Nonspecific labeling limits the utility of Cre-Lox bred CST-YFP mice for studies of corticospinal tract regeneration

Astroglial exosome HepaCAM signaling and ApoE antagonization coordinates early postnatal cortical pyramidal neuronal axon growth and dendritic spine formation

Developing astroglia play important roles in regulating synaptogenesis through secreted and contact signals. Whether they regulate postnatal axon growth is unknown. By selectively isolating exosomes using size-exclusion chromatography (SEC) and employing cell-type specific exosome reporter mice, our current results define a secreted astroglial exosome pathway that can spread long-range in vivo and stimulate axon growth of cortical pyramidal neurons. Subsequent biochemical and genetic studies found that surface expression of glial HepaCAM protein essentially and sufficiently mediates the axon-stimulating effect of astroglial exosomes. Interestingly, apolipoprotein E (ApoE), a major astroglia-secreted cholesterol carrier to promote synaptogenesis, strongly inhibits the stimulatory effect of astroglial exosomes on axon growth. Developmental ApoE deficiency also significantly reduces spine density of cortical pyramidal neurons. Together, our study suggests a surface contact mechanism of astroglial exosomes in regulating axon growth and its antagonization by ApoE, which collectively coordinates early postnatal pyramidal neuronal axon growth and dendritic spine formation.

Astroglial exosome HepaCAM signaling and ApoE antagonization coordinates early postnatal cortical pyramidal neuronal axon growth and dendritic spine formation

Developing astroglia play important roles in regulating synaptogenesis through secreted and contact signals. Whether they regulate postnatal axon growth is unknown. By selectively isolating exosomes using size-exclusion chromatography (SEC) and employing cell-type specific exosome reporter mice, our current results define a secreted astroglial exosome pathway that can spread long-range in vivo and stimulate axon growth of cortical pyramidal neurons. Subsequent biochemical and genetic studies found that surface expression of glial HepaCAM protein essentially and sufficiently mediates the axon-stimulating effect of astroglial exosomes. Interestingly, apolipoprotein E (ApoE), a major astroglia-secreted cholesterol carrier to promote synaptogenesis, strongly inhibits the stimulatory effect of astroglial exosomes on axon growth. Developmental ApoE deficiency also significantly reduces spine density of cortical pyramidal neurons. Together, our study suggests a surface contact mechanism of astroglial exosomes in regulating axon growth and its antagonization by ApoE, which collectively coordinates early postnatal pyramidal neuronal axon growth and dendritic spine formation.

In-Depth Characterization of Layer 5 Output Neurons of the Primary Somatosensory Cortex Innervating the Mouse Dorsal Spinal Cord

Neuronal circuits of the spinal dorsal horn integrate sensory information from the periphery with inhibitory and facilitating input from higher central nervous system areas. Most previous work focused on projections descending from the hindbrain. Less is known about inputs descending from the cerebral cortex. Here, we identified cholecystokinin (CCK) positive layer 5 pyramidal neurons of the primary somatosensory cortex (CCK ⁺ S1-corticospinal tract [CST] neurons) as a major source of input to the spinal dorsal horn. We combined intersectional genetics and virus-mediated gene transfer to characterize CCK⁺ S1-CST neurons and to define their presynaptic input and postsynaptic target neurons. We found that S1-CST neurons constitute a heterogeneous population that can be subdivided into distinct molecular subgroups. Rabies-based retrograde tracing revealed monosynaptic input from layer 2/3 pyramidal neurons, from parvalbumin positive cortical interneurons, and from thalamic relay neurons in the ventral posterolateral nucleus. Wheat germ agglutinin-based anterograde tracing identified postsynaptic target neurons in dorsal horn laminae III and IV. About 60% of these neurons were inhibitory and about 60% of all spinal target neurons expressed the transcription factor c-Maf. The heterogeneous nature of both S1-CST neurons and their spinal targets suggest complex roles in the fine-tuning of sensory processing.

Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Rodent spinal cord injury models for studies of axon regeneration

For over a century, axon regeneration has been considered the Holy Grail for spinal cord injury (SCI) repair. Although there are other factors that could contribute to improving function, restoring the long motor and sensory tracts that are interrupted by SCI has the greatest potential for actually reversing paralysis, restoring the brain's control of autonomic functions mediated by sympathetic and parasympathetic circuits of the spinal cord and restoring sensation. Accordingly and in keeping with the overall theme of this special issue, this review focuses narrowly on rodent SCI models for studies of axon regeneration.

