I. H. Hwang

Cyclic mechanical stress suppresses myogenic differentiation of adult bovine satellite cells through activation of extracellular signal-regulated kinase

Mechanical stress leads to satellite cell activation, which is an important event in the development, growth, and remodeling of postnatal skeletal muscle. Although there is a considerable knowledge on the events involved in skeletal muscle regeneration and development, the precise role of mechanical stress on activation of satellite cells remains unclear. Previously, satellite cells were isolated from adult bovine muscle and it was shown that the cells are multipotent, i.e., capable of proliferating and to differentiating into both myoblasts and adipocytes. This study investigated the cellular mechanisms by which cyclic mechanical stretching modulates the proliferation and differentiation of adult bovine satellite cells. The application of cyclic stretch induced the proliferation of satellite cells and inhibited their differentiation into myotubes. This response is believed to be closely related to the stretch-mediated changes in the expression of myogenic and cell cycle regulatory factors. Cyclic stretching increased the level of extracellular signal-regulated kinase (ERK) phosphorylation, whereas a specific ERK inhibitor (PD98058) blocked the stretch-mediated inhibition of myogenesis in a dose-dependent manner. Overall, this study demonstrates for the first time that cyclic mechanical stretch induces the proliferation of bovine satellite cells and suppresses their myogenic differentiation through the activation of ERK.

Effect of fiber type on postmortem proteolysis in longissimus muscle of Landrace and Korean native black pigs.

The current study was conducted to characterize objective meat quality, fiber type and their relations to postmortem proteolysis in longissimus muscle of Landrace and Korean native black (KNP) pigs. Longissimus muscles from each 10 market-weighted male pigs were removed after conventional slaughtering and chilling procedures, and aged for 1 or 7 days at 4°C to determine WB-shear force, objective meat color, proportion of myosin heavy chain I (MyHC I), intramuscular fat content and rate of proteolysis by a proteomics approach. KNP had a significantly (p<0.05) higher content of MyHC I, and that concurred with greatly (p<0.05) higher intramuscular fat content and Hunter a(∗) value, and significantly (p<0.05) lower drip loss than those seen in Landrace. One-dimension SDS-PAGE indicated that GAPDH, troponin I and creatine kinase were prominent proteolytic products during chiller ageing. By applying a gel-based proteome analysis, 26 proteins were identified, which showed different degradation properties during ageing between the breeds. Biopsied sample revealed that myosin regulatory light chain 2, myosin light chain isotype v/sb, fatty acid-binding protein and albumin were expressed at a greatly higher level for KNP, but their relation to fiber type (or genetic background) is unclear. It was particular noticeable that different actin isoforms showed various degradation behavior during ageing time.

Beef quality grades as determined by Korean and Australian consumers

Consumer responses were examined in an incomplete factorial design where Australian consumers evaluated 216 beef samples derived from 18 cattle killed in Australia and Korean consumers evaluated 216 samples from the same 18 cattle, plus 216 similar samples from 18 Korean cattle. Samples of the Mm. triceps brachii, longissimus dorsi and semimembranosus were cooked using grill and Korean barbeque methods. Each sample was sensory tested by 10 consumers, who scored it for tenderness, juiciness, like flavour, and overall liking. Consumers then graded each sample as either unsatisfactory (2 star), good every day (3 star), better than every day (4 star), or premium (5 star) quality. For those samples assessed by both Australian and Korean consumers, the Korean consumers graded a higher proportion of samples ‘unsatisfactory’ and a lower proportion of samples ‘premium’ grade product than Australian consumers. Using a composite meat quality score (MQ4) to predict grade, a discriminant analysis showed that the Korean consumers had boundary cut-offs for the lower grades, which were ~4–10 palatability units higher than the Australian consumers. Analysis of the residuals between actual and predicted palatability scores showed that the Meat Standards Australia (MSA) grading model produced relatively unbiased estimates within ±2 MQ4 units for the different consumer groups, muscle and carcass suspension treatments, with the exception of the M. semimembranosus samples. Implications of the results for both Korean and Australian beef markets through the use of an empirical grading model to predict palatability are discussed.

