Cellular/Molecular
Some Active Zones Exhibit Only Spontaneous Vesicle Release
Jan E. Melom, Yulia Akbergenova, Jeffrey P. Gavornik, and J. Troy Littleton
(see pages 17253–17263)
Synaptic vesicles release neurotransmitter not only when action potentials arrive at the synaptic terminal, but also when single vesicles spontaneously fuse with the membrane. The extent to which evoked and spontaneous release differ is unknown, but evidence that the two types of fusion use different regulatory proteins and/or vesicle pools indicates that spontaneous release may serve distinct functions. Melom et al. further support this hypothesis, suggesting that some synaptic active zones of Drosophila neuromuscular junctions (NMJs) are dedicated to spontaneous release. Using a membrane-tethered, calcium-sensitive form of green fluorescent protein (GCaMP5) to detect postsynaptic calcium influx in response to spontaneous and evoked neurotransmitter release, the authors found that ∼22% of active zones exhibited only spontaneous release. Both evoked and spontaneous release occurred at ∼40% of active zones, but the release probabilities for the two types of fusion were uncorrelated at these sites, suggesting they are independently regulated. These results support the possibility that spontaneous and evoked neurotransmission engage independent information channels.
Development/Plasticity/Repair
Thyroid Hormone Treatment Might Help Preterm Infants
Linnea R. Vose, Govindaiah Vinukonda, Sungro Jo, Omid Miry, Daniel Diamond, et al.
(see pages 17232–17246)
Intraventricular hemorrhage (IVH) occurs in 20–25% of preterm infants, and it is a major cause of mental retardation and cerebral palsy in these infants. IVH causes oxidative stress and inflammation resulting in hypomyelination, which is thought to underlie adverse neurological outcomes. Because oxidative stress reduces activation of thyroid hormone (TH), which promotes oligodendrocyte maturation and myelination, Vose et al. asked whether IVH disrupts TH-mediated signaling. Indeed, expression of type-2 deiodinase—the enzyme that converts the prohormone thyroxine (T4) to the active form, triiodothyronine (T3)—was lower in germinal matrix of IVH-exposed preterm infants than in controls, whereas expression of type-3 deiodinase—which inactivates T3—was elevated in IVH tissue. Similar changes in a rabbit model resulted in reduced expression of T3-dependent genes, including those encoding myelin proteins; T4 administration reversed these effects in rabbits and restored motor function. More importantly, TH treatment promoted oligodendrocyte maturation in preterm human infants that experienced IVH, indicating TH might improve neurological outcomes in these infants.
Systems/Circuits
Frog Olfactory Receptor Neurons Innervate Multiple Glomeruli
Thomas Hassenklöver and Ivan Manzini
(see pages 17247–17252)
In the mature olfactory system of rodents and zebrafish, each olfactory receptor neuron (ORN) expresses a single olfactory receptor gene and sends a single, unbranched axon to a single glomerulus in the main olfactory bulb. This projection pattern has been assumed to be conserved across all vertebrates. But Hassenklöver and Manzini now present evidence that in Xenopus laevis, ORN axons branch and often innervate two or more glomeruli in the olfactory bulb. This was not only true in tadpoles, but also in postmetamorphic frogs. Moreover, both mature and immature ORNs had branched axons. Similarly, axons of vomeronasal receptor neurons were branched and innervated multiple glomeruli in the accessory olfactory bulb. The functional consequences of this unique wiring pattern have yet to be revealed.
Neurobiology of Disease
TRPM2 Affects Survival by Regulating NMDA Receptor Expression
Ishraq Alim, Lucy Teves, Rongwen Li, Yasuo Mori, and Michael Tymianski
(see pages 17264–17277)
Ischemia causes a rapid increase in extracellular glutamate, which activates NMDA receptors (NMDARs), leading to excessive calcium influx and excitotoxic neuronal death. Two members of the transient receptor potential melastatin (TRPM) family of cation channels, TRPM7 and TRPM2, are activated by reactive oxygen species (ROS) during postischemic reperfusion and are thought to contribute to calcium influx and neuron death. Alim et al. used TRPM2-null mice to elucidate the role of this channel. As predicted, the extent of neuronal damage caused by transient ischemia and reperfusion was smaller in TRPM2-null mice than in controls. Unexpectedly, however, neuronal excitability was increased by the ROS H2O2 in TRPM2-null, but not wild-type mice. Increased excitability normally increases susceptibility to excitotoxic death, but this was counteracted in TRPM2-null mice by reduced expression of the death-promoting, extrasynaptic NMDAR subunit GluN2B and increased expression of the survival-promoting, synaptic NMDAR subunit GluN2A. Accordingly, GluN2A antagonist reversed the effects of TRPM2 knockout on neuronal excitability and survival after exposure to H2O2 or oxygen–glucose deprivation, and GluN2B antagonist increased survival of wild-type neurons.