Synaptic Activation of Glutamate Transporters in Hippocampal Astrocytes

AbstractGlutamate transporters in the CNS are expressed in neurons and glia and mediate high affinity, electrogenic uptake of extracellular glutamate. Although glia have the highest capacity for glutamate uptake, the amount of glutamate that reaches glial membranes following release and the rate that glial transporters bind and sequester transmitter is not known. We find that stimulation of Schaffer collateral/commissural fibers in hippocampal slices evokes glutamate transporter currents in CA1 astrocytes that activate rapidly, indicating that a significant amount of transmitter escapes the synaptic cleft shortly after release. Transporter currents in outside-out patches from astrocytes have faster kinetics than synaptically elicited currents, suggesting that the glutamate concentration attained at astrocytic membranes is lower but remains elevated for longer than in the synaptic cleft

Spontaneous activity in the developing auditory system

Abstract Spontaneous electrical activity is a common feature of sensory systems during early development. This sensoryindependent neuronal activity has been implicated in promoting their survival and maturation, as well as growth and refinement of their projections to yield circuits that can rapidly extract information about the external world. Periodic bursts of action potentials occur in auditory neurons of mammals before hearing onset. This activity is induced by inner hair cells (IHCs) within the developing cochlea, which establish functional connections with spiral ganglion neurons (SGNs) several weeks before they are capable of detecting external sounds. During this pre-hearing period, IHCs fire periodic bursts of Ca 2+ action potentials that excite SGNs, triggering brief but intense periods of activity that pass through auditory centers of the brain. Although spontaneous activity requires input from IHCs, there is ongoing debate about whether IHCs are intrinsically active and their firing periodically interrupted by external inhibitory input (IHC-inhibition model), or are intrinsically silent and their firing periodically promoted by an external excitatory stimulus (IHC-excitation model). There is accumulating evidence that inner supporting cells in Kölliker's organ spontaneously release ATP during this time, which can induce bursts of Ca 2+ spikes in IHCs that recapitulate many features of auditory neuron activity observed in vivo. Nevertheless, the role of supporting cells in this process remains to be established in vivo. A greater understanding of the molecular mechanisms responsible for generating IHC activity in the developing cochlea will help reveal how these events contribute to the maturation of nascent auditory circuits

Degeneration and impaired regeneration of gray matter oligodendrocytes in amyotrophic lateral sclerosis.

Oligodendrocytes associate with axons to establish myelin and provide metabolic support to neurons. In the spinal cord of amyotrophic lateral sclerosis (ALS) mice, oligodendrocytes downregulate transporters that transfer glycolytic substrates to neurons and oligodendrocyte progenitors (NG2(+) cells) exhibit enhanced proliferation and differentiation, although the cause of these changes in oligodendroglia is unknown. We found extensive degeneration of gray matter oligodendrocytes in the spinal cord of SOD1 (G93A) ALS mice prior to disease onset. Although new oligodendrocytes were formed, they failed to mature, resulting in progressive demyelination. Oligodendrocyte dysfunction was also prevalent in human ALS, as gray matter demyelination and reactive changes in NG2(+) cells were observed in motor cortex and spinal cord of ALS patients. Selective removal of mutant SOD1 from oligodendroglia substantially delayed disease onset and prolonged survival in ALS mice, suggesting that ALS-linked genes enhance the vulnerability of motor neurons and accelerate disease by directly impairing the function of oligodendrocytes

Purinergic Signaling Controls Spontaneous Activity in the Auditory System throughout Early Development

