Denervation-Induced Activation of the Standard Proteasome and Immunoproteasome

Abstract

The standard 26S proteasome is responsible for the majority of myofibrillar protein degradation leading to muscle atrophy. The immunoproteasome is an inducible form of the proteasome. While its function has been linked to conditions of atrophy, its contribution to muscle proteolysis remains unclear. Therefore, the purpose of this study was to determine if the immunoproteasome plays a role in skeletal muscle atrophy induced by denervation. Adult male C57BL/6 wild type (WT) and immunoproteasome knockout lmp7-/-/mecl-1-/- (L7M1) mice underwent tibial nerve transection on the left hindlimb for either 7 or 14 days, while control mice did not undergo surgery. Proteasome activity (caspase-, chymotrypsin-, and trypsin- like), protein content of standard proteasome (β1, β5 and β2) and immunoproteasome (LMP2, LMP7 and MECL-1) catalytic subunits were determined in the gastrocnemius muscle. Denervation induced significant atrophy and was accompanied by increased activities and protein content of the catalytic subunits in both WT and L7M1 mice. Although denervation resulted in a similar degree of muscle atrophy between strains, the mice lacking two immunoproteasome subunits showed a differential response in the extent and duration of proteasome features, including activities and content of the β1, β5 and LMP2 catalytic subunits. The results indicate that immunoproteasome deficiency alters the proteasome’s composition and activities. However, the immunoproteasome does not appear to be essential for muscle atrophy induced by denervation.

Downhill exercise alters immunoproteasome content in mouse skeletal muscle

“…Together, no measureable differences in β1, β5, PA28α, Rpt1 or α7 protein content were present between WT and LMP7/MECL1 deficient muscle in the unstressed state, similar to what we have recently reported in 5-7-month-old mice (Liu et al 2016). However, when stressed by a bout of downhill exercise, WT muscle significantly upregulated LMP7 content while L7M1 muscle instead increased β5 content.…”

“…Although this is the first report to demonstrate LMP7 content increases in skeletal muscle due to unaccustomed exercise, previous studies have observed LMP7 is upregulated in numerous atrophic models. In fact, data from several laboratories, including our own, have shown elevated LMP7 content in aged (Ferrington et al 2005;Husom et al 2004), denervated (Liu et al 2016), dystrophic (Chen et al 2014;Farini et al 2016) and myopathic (Bhattarai et al 2016) skeletal muscles. Most of these reports also demonstrated LMP2 and/or MECL1 content, the immunoproteasome's other catalytic subunits, increased in parallel to LMP7.…”

“…In skeletal muscle, immunoproteasome content increases across various catabolic conditions, as seen with aging (Ferrington et al 2005;Husom et al 2004), denervation (Liu et al 2016), muscular dystrophy (Chen et al 2014;Farini et al 2016) and idiopathic inflammatory myopathies (Bhattarai et al 2016). These data support that similar to the standard proteasome, the immunoproteasome is involved in regulating and possibly maintaining cellular homeostasis in the skeletal muscle.…”

“…Male 8-16-week-old WT and L7M1 mice with a C57BL/6 genetic background were used in this study (Hussong et al 2011;Liu et al 2016;Schuld et al 2015). Mice were housed in groups of no more than four animals per cage, supplied with food and water ad libitum and maintained in a room at 20-23°C with a 12-h photoperiod.…”

“…Because we have previously observed L7M1 mice weigh significantly more than age-matched WT mice (Liu et al 2016), body fat percentage was measured on a subset of WT and L7M1 mice before any testing took place. Briefly, at 8-10 weeks of age, mice were anesthetized with isoflurane, carefully placed on a specimen tray and scanned using a Lunar PIXImus densitometer (GE Lunar Corporation, Madison, WI, USA) according to the specifications set by the manufacturer.…”