Metabolomics: Impact of Comorbidities and Inflammation on Sickness Behaviors for Individuals with Chronic Wounds

Abstract

Significance:

Approximately 6.5 million people in the United States suffer from chronic wounds. The chronic wound population is typically older and is characterized by a number of comorbidities associated with inflammation. In addition to experiencing wound-related pain, individuals with chronic wounds commonly experience multiple concurrent psychoneurological symptoms such as fatigue and depression, which delay wound healing. However, these distressing symptoms have been relatively overlooked in this population, although their adverse effects on morbidity are well established in other chronic disease populations.

Recent Advances:

Inflammation is involved in multiple pathways, which activate brain endothelial and innate immune cells that release proinflammatory cytokines, which produce multiple symptoms known as sickness behaviors. Inflammation-based activation of the kynurenine (KYN) pathway and its metabolites is a mechanism associated with chronic illnesses.

Critical Issues:

Although putative humoral and neuronal routes have been identified, the specific metabolic variations involved in sickness behaviors in chronic wound patients remain unclear. To improve health outcomes in the chronic wound population, clinicians need to have better understanding of the mechanisms underlying sickness behaviors to provide appropriate treatments.

Future Directions:

This article presents a synthesis of studies investigating associations between inflammation, metabolic pathways, and sickness behaviors in multiple chronic diseases. The presentation of a theoretical framework proposes a mechanism underlying sickness behaviors in the chronic wound population. By mediating the immune system response, dysregulated metabolites in the KYN pathway may play an important role in sickness behaviors in chronic inflammatory conditions. This framework may guide researchers in developing new treatments to reduce the disease burden in the chronic wound population.

Junglyun Kim, PhD

SCOPE AND SIGNIFICANCE

As many as 6.5 million people in the United States, or ∼2% of the total population, suffer from chronic wounds.^1,2 Chronic wounds are a significant medical and emotional burden for patients and their caregivers.^3

–6 The chronic wound population is typically in older individuals who also have a number of chronic illnesses, including obesity, diabetes, heart disease and other inflammatory states. Nationally, chronic wound care costs about $25 billion a year.⁷ Infections increase the treatment cost to between $28.1 and $96.8 billion annually.⁸ Individuals with chronic wounds commonly experience significant concurrent psychoneurological symptoms such as pain, fatigue, depression, sleep disturbances, and anxiety,^9,10 which collectively are known as “sickness behaviors.” Sickness behaviors result from the coordinated set of molecular and behavioral changes linked to inflammatory activity in sick individuals.^11,12 Given the decreased quality of life associated with delayed wound healing and chronic wounds, this patient population requires comprehensive early assessment to detect and prevent the sickness behaviors associated with chronic inflammation, in addition to pain and other symptoms that may be more associated with local wound healing.

TRANSLATIONAL RELEVANCE

Metabolites refer to substrates, intermediates, and metabolic products resulting from energy production and storage, signal transduction, and the impact of cellular apoptosis on bodily tissues, organs, fluids, or organisms at a specific time point that are identified and quantified in a high-throughput manner, such as liquid chromatography-mass spectrometry.¹³ Metabolites as a biomarker can provide insights into disease progression, including the effect of environmental factors that develop pharmaceutical and other clinical intervenitions.¹⁴ Metabolite levels are also useful in determining physiological status in individuals and metabolism changes in microbiome.¹⁵ However, current investigations of metabolites in individuals with chronic wounds are insufficient. We hypothesize that metabolites are involved in the complex cascade associated with the presence of sickness behaviors and may serve as a biomarker for prediction and control of sickness behaviors in individuals with chronic wounds. Metabolites may play as biomarkers to measure cellular mechanisms that, through immunity activation, relate to sickness behaviors in patients suffering from chronic wounds. Future investigations of the association between clinical symptoms, dysregulated metabolites, and inflammatory markers may help researchers better understand sickness behaviors in patients with chronic wounds.

CLINICAL RELEVANCE

The chronic wound population commonly experiences symptoms such as depression, anxiety, pain, fatigue, and sleep disturbances that significantly reduce the quality of life for both these individuals and their caregivers. Despite the prevalence and impact of negative physical and psychological symptoms in individuals with chronic wounds, the underlying biomolecular pathway explaining sickness behaviors in this population remains uncertain. No clear research-based explanation exists for a link between inflammation and the biomolecular mechanisms associated with sickness behaviors in the chronic wound population. Reviewing current evidence of biomolecular pathways, this article proposes a conceptual framework to explain sickness behaviors in people with chronic wounds. The proposed conceptual framework provides an understanding of biomolecular pathways underlying sickness behaviors associated with chronic inflammation in delayed wound healing and may lead to the development of therapeutic interventions to facilitate wound healing in this population.

