Targeting the TCA cycle can ameliorate widespread axonal energy deficiency in neuroinflammatory lesions.

Inflammation in the central nervous system can impair the function of neuronal mitochondria and contributes to axon degeneration in the common neuroinflammatory disease multiple sclerosis (MS). Here we combine cell-type-specific mitochondrial proteomics with in vivo biosensor imaging to dissect how inflammation alters the molecular composition and functional capacity of neuronal mitochondria. We show that neuroinflammatory lesions in the mouse spinal cord cause widespread and persisting axonal ATP deficiency, which precedes mitochondrial oxidation and calcium overload. This axonal energy deficiency is associated with impaired electron transport chain function, but also an upstream imbalance of tricarboxylic acid (TCA) cycle enzymes, with several, including key rate-limiting, enzymes being depleted in neuronal mitochondria in experimental models and in MS lesions. Notably, viral overexpression of individual TCA enzymes can ameliorate the axonal energy deficits in neuroinflammatory lesions, suggesting that TCA cycle dysfunction in MS may be amendable to therapy.

Synaptogenic gene therapy with FGF22 improves circuit plasticity and functional recovery following spinal cord injury.

Functional recovery following incomplete spinal cord injury (SCI) depends on the rewiring of motor circuits during which supraspinal connections form new contacts onto spinal relay neurons. We have recently identified a critical role of the presynaptic organizer FGF22 for the formation of new synapses in the remodeling spinal cord. Here, we now explore whether and how targeted overexpression of FGF22 can be used to mitigate the severe functional consequences of SCI. By targeting FGF22 expression to either long propriospinal neurons, excitatory interneurons, or a broader population of interneurons, we establish that FGF22 can enhance neuronal rewiring both in a circuit-specific and comprehensive way. We can further demonstrate that the latter approach can restore functional recovery when applied either on the day of the lesion or within 24 h. Our study thus establishes viral gene transfer of FGF22 as a new synaptogenic treatment for SCI and defines a critical therapeutic window for its application.

Coordinated neurostimulation promotes circuit rewiring and unlocks recovery after spinal cord injury.

Functional recovery after incomplete spinal cord injury depends on the effective rewiring of neuronal circuits. Here, we show that selective chemogenetic activation of either corticospinal projection neurons or intraspinal relay neurons alone led to anatomically restricted plasticity and little functional recovery. In contrast, coordinated stimulation of both supraspinal centers and spinal relay stations resulted in marked and circuit-specific enhancement of neuronal rewiring, shortened EMG latencies, and improved locomotor recovery.

Selective plasticity of callosal neurons in the adult contralesional cortex following murine traumatic brain injury.

Traumatic brain injury (TBI) results in deficits that are often followed by recovery. The contralesional cortex can contribute to this process but how distinct contralesional neurons and circuits respond to injury remains to be determined. To unravel adaptations in the contralesional cortex, we used chronic in vivo two-photon imaging. We observed a general decrease in spine density with concomitant changes in spine dynamics over time. With retrograde co-labeling techniques, we showed that callosal neurons are uniquely affected by and responsive to TBI. To elucidate circuit connectivity, we used monosynaptic rabies tracing, clearing techniques and histology. We demonstrate that contralesional callosal neurons adapt their input circuitry by strengthening ipsilateral connections from pre-connected areas. Finally, functional in vivo two-photon imaging demonstrates that the restoration of pre-synaptic circuitry parallels the restoration of callosal activity patterns. Taken together our study thus delineates how callosal neurons structurally and functionally adapt following a contralateral murine TBI.

A deep learning-based toolbox for Automated Limb Motion Analysis (ALMA) in murine models of neurological disorders.

