Recent findings regarding the response of fibronectin type III domain-containing protein 5 (Fndc5) and irisin to exercise are partly controversial. While the 25 kDa form of Fndc5 can be observed in muscle and serum of different species, the ~12 kDa irisin band was not detectable up to now. The present study aimed to clarify whether irisin exists in its theoretical size of ~12 kDa in mice and if it is affected by exercise. Male mice were randomly assigned to a sedentary control group (CO), a group with free access to running wheels (RW), and a treadmill group (TM). Blood and leg muscles were collected to investigate the regulatory cascade including peroxisome proliferator-activated receptor gamma co-activator 1-alpha (Ppargc1a) and Fndc5. In western blot analysis, antibodies were used capable of differentiation between full-length Fndc5 and irisin. This enabled us to demonstrate that irisin exists in muscle and serum of mice independent of exercise and that it is increased immediately after acute exercise. Different transcripts of Ppargc1a mRNA, but not Fndc5 mRNA, were up-regulated in the TM group. Furthermore, neither Fndc5 (25 kDa) nor Ppargc1a protein was elevated in muscle tissue. The Ppargc1a-Fndc5/irisin pathway did not clearly respond to mild exercise in the RW group. Our results provide evidence for the existence of irisin and for its immediate response to acute exercise in mice.

Adipose tissue is considered as a major endocrine organ that secretes numerous proteins called adipokines. The heterogeneous nature of adipose tissue in different parts of the body suggests respective heterogeneity of proteomes and secretomes. This review consolidates knowledge from recent studies targeting the diversity of different adipose depots affecting the pattern of secreted adipokines and discusses potential consequences for the cross-talk between adipose and skeletal muscle in humans, rodent models and farm animals. Special attention is paid to muscle-associated fat depots like inter- and intramuscular fat that become focus of attention in the context of the rather new notion of skeletal muscle as a major endocrine organ. Understanding the complexity of communication between adipocytes and skeletal muscle cells will allow developing strategies for improvement of human health and for sustainable production of high quality meat.

Beside its role in melanogenesis, the agouti signaling protein (ASIP) has been related to obesity. The potentially crucial role in adipocyte development makes it a tempting candidate for economic relevant, fat related traits in farm animals. The objective of our study was to characterize the mRNA expression of different ASIP transcripts and of putative targets in different bovine tissues, as well as to study consequences on protein abundance and localization. ASIP mRNA abundance was determined by RT-qPCR in adipose and further tissues of cattle representing different breeds and crosses. ASIP mRNA was up-regulated more than 9-fold in intramuscular fat of Japanese Black cattle compared to Holstein (p<0.001). Further analyses revealed that a transposon-derived transcript was solely responsible for the increased ASIP mRNA abundance. This transcript was observed in single individuals of different breeds indicating a wide spread occurrence of this insertion at the ASIP locus in cattle. The protein was detected in different adipose tissues, skin, lung and liver, but not in skeletal muscle by Western blot with a bovine-specific ASIP antibody. However, the protein abundance was not related to the observed ASIP mRNA over-expression. Immuno-histochemical analyses revealed a putative nuclear localization of ASIP additionally to the expected cytosolic signal in different cell types. The expression of melanocortin receptors (MCR) 1 to 5 as potential targets for ASIP was analyzed by RT-PCR in subcutaneous fat. Only MC1R and MC4R were detected indicating a similar receptor expression like in human adipose tissue. Our results provide evidence for a widespread expression of ASIP in bovine tissues at mRNA and, for the first time, at protein level. ASIP protein is detectable in adipocytes as well as in further cells of adipose tissue. We generated a basis for a more detailed investigation of ASIP function in peripheral tissues of various mammalian species.

The transmembrane protein FNDC5 was recently characterized as precursor of an exercise induced myokine named irisin. Previous studies found a relationship between circulating irisin levels and muscle mass in humans. Consequently, we tested the hypothesis whether FNDC5/irisin is involved in the modulation of body composition in cattle. Since information on the bovine FNDC5 locus was scarce, we characterized the gene experimentally as prerequisite for these investigations. We provide here a revised and extended gene model for bovine FNDC5. Although similarly organized like the human and murine loci, a higher variability was observed at transcript level in the bovine locus. FNDC5 mRNA was abundant in bovine skeletal muscle and was detected at lower levels in adipose tissue and liver. There were no expression differences between two groups of bulls highly different in muscularity and adiposity. Full-length FNDC5 protein (25 kDa) was present in bovine skeletal muscle independent of muscularity. Neither FNDC5 nor its putatively secreted peptide irisin were found in circulation of bulls. In contrast, we demonstrated that FNDC5 (25 kDa) and irisin (12 kDa) were present in murine skeletal muscle and that irisin was circulating in murine serum. This indicates fundamental differences in the regulation of FNDC5 and irisin between rodents and cattle.

Thyroid hormone responsive protein (THRSP) is known to be involved in lipogenic processes in rodents. In cattle, THRSP could be a potential molecular marker for intramuscular fat (IMF) deposition since mRNA abundance was frequently found to be increased in skeletal muscle with high IMF content compared to those with low IMF. The aim of this study was to elucidate the background of this differential expression and to evaluate the role of THRSP as candidate for increased IMF content in cattle. By combination of mRNA and protein analyses, we could demonstrate that THRSP is present mainly in nuclei of adipose tissue, in intramuscular fat cells and associated cells, and in cells of the portal triad of liver, whereas muscle cells did not express THRSP. Cell culture analyses revealed furthermore that THRSP is expressed in mature adipocytes rather than in early stages of adipogenesis. Collectively, our data support the putative role of THRSP as transcriptional regulator and demonstrate that an increased expression of THRSP in M. longissimus is a consequence of but not the reason for a higher number of intramuscular adipocytes in cattle with enhanced IMF deposition.