A Surgical Mouse Model for Advancing Laryngeal Nerve Regeneration Strategies

Postnatal changes in thyroid cartilage shape and cartilage matrix composition are not synchronized in Mus musculus

The study was conducted to quantify laryngeal cartilage matrix composition and to investigate its relationship with cartilage shape in a mouse model. A sample of 30 mice (CD-1 mouse, Mus musculus) from five age groups (postnatal Days 2, 21, 90, 365, and 720) were used. Three-dimensional mouse laryngeal thyroid cartilage reconstructions were generated from contrast-enhanced micro-computed tomography (CT) image stacks. Cartilage matrix composition was estimated as Hounsfield units (HU). HU were determined by overlaying 3D reconstructions as masks on micro-CT image stacks and then measuring the attenuation. Cartilage shape was quantified with landmarks placed on the surface of the thyroid cartilage. Shape differences between the five age groups were analyzed using geometric morphometrics and multiparametric analysis of landmarks. The relationship between HU and shape was investigated with correlational analyses. Among five age groups, HU became higher in older animals. The shape of the thyroid cartilage changes with age throughout the entire life of a mouse. The changes in shape were not synchronized with changes in cartilage matrix composition. The thyroid cartilage of young and old M. musculus larynx showed a homogenous mineralization pattern. High-resolution contrast-enhanced micro-CT imaging makes the mouse larynx accessible for analysis of genetic and environmental factors affecting shape and matrix composition.

Method for Collecting Single Epithelial Cells from the Mouse Larynx

OBJECTIVE

The larynx is lined by specialized epithelial cell populations. Studying molecular changes occurring in individual epithelial cell types requires a reliable method for removing these cells from the larynx. Our objective was to develop a method to harvest individual epithelial cells from the mouse larynx while minimizing contamination from non-laryngeal sites and non-epithelial laryngeal cells.

METHODS

Mice were euthanized, and the larynx was carefully exposed and separated from non-laryngeal sites. A small dental brush was inserted into the laryngeal inlet and rotated to obtain epithelial cells. Cells were transferred to collection media, counted, and cytospin preparations stained for laryngeal epithelial (i.e., Pan-Keratin, EpCAM, NGFR, p63, K5, β-tubulin, MUC5AC) and non-epithelial (i.e., vimentin) cell markers. Histopathology was completed on brushed laryngeal tissue sections to evaluate the depth of cell collection. Preliminary Single-cell RNA sequencing (scRNA-seq) was performed to confirm this method can capture diverse laryngeal cell types.

RESULTS

We collected 6000-8000 cells from a single larynx and 35000-40000 cells from combining brushings from three tissues. Histopathology demonstrated brushing removed the epithelial layer of the larynx and some underlying tissue. Immunofluorescence staining demonstrated the phenotype of harvested cells was primarily epithelial. Preliminary scRNA-seq was successfully conducted and displayed nine unique cell clusters.

CONCLUSION

We developed a reliable method of harvesting individual epithelial cells from the mouse larynx. This method will be useful for collection of laryngeal cells for a variety of downstream cellular and molecular assays, including scRNA-seq, protein analyses, and cell-culture-based experiments, following laryngeal injury.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:786-794, 2024.

Minimally Invasive Murine Laryngoscopy for Close-Up Imaging of Laryngeal Motion during Breathing and Swallowing