In neuroscience research, the refined analysis of rodent locomotion is complex and cumbersome, and access to the technique is limited because of the necessity for expensive equipment. In this study, we implemented a new deep learning-based open-source toolbox for Automated Limb Motion Analysis (ALMA) that requires only basic behavioral equipment and an inexpensive camera. The ALMA toolbox enables the consistent and comprehensive analyses of locomotor kinematics and paw placement and can be applied to neurological conditions affecting the brain and spinal cord. We demonstrated that the ALMA toolbox can (1) robustly track the evolution of locomotor deficits after spinal cord injury, (2) sensitively detect locomotor abnormalities after traumatic brain injury, and (3) correctly predict disease onset in a multiple sclerosis model. We, therefore, established a broadly applicable automated and standardized approach that requires minimal financial and time commitments to facilitate the comprehensive analysis of locomotion in rodent disease models.

Semaphorin7A: its role in the control of serotonergic circuits and functional recovery following spinal cord injury.

Serotonin is a monoamine neurotransmitter synthetized in various populations of brainstem neurons. In the spinal cord, descending serotonergic projections regulate postural muscle tone, locomotion and rhythm and coordination of movements via the Central Pattern Generator. Following a spinal cord injury, serotonergic projections to the lumbar spinal cord, where the Central Pattern Generators are located, are interrupted resulting in devastating locomotor impairments and changes in the expression and activation of serotonin and its spinal receptors. The molecular cues that control the precise patterning and targeting of serotonergic inputs onto Central Pattern Generator networks in healthy animals or after injury are still unknown. In our recent research work, we have been particularly interested in Semaphorin7A, which belongs to the Semaphorins family involved in guiding growing axons and controlling plasticity of synaptic connections. In this review, we discuss the role of Semaphorin7A signaling as an important molecular actor that instructs the patterning of serotonin inputs to spinal Central Pattern Generator networks. We show that Semaphorin7A controls the wiring of descending serotonin axons in the spinal cord. Our results reveal that mistargetting of serotonin fibers in the spinal cord is compensated in healthy uninjured Semaphorin7A deficient mice so that their gross locomotion proceeds accurately. We also demonstrate that when the system is challenged with a spinal lesion, the pattern of post-injury serotonin expression is significantly altered in Semaphorin7A deficient mice with specific ectopic targeting of serotonin fibers in the lumbar spinal cord. Compensatory mechanisms in place in uninjured Semaphorin7A deficient mice are lost and injured Semaphorin7A deficient mice exhibit a worsening of their post-injury locomotor abilities. Our findings identify Semaphorin7A as a critical determinant of serotonergic circuit formation in healthy or spinal cord injured mice.

Chemogenetic approaches to unravel circuit wiring and related behavior after spinal cord injury.

A critical shortcoming of the central nervous system is its limited ability to repair injured nerve connections. Trying to overcome this limitation is not only relevant to understand basic neurobiological principles but also holds great promise to advance therapeutic strategies related, in particular, to spinal cord injury (SCI). With barely any SCI patients re-gaining complete neurological function, there is a high need to understand how we could target and improve spinal plasticity to re-establish neuronal connections into a functional network. The development of chemogenetic tools has proven to be of great value to understand functional circuit wiring before and after injury and to correlate novel circuit formation with behavioral outcomes. This review covers commonly used chemogenetic approaches based on metabotropic receptors and their use to improve our understanding of circuit wiring following spinal cord injury.

Semaphorin 7A restricts serotonergic innervation and ensures recovery after spinal cord injury.

Descending serotonergic (5-HT) projections originating from the raphe nuclei form an important input to the spinal cord that control basic locomotion. The molecular signals that control this projection pattern are currently unknown. Here, we identify Semaphorin7A (Sema7A) as a critical cue that restricts serotonergic innervation in the spinal cord. Sema7A deficient mice show a marked increase in serotonergic fiber density in all layers of the spinal cord while the density of neurons expressing the corresponding 5-HTR2α receptor remains unchanged. These alterations appear to be successfully compensated as no obvious changes in rhythmic locomotion and skilled stepping are observed in adult mice. When the system is challenged with a spinal lesion, serotonergic innervation patterns in both Sema7A-deficient and -competent mice evolve over time with excessive innervation becoming most pronounced in the dorsal horn of Sema7A-deficient mice. These altered serotonergic innervation patterns correlate with diminished functional recovery that predominantly affects rhythmic locomotion. Our findings identify Sema7A as a critical regulator of serotonergic circuit formation in the injured spinal cord.

