mitochondria towards the distal ends of neuronal processes. Uncoupling Grif-1 recruitment to mitochondria by expressing a Grif-1/Miro1 binding fragment dramatically reduces mitochondrial transport into neuronal processes. Altering Miro1 function by mutating its first GTPase domain affects Miro's ability to recruit Grif-1 to mitochondria and in addition alters mitochondrial distribution and shape along neuronal processes. These data suggest that Miro1 and the kinesin adaptor Grif-1 play an important role in regulating mitochondrial transport in neurons.
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