Biology subjects

Ruthazer, E. S.

Publications and source records attributed to Ruthazer, E. S..

10 recordsLinked to original sources

bioRxiv2025

Norepinephrine acts through radial astrocytes in the developing optic tectum to enhance threat detection and escape behavior

The ability to switch behavioral states is essential for animals to adapt and survive. We investigated how norepinephrine (NE) activation of radial astrocytes alters visual processing in the optic tectum (OT) of developing Xenopus laevis. NE activates calcium transients in radial astrocytes through 1-adrenergic receptors. NE and radial astrocyte activation shifted OT response selectivity to preferentially respond to looming stimuli, associated with predation threat. NE-mediated astrocytic release of ATP/adenosine reduced excitatory transmission by retinal ganglion cell axons, without affecting inhibitory transmission in the OT. Blockade of adenosine receptors prevented both decreased neurotransmission and the selectivity shift. Chemogenetic activation of tectal radial astrocytes reproduced NEs effects and enhanced behavioral detection of looming stimuli in freely swimming animals. NE signaling via radial astrocytes enhances network signal-to-noise for detecting threatening stimuli, with important implications for sensory processing and behavior.

neuroscience↗

bioRxiv2025

Parallel morphological and functional development in the Xenopus retinotectal system

The retinotectal projection in Xenopus laevis is topographically organized. During the early development of the Xenopus visual system, the optic tectum increases considerably in volume, and retinotectal axons and dendrites undergo extensive activity-dependent remodeling. We have previously observed marked changes in the three-dimensional layout of the tectal retinotopic functional map over the course of a few days. This raised the question of whether such functional reorganization might be attributable to the migration and structural remodeling of tectal neurons as the brain grows. To examine changes in map topography in the context of individual tectal neuron morphology and location, we performed calcium imaging in the optic tecta of GCaMP6s-expressing tadpoles in parallel with structural imaging of tectal cells that were sparsely labelled with Alexa 594-dextran dye. We performed functional and structural imaging of the optic tectum at two developmental time points, recording the morphology of the dextran-labelled cells and quantifying the changes in their positions and the spanning volume of their dendritic fields. Comparing anatomical growth to changes in the functional retinotopic map at these early stages, we found that dendritic arbor growth kept pace with the overall growth of the optic tectum, and that individual neurons continued to receive widespread visual field input, even as the tectal retinotopic map evolved markedly over time.

neuroscience↗

bioRxiv2021

BDNF signaling in Hebbian and Stentian structural plasticity in the developing visual system

During development, patterned neural activity instructs topographic map refinement. Axons with similar patterns of neural activity, converge onto target neurons and stabilize their synapses with these postsynaptic partners, restricting exploratory branch elaboration (Hebbian structural plasticity). On the other hand, non-correlated firing in inputs leads to synapse weakening and increased exploratory growth of axons (Stentian structural plasticity). We used visual stimulation to control the correlation structure of neural activity in a few ipsilaterally projecting (ipsi) retinal ganglion cell (RGC) axons with respect to the majority contralateral eye inputs in the optic tectum of albino Xenopus laevis tadpoles. Multiphoton live imaging of ipsi axons, combined with specific targeted disruptions of brain-derived neurotrophic factor (BDNF) signaling, revealed that both presynaptic p75NTR and TrkB are required for Stentian axonal branch addition, whereas presumptive postsynaptic BDNF signaling is necessary for Hebbian axon stabilization. Additionally, we found that BDNF signaling mediates local suppression of branch elimination in response to correlated firing of inputs. Daily In vivo imaging of contralateral RGC axons demonstrated that p75NTR knockdown reduces axon branch elongation and arbor spanning field volume.

neuroscience↗

bioRxiv2021

The effects of the NMDAR co-agonist D-serine on the structure and function of the optic tectum

