Requirement for Dicer in Maintenance of Monosynaptic Sensory-Motor Circuits in the Spinal Cord

SummaryIn contrast to our knowledge of mechanisms governing circuit formation, our understanding of how neural circuits are maintained is limited. Here, we show that Dicer, an RNaseIII protein required for processing microRNAs (miRNAs), is essential for maintenance of the spinal monosynaptic stretch reflex circuit in which group Ia proprioceptive sensory neurons form direct connections with motor neurons. In postnatal mice lacking Dicer in proprioceptor sensory neurons, there are no obvious defects in specificity or formation of monosynaptic sensory-motor connections. However, these circuits degrade through synapse loss and retraction of proprioceptive axonal projections from the ventral spinal cord. Peripheral terminals are also impaired without retracting from muscle targets. Interestingly, despite these central and peripheral axonal defects, proprioceptive neurons survive in the absence of Dicer-processed miRNAs. These findings reveal that Dicer, through its production of mature miRNAs, plays a key role in the maintenance of monosynaptic sensory-motor circuits

RhoA is dispensable for axon guidance of sensory neurons in the mouse dorsal root ganglia

RhoA, a member of the Rho family small GTPases, has been shown to play important roles in axon guidance. However, to date, the physiological function of RhoA in axon guidance events in vivo has not been determined genetically in animals. Here we show that RhoA mRNA is strongly expressed by sensory neurons in the developing mouse dorsal root ganglia (DRG). We have deleted RhoA in sensory neurons of the DRG using RhoA-floxed mice under the Wnt1-Cre driver in which Cre is strongly expressed in sensory neurons. Peripheral projections of sensory neurons appear normal and there are no detectable defects in the central projections of either cutaneous or proprioceptive sensory neurons in RhoAf/f; Wnt1-Cre mice. Furthermore, a co-culture assay using DRG explants from RhoAf/f; Wnt1-Cre embryos, and 293T cells expressing semaphorin3A (Sema3A) reveals that RhoA is not required for Sema3A-mediated axonal repulsion of sensory neurons. Expression of RhoC, a closely related family member, is increased in RhoA-deficient sensory neurons and may play a compensatory role in this context. Taken together, these genetic studies demonstrate that RhoA is dispensable for peripheral and central projections of sensory neurons in the DRG

Deletion of Sema3a or plexinA1/plexinA3 causes defects in sensory afferent projections of statoacoustic ganglion neurons.

Statoacoustic ganglion (SAG) neurons project sensory afferents to appropriate targets in the inner ear to form functional vestibular and auditory circuits. Neuropilin1 (Npn1), a receptor for class 3 semaphorins, is required to generate appropriate afferent projections in SAG neurons; however, the ligands and coreceptors involved in Npn1 functioning remain unknown. Here we show that both plexinA1 and plexinA3 are expressed by SAG neurons, and plexinA1/plexinA3 double mutant mice show defects in afferent projections of SAG neurons in the inner ear. In control mice, sensory afferents of SAG neurons terminate at the vestibular sensory patches, whereas in plexinA1/plexinA3 double mutants, they extend more dorsally in the inner ear beyond normal vestibular target areas. Moreover, we find that semaphorin3a (Sema3a) is expressed in the dorsal otocyst, and Sema3a mutant mice show defects in afferent projections of SAG neurons similar to those observed in plexinA1/plexinA3 double mutants and in mice lacking a functional Npn1 receptor. Taken together, these genetic findings demonstrate that Sema3a repellent signaling plays a role in the establishment of proper afferent projections in SAG neurons, and this signaling likely occurs through a receptor complex involving Npn1 and either plexinA1 or plexinA3

