Regulatory factors controlling muscle mass: Competition between innate immune function and anabolic signals in regulation of atrogin-1 in Atlantic salmon

Supplementation of arginine, ornithine and citrulline in rainbow trout (Oncorhynchus mykiss): Effects on growth, amino acid levels in plasma and gene expression responses in liver tissue

Functional amino acids (FAA) regulate metabolic pathways directly linked to health, survival, growth and development. Arginine is a FAA with crucial roles in protein deposition and the immune response. In mammals, supplementation of arginine's precursor amino acid, citrulline, is known to increase circulating arginine to levels beyond direct arginine supplementation, however, citrulline supplementation is poorly studied in fish. To address this knowledge gap, we supplemented the diet of rainbow trout with arginine and its precursor amino acids, ornithine and citrulline, at 3 levels (0.5%, 1% and 2% of the total diet) during a 14-week experiment. We sampled fish at 3 h and 24 h post-feeding to investigate immediate and steady-state effects, respectively. There were no differences in fish growth for any of the diets across a range of indicators. In blood plasma, out of 26 amino acids detected, 11 and 6 displayed significant changes 24 h and 3 h post-prandial, respectively. Arginine, ornithine and citrulline levels were all significantly increased by the citrulline supplemented diets. In muscle, 8 amino acids were significantly altered by supplemented diets, while there were no significant changes in liver. Arginine was increased by 2% citrulline supplementation in muscle tissue. We also investigated the transcriptional responses of urea cycle, nitric oxide cycle and rate-limiting polyamine synthesis enzymes, related to arginine's metabolism, in liver. At both time points, only 2 enzymes were significantly altered by the supplemented diets, however several significant changes were observed comparing 3 h and 24 h post-prandial expression levels. Of these, the paralogous polyamine synthesis enzyme encoding genes ODC1 and ODC2 displayed the largest increases in 3 h post-prandial fish. These findings demonstrate that endogenous synthesis of arginine is possible from a citrulline supplemented diet and improve our understanding of arginine metabolism in fish.

β-Glucan improves wound healing in silver catfish (Rhamdia quelen)

The immune modulating activity of β-glucan on aquatic species has been a matter of intense investigation. Here, we aimed to investigate the effect of β-glucan on wound healing of silver catfish, a Neotropical South American scale-free fish. Small sections of skin and muscle (3 mm in diameter) were removed and fish were bathed daily with β-glucan (0.1% and 0.5%) up to 28 days when cicatrization was complete. A group of fish similarly injured and non-exposed to β-glucan was used as control. Wound closure and healing was monitored visually and by histopathological analysis. In fish bathed with 0.5% β-glucan we found reduced blood cortisol levels at day one post-wounding and, by day 7 post wounding, the deposition of granulation tissue was higher compared to non-exposed fish. In addition, from day 7 forward, wound size was significantly lower in fish bathed with 0.5% β-glucan. Histopathological analysis of the wounded site indicated a thin layer of immature epidermal cells at day one post wounding. A discrete inflammation with mixed inflammatory cell infiltrate was observed on wounded muscle and was lower by day 7 post wounding on fish bathed with 0.5% β-glucan. By day 14 post wounding, the deposition of collagen fibers and the presence of fibroblast and new muscle fibers were higher in fish exposed to 0.5% β-glucan, and dermis restoration was complete. Thus, our results indicate that in silver catfish wound healing occurs rapidly and improves greatly by daily bathing with β-glucan.

Rainbow trout (Oncorhynchus mykiss) urea cycle and polyamine synthesis gene families show dynamic expression responses to inflammation

The urea cycle is an endogenous source of arginine that also supports removal of nitrogenous waste following protein metabolism. This cycle is considered inefficient in salmonids, where only 10-15% of nitrogenous waste is excreted as urea. In rainbow trout, arginine is an essential amino acid that has attracted attention due to its many functional roles. These roles include the regulation of protein deposition, immune responses and polyamine synthesis; the latter is directly linked to the urea cycle and involved in tissue repair. The key enzymes used in the urea cycle, namely arginase, ornithine transcarbamylase, argininosuccinate synthase and argininosuccinate lyase, in addition to two rate limiting enzymes required for polyamine synthesis (ornithine decarboxylase and s-adenosylmethionine decarboxylase) are poorly studied in fishes, and their responses to inflammation remain unknown. To address this knowledge gap, we characterised these gene families using phylogenetics and comparative genomics, investigated their mRNA distribution among a panel of tissues and established their transcriptional responses to an acute inflammatory response caused by bacterial infection in liver and muscle. Gene duplicates (paralogues) were identified for arginase (ARG1a, 1b, 2a and 2b), ornithine decarboxylase (ODC1 and 2) and s-adenosylmethionine decarboxylase (SAMdc1 and 2), including paralogues retained from an ancestral salmonid-specific whole genome duplication. ARG2a and 2b were highly upregulated following bacterial infection in liver, whereas ARG1b was downregulated, while both paralogues of SAMdc and ODC were upregulated in liver and unchanged in muscle. Overall, these findings improve our understanding of the molecules supporting the urea cycle and polyamine synthesis in fish, highlighting major changes in the regulation of these systems during inflammation.