DISCUSSION

The presented conceptual framework (Fig. 1) explains sickness behaviors in the chronic wound population, with a focus on pathways connecting peripheral inflammation and the brain. This framework addresses the influence of dysregulated metabolites induced by cytokines, which are released from the immune system in reaction to inflammation associated and the chronic wound population. Notably, evidence in human studies supports that inflammatory cytokines induce dysregulation of the kynurenine (KYN) pathway, which is associated with symptoms (e.g., depression, anxiety, fatigue, cognitive impairment and sleep disturbance; Table 1). By synthesizing current findings, we found that sickness behaviors and tryptophan (TRP) metabolism in the KYN pathway are associated with inflammation during disease progression. As seen in Fig. 1, our hypothetical model proposes that wound inflammation caused by biofilm-related infection interrupts the normal healing process. This triggers persistent inflammation leading to various symptoms associated with other factors in the chronic wound population. Prolonged inflammation causes the upregulation of proinflammatory cytokines, which may dysregulate metabolites involved in the biosynthesis of certain neurotransmitters—specifically, as seen in the KYN pathway. Such dysregulated metabolites in the central nervous system (CNS) are associated with sickness behaviors in the chronic wound population.

Figure 1.

A conceptual framework for the association of dysregulated metabolites with sickness behaviors in the chronic wound population.

Table 1.

Included studies that examine associations between inflammation, metabolites, and sickness behaviors

