A porcine model for studying the cardiovascular consequences of high-thoracic spinal cord injury

A 72-h sedated porcine model of traumatic spinal cord injury

Introduction

There is an increasing focus on the prevention of secondary injuries following traumatic spinal cord injury (TSCI), especially through improvement of spinal cord perfusion and immunological modulation. Such therapeutic strategies require translational and controlled animal models of disease progression of the acute phases of human TSCI.

Research question

Is it possible to establish a 72-h sedated porcine model of incomplete thoracic TSCI, enabling controlled use of continuous, invasive, and non-invasive modalities during the entire sub-acute phase of TSCI?

Material and methods

A sham-controlled trial was conducted to establish the model, and 10 animals were assigned to either sham or TSCI. All animals underwent a laminectomy, and animals in the TSCI group were subjected to a weight-drop injury. Animals were then kept sedated for 72 h. The amount of injury was assessed by ex-vivo measures MRI-based fiber tractography, histology and immunohistochemistry.

Results

In all animals, we were successful in maintaining sedation for 72 h without comprising vital physiological parameters. The MRI-based fiber tractography showed that all TSCI animals revealed a break in the integrity of spinal neurons, whereas histology demonstrated no transversal sections of the spine with complete injury. Notably, some animals displayed signs of secondary ischemic tissue in the cranial and caudal sections.

Discussion and conclusions

This study succeeded in producing a porcine model of incomplete TSCI, which was physiologically stable up to 72 h. We believe that this TSCI model will constitute a potential translational model to study the pathophysiology secondary to TSCI in humans.

Lesion level-dependent systemic muscle wasting after spinal cord injury is mediated by glucocorticoid signaling in mice

An incomplete mechanistic understanding of skeletal muscle wasting early after spinal cord injury (SCI) precludes targeted molecular interventions. Here, we demonstrated systemic wasting that also affected innervated nonparalyzed (supralesional) muscles and emerged within 1 week after experimental SCI in mice. Systemic muscle wasting caused muscle weakness, affected fast type 2 myofibers preferentially, and became exacerbated after high (T3) compared with low (T9) thoracic paraplegia, indicating lesion level-dependent ("neurogenic") mechanisms. The wasting of nonparalyzed muscle and its rapid onset and severity beyond what can be explained by disuse implied unknown systemic drivers. Muscle transcriptome and biochemical analysis revealed a glucocorticoid-mediated catabolic signature early after T3 SCI. SCI-induced systemic muscle wasting was mitigated by (i) endogenous glucocorticoid ablation (adrenalectomy) and (ii) pharmacological glucocorticoid receptor (GR) blockade and was (iii) completely prevented after T3 relative to T9 SCI by genetic muscle-specific GR deletion. These results suggest that neurogenic hypercortisolism contributes to a rapid systemic and functionally relevant muscle wasting syndrome early after paraplegic SCI in mice.

Porcine Models of Spinal Cord Injury

Large animal models of spinal cord injury may be useful tools in facilitating the development of translational therapies for spinal cord injury (SCI). Porcine models of SCI are of particular interest due to significant anatomic and physiologic similarities to humans. The similar size and functional organization of the porcine spinal cord, for instance, may facilitate more accurate evaluation of axonal regeneration across long distances that more closely resemble the realities of clinical SCI. Furthermore, the porcine cardiovascular system closely resembles that of humans, including at the level of the spinal cord vascular supply. These anatomic and physiologic similarities to humans not only enable more representative SCI models with the ability to accurately evaluate the translational potential of novel therapies, especially biologics, they also facilitate the collection of physiologic data to assess response to therapy in a setting similar to those used in the clinical management of SCI. This review summarizes the current landscape of porcine spinal cord injury research, including the available models, outcome measures, and the strengths, limitations, and alternatives to porcine models. As the number of investigational SCI therapies grow, porcine SCI models provide an attractive platform for the evaluation of promising treatments prior to clinical translation.

Porcine spinal cord injury model for translational research across multiple functional systems

Animal models are necessary to identify pathological changes and help assess therapeutic outcomes following spinal cord injury (SCI). Small animal models offer value in research in terms of their easily managed size, minimal maintenance requirements, lower cost, well-characterized genomes, and ability to power research studies. However, despite these benefits, small animal models have neurologic and anatomical differences that may influence translation of results to humans and thus limiting the success of their use in preclinical studies as a direct pipeline to clinical studies. Large animal models, offer an attractive intermediary translation model that may be more successful in translating to the clinic for SCI research. This is largely due to their greater neurologic and anatomical similarities to humans. The physical characteristics of pig spinal cord, gut microbiome, metabolism, proportions of white to grey matter, bowel anatomy and function, and urinary system are strikingly similar and provide great insight into human SCI conditions. In this review, we address the variety of existing porcine injury models and their translational relevance, benefits, and drawbacks in modeling human systems and functions for neurophysiology, cardiovascular, gastrointestinal and urodynamic functions.

