Introduction
In the past fifty years, artificial selection, which relies on phenotypic characteristics, has made substantial progress in the industrial breeding of poultry, and, as a result, modern broiler chickens and ducks have significantly improved in terms of meat production and daily weight gain (Lin et al., Reference LIN, CHEN, ROUVIER and MARIE-ETANCELIN2016; Wang et al., Reference WANG, LI, MA, ZHANG and ZHANG2017b). The time for broilers to attain market weight is now less than 42 days for a body weight of 2.0 kg. However, modern breeding strategies have resulted in greater body fat deposition and metabolic disorders as well as decreased immunity and increased mortality (sometimes up to 42%; Hocking, Reference HOCKING2014). Among these factors, excessive body fat deposition has become a particularly prominent problem that affects carcass quality and increases the difficulty of processing, resulting in substantial losses to producers and consumers. Therefore, controlling excessive body fat deposition in poultry has become a major goal in poultry breeding (Wang et al., Reference WANG, KIM, CLINE and GILBERT2017a; Zhang et al., Reference ZHANG, WU, WANG, ZHANG, DU, LI, CAO, LUAN, LENG and LI2017c).
The formation of body fat in animals consists of two aspects: adipocyte proliferation (i.e. the increase in number of adipocytes) and hypertrophy (increase in volume of adipocytes), and excessive proliferation and differentiation of adipocytes in tissue leads to excessive adipogenesis, thus causing excessive body fat deposition (Wang et al., Reference WANG, LI, MA, ZHANG and ZHANG2017b). To date, researchers have successively established primary culture models of preadipocytes from several species, and extensive studies on the regulation of adipocyte proliferation and differentiation have been performed (Hassan et al., Reference HASSAN, AHN, SUH, CHOI, CHEN and LEE2014; Yan et al., Reference YAN, YANG, GAN and SUN2014). However, there is a lack in research on adipogenic differentiation in poultry. An in-depth understanding of the biological processes of proliferation and differentiation of poultry fat cells will enable the reduction in abdominal fat deposition, resulting in healthier poultry products for human consumption and provide information for animal breeding, genetics and other techniques. To provide scientific guidance and insight into poultry preadipocyte differentiation mechanisms, this review focusses on the research progress in the following three aspects: the isolation and culture of preadipocytes, induction of differentiation and identification of molecular control networks in poultry (mainly chickens and ducks).
Isolation, in vitro culture and identification of poultry preadipocytes
Isolation of preadipocytes commonly involves obtaining white adipose tissue under aseptic conditions, and a stromal vascular section is the acquired after collagenase digestion, filtration and centrifugation to isolate an aliquot. Newly isolated preadipocytes are complex in composition, containing a large number of blood cells and a small number of mature adipocytes, macrophages and other cells, which can be removed by liquid exchange or passaging, leading to purer and more uniform preadipocytes (Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015). Another method is referred to as ‘ceiling culture’, where a piece of adipose tissue is attached to the top of a culture flask. After one to two weeks, undifferentiated preadipocytes slowly grow around the adipose tissue, which is dedifferentiated. This method is widely used to study the process of adipocyte dedifferentiation. Studies have shown that although the ceiling culture method can isolate new preadipocytes, this method inhibits adipogenic differentiation ability of 3T3-L1 preadipocytes, resulting in decreased intracellular fat deposition (Song et al., Reference SONG, CHENG, YANG, LI, GAO, SHI and YANG2015). Therefore, the most commonly used method for isolating primary preadipocytes is collagenase digestion where the cells are cultured in an incubator with 5% CO2 at 37°C. Studies have shown that porcine preadipocytes proliferate well at 39°C, whereas a 37°C culture environment inhibits the proliferation and differentiation ability of porcine primary preadipocytes (Bohan et al., Reference BOHAN, PURVIS, BARTOSH and BRANDEBOURG2014). In vitro isolation and culture of poultry preadipocytes is a relatively recent development, and chicken primary preadipocytes were first successfully isolated in the 1980s (Cryer et al., Reference CRYER, WOODHEAD and CRYER1987). Using this model, researchers began to investigate the growth characteristics of preadipocytes in chickens. Duck preadipocytes were isolated for the first time in 2012 (He et al., Reference HE, TIAN, LI, SHEN, TAO, FU, NIU and LU2012) where the effect of fatty acid-binding protein (fatty acid synthase, FABP4) on the expression of fat differentiation-related genes was investigated. Beijing duck primary preadipocytes have been isolated by our group, and a systematic investigation of cell growth characteristics and methods for inducing differentiation have been conducted (Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015). The identifying characteristics of preadipocytes are as follows: 1) the cells are adherent in culture, and the shape is triangular or fusiform; 2) under certain induction conditions, the fusiform preadipocytes can differentiate into round fat cells, and intracellular deposition of lipid droplets can be stained red by Oil Red O; 3) preadipocytes can specifically express certain genetic markers associated with fat differentiation, such as FABP4 (He et al., Reference HE, TIAN, LI, SHEN, TAO, FU, NIU and LU2012; Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015).