Symptoms	Author, Year	Study Design	Study Population	Comparison Group	Sample Size	Inflammatory Markers	Metabolites (Assay)	Sample	Symptom Measurement	Results
Depression, anxiety	Abautret-Daly et al. (2017)	Cross-sectional	IBD patients who were confirmed with ulcerative colitis and Crohn's disease (N = 18; female = 11; mean age = 49 ± 2.69)	Age-balanced patient controls with normal colonoscopy (N = 19; 9 female; age = 54.3 ± 2.3)	37 (Ireland)	IL-6, IFN-γ, CRP (ELISA), MMP-9 (zymography), IFN-γ, MMP-9, TNF-α, IL-6, IL-1β (PCR)	KYN, TRP (HPLC)	Blood	- Profile of mood state test - HAM-D - HAM-A	Increased expression levels of IL-1β, IL-6, MMP-9, and CRP, as well as an increased KYN/TRP ratio, were observed in symptomatic IBD patients. Changes of inflammatory mediators and TRP may be related to the increased anxiety and depression in IBD patients.
Fatigue	Barat et al. (2016)	Cross-sectional	Obese children (N = 41; median age = 12 [9–15])	N/A	41 (France) (metabolic markers: 17 samples)	hs-CRP	TRP, KYN, neopterin	Blood	- Pediatric quality-of-life inventory (Peds-QL^®) multidimensional fatigue scale	BMI and insulin resistance were positively correlated with cognitive fatigue. In obese children, fatigue was not associated with TRP metabolism; however, KYN and KYN/TRP ratios were associated with insulin resistance.
Depressive symptoms, suicidality	Bay-Richter et al. (2015)	Cohort (2 years)	Patients with depression and suicidality (N = 30; 21 females; age = 40.0 ± 8.7)	Healthy controls (N = 36; 7 females; 29.7 ± 7.3 years)	67 (Denmark)	IL-1β, IL-6, TNF-α, IL-8 (CSF)	QA, KA (CSF and serum; HPLC)	CSF and blood	- MADRS - Suicide assessment scale	Depressed patients exhibited elevated CSF QA levels compared to healthy controls over 2 years. CSF KA level was negatively correlated with depressive symptoms and suicidal ideation. A dysregulated KYN pathway may be associated with glutamate-based mechanisms in the development of depressive symptoms and suicidality based on the affinity of QA and KA to the NMDA receptor.
Depression, sleep disturbance	Cho et al. (2017)	Cross-sectional	Currently depressed (N = 68; 64% female; age = 36.2 ± 9.9) and previously depressed (N = 26; 65% female; age = 33.3 ± 11.1) subjects	Never depressed subjects (N = 66; 81% female; age = 32.3 ± 10.2)	160 (United States of America)	hs-CRP (immunoturbidimetric assay)	3-HK, QA, KA, KYN (HPLC)	Blood	- PSQI - MADRS	Sleep disturbance was associated with reduced KA/QA ratio and increased hs-CRP in the currently depressed group, adjusting for age, sex, analysis batch, BMI, and depressive symptoms (KA/QA ratio: adjusted β = −0.30, p = 0.02; hs-CRP: adjusted β = 0.33, p = 0.02). No significant association was found between sleep disturbance and KA/3-HK in any of the groups.
Depressive symptoms	Dahl et al. (2015)	Cohort (baseline, 12-week follow-up)	Psychotropic and immune-modulating medication-free MDD patients with a depressive episode (N = 50; 38 females; age = 40.0 ± 12.0)	Healthy controls (N = 34; 19 females; age = 38.3 ± 13.9)	84 (Norway) (metabolic markers: 43 subjects; cytokine markers: 43 controls, 50 patients)	IL-1β, IL-1Ra, IL-5, IL-6, IL-7, IL-8, IL-10, G-CSF, IFN-γ, MIP1α, TNF-α	TRP, KYN, KA, QA, CAA, tyrosine, valine, phenyalanine, leucine, isoleucine (HPLC)	Blood	- Mini-international Neuropsychiatric interview - DSM-IV - MADRS - Inventory of depressive symptomatology	Depressive symptoms decreased over 12 weeks, but there was no significant change of KYN plasma levels. However, cytokine levels were elevated in patients with a depressive episode.
Depression	Doolin et al. (2018)	Cross-sectional	Depressed patients (first presentation with depression: N = 39, 23 females, age = 24.36 ± 7.66; recurrent episodes of depression: N = 35; 24 females; age = 42.46 ± 11.50)	Healthy controls (N = 39; 19 females; age = 30.86 years)	113 (Ireland)	CRP (ELISA)	TRP, QA, KA (LC-MS/MS), IDO1, KAT1 (PCR)	Blood	- HAM-D - PSQI	Patients with MDD showed significantly lower TRP concentrations compared to healthy controls, but neither group displayed any difference in KYN concentrations. Higher QA was observed in the recurrent MDD group compared to the first presentation MDD and healthy control groups. However, lower KA was found in the first presentation MDD group compared to healthy individuals. The recurrent MDD group had higher KYN/TRP and QUIN/KYM ratios. The MDD and healthy control groups exhibited no difference in KAT1 expression and KA concentration. In addition, in the MDD group, there was a positive correlation between QA and both CRP concentrations and IDO1 expression, and between IDO1 expression and the KYN/TRP ratio.
Depression, fatigue, cognitive dysfunction	Georgin-Lavialle et al. (2016)	Cross-sectional	Patients with mastocytosis (N = 54; age = 50.1)	Healthy age-matched controls (N = 54; age = 50.0 years)	108 (France)	IFN-γ	TRP, serotonin, KYN, KA, QA (HPLC)	Blood	- BDI-revised - Perceived Stress scale	Patients with mastocytosis showed significantly higher plasma IDO1 activity, KA and QA, and lower levels of plasma TRP and serotonine compared with healthy individuals. IDO1 activity and TRP levels were negatively correlated with perceived stress and depression scores. In addition, no association of IFN-γ with depression and IDO was found.
Depressive symptoms, anxiety	Hüfner et al. (2015)	Cross-sectional	Breast cancer patients without depressive symptoms (N = 51, age = 45.7 ± 0.8), breast cancer patients with depressive symptoms (N = 29, age = 53.9 ± 1.6)	Control patients with depression (N = 46, 43.9 ± 1.6), healthy subjects (N = 28, 42.9 ± 2.1)	97 (Austria)	Neopterin	KYN, TRP, phenylalanine, tyrosine	Blood	- HADS - HAM-D - BDI - Spielberger state trait anxiety inventory	Depressive symptoms were significantly associated with neopterin (a marker of T helper cell type 1-derived inflammation) and the phenylalanine/tyrosine ratio. Anxiety was associated with neopterin and the KYN/TRP ratio. The phenylalanine/tyrosine ratio was associated with the interaction of breast cancer and anxiety. The KYN/TRP ratio predicted breast cancer and the interaction of breast cancer with depressive symptoms.
