3Dimaging of supraspinal inputs to the thoracic and lumbar spinal cord mapped by retrograde tracing and light‐sheet microscopy

Abstract

The supraspinal inputs play a major role in tuning the hindlimb locomotion function. While most research on spinal cord injury (SCI) with rodents is based on thoracic segments, the difference in connectivity of the supraspinal centers to the thoracic and lumbar cord is still unknown. Here, we combined retrograde tracing and 3D imaging to map the connectivity of supraspinal neurons projecting to thoracic (T9‐vertebral) and lumbar (T13‐vertebral) spinal levels in adult female mice. We dissected the difference in connections of corticospinal neurons (CSNs), rubrospinal neurons, and reticulospinal neurons projecting to thoracic and lumbar cords. The ratio of double‐labeled neurons is higher in T13‐vertebral projection CSNs and parvocellular part of the red nucleus (RPC) than in T9‐vertebral projection. Using the Cre‐DIO system, we precisely targeted CSNs projecting to T9‐vertebral or T13‐vertebral. We found that abundant axon branches communicated with the red nucleus and reticular formation and distributed from cervical gray matter to the lumbar cord. Their collateral branches showed a distinct innervation pattern in thoracic and lumbar gray matters and a similar distribution pattern in the cervical spinal cord. These results revealed the difference in connectivity between the thoracic and lumbar projection supraspinal centers and clarified the collateralization of thoracic/lumbar projection CSNs throughout the brain and spinal cord. This study highlights brain‐spinal cord neural networks and the complexity of the axon terminals of spinal projection CSNs, which could contribute to the development of targeted therapeutic strategies connecting CST fibers and hindlimb function recovery. Cover Image for this issue: https://doi.org/10.1111/jnc.15414

A shift of brain network hub after spinal cord injury

“…Why, then, have changes occurred in such a large network? First, the cells of the rodent motor cortex have fibers that extend to the lumbar spinal cord, but there are also branches in the brainstem and cervical spinal cord ( Lu et al, 2022 ), suggesting that the axonal transection affected the larger network. Second, although we could not subdivide the motor cortex because there were only 244 nodes in the entire brain, analysis by the motor cortex subtype might have revealed that the upper limb region is relatively preserved.…”

Long Preservation of AAV-Transduced Fluorescence by a Modified Organic Solvent-Based Clearing Method

“…The laminal plates of C4/5 vertebrae were removed. Then, AAV2-Retro-EGFP (500 nL/site) was injected into the right spinal cord at 0.5 mm lateral to the midline and a depth of 0.7 mm [ 32 , 33 ]. AAV2-Retro -mCherry (500 nL/site) was injected into the left cord using the same method.…”

In Vivo and 3D Imaging Technique(s) for Spatiotemporal Mapping of Pathological Events in Experimental Model(s) of Spinal Cord Injury

“…Several efforts have been made to improve the resolution or shortcomings of CLSM by using advanced imaging techniques or tools to assess the spatiotemporal expression of proteins. The tools include the multiphoton microscopy two/three-photon imaging (2PEF/2PEF), second-harmonic imaging microscopy (SHIM), light sheet fluorescence microscopy (LSFM), in vivo imaging system (IVIS), chemical tracers, and genetically modified viral tracers such as chemogenetics, optogenetics, and dual virus transduction. − …”