Morphological and phenotypical characteristics of porcine satellite glial cells of the dorsal root ganglia

Satellite glial cells (SGCs) of the dorsal root ganglia (DRG) ensure homeostasis and proportional excitability of sensory neurons and gained interest in the field of development and maintenance of neuropathic pain. Pigs represent a suitable species for translational medicine with a more similar anatomy and physiology to humans compared to rodents, and are used in research regarding treatment of neuropathic pain. Knowledge of anatomical and physiological features of porcine SGCs is prerequisite for interpreting potential alterations. However, state of knowledge is still limited. In the present study, light microscopy, ultrastructural analysis and immunofluorescence staining was performed. SGCs tightly surround DRG neurons with little vascularized connective tissue between SGC-neuron units, containing, among others, axons and Schwann cells. DRG were mainly composed of large sized neurons (∼59%), accompanied by fewer medium sized (∼36%) and small sized sensory neurons (∼6%). An increase of neuronal body size was concomitant with an increased number of surrounding SGCs. The majority of porcine SGCs expressed glutamine synthetase and inwardly rectifying potassium channel Kir 4.1, known as SGC-specific markers in other species. Similar to canine SGCs, marked numbers of porcine SGCs were immunopositive for glial fibrillary acidic protein, 2',3'-cyclic-nucleotide 3'-phosphodiesterase and the transcription factor Sox2. Low to moderate numbers of SGCs showed aquaporin 4-immunoreactivity (AQP4) as described for murine SGCs. AQP4-immunoreactivity was primarily found in SGCs ensheathing small and medium sized neuronal somata. Low numbers of SGCs were immunopositive for ionized calcium-binding adapter molecule 1, indicating a potential immune cell character. No immunoreactivity for common leukocyte antigen CD45 nor neural/glial antigen 2 was detected. The present study provides essential insights into the characteristic features of non-activated porcine SGCs, contributing to a better understanding of this cell population and its functional aspects. This will help to interpret possible changes that might occur under activating conditions such as pain.

Porcine Model of Spinal Cord Injury: A Systematic Review

Spinal cord injury (SCI) is a devastating disease with limited effective treatment options. Animal paradigms are vital for understanding the pathogenesis of SCI and testing potential therapeutics. The porcine model of SCI is increasingly favored because of its greater similarity to humans. However, its adoption is limited by the complexities of care and range of testing parameters. Researchers need to consider swine selection, injury method, post-operative care, rehabilitation, behavioral outcomes, and histology metrics. Therefore, we systematically reviewed full-text English-language articles to evaluate study characteristics used in developing a porcine model and summarize the interventions that have been tested using this paradigm. A total of 63 studies were included, with 33 examining SCI pathogenesis and 30 testing interventions. Studies had an average sample size of 15 pigs with an average weight of 26 kg, and most used female swine with injury to the thoracic cord. Injury was most commonly induced by weight drop with compression. The porcine model is amenable to testing various interventions, including mean arterial pressure augmentation (n = 7), electrical stimulation (n = 6), stem cell therapy (n = 5), hypothermia (n = 2), biomaterials (n = 2), gene therapy (n = 2), steroids (n = 1), and nanoparticles (n = 1). It is also notable for its clinical translatability and is emerging as a valuable pre-clinical study tool. This systematic review can serve as a guideline for researchers implementing and testing the porcine SCI model.

Influence of respiratory loading on left-ventricular function in cervical spinal cord injury

KEY POINTS

Cervical spinal cord injury (C-SCI) alters both the cardiac and respiratory systems, however little is known as to how these systems interact following injury. Here, we manipulated inspiratory or expiratory intrathoracic pressure (ITP) to mechanistically test the role of the respiratory pump on circulatory function in highly-trained individuals with C-SCI and an able-bodied reference group. In individuals with C-SCI, greater ITP during expiratory loading caused dynamic hyperinflation that was associated with impaired left-ventricular filling. More negative ITP during inspiratory loading did not significantly alter left-ventricular volumes in either group. Interventions that prevent dynamic hyperinflation and/or enhance the ability to generate expiratory pressures may help preserve left-ventricular filling in individuals with C-SCI.