Spontaneous bursts of electrical activity in the developing auditory system arise within the cochlea before hearing onset and propagate through future sound-processing circuits of the brain to promote maturation of auditory neurons. Studies in isolated cochleae revealed that this intrinsically generated activity is initiated by ATP release from inner supporting cells (ISCs), resulting in activation of purinergic autoreceptors, K+ efflux, and subsequent depolarization of inner hair cells. However, it is unknown when this activity emerges or whether different mechanisms induce activity during distinct stages of development. Here we show that spontaneous electrical activity in mouse cochlea from both sexes emerges within ISCs during the late embryonic period, preceding the onset of spontaneous correlated activity in inner hair cells and spiral ganglion neurons, which begins at birth and follows a base to apex developmental gradient. At all developmental ages, pharmacological inhibition of P2Y1 purinergic receptors dramatically reduced spontaneous activity in these three cell types. Moreover, in vivo imaging within the inferior colliculus revealed that auditory neurons within future isofrequency zones exhibit coordinated neural activity at birth. The frequency of these discrete bursts increased progressively during the postnatal prehearing period yet remained dependent on P2RY1. Analysis of mice with disrupted cholinergic signaling in the cochlea indicate that this efferent input modulates, rather than initiates, spontaneous activity before hearing onset. Thus, the auditory system uses a consistent mechanism involving ATP release from ISCs and activation of P2RY1 autoreceptors to elicit coordinated excitation of neurons that will process similar frequencies of sound.Fil: Babola, Travis A.. University Johns Hopkins; Estados UnidosFil: Li, Sally. University Johns Hopkins; Estados UnidosFil: Wang, Zhirong. University Of Georgetown; Estados UnidosFil: Kersbergen, Calvin J.. University Johns Hopkins; Estados UnidosFil: Elgoyhen, Ana Belen. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor N. Torres"; ArgentinaFil: Coate, Thomas M.. University Johns Hopkins; Estados UnidosFil: Bergles, Dwight E.. University Johns Hopkins; Estados Unido

Problems and Pitfalls of Identifying Remyelination in Multiple Sclerosis.

Regenerative medicines that promote remyelination in multiple sclerosis (MS) are making the transition from laboratory to clinical trials. While animal models provide the experimental flexibility to analyze mechanisms of remyelination, here we discuss the challenges in understanding where and how remyelination occurs in MS.The authors acknowledge the following support: The UK Multiple Sclerosis Society (RTK, CZ, RJMF), The Adelson Medical Research Foundation (DSR, DEB, RJMF), Intramural Research Program of NINDS/NIH (DSR), European Research Council (ERC) under the European Union Horizon 2020 Re- search and Innovation Program (RTK), The Lister Institute (RTK), and a core support grant from the Wellcome and MRC to the Wellcome-Medical Research Council Cambridge Stem Cell Institute (RTK, RJMF)

Changes in the Oligodendrocyte Progenitor Cell Proteome with Ageing.

Following central nervous system (CNS) demyelination, adult oligodendrocyte progenitor cells (OPCs) can differentiate into new myelin-forming oligodendrocytes in a regenerative process called remyelination. Although remyelination is very efficient in young adults, its efficiency declines progressively with ageing. Here we performed proteomic analysis of OPCs freshly isolated from the brains of neonate, young and aged female rats. Approximately 50% of the proteins are expressed at different levels in OPCs from neonates compared with their adult counterparts. The amount of myelin-associated proteins, and proteins associated with oxidative phosphorylation, inflammatory responses and actin cytoskeletal organization increased with age, whereas cholesterol-biosynthesis, transcription factors and cell cycle proteins decreased. Our experiments provide the first ageing OPC proteome, revealing the distinct features of OPCs at different ages. These studies provide new insights into why remyelination efficiency declines with ageing and potential roles for aged OPCs in other neurodegenerative diseases

Problems and Pitfalls of Identifying Remyelination in Multiple Sclerosis

The authors acknowledge the following support: The UK Multiple Sclerosis Society (MS50 to R.T.K., C.Z., and R.J.M.F.), The Adelson Medical Research Foundation (D.S.R., D.E.B., and R.J.M.F.), Intramural Research Program of NINDS/NIH (D.S.R.), European Research Council (ERC) under the European Union Horizon 2020 Research and Innovation Program (771411 to R.T.K.), The Lister Institute (R.T.K.), and a core support grant from the Wellcome and MRC to the Wellcome-Medical Research Council Cambridge Stem Cell Institute (203151/Z/16/Z to R.T.K. and R.J.M.F.). Publisher Copyright: © 2020 Elsevier Inc.Regenerative medicines that promote remyelination in multiple sclerosis (MS) are making the transition from laboratory to clinical trials. While animal models provide the experimental flexibility to analyze mechanisms of remyelination, here we discuss the challenges in understanding where and how remyelination occurs in MS.Peer reviewe

b-Lactam antibiotics offer neuroprotection by increasing glutamate transporter expression