The larynx is an essential organ in mammals with three primary functions - breathing, swallowing, and vocalizing. A wide range of disorders are known to impair laryngeal function, which results in difficulty breathing (dyspnea), swallowing impairment (dysphagia), and/or voice impairment (dysphonia). Dysphagia, in particular, can lead to aspiration pneumonia and associated morbidity, recurrent hospitalization, and early mortality. Despite these serious consequences, existing treatments for laryngeal dysfunction are largely aimed at surgical and behavioral interventions that unfortunately do not typically restore normal laryngeal function, thus highlighting the urgent need for innovative solutions. To bridge this gap, we have been developing an experimental endoscopic approach to investigate laryngeal dysfunction in murine (i.e., mouse and rat) models. However, endoscopy in rodents is quite challenging due to their small size relative to current endoscope technology, anatomical differences in the upper airway, and the necessity for anesthesia to optimally access the larynx. Here, we describe a novel transoral laryngoscopy approach that permits close-up, unobstructed video imaging of laryngeal motion in mice and rats. Critical steps in the protocol include precise anesthesia management (to prevent overdosing that abolishes swallowing and/or risks respiratory distress-related mortality) and micromanipulator control of the endoscope (for stable video recording of laryngeal motion by a single researcher for subsequent quantification). Importantly, the protocol can be performed over time in the same animals to study the impact of various pathological conditions specifically on laryngeal function. A novel advantage of this protocol is the ability to visualize airway protection during swallowing, which is not possible in humans due to epiglottic inversion over the laryngeal inlet that obstructs the glottis from view. Rodents therefore provide a unique opportunity to specifically investigate the mechanisms of normal versus pathological laryngeal airway protection for the ultimate purpose of discovering treatments to effectively restore normal laryngeal function.

Effects of Unilateral Vagotomy on LPS-Induced Aspiration Pneumonia in Mice

Respiratory-related dysphagia and aspiration pneumonia can be attributed to multiple causes. However, reproduction of multiple factor-related respiratory distress and aspiration pneumonia in a single animal model is challenging. To validate animals with vagal nerve palsy as novel models for severe aspiration pneumonia associated with respiratory distress, we investigated the effects of unilateral vagotomy on the swallowing function and severity of pneumonia after forced aspiration in mice. Unilateral vagotomy was performed in C57BL6 male mice that subsequently underwent evaluation of swallowing function by videofluoroscopic swallow study (VFSS) and histological assessments for aspiration pneumonia induced by lipopolysaccharide (LPS). VFSS examinations demonstrated that unilateral vagotomy did not cause apparent aspiration in mice, but it resulted in a significant loss of body weight (BW) due to decreased oral intake. In addition, when aspiration pneumonia was induced by forced administration of LPS, significantly prolonged BW loss and severe infiltration of inflammatory cells associated with aspiration pneumonia were observed in the mice that underwent unilateral vagotomy. In conclusion, the vagotomized mice showed appropriate characteristics as a model of aspiration pneumonia caused by multiple factors, including the paralysis of vocal fold movement and respiratory distress. This model can help elucidate the pathogenesis of aspiration pneumonia and the treatment methods for the respiration-compromised model.

Electroacupuncture improves swallowing function in a post-stroke dysphagia mouse model by activating the motor cortex inputs to the nucleus tractus solitarii through the parabrachial nuclei

As a traditional medical therapy, stimulation at the Lianquan (CV23) acupoint, located at the depression superior to the hyoid bone, has been shown to be beneficial in dysphagia. However, little is known about the neurological mechanism by which this peripheral stimulation approach treats for dysphagia. Here, we first identified a cluster of excitatory neurons in layer 5 (L5) of the primary motor cortex (M1) that can regulate swallowing function in male mice by modulating mylohyoid activity. Moreover, we found that focal ischemia in the M1 mimicked the post-stroke dysphagia (PSD) pathology, as indicated by impaired water consumption and electromyographic responses in the mylohyoid. This dysfunction could be rescued by electroacupuncture (EA) stimulation at the CV23 acupoint (EA-CV23) in a manner dependent on the excitatory neurons in the contralateral M1 L5. Furthermore, neuronal activation in both the parabrachial nuclei (PBN) and nucleus tractus solitarii (NTS), which was modulated by the M1, was required for the ability of EA-CV23 treatment to improve swallowing function in male PSD model mice. Together, these results uncover the importance of the M1-PBN-NTS neural circuit in driving the protective effect of EA-CV23 against swallowing dysfunction and thus reveal a potential strategy for dysphagia intervention.