Compared with traditional cell lines, preadipocytes isolated in vitro can often reflect the body condition more accurately. Furthermore, primary cells can be isolated from different tissues from animals of varying ages to make the research more accurate and precise. Despite this, there are evident limitations to the use of primary preadipocytes: 1) the number of preadipocytes in the adipose tissue of ducks is small, and many birds need to be sacrificed to obtain a sufficient amount of preadipocytes; 2) although adipose tissue is more prevalent in older birds, adipocyte proliferation in young birds is comparatively greater, therefore, ducks aged one to two weeks are used as experimental animals; 3) the time of preadipocyte culture in vitro should not be too long because too many passes may lead to slower cell growth and decreased differentiation. Therefore, cells cultured within four passes are usually used to conduct experiments to obtain more accurate and precise results (Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015).
Induction and differentiation modes of poultry preadipocytes
Using preadipocytes cultured in vitro, many in-depth studies have been performed that are focused on the induction and differentiation of preadipocytes. In different species, induced differentiation of preadipocytes is not the same. In most species, preadipocytes differentiate into mature adipocytes using a combination of growth factors (referred to as a ‘hormone cocktail’), such as 3-isobutyl-1-methylxanthine (IBMX), dexamethasone (DEX) and insulin. Preadipocyte differentiation in several species (e.g., human, pig and sheep) requires the addition of a certain amount of rosiglitazone or troglitazone to stimulate adipogenic differentiation (Witte et al., Reference WITTE, MUENZNER, RIETSCHER, KNAUER, HEIDENREICH, NUOTIO-ANTAR, GRAEF, FEDDERS, TOLKACHOV, GOEHRING and SCHUPP2015). These methods do not induce differentiation in poultry preadipocytes; however, after addition of oleic acid, more lipid droplets deposition, cell differentiation and maturation were observed (Matsubara et al., Reference MATSUBARA, SATO, ISHII and AKIBA2005). Thus, the effect of exogenous fatty acids on preadipocyte differentiation in chickens was investigated, and the results showed that only the addition of a fatty acid mixture together with transferrin, insulin and albumin induced primary chicken preadipocyte differentiation, indicating that exogenous fatty acids are key factors (Matsubara et al., Reference MATSUBARA, ENDO and KANO2008). A subsequent study indicated that addition of oleic acid separately, regardless of simultaneous addition of DEX, promoted the formation of intracellular lipid droplets which led to differentiation into mature adipocytes, indicating that oleic acid is a key inducer of chicken preadipocyte differentiation (Shang et al., Reference SHANG, GUO, WANG, SHI, WANG and LI2014). One group has performed many studies on the induction and differentiation of duck preadipocytes at the early stage of differentiation. The results indicated that, when the abovementioned hormone cocktail and oleic acid were added simultaneously, duck preadipocytes differentiated successfully. When oleic acid was added alone, lipid droplets were deposited in the cells and the expression of adipocyte differentiation-related genes was upregulated (He et al., Reference HE, TIAN, LI, SHEN, TAO, FU, NIU and LU2012; Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015). The specific regulatory mechanisms of preadipocyte differentiation in ducks by the two induction methods are currently not clear.
Insulin and DEX are the main preadipocyte inducers for most species, except for chicken, where neither promoted fat deposition (Shang et al., Reference SHANG, GUO, WANG, SHI, WANG and LI2014). Experimental results with duck preadipocytes fully demonstrated this observation (Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015). It has been found that rosiglitazone promoted the differentiation and maturation of duck preadipocytes by promoting fat synthesis and ester hydrolysis (Ding et al., Reference DING, QIU, LI, HU, SONG, HAN, HE and WANG2016). Addition of the hormone cocktail with a certain amount of rosiglitazone or troglitazone during the chicken preadipocyte differentiation process increased lipid production and ester hydrolysis in cells, further indicating that rosiglitazone promotes poultry adipocyte differentiation (Cheng et al., Reference CHENG, WU, XU, ZHANG, WANG, WANG, LENG and LI2016). However, although addition of oleic acid alone induced chicken preadipocyte differentiation, the cellular ester hydrolysis ability was weak, whereas the normal ability of ester hydrolysis was associated with the expression of mature adipocytes. This study indicated that the combination of the hormone cocktail and rosiglitazone was a more effective method for inducing preadipocytes to differentiate into normal functional adipocytes (Cheng et al., Reference CHENG, WU, XU, ZHANG, WANG, WANG, LENG and LI2016).
