Effects of arachidonic acid and its major prostaglandin derivatives on bovine myoblast proliferation, differentiation, and fusion

Comparison of differentially expressed genes in longissimus dorsi muscle of Diannan small ears, Wujin and landrace pigs using RNA-seq

Introduction

Pig growth is an important economic trait that involves the co-regulation of multiple genes and related signaling pathways. High-throughput sequencing has become a powerful technology for establishing the transcriptome profiles and can be used to screen genome-wide differentially expressed genes (DEGs). In order to elucidate the molecular mechanism underlying muscle growth, this study adopted RNA sequencing (RNA-seq) to identify and compare DEGs at the genetic level in the longissimus dorsi muscle (LDM) between two indigenous Chinese pig breeds (Diannan small ears [DSE] pig and Wujin pig [WJ]) and one introduced pig breed (Landrace pig [LP]).

Methods

Animals under study were from two Chinese indigenous pig breeds (DSE pig, n = 3; WJ pig, n = 3) and one introduced pig breed (LP, n = 3) were used for RNA sequencing (RNA-seq) to identify and compare the expression levels of DEGs in the LDM. Then, functional annotation, Gene Ontology (GO) enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, and Protein-Protein Interaction (PPI) network analysis were performed on these DEGs. Then, functional annotation, Gene Ontology (GO) enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, and Protein-Protein Interaction (PPI) network analysis were performed on these DEGs.

Results

The results revealed that for the DSE, WJ, and LP libraries, more than 66, 65, and 71 million clean reads were generated by transcriptome sequencing, respectively. A total of 11,213 genes were identified in the LDM tissue of these pig breeds, of which 7,127 were co-expressed in the muscle tissue of the three samples. In total, 441 and 339 DEGs were identified between DSE vs. WJ and LP vs. DSE in the study, with 254, 193 up-regulated genes and 187, 193 down-regulated genes in DSE compared to WJ and LP. GO analysis and KEGG signaling pathway analysis showed that DEGs are significantly related to contractile fiber, sarcolemma, and dystrophin-associated glycoprotein complex, myofibril, sarcolemma, and myosin II complex, Glycolysis/Gluconeogenesis, Propanoate metabolism, and Pyruvate metabolism, etc. In combination with functional annotation of DEGs, key genes such as ENO3 and JUN were identified by PPI network analysis.

Discussion

In conclusion, the present study revealed key genes including DES, FLNC, PSMD1, PSMD6, PSME4, PSMB4, RPL11, RPL13A, ROS23, RPS29, MYH1, MYL9, MYL12B, TPM1, TPM4, ENO3, PGK1, PKM2, GPI, and the unannotated new gene ENSSSCG00000020769 and related signaling pathways that influence the difference in muscle growth and could provide a theoretical basis for improving pig muscle growth traits in the future.

Analysis of commercial fetal bovine serum (FBS) and its substitutes in the development of cultured meat

Fetal bovine serum (FBS) is an extremely important culture growth supplement, accounting for approximately 60 % of cell-culture-media costs; therefore, lowering FBS-acquisition costs for the industrialization of cultured meat is imperative. This study attempted to produce an FBS substitute using discarded livestock by-products, with particular focus on formulating a product with a composition similar to that of FBS to improve effectiveness. However, to date, no study has precisely analyzed the commercial components of FBS, and this study is the first to compare the chemical composition of FBS and commercially available horse serum purchased from the United States or Europe with that of FBS substitutes developed by our team. This study analyzed the chemical composition of the FBS products purchased by our team over the past 3 years via blood, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and independent composition analyses. While the composition and quality of commercial FBS products are known to vary, the FBS composition of our purchased products was relatively uniform regardless of company, brand, or country of origin. In contrast, FBS substitutes obtained from three major livestock species (cattle, pig, and chicken) clearly exhibited differences in composition, a phenomenon that was also observed upon comparing with FBS as well as among different species. Therefore, to replace commercial FBS entirely, the production of a proportionately effective substitute product comprising an equal or similar composition is required, and the results of this study can be a steppingstone to achieving this. In addition, FBS substitutes manufactured using inexpensive slaughter by-products as raw materials are expected to ultimately reduce the unit cost of cultured meat production.