The N-methyl-D-aspartate type glutamate receptor (NMDAR) is a molecular coincidence detector which converts correlated patterns of neuronal activity into cues for the structural and functional refinement of developing circuits in the brain. D-serine is an endogenous co-agonist of the NMDAR. In this study, we investigated the effects of potent enhancement of NMDAR-mediated currents by chronic administration of saturating levels of D-serine on the developing Xenopus retinotectal circuit. Chronic exposure to the NMDAR co-agonist D-serine resulted in structural and functional changes to the optic tectum. D-serine administration affected synaptogenesis and dendritic morphology in recently differentiated tectal neurons, resulting in increased arbor compaction, reduced branch dynamics, and higher synapse density. These effects were not observed in more mature neurons. Calcium imaging to examine retinotopic map organization revealed that tectal neurons of animals raised in D-serine had sharper visual receptive fields. These findings suggest that the availability of endogenous NMDAR co-agonists like D-serine at glutamatergic synapses may regulate the refinement of circuits in the developing brain. Significance statementN-methyl-D-aspartate receptors (NMDAR) are implicated in activity-dependent circuit plasticity. We used administration of the NMDAR co-agonist D-serine to further examine the role of the NMDAR in circuit development in vivo. D-serine stabilised dendritic arbors specifically of recently differentiated neurons, promoted synaptogenesis, and led to sharper retinotopic receptive fields in the optic tectum. Together, these results support the idea that signaling in response to synaptic current through NMDARs promotes the maturation of developing brain circuits.

neuroscience↗

bioRxiv2021

Activity-dependent alteration of early myelin ensheathment in a developing sensory circuit

Adaptive myelination has been reported in response to experimental manipulations of neuronal activity, but the links between sensory experience, corresponding neuronal activity, and resultant alterations in myelination require investigation. To study this, we used the Xenopus laevis tadpole, which is a classic model for studies of visual system development and function because it is translucent and visually responsive throughout the formation of this retinotectal system. Here, we report the timecourse of early myelin ensheathment in the Xenopus retinotectal system using immunohistochemistry of myelin basic protein (MBP) along with third-harmonic generation (THG) microscopy, a label-free structural imaging technique. Characterization of the myelination progression revealed an appropriate developmental window to address the effects of early patterned visual experience on myelin ensheathment. To alter patterned activity, we showed tadpoles stroboscopic stimuli and measured the calcium responses of retinal ganglion cell axon terminals. We identified strobe frequencies that elicited robust versus dampened calcium responses, reared animals in these strobe conditions for 7 d, and subsequently observed differences in the amount of early myelin ensheathment at the optic chiasm. This study provides evidence that it is not just the presence but also to the specific temporal properties of sensory stimuli that are important for myelin plasticity.

neuroscience↗

bioRxiv2020

Sodium-Calcium Exchanger Mediates Sensory-Evoked Glial Calcium Transients in the Developing Retinotectal System

Various types of sensory stimuli have been shown to induce calcium elevations in glia. However, a mechanistic understanding of the signalling pathways mediating sensory-evoked activity in glia in intact animals is still emerging. Here we demonstrate that during early development of the Xenopus laevis visual system, radial astrocytes in the optic tectum are highly responsive to sensory stimulation. Calcium transients occur spontaneously in radial astrocytes at rest and are abolished by silencing neuronal activity with tetrodotoxin. Visual stimulation drives temporally correlated increases in the activity patterns of neighbouring radial astrocytes. Following blockade of all glutamate receptors, visually-evoked calcium activity in radial astrocytes is enhanced, rather than suppressed, while the additional blockade of either glutamate transporters or sodium-calcium exchangers (NCX) fully prevents visually-evoked responses. Additionally, we demonstrate that blockade of NCX alone is sufficient to prevent visually-evoked responses in radial astrocytes, highlighting a pivotal role for NCX in glia during development.

neuroscience↗

bioRxiv2020

In vivo imaging of microglia-mediated axonal pruning and modulation by the complement system

Partial phagocytosis--called trogocytosis--of axons by microglia has been documented in ex vivo preparations but has not been directly observed in vivo. The mechanisms that modulate microglial trogocytosis of axons and its function in neural circuit development remain poorly understood. Here we directly observe axon trogocytosis by microglia in vivo in the developing Xenopus laevis retinotectal circuit. We show that microglia regulate pruning of retinal ganglion cell axons and are important for proper behavioral response to dark and bright looming stimuli. We identify amphibian regulator of complement activation 3, a homolog of human CD46, as a neuronally-expressed synapse-associated complement inhibitory molecule that inhibits trogocytosis and axonal pruning. Using a membrane-bound complement C3 fusion protein, we demonstrate that enhancing complement activity enhances axonal pruning. Our results support the model that microglia remodel axons via trogocytosis and that neurons can control this process through expression of complement inhibitory proteins.

