Unraveling the molecular pathobiology of vocal fold systemic dehydration using an in vivo rabbit model

Molecular changes, histopathology, and ultrasonic vocalization acoustic profiles of systemically dehydrated rats

Systemic hydration is known to promote optimal functioning of bodily systems-including the vocal folds. The impact of systemic dehydration on the biology of the vocal folds and the downstream effects of dehydration on voice output are not well understood. An in vivo rat model of systemic dehydration was employed to investigate vocal fold gene expression, histological changes, and acoustic changes in vocalization. Ultrasonic vocalizations (USVs) were recorded every day for 5 days (baseline), in male and female Long-Evans rats (N = 36, ages: 3-4 months) using an anticipatory reward paradigm. Next, rats were dehydrated (N = 18) using a published water-restriction model for 5 days or euhydrated (N = 18) and provided ad libitum access to water for 5 days. USVs were recorded daily during the dehydration/euhydration period. The USV variables were averaged at baseline and following dehydration/euhydration for individual animals, and the difference between these time periods was used for statistical analysis. USV analysis included total USV count, complexity ratio, duration (s), frequency range (kHz), and maximum intensity (dB). At the end of dehydration/euhydration, animals were euthanized, and kidney and vocal fold tissue samples were dissected and processed for histology and gene expression analysis. Compared to euhydrated rats, dehydrated male and female rats had significantly up-regulated gene expression of kidney renin (male p = 0.047; female p = 0.018), indicating physiologic dehydration. There were no statistically significant differences in the USV acoustic profile or histopathology between the two groups. Differential expression (p < 0.05) of several genes related to extracellular matrix remodeling, inflammatory responses, and water ion transport in the vocal folds was present. Our results indicate that mild systemic dehydration impacts gene expression in the vocal fold mucosa; however, these gene expression changes are not evident in the acoustic profile of vocalizations.

Effectiveness of Systemic Hydration on Thermal and Multi-Dimensional Voice Outcome in Speech-Language Pathologists

PURPOSE

Professional voice users utilize systematic hydration to preserve the quality of their voice. However, the existing literature provides limited research on the systemic hydration of voice quality from a thermal and multi-dimensional perspective, particularly in speech-language pathologists (SLPs). Thus, using thermal and multidimensional measures, the current study inspected the effectiveness of systemic hydration in SLPs' voice quality.

METHOD

The study involved 20 vocally healthy female SLPs allocated to vocal loading (control group), who read passage loudly for 1 hour, and systemic hydration (experimental group), who consumed water during the vocal loading task. Pre evaluation and post evaluation of multidimensional voice measures: thermal (front view of throat), acoustic (fundamental frequency, perturbation [frequency and amplitude], and noise-related measures), aerodynamic (maximum phonation time: MPT and s/z ratio), and self-perceptual [perceived phonatory effort (PPE), perceived vocal tiredness (PVT)] measures were collected for two groups.

RESULTS

Pre-post comparison in the vocal loading group had a significant increase in thermal (FVT), acoustic [STD of F0, vF0, jitter, shimmer, amplitude perturbation quotient (APQ), and noise-to-harmonic ratio (NHR)], self-perceptual (PPE and PVT), and decreased aerodynamic measures MPT. However, no significant differences were noted in the systemic hydration group's thermal and multidimensional voice outcome measures. Between-group post test comparisons, the systemic hydration group showed a significant decrease in thermal front view of the throat (FVT), acoustic [jitter, NHR], and an increase in aerodynamic (MPT) compared with the vocal loading group.

CONCLUSION

The voice quality was decreased in the vocal loading group, while the systemic hydration group remained stable, as depicted on multidimensional voice measures. This study confirms the positive benefits of systemic hydration on voice quality and recommends its incorporation into clinical practice for SLPs.

