Biology subjects

Kjaer, M.

Publications and source records attributed to Kjaer, M..

7 recordsLinked to original sources

bioRxiv2025

Spatially distinct ECM-producing fibroblasts and myonuclei orchestrate early adaptation to mechanical loading in the human muscle-tendon unit

Mechanical loading drives structural and functional improvements in muscle and tendon, protecting against injury at their interface - the myotendinous junction (MTJ) - and within the tendon matrix. However, the early cellular and molecular events that initiate these adaptations in humans remain poorly understood. To investigate this, we applied single nucleus RNA sequencing and in situ hybridization to map the acute transcriptional response of the human muscle-tendon unit to a single bout of eccentric resistance exercise, with a focus on extracellular matrix (ECM) regulation. We identified four transcriptionally distinct fibroblast subtypes expressing key ECM components, including COL1A1 and DCN. Three of these subtypes were localized to tendon and responded to exercise: two were spatially restricted to the collagen fascicles or the MTJ, while the third, enriched in the interfascicular matrix (IFM), exhibited the strongest response. This IFM population, marked by PDGFRA, upregulated PRG4 and VCAN, ECM genes linked to tissue lubrication and resilience. In parallel, exercise induced dynamic ECM regulation in myonuclei, particularly in a distinct subset of type II myonuclei at the MTJ that expanded in number and robustly upregulated COL22A1, a collagen essential for MTJ integrity. Together, these findings uncover a spatially organized, cell type-specific program of ECM remodeling in response to mechanical load, offering new insight into the early molecular events of human muscle-tendon adaptation.

cell biology↗

bioRxiv2025

Heavy Resistance Exercise Training in Older Men: A Responder and Inter-individual Variability Analysis

BackgroundHeavy resistance exercise training (HReT) effectively increases muscle mass and strength in groups of older individuals. However, the extent of inter-individual variability in response to HReT, and the possible existence of non-responders, remains unclear. The primary aim was to determine the presence of inter-individual variability in the response to prolonged HReT in healthy older individuals. Secondary aims were to classify individual responsiveness using a combination of gold-standard assessment methods, and to explore training progression and baseline levels as possible response moderators. MethodsWe conducted a secondary analysis of an 8- and 16-week intervention of thrice weekly HReT (EX) or continuation of a sedentary lifestyle (SED). 58 healthy men (age 72{+/-}5) were randomized into EX (n=38) or SED (n=20). Assessments were conducted at baseline, mid-intervention (8wk), and post-intervention (16wk) for five outcomes: Maximal voluntary contraction strength (MVC), rate of force development (RFD), quadriceps cross-sectional area (qCSA), and type I and II myofibre cross-sectional area (fCSA). Training compliance and progression (as measured by 1-repetition maximum (1RM)) were monitored. Inter-individual variability was assessed using the standard deviation of individual responses (SDIR). Individual change scores relative to a Typical Error (TE) were used to classify individuals as Poor, Trivial, Robust or Excellent responders. ResultsAt the group level, EX increased MVC, RFD, qCSA and type II fCSA by 19{+/-}14 %, 58{+/-}80 %, 3{+/-}4 % and 14{+/-}25 %, respectively, with no changes in SED. Inter-individual variability was observed for all outcomes. Individual responses to HReT were outcome- and time-dependent, with 31 participants (82%) being classified as Robust or Excellent responders, and two participants (5%) being Poor responders, following 16wk of HReT. Training compliance and 1RM progression did not account for the observed response variability following HReT. Lower baseline levels were associated with greater improvements; however, this did not account for the observed inter-individual variability. ConclusionsThis study provides strong evidence of inter-individual variability in response to HReT among healthy older men. Given the rarity of true non-responders, HReT should remain the universally recommended first-line strategy for enhancing muscle mass and strength in this population.

physiology↗

bioRxiv2024

Muscle Fibroblasts and Stem Cells Stimulate Motor Neurons in An Age and Exercise-Dependent Manner

Exercise preserves neuromuscular function in ageing through unknown mechanisms. Skeletal muscle fibroblasts (FIB) and stem cells (MuSC) are abundant in skeletal muscle and reside close to neuromuscular junctions, but their relative roles in motor neuron maintenance remain undescribed. Using direct co-cultures of embryonic rat motor neurons with either human MuSC or FIB, RNA sequencing revealed profound differential regulation of the motor neuron transcriptome, with FIB generally favoring neuron growth and cell migration and MuSC favoring production of ribosomes and translational machinery. Conditioned medium from FIB was superior to MuSC in preserving motor neurons and increasing their maturity. Lastly, we established the importance of donor age and exercise status and found an age-related distortion of motor neuron and muscle cell interaction that was fully mitigated by lifelong physical activity. In conclusion, we show that human muscle FIB and MuSC synergistically stimulate the growth and viability of motor neurons, which is further amplified by regular exercise.

physiology↗

bioRxiv2024

Fibre type differences in the organisation of mononuclear cells and myonuclei at the tips of human myofibres

The myotendinous junction (MTJ) is a weak link in the musculoskeletal system. Here, we isolated the tips of single myofibres from healthy human hamstring muscles for confocal microscopy (n=6) and RNAscope in situ hybridization (n=6) to gain insight into the profiles of cells and myonuclei in this region. A marked presence of mononuclear cells was observed coating the fibre tips, with a median of 29 (range 16-63) and 16 (9-23) cells per fibre for type I and II myofibres, respectively (p<0.05). The number and density of myonuclei gradually increased from the myofibre proper towards the tip (p<0.05), similarly for both fibre types, and a greater number of COL22A1-expressing nuclei was seen in type II vs type I myofibres (p<0.05). These divergent fibre type-specific characteristics of the MTJ reflect the respective demands for remodelling of the tendon and myofibre sides of the junction according to loading patterns. This insight refines our fundamental understanding of the human MTJ at the cell and structural levels. Summary statementAt the site of attachment to tendon, type I and II human myofibre tips display divergent numbers of mononuclear cells and COL22A1+ nuclei, changing our understanding of myotendinous junction biology.