neuroscience↗

bioRxiv2020

Improved genetically encoded near-infrared fluorescent calcium ion indicators for in vivo imaging

Near-infrared (NIR) genetically-encoded calcium ion (Ca2+) indicators (GECIs) can provide advantages over visible wavelength fluorescent GECIs in terms of reduced phototoxicity, minimal spectral cross-talk with visible-light excitable optogenetic tools and fluorescent probes, and decreased scattering and absorption in mammalian tissues. Our previously reported NIR GECI, NIR-GECO1, has these advantages but also has several disadvantages including lower brightness and limited fluorescence response compared to state-of-the-art visible wavelength GECIs, when used for imaging of neuronal activity. Here, we report two improved NIR GECI variants, designated NIR-GECO2 and NIR-GECO2G, derived from NIR-GECO1. We characterized the performance of the new NIR GECIs in cultured cells, acute mouse brain slices, and C. elegans and Xenopus laevis in vivo. Our results demonstrate that NIR-GECO2 and NIR-GECO2G provide substantial improvements over NIR-GECO1 for imaging of neuronal Ca2+ dynamics.

molecular biology↗

bioRxiv2019

Pre- and post-synaptic roles for DCC in memory consolidation in the adult mouse hippocampus.

The receptor deleted in colorectal cancer (DCC) and its ligand netrin-1 are essential for axon guidance during development and are expressed by neurons in the mature brain. Netrin-1 recruits GluA1-containing -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and is critical for long-term potentiation (LTP) at CA3-CA1 hippocampal Schaffer collateral synapses, while conditional DCC deletion from glutamatergic neurons impairs hippocampal-dependent spatial memory and severely disrupts LTP induction. DCC co-fractionates with the detergent-resistant component of the postsynaptic density, yet is enriched in axonal growth cones that differentiate into presynaptic terminals during development. Specific presynaptic and postsynaptic contributions of DCC to the function of mature neural circuits have yet to be identified. Employing hippocampal subregion-specific conditional deletion of DCC, we show that DCC loss from CA1 hippocampal pyramidal neurons results in deficits in spatial memory, increased resting membrane potential, abnormal dendritic spine morphology, and weaker spontaneous excitatory postsynaptic activity. In contrast, deletion of DCC from CA3 neurons did not induce detectable changes in spine morphology or intrinsic electrophysiological properties of CA1 pyramidal neurons, but resulted in impaired performance on the novel object place recognition task as well as compromised excitatory synaptic transmission and long-term potentiation (LTP) at the Schaffer collateral synapse. Together, these findings reveal that DCC makes specific pre- and post-synaptic contributions to hippocampal synaptic plasticity underlying spatial memory.

neuroscience↗

bioRxiv2018

Spatial memory formation requires netrin-1 expression by neurons in the adult mammalian brain.

Netrin-1 was initially characterized as an axon guidance molecule that is essential for normal embryonic neural development; however, many types of neurons continue to express netrin-1 in the post-natal and adult mammalian brain. Netrin-1 and the netrin receptor DCC are both enriched at synapses. In the adult hippocampus, activity-dependent secretion of netrin-1 by neurons potentiates glutamatergic synapse function, and is critical for long-term potentiation, an experimental cellular model of learning and memory. Here, we assessed the impact of neuronal expression of netrin-1 in the adult brain on behavior using tests of learning and memory. We show that adult mice exhibit impaired spatial memory following conditional deletion of netrin-1 from glutamatergic neurons in the hippocampus and neocortex. Further, we provide evidence that mice with conditional deletion of netrin-1 do not display aberrant anxiety-like phenotypes and show a reduction in self-grooming behaviour. These findings reveal a critical role for netrin-1 expressed by neurons in the regulation of spatial memory formation.

neuroscience↗