Amniotic Fluid as a Potential Treatment for Vocal Fold Scar in a Rabbit Model

OBJECTIVES/HYPOTHESIS

Vocal fold (VF) injury and chronic inflammation can progress to scarring, which is notoriously difficult to treat. Human amniotic fluid (AF) has potential for VF wound healing in a rabbit model, and we hypothesized that AF would demonstrate wound healing properties superior to hyaluronic acid (HA) over time.

STUDY DESIGN

Randomized, controlled trial.

METHODS

In this pilot study, 60 New Zealand white rabbits (10 rabbits in each of six groups) received unilateral VF biopsy and immediate injection treatment of AF, HA, or saline (SA). The contralateral VF served as an uninjured control. Rabbits were sacrificed and VFs removed at 4 or 10 weeks postinjury. Rheology and reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) were employed to assess viscoelastic properties and inflammation, respectively.

RESULTS

Rheology differences were seen between four and 10 weeks postinjury in treatment groups but not in controls. Values for the AF treatment group differed from the SA and HA groups at week 10 (elastic P = 0.0002, viscous P < 0.0001). RT-qPCR: AF and HA had higher levels of inflammatory cytokines than SA at week 4 [TNFα: SA4 < HA4 (P = 0.0086), SA4 < AF4 (P = 0.0112)]. Presumptive inflammation was still present at 10 weeks in all treatment groups [IL-1β: AF control < AF (P = 0.0002), SA control < SA (P = 0.0212); IL-6: HA control < HA (P = 0.0312)]. AF demonstrated reduced inflammation at 10 weeks compared with SA [IL-6: SA > AF (P = 0.0141)]. AF was the only treatment group that had significant reductions in inflammation at 10 versus 4 weeks [IL-1β: AF4 > AF10 (P = 0.0249)].

CONCLUSIONS

The main finding from this pilot study was that AF demonstrated wound healing effects over time compared with HA and SA by reducing inflammation and improving VF viscoelastic properties. Continued research to further investigate the use of AF in VF wound healing over longer periods of time is necessary before translation to human VFs.

A Review on In Vivo Research Dehydration Models and Application of Rehydration Strategies

Background: Dehydration, a common condition where the amount water lost from the body exceeds intake, disrupts metabolic processes and negatively impacts health and performance. Rehydration, the process of restoring body fluids and electrolytes to normal levels, is crucial for maintaining physiological health. In vivo dehydration models are experimental systems used to study the effects of dehydration on living organisms. However, a comprehensive summary of in vivo models and the application of human rehydration strategies is lacking. Methods: This review provides a comprehensive overview of various in vivo models and rehydration strategies. Results: In vivo models, stimulated by fluid restriction, exercise, thermal exposure, and chemicals, have been used to study dehydration. Importantly, the principles, characteristics, and limitations of the in vivo models are also discussed, along with rehydration administration methods, including oral, intestinal, intravenous, subcutaneous, and intraperitoneal routes. Additionally, rehydration strategies and the application for managing different dehydration conditions both in daily life and clinical settings have been summarized. Conclusions: Overall, this review aims to enhance the understanding of the conditions in which in vivo dehydration models and rehydration strategies are applicable, thereby advancing research into the physiological and pathological mechanisms of dehydration and supporting the development of effective rehydration therapies.

High-Resolution Profiling of Human Vocal Fold Cellular Landscapes With Single-Nuclei RNA Sequencing

INTRODUCTION

The function of the vocal folds (VFs) is determined by the phenotype, abundance, and distribution of differentiated cells within specific microenvironments. Identifying this histologic framework is crucial in understanding laryngeal disease. A paucity of studies investigating VF cellular heterogeneity has been undertaken. Here, we examined the cellular landscape of human VFs by utilizing single-nuclei RNA-sequencing.

METHODS

Normal true VF tissue was excised from five patients undergoing pitch elevation surgery. Tissue was snap frozen in liquid nitrogen and subjected to cellular digestion and nuclear extraction. Nuclei were processed for single-nucleus sequencing using the 10X Genomics Chromium platform. Sequencing reads were assembled using cellranger and analyzed with the scanpy package in python.