ABSTRACT

Cervical spinal cord injury (C-SCI) negatively impacts cardiac and respiratory function. As the heart and lungs are linked via the pulmonary circuit these systems are interdependent. Here, we utilized inspiratory and expiratory loading to assess whether augmenting the respiratory pump improves left-ventricular (LV) filling and output in individuals with motor-complete C-SCI. We hypothesized LV end-diastolic volume (LVEDV) would increase and decrease with inspiratory and expiratory loading, respectively. Participants (C-SCI: 7M/1F, 35±7 years; able-bodied: 7M/1F, 32±6 years) were assessed under five conditions during 45° head-up tilt; unloaded, inspiratory loading with -10 and -20cmH₂ O esophageal pressure (Pes) on inspiration, and expiratory loading with +10 and +20cmH₂ O Pes on expiration. An esophageal balloon catheter monitored Pes and LV structure and function were assessed by echocardiography. In C-SCI only, (1) +20cmH₂ O reduced LVEDV vs. unloaded (81±15 vs. 88±11 mL, p = 0.006); (2) heart rate was higher during +20cmH₂ O compared to unloaded (p = 0.001) and +10cmH₂ O (p = 0.002); (3) cardiac output was higher during +20cmH₂ O than unloaded (p = 0.002); and (4) end-expiratory lung volume was higher during +20cmH₂ O vs. unloaded (63±10 vs. 55±13% total lung capacity, p = 0.003) but was unaffected by inspiratory loading. In both groups, -10 and -20cmH₂ O had no significant effect on LVEDV. These findings suggest greater expiratory positive pressure acutely impairs LV filling in C-SCI, potentially via impaired venous return, mediastinal constraint and/or direct ventricular interaction subsequent to dynamic hyperinflation. Inspiratory loading did not significantly improve LV function in C-SCI and neither inspiratory nor expiratory loading affected cardiac function or lung volumes in able-bodied participants. Abstract figure legend Background: Cervical spinal cord injury (C-SCI) alters both the cardiac and respiratory systems. However, expiratory function is compromised to a greater extent than inspiratory function. Experimental set up: To examine how the cardiac and respiratory systems interact following C-SCI we manipulated inspiratory or expiratory intrathoracic pressure (ITP) to mechanistically test how changes in ITP and lung volumes influence cardiac function in highly-trained individuals with C-SCI and an able-bodied reference group. Participants were assessed under five conditions during 45° head-up tilt; unloaded, two inspiratory loading, and two expiratory loading conditions.

KEY FINDINGS

Following C-SCI, greater ITP during expiratory loading increased lung volumes and was associated with impaired left-ventricular filling. Interventions that prevent increases in lung volumes and/or enhance the ability to generate expiratory pressures may help preserve left-ventricular filling in individuals with C-SCI. A portion of this figure was created with biorender.com This article is protected by copyright. All rights reserved.

Evaluation of the Cardiometabolic Disorders after Spinal Cord Injury in Mice

Changes in cardiometabolic functions contribute to increased morbidity and mortality after chronic spinal cord injury. Despite many advancements in discovering SCI-induced pathologies, the cardiometabolic risks and divergences in severity-related responses have yet to be elucidated. Here, we examined the effects of SCI severity on functional recovery and cardiometabolic functions following moderate (50 kdyn) and severe (75 kdyn) contusions in the thoracic-8 (T8) vertebrae in mice using imaging, morphometric, and molecular analyses. Both severities reduced hindlimbs motor functions, body weight (g), and total body fat (%) at all-time points up to 20 weeks post-injury (PI), while only severe SCI reduced the total body lean (%). Severe SCI increased liver echogenicity starting from 12 weeks PI, with an increase in liver fibrosis in both moderate and severe SCI. Severe SCI mice showed a significant reduction in left ventricular internal diameters and LV volume at 20 weeks PI, associated with increased LV ejection fraction as well as cardiac fibrosis. These cardiometabolic dysfunctions were accompanied by changes in the inflammation profile, varying with the severity of the injury, but not in the lipid profile nor cardiac or hepatic tyrosine hydroxylase innervation changes, suggesting that systemic inflammation may be involved in these SCI-induced health complications.