Electrospun conductive nanofiber yarns for accelerating mesenchymal stem cells differentiation and maturation into Schwann cell-like cells under a combination of electrical stimulation and chemical induction

Development of multifunctional tube-filling materials is required to improve the performances of the existing nerve guidance conduits (NGCs) in the repair of long-gap peripheral nerve (PN) injuries. In this study, composite nanofiber yarns (NYs) based on poly(p-dioxanone) (PPDO) biopolymer and different concentrations of carbon nanotubes (CNTs) were manufactured by utilizing a modified electrospinning apparatus. We confirmed the successful incorporation of CNTs into the PPDO nanofibers of as-fabricated composite NYs. The PPDO/CNT NYs exhibited similar morphology and structure in comparison with pure PPDO NYs. However, the PPDO/CNT NYs showed obviously enhanced mechanical properties and electrical conductivity compared to PPDO NYs. The biological tests revealed that the addition of CNTs had no negative effects on the cell growth, and proliferation of rabbit Schwann cells (rSCs), but it better maintained the phenotype of rSCs. We also demonstrated that the electrical stimulation (ES) significantly enhanced the differentiation capability of human adipose-derived mesenchymal stem cells (hADMSCs) into SC-like cells (SCLCs) on the PPDO/CNT NYs. More importantly, a unique combination of ES and chemical induction was found to further enhance the maturation of hADMSC-SCLCs on the PPDO/CNT NYs by notably upregulating the expression levels of SC myelination-associated gene markers and increasing the growth factor secretion. Overall, this study showed that our electrically conductive PPDO/CNT composite NYs could provide a beneficial microenvironment for various cell activities, making them an attractive candidate as NGC-infilling substrates for PN regeneration applications. STATEMENT OF SIGNIFICANCE: The morphology, microstructure, and bioelectrical properties of conductive PPDO/CNT NYs have been explored for guiding or controlling cell behaviors. The PPDO/CNT NYs exhibited improved mechanical properties and increased electrical conductivity compared to the CNT-free PPDO NYs. They also presented an obviously enhanced biocompatibility by effectively maintaining the phenotype of rSCs. In addition, when hADMSCs were seeded and cultured on the conductive PPDO/CNT NYs, CI was demonstrated to promote the SC-related growth factor secretion of hADMSCs, and ES was demonstrated to improve the phenotypic maturation of hADMSCs into myelinating SCLCs. Moreover, the combination of CI and ES was found to further synergistically enhance the maturation of hADMSC-SCLCs. The achievement of conductive PPDO/CNT NYs shows potential for application as NGC-infilling substrates for PN regeneration.

Experimental Injury Rodent Models for Oropharyngeal Dysphagia

Oropharyngeal dysphagia is a disorder that can make swallowing difficult and reduce the quality of life. Recently, the number of patients with swallowing difficulty has been increasing; however, no comprehensive treatment for such patients has been developed. Various experimental animal models that mimic oropharyngeal dysphagia have been developed to identify appropriate treatments. This review aims to summarize the experimentally induced oropharyngeal dysphagia rodent models that can be used to provide a pathological basis for dysphagia. The selected studies were classified into those reporting dysphagia rodent models showing lingual paralysis by hypoglossal nerve injury, facial muscle paralysis by facial nerve injury, laryngeal paralysis by laryngeal and vagus nerve injury, and tongue dysfunction by irradiation of the head and neck regions. The animals used in each injury model, the injury method that induced dysphagia, the screening method for dysphagia, and the results are summarized. The use of appropriate animal models of dysphagia may provide adequate answers to biological questions. This review can help in selecting a dysphagia animal system tailored for the purpose of providing a possible solution to overcome dysphagia.

An Experimental Swallow Evoked Potential Protocol to Investigate the Neural Substrates of Swallowing

Advancement in dysphagia intervention is hindered by our lack of understanding of the neural mechanisms of swallowing in health and disease. Evoking and understanding neural activity in response to normal and disordered swallowing is essential to bridge this knowledge gap. Building on sensory evoked potential methodology, we developed a minimally invasive approach to generate swallow evoked potentials (SwEPs) in response to repetitive swallowing induced by citric acid stimulation of the oropharynx in lightly anesthetized healthy adult rats. The SwEP waveform consisted of 8 replicable peaks within 10 milliseconds immediately preceding the onset of electromyographic swallowing activity. Methodology refinement is underway with healthy rats to establish normative SwEP waveform morphology before proceeding to models of advanced aging and age-related neurodegenerative diseases. Ultimately, we envision that this experimental protocol may unmask the pathologic neural substrates contributing to dysphagia to accelerate the discovery of targeted therapeutics.

