Unlocking the role of small heat shock proteins and apoptosis in postmortem proteolysis and meat quality characteristics of skeletal muscles under different conditions

Postmortem aging has been extensively practiced as value-adding process due to the beneficial impacts on meat palatability. Meat tenderization occurred through proteolytic fragmentation of myofibrillar structural proteins via endogenous protease systems, which is considered as the primary drive to enhance major palatability attributes including tenderness, juiciness, and flavor. Recent theoretical framework proposes apoptosis, or programmed cell death, as the preceding step that initiates postmortem proteolysis. Whereas small heat shock proteins have been consistently recognized as meat quality biomarkers, probably due to their protective activities against proteolysis through anti-stress, anti-apoptotic, and chaperoning functionalities. To shed light on detailed mechanisms controlling postmortem proteolysis and consequential impacts on the development of fresh meat quality characteristics, postmortem proteolytic changes of small heat shock proteins, apoptotic factors, and myofibrillar structural proteins were profiled in postmortem skeletal muscles under different metabolic backgrounds and across species.

In beef, three muscles, longissimus lumborum (LL), semimembranosus (SM), and psoas major (PM), have been selected to represent glycolytic, intermediate, and oxidative muscle types. Tenderness and water - holding capacity were determined, and proteolysis, apoptotic features, and small heat shock proteins were measured in 8 beef carcasses at 1, 2, 9, 16, and 23 days of aging. PM exhibited limited aging potential in quality developments shown by lower extents of shear force, water-holding capacity, and proteolytic changes, including calpain 1 autolysis, troponin T, and HSP27 compared to LL and SM. Conversely, LL had an increase in tenderization and water-holding capacity, which was accompanied with more extended calpain 1 autolysis, proteolysis and HSP27 degradation, compared with other muscles. The results of this study suggest that postmortem proteolytic changes of myofibrillar proteins, small HSPs and apoptotic factors occur in a muscle-specific manner, which is likely attributed to different rate and extent of meat quality developments of each muscle during aging.

Callipyge lambs are a unique genetic background showing calpastatin over-expression, muscle hypertrophy in loin and hindquarter area, substantially compromised meat tenderization potential, and a shift of muscle fiber composition towards fast-glycolytic directions. Proteome and metabolome changes in muscles from callipyge mutation (+/C) and non-callipyge phenotype (+/+, C/+, and C/C) lambs were profiled to provide insight into the biochemical changes affecting meat quality attributes. M. longissimus thoracis from lambs with all four possible callipyge genotype (n = 4, C/+, C/C, +/C, and +/+) were collected after 3d aging and analyzed using mass-spectrometry based platforms. Among identified proteomes, cytochrome c (pro-apoptotic protein) was detected with significantly lower abundances in +/C. Anti-apoptotic HSP70, BAG3, and PARK7 were over-abundant in +/C, which could result in delayed apoptosis and possibly attributed to tougher meat in callipyge lambs. Eight glycolysis enzymes were overabundant in +/C lambs, whereas 3 enzymes involved in TCA cycle were overabundant in non-callipyge ones (C/C and/or C/+). Twenty-five metabolites were affected by genotypes (P < 0.05), including metabolic co-factors, polyphenols, and AA/short peptides.

Pig production is facing increased public pressure regarding antibiotic usage restriction. Recently, dietary L-glutamine at cost effective level (0.2%) was identified as an effective antibiotic alternative in post-transport nursery pig diets. To evaluate carcass and meat quality characteristics in market-ready pigs when 0.2% dietary L-glutamine was applied as for early-life post-weaning and transport recovery, pigs (N=480) were weaned and transported in two replication trials in SPRING (April of 2017) vs. SUMMER (July of 2016), fed 3 different diets (Non: no antibiotic, Anti: 441 ppm chlortetracycline and + 38.6 ppm tiamulin, Gln: 0.20% L-glutamine) for 14 days after transport, and fed basal diet until reaching market weight. Pairs of longissimus dorsi (LD) and psoas major (PM) muscles from each carcass (n=10/diet/trial) were separated at 1 d and 7 d postmortem, respectively. Carcass yield and meat physical and quality attributes were evaluated. Overall impacts of Gln on physical attributes of carcasses and porcine muscles were minimal. No dietary effects were found in carcass, proximate composition, water-holding capacity, or shear force. Significant difference between trials were found in terms of productivity and pork/carcass qualities, where SPRING replicates showed increased body weight, faster pH decline, paler surface color, higher intra-muscular fat deposition, and improved tenderness and water-holding capacity as indicated by lower shear force values, thaw-purge loss, and cooking loss (P < 0.05).

The pork and carcass quality results give rise to a postulation that different metabolism and animal growth might have been occured between the two production trials, consequentially differentiated meat quality development. In this regard, myofibrillar proteolysis, small heat shock proteins, and apoptotic factors were characterized during 7 d postmortem aging in porcine LD and PM muscles from both seasonal trials, combined with metabolomics profiles of 1d samples using the GC-TOF-MS/MS platform. Compared to SUMMER counterparts, SPRING muscles showed concurrence of more extended apoptosis, further calpain 1 autolysis, and increased structural protein degradation (P<0.05). SPRING muscles showed more ATP catabolism compounds and increase in carbohydrates, branched-chain amino acids, and 16-18 carbon fatty acids, which could be chemistry fingerprints of increased cellular oxidative stress, consequentially favoring onset of apoptosis and proteolysis. Meanwhile, SUMMER pigs showed increased stress-defending metabolites, such as ascorbic acid, antioxidant amino acids, and decreased inhibitory neuro-transmitter GABA, which may indicate elevated stress-defending activity in SUMMER pigs that possibly inhibited apoptosis and proteolysis.