Depressive symptoms	Hughes et al. (2012)	Cross-sectional	Patients with MDD (N = 39, 23 females; age: 41.9 ± 1.8 years)	Age- and sex-matched controls (N = 39; 22 females; age = 37.1 ± 2.1 years)	78 (Ireland)	IFN-γ, TNF-α, IL-1β, IL-6, CRP	TRP, KYN, KA, 3-HAA (HPLC), mRNA expression of IDO, KAT1/II, KMO, kynureninase, serotonin	Blood	- HAM-D	Elevated plasma concentrations of IL-6, IFN-γ, and CRP, but not TNF-α and IL-1β, were observed in the patients with MDD. No significant differences in plasma concentrations of KA, 3-HAA, and KYN, and in gene expressions of IDO, KAT, KMO, and kynureninase were found between patients with MDD and control subjects.
Neurocognitive deficits and impairment, depression, anxiety	Kanchanatawan et al. (2017)	Cross-sectional	Patients with schizophrenia (N = 43; 21 females; age: 41.8 ± 11.0) and without symptoms (N = 36; 22 females; age: 40.0 ± 11.1)	Healthy controls (N = 40; 10 females; age = 37.4 ± 12.8)	119 (Thailand)	IgA, IgM	QA, 3-HK, PA, XA, KA, AA	Blood	- Fibromyalgia and Chronic Fatigue Syndrome rating scale - Cambridge Neuropsychological test automated battery (CANTAB) -HAM-A -HAM-D -Positive and negative syndrome scale	Patients with schizophrenia exhibited increased levels of physiosomatic symptoms, including depression and anxiety. TRYCAT-related IgA/IgM responses to 3-HK, PA, and XA were strongly associated with physiosomatic symptoms.
Depression, Anxiety	Kanchanatawan et al. (2018)	Cross-sectional	Patients with schizophrenia confirmed with DSM-IV-TR diagnostic criteria (with symptoms; N = 40; 21 females; age = 40.9 ± 11.2; without symptoms; N = 40; 16 females; age = 41.3 ± 11.0)	Healthy controls (N = 40)	120 (Thailand)	IgA, IgM	QA, XA, PA, 3-HK, AA, KA	Blood	- HAM-A - HAM-D	Depressive symptoms and anxiety were associated with increased IgA as a response to PA or to the noxious (XA +3-HK + QA + PA)/protective (AA + KA) TRYCAT ratio. Decreased IgM appeared as a response to 3-HK in schizophrenia.
Depression, anxiety, cognitive function	Karu et al. (2016)	Cohort (a 2-year follow-up)	Patients with chronic kidney disease stage 3b,4, and 5 (33% female, age: 76.4 ± 7.3)	N/A	27 (Tasmania, Australia)	Neopterin, cortisol	Indoxyl sulfate, IAA, TRP, serotonin, 5-OH IAA, KYN, KA, QA, XA, 3-HAA (LC-MS/MS)	Blood (serum)	- Patient Health Questionnaire-9 - Beck Anxiety Inventory	Neopterin was positively associated with metabolites involved in the KYN pathway, including 5-OH IAA (R = 0.50, p < 0.01); KYN (R = 0.60, p < 0.001); QA (R = 0.52, p < 0.01), 3-HAA (R = 0.67, p < 0.001); indoxyl sulfate (R = 0.68, p < 0.001); and KYN/TRP (R = 0.60, p < 0.001). Anxiety and depression were positively correlated with IAA (R = 0.52, p = 0.005; R = 0.39, p < 0.05, respectively), while cognitive function was negatively associated with KA (R = −0.39, p < 0.05).
Sleep disturbance, depression severity	Mukherjee (2018)	Cohort	Symptomatic bipolar disorder participants (N = 21; 10 females; age = 36.10 ± 11.33)	Healthy controls (N = 28; 16 females; age = 31.57 ± 10.33)	49 (United States of America)	Neopterin (ELISA)	TRP, KYN (HPLC)	Blood	- HAM-D + atypical items - Actigraph (Sleepwatch-O, Ambulatory Monitoring) - Sleep log	Depressive symptom severity was positively associated with total sleep time and KYN/TRP adjusting for BMI in participants with bipolar disorder compared to healthy controls. KYN, TRP, and the KYN/TRP ratio were positively correlated with depressive symptom severity, but no correlation was observed between neopterin and total sleep time and mood severity.
Depression	Nikkheslat (2015)	Cross-sectional	CHD patients with comorbid depression (N = 28; 46% female; age = 69 ± 2)	CHD patients without comorbid depression (N = 55; 11% female; age = 70 ± 1)	83 (United Kingdom)	CRP, plasma VEGF, plasma, and salivary cortisol (ELISA), gene expression levels of GR, IL-6 (qPCR)	TRP, KYN, KA, and 3-HAA (HPLC)	Salivary/serum (cortisol), Blood serum (CRP, VEGF, metabolic markers)	- BDI	CHD patients with depression were reported to have higher levels of CRP, IL-6 gene expression, and VEGF, as well as lower cortisol and TRP levels than CHD patients without depression. An elevated KYN/TRP ratio was observed in the depression group. Overall, CHD patients with depression had increased inflammation and activation of the KYN pathway, as well as GR resistance.
Fatigue, depression	Pertl (2013)	Cohort	Breast cancer patients before chemotherapy (N = 61; age = 50.15 ± 9.87)	N/A	61 (Ireland)	IFN-γ, IL-6, TNF-α, CRP (ELISA)	TRP, KYN (HPLC)	Blood	- Functional assessmentof cancer therapy—Fatigue scale - HADS	More fatigue was significantly associated with higher levels of CRP, but not with IFN-γ, IL-6, or TNF-α. Fatigue was not significantly correlated with KYN, TRP, or the KYN/TRP ratio. Inflammatory markers, KYN, and TRP did not predict initial fatigue status or changes in fatigue.
Depressive symptoms	Quak (2014)	Cross-sectional (at baseline)	Patients with current MDD (N = 1,042; 67.9% female; age = 40.9 ± 12.1)	Patients with no current MDD (N = 1,770; 65.8% female; age = 42.6 ± 13.5)	2,812 (Netherlands)	CRP, IL-6, TNF-α (ELISA)	TRP, KYN (XLC-MS/MS; an automated online solid-phase extraction-liquid chromatographic-tandem mass spectrometry)	Blood	Inventory of Depressive Symptomatology self-report (IDS-SR)	Depressive symptoms were significantly associated with inflammation (CRP and IL-6), but not KYN/TRP for the total patient group and those with current MDD. CRP and IL-6 did not indirectly affect depressive symptoms through KYN/TRP. The result indicates that KYN/TRP (i.e., tryptophan degradation) does not mediate the relationship between depressive symptoms and inflammation.

