Modeling spinal locomotor circuits for movements in developing zebrafish

Many spinal circuits dedicated to locomotor control have been identified in the developing zebrafish. How these circuits operate together to generate the various swimming movements during development remains to be clarified. In this study, we iteratively built models of developing zebrafish spinal circuits coupled to simplified musculoskeletal models that reproduce coiling and swimming movements. The neurons of the models were based upon morphologically or genetically identified populations in the developing zebrafish spinal cord. We simulated intact spinal circuits as well as circuits with silenced neurons or altered synaptic transmission to better understand the role of specific spinal neurons. Analysis of firing patterns and phase relationships helped to identify possible mechanisms underlying the locomotor movements of developing zebrafish. Notably, our simulations demonstrated how the site and the operation of rhythm generation could transition between coiling and swimming. The simulations also underlined the importance of contralateral excitation to multiple tail beats. They allowed us to estimate the sensitivity of spinal locomotor networks to motor command amplitude, synaptic weights, length of ascending and descending axons, and firing behavior. These models will serve as valuable tools to test and further understand the operation of spinal circuits for locomotion. eLife digest The spinal cord is a column of nerve tissue that connects the brain to the rest of the body in vertebrate animals. Nerve cells in the spinal cord, called neurons, help to control and coordinate the body's movements. As the spinal cord develops, new neurons are born and new connections are made between neurons and muscles, resulting in more coordinated and skillful movements as time goes on. Zebrafish, for example, display body-bending maneuvers called coils within 24 hours of the egg being fertilized. Next, bursts of swimming movements emerge, which are driven by sporadic tail beats. These tail maneuvers become more consistent as the fish develops, and eventually result in smooth movements called beat-and-glide swimming. The groups of spinal cord neurons that appear at each stage of zebrafish development have been characterized, but it remains unclear how newly formed circuits (groups of neurons recently connected to each other) work together to produce swimming maneuvers. To answer this question, Roussel et al. simulated changes in the spinal cord that help zebrafish acquire new swimming movements as they grow. The computer models encoded neural circuits based on cell populations identified in experimental studies, and replicated swimming behaviors that emerge during the first few days of zebrafish development. Simulations tested how specific neural circuits generate the characteristic swimming movements that represent key developmental milestones in zebrafish. The results showed that adding new neurons and more cell-to-cell connections led to increasingly sophisticated swimming maneuvers. As the zebrafish spinal cord matured, the fish were better able to control the pace and duration of their swimming movements. Roussel et al. also identified specific patterns of neural activity linked to particular maneuvers. For example, tail beats switch direction when neurons on one side of the spinal cord excite neurons on the opposite side. This activity, which becomes more rhythmic, also needs to be exquisitely timed to produce and coordinate the right motion. Roussel et al.'s modelling of developmental milestones in growing zebrafish provides insights into how neural networks control movement. The computer models are among the first to accurately reproduce swimming behaviors in developing zebrafish. More experimental data could be added to the models to capture the full range of early zebrafish movements, and to further investigate how maturing spinal cord circuits control swimming. Since zebrafish and mammals have many spinal neurons in common, further research may aid our understanding of movement disorders in humans.

Uncovering the neural encoding of behavior in the adult Drosophila motor system

Chin-Lin Chen

One of the most important goals in neuroscience research has always been to understand how animals control their behavior. However, the long focus on the role of brain neurons in behavioral control might be missing the full story. In fact, brain-wide fluctuations in neural activity are highly correlated to the behavioral state, suggesting a closed-loop system where the behavioral command and the body state interact to interpret sensory inputs and dynamically guide action selection. However, to date, little is known about how and where information about the behavioral state is generated. In principle, information about the behavioral state is a relatively high-level encoding of the body state derived from the integration of signals from primary sensors and the local network. Local motor circuits residing in the spinal cord of mammals or the ventral nerve cord (VNC) of insects may give rise to the behavioral state and pass to the brain via brain-projecting interneurons---termed ascending neurons (ANs). However, what information ANs carry and where this information originates is still a mystery due to the technical challenges in measuring these signals in behaving animals. Therefore, it is critical to develop tools to track neuron identity and record neuronal activity during behavior to crack the functional network. In this work, a method is introduced for imaging the motor circuit in the VNC of Drosophila melanogaster. This preparation demonstrates how to retain fly limb function after dissection and allows simultaneous recording of a tethered fly's behavior and its neural activity in the VNC. Incorporating this methodology with other approaches can verify the conclusions from the experiments such as those manipulating neuronal excitability, and motivate further investigation into neuronal function in specific types of behavior. Since it has long been theorized that ANs are the cellular basis of the behavioral state information sent in the brain, the technique introduced in this work was designed to resolve questions relating to what type of information ANs convey. Here, a large-scale functional screen was performed to relate neural activity, behavior, and neuronal morphology for 247 genetically identified ANs in the adult fly. This is the first large-scale screen of neural activity in a behaving animal, and this work uncovered three fundamental features of AN populations. First, rather than low-level limb movements, ANs encode high-level behaviors like walking, grooming, and resting. Second, ANs project nearly exclusively to an integrative sensory center (AVLP) and an action selection hub (GNG). Remarkably, the encodings projected to these distinct areas reflect their potential roles in contextualizing sensory cues (AVLP) and guiding future actions (GNG). Third, the behavioral encodings of ANs are closely linked to their motor system patterning. In conclusion, this work provides an indispensable tool to study the motor system beyond the brain network by overcoming a major technical barrier in in-vivo recording of the VNC of a behaving fly. This tool was used to help resolve what is encoded by ANs as a higher-level behavioral state, suggesting that the primary cellular origin of the behavioral information that is used to modulate brain function comes from the VNC. Overall, the technique and the findings on AN encoding should inspire follow-up studies to uncover additional neural mechanisms used in adaptive behavior generation.