Formation of somatosensory detour circuits mediates functional recovery following dorsal column injury.

Anatomically incomplete spinal cord injuries can be followed by functional recovery mediated, in part, by the formation of intraspinal detour circuits. Here, we show that adult mice recover tactile and proprioceptive function following a unilateral dorsal column lesion. We therefore investigated the basis of this recovery and focused on the plasticity of the dorsal column-medial lemniscus pathway. We show that ascending dorsal root ganglion (DRG) axons branch in the spinal grey matter and substantially increase the number of these collaterals following injury. These sensory fibers exhibit synapsin-positive varicosities, indicating their integration into spinal networks. Using a monosynaptic circuit tracing with rabies viruses injected into the cuneate nucleus, we show the presence of spinal cord neurons that provide a detour pathway to the original target area of DRG axons. Notably the number of contacts between DRG collaterals and those spinal neurons increases by more than 300% after injury. We then characterized these interneurons and showed that the lesion triggers a remodeling of the connectivity pattern. Finally, using re-lesion experiments after initial remodeling of connections, we show that these detour circuits are responsible for the recovery of tactile and proprioceptive function. Taken together our study reveals that detour circuits represent a common blueprint for axonal rewiring after injury.

Corticospinal circuit remodeling after central nervous system injury is dependent on neuronal activity.

The remodeling of supraspinal axonal circuits mediates functional recovery after spinal cord injury. This process critically depends on the selection of appropriate synaptic connections between cortical projection and spinal relay neurons. To unravel the principles that guide this target selection, we used genetic and chemogenetic tools to modulate NMDA receptor (NMDAR) integrity and function, CREB-mediated transcription, and neuronal firing of relay neurons during injury-induced corticospinal remodeling. We show that NMDAR signaling and CREB-mediated transcription maintain nascent corticospinal tract (CST)-relay neuron contacts. These activity-dependent signals act during a defined period of circuit remodeling and do not affect mature or uninjured circuits. Furthermore, chemogenetic modulation of relay neuron activity reveals that the regrowing CST axons select their postsynaptic partners in a competitive manner and that preventing such activity-dependent shaping of corticospinal circuits limits motor recovery after spinal cord injury.

Combining molecular intervention with in vivo imaging to untangle mechanisms of axon pathology and outgrowth following spinal cord injury.

In vivo imaging of the spinal cord has allowed the observation of single axons over relatively long periods in the living mouse. After spinal cord injury, this methodology has helped to differentiate several pathological stages and tissue processes which impact axon morphology. In addition, the combination of in vivo imaging techniques with particular molecular intervention has shown that specific pathological axon changes can respond to distinct treatments. Combining in vivo imaging with molecular interventions is, hence, a powerful approach to extend our knowledge of the pathological processes leading to axonal loss. It also allows testing possible treatment options to, for example, increase axonal outgrowth. This review will provide a detailed description and critical examination of several studies that have combined the two methodologies in spinal cord injury research and pinpoints the specificities of the approach.

Four N-(E)-cinnamoyl (cinnamamide) derivatives of aminoalkanols with promising anticonvulsant and analgesic activity.