The induction of poultry adipocyte differentiation differs from the induction of mammalian adipocyte differentiation because de novo adipose synthesis differs between species and tissue types. De novo synthesis of adipose tissue refers to the process by which fat-related tissues (liver and adipose tissue) use non-fat precursors (e.g. carbohydrates produced by sugar metabolism) to synthesise fatty acids de novo with a variety of different carbon chains, and these products are further esterified into lipids. De novo fat synthesis in rodents typically occurs in the liver and adipose tissues (Smith and Kahn, Reference SMITH and KAHN2016). Animals such as pigs, cattle and sheep mainly synthesise fat in adipose tissues (Yue et al., Reference YUE, ZHANG, ZHANG, LI and YU2018). De novo synthesis of poultry fat mainly occurs in the liver and is then transported to the extracellular space through lipoproteins, promoting fat deposition in adipose tissues (Ding et al., Reference DING, PAN, KOU, LI, XIA, HU, LIU and WANG2012). Poultry adipose tissue itself has a low ability to synthesize fat. Therefore, preadipocytes isolated in vitro require exogenous fatty acids to initiate differentiation. Oleic acid can induce mouse 3T3-L1 cells to differentiate into mature adipocytes, but the differentiated cells have impaired insulin sensitivity, whereas this phenomenon is not aPPARent in adipocytes induced by the use of the hormone cocktail (Xie et al., Reference XIE, HAMILTON, KIRKLAND, CORKEY and GUO2006). Whether oleic acid-induced preadipocyte differentiation impairs the insulin sensitivity of poultry adipocytes remains to be investigated.
Molecular regulation of poultry preadipocytes
Adipocyte differentiation is a process of coregulation by several transcription factors. Studies have found several transcription factors that regulate adipose differentiation (Shipp et al., Reference SHIPP, CLINE and GILBERT2016). The most critical transcription regulators include peroxisome proliferator-activated receptors (PPARs), CCAAT/enhancer binding protein (C/EBPs) and sterol response element-binding proteins (SREBPs). These transcription factors regulate adipocyte differentiation by affecting the expression levels or activity of target genes (Mota et al., Reference MOTA, RICHARD, HANG and STEPHENS2017).
PPARs
The PPARγ gene belongs to the PPAR family, which includes three members, α, β (δ) and γ, and plays a transcription regulatory role by binding to the peroxisome proliferator response elements upstream of the target-gene promoter. PPARγ is mainly expressed in adipose tissues and participates in the regulation of lipid metabolism and adipose differentiation. In the regulatory network of adipocyte differentiation, PPARγ plays a key role in early differentiation, and its expression precedes most adipocyte-specific genes (Lee and Ge, Reference LEE and GE2014). It has been confirmed that differentiation factors cannot initiate the differentiation process in the absence of PPARγ, and most adipocyte differentiation factors play roles in activating PPARγ (Tontonoz and Spiegelman, Reference TONTONOZ and SPIEGELMAN2008). In mouse fibroblasts lacking the C/EBPα gene, cells undergo adipose differentiation after ectopic expression of PPARγ; however, without this expression, differentiation does not occur, indicating that PPARγ is essential for adipocyte differentiation. It is unclear whether other members of the C/EBP gene family partially compensate for the function of C/EBPα to promote adipose differentiation when this gene is absent (Rosen et al., Reference ROSEN, HSU, WANG, SAKAI, FREEMAN, GONZALEZ and SPIEGELMAN2002).
Chicken PPARγ contains five spliceosomes and its open reading frame encodes two isomers; PPARγ1 and PPARγ2. PPARγ1 is highly expressed in the abdominal adipose tissue of chickens, and its expression in two to seven-week-old obese chickens is significantly higher than that in lean meat-type chickens, which indicates that it is closely related to chicken fat deposition (Guo et al., Reference GUO, SUN, SHANG, LENG, WANG, WANG and LI2011; Duan et al., Reference DUAN, SUN, ZHANG, ZHANG, ZHANG, ZHANG, WANG, WANG, LENG, LI and WANG2015). At the cellular level, in oleic acid-induced differentiated chicken preadipocytes, the expression level of PPARγ reached its peak at 9 h after differentiation, preceding other differentiation genes; therefore, it was speculated that PPARγ is an important gene that regulates the early stage of chicken adipocyte differentiation (Matsubara et al., Reference MATSUBARA, SATO, ISHII and AKIBA2005). Furthermore, PPARγ inhibits the proliferation and growth of preadipocytes, which promotes the differentiation of preadipocytes into mature adipocytes. When the expression of this gene was suppressed, compared with the relevant interference group, the proliferation ability of chicken adipocytes significantly increased (P<0.05), the differentiation ability significantly decreased and the expression of SREBP1, FABP4 and lipoprotein lipase (LPL) significantly decreased (P<0.05), which hindered adipocyte differentiation (Wang et al., Reference WANG, NA, WANG, WANG, WANG, WANG, LI and LI2012).