N-3 Polyunsaturated Fatty Acids as a Nutritional Support of the Reproductive and Immune System of Cattle-A Review

This paper focuses on the role of n-3 fatty acids as a nutrient crucial to the proper functioning of reproductive and immune systems in cattle. Emphasis was placed on the connection between maternal and offspring immunity. The summarized results confirm the importance and beneficial effect of n-3 family fatty acids on ruminant organisms. Meanwhile, dietary n-3 fatty acids supplementation, especially during the critical first week for dairy cows experiencing their peripartum period, in general, is expected to enhance reproductive performance, and the impact of its supplementation appears to be dependent on body condition scores of cows during the drying period, the severity of the negative energy balance, and the amount of fat in the basic feed ration. An unbalanced, insufficient, or excessive fatty acid supplementation of cows' diets in the early stages of pregnancy (during fetus development) may affect both the metabolic and nutritional programming of the offspring. The presence of the polyunsaturated fatty acids of the n-3 family in the calves' ration affects not only the performance of calves but also the immune response, antioxidant status, and overall metabolism of the future adult cow.

The dose-response effects of arachidonic acid on primary human skeletal myoblasts and myotubes

Background

Cellular inflammatory response, mediated by arachidonic acid (AA) and cyclooxygenase, is a highly regulated process that leads to the repair of damaged tissue. Recent studies on murine C2C12 cells have demonstrated that AA supplementation leads to myotube hypertrophy. However, AA has not been tested on primary human muscle cells. Therefore, the purpose of this study was to determine whether AA supplementation has similar effects on human muscle cells.

Methods

Proliferating and differentiating human myoblasts were exposed to AA in a dose-dependent manner (50-0.80 µM) for 48 (myoblasts) or 72 (myotubes) hours. Cell viability was tested using a 3-(4,5-Dimethylthiazol-2-Yl)-2,5-Diphenyltetrazolium Bromide (MTT) assay and cell counting; myotube area was determined by immunocytochemistry and confocal microscopy; and anabolic signaling pathways were evaluated by western blot and RT-PCR.

Results

Our data show that the treatment of primary human myoblasts treated with 50 µM and 25 µM of AA led to the release of PGE₂ and PGF_2α at levels higher than those of control-treated cells (p < 0.001 for all concentrations). Additionally, 50 µM and 25 µM of AA suppressed myoblast proliferation, myotube area, and myotube fusion. Anabolic signaling indicated reductions in total and phosphorylated TSC2, AKT, S6, and 4EBP1 in myoblasts at 50 µM of AA (p < 0.01 for all), but not in myotubes. These changes were not affected by COX-2 inhibition with celecoxib.

Conclusion

Together, our data demonstrate that high concentrations of AA inhibit myoblast proliferation, myotube fusion, and myotube hypertrophy, thus revealing potential deleterious effects of AA on human skeletal muscle cell health and viability.

Influence of COPD systemic environment on the myogenic function of muscle precursor cells in vitro

Background:

Loss of muscle mass and function are well-recognized systemic manifestations of chronic obstructive pulmonary disease (COPD). Acute exacerbations, in turn, significantly contribute to upgrade these systemic comorbidities. Involvement of myogenic precursors in muscle mass maintenance and recovery is poorly understood. The aim of the present study was to investigate the effects of the vascular systemic environment from stable and exacerbated COPD patients on the myogenic behavior of human muscle precursor cells (MPC) in vitro.

Methods:

Serum from healthy controls and from stable and exacerbated COPD patients (before and after Methylprednisolone treatment) was used to stimulate human MPC cultures. Proliferation analysis was assessed through BrdU incorporation assays. MPC differentiation was examined through real-time RT-PCR, western blot and immunofluorescence analysis.

Results:

Stimulation of MPCs with serum obtained from stable COPD patients did not affect myogenic precursor cell function. The vascular systemic environment during an acute exacerbation exerted a mitotic effect on MPCs without altering myogenic differentiation outcome. After Methylprednisolone treatment of acute exacerbated COPD patients, however, the mitotic effect was further amplified, but it was followed by a deficient differentiation capacity. Moreover, these effects were prevented when cells were co-treated with the glucocorticoid receptor antagonist Mifepristone.

