Peripheral neurons, CRH, and sickness behavior

jnmaciuch

jnmaciuch

Senior Member (Voting Rights)
Rather than do a separate thread for each paper, I figured I would make one thread pulling together a couple findings and see if there's any potential relevance to ME/CFS.

CRH is of interest from the ME/CFS autopsy study preliminary results, discussed here:
J

Thread 'Loss of CRH neurons and other neural changes in ME/CFS autopsy study - University of Amsterdam'


  • All in all, the HPA axis with the dramatic reduction of CRH-producing neurons in the hypothalamus, a shutdown of the pituitary, and lower cortisol levels found appeared to be pretty much of a wreck in these autopsied patients.
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Biofilm exopolysaccharides alter sensory-neuron-mediated sickness during lung infection, 2024, Granton et al

Granton, Elise; Brown, Luke; Defaye, Manon; Moazen, Parisa; Almblad, Henrik; Randall, Trevor E.; Rich, Jacquelyn D.; Geppert, Andrew; Abdullah, Nasser S.; Hassanabad, Mortaza F.; Hiroki, Carlos H.; Farias, Raquel; Nguyen, Angela P.; Schubert, Courtney; Lou, Yuefei; Andonegui, Graciela; Iftinca, Mircea; Raju, Deepa; Vargas, Mario A.; Howell, P. Lynne; Füzesi, Tamás; Bains, Jaideep; Kurrasch, Deborah; Harrison, Joe Jonathan; Altier, Christophe; Yipp, Bryan G.

Highlights
  • Non-biofilm P. aeruginosa induce greater sickness than biofilm-producing strains
  • Lung TRPV1+ nociceptors detect LPS from non-biofilm bacterial pneumonias via TLR4
  • Vagal nociceptors in the lung activate acute stress neurocircuits in the hypothalamus
  • CRH from the PVN of the hypothalamus drives sickness behavior in non-biofilm infections
Summary

Infections of the lung cause observable sickness thought to be secondary to inflammation. Signs of sickness are crucial to alert others via behavioral-immune responses to limit contact with contagious individuals. Gram-negative bacteria produce exopolysaccharide (EPS) that provides microbial protection; however, the impact of EPS on sickness remains uncertain. Using genome-engineered Pseudomonas aeruginosa (P. aeruginosa) strains, we compared EPS-producers versus non-producers and a virulent Escherichia coli (E. coli) lung infection model in male and female mice. EPS-negative P. aeruginosa and virulent E. coli infection caused severe sickness, behavioral alterations, inflammation, and hypothermia mediated by TLR4 detection of the exposed lipopolysaccharide (LPS) in lung TRPV1+ sensory neurons. However, inflammation did not account for sickness. Stimulation of lung nociceptors induced acute stress responses in the paraventricular hypothalamic nuclei by activating corticotropin-releasing hormone neurons responsible for sickness behavior and hypothermia. Thus, EPS-producing biofilm pathogens evade initiating a lung-brain sensory neuronal response that results in sickness.

Web | DOI | Cell
 
Granton et al. was inspired by the mystery behind indolent lung infections, where the local inflammation and damage might be very severe but the patient only experiences mild feelings of sickness. They thought that the answer might lie in an interaction between a specific secretion from bacteria (biofilm) and lung sensory neurons.

Basically, what they're showing is that sickness behavior in bacterially infected mice is dependent on activation of TLR4 on peripheral sensory neurons. TLR4 detects specific fragments of pathogens and is highly expressed on immune cells, but can also be expressed on other cell types. Bacteria that produced a lot of biofilm were able to block those sensory neurons and substantially reduced sickness behavior.

They also found that sickness behavior was dependent on upregulation of CRH production in the hypothalamus. CRH neuron activation is known to happen during infection, and it was assumed to be mediated by circulating cytokines, which those neurons have receptors for. But this study shows that it's indirectly mediated by the CRH neurons getting peripheral signals from sensory neurons. Get rid of TLR4 on sensory neurons, no sickness behavior. Block CRH, no sickness behavior.

However, this shows that CRH upregulation is necessary but not sufficient for generating sickness behavior.
 
Just looking at the title and abstract it is a bit odd that the title talks of biofilm when the findings seem not to relate to biofilm - just polysaccharides, which have always been implicated in 'sickness behaviour'.

It would be interesting if CRH was involved. It seems surprising, since CRH is supposed to stimulate ACTH and adrenal steroid production since adrenal steroids reduce 'sickness behaviour'.
 
