Biology subjects

Birk, J. B.

Publications and source records attributed to Birk, J. B..

4 recordsLinked to original sources

bioRxiv2026

Personalized phosphoproteomics establish mTORC1 as a regulator of exercise-induced insulin sensitization in human skeletal muscle

Exercise enhances skeletal muscle insulin sensitivity, but the signaling mechanisms responsible are poorly understood. Understanding them may open new therapeutic avenues for individuals with limited exercise capacity. Here, we used rapamycin to inhibit mTORC1 in combination with exercise and insulin stimulation in healthy men. A single dose of rapamycin enhanced the insulin-sensitizing effect of exercise by 53% on average compared to placebo. Responses varied widely across individuals (-40% to 218%), and we leveraged this variance through personalized phosphoproteomics to map the mTORC1-dependent signaling network in skeletal muscle. This identified the protein kinase MKNK2 as a candidate downstream effector, which we then targeted for functional validation. Pharmacological inhibition of MKNK2 with eFT508 in insulin-clamped mice reduced both whole-body and skeletal muscle insulin sensitivity, confirming a functional role for MKNK2 activity in muscle glucose uptake. We then used eFT508 in ex vivo incubated human skeletal muscle to map the signaling network downstream of MKNK2, identifying the translational initiator eIF4G1 as a further regulatory node. Together, these findings indicate that exercise-induced insulin sensitization is actively constrained by a negative feedback pathway running from mTORC1 through the translational regulators MKNK2 and eIF4G1, raising the possibility that rapid translation of unidentified target proteins contributes to fine-tuning glucose uptake.

physiology↗

bioRxiv2026

Functionally Mature Bioengineered Human Skeletal Muscle Tissues Capture Essential Aspects of Glucose Metabolism

Human skeletal muscle is a major regulator of whole-body metabolic homeostasis, yet mechanistic insight into human muscle plasticity is limited by the lack of in vitro models with adult-like metabolic and functional maturity. Here, we develop a workflow for generating bioengineered human skeletal muscle tissues that undergo coordinated structural, molecular, and functional maturation and stabilize in an adult-like state by day 21. Time-resolved RNA-seq and protein profiling reveal consolidation of contractile programs alongside progressive metabolic maturation, including increased mitochondrial electron transport chain content, mature mitochondrial network organization, and upregulation of glucose- and glycogen-handling proteins as well as muscle-enriched AMPK isoforms. Functionally, the tissues develop physiological force-frequency behavior, post-tetanic potentiation, and reproducible fatigue responses that are exacerbated by hypoxia and glucose withdrawal. Exercise-like chronic stimulation increases force and endurance with hypertrophy-like remodeling, and these adaptations reverse with detraining. The model also captures pharmacological responsiveness. {beta}2-adrenergic stimulation activates canonical signaling, increases force, limits disuse-related decline, and improves endurance in a glucose-dependent manner. Under physiological insulin and IGF-1 conditions, tissues show robust insulin-stimulated glucose uptake and glycogen synthesis, with punctate glucose transporter 4 (GLUT4) localization. Finally, knockdown of muscle glycogen synthase (GYS1) preserves peak tetanic force but impairs endurance and force recovery under fuel stress, indicating that glycogen metabolism is a key determinant of human muscle resilience.

cell biology↗

bioRxiv2023

The human AMPKgamma3 R225W mutation does neither enhance basal AMPKgamma3-associated activity nor glycogen in human or mouse skeletal muscle

BackgroundAMP-activated protein kinase (AMPK) is activated during cellular energy perturbation. AMPK is composed of three subunits and several variants of AMPK complexes are expressed in skeletal muscle. The regulatory AMPK{gamma}3 subunit is predominantly expressed in fast-twitch muscle fibers. A human AMPK{gamma}3 R225W mutation has been described in two families. In cultured cells derived from R225W carrier muscle, the mutation was reported to increase total AMPK activity. In addition, elevated glycogen levels were observed in skeletal muscle. This led to the idea of AMPK{gamma}3 being involved in glycogen levels in skeletal muscle. Evidence for such a causative link has never been provided. ResultsWe studied newly obtained muscle biopsies from three human carriers of the AMPK{gamma}3 R225W mutation and matched controls and we developed a novel knock-in mouse model carrying the AMPK{gamma}3 R225W mutation (KI HOM). In all three human pairs, the basal AMPK{gamma}3-associated activity was reduced when assayed in the absence of exogenous AMP. No difference was observed when assayed under AMP saturation, which was supported by findings in muscle of KI HOM mice. Furthermore, the well-established stimulatory effects of AICAR/muscle contraction on AMPK{gamma}3-associated activity were absent in muscle from KI HOM mice. Muscle glycogen levels were not affected by the mutation in human carriers or in KI HOM mice. ConclusionsThe AMPK{gamma}3 R225W mutation does not impact AMPK-associated activity in mature human skeletal muscle and the mutation is not linked to glycogen accumulation. The R225W mutation ablates AMPK{gamma}3-associated activation by AICAR/muscle contractions, presumably through loss of nucleotide binding.

molecular biology↗

bioRxiv2022

Metformin improves glycemia independently of skeletal muscle AMPK via enhanced intestinal glucose clearance

Metformin is an inexpensive oral anti-hyperglycemic agent used worldwide as a first-choice drug for the prevention of type 2 diabetes mellitus (T2DM). Although current view suggests that metformin exerts its anti-hyperglycemic effect by lowering hepatic glucose production, it has been proposed that metformin also reduce hyperglycemia by increasing glucose uptake in skeletal muscle via activation of AMP-activated protein kinase (AMPK). Herein, we demonstrate in lean and diet-induced obese (DIO) male and female mouse models that the anti-hyperglycemic effect of metformin occurs independently of muscle AMPK, and instead relies on elevated intestinal glucose clearance. Furthermore, we report that the AMPK activity is elevated in skeletal muscle from patients with T2DM following chronic metformin treatment, but this is not associated with enhanced peripheral insulin sensitivity. These results argue against existing paradigms and emphasize the non-essential role of muscle AMPK but important role of the intestine for the anti-hyperglycemic effect of metformin.

physiology↗