Kynurenine aminotransferase isoforms display fiber-type specific expression in young and old human skeletal muscle

https://doi.org/10.1016/j.exger.2020.110880Get rights and content

Abstract

Conversion of kynurenine (KYN) to kynurenic acid (KYNA) is the main pathway for free tryptophan degradation in skeletal muscle and has emerged as an important mechanism of how exercise is linked to promotion of mental health. Metabolism of KYN to KYNA mainly depends on the expression of kynurenine aminotransferases (KATs) that is under control of the mitochondria biogenesis regulator PGC-1α. We therefore hypothesized that expression of KATs would vary between muscle fibers that differ in mitochondrial content, i.e. oxidative type I vs more glycolytic type II muscle fibers. Moreover, we tested the hypothesis that KAT expression differs with age. Single muscle fibers were isolated from biopsies taken from the vastus lateralis muscle in young and old healthy subjects. In young and old subjects the abundance of KAT I, KAT III and KAT IV was greater in Type I than Type II fibers without age-dependent difference in the KAT isoform expressions. The link to mitochondrial content was further seen as the expression of KAT IV correlated to mitochondrial cytochrome c oxidase IV (COX IV) abundance in both fiber types. In conclusion, we describe for the first time the expression pattern of KAT isoforms with respect to specific fiber types and age in human skeletal muscle.

Introduction

The essential amino acid tryptophan can be metabolized through several routes leading to a variety of active substances including well known mediators of neuronal function, such as serotonin and melatonin. Perhaps less known is that >90% of free tryptophan is converted into the broadly conserved kynurenine (KYN) metabolism pathway, in which metabolites have important roles in inflammation, metabolism and in maintaining neuronal functions (Han et al., 2010; Cervenka et al., 2017). Degradation of tryptophan initially leads to formation of KYN that then can be processed either into the neurotoxic NMDA-agonist quinolinic acid (QUIN) or to the NMDA-antagonist kynurenic-acid (KYNA) that is considered neuroprotective (Kessler et al., 1989; Cervenka et al., 2017). Indeed, perturbations in the KYN metabolism system are linked to mental health (Brundin et al., 2016; Erhardt et al., 2017) and low levels of serum KYNA correlate with mental deficiency in the elderly (Gulaj et al., 2010). In contrast, an increase in serum KYNA is associated with resilience to stress-induced depression (Agudelo et al., 2014). The synthesis of KYNA is catalyzed by a group of four enzymes called kynurenine aminotransferases (KATs). The four KATs (KAT I, II, III, and IV) are all expressed in the brain, liver and in skeletal muscle (Guidetti et al., 1997; Yu et al., 2006; Guidetti et al., 2007; Agudelo et al., 2014; Schlittler et al., 2016). Three of the KAT isoforms are targeted to the mitochondria (KAT II, III and IV) (Han et al., 2010) and mice which overexpress the mitochondrial biogenesis regulator peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) have an increased abundance of KAT isoforms and are resistance to stress-induced depression (Agudelo et al., 2014). In human, skeletal muscle expression of the PGC-1α genes are correlated to KAT isoform mRNA expression (Schlittler et al., 2016; Agudelo et al., 2019; Allison et al., 2019). We recently showed that the protein expression of KATs is higher in endurance athletes, which presumably have a higher mitochondrial content, compared to recreationally active people (Schlittler et al., 2016). However, the correlation between expression of KATs and mitochondria proteins has not yet been shown.

Skeletal muscle is a heterogeneous tissue, comprised of both slow-oxidative (Type I) and fast-glycolytic (Type II) muscle fibers, which is defined by the respective myosin heavy chain isoform present. Each fiber type has unique metabolic and contractile properties and are differentially affected by age (Lamboley et al., 2015) and exercise (Thomassen et al., 2013; Wyckelsma et al., 2017a; Edman et al., 2019). Often protein analyses using whole muscle homogenate can overlook changes seen in single fibers (Wyckelsma et al., 2017a; Wyckelsma et al., 2017b). In addition, single fiber analyses can complement homogenate findings by providing information of fiber-type specific properties in the muscle. Given the higher mitochondrial content within Type I than Type II fibers (MacInnis et al., 2017; Wyckelsma et al., 2017a), we examined the fiber-type specificity of the KAT isoforms in human skeletal muscle and how this correlated with the abundance of a marker mitochondrial protein content i.e. cytochrome C oxidase subunit IV (COX IV).