A Mouse Model of Dysphagia After Facial Nerve Injury

OBJECTIVE

Dysphagia is common following facial nerve injury; however, research is sparse regarding swallowing-related outcomes and targeted treatments. Previous animal studies have used eye blink and vibrissae movement as measures of facial nerve impairment and recovery. The purpose of this study was to create a mouse model of facial nerve injury that results in dysphagia to enhance translational research outcomes.

STUDY DESIGN

Prospective animal study.

METHODS

Twenty C57BL/6J mice underwent surgical transection of the main trunk (MT) (n = 10) or marginal mandibular branch (MMB) (n = 10) of the left facial nerve. Videofluoroscopic swallow study (VFSS) assays for drinking and eating were performed at baseline and 14 days postsurgery to quantify several deglutition-related outcome measures.

RESULTS

VFSS analysis revealed that MT transection resulted in significantly slower lick and swallow rates during drinking (P ≤ .05) and significantly slower swallow rates and longer inter-swallow intervals during eating (P ≤ .05), congruent with oral and pharyngeal dysphagia. After MMB transection, these same VFSS metrics were not statistically significant (P > .05).

CONCLUSION

The main finding of this study was that transection of the facial nerve MT leads to oral and pharyngeal stage dysphagia in mice; MMB transection does not. These results from mice provide novel insight into specific VFSS metrics that may be used to characterize dysphagia in humans following facial nerve injury. We are currently using this surgical mouse model to explore promising treatment modalities such as electrical stimulation to hasten recovery and improve outcomes following various iatrogenic and idiopathic conditions affecting the facial nerve.

LEVEL OF EVIDENCE

NA Laryngoscope, 131:17-24, 2021.

Persistent Feeding and Swallowing Deficits in a Mouse Model of 22q11.2 Deletion Syndrome

Disrupted development of oropharyngeal structures as well as cranial nerve and brainstem circuits may lead to feeding and swallowing difficulties in children with 22q11. 2 deletion syndrome (22q11DS). We previously demonstrated aspiration-based dysphagia during early postnatal life in the LgDel mouse model of 22q11DS along with disrupted oropharyngeal morphogenesis and divergent differentiation and function of cranial motor and sensory nerves. We now ask whether feeding and swallowing deficits persist in adult LgDel mice using methods analogous to those used in human patients to evaluate feeding and swallowing dysfunction. Compared to wild-type mice, videofluoroscopic swallow study revealed that LgDel mice have altered feeding and swallowing behaviors, including slower lick rates, longer inter-lick intervals, and longer pharyngeal transit times with liquid consistency. Transoral endoscopic assessment identified minor structural anomalies of the palate and larynx in one-third of the LgDel mice examined. Video surveillance of feeding-related behaviors showed that LgDel mice eat and drink more frequently. Furthermore, LgDel animals engage in another oromotor behavior, grooming, more frequently, implying that divergent craniofacial and cranial nerve structure and function result in altered oromotor coordination. Finally, LgDel mice have significantly increased lung inflammation, a potential sign of aspiration-based dysphagia, consistent with results from our previous studies of early postnatal animals showing aspiration-related lung inflammation. Thus, oromotor dysfunction, feeding, and swallowing difficulties and their consequences persist in the LgDel 22q11DS mouse model. Apparently, postnatal growth and/or neural plasticity does not fully resolve deficits due to anomalous hindbrain, craniofacial, and cranial nerve development that prefigure perinatal dysphagia in 22q11DS. This new recognition of persistent challenges with feeding and swallowing may provide opportunities for improved therapeutic intervention for adolescents and adults with 22q11DS, as well as others with a history of perinatal feeding and swallowing disorders.