Effect of carcass suspension and cooking method on the palatability of three beef muscles as assessed by Korean and Australian consumers

A total of 36 steer carcasses (18 slaughtered in Australia and 18 slaughtered in Korea), where one side had been suspended by the hip (tenderstretch) and the other by the Achilles tendon were used to provide sensory and shear force samples from the Mm. triceps brachii, longissimus lumborum and semimembranosus. Sensory samples were cooked using grill (25 mm thick) and barbeque (BBQ, 4 mm thick) methods and served to 360 untrained Australian and 720 untrained Korean consumers. Australian consumers sensory tested grill and BBQ samples from Australian carcasses (216 samples), while Korean consumers sensory tested grill and BBQ samples from both Australian and Korean carcasses (a total of 432 samples). The three-way interaction between carcass suspension, cooking method and muscle was significant (P < 0.05) for tenderness, overall liking and a composite palatability score (MQ4), where the combination of BBQ cooking and hip suspension resulted in large increases in sensory scores for the M. semimembranosus. Variation in sensory scores and shear force are discussed in the context of possible interactions with cooking temperature. There was a significant (P < 0.05) first order interaction between consumer group and muscle for juiciness score. Consumer effects were significant (P < 0.05) for all sensory scores, being largest for juiciness (~8 sensory units), like flavour and overall liking (both ~6 sensory units) and MQ4 (~5 sensory units) scores, with the smallest effect on tenderness (~2 sensory units).

Involvement of p38 MAPK-mediated signaling in the calpeptin-mediated suppression of myogenic differentiation and fusion in C2C12 cells

Calpeptin inhibits myoblast fusion by inhibiting the activity of calpain. However, the mechanism by which calpeptin inhibits myogenesis is not completely understood. This study examined how calpeptin affects the expression of the myogenic regulatory factors (MRFs) and the phosphorylation of p38 mitogen-activated protein kinase (MAPK) in differentiating C2C12 myoblasts. Consistent with previous reports, calpeptin inhibited the induction of μ-calpain and the formation of myotubes in these cells. In particular, calpeptin inhibited the expression of the early and mid differentiation markers including MyoD, Myf5, myogenin, and MRF4 as well as the expression of the late markers such as troponin T and myosin heavy chain (MyHC). Calpeptin also suppressed the phosphorylation of p38 MAPK in C2C12 cells. SB203580, a specific p38 inhibitor, prevented the expression of the muscle-specific markers and their fusion into myotubes in these cells, which was further accelerated in the presence of calpeptin. These findings suggest that calpeptin inhibits the myogenesis of skeletal muscle cells by down-regulating the MRFs and involving p38 MAPK signaling.

Determination of Genetic Diversity among Korean Hanwoo Cattle Based on Physical Characteristics

This study was conducted to establish genetic criteria for phenotypic characteristics of Hanwoo cattle based on allele frequencies and genetic variance analysis using microsatellite markers. Analysis of the genetic diversity among 399 Hanwoo cattle classified according to nose pigmentation and coat color was carried out using 22 microsatellite markers. The results revealed that the INRA035 locus was associated with the highest Fis (0.536). Given that the Fis value for the Hanwoo INRA035 population ranged from 0.533 (white) to 1.000 (white spotted), this finding was consistent with the loci being fixed in Hanwoo cattle. Expected heterozygosities of the Hanwoo groups classified by coat colors and degree of nose pigmentation ranged from 0.689±0.023 (Holstein) to 0.743±0.021 (nose pigmentation level of d). Normal Hanwoo and animals with a mixed white coat showed the closest relationship because the lowest DA value was observed between these groups. However, a pair-wise differentiation test of Fst showed no significant difference among the Hanwoo groups classified by coat color and degree of nose pigmentation (p<0.01). Moreover, results of the neighbor-joining tree based on a DA genetic distance matrix within 399 Hanwoo individuals and principal component analyses confirmed that different groups of cattle with mixed coat color and nose pigmentation formed other specific groups representing Hanwoo genetic and phenotypic characteristics. The results of this study support a relaxation of policies regulating bull selection or animal registration in an effort to minimize financial loss, and could provide basic information that can be used for establishing criteria to classify Hanwoo phenotypes.