Glutamate is the principal excitatory neurotransmitter in the nervous system. Inactivation of synaptic glutamate is handled by the glutamate transporter GLT1 (also known as EAAT2; refs 1, 2), the physiologically dominant astroglial protein. In spite of its critical importance in normal and abnormal synaptic activity, no practical pharmaceutical can positively modulate this protein. Animal studies show that the protein is important for normal excitatory synaptic transmission, while its dysfunction is implicated in acute and chronic neurological disorders, including amyotrophic lateral sclerosis (ALS) 3 , stroke 4 , brain tumours 5 and epilepsy To identify compounds capable of increasing rodent GLT1 expression, a structurally diverse library of 1,040 FDA-approved drugs and nutritionals were individually added to organotypic spinal cord slice cultures prepared from postnatal day 9 rats To better understand the mechanism of action, the effect of the drugs on the GLT1 promoter was examined in cell lines fro

Synaptic signaling between GABAergic interneurons and oligodendrocyte precursor cells in the hippocampus. Nat. Neurosci 7:24–32

Glia express receptors for many neurotransmitters 1 , which enables neurons to communicate rapidly with surrounding glia and allows glia to assess the level of neuronal activity. This neuron-to-glia cell signaling influences the differentiation of glia progenitors 2,3 , maintains astroglia ensheathment of synapses 4 and regulates the efficacy of inhibitory transmission 5 . Neurotransmitter receptors on glia have also been implicated in several human pathologies, as they regulate the growth and migration of gliomas 6 and predispose preterm infants to ischemia-induced damage to white matter 7 . The presence of receptors on glia represents an additional site of action for many therapeutics. Despite the universal expression of these receptors by glia and their demonstrated involvement in essential physiological processes, little is known about the patterns of neuronal activity that are necessary to elicit their activation in situ and the dynamics of receptor signaling that result. Such information is essential for understanding when these forms of signaling occur and how they influence the physiology of glia. Direct communication between neurons and glia progenitor cells seems to be important for the continued generation of glia in the adult central nervous system, as neuronal activity and factors derived from neurons influence both the proliferation 8,9 and maturation of glia Rapid and precise signaling between neurons is ensured by the distribution of receptor-ion channels in clusters at synapses, the rapid dilution of transmitter in the extracellular space, receptor desensitization and isolation through astroglia ensheathment 4 and through the activity of transporters 17 . Although transmitter can reach receptors on astroglia by spilling out of the synaptic cleft Synaptic signaling between GABAergic interneurons and oligodendrocyte precursor cells in the hippocampus Shih-chun Lin & Dwight E Bergles Oligodendrocyte precursor cells (OPCs) express receptors for many neurotransmitters, but the mechanisms responsible for their activation are poorly understood. We have found that quantal release of GABA from interneurons elicits GABA A receptor currents with rapid rise times in hippocampal OPCs. These currents did not exhibit properties of spillover transmission or release by transporters, and immunofluorescence and electron microscopy suggest that interneuronal terminals are in direct contact with OPCs, indicating that these GABA currents are generated at direct interneuron-OPC synapses. The reversal potential of OPC GABA A currents was -43 mV, and interneuronal firing was correlated with transient depolarizations induced by GABA A receptors; however, GABA application induced a transient inhibition of currents mediated by AMPA receptors in OPCs. These results indicate that OPCs are a direct target of interneuronal collaterals and that the GABA-induced Cl -flux generated by these events may influence oligodendrocyte development by regulating the efficacy of glutamatergic signaling in OPCs