Human, hairy skin contains a subgroup of C-fibers, the C-low threshold mechanoreceptive afferents ((C-LTMR) C-tactile or C-touch (CT) fibers) that are linked with the signaling of affective aspects of human touch. Recent studies suggest an involvement of these afferents in the modulation of pain in healthy volunteers. Small fiber neuropathy (SFN) is associated with a damage of C-fibers. Therefore, an impairment of C-LTMRs can be assumed. We aimed to elaborate a possible role of CT-afferents in pain modulation by investigating healthy volunteers and SFN-patients. Experiment I: 20 SFN-patients (12 women, median age 52.0 years) and 20 healthy controls (14 women, median age 43.0 years) participated in this prospective fMRI and psychophysical study. Heat-pain (HP), CT-targeted touch (slow brushing) and HP combined with CT-targeted touch were applied in randomized order to the left shank in a block design. The participants rated pain intensity on a visual analogue scale. Experiment II: We investigated a possible impact of pain intensity on CT induced pain modulation (10 healthy participants). The intensity of HP stimulation was chosen to induce pain intensity 50/100 (NRS). HP stimulation was applied with and without CT-targeted touch. Experiment I: CT-stimulation was sufficient to reduce heat pain in healthy participants (p = 0.016), but not in SFN-patients. HP induced pain intensity was significantly higher (32,2 vs 52,6) in SFN-patients. During HP, bold responses in pain associated areas were observed in both groups. Additional CT-stimulation elicited no significant difference of bold responses compared to HP. Experiment II: In healthy volunteers, we reproduced a significant reduction of HP intensity by CT-stimulation (p = 0.038). CT input seems to be sufficient to modulate pain, independent of intensity of the pain stimulus. As a prerequisite, the CT fibers have to be intact as in healthy volunteers. If CT fibers are impaired – as in SFN -, CT-targeted touch does not modulate pain intensity. The location of CT-induced pain modulation might be attributed to the level of the dorsal horn since the cortical activation pattern of heat pain with and without CT-targeted touch did not differ in healthy subjects and in SFN-patients.

Polymyositis with mitochondrial pathology (PM-Mito) was first identified in 1997 as a subtype of idiopathic inflammatory myopathy. Recent findings demonstrated significant molecular similarities between PM-Mito and inclusion body myositis (IBM), suggesting a trajectory from early to late IBM and prompting the inclusion of PM-Mito as an IBM precursor (early IBM) within the IBM spectrum. Both PM-Mito and IBM show mitochondrial abnormalities, suggesting that mitochondrial disturbance is a crucial element of IBM pathogenesis. The primary objective of this cross-sectional study was to characterize the mitochondrial phenotype in PM-Mito at histological, ultrastructural and molecular levels and to study the interplay between mitochondrial dysfunction and inflammation. Skeletal muscle biopsies of 27 patients with PM-Mito and 27 with typical IBM were included for morphological and ultrastructural analysis. Mitochondrial DNA (mtDNA) copy number and deletions were assessed by quantitative PCR and long-range PCR, respectively. In addition, full-length single-molecule sequencing of the mtDNA enabled precise mapping of deletions. Protein and RNA levels were studied using unbiased proteomic profiling, immunoblotting and bulk RNA sequencing. Cell-free mtDNA was measured in the serum of IBM patients. We found widespread mitochondrial abnormalities in both PM-Mito and IBM, illustrated by elevated numbers of cytochrome c oxidase (COX)-negative and succinate dehydrogenase (SDH)-positive fibres and prominent ultrastructural abnormalities, with disorganized and concentric cristae within enlarged and dysmorphic mitochondria. mtDNA copy numbers were significantly reduced, and multiple large-scale mtDNA deletions were already evident in PM-Mito compared with healthy age-matched controls, similar to the IBM group. The canonical cyclic GMP–AMP synthase/stimulator of interferon genes inflammatory pathway was activated in PM-Mito and IBM, and we detected elevated levels of circulating cell-free mtDNA indicative of mtDNA leakage. In PM-Mito and IBM, these findings were accompanied by dysregulation of proteins and transcripts linked to the mitochondrial membranes. In summary, we identified that mitochondrial dysfunction with multiple mtDNA deletions and depletion, disturbed mitochondrial ultrastructure and defects of the inner mitochondrial membrane are features of PM-Mito and IBM, underlining the concept of an IBM-spectrum disease. Notably, mitochondrial abnormalities precede tissue remodelling and infiltration by specific T-cell subpopulations (e.g. KLRG1+) characteristic of late IBM. The activation of inflammatory, DNA-sensing pathways might be related to mtDNA release, which would indicate a significant role of mitochondria-associated inflammation in the pathogenesis of IBM-spectrum disease. This study highlights the crucial role of early mitochondrial abnormalities in the pathomechanism of IBM, which might lead to new approaches to therapy.