Conclusion:

Our findings suggest that MPC capacity is inherently preserved in COPD patients, but is compromised after systemic administration of MP. This finding strengthens the concept that glucocorticoid treatment over the long term can negatively impact myogenic stem cell fate decisions and interfere with muscle mass recovery.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-02203-6.

Fatty Acid Fingerprints and Hyaluronic Acid in Extracellular Vesicles from Proliferating Human Fibroblast-like Synoviocytes

Extracellular vesicles (EVs) function as conveyors of fatty acids (FAs) and other bioactive lipids and can modulate the gene expression and behavior of target cells. EV lipid composition influences the fluidity and stability of EV membranes and reflects the availability of lipid mediator precursors. Fibroblast-like synoviocytes (FLSs) secrete EVs that transport hyaluronic acid (HA). FLSs play a central role in inflammation, pannus formation, and cartilage degradation in joint diseases, and EVs have recently emerged as potential mediators of these effects. The aim of the present study was to follow temporal changes in HA and EV secretion by normal FLSs, and to characterize the FA profiles of FLSs and EVs during proliferation. The methods used included nanoparticle tracking analysis, confocal laser scanning microscopy, sandwich-type enzyme-linked sorbent assay, quantitative PCR, and gas chromatography. The expression of hyaluronan synthases 1-3 in FLSs and HA concentrations in conditioned media decreased during cell proliferation. This was associated with elevated proportions of 20:4n-6 and total n-6 polyunsaturated FAs (PUFAs) in high-density cells, reductions in n-3/n-6 PUFA ratios, and up-regulation of cluster of differentiation 44, tumor necrosis factor α, peroxisome proliferator-activated receptor (PPAR)-α, and PPAR-γ. Compared to the parent FLSs, 16:0, 18:0, and 18:1n-9 were enriched in the EV fraction. EV counts decreased during cell growth, and 18:2n-6 in EVs correlated with the cell count. To conclude, FLS proliferation was featured by increased 20:4n-6 proportions and reduced n-3/n-6 PUFA ratios, and FAs with a low degree of unsaturation were selectively transferred from FLSs into EVs. These FA modifications have the potential to affect membrane fluidity, biosynthesis of lipid mediators, and inflammatory processes in joints, and could eventually provide tools for translational studies to counteract cartilage degradation in inflammatory joint diseases.

Effect of Different Dietary Regimes on the Gut Microbiota and Fecal Metabolites of Père David’s Deer

Simple Summary

Père David’s deer is native to the middle and lower reaches of the Yangtze River and the Yellow River in China. However, the wild population became extinct in China around 1900. In 1986, 39 Père David’s deer were reintroduced into Dafeng. Up until now, its wild population has reached 2658, with a total of 6119 in 2021. At present, due to the continuous increase in the population, the repeated grazing on the same plants by the Père David’s deer has affected the re-growth of plants, which has led to insufficient natural food. Therefore, feeding supplement with silage is necessary. As a key nutritional factor, diet is the most important for the gut microbiota and metabolites of wild animals. In order to determine the effect of different dietary patterns on the nutrition and health of Père David’s deer in Dafeng Reserve in spring, we conducted a comprehensive analysis of Père David’s deer feces by UPLC-MS/MS and 16S rRNA gene sequencing to reveal its intestinal chemical environment and the differences in the fecal microbiome. Altogether, our data explored the significant changes in the gut microbiota and metabolic pathways during the transition from full silage to a combination diet with silage and plant in spring. These data provided important information to make more reasonable measures for Père David’s deer’s protection.