Age was presented with mean ± standard deviation (years).

3-HAA, 3-hydroxyanthranilic acid; 3-HK, 3-hydroxykunurenine; 5-OH IAA, 5-hydroxy-3-indole acetic acid; AA, anthranilic acid; BDI, Beck Depression Inventory; BMI, body mass index; CAA, competitive amino acid; CHD, coronary heart disease; CRP, C-reactive protein; CSF, cerebrospinal fluid; DSM-IV, diagnostic and statistical manual of mental disorders-4th edition; ELISA, enzyme-linked immunosorbent assay; G-CSF, granulocyte colony-stimulating factor; GR, glucocorticoid receptor; HADS, Hospital anxiety and depression scale; HAM-A, Hamilton anxiety rating scale; HAM-D, Hamilton depression rating scale; HPLC, high-performance liquid chromatography; hs-CRP, high-sensitivity C-reactive protein; IAA, indole-3 acetic acid; IBD, inflammatory bowel disease; IDO, indolamine 2,3 dioxygenase; IgA, immunoglobulin A; IgM, immunoglobulin M; IL, interleukin; IL-1Ra, interleukin-1 receptor antagonist; IFN-γ, interferon-gamma; KA, kynurenic acid; KAT, kynurenine aminotransferases; KMO, kynurenine-3-monooxygenase; KYN, kynurenine; MADRS, Montgomery Åsberg depression rating scale; MDD, major depressive disorder; MIP1α, macrophage inflammatory protein 1α; MMP-9, metalloproteinase-9; N/A, not applicable; NMDA, N-methyl-d-aspartic acid; PA, picolinic acid; PSQI, Pittsburgh sleep quality index; QA, quinolinic acid; TNF-α, tumor necrosis factor-alpha; TRP, tryptophan; TRYCAT, tryptophan catabolite; VEGF, vascular endothelial growth factor; XA, xanthurenic acid; XLC-MS/MS, solid-phase extraction-liquid chromatographic-tandem mass spectrometry.

Host factors

Demographic factors, including age, gender, race/ethnicity, comorbidity, nutritional status, life style habit, activity level, pressure on wound area,^16

–21 and stress,²² have been addressed as influential factors of wound healing in the chronic wound population. Comorbidity, such as diabetes, cardiovascular disease, cancer, and neurodegenerative disease resulting from the systemic inflammatory responses and altered metabolic homeostasis, promotes aging and increases mortality.²³ However, there is a lack of evidence that supports influences of host factors on sickness behaviors resulted from systemic inflammatory response in individuals with chronic wounds. In this conceptual framework, we hypothesized that host factors contribute to sickness behaviors in chronic wound populations by influencing wound healing process and contributing to the generation of a systemic inflammatory response.

Sickness behaviors

Inflammatory activity plays a key role not only in protecting individuals from infection and tissue damage but also in psychological and behavioral regulation.²⁴ Cytokines are chemical messengers that regulate the peripheral immune system and transport signals to the brain through the blood–brain barrier to impact behaviors.^12,24 Changes triggered by inflammatory activity are termed “sickness behaviors.” This constellation of behavioral responses include the following symptoms: fatigue and lethargy, depression, cognitive disturbances, sleep disturbances, pain, anorexia, appetite changes, altered sexual behavior, and social withdrawal.^25
–27 The manifestation of these symptoms is generally thought to involve the reorganization of energetic priorities or to reflect immune activation meant to help heal the sick host.²⁸

Dominant cytokines, including interleukin (IL)-1β, IL-6, and tumor necrosis factor-alpha (TNF-α), are strongly linked to sickness behaviors in humans and animals.^29

–32 Changes in hormone levels (e.g., epinephrine, testosterone, cortisol, oxytocin, vasopressin, leptin, ghrelin, and melatonin) are psychological and physical stressors that activate the hypothalamic-pituitary-adrenal (HPA) axis.^22,28 Other factors that may contribute to the expression of sickness behavior and related symptoms include the patient's age^33,34; environmental conditions (e.g., pathogen exposure, inadequate nutrition, and psychosocial stress) during infancy and childhood³⁵; sex of the patient³⁶; and inflammatory cytokine gene variation.³⁷ However, there is a gap of knowledge in the explanation of associations between changing levels of cytokines, either locally or systemically, and sickness behaviors in chronic wound population. We have preliminary results that show systemic C-reactive protein (CRP) and cytokines measured from plasma are associated with pain, fatigue, and depression in patients with chronic venous leg ulcers, who have multiple commodities.^*

Evidence for dysregulated metabolites regulating sickness behaviors

Evidence indicates that imbalanced or dysregulated metabolites in the KYN pathway is associated with symptoms (e.g., depression, anxiety, and fatigue are collectively called sickness behaviors) in patients with inflammatory conditions. Table 1 lists studies that explored relationships between inflammatory markers, metabolites involved in neurotransmitter synthesis, and symptoms in individuals with diverse diseases. For example, among individuals with mastocytosis, higher stress and depression scores correlated with high indolamine 2,3 dioxygenase (IDO)1 activity and low TRP levels.³⁸ People with frequent symptoms of inflammatory bowel disease, meanwhile, had higher IL-6 and CRP levels, increased KYN/TRP ratios, and higher depression scores.³⁹

Sickness behaviors, inflammatory markers, and metabolites in psychological diseases

