SummaryEnergy use, mainly to reverse ion movements in neurons, is a fundamental constraint on brain information processing. Trafficking of mitochondria to locations in neurons where there are large ion fluxes is essential for powering neural function. Mitochondrial trafficking is regulated by Ca2+ entry through ionotropic glutamate receptors, but the underlying mechanism is unknown. We show that the protein Miro1 links mitochondria to KIF5 motor proteins, allowing mitochondria to move along microtubules. This linkage is inhibited by micromolar levels of Ca2+ binding to Miro1. With the EF hand domains of Miro1 mutated to prevent Ca2+ binding, Miro1 could still facilitate mitochondrial motility, but mitochondrial stopping induced by glutamate or neuronal activity was blocked. Activating neuronal NMDA receptors with exogenous or synaptically released glutamate led to Miro1 positioning mitochondria at the postsynaptic side of synapses. Thus, Miro1 is a key determinant of how energy supply is matched to energy usage in neurons.

SummaryMicroglia exhibit two modes of motility: they constantly extend and retract their processes to survey the brain, but they also send out targeted processes to envelop sites of tissue damage. We now show that these motility modes differ mechanistically. We identify the two-pore domain channel THIK-1 as the main K+ channel expressed in microglia in situ. THIK-1 is tonically active, and its activity is potentiated by P2Y12 receptors. Inhibiting THIK-1 function pharmacologically or by gene knockout depolarizes microglia, which decreases microglial ramification and thus reduces surveillance, whereas blocking P2Y12 receptors does not affect membrane potential, ramification, or surveillance. In contrast, process outgrowth to damaged tissue requires P2Y12 receptor activation but is unaffected by blocking THIK-1. Block of THIK-1 function also inhibits release of the pro-inflammatory cytokine interleukin-1β from activated microglia, consistent with K+ loss being needed for inflammasome assembly. Thus, microglial immune surveillance and cytokine release require THIK-1 channel activity.

Myelination speeds conduction of the nerve impulse, enhancing cognitive power. Changes of white matter structure contribute to learning, and are often assumed to reflect an altered number of myelin wraps. We now show that, in rat optic nerve and cerebral cortical axons, the node of Ranvier length varies over a 4.4-fold and 8.7-fold range respectively and that variation of the node length is much less along axons than between axons. Modelling predicts that these node length differences will alter conduction speed by ~20%, similar to the changes produced by altering the number of myelin wraps or the internode length. For a given change of conduction speed, the membrane area change needed at the node is >270-fold less than that needed in the myelin sheath. Thus, axon-specific adjustment of node of Ranvier length is potentially an energy-efficient and rapid mechanism for tuning the arrival time of information in the CNS.DOI: http://dx.doi.org/10.7554/eLife.23329.001

The density of GABAA receptors (GABAARs) at synapses regulates brain excitability, and altered inhibition may contribute to Huntington’s disease, which is caused by a polyglutamine repeat in the protein huntingtin. However, the machinery that delivers GABAARs to synapses is unknown. We demonstrate that GABAARs are trafficked to synapses by the kinesin family motor protein 5 (KIF5). We identify the adaptor linking the receptors to KIF5 as the huntingtin associated protein 1 (HAP1). Disrupting the HAP1-KIF5 complex decreases synaptic GABAAR number, and reduces the amplitude of inhibitory postsynaptic currents. When huntingtin is mutated as in Huntington’s disease, GABAAR transport and inhibitory synaptic currents are reduced. Thus, HAP1-KIF5 dependent GABAAR trafficking is a fundamental mechanism controlling the strength of synaptic inhibition in the brain. Its disruption by mutant huntingtin may explain some of the defects in brain information processing occurring in Huntington’s disease, and provides a new molecular target for therapeutic approaches.

The efficacy of synaptic inhibition depends on the number of ␥-aminobutyric acid type A receptors (GABA A Rs) expressed on the cell surface of neurons. The clathrin adaptor protein 2 (AP2) complex is a critical regulator of GABAAR endocytosis and, hence, surface receptor number. Here, we identify a previously uncharacterized atypical AP2 binding motif conserved within the intracellular domains of all GABAAR ␤ subunit isoforms. This AP2 binding motif (KTHLRRRSSQLK in the ␤3 subunit) incorporates the major sites of serine phosphorylation within receptor ␤ subunits, and phosphorylation within this site inhibits AP2 binding. Furthermore, by using surface plasmon resonance, we establish that a peptide (pep␤3) corresponding to the AP2 binding motif in the GABAAR ␤3 subunit binds to AP2 with high affinity only when dephosphorylated. Moreover, the pep␤3 peptide, but not its phosphorylated equivalent (pep␤3-phos), enhanced the amplitude of miniature inhibitory synaptic current and whole cell GABAAR current. These effects of pep␤3 on GABAAR current were occluded by inhibitors of dynamin-dependent endocytosis supporting an action of pep␤3 on GABAAR endocytosis. Therefore phosphodependent regulation of AP2 binding to GABAARs provides a mechanism to specify receptor cell surface number and the efficacy of inhibitory synaptic transmission. endocytosis ͉ phosphorylation G ABA A receptors (GABA A Rs) are the major sites of fast synaptic inhibition in the brain (1). These pentameric ligandgated ion channels can be constructed from seven subunit classes: ␣1-6, ␤ 1-3, ␥ 1-3, ␦, , , and (2), with the majority of benzodiazepine-sensitive receptor subtypes being assembled from ␣, ␤, and ␥2 subunits (1, 2). A primary determinant for the efficacy of synaptic inhibition and, hence, neuronal excitation is the number of functional GABA A Rs expressed on the surface of neurons (3-10). Therefore, there has been considerable interest in understanding the cellular mechanism that neurons use to regulate GABA A R cell surface stability and activity. Collectively these studies have revealed that neuronal GABA A Rs undergo significant rates of constitutive endocytosis (3,8,(11)(12)(13)(14)(15), a process that has been established to regulate synaptic inhibition (8). GABA A Rs enter the endocytic pathway by a clathrin-mediated dynamindependent mechanism (8,(11)(12)(13)(14), a process that is facilitated by the clathrin adaptor protein 2 (AP2) complex, which is intimately associated with these receptors in neurons (8, 13). Internalized GABA A Rs are then subjected to either rapid recycling or targeted for lysozomal degradation, an endoctytic sorting decision that is regulated by the Huntingtin associated protein-1 (15). Therefore, changes in the rates of GABA A R endocytosis and͞or endocytic sorting represent potentially powerful mechanisms to regulate GABA A R cell surface number and inhibitory synaptic transmission (8,15).A potential mechanism to regulate target protein endocytosis is modulating interaction with the AP2 adaptor protein complex (1...