EPFL2022

Spinal V1 neurons inhibit motor targets locally and sensory targets distally

Yann Roussel

Rostro-caudal coordination of spinal motor output is essential for locomotion. Most spinal interneurons project axons longitudinally to govern locomotor output, yet their connectivity along this axis remains unclear. In this study, we use larval zebrafish to map synaptic outputs of a major inhibitory population, V1 (Eng1(+)) neurons, which are implicated in dual sensory and motor functions. We find that V1 neurons exhibit long axons extending rostrally and exclusively ipsilaterally for an average of 6 spinal segments; however, they do not connect uniformly with their post-synaptic targets along the entire length of their axon. Locally, V1 neurons inhibit motor neurons (both fast and slow) and other premotor targets, including V2a, V2b, and commissural premotor neurons. In contrast, V1 neurons make robust long-range inhibitory contacts onto a dorsal horn sensory population, the commissural primary ascending neurons (CoPAs). In a computational model of the ipsilateral spinal network, we show that this pattern of short-range V1 inhibition to motor and premotor neurons underlies burst termination, which is critical for coordinated rostro-caudal propagation of the loco motor wave. We conclude that spinal network architecture in the longitudinal axis can vary dramatically, with differentially targeted local and distal connections, yielding important consequences for function.

CELL PRESS2021

Transgenic SOD1 mice as a model for developing amyotrophic lateral sclerosis therapies

Sandrine Guillot

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by a loss of motor neurons in the brain (brainstem and cortex) and the spinal cord that leads to a motor neurological symptomatology. Approximately 10% of ALS cases have a familial form of ALS. Among the familial cases, 20% are caused by dominantly inherited mutations (around 100 different mutations) in the protein Cu/Zn superoxide dismutase (SOD1). To date, genetic ALS models on mutated SOD1 (SOD1G93A, SOD1G85R and SOD1G37R) transgenesis in rodents have been successful in recapitulating the main features of the human pathology, especially the loss of spinal or facial motoneurons, the increased astrogliosis, the activation of microglia and the cellular and molecular disruptions in motoneurons. The therapies toward a treatment or cure for ALS have been met with limited success. In the present study, SOD1G93A transgenic mice have been used to test new gene therapies to slow disease progression and pharmacological approach to obtain a better comprehension of the pathological process, more precisely, to define the involvement of microglial activation in the spread of the disease. For the approach by gene therapy, injection of viral vectors in a localized region of central nervous system (CNS) constitutes an excellent tool for the development of new therapies. In the present study, the viral-mediated expression of potential neuroprotective factors was explored in the lumbar spinal cord or facial nucleus of SOD1G93A transgenic mice. As HIV-1-derived lentiviral vectors can efficiently transduce neurons in CNS, these retroviral vectors were used to over-express the neurotrophic factor GDNF (glial cell line-derived neurotrophic factor) to supply a trophic support to lumbar and facial motoneurons, or RNAi molecules specifically targeting the SOD1G93A gene to inhibit the mutant SOD1 expression, the cause of the disease, in the lumbar spinal cord of SOD1G93A transgenic mice. For the study of the level of microglial involvement in the pathology, SOD1G93A transgenic mice had a daily pharmacological treatment with an inhibitor of microglial activation, the minocycline. The present thesis demonstrates that, on the one hand, at the difference of laboratories that delayed the disease progression by intramuscular injection of adenoviral vectors or adenoassociated vectors encoding for GDNF, the intraspinal injection of lentivirus encoding for GDNF did not lead to a protection of lumbar motoneurons and a delay of the disease. But this study showed a significantly rescue of facial motoneurons by the injection of lentivirus encoding for GDNF directly in facial nucleus. A selective vulnerability between motoneurons in facial nucleus and these in lumbar spinal cord has been therefore underlined. On the other hand, we showed that the therapeutic approach to inhibit the mutant SOD1 expression look as the most effective to delay the progression of the ALS pathology. Both studies of gene therapy, compared to others studies using the same therapeutic factors, demonstrated clearly that to optimize a therapeutic treatment, it seems better to act at muscular junction level with a retrograde transport form muscle to motoneuronal bodies of the therapeutic molecule or viral vector. Nevertheless, motoneuron-restricted silencing of mutant SOD1 would be useful for better understanding of neuronal and glial cellular mechanism mediating ALS-linked mutated SOD1 toxicity. Regarding the involvement of microglia in the pathological process, the inhibition of microglial activation by the minocycline treatment, suggests, on the one hand, that microglial activation do not initiate the pathological process and that this microgliosis might be a consequence and triggered by multiple disruptions. On the other hand, that minocycline could protect completely facial motoneurons and partially spinal motoneurons by its anti-apoptotic property. Therapeutic approaches aiming to act directly on the cause of the disease as well as a better comprehension of the role of non-neuronal cells in the pathological process may therefore open new perspectives for the treatment of ALS pathology.