Abstract

A deep understanding of the effect of seasonal dietary changes on the nutrition and health of Père David’s deer in Dafeng Reserve will contribute greatly to Père David’s deer’s protection. In this reserve, there were three seasonal dietary regimes: feeding on naturally occurring plants (PLANT diet), silage (SILAGE diet), and a combination of natural plants and silage (COMB diet). To some extent, the COMB diet reflects the seasonal transition from silage to the all-natural plant diet, especially in early spring. However, little is known regarding the gut microbiota changes and metabolic consequences under the COMB diet. Based on 16S rRNA sequencing and ultra-high performance liquid chromatography combined with tandem mass spectrometry, the gut microbiota and fecal metabolites of Père David’s deer under these three diets were compared. Results showed the alpha diversity of the gut microbiota was significantly lower under the COMB diet compared to either the SILAGE or PLANT diets. Although no significant changes were observed in the core phyla, Firmicutes and Bacteroidetes, among the three dietary regimes, a significant lower abundance of several other phyla (Spirochaetes, Melainabacteria, Proteobacteria, and Verrucobacteria) was observed in the COMB diet compared to the SILAGE diet. A greater number of fecal metabolite differences was identified between the COMB and SILAGE or COMB and PLANT diets than between the SILAGE and PLANT diets, suggesting that the COMB diet had more of an effect on the metabolism of Père David’s deer. The integrated pathway analysis showed that several metabolic pathways were significantly affected by the different dietary regimes, such as tryptophan metabolism, vitamin metabolism, and the platelet activation pathways. These metabolic changes reflect the responses and adaptations of Père David’s deer to different diets. Taken overall, our data reveal the difference in the gut microbiota and metabolic pathways of Père David’s deer under three dietary regimes in Dafeng Reserve, which provides important information for Père David’s deer conservation.

Single-cell RNA Sequencing Reveals Heterogeneity of Cultured Bovine Satellite Cells

Skeletal muscle from meat-producing livestock such as cattle is a major source of food for humans. To improve skeletal muscle growth efficiency or quality in cattle, it is necessary to understand the genetic and physiological mechanisms that govern skeletal muscle composition, development, and growth. Satellite cells are the myogenic progenitor cells in postnatal skeletal muscle. In this study we analyzed the composition of bovine satellite cells with single-cell RNA sequencing (scRNA-seq). We isolated satellite cells from a 2-week-old male calf, cultured them in growth medium for a week, and performed scRNA-seq using the 10x Genomics platform. Deep sequencing of two scRNA-seq libraries constructed from cultured bovine satellite cells yielded 860 million reads. Cell calling analyses revealed that these reads were sequenced from 19,096 individual cells. Clustering analyses indicated that these reads represented 15 cell clusters that differed in gene expression profile. Based on the enriched expression of markers of satellite cells (PAX7 and PAX3), markers of myoblasts (MYOD1, MYF5), and markers of differentiated myoblasts or myocytes (MYOG), three clusters were determined to be satellite cells, two clusters myoblasts, and two clusters myocytes. Gene ontology and trajectory inference analyses indicated that cells in these myogenic clusters differed in proliferation rate and differentiation stage. Two of the remaining clusters were enriched with PDGFRA, a marker of fibro-adipogenic (FAP) cells, the progenitor cells for intramuscular fat, and are therefore considered to be FAP cells. Gene ontology analyses indicated active lipogenesis in one of these two clusters. The identity of the remaining six clusters could not be defined. Overall, the results of this study support the hypothesis that bovine satellite cells are composed of subpopulations that differ in transcriptional and myogenic state. The results of this study also support the hypothesis that intramuscular fat in cattle originates from fibro-adipogenic cells.

Effects of Arachidonic Acid Metabolites on Cardiovascular Health and Disease

Arachidonic acid (AA) is an essential fatty acid that is released by phospholipids in cell membranes and metabolized by cyclooxygenase (COX), cytochrome P450 (CYP) enzymes, and lipid oxygenase (LOX) pathways to regulate complex cardiovascular function under physiological and pathological conditions. Various AA metabolites include prostaglandins, prostacyclin, thromboxanes, hydroxyeicosatetraenoic acids, leukotrienes, lipoxins, and epoxyeicosatrienoic acids. The AA metabolites play important and differential roles in the modulation of vascular tone, and cardiovascular complications including atherosclerosis, hypertension, and myocardial infarction upon actions to different receptors and vascular beds. This article reviews the roles of AA metabolism in cardiovascular health and disease as well as their potential therapeutic implication.