Local and Systemic Cytokine Profiling for Pancreatic Ductal Adenocarcinoma to Study Cancer Cachexia in an Era of Precision Medicine

Cancer cachexia is a debilitating condition seen frequently in patients with pancreatic ductal adenocarcinoma (PDAC). The underlying mechanisms driving cancer cachexia are not fully understood but are related, at least in part, to the immune response to the tumor both locally and systemically. We hypothesize that there are unique differences in cytokine levels in the tumor microenvironment and systemic circulation between PDAC tumors and that these varying profiles affect the degree of cancer cachexia observed. Patient demographics, operative factors, oncologic factors, and perioperative data were collected for the two patients in the patient derived xenograft (PDX) model. Human pancreatic cancer PDX were created by implanting fresh surgical pancreatic cancer tissues directly into immunodeficient mice. At PDX end point, mouse tumor, spleen and muscle tissues were collected and weighed, muscle atrophy related gene expression measured, and tumor and splenic soluble proteins were analyzed. PDX models were created from surgically resected patients who presented with different degrees of cachexia. Tumor free body weight and triceps surae weight differed significantly between the PDX models and control (P < 0.05). Both PDX groups had increased atrophy related gene expression in muscle compared to control (FoxO1, Socs3, STAT3, Acvr2b, Atrogin-1, MuRF1; P < 0.05). Significant differences were noted in splenic soluble protein concentrations in 14 of 15 detected proteins in tumor bearing mice when compared to controls. Eight splenic soluble proteins were significantly different between PDX groups (P < 0.05). Tumor soluble proteins were significantly different between the two PDX groups in 15 of 24 detected proteins (P < 0.05). PDX models preserve the cachectic heterogeneity found in patients and are associated with unique cytokine profiles in both the spleen and tumor between different PDX. These data support the use of PDX as a strategy to study soluble cachexia protein markers and also further efforts to elucidate which cytokines are most related to cachexia in order to provide potential targets for immunotherapy.

Growth hormone transgenesis in coho salmon disrupts muscle immune function impacting cross-talk with growth systems

Suppression of growth during infection may aid resource allocation towards effective immune function. Past work supporting this hypothesis in salmonid fish revealed an immune-responsive regulation of the insulin-like growth factor (IGF) system, an endocrine pathway downstream of growth hormone (GH). Skeletal muscle is the main target for growth and energetic storage in fish, yet little is known about how its growth is regulated during an immune response. We addressed this knowledge gap by characterizing muscle immune responses in size-matched coho salmon (Oncorhynchus kisutch) achieving different growth rates. We compared a wild-type strain with two GH transgenic groups from the same genetic background achieving either maximal or suppressed growth, a design separating GH's direct effects from its influence on growth rate and nutritional state. Fish were sampled 30h post-injection with PBS (control) or mimics of bacterial or viral infection. We quantified mRNA expression levels for genes from the GH, GH receptor, IGF hormone, IGF1 receptor and IGF-binding protein families, along with immune genes involved in inflammatory or antiviral responses and muscle growth status marker genes. We demonstrate dampened immune function in GH transgenics compared to wild-type. The muscle of GH transgenics achieving rapid growth showed no detectable antiviral response, coupled with evidence of a constitutive inflammatory state. GH and IGF system gene expression was strongly altered by GH transgenesis and fast growth, both for baseline expression and responses to immune stimulation. Thus, GH transgenesis strongly disrupts muscle immune status and normal GH and IGF system expression responses to immune stimulation.

Divergent regulation of insulin-like growth factor binding protein genes in cultured Atlantic salmon myotubes under different models of catabolism and anabolism

Much attention has been given to insulin-like growth factor (Igf) pathways that regulate the balance of skeletal muscle protein synthesis and breakdown in response to a range of extrinsic and intrinsic signals. However, we have a less complete understanding of how the same signals modulate muscle mass upstream of such signalling, through a family of functionally-diverse Igf-binding proteins (Igfbps) that modify the availability of Igfs to the cell receptor Igf1r. We exposed cultured myotubes from Atlantic salmon (Salmo salar L.) to treatments recapturing three catabolic signals: inflammation (interleukin-1β), stress (dexamethasone) and fasting (amino acid deprivation), plus one anabolic signal: recovery of muscle mass post-fasting (supplementation of fasted myotubes with Igf-I and amino acids). The intended phenotype of treatments was confirmed by significant changes in myotube diameter and immunofluorescent staining of structural proteins. We quantified the mRNA-level regulation of the full expressed Igf and Igfbp gene complement across a post-treatment time course, along with marker genes for muscle structural protein synthesis, as well as muscle breakdown, via the ubiquitin-proteasome and autophagy systems. Our results highlight complex, non-overlapping responses of Igfbp family members to the different treatments, suggesting that the profile of expressed Igfbps is differentially regulated by distinct signals promoting similar muscle remodelling phenotypes. We also demonstrate divergent regulation of salmonid-specific gene duplicates of igfbp5b1 and igfbp5b2 under distinct catabolic and anabolic conditions. Overall, this study increases our understanding of the regulation of Igfbp genes in response to signals that promote remodelling of skeletal muscle.

The Function of Fish Cytokines

What is known about the biological activity of fish cytokines is reviewed. Most of the functional studies performed to date have been in teleost fish, and have focused on the induced effects of cytokine recombinant proteins, or have used loss- and gain-of-function experiments in zebrafish. Such studies begin to tell us about the role of these molecules in the regulation of fish immune responses and whether they are similar or divergent to the well-characterised functions of mammalian cytokines. This knowledge will aid our ability to determine and modulate the pathways leading to protective immunity, to improve fish health in aquaculture.