RESULTS

RNA sequencing revealed 18 global cell clusters. While many were of epithelial origin, expected cell types, such as fibroblasts, immune cells, muscle cells, and endothelial cells were present. Subcluster analysis defined unique epithelial, immune, and fibroblast subpopulations.

CONCLUSION

This study evaluated the cellular heterogeneity of normal human VFs by utilizing single-nuclei RNA-sequencing. With further confirmation through additional spatial sequencing and microscopic imaging, a novel cellular map of the VFs may provide insight into new cellular targets for VF disease.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:3193-3200, 2024.

In Vivo Visualization and Quantification of Rat Laryngeal Blood Supply After Hydration Challenge

OBJECTIVES

Systemic dehydration decreases total body blood volume; however, hemodynamic alterations at the level of local organs, such as the larynx, remain unclear. Here we sought to quantify superior thyroid artery (STA) blood flow after dehydration and rehydration using in vivo magnetic resonance angiography (MRA) and ultrasound imaging in a rat model.

METHODS

Male Sprague-Dawley rats (N = 17) were included in this prospective, repeated measures design. Rats first underwent MRA to determine baseline STA cross-sectional area, followed by high-frequency in vivo ultrasound imaging to measure STA blood velocity at baseline. Next, rats were systemically dehydrated (water withholding), followed by rehydration (water ad-lib). Ultrasound imaging was repeated immediately after dehydration and following rehydration. The STA blood velocity and STA cross-sectional area were used to compute STA blood flow. Three rats served as temporal controls for ultrasound imaging. To determine if the challenges to hydration status affected the STA cross-sectional area, four rats underwent only MRA at baseline, dehydration, and rehydration.

RESULTS

Systemic dehydration resulted in 10.5% average body weight loss. Rehydration resulted in average body weight gain of 10.9%. Statistically significant reductions were observed in STA mean blood flow rate after dehydration. Rehydration reversed these changes to pre-dehydration levels. No significant differences were observed in STA cross-sectional area with dehydration or rehydration.

CONCLUSION

Systemic dehydration decreased blood flow in the superior thyroid artery. Rehydration restored blood flow in the STA. Change in hydration status did not alter the STA cross-sectional area. These preliminary findings demonstrate the feasibility of using ultrasound and MRA to quantify hemodynamic changes and visualize laryngeal blood vessels.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:779-785, 2024.

Impact of Rehydration Following Systemic Dehydration on Vocal Fold Gene Expression

OBJECTIVE

Biological data on the beneficial effects of vocal fold rehydration are lacking. This study aimed to examine the effects of acute systemic dehydration on vocal fold gene expression and determine whether rehydration would reverse these changes.

METHODS

Male New Zealand White rabbits (N = 24, n = 8/group) provided the animal model. Systemic dehydration was induced by 5 days of water volume restriction. Rehydration was provided by ad-lib water for 3 days following dehydration. Euhydrated rabbits were used as the control group. Vocal fold tissue was dissected. Seventeen genes were selected based on physiological function and role in supporting vocal fold structure, oxidative stress, hemodynamics, and extracellular matrix turnover. Relative gene expression was assessed by RT-qPCR.

RESULTS

Rehydration following systemic dehydration can modulate gene expression, with expression patterns suggesting that rehydration reverses dehydration-induced changes in over half of the tested genes. CLIC5 (chloride intracellular channel 5) and EFEMP1 (EGF containing fibulin extracellular matrix protein 1) genes were significantly upregulated in the dehydration group compared with the euhydrated control. A1BG (alpha-1B-glycoprotein) and IL1RAP (interleukin 1 receptor accessory protein) were downregulated by rehydration compared with the dehydration group.

CONCLUSION

This study provides molecular evidence for a transcriptional response to rehydration following acute systemic dehydration in the vocal folds. These data are the first to study gene expression following realistic dehydration and rehydration paradigms and provide biological data to support clinical recommendations to increase water intake after acute dehydration.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:3499-3505, 2023.