Altered mitochondrial function is a typical feature of aging (Andersson et al., 2011; Joseph et al., 2012; Romanello and Sandri, 2015; Kauppila et al., 2017), although mitochondrial health is adaptive as suggested by findings that mitochondria from physically active older adults are not overtly different from the young (Gram et al., 2015; Wyckelsma et al., 2017a). Given the link to mitochondria and that KAT expression is under control of PGC-1α we investigated whether the abundance of KATs is differently expressed in young and elderly individuals and how the expression correlated with COX IV within Type I and II fibers.

Section snippets

Participant recruitment and muscle sampling

Eight older adults aged 68 ± 0.3 years (mean ± SEM) and ten young adults aged 29.6 ± 0.6 were recruited to the Lithuanian Sports University and University of Tartu for a resting muscle biopsy. The study was approved by the local ethics committee (ethics number BE-2-35) which conforms to the Declaration of Helsinki. Each subject provided written informed consent before participation. All volunteers were male, healthy and free from known metabolic diseases. The elderly cohort completed the short

Fiber-type specificity of KAT isoforms in young and old adults

The expression of KAT isoforms has not previously been investigated in a fiber-type specific manner. In both young and older adults there was a greater abundance of the KAT I, III and IV isoforms in Type I compared to Type II fibers (KAT I p = .006, KAT III p = .03 and KAT IV p = .02, two-way ANOVA; Fig. 1). Age had no effect on the abundance of the KAT I (p = .52), III (p = .19) and IV (p = .51) isoforms in either fiber-type. As per previous published studies, we confirmed that the abundance

Discussion

We show for the first time that expression of KAT isoforms differs between fiber types in human skeletal muscle, where higher expression of KAT isoforms is seen in oxidative type I muscle fibers without difference in young and older humans. The link between KAT expression and the oxidative phenotype was further seen as the expression of mitochondria-targeted KAT isoforms displayed a general correlation to the abundance of the respiratory chain component COX IV.

The link between skeletal muscle,

Conclusion

Our study shows, for the first time, differential expression of KAT isoforms between human oxidative and more glycolytic muscle fiber types, without difference in young and older individuals. KAT isoforms are generally more expressed in oxidative muscle fibers, which is furthermore seen with an association between KATs and mitochondrial abundance. This highlights that skeletal muscle is a heterogeneous tissue composed of cell types with different metabolic profile, also including the KYN

CRediT authorship contribution statement

V.L. Wyckelsma: Conceptualization, Investigation, Formal analysis, Writing - original draft, Validation. W. Lindkvist: Conceptualization, Writing - review & editing, Validation. T. Venckunas: Conceptualization, Writing - review & editing. M. Brazaitis: Conceptualization, Writing - review & editing, Validation. S. Kamandulis: Conceptualization, Writing - review & editing, Validation. M. Pääsuke: Conceptualization, Writing - review & editing, Validation. J. Ereline: Conceptualization, Writing -

Acknowledgements

We thank Karin Söderlund from the Swedish School of Sport and Health Sciences for assistance with freeze drying muscle samples. Also, thanks to the participants for their time and contribution to the study. The research was supported by the following grants Swedish Heart-Lung Foundation (D.C.A., 20160741; 20180803), Harald och Greta Jeanssons Stiftelse (D.C.A), Swedish Society for Medical Research (D.C.A, SSMF; S16-0159), Svenska Läkaresällskapet (SLS-891461), Swedish Research Council (H.W.,

References (29)

  • L. Brundin et al.

    An enzyme in the kynurenine pathway that governs vulnerability to suicidal behavior by regulating excitotoxicity and neuroinflammation

    Trans Psychiatry

    (2016)
  • I. Cervenka et al.

    Kynurenines: Tryptophan's metabolites in exercise, inflammation, and mental health

    Science

    (2017)
  • D. Christiansen et al.

    A fast, reliable and sample-sparing method to identify fibre types of single muscle fibres

    Sci. Rep.

    (2019)
  • C.L. Craig et al.

    International physical activity questionnaire: 12-country reliability and validity

    Med. Sci. Sports Exerc.

    (2003)
  • View full text