Comprehensive Corticospinal Labeling with mu-crystallin Transgene Reveals Axon Regeneration after Spinal Cord Trauma in ngr1-/- Mice

Spinal cord injury interrupts descending motor tracts and creates persistent functional deficits due to the absence of spontaneous axon regeneration. Of descending pathways, the corticospinal tract (CST) is thought to be the most critical for voluntary function in primates. Even with multiple tracer injections and genetic tools, the CST is visualized to only a minor degree in experimental studies. Here, we identify and validate the mu-crystallin (crym) gene as a high-fidelity marker of the CST. In transgenic mice expressing green fluorescent protein (GFP) under crym regulatory elements (crym-GFP), comprehensive and near complete CST labeling is achieved throughout the spinal cord. Bilateral pyramidotomy eliminated the 17,000 GFP-positive CST axons that were reproducibly labeled in brainstem from the spinal cord. We show that CST tracing with crym-GFP is 10-fold more efficient than tracing with biotinylated dextran amine (BDA). Using crym-GFP, we reevaluated the CST in mice lacking nogo receptor 1 (NgR1), a protein implicated in limiting neural repair. The number and trajectory of CST axons in ngr1(-/-) mice without injury was indistinguishable from ngr1(+/+) mice. After dorsal hemisection in the midthoracic cord, CST axons did not significantly regenerate in ngr1(+/+) mice, but an average of 162 of the 6000 labeled thoracic CST axons (2.68%) regenerated >100 μm past the lesion site in crym-GFP ngr1(-/-) mice. Although traditional BDA tracing cannot reliably visualize regenerating ngr1(-/-) CST axons, their regenerative course is clear with crym-GFP. Therefore the crym-GFP transgenic mouse is a useful tool for studies of CST anatomy in experimental studies of motor pathways.

SIGNIFICANCE STATEMENT

Axon regeneration fails in the adult CNS, resulting in permanent functional deficits. Traditionally, inefficient extrinsic tracers such a biotinylated dextran amine (BDA) are used to label regenerating fibers after therapeutic intervention. We introduce crym-green fluorescent protein (GFP) transgenic mice as a comprehensive and specific tool with which to study the primary descending motor tract, the corticospinal tract (CST). CST labeling with crym-GFP is 10 times more efficient compared with BDA. The enhanced sensitivity afforded by crym-GFP revealed significant CST regeneration in NgR1 knock-out mice. Therefore, crym-GFP can be used as a standardized tool for future CST spinal cord injury studies.

A novel closed-body model of spinal cord injury caused by high-pressure air blasts produces extensive axonal injury and motor impairments

Diffuse axonal injury is thought to be the basis of the functional impairments stemming from mild traumatic brain injury. To examine how axons are damaged by traumatic events, such as motor vehicle accidents, falls, sports activities, or explosive blasts, we have taken advantage of the spinal cord with its extensive white matter tracts. We developed a closed-body model of spinal cord injury in mice whereby high-pressure air blasts targeted to lower thoracic vertebral levels produce tensile, compressive, and shear forces within the parenchyma of the spinal cord and thereby cause extensive axonal injury. Markers of cytoskeletal integrity showed that spinal cord axons exhibited three distinct pathologies: microtubule breakage, neurofilament compaction, and calpain-mediated spectrin breakdown. The dorsally situated axons of the corticospinal tract primarily exhibited microtubule breakage, whereas all three pathologies were common in the lateral and ventral white matter. Individual axons typically demonstrated only one of the three pathologies during the first 24h after blast injury, suggesting that the different perturbations are initiated independently of one another. For the first few days after blast, neurofilament compaction was frequently accompanied by autophagy, and subsequent to that, by the fragmentation of degenerating axons. TuJ1 immunolabeling and mice with YFP-reporter labeling each revealed more extensive microtubule breakage than did βAPP immunolabeling, raising doubts about the sensitivity of this standard approach for assessing axonal injury. Although motor deficits were mild and largely transient, some aspects of motor function gradually worsened over several weeks, suggesting that a low level of axonal degeneration continued past the initial wave. Our model can help provide further insight into how to intervene in the processes by which initial axonal damage culminates in axonal degeneration, to improve outcomes after traumatic injury. Importantly, our findings of extensive axonal injury also caution that repeated trauma is likely to have cumulative adverse consequences for both brain and spinal cord.