Comparative proteomic changes in rabbit vocal folds undergoing systemic dehydration and systemic rehydration

BACKGROUND

A considerable body of clinical evidence suggests that systemic dehydration can negatively affect voice production, leading to the common recommendation to rehydrate. Evidence for the corrective benefits of rehydration, however, is limited with mixed conclusions, and biological data on the underlying tissue changes with rehydration is lacking. In this study, we used a rabbit model (n = 24) of acute (5 days) water restriction-induced systemic dehydration with subsequent rehydration (3 days) to explore the protein-level changes underlying the molecular transition from euhydration to dehydration and following rehydration using LC-MS/MS protein quantification in the vocal folds. We show that 5-day water restriction led to an average 4.3% decrease in body weight with relative increases in anion gap, Cl-, creatinine, Na+, and relative decreases in BUN, iCa2+, K+, and tCO2 compared to control (euhydrated) animals. A total of 309 differentially regulated (p < 0.05) proteins were identified between the Control and Dehydration groups. We observed a noteworthy similarity between the Dehydration and Rehydration groups, both well differentiated from the Control group, highlighting the distinct timelines of resolution of the clinical symptoms of systemic dehydration and the underlying molecular changes.

SIGNIFICANCE

Voice disorders are a ubiquitous problem with considerable economic and psychological impact. Maintenance of proper hydration is commonly prescribed as a general vocal hygiene practice. There is evidence that dehydration negatively impacts phonation, but our understanding of the state of vocal folds in the context of systemic dehydration are limited, particular from a molecular perspective. Further, ours is a novel molecular study of the short-term impact of rehydration on the tissue. Given the relatively minimal difference in vocal fold proteomic profiles between the Dehydration and Rehydration groups, our data demonstrate a complex physiological response to acute systemic dehydration, and highlight the importance of considering persistent underlying molecular pathology despite the rapid resolution of clinical measures. This study sets a foundation for future research to confirm the nature of potential beneficial outcomes of clinical recommendations related to hydration.

An optimized method for high-quality RNA extraction from distinctive intrinsic laryngeal muscles in the rat model

Challenges related to high-quality RNA extraction from post-mortem tissue have limited RNA-sequencing (RNA-seq) application in certain skeletal muscle groups, including the intrinsic laryngeal muscles (ILMs). The present study identified critical factors contributing to substandard RNA extraction from the ILMs and established a suitable method that permitted high-throughput analysis. Here, standard techniques for tissue processing were adapted, and an effective means to control confounding effects during specimen preparation was determined. The experimental procedure consistently provided sufficient intact total RNA (N = 68) and RIN ranging between 7.0 and 8.6, which was unprecedented using standard RNA purification protocols. This study confirmed the reproducibility of the workflow through repeated trials at different postnatal time points and across the distinctive ILMs. High-throughput diagnostics from 90 RNA samples indicated no sequencing alignment scores below 70%, validating the extraction strategy. Significant differences between the standard and experimental conditions suggest circumvented challenges and broad applicability to other skeletal muscles. This investigation remains ongoing given the prospect of therapeutic insights to voice, swallowing, and airway disorders. The present methodology supports pioneering global transcriptome investigations in the larynx previously unfounded in literature.

The role of systemic dehydration in vocal fold healing: Preliminary findings

Rationale

Systemic dehydration negatively alters the expression of vocal fold inflammatory and cell junction markers. These biological changes can have downstream effects on the healing processes of injured vocal folds. In the dermis, reduced hydration prolongs inflammation and delays healing. It is unknown whether this biological effect is observed in vocal fold tissue.

Objective

To investigate the effects of systemic dehydration on vocal fold healing outcomes following acute, bilateral vocal fold injury in a rodent model.

Methods

Eighteen systemic dehydrated and 18 euhydrated adult male Sprague Dawley rats experienced bilateral vocal fold injuries or no injury (N = 9/group). Vocal fold gene expression levels of inflammatory mediators and epithelial cell junction markers were measured 24 h post-injury.