Epilepsy and neuropathic pain are frequent neurological disorders with pathomechanism based on abnormal neuronal discharges. Secondary tissue impairment observed after traumatic brain injury is also connected with neuronal dysfunction. Those three neurological disorders are ineffectively treated with currently available pharmacotherapy options so great effort is made in searching for new effective drugs. Four N-(E)-cinnamoyl (cinnamamide) derivatives of aminoalkanols: S-(2E)-N-(1-hydroxypropan-2-yl)-3-(2-methylphenyl)prop-2-enamide (1), R,S-(2E)-3-(4-chlorophenyl)-N-(1-hydroxybutan-2-yl)prop-2-enamide (2), R,S-(2E)-3-(4-chlorophenyl)-N-(2-hydroxypropyl)prop-2-enamide (3), (2E)-3-(4-chlorophenyl)-N-(4-hydroxycyclohexyl)prop-2-enamide (4) were evaluated in vivo and in vitro for anticonvulsant, neuroprotective and/or analgesic activity. In intravenous metrazol seizure threshold test compounds 1-3 did not show pro-convulsive effect but proved anticonvulsant potential. In corneal kindled mice model the tested compounds showed beneficial anticonvulsant properties with ED50 of 36.8 mg/kg for 1, 25.7 mg/kg for 2, and 51.1 mg/kg for 3. Compound 2 tested in vitro in spontaneously bursting rat hippocampal slice model significantly reduced burst rate. Compounds 1 and 2 did not decrease lesion volume in acute model of traumatic brain injury. In formalin test of hyperalgesia in mice, compound 1 was active in the acute phase of the test, while compound 4 caused reduction of the time of licking of the affected paw by approx. 88% during the acute phase and 100% during the inflammatory phase. In rat sciatic ligation model of neuropathic pain, compound 1 significantly increased the paw withdrawal threshold starting from one hour after oral administration and the activity continued up to six hours. Reported here four N-(E)-cinnamoyl derivatives of aminoalkanols possess promising activity as anticonvulsant and/or analgesic agents.

Rehabilitation following spinal cord injury: how animal models can help our understanding of exercise-induced neuroplasticity.

Spinal cord injury is a devastating condition that is followed by long and often unsuccessful recovery after trauma. The state of the art approach to manage paralysis and concomitant impairments is rehabilitation, which is the only strategy that has proven to be effective and beneficial for the patients over the last decades. How rehabilitation influences the remodeling of spinal axonal connections in patients is important to understand, in order to better target these changes and define the optimal timing and onset of training. While clinically the answers to these questions remain difficult to obtain, rodent models of rehabilitation like bicycling, treadmill training, swimming, enriched environments or wheel running that mimic clinical rehabilitation can be helpful to reveal the axonal changes underlying motor recovery. This review will focus on the different animal models of spinal cord injury rehabilitation and the underlying changes in neuronal networks that are improved by exercise and rehabilitation.

Enhanced Voluntary Exercise Improves Functional Recovery following Spinal Cord Injury by Impacting the Local Neuroglial Injury Response and Supporting the Rewiring of Supraspinal Circuits.

Recent reports suggest that rehabilitation measures that increase physical activity of patients can improve functional outcome after incomplete spinal cord injuries (iSCI). To investigate the structural basis of exercise-induced recovery, we examined local and remote consequences of voluntary wheel training in spinal cord injured female mice. In particular, we explored how enhanced voluntary exercise influences the neuronal and glial response at the lesion site as well as the rewiring of supraspinal tracts after iSCI. We chose voluntary exercise initiated by providing mice with free access to running wheels over "forced overuse" paradigms because the latter, at least in some cases, can lead to worsening of functional outcomes after SCI. Our results show that mice extensively use their running wheels not only before but also after injury reaching their pre-lesion exercise levels within five days after injury. Enhanced voluntary exercise improved their overall and skilled motor function after injury. In addition, exercising mice started to recover earlier and reached better sustained performance levels. These improvements in motor performance are accompanied by early changes of axonal and glial response at the lesion site and persistent enhancements of the rewiring of supraspinal connections that resulted in a strengthening of both indirect and direct inputs to lumbar motoneurons.

Heterotopic Transcallosal Projections Are Present throughout the Mouse Cortex.

Transcallosal projection neurons are a population of pyramidal excitatory neurons located in layers II/III and to a lesser extent layer V of the cortex. Their axons form the corpus callosum thereby providing an inter-hemispheric connection in the brain. While transcallosal projection neurons have been described in some detail before, it is so far unclear whether they are uniformly organized throughout the cortex or whether different functional regions of the cortex contain distinct adaptations of their transcallosal connectivity. To address this question, we have therefore conducted a systematic analysis of transcallosal projection neurons and their axons across six distinct stereotactic coordinates in the mouse cortex that cover different areas of the motor and somatosensory cortices. Using anterograde and retrograde tracing techniques, we found that in agreement with previous studies, most of the transcallosal projections show a precise homotopic organization. The somata of these neurons are predominantly located in layer II/III and layer V but notably smaller numbers of these cells are also found in layer IV and layer VI. In addition, regional differences in the distribution of their somata and the precision of their projections exist indicating that while transcallosal neurons show a uniform organization throughout the mouse cortex, there is a sizeable fraction of these connections that are heterotopic. Our study thus provides a comprehensive characterization of transcallosal connectivity in different cortical areas that can serve as the basis for further investigations of the establishment of inter-hemispheric projections in development and their alterations in disease.