Catalytically active phospholipase A2 myotoxin from Crotalus durissus terrificus induces proliferation and differentiation of myoblasts dependent on prostaglandins produced by both COX-1 and COX-2 pathways

Although crotoxin B (CB) is a well-established catalytically active secretory phospholipase A₂ group IIA (sPLA₂-IIA) myotoxin, we investigated its potential stimulatory effect on myogenesis with the involvement of prostaglandins (PGs) produced by cyclooxygenase (COX)-1 and -2 pathways. Myoblast C2C12 were cultured in proliferation or commitment protocols and incubated with CB followed by lumiracoxib (selective COX-2 inhibitor) or valeryl salicylate (selective COX-1 inhibitor) and subjected to analysis of PG release, cell proliferation and activation of myogenic regulatory factors (MRFs). Our data showed that CB in non-cytotoxic concentrations induces an increase of COX-2 protein expression and stimulates the activity of both COX isoforms to produce PGE₂, PGD₂ and 15d-PGJ₂. CB induced an increase in the proliferation of C2C12 myoblast cells dependent on PGs from both COX-1 and COX-2 pathways. In addition, CB stimulated the activity of Pax7, MyoD, Myf5 and myogenin in proliferated cells. Otherwise, CB increased myogenin activity but not MyoD in committed cells. Our findings evidence the role of COX-1- and COX-2-derived PGs in modulating CB-induced activation of MRFs. This study contributes to the knowledge that CB promote early myogenic events via regulatory mechanisms on PG-dependent COX pathways, showing new concepts about the effect of sPLA₂-IIA in skeletal muscle repair.

Identification of Meat Quality Determining Marker Genes in Fibroblasts of Bovine Muscle Using Transcriptomic Profiling

In the present study, we comparatively analyzed the transcriptomic profiling of fibroblasts derived from two different muscles, biceps femoris and longissimus dorsi with significant difference in the meat quality and tenderness. EBSeq algorithm was applied to analyze the data, and genes were considered to be significantly differentially expressed if the false discovery rate value was <0.05, the P value was <0.01, and the fold change was >0.585. The results revealed that 253 genes were differentially expressed genes (DEGs) (170 genes were upregulated, and 83 were downregulated) and more than 100 DEGs were probably associated with intramuscular fat deposition, tenderness, and toughness, which are driving the meat quality and were involved in biological processes such as collagen synthesis, cell differentiation, and muscle tissue and fiber development; molecular functions such as chemokine activity and collagen activity; cellular components such as cytoplasm and myofibril; and pathways such as collagen signaling and metabolic pathways. A gene-act network and a co-expression network revealed the close relationship between intramuscular fat deposition and meat tenderness. The expressions of 20 DEGs were validated by real-time PCR, and the results suggested that the DEGs are correlated with RNA-seq data and play crucial roles in muscle growth, development processes, toughness, and tenderness of the meat. Together, the genome-wide transcriptome analysis revealed that various genes are responsible for toughness and tenderness variance in the difference muscles of beef.

Muscle stem cell isolation and in vitro culture for meat production: A methodological review

Cultured muscle tissue-based protein products, also known as cultured meat, are produced through in vitro myogenesis involving muscle stem cell culture and differentiation, and mature muscle cell processing for flavor and texture. This review focuses on the in vitro myogenesis for cultured meat production. The muscle stem cell-based in vitro muscle tissue production consists of a sequential process: (1) muscle sampling for stem cell collection, (2) muscle tissue dissociation and muscle stem cell isolation, (3) primary cell culture, (4) upscaled cell culture, (5) muscle differentiation and maturation, and (6) muscle tissue harvest. Although muscle stem cell research is a well-established field, the majority of these steps remain to be underoptimized to enable the in vitro creation of edible muscle-derived meat products. The profound understanding of the process would help not only cultured meat production but also business sectors that have been seeking new biomaterials for the food industry. In this review, we discuss comprehensively and in detail each step of cutting-edge methods for cultured meat production. This would be meaningful for both academia and industry to prepare for the new era of cellular agriculture.

Prostaglandin F2α in vitro can affect basic inflammatory parameters of mesenchymal stem cells and slight modulating some of their immunomodulatory properties