Results

Pro-inflammatory gene markers (IL-1β; TNF-α) were differentially expressed in response to systemic dehydration with vocal fold injury compared to non-injury. Epithelial cell junction markers (Cadherin-3, Desmoglein-1) also exhibited divergent trends following systemic dehydration, but these data were not statistically significant.

Conclusions

Systemic dehydration may affect cellular vocal fold healing processes within 24 h. These findings lay the groundwork for further investigation of how hydration status can affect vocal fold tissue recovery and influence clinical care.

Proteomic analysis reveals that aging rabbit vocal folds are more vulnerable to changes caused by systemic dehydration

BACKGROUND

Older adults are more prone to develop systemic dehydration. Systemic dehydration has implications for vocal fold biology by affecting gene and protein expression. The objective of this study was to quantify vocal fold protein changes between two age groups and hydration status, and to investigate the interaction of age and hydration status on protein expression, which has not been investigated in the context of vocal folds before. Comparative proteomics was used to analyze the vocal fold proteome of 6.5-month-old and > 3-year-old rabbits subjected to water ad libitum or water volume restriction protocol.

RESULTS

Young and older adult rabbits (n = 22) were either euhydrated (water ad libitum) or dehydrated by water volume restriction. Dehydration was confirmed by body weight loss of - 5.4% and - 4.6% in young and older groups, respectively, and a 1.7-fold increase of kidney renin gene expression in the young rabbits. LC-MS/MS identified 2286 proteins in the rabbit vocal folds of young and older adult rabbits combined. Of these, 177, 169, and 81 proteins were significantly (p ≤ 0.05) affected by age, hydration status, or the interaction of both factors, respectively. Analysis of the interaction effect revealed 32 proteins with opposite change patterns after dehydration between older and young rabbit vocal folds, while 31 proteins were differentially regulated only in the older adult rabbits and ten only in the young rabbits in response to systemic dehydration. The magnitude of changes for either up or downregulated proteins was higher in the older rabbits. These proteins are predominantly related to structural components of the extracellular matrix and muscle layer, suggesting a disturbance in the viscoelastic properties of aging vocal fold tissue, especially when subjected to systemic dehydration.

CONCLUSIONS

Water restriction is a laboratory protocol to assess systemic dehydration-related changes in the vocal fold tissue that is translatable to human subjects. Our findings showed a higher number of proteins differentially regulated with a greater magnitude of change in the vocal folds of older adult rabbits in the presence of systemic dehydration compared to younger rabbits. The association of these proteins with vocal fold structure and biomechanical properties suggests that older human subjects may be more vulnerable to the effects of systemic dehydration on vocal function. The clinical implications of these protein changes warrant more investigation, but age should be taken into consideration when evaluating vocal treatment recommendations that interfere with body fluid balance.

Recurring exposure to low humidity induces transcriptional and protein level changes in the vocal folds of rabbits

Voice disorders are an important human health condition. Hydration is a commonly recommended preventive measure for voice disorders though it is unclear how vocal fold dehydration is harmful at the cellular level. Airway surface dehydration can result from exposure to low humidity air. Here we have induced airway surface dehydration in New Zealand White rabbits exposed to a recurring 8-h low humidity environment over 15 days. This model mimics an occupational exposure to a low humidity environment. Exposure to moderate humidity was the control condition. Full thickness soft-tissue samples, including the vocal folds and surrounding laryngeal tissue, were collected for molecular analysis. RT-qPCR demonstrated a significant upregulation of MUC4 (mucin 4) and SCL26A9 (chloride channel) and a large fold-change though statistically non-significant upregulation of SCNNA1 (epithelial sodium channel). Proteomic analysis demonstrated differential regulation of proteins clustering into prospective functional groups of muscle structure and function, oxidative stress response, and protein chaperonin stress response. Together, the data demonstrate that recurring exposure to low humidity is sufficient to induce both transcriptional and translational level changes in laryngeal tissue and suggest that low humidity exposure induces cellular stress at the level of the vocal folds.