Increased immunoglobulin A (IgA) is associated with depression and anxiety, which are linked to an increased ratio of neurotoxic to neuroprotective metabolites in the KYN pathway among people with schizophrenia.^40,41 Meanwhile, levels of quinolinic acid, CRP, and IDO1 positively correlate with major depressive disorder (MDD).⁴² Individuals with depression can also experience sleep disturbances associated with a higher CRP level and lower ratio of kynurenine acid/quinolinic acid.⁴³

While KYN/TRP and neopterin levels positively correlate with each other, they are associated with depressive symptoms or sleep quality in people with bipolar disorder.⁴⁴ In one large study sample (N = 2,812) consisting of individuals with MDD and a healthy control population, KYN/TRP did not mediate between CRP, IL-6, and depressive symptoms.⁴⁵ Despite the significant association between inflammatory markers (CRP and IL-6) and depressive symptoms, this research found no relationship between KYN/TRP and depression. Similarly, a longitudinal study found no correlation between metabolic changes in the KYN pathway and depressive symptoms, despite high cytokine levels among patients with MDD.⁴⁶ When comparing a control group to depressed individuals with elevated IL-6, interferon-gamma (IFN-γ), and CRP levels, no differences in kynurenine acid and 3 hydroxyanthranilic acid were observed.⁴⁷

The recent literature has not provided clear answers in this research area. Some studies suggest significant relationships between metabolites in the KYN pathway, sickness behaviors such as depression and sleep disturbances, and elevated inflammatory markers (e.g., IgA and proinflammatory cytokines). Other studies, however, have found no such association.

Sickness behaviors, inflammatory markers, and metabolites in major disease states

Karu et al. (2016)⁴⁸ reported a correlation between neopterin and KYN/TRP, and between indole-3 acetic acid and anxiety and depression. They also found an association between low cognitive function and high kynurenine acid levels in individuals with chronic kidney disease. Similarly, people with comorbid coronary heart disease (CHD) and depression have exhibited higher CRP levels and more KYN pathway activity than CHD patients without depression.⁴⁹ Pertl et al. (2013)⁵⁰ reported no relationship between fatigue, depression, proinflammatory cytokines, and the KYN pathway, although CRP did correlate with fatigue and depression before chemotherapy. Another study of breast cancer patients by Hüfner et al. (2015)⁵¹ found high overall rates of anxiety and depression. The researchers also reported a correlation between KYN/TRP ratios and anxiety, and between phenylalanine/tyrosine ratios and depression. In addition, patients with depression and anxiety had higher neopterin levels than patients without these psychological symptoms. Dominantly, inflammatory markers, sickness behaviors, and metabolites involved in neurotransmitter synthesis are linked together in major disease.

Metabolites and chronic wounds

For examples of metabolites in wounds, arginine has an effect on healing in human subjects with chronic wounds. Arginine is metabolized by nitric oxide synthase into nitric oxide, which plays essential roles in wound healing in terms of immune response, angiogenesis, cell migration, and proliferation.^52,53 Nitric oxide plays an important role during initial wound healing.⁵⁴ Arginine is also metabolized by arginase into ornithine,⁵⁵ an essential precursor of collagen.⁵⁶ Finally, arginase affects the immune system response and tissue repair. This is especially true of arginase 1, the cytosolic isoform, which is more active in pathological skin healing in humans, and arginase 2, the mitochondrial isoform, which is prominent in normal healing.⁵⁶

A few enzymes were identified as regulators of wound healing in animal studies. Examples of enzyme reactions that may accelerate wound healing include decreased activity of IDO, which converts TRP to KYN⁵⁷; inhibition of 11b-hydroxysteroid dehydrogenase type 1, which converts 11-dehydrocorticosterone to corticosterone⁵⁸; and activation of Sphingosine kinase 1, which regulates glucose metabolism.⁵⁹ Also, distinct differences in metabolite profiles between diabetic and nondiabetic wounds in mouse models were reported: 1-methylguanosine, aconitate, adenylosuccinate, and cyclic guanosine monophosphate only respond to nondiabetic wounds, while pantothenate, guanosine-5-triphosphate, and inositol only respond to diabetic wounds.⁶⁰

Although metabolic investigations in this field are preliminary and limited to local wounds and animal studies, they provide insights into how sickness behaviors develop in chronic wound patients based on the brain's response to immunological information. Of particular interest is how the metabolic products from a biological system—such as amino acids, nucleic acids, and peptides, lipids, vitamins, and carbohydrates—impact cell and tissue metabolism linked to sickness behaviors in patients with chronic wounds.^13,16,61

Evidence for the immune response to wound microbiome

In chronic wounds, multicellular polymeric substance matrix, called biofilm, is embedded,⁶² which helps maintain the bacterial community. In fact, a current meta-analysis reports that about 78.2% of chronic wounds have biofilm.⁶³ Biofilm interferes with the host's normal immune response to repair and regenerate tissue and consequently prolongs inflammation, which causes the excessive release of inflammatory cytokines⁶⁴ and increases oxidative stress and accumulation of cellular metabolites.⁶⁵