In the last decade, mesenchymal stem cells (MSCs) have been gaining attention due their ability to influence the function of other cells as well as modulate the inflammatory response. This occurs via their immunomodulatory functions,  acting through direct cell-cell interaction or by releasing a broad spectrum of bioactive factors such as cytokines and growth factors. In addition, prostaglandins are arachidonic acid metabolites that play a key role in the generation and modulation of the inflammatory response. Among the bioactive prostaglandins, PGF_2α is able to stimulate cell proliferation as well as act to inhibit progenitor cell differentiation, but no information about this prostaglandin's action on the immunoregulatory function of MSCs has been reported. In this study we evaluate important aspects of the influence of PGF_2α analog (17-phenyl-trinor PGF_2α), which is a potent prostaglandin FP receptor agonist, on some mechanisms that control the main functions of MSCs. C3H10T1/2, a mesenchymal stem cell linage, was stimulated with PGF_2α under inflammatory conditions trigged by LPS in order to investigate PGF_2α inflammatory parameters as well as its ability to immunoregulate macrophages and lymphocytes. PGF_2α has the ability to increase proliferation tax without altering the cell viability of LPS-stimulated MSCs, while also diminishing the phosphorylation of NFκB transcription factor leading to attenuation of IL-1β and GM-CSF production. Additionally, MSC-s conditioned media from cells stimulated with PGF_2α was able to increase the lymphocytes' IL-10 production. Overall, this study implied that PGF_2α are able to modify some properties of MSCs.

Ibuprofen does not impair skeletal muscle regeneration upon cardiotoxin-induced injury

Muscle regeneration is regulated through interaction between muscle and immune cells. Studies showed that treatment with supra-physiological doses of Non-Steroidal Anti-Inflammatory Drug (NSAID) abolished inflammatory signaling and impaired muscle recovery. The present study examines the effects of pharmacologically-relevant NSAID treatment on muscle regeneration. C57BL/6 mice were injected in the tibialis anterior (TA) with either PBS or cardiotoxin (CTX). CTX-injected mice received ibuprofen (CTX-IBU) or were untreated (CTX-PLAC). After 2 days, Il-1beta and Il-6 expression was upregulated in the TA of CTX-IBU and CTX-PL vs. PBS. However, Cox-2 expression and macrophage infiltration were higher in CTX-PL vs. PBS, but not in CTX-IBU. At the same time, anabolic markers were higher in CTX-IBU vs. PBS, but not in CTX-PL. Nevertheless, ibuprofen did not affect muscle mass or muscle fiber regeneration. In conclusion, mild ibuprofen doses did not worsen muscle regeneration. There were even signs of a transient improvement in anabolic signaling and attenuation of inflammatory signaling.

IL-1β and TNF-α Modulation of Proliferated and Committed Myoblasts: IL-6 and COX-2-Derived Prostaglandins as Key Actors in the Mechanisms Involved

In this study, we investigated the effects and mechanisms of the pro-inflammatory cytokines IL-1β and TNF-α on the proliferation and commitment phases of myoblast differentiation. C2C12 mouse myoblast cells were cultured to reach a proliferated or committed status and were incubated with these cytokines for the evaluation of cell proliferation, cyclooxygenase 2 (COX-2) expression, release of prostaglandins (PGs) and myokines, and activation of myogenic regulatory factors (MRFs). We found that inhibition of the IL-6 receptor reduced IL-1β- and TNF-α-induced cell proliferation, and that the IL-1β effect also involved COX-2-derived PGs. Both cytokines modulated the release of the myokines myostatin, irisin, osteonectin, and IL-15. TNF-α and IL-6 reduced the activity of Pax7 in proliferated cells and reduced MyoD and myogenin activity at both proliferative and commitment stages. Otherwise, IL-1β increased myogenin activity only in committed cells. Our data reveal a key role of IL-6 and COX-2-derived PGs in IL-1β and TNF-α-induced myoblast proliferation and support the link between TNF-α and IL-6 and the activation of MRFs. We concluded that IL-1β and TNF-α induce similar effects at the initial stages of muscle regeneration but found critical differences between their effects with the progression of the process, bringing new insights into inflammatory signalling in skeletal muscle regeneration.

The Microenvironment Is a Critical Regulator of Muscle Stem Cell Activation and Proliferation

Skeletal muscle has a remarkable capacity to regenerate following injury, a property conferred by a resident population of muscle stem cells (MuSCs). In response to injury, MuSCs must double their cellular content to divide, a process requiring significant new biomass in the form of nucleotides, phospholipids, and amino acids. This new biomass is derived from a series of intracellular metabolic cycles and alternative routing of carbon. In this review, we examine the link between metabolism and skeletal muscle regeneration with particular emphasis on the role of the cellular microenvironment in supporting the production of new biomass and MuSC proliferation.