Physicochemical and biological evaluation of a cinnamamide derivative R,S-(2E)-1-(3-hydroxypiperidin-1-yl)-3-phenylprop-2-en-1-one (KM-608) for nervous system disorders.

A cinnamamide scaffold has been successfully incorporated in several compounds possessing desirable pharmacological activities in central and peripheral nervous system such as anticonvulsant, antidepressant, neuroprotective, analgesic, anti-inflammatory, muscle relaxant, and sedative/hypnotic properties. R,S-(2E)-1-(3-hydroxypiperidin-1-yl)-3-phenylprop-2-en-1-one (KM-608), a cinnamamide derivative, was synthesized, its chemical structure was confirmed by means of spectroscopy and crystallography, and additionally, thermal analysis showed that it exists in one crystalline form. The compound was evaluated in vivo in rodents as anticonvulsant, antiepileptogenic, analgesic, and neuroprotective agent. The beneficial properties of the compound were found in animal models of seizures evoked electrically (maximal electroshock test, 6-Hz) and chemically (subcutaneous pentylenetetrazole seizure test) as well as in three animal models of epileptogenesis: corneal-kindled mice, hippocampal-kindled rats, and lamotrigine-resistant amygdala-kindled rats. Quantitative pharmacological parameters calculated for the tested compound were comparable to those of currently used antiepileptic drugs. In vivo pharmacological profile of KM-608 corresponds with the activity of valproic acid.

FGF22 signaling regulates synapse formation during post-injury remodeling of the spinal cord.

The remodeling of axonal circuits after injury requires the formation of new synaptic contacts to enable functional recovery. Which molecular signals initiate such axonal and synaptic reorganisation in the adult central nervous system is currently unknown. Here, we identify FGF22 as a key regulator of circuit remodeling in the injured spinal cord. We show that FGF22 is produced by spinal relay neurons, while its main receptors FGFR1 and FGFR2 are expressed by cortical projection neurons. FGF22 deficiency or the targeted deletion of FGFR1 and FGFR2 in the hindlimb motor cortex limits the formation of new synapses between corticospinal collaterals and relay neurons, delays their molecular maturation, and impedes functional recovery in a mouse model of spinal cord injury. These results establish FGF22 as a synaptogenic mediator in the adult nervous system and a crucial regulator of synapse formation and maturation during post-injury remodeling in the spinal cord.

Multiparametric optical analysis of mitochondrial redox signals during neuronal physiology and pathology in vivo.

Mitochondrial redox signals have a central role in neuronal physiology and disease. Here we describe a new optical approach to measure fast redox signals with single-organelle resolution in living mice that express genetically encoded redox biosensors in their neuronal mitochondria. Moreover, we demonstrate how parallel measurements with several biosensors can integrate these redox signals into a comprehensive characterization of mitochondrial function. This approach revealed that axonal mitochondria undergo spontaneous 'contractions' that are accompanied by reversible redox changes. These contractions are amplified by neuronal activity and acute or chronic neuronal insults. Multiparametric imaging reveals that contractions constitute respiratory chain-dependent episodes of depolarization coinciding with matrix alkalinization, followed by uncoupling. In contrast, permanent mitochondrial damage after spinal cord injury depends on calcium influx and mitochondrial permeability transition. Thus, our approach allows us to identify heterogeneity among physiological and pathological redox signals, correlate such signals to functional and structural organelle dynamics and dissect the underlying mechanisms.