The impact of damage‐associated molecular patterns on the neurotransmitter release and gene expression in the ex vivo rat carotid body

Abstract

New Findings What is the central question of this study?Are carotid bodies (CBs) modulated by the damage‐associated molecular patterns (DAMPs) and humoral factors of aseptic tissue injury? What are the main findings and their importance?DAMPs (HMGB1, S100 A8/A9) and blood plasma from rats subjected to tibia surgery, a model of aseptic injury, stimulate the release of neurotransmitters (ATP, dopamine) and TNF‐α from ex vivo rat CBs. All‐thiol HMGB1 mediates upregulation of immune‐related biological pathways. These data suggest regulation of CB function by endogenous mediators of innate immunity. Abstract The glomus cells of carotid bodies (CBs) are the primary sensors of arterial partial O2 and CO2 tensions and moreover serve as multimodal receptors responding also to other stimuli, such as pathogen‐associated molecular patterns (PAMPs) produced by acute infection. Modulation of CB function by excessive amounts of these immunomodulators is suggested to be associated with a detrimental hyperinflammatory state. We have hypothesized that yet another class of immunomodulators, endogenous danger‐associated molecular patterns (DAMPs), released upon aseptic tissue injury and recognized by the same pathogen recognition receptors as PAMPs, might modulate the CB activity in a fashion similar to PAMPs. We have tested this hypothesis by exposing rat CBs to various DAMPs, such as HMGB1 (all‐thiol and disulfide forms) and S100 A8/A9 in a series of ex vivo experiments that demonstrated the release of dopamine and ATP, neurotransmitters known to mediate CB homeostatic responses. We observed a similar response after incubating CBs with conditioned blood plasma obtained from the rats subjected to tibia surgery, a model of aseptic injury. In addition, we have investigated global gene expression in the rat CB using an RNA sequencing approach. Differential gene expression analysis showed all‐thiol HMGB1‐driven upregulation of a number of prominent pro‐inflammatory markers including Il1α and Il1β. Interestingly, conditioned plasma had a more profound effect on the CB transcriptome resulting in inhibition rather than activation of the immune‐related pathways. These data are the first to suggest potential modulation of CB function by endogenous mediators of innate immunity.

Growth Factors in the Carotid Body—An Update

“…In particular, inflammatory cytokines (i.e., IL-1α, IL-1β, IL-6, and TNFα) were proved to be expressed into type I cells of the rat CB by RT-PCR [ 49 , 65 , 66 ], quantitative Real-Time PCR (qPCR) [ 67 , 68 ], amplified RNA (aRNA)/qPCR technology [ 69 ], RNA sequencing [ 68 ], ELISA [ 68 ], double immunofluorescence [ 49 ], and immunohistochemical studies [ 70 , 71 , 72 , 73 ] ( Table 8 ).…”

Damage control: carotid body activation and remodelling in response to aseptic tissue injury

“…In addition to hypoxia, it is becoming more apparent that the CB also responds to other threatening stimuli including hypercapnia, acidosis, hypoglycaemia and the stress hormone adrenaline (Thompson et al., 2016). In this issue of Experimental Physiology , Mkrtchian and co‐workers now provide the first evidence that the CB is able to detect and respond to damage‐associated molecular patterns (DAMPs) – key inflammatory mediators released into the systemic circulation upon aseptic cell/tissue injury or death (Mkrtchian et al., 2020). This is an exciting and timely translational study that uses a range of in vivo , biochemical and genetic approaches to emphasise the importance of immune signalling in regulating CB function and remodelling in this novel context.…”

“…In their initial experiments, the authors demonstrate that exogenous application of two well‐known DAMPs, high mobility group box 1 (HMGB1) and S100 A8/A9, to ex vivo isolated rat CBs causes an elevation in extracellular dopamine, consistent with CB stimulation and neurotransmitter release (Mkrtchian et al., 2020). To strengthen these findings and increase the translational impact, the investigators then obtained plasma from animals following tibial surgery, a well‐established model of aseptic tissue injury.…”

“…Using elegant and highly skilled cutting‐edge molecular biology techniques, the authors go on to evaluate the impact of DAMPs on gene expression in the CB. Both exogenous application of HMGB1 and the conditioned plasma evoked significant alterations in gene expression, measured using RNA sequencing (Mkrtchian et al., 2020). Thus, exposure to DAMPs and tissue injury not only evokes acute CB activation, but also has the potential to produce long term CB remodelling.…”

“…The study by Mkrtchian and co‐workers also paves the way for future experiments to explore the pathological impact that the genetic alterations induced by DAMPs may have on CB function. Although the acute action of tissue injury and conditioned plasma was to elevate type I cell transmitter release, it appears that this was not well maintained and after 24 h may even have started to fall below baseline (Mkrtchian et al., 2020). Thus, the longer lasting actions of DAMPs may be to impair CB function.…”