I've previously discussed another paper showing that sickness behavior is driven by interferon and interferon-stimulated CXCL10 in the brain:

Hutan

Thread 'Brain Endothelial- and Epithelial-Specific Interferon Receptor Chain 1 Drives Virus-Induced Sickness Behavior and Cognitive Impairment, 2016, Blank +'

Link

Thomas Blank 1 ∙ Claudia N. Detje 2 ∙ Alena Spieß 1 ∙ Nora Hagemeyer 1 ∙ Stefanie M. Brendecke 1 ∙ Jakob Wolfart 3 ∙ Ori Staszewski 1 ∙ Tanja Zöller 1 ∙ Ismini Papageorgiou 4 ∙ Justus Schneider 4 ∙ Ricardo Paricio-Montesinos 1 ∙ Ulrich L.M. Eisel 5∙ Denise Manahan-Vaughan 6 ∙ Stephan Jansen 6 ∙ Stefan Lienenklaus 2, 7 ∙ Bao Lu 8 ∙ Yumiko Imai 9 ∙ Marcus Müller 10 ∙ Susan E. Goelz 11 ∙ Darren P. Baker 12 ∙ Markus...
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In that study, they showed that CXCL10 alone is sufficient to induce sickness behavior in mice, and that the receptor for CXCL10 (CXCR3) is necessary for sickness behavior induced by interferon beta. CXCR3 is expressed on many neuron types, though CRH was not specifically assessed. Other studies (cited in Blank et al) have shown that CXCL10 modulates neuron firing rates and neurotransmitter release.

So these two findings are not necessarily contradictory, if they represent two different ways that sickness behavior can be activated. There might also be overlap, if sickness behavior in Blank et al. is dependent on the effect of CXCL10 on CRH neurons
 
jnmaciuch said:
Basically, what they're showing is that sickness behavior in bacterially infected mice is dependent on activation of TLR4 on peripheral sensory neurons. TLR4 detects specific fragments of pathogens and is highly expressed on immune cells, but can also be expressed on other cell types.
Click to expand...

That is neat and makes good sense.
jnmaciuch said:
Block CRH, no sickness behavior.
Click to expand...

So this is the surprising finding to me. I would expect CRH to be switched on but not to mediate the response. If this is right it seems as if there are branches to the causal tree around the CRH cell area that might be very significant. There was previous discussion of symptoms in ME/CFS arising because one thing was dodgy but not another that ought to go with it. The specific suggestion might have been wrong but this raises the same sort of paradox to me.
 
Jonathan Edwards said:
Just looking at the title and abstract it is a bit odd that the title talks of biofilm when the findings seem not to relate to biofilm - just polysaccharides, which have always been implicated in 'sickness behaviour'.
Click to expand...
EPS is specifically a biofim-associated polysaccharide, necessary for aggregation. Unlike LPS it doesn't seem to stimulate TLR4 directly. So I guess the authors were driven by the question of why EPS+ vs. EPS- strains produce such different strengths of sickness behavior response
 
There are precedents for this sort of paradox when it comes to the 'HPA axis'. For instance, corticosteroid treatment induces water retention but baseline corticosteroid is necessary for water excretion.
 
Jonathan Edwards said:
I would expect CRH to be switched on but not to mediate the response.
Click to expand...
Agreed, especially when no one is getting sickness behavior-like responses from a stress-induced CRH spike (or any of the other things that stimulate CRH). So it seems to be a "necessary but not sufficient" piece, at least for LPS-mediated sickness behavior. It has to be CRH + some other immune-related signaling
 
Paraventricular nucleus CRH neurons regulate acute lung injury via sympathetic nerve–neutrophil axis, 2025, Li et al

Li, Hui; Liu, Tao; Wang, Yang; Miao, Xue-Mei; Xiong, Yi-Yu; Zhao, Qian; Shen, Wei-Yun; Su, Fu-Hong; Chen, Kang; Dai, Ru-Ping

Abstract

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are severe conditions with high morbidity and mortality with limited effective therapies. Neuroimmune interactions play a critical role in lung homeostasis, but it remains unclear if specific brain regions regulate lung inflammation. Here, we perform anatomical tracing, chemogenetic modulation, and pharmacological interventions in male mice and identify a neural circuit from corticotropin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus (CRHPVN neurons) to the lung. The activation of these neurons protects mice from ALI and promotes survival, reduces neutrophil infiltration and effector functions in the lung, whereas inhibiting CRHPVN neurons worsens ALI. The protective effect is mediated by increased sympathetic nervous activity, with locally released norepinephrine modulating neutrophil functions via β2-AR–β-arrestin2 signaling to inhibit the NF-κB pathway. These findings uncover a brain-lung neural circuit that modulates immune responses during ALI, offering a potential therapeutic target for ALI and ARDS.

Web | DOI | PMC | PDF | Nature Communications
 
Another paper which cites Granton et al. as its inspiration. The part that specifically interested me was this:
Li et al. said:
The activation of CRHPVN neurons was further examined using a cecal ligation and puncture (CLP)-induced polymicrobial sepsis model. Similar to the effects of intranasal LPS administration, CLP caused a marked increase in c-Fos⁺ cells, which are predominantly expressed in CRH neurons at 3-h post-CLP, indicating CRHPVN neuronal activation in response to CLP (Supplementary Fig. 3a–d). The increased calcium activity further confirmed the CRHPVN neuronal activation upon CLP treatment (Supplementary Fig. 3e–g). CRHPVN neuronal activation in response to CLP may be mediated by the nerve transmission or sepsis-related systemic inflammation.

