Myofiber “Lipotoxicity” Does Not Disrupt Cellular Biophysical Properties
Authors: Shen Karen , Collins Kelsey , Guilak Farshid , Meyer Gretchen 
Published: May 2024

Obesity is associated with pathological changes in skeletal muscle that contribute to functional loss and disability. While it is unclear which specific factors cause this impairment in muscle performance, intramyocellular lipid (IMCL) within the muscle fibers, termed “lipotoxicity”, is implicated. Some studies have suggested the mechanism for this association is mitochondrial dysfunction, whereas others propose the increase in IMCL leads to structural changes within the muscle fibers. The objective of this study was to determine whether IMCL, independent of obesity, impacted active force generation and passive viscoelasticity of the myofiber sarcomere lattice. This study utilized soleus muscles from wildtype (WT) and lipodystrophic (LD) mice and calf muscle biopsies from individuals with (DPN) and without (CTL) type 2 diabetes over a range of obesity status to isolate the effects of IMCL from the effects of obesity in general. IMCL was quantified for each fiber type by co-registration of Oil Red O (ORO) stained cryosections or fluorescent BODIPY stained section and myosin heavy chain isoform stained sections. BODIPY stained sections were used with confocal microscopy to quantify individual lipid droplet metrics (average area, total number). Individual muscle fibers were isolated from soleus muscles and calf muscle biopsies and chemically permeabilized to isolate the myofibrillar lattice and were tested in a specialized system (1400A; Aurora Scientific). Active and passive forces were recorded, fit with a 3-element model, and correlated with metrics of IMCL accumulation by fiber type. LD muscle had 2-3 fold higher IMCL accumulation (p<0.05) in type 2a and 2x fibers than WT quantified by ORO and BODIPY. This was due to an increase in both lipid droplet area and number. DPN muscle had ~2 fold higher IMCL accumulation (p<0.05) in type 2 fibers compared with CTL. Both type 1 and type 2 fibers had significantly increased area of lipid droplets, but only in type 2 fibers was there an increase in lipid droplet number and consequently in area fraction. Despite the large accumulation of IMCL, LD permeabilized muscle fibers had no deficit in active force generation and no change in any of the 5 measures of passive viscoelasticity compared with WT. Conversely, in humans, DPN type 2 fibers had significantly lower (-18%) active force generation and both type 1 and type 2 DPN fibers had elevated resting viscosity (+5%) compared with CTL. Across human samples, active force generation was significantly correlated with average lipid droplet area (r2=0.53, p=0.04), but no other correlations were significant. This study suggests that an increase in IMCL accumulation in and of itself does not physically disrupt sarcomere contraction. Dramatic accumulation of IMCL in LD mouse fibers did not impact active or passive forces, likely due to an absence of systemic complications (obesity, advanced diabetes). Human fibers of comparable IMCL accumulation have a deficit in active force generation correlated with IMCL parameters, but we conclude this relationship is more likely correlative than causative as type 1 fibers with similar lipid droplet areas have no association between lipid droplet area and active force generation. These data suggest that while accumulation of IMCL is associated with contractile disfunction across a number of conditions, the mechanism is likely either indirect or biochemical rather than biophysical. Shriners Hospitals for Children, P30 AR074992, AR075773, AG15768, AG46927, AR072999, AR073752. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.