Dual-Viral Transduction Utilizing Highly Efficient Retrograde Lentivirus Improves Labeling of Long Propriospinal Neurons

Long ascending propriospinal neurons are heterogenous and subject to spinal cord injury induced anatomic plasticity

Long ascending propriospinal neurons (LAPNs) are a subset of spinal interneurons that provide direct connectivity between distant spinal segments. Here, we focus specifically on an anatomically defined population of "inter-enlargement" LAPNs with cell bodies at L2/3 and terminals at C5/6. Previous studies showed that silencing LAPNs in awake and freely moving animals disrupted interlimb coordination of the hindlimbs, forelimbs, and heterolateral limb pairs. Surprisingly, despite a proportion of LAPNs being anatomically intact post- spinal cord injury (SCI), silencing them improved locomotor function but only influenced coordination of the hindlimb pair. Given the functional significance of LAPNs pre- and post-SCI, we characterized their anatomy and SCI-induced anatomical plasticity. This detailed anatomical characterization revealed three morphologically distinct subsets of LAPNs that differ in soma size, neurite complexity and/or neurite orientation. Following a mild thoracic contusive SCI there was a marked shift in neurite orientation in two of the LAPN subsets to a more dorsoventral orientation, and collateral densities decreased in the cervical enlargement but increased just caudal to the injury epicenter. These post-SCI anatomical changes potentially reflect maladaptive plasticity and an effort to establish new functional inputs from sensory afferents that sprout post-SCI to achieve circuitry homeostasis.

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

Endothelial damage, astrogliosis, microgliosis, and neuronal degeneration are the most common events after spinal cord injury (SCI). Studies highlighted that studying the spatiotemporal profile of these events might provide a deeper understanding of the pathophysiology of SCI. For imaging of these events, available conventional techniques such as 2-dimensional histology and immunohistochemistry (IHC) are well established and frequently used to visualize and detect the altered expression of the protein of interest involved in these events. However, the technique requires the physical sectioning of the tissue, and results are also open to misinterpretation. Currently, researchers are focusing more attention toward the advanced tools for imaging the spinal cord's various physiological and pathological parameters. The tools include two-photon imaging, light sheet fluorescence microscopy, in vivo imaging system with fluorescent probes, and in vivo chemical and fluorescent protein-expressing viral-tracers. These techniques outperform the limitations associated with conventional techniques in various aspects, such as optical sectioning of tissue, 3D reconstructed imaging, and imaging of particular planes of interest. In addition to this, these techniques are minimally invasive and less time-consuming. In this review, we will discuss the various advanced imaging methodologies that will evolve in the future to explore the fundamental mechanisms after SCI.

Double viral vector intersectional approaches for pathway-selective manipulation of motor functions and compensatory mechanisms

Selective manipulation of particular subcomponent of neural circuits is crucial for understanding the functional architecture of neural systems and for development of the future therapeutic strategies against neurological disorders. In this article, I review how the intersectional approaches with double viral vector technique was introduced for the pathway-selective manipulation of spinal circuits. In this technique, a retrograde gene transfer vector is injected into the terminal area of the targeted neurons and an anterograde vector is injected at the location of their somata. Either by using the Tet-transactivator or Cre-loxP system, the experimenter can chemogenetically or optogenetically manipulate the transmission of the target pathway originated from the double-infected neurons. This technique was first developed for manipulation of spinal cord interneurons in the macaque monkeys by selective expression of tetanus neurotoxin and successfully affected the dexterous hand movements. Currently, this technique is widely used on a variety of neural pathways both in rodents and primates in combination with a variety of retrograde vectors and a variety of optogenetic and chemogenetic tools. The advantage of this technique is that it is not necessary to generate transgenic animals. Knowledge of the cell-type specific promotors is not needed. Manipulation is achieved primarily by injection of two viral vectors based on the anatomical knowledge and it is applicable in a variety of animal species including primates. The pros, cons and future direction of this technique are discussed.

Exploring propriospinal neuron-mediated neural circuit plasticity using recombinant viruses after spinal cord injury

Propriospinal neurons (PSNs) play a crucial role in motor control and sensory processing and contribute to plastic reorganization of spinal circuits responsible for recovery from spinal cord injury (SCI). Due to their scattered distribution and various intersegmental projection patterns, it is challenging to dissect the function of PSNs within the neuronal network. New genetically encoded tools, particularly cell-type-specific transgene expression methods using recombinant viral vectors combined with other genetic, pharmacologic, and optogenetic approaches, have enormous potential for visualizing PSNs in the neuronal circuits and monitoring and manipulating their activity. Furthermore, recombinant viral tools have been utilized to promote the intrinsic regenerative capacities of PSNs, towards manipulating the 'hostile' microenvironment for improving functional regeneration of PSNs. Here we summarize the latest development in this fast-moving field and provide a perspective for using this technology to dissect PSN physiological role in contributing to recovery of function after SCI.

Silencing long ascending propriospinal neurons after spinal cord injury improves hindlimb stepping in the adult rat

Long ascending propriospinal neurons (LAPNs) are a subpopulation of spinal cord interneurons that directly connect the lumbar and cervical enlargements. Previously we showed, in uninjured animals, that conditionally silencing LAPNs disrupted left-right coordination of the hindlimbs and forelimbs in a context-dependent manner, demonstrating that LAPNs secure alternation of the fore- and hindlimb pairs during overground stepping. Given the ventrolateral location of LAPN axons in the spinal cord white matter, many likely remain intact following incomplete, contusive, thoracic spinal cord injury (SCI), suggesting a potential role in the recovery of stepping. Thus, we hypothesized that silencing LAPNs after SCI would disrupt recovered locomotion. Instead, we found that silencing spared LAPNs post-SCI improved locomotor function, including paw placement order and timing, and a decrease in the number of dorsal steps. Silencing also restored left-right hindlimb coordination and normalized spatiotemporal features of gait such as stance and swing time. However, hindlimb-forelimb coordination was not restored. These data indicate that the temporal information carried between the spinal enlargements by the spared LAPNs post-SCI is detrimental to recovered hindlimb locomotor function. These findings are an illustration of a post-SCI neuroanatomical-functional paradox and have implications for the development of neuronal- and axonal-protective therapeutic strategies and the clinical study/implementation of neuromodulation strategies.