Real-Time Tsunami Prediction System Using DONET

We constructed a real-time tsunami prediction system using the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET). This system predicts the arrival time of a tsunami, the maximum tsunami height, and the inundation area around coastal target points by extracting the proper fault models from 1,506 models based on the principle of tsunami amplification. Since DONET2, installed in the Nankai earthquake rupture zone, was constructed in 2016, it has been used in addition to DONET1 installed in the Tonankai earthquake rupture zone; we revised the system using both DONET1 and DONET2 to improve the accuracy of tsunami prediction. We introduced a few methods to improve the prediction accuracy. One is the selection of proper fault models from the entire set of models considering the estimated direction of the hypocenter using seismic and tsunami data. Another is the dynamic selection of the proper DONET observatories: only DONET observatories located between the prediction point and tsunami source are used for prediction. Last is preparation for the linked occurrence of double tsunamis with a time-lag. We describe the real-time tsunami prediction system using DONET and its implementation for the Shikoku area

Synapse Formation in Monosynaptic Sensory–Motor Connections Is Regulated by Presynaptic Rho GTPase Cdc42

Spinal reflex circuit development requires the precise regulation of axon trajectories, synaptic specificity, and synapse formation. Of these three crucial steps, the molecular mechanisms underlying synapse formation between group Ia proprioceptive sensory neurons and motor neurons is the least understood. Here, we show that the Rho GTPase Cdc42 controls synapse formation in monosynaptic sensory–motor connections in presynaptic, but not postsynaptic, neurons. In mice lacking Cdc42 in presynaptic sensory neurons, proprioceptive sensory axons appropriately reach the ventral spinal cord, but significantly fewer synapses are formed with motor neurons compared with wild-type mice. Concordantly, electrophysiological analyses show diminished EPSP amplitudes in monosynaptic sensory–motor circuits in these mutants. Temporally targeted deletion of Cdc42 in sensory neurons after sensory–motor circuit establishment reveals that Cdc42 does not affect synaptic transmission. Furthermore, addition of the synaptic organizers, neuroligins, induces presynaptic differentiation of wild-type, but not Cdc42-deficient, proprioceptive sensory neurons in vitro. Together, our findings demonstrate that Cdc42 in presynaptic neurons is required for synapse formation in monosynaptic sensory–motor circuits

<i>PlexinA1</i> and <i>plexinA3</i> are required for proper sensory innervation of the inner ear.

<p>Some SAG neurons projected beyond the sensory patches and extended dorsally in E13.5 <i>plexinA1</i>^+/−; <i>plexinA3</i>^+/− (A, B), <i>plexinA1</i>^+/−; <i>plexinA3</i>^null (C) and <i>plexinA1</i>^−/−; <i>plexinA3</i>^null (D) embryos (arrowheads). AC, anterior crista; Co, cochlea; HC, horizontal crista; PC, posterior crista; S, saccule; U, utricle. Scale bar, 500 µm.</p

Incidence of phenotypes exhibiting overprojections of SAG neurons in <i>plexinA1</i>/<i>plexinA3</i>-knockout embryos at E13.5.

<p>Incidence of phenotypes exhibiting overprojections of SAG neurons in <i>plexinA1</i>/<i>plexinA3</i>-knockout embryos at E13.5.</p

Expression patterns of <i>Npn1</i> and <i>plexinAs</i> in the developing inner ear at E11.5.

<p>(A) A schematic diagram of a typical mouse embryo showing the orientation of the SAG and the otocyst. (B) Expression of <i>Npn1</i> was found in SAG neurons (arrowheads). (C) Expression of <i>plexinA1</i> was found in SAG neurons and the otocyst (arrowheads). (D, E) <i>PlexinA2</i> and <i>plexinA3</i> were expressed in SAG neurons only (arrowheads). (F) <i>PlexinA4</i> transcripts were detected in the posterior part of the otocyst (arrowheads). Scale bar, 100 µm.</p

<i>Sema3a</i>, but not <i>Sema3e</i>, is required for proper sensory innervation of the inner ear.

<p>(A, B) Overall innervation patterns of <i>Sema3e</i>^−/− embryos appeared normal at E13.5. (C, D) Some SAG neurons projected beyond the sensory patches and extended dorsally in <i>Sema3a</i>^−/− embryos at E13.5 (arrowheads). AC, anterior crista; Co, cochlea; HC, horizontal crista; PC, posterior crista; S, saccule; U, utricle. Scale bar, 500 µm.</p