Subcellular location and colocalization of lipid droplet proteins, ADRP and OXPAT, in resting and stimulated rat soleus

Abstract

Skeletal muscle lipid droplet proteins are thought to regulate lipolysis through protein-protein interactions on the lipid droplet surface. In cultured cells ADRP is found only on lipid droplets, while OXPAT (expressed only in oxidative tissues) is found both on and off the lipid droplet, and may be recruited when needed. Our purpose was to determine if OXPAT is recruited to ADRP-coated lipid droplets with contraction, and to investigate the myocellular location and colocalization of OXPAT and ADRP. Rat solei were isolated and assigned one of two groups: 1) a 30 min resting incubation; 2) a 30 min stimulation; 150 ms trains of 0.1ms impulses (60 Hz) at 20 tetani/min for maximal lipid utilization (n=5 each, 25°C). Immunofluorescence microscopy was used to determine subcellular distribution and colocalization. ADRP and OXPAT distribution showed an exponentially higher content from the sarcolemma toward the central region (p<0.001), with no effect of contraction (p=.99). ADRP-OXPAT colocalized at rest and there was no significant difference post stimulation (p=0.11; rest r2=0.64±0.03, stimulated r2=0.71±0.01). There was no difference in either ADRP (p=0.58) or OXPAT (p=0.75) content post stimulation. Contrary to our hypothesis these results show that OXPAT is not recruited to the lipid droplet during contraction and is highly colocalized with ADRP at all times. Supported by NSERC, Canada.