physiology↗

bioRxiv2023

The mitochondrial mRNA stabilizing protein, SLIRP, regulates skeletal muscle mitochondrial structure and respiration by exercise-recoverable mechanisms

Summary and graphical abstractDecline in mitochondrial function associates with decreased muscle mass and strength in multiple conditions, including sarcopenia and type 2 diabetes. Optimal treatment could include improving mitochondrial function, however, there are limited and equivocal data regarding the molecular cues controlling muscle mitochondrial plasticity. Here we uncover the mitochondrial-mRNA-stabilizing protein SLIRP, in complex with LRPPRC, as a PGC-1 target that regulates mitochondrial structure, respiration, and mitochondrially-encoded-mRNA pools in skeletal muscle. Exercise training effectively counteracted mitochondrial defects induced by loss of LRPPRC/SLIRP, despite sustained low mitochondrially-encoded-mRNA pools, via increased mitoribosome translation capacity. In humans, exercise training robustly increased muscle SLIRP and LRPPRC protein content across exercise modalities and sexes, yet this increase was less prominent in subjects with type 2 diabetes. Our work identifies a mechanism of post-transcriptional mitochondrial regulation in skeletal muscle through mitochondrial mRNA stabilization. It emphasizes exercise as an effective approach to alleviate mitochondrial defects by possibly increasing mitoribosome capacity. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=142 SRC="FIGDIR/small/564600v2_ufig1.gif" ALT="Figure 1"> View larger version (61K): org.highwire.dtl.DTLVardef@1cdc6c5org.highwire.dtl.DTLVardef@b4cb1borg.highwire.dtl.DTLVardef@182871corg.highwire.dtl.DTLVardef@7462ea_HPS_FORMAT_FIGEXP M_FIG C_FIG

molecular biology↗

bioRxiv2023

Marked irregular myofiber shape is a hallmark of human skeletal muscle aging and is reversed by heavy resistance training

BackgroundAge-related loss of strength is disproportionally greater than the loss of mass, suggesting maladaptations in the neuro-myo-tendinous system. Myofibers are often misshaped in aged and diseased muscle, but systematic analyses of large sample sets are lacking. Our aim was to investigate myofiber shape in relation to age, exercise, myofiber type, species, and sex. MethodsPreviously collected vastus lateralis muscle biopsies (n=265) from 197 males and females, covering an age-span of 20 to 97 years, were examined. The gastrocnemius and soleus muscles of 7 C57BL/6 mice were also examined. Immunofluorescence and ATPase stainings of muscle cross-sections were used to measure myofiber cross-sectional area (CSA) and perimeter, from which a shape factor index (SFI) was calculated in a fiber type specific manner (type I and II in humans; type I, IIa, IIx and IIb in mice). Heavy resistance training (RT) was performed 3 times per week for 3-4 months by a subgroup (n=59). Correlation analyses were performed comparing SFI and CSA with age, muscle mass, maximal voluntary contraction (MVC), rate of force development (RFD), and specific force (MVC/muscle mass). ResultsIn human muscle, SFI was positively correlated with age for both type I (R2=0.20) and type II (R2=0.38) myofibers. When subjects were separated into age cohorts, SFI was lower for type I (p<0.001) and II (p<0.001) myofibers in Young (20-36) compared to Old (60-80), and higher for type I (p<0.05) and II (p<0.001) myofibers in the Oldest Old (>80) compared to Old. The increased SFI in old muscle was observed in myofibers of all sizes. Within all three age cohorts, type II myofibers SFI was higher than for type I myofibers (p<0.001), which was also the case in mice muscles (p<0.001). Across age cohorts, there was no difference between males and females in SFI for either type I (p=0.496/0.734) or II (p=0.176/0.585) myofibers. Multiple linear regression revealed that SFI, after adjusting for age and myofiber CSA, has independent explanatory power for 8 out of 10 indices of muscle mass and function. RT reduced SFI of type II myofibers in both Young and Old (p<0.001). ConclusionsHere, we identify type I and II myofiber shape in humans and mice as a hallmark of muscle ageing, that independently predicts volumetric and functional assessments of muscle health. RT reverts the shape of type II myofibers, indicating that lack of neuromuscular activation might lead to myofiber deformity.

physiology↗

bioRxiv2022

Distinct myofibre domains of the human myotendinous junction revealed by single nucleus RNA-seq

The myotendinous junction (MTJ) is a specialized domain of the multinucleated myofibre, faced with the challenge of maintaining robust cell-matrix contact with the tendon under high mechanical stress and strain. Here, we profiled 24,161 nuclei in semitendinosus muscle-tendon samples from 3 healthy males by single nucleus RNA-sequencing (snRNA-seq), alongside spatial transcriptomics, to gain insight into the genes characterizing this specialization in humans. We identified a cluster of MTJ myonuclei, represented by 47 enriched transcripts, of which the presence of ABI3BP, ABLIM1, ADAMTSL1, BICD1, CPM, FHOD3, FRAS1 and FREM2 was confirmed at the MTJ at the protein level by immunofluorescence. Four distinct subclusters of MTJ myonuclei were apparent and segregated into two COL22A1-expressing subclusters and two lacking COL22A1 but with a clear fibre type profile expressing MYH7 or MYH1/2. Our findings reveal distinct myonuclei profiles of the human MTJ, a weak link in the musculoskeletal system, which is selectively affected in pathological conditions, from muscle strains to muscular dystrophies.

molecular biology↗