Notably, c-Fos activation persisted for up to 24 h following CLP (Supplementary Fig. 4a). Bilateral subdiaphragmatic vagotomy (SDV) effectively blocked the early phase of CRHPVN neuronal activation (1-h post-CLP), suggesting critical involvement of vagal afferent signaling (Supplementary Fig. 4a, b). In contrast, during the later phase (6-h post-CLP and beyond), SDV only mildly inhibited c-Fos upregulation (Supplementary Fig. 4a, b). Combined anti-TNF and anti-IL-1β antibody treatment potently suppressed CRHPVN neuronal activation during this later phase (Supplementary Fig. 4c, d). These results reveal that the activation of CRHPVN neurons in CLP is mediated by neural mechanisms in the initial phase, and primarily driven by systemic inflammation along with neural transmission in the later phase.
Click to expand...
 
And finally there's the paradox with depleted CRH neurons in ME/CFS, if CRH is supposed to mediate sickness behavior.

For that, the only potential answer I have is this paper:
Cyclic AMP Inducible Early Repressor Mediates the Termination of Corticotropin Releasing Hormone Transcription in Hypothalamic Neurons, 2009, Liu et al

Liu, Ying; Aguilera, Greti

Abstract

Elevations of inducible cAMP early repressor (ICER), the repressor isoform of the cAMP-responsive element modulator (CREM), are associated with protein binding to the corticotrophin releasing hormone (CRH) promoter and termination of CRH transcriptional responses to stress. To determine whether endogenous ICER production represses CRH transcription, we examined the effect of CREM siRNA on forskolin-stimulated ICER formation and CRH transcription in the hypothalamic cell line, 4B, and in primary cultures of hypothalamic neurons. Cotransfection of 4B cells with CREM siRNA and a CRH promoter-driven luciferase reporter gene markedly reduced the induction of ICER by forskolin and potentiated the stimulatory effect of forskolin on CRH promoter activity, compared with cells cotransfected with a nonspecific oligonucleotide. The role of ICER on endogenous CRH expression was studied in primary cultures of hypothalamic neurons by examining the effect of CREM siRNA on forskolin-induced primary transcript (CRH hnRNA) using intronic real-time PCR. As observed during stress in vivo, forskolin-stimulated CRH hnRNA was transient, increasing up to 60 min and declining to near basal values by 3 h. Transfection of CREM siRNA reduced forskolin-induced ICER by about 45% 48-h later and partially reversed the declining phase of CRH hnRNA production at 3 h. The data provide evidence that endogenous ICER formation is required for termination of CRH transcription and support the hypothesis that ICER is part of an intracellular feedback mechanism limiting the activation of CRH transcription during stress.

Web | DOI | PMC | PDF | Cellular and Molecular Neurobiology
 
Basically, CRH production has a pretty responsive negative regulatory mechanism. So repeatedly stimulated CRH neurons have at least one known way to shut down their own CRH production. So the question is: if the CRH depletion in ME/CFS is real, and CRH is potentially relevant to sickness behavior-like symptoms in ME/CFS, is it possible that in the severe ppts who donated their brains we are just seeing the effects of initially overactive CRH?

[Edit: alternatively, there may be another epigenetic regulatory mechanism on CRH from the immune signaling, like what ME/CFS Science Blog linked in the CRH autopsy thread]
 
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jnmaciuch said:
Here, we perform anatomical tracing, chemogenetic modulation, and pharmacological interventions in male mice and identify a neural circuit from corticotropin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus (CRHPVN neurons) to the lung. The activation of these neurons protects mice from ALI and promotes survival, reduces neutrophil infiltration and effector functions in the lung, whereas inhibiting CRHPVN neurons worsens ALI. The protective effect is mediated by increased sympathetic nervous activity, with locally released norepinephrine modulating neutrophil functions via β2-AR–β-arrestin2 signaling to inhibit the NF-κB pathway. These findings uncover a brain-lung neural circuit that modulates immune responses during ALI, offering a potential therapeutic target for ALI and ARDS.
Click to expand...

That further complicates things, suggesting that CRH neurons have an anti-inflammatory effect other than through corticosteroid.

I wonder whether there is a problem with the 'sickness behaviour ' tag here. People often equate it with feeling nauseous and febrile and otherwise ghastly in us but maybe there is also a sympathetic element that makes mice jumpy and is controlled differently?
 
Jonathan Edwards said:
That further complicates things, suggesting that CRH neurons have an anti-inflammatory effect other than through corticosteroid.

I wonder whether there is a problem with the 'sickness behaviour ' tag here. People often equate it with feeling nauseous and febrile and otherwise ghastly in us but maybe there is also a sympathetic element that makes mice jumpy and is controlled differently?
Click to expand...
Maybe? I think not, though, because in the CXCL10 paper "sickness behavior" was explicitly defined as lethargy with way less movement around the enclosure. I suppose a mouse could be jumpy and non-mobile but that seems a bit weird. The alternative explanation is that CRH doesn't make people feel jumpy--CRH production is triggered by adrenaline after all. So maybe it's the adrenaline or something else that induces the jumpiness, but direct stimulation of CRH doesn't
 
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