Discussion
Neuropeptides are a kind of special information substances, which generally refer to the endogenous active substances that exist in nervous tissues and participate in the function of the nervous system. It is characterized by low content, high activity, extensive and complex effects, and regulates a variety of physiological functions in the body, such as pain, sleep, emotions, learning and memory, and even the differentiation and development of the nervous system itself are regulated by neuropeptides. Neuropeptides not only exist in the nervous system, but also widely exist in various systems throughout the body, playing various roles such as neurohormones, neurotransmitters, neuromodulators, and cytokines, and playing important regulatory roles in various physiological functions at various stages of biological development7. In recent years, with the development of molecular biology technology, the research of neuropeptides is making rapid progress with each passing day.
The novel coronavirus may invade sensory neurons and glial cells of the vagus nerve, inducing the release of neuropeptides and inflammatory mediators. Epithelial cells and inflammatory cells (such as macrophages, neutrophils, lymphocytes, etc.) involved in COVID-19 infection and recognition can release a variety of cytokines and inflammatory mediators8. Neuropeptides and neuroinflammatory mediators can further recruit and activate immune cells, cause lung and airway inflammation, and enhance cough sensitivity. The nerve-phagocytic nature of the novel coronavirus leads to the release of various inflammatory mediators such as histamine, triggering neurogenic inflammation, and directly or indirectly stimulating nerve receptors, resulting in increased cough sensitivity9. COVID-19 infection can induce eosinophilic bronchitis or airway hyperresponsiveness, a subacute stage of eosinophilic bronchial inflammation, or cough-variant asthma. At present, a large number of studies have found that local glucocorticoids can reduce bronchial inflammation caused by COVID-19 infection or the subacute stage of variant asthma, mainly through a wide range of effects on structural cells and inflammatory cells, and multi-target blocking airway inflammation. It also reduces neuroreceptor sensitivity by reducing neurogenic inflammation. This suggests that the abnormal release of neuropeptides plays a major role in the pathogenesis of COVID-1910,11.
ACE2 is distributed in almost all immune cells and endothelial cells, and its loss in these two cells will lead to the weakening and failure of many important organs in the body12. COVID-19 has a high affinity with ACE2 receptors, and ACE2 is highly expressed in some COVID-19 infections with chronic underlying diseases and elderly people, suggesting that more ACE2 is needed to counteract the adverse effects of angiotensin II13,14. The number of ACE2 receptors is related to age and body health. Studies have shown that the expression of ACE2 in children’s nasal epithelial cells is low. In this study, the expression of ACE and ACE2 in children’s COVID-19 group was not high. There was no significant correlation with the expression of LDH/N%/L%/CRP, an inflammatory indicator of the disease, similar to foreign studies, which may be one of the reasons why children are more tolerant to COVID-19 and have a lower incidence of severe diseases.
ASCL1, a key transcription factor in the development of autonomic ganglia that is overexpressed in fibroblasts, has not been detected in COVID-19-infected patients. Studies have shown that specifically knocking out the ASCL1 gene in asthmatic mice can restrict nerve conduction in lung tissue. At the same time, the levels of inflammatory cells such as Th2, eosinophil, and ILC2 cells were significantly lower than those of wild-type mice, and the expressions of IL-5 and IL-13 were also significantly lower than those of the control group. It is suggested that ASCL1 plays an important role in airway neuroendocrine regulation. In particular, it plays an important role in the occurrence of human wheezing. In this study, the expression level of ASCL1 was negatively correlated with N% and CRP, suggesting that ASCL1 may play a certain role in the inflammation of COVID-19. Further studies on the mechanism of action and larger samples will be discussed in the future.
Gastrin-releasing peptide (GRP) is a neuropeptide widely available in the central nervous system. It has a high affinity with GRP receptor (GRPR) and its expression in different brain regions can participate in different functions. Such as itch, sighing, fear memory, and so on. Recent studies have shown that GRP is significantly elevated in children with asthma, and the use of GRP blockers and GRP antibodies can prevent the abnormal increase of airway reactivity in asthmatic mouse models stimulated by OVA, reduce the number of macrophages and granulocytes in bronchoalveolar lavage fluid, and reduce the levels of IL-5, IL-13, and other cytokines. In this study, there was no statistically significant difference in GRP/VIP expression between COVID-19 infection and non-COVID-19 infection. These results suggest that GRP/VIP has different roles in asthma and COVID-19 infection, so its expression level may be conducive to distinguishing acute asthma attacks or asthma combined with COVID-19 infection.
VIP is distributed in the nerve endings of the bronchial smooth muscle near the human lung and around the submucosal glands of the lung and bronchus. The enzymes released by inflammatory cells in children with asthma will degrade and destroy VIP, which is conducive to cholinergic nerve action and leads to bronchospasm. Therefore, VIP mainly acts as an inhibitory neurotransmitter in the body.
SP is the most abundant sensory neuropeptide in the lung. When SP is released from the sensory nerve by the axonal reflex after inhalation of stimuli, it can induce bronchoconstriction by binding to the specific opposite receptors in the lung, resulting in wheezing. At the same time, SP is involved in the occurrence of lung immune regulation and inflammatory response, and SP has a chemotactic effect on T cells, mononuclear macrophages, and eosinophils15,16. SP level in the sputum of asthmatic patients and chronic bronchitis patients is significantly higher than that of the control group, and SP level in asthmatic patients induced sputum is positively correlated with eosinophils level17. SP has not been studied in children with COVID-19 infection. Cough and wheezing are usually one of the main symptoms in children infected with COVID-19. The specific mechanism of cough and wheezing remains unclear. Non-specific inflammation of the lung interstitial is the main pathophysiological change.
In this study, the expression of neuropeptide-related molecules in COVID-19-infected patients and their correlation with inflammatory indicators were preliminarily analyzed. It was found that, unlike adults, ACE and ACE2 expression was not high in children with COVID-19. The level of neuropeptide ASCL1 in children with COVID-19 was lower than that in non-COVID-19 children, which may indicate that the expression level of ASCL1 may be decreased in COVID-19 patients. At the same time, the expression level of ASCL1 is negatively correlated with N% and CRP, suggesting that ASCL1 may play a certain role in COVID-19 inflammation. Limited by the size of the sample size, this project failed to conduct large-sample and multi-center clinical studies. However, we have provided a new research entry point for COVID-19 infection, focusing on the level of neuropeptides. We will further explore the role of neuropeptides in lung diseases caused by COVID-19 infection through in vivo and in vitro experiments to provide accurate treatment for COVID-19 Therapy provides a new theoretical basis.
Acknowledgments Not applicable.
Funding This work was supported by the Basic and Applied Basic Research Foundation of Guangdong Medical Research Foundation (grant number A2023345), Zhong Nanshan Medical Foundation of Guangdong Province (grant number 202102010343) and the Science and Technology Program of Guangzhou (grant number 202102010276).