Hemodynamic Changes in Response to Hyperacute Spinal Trauma in a Swine Model

Acute spinal cord injury (ASCI) is a devastating event that can have severe hemodynamic consequences, depending on location and severity of the lesion. Knowledge of hyperacute hemodynamic changes is important for researchers using porcine models of thoracic ASCI. The goal of this study was to determine the hyperacute hemodynamic changes observed after ASCI when using pigs as their own controls. Five Yucatan gilts were anesthetized, and a dorsal laminectomy performed at T10-T12. Standardized blunt trauma was applied for 5 consecutive min, and hemodynamic variables were collected 5 min before ASCI, and at 2, 4, 6, 8, 10, 20, 30, 60, 80 and 120 min after ASCI. Arterial blood gas samples were collected at 60 min and 10 min before, and at 30 min and between 120 and 240 min after ASCI. Parametric data were analyzed using a mixed effects model with time point as the fixed factor and subject as the random factor. We found no effect on heart rate, pulse pressure, SpO2, EtCO2, and respiratory rate between baseline and timepoints after ASCI. Diastolic arterial pressure, mean arterial pressure, and systolic arterial pressure fell significantly by 18%, 16%, and 15%, respectively, at 2 min after ASCI. However, none of the decrements in arterial pressures resulted in hypotension at any time point. Heart rate did not change significantly after ASCI. Blood glucose progressively increased to 50% above baseline between 120 and 240 minutes after ASCI. Low-thoracic ASCI caused a consistent and statistically significant but clinically minor hyperacute decrease in arterial pressures (-15%) that did not produce hypotension or metabolic changes suggestive of tissue hypoperfusion. Our findings using this model suggest that mean arterial pressures should be maintained above 85 mm Hg prior to spinal trauma in order to avoid hypotensive states after ASCI.

Characterization of the gut microbiome in a porcine model of thoracic spinal cord injury

BACKGROUND

The gut microbiome is a diverse network of bacteria which inhabit our digestive tract and is crucial for efficient cellular metabolism, nutrient absorption, and immune system development. Spinal cord injury (SCI) disrupts autonomic function below the level of injury and can alter the composition of the gut microbiome. Studies in rodent models have shown that SCI-induced bacterial imbalances in the gut can exacerbate the spinal cord damage and impair recovery. In this study we, for the first time, characterized the composition of the gut microbiome in a Yucatan minipig SCI model. We compared the relative abundance of the most dominant bacterial phyla in control samples to those collected from animals who underwent a contusion-compression SCI at the 2nd or 10th Thoracic level.

RESULTS

We identify specific bacterial fluctuations that are unique to SCI animals, which were not found in uninjured animals given the same dietary regimen or antibiotic administration. Further, we identified a specific time-frame, "SCI-acute stage", during which many of these bacterial fluctuations occur before returning to "baseline" levels.

CONCLUSION

This work presents a dynamic view of the microbiome changes that accompany SCI, establishes a resource for future studies and to understand the changes that occur to gut microbiota after spinal cord injury and may point to a potential therapeutic target for future treatment.

Cardio-centric hemodynamic management improves spinal cord oxygenation and mitigates hemorrhage in acute spinal cord injury

Chronic high-thoracic and cervical spinal cord injury (SCI) results in a complex phenotype of cardiovascular consequences, including impaired left ventricular (LV) contractility. Here, we aim to determine whether such dysfunction manifests immediately post-injury, and if so, whether correcting impaired contractility can improve spinal cord oxygenation (SCO₂), blood flow (SCBF) and metabolism. Using a porcine model of T2 SCI, we assess LV end-systolic elastance (contractility) via invasive pressure-volume catheterization, monitor intraparenchymal SCO₂ and SCBF with fiberoptic oxygen sensors and laser-Doppler flowmetry, respectively, and quantify spinal cord metabolites with microdialysis. We demonstrate that high-thoracic SCI acutely impairs cardiac contractility and substantially reduces SCO₂ and SCBF within the first hours post-injury. Utilizing the same model, we next show that augmenting LV contractility with the β-agonist dobutamine increases SCO₂ and SCBF more effectively than vasopressor therapy, whilst also mitigating increased anaerobic metabolism and hemorrhage in the injured cord. Finally, in pigs with T2 SCI survived for 12 weeks post-injury, we confirm that acute hemodynamic management with dobutamine appears to preserve cardiac function and improve hemodynamic outcomes in the chronic setting. Our data support that cardio-centric hemodynamic management represents an advantageous alternative to the current clinical standard of vasopressor therapy for acute traumatic SCI.