In ducks, PPARγ has two isomers: PPARγ1 and PPARγ2. PPARγ1 is related to peroxisome proliferation, while PPARγ2 mainly regulates lipid metabolism. At the cellular level, the expression level of PPARγ reached its peak on the third day after induction of duck preadipocytes, preceding C/EBPα and FABP4 expression, which suggests that duck PPARγ plays an important role in the early stage of adipocyte differentiation (Ding et al., Reference DING, LI, LI, GAN, YUAN, GOU, HE, HAN and WANG2015). In vivo, when ducklings were intravenously injected with PPARγ-specific siRNA, duck abdominal fat deposition was significantly reduced on days 21 and 35, and the expression of key genes (LPL and FABP4) for adipose differentiation significantly decreased, indicating that PPARγ plays an important regulatory role in duck fat deposition and is a key transcription factor in adipocyte differentiation (Xiong et al., Reference XIONG, LI, PENG, FENG, YU, XIN and GONG2010).
C/EBP family
The C/EBP family consists of several members, and C/EBPα, β and δ are closely associated with adipocyte differentiation. The expression of C/EBPα precedes other genes, which is essential for inducing adipocyte differentiation. After inducing differentiation of chicken preadipocytes, the expression of C/EBPα reached its peak at 24 h after induction, while the expression of C/EBPβ and δ reached its peak at 48 and 36 h after induction, respectively, indicating that C/EBPα plays an important role in initiating preadipocyte differentiation (Bai et al., Reference BAI, WANG, ZHANG, ZHANG, RICE, CLINE and GILBERT2015). In addition, there are several SREBP1 and C/EBPβ-binding sites on the C/EBPα promoter, and overexpression of SREBP1 or C/EBPβ in a chicken embryo fibroblast line reduced the transcriptional activity of the C/EBPα promoter. By contrast, overexpression of SREBP1 or C/EBPβ in 3T3-L1 preadipocytes promoted transcription of the C/EBPα gene (Lay et al., Reference LAY, LEFRÈRE, TRAUTWEIN, DUGAIL and KRIEF2002). This discrepancy could be attributed to the different genetic backgrounds of preadipocytes from different species, however, the specific molecular mechanism of this difference has not been elucidated. In addition, it has been reported that multiple C/EBPα and C/EBPβ-binding sites exist on the PPARγ promoter, and overexpression of C/EBPα and C/EBPβ in chicken embryo fibroblast lines stimulated PPARγ promoter activity. Early studies found that C/EBPα activated the transcription of PPARγ by directly binding to the binding site of the chicken PPARγ promoter. These results suggested that chicken C/EBPα and C/EBPβ promoted the activation of PPARγ by direct interactions with its promoter (Ding et al., Reference DING, GAO, WANG and LI2011). Overexpression of PPARγ has been shown to improve the promoter activity of the downstream target gene, FAS, but had no significant effect on the activity of the C/EBPα promoter. Chicken C/EBPα and C/EBPβ can regulate PPARγ transcriptional activity and promote downstream target gene expression, whereas PPARγ does not directly regulate C/EBPα and C/EBPβ. Moreover, it has been reported that ectopic expression of C/EBPα or PPARγ in chicken embryonic fibroblast cells induced their differentiation into adipocyte-like cells. Specifically, lipid droplet deposition occurred in these cells, and the FABP4 genetic marker of adipose differentiation was expressed in the cells, suggesting that these two transcription factors, C/EBPα and PPARγ, play key roles in regulating poultry preadipocyte differentiation (Liu et al., Reference LIU, WANG, WANG, WANG, LI and LI2010).
Krüppel-like factors (KLFs)
KLFs regulate the expression of many genes by binding to the CACCC/GC/GT region of the target gene promoter. They are involved in regulating a series of cellular processes, including embryonic development, adipocyte differentiation, lipid metabolism, insulin resistance and cardiovascular disease (Fan et al., Reference FAN, HSIEH, SWEET and JAIN2017; Klein et al., Reference KLEIN, HU