Functioning as part of the body's defense system, toll-like receptors (TLRs) exist on the membrane of immune and epithelial cells.⁶⁶ This group of pattern-recognition receptors bind with bacteria or viruses to trigger an individual's innate or adaptive immune system.⁶⁷ Several in vivo studies have demonstrated that TLRs are involved in wound healing.^68
–70 Upon recognizing and binding to diverse pathogenic ligands in circulating immune cells, TLRs activate nuclear factor-kappa B (NF-κB) pathways that stimulate the production of proinflammatory cytokines (e.g., IFN-α/β, IL-1, TNF-α, and IL-12) and anti-inflammatory cytokines (e.g., IL-10, IL-6, and IL-12) in dendritic cells, macrophages, and granulocytes.^71,72 NF-κB also regulates T cell differentiation, including T helper 1 (Th1), Th2, Th17, and T-follicular helper cells, all of which secrete cytokines.⁷² Pathogenic bacteria thus activate TLRs, leading to immune cell recruitment. In chronic wounds, TLRs are upregulated, which results in an excessive, prolonged inflammatory response that elevates proinflammatory cytokines. This is problematic for patients as excessive cytokines impair the normal wound healing process.

Peripheral inflammation, the CNS and sickness behaviors

The bidirectional communication between an individual's brain and peripheral immune system through the HPA axis is integral in regulating stress levels and emotional behaviors.⁷³ Elevated levels of cytokines induced by T cells directly affect sickness behaviors by humoral and neural routes that connect peripheral immunity and the brain.⁷⁴ Recent research suggests that lymphatic vessels in the meninges of the CNS connect the neurological system and immunological system.^75,76 Activation of peripheral immunity against a local inflammatory condition affects systemic immunity, which may regulate sickness behaviors. This may be demonstrated through the neuropsychiatric behavioral changes, such as the onset of depression that directly corresponds to activation of the cytokine response to infection, transmitted by neural communication with the brain.^74,77,78

Influence of local inflammation on sickness behaviors

Peripheral inflammation results from endothelial-immune cell interactions in distant tissues.⁷⁷ Recruited T cells, B cells, macrophages, mast cells, and dendritic cells respond to tissue damage or infection, releasing chemokines and proinflammatory cytokines that stimulate the neuronal and humoral routes of brain communication.⁷⁴ Studies suggest cytokines can alter neuropsychiatric behaviors and symptoms and overall mood. Below are the effects of several different cytokines:

Elevated TNF-α levels and increased microglial activation result from inflammation of the gut⁷⁹ and cause depressive-like behavior in vivo. ⁸⁰

IFN-γ delivered by T cells⁸¹ regulates meningeal immunity and social behaviors and strongly influences depressive-like behaviors in mice.⁸⁰

IL-1β, when delivered through the humoral pathway, induces sickness behaviors.^82,83

Patients with depression exhibit elevated serum levels of IL-6,⁸⁴ which influences depressive behaviors.⁷⁴

IFN-α therapy for viral infections causes sickness behaviors.⁸⁵

IL-1β, IL-6, and TNF-α increase in the peripheral blood of patients with depression.⁸⁶

Cytokines interact with metabolites in biosynthesis pathways of neurotransmitters

Ongoing inflammation alters metabolic activity in homeostasis when excessive inflammatory cells migrate, causing metabolic stress,^87,88 Circulating immune cells are activated and start the inflammation process around infected or damaged areas, while metabolic pathways shift to support immune cell activity.^87
–89 The intracellular metabolism is reprogrammed by cytokines: IFN-γ, IL-1β, and IL-6 induce M1 macrophages, while IL-4 and IL-13 induce M2 macrophages.^87,89 Consequently, immune cells activated by inflammation undergo aerobic glycolysis to meet the increased demands for energy in immune cells and invaded bacteria.⁸⁷

Previous animal studies report that cytokines impact neurotransmitter signaling pathways in the brain.^90,91 These include the serotonin, dopamine, and glutamate pathways induced by peripheral inflammation from chronic wounds.⁹² In particular, the kynurenine pathway is essential to the biosynthesis of neurotransmitters.⁹³ Proinflammatory cytokines are linked to dysregulation of the KYN pathway caused by activation of the IDO enzyme. This results in an imbalance of TRP, a precursor of serotonin that plays a role in stress response.^94

–97 Research indicates that metabolites involved in the KYN pathway act as either neurotoxic molecules (e.g., hydroxykynurenine, 3-hydroxyanthranilic acid, and quinolinic acid) or neuroprotective molecules (i.e., kynurenic acid).⁴³ Proinflammatory cytokines also stimulate 5,6,7,8-tetrahydrobiopterin (BH4), which reduces the biosynthesis of catecholamine.^95,98,99 In chronic inflammation, altered amino acid metabolism is linked to depression and other neuropsychiatric symptoms.^99,100 Dysregulated metabolites involved in the KYN pathway are also linked to neuropsychiatric symptoms in patients with HIV,^99,101 hepatitis B,¹⁰² and hepatitis C.¹⁰³ In addition, this dysregulation accelerates the progression of sepsis,^104,105 psychiatric disorders,⁹⁴ neurodegenerative diseases, cancers, and autoimmune diseases.⁹⁵ Such diseases share common symptoms connected to dysregulated metabolites involved in neurotransmitter biosynthesis.^94,95 In particular, the TRP metabolism pathway is potentially altered by the dysbiosis of gut microbiota.¹⁰⁶

Limitations of the current state of the science

Although microbiome and biofilm are hypothesized to hinder wound healing, only limited research has been conducted to verify this hypothesis.^107,108 Meanwhile, the impact of bioburden in wound healing and health outcomes remains unclear.¹⁰⁹ No study has examined the impact of microbiomes on sickness behaviors in the chronic wound population. Another concern is that evidence about the relationship between metabolomics and sickness behaviors is limited to certain populations and it has never been investigated in the chronic wound population. In addition, another limitation is that it is unknown if comorbid conditions also influence the KYN pathway in patients with chronic wounds. Given that a certain metabolite, IDO1, in the l-TRP–l-KYN pathway changed the wound healing process by mediating cytokines in a mouse model (i.e., IDO1 knockout mice showed the significantly higher rate of wound healing than wild-type mice),⁵⁷ this could provide insight regarding the association between a systemic change of metabolites in the KYN pathway and chronic wounds in human. The prior research supports that some comorbid conditions, such as diabetes and obesity, which are commonly found and predisposed in the chronic wound population, may induce the recruitment of macrophages and cytokines and lead to the activation of KYN pathway.^110
–112 In addition, the imbalanced TRP metabolism in chronic inflammatory conditions such as diabetes and obesity is well described.^23,106 Therefore, the local and systematic dysregulation of the KYN pathway may play a key role in sickness behaviors in people with chronic wounds. Currently, there is no evidence as to whether the underlying comorbidities affect the dysregulation of KYN pathway or produce a joint prognostic effect along with chronic wounds. Further research is warranted to determine the effect of comorbidities on chronic wounds in aspects of KYN pathway.

Despite these limitations, sufficient evidence exists that inflammation-induced alterations of neurotransmitter synthesis pathways are associated with sickness behaviors. By investigating metabolites in the KYN pathway, we hope to provide substantial evidence that localized immune responses to chronically infected wounds in the chronic wound population correspond to a systemic impact on sickness behaviors. Detailing metabolite changes in local wounds and circulating systems may allow us to identify modifiable biomolecules in the chronic wound population. These biomolecules can then be targeted to reduce the disease burden in the chronic wound population.

Our hypothesized conceptual framework will thus address relationships between inflammation, alteration of metabolites involved in neurotransmitter synthesis (i.e., the KYN pathway), host immune responses, and sickness behaviors in the chronic wound population. The conceptual framework will guide experimental studies to investigate biomolecular pathways underlying sickness behaviors in the chronic wound population. It will also help to develop therapeutic interventions to reduce the physical and psychological health burden among chronic wound patients.

SUMMARY

Understanding symptoms can help practitioners implement more effective treatment of individuals with chronic wounds. While prior research has strongly focused on the influence of inflammation activation, the underlying molecular mechanisms of symptoms remain uncertain. To address this knowledge gap, we propose a theoretical framework to explore biomolecular pathways that modulate sickness behaviors in the chronic wound populations. In particular, the KYN pathway and relevant metabolites mediated by the immunological response could play a critical role in sickness behaviors in these complex patients. Guided by this framework, future research can explore the association of metabolic mechanisms with sickness behaviors in the chronic wound population.

TAKE-HOME MESSAGE

Activation of peripheral immunity against a local inflammatory condition affects systemic immunity that may regulate sickness behaviors such as pain, fatigue, and depression.

The specific variations of metabolites that are associated with sickness behaviors in the chronic wound population remain unclear.

The proposed theoretical framework characterizes dysregulated metabolites generated during chronic inflammation, which can guide researchers in developing new treatments to reduce sickness behaviors and the disease burden in the chronic wound population.

Footnotes

ACKNOWLEDGMENTS AND FUNDING SOURCES

No funding supported this work. G.S.Y.'s effort is supported by the National Institutes of Health/National Institute of Nursing Research (F32NR018367 to G.S.Y.).

AUTHOR DISCLOSURE AND GHOSTWRITING

All authors declared no conflict of interests. J.K. and J.S. conceived the idea for the review; and J.K. and G.S.Y. reviewed the literature and wrote the article. All authors wrote, critically reviewed, and revised the article. No ghostwriters were used to write this article.

ABOUT THE AUTHORS

Junglyun Kim, PhD, is an Assistant Professor at the University of Minnesota School of Nursing. Dr. Kim has extensive clinical experience in chronic wounds and knowledge in biobehavioral symptom science. Gee Su Yang, PhD, is a Post-doctoral fellow at the University of Florida College of Nursing. Debra Lyon, PhD, is a Professor and the Kirbo Endowed Chair and Executive Associate Dean of Biobehavioral Nursing Science of the University of Florida College of Nursing. Debra L. Kelly, PhD, is an Assistant Professor at the University of Florida College of Nursing. Joyce Stechmiller PhD is a Professor at the University of Florida College of Nursing. Dr. J.S. has extensive experience and expertise in wound science, with a special emphasis on biobehavioral approaches to enhancing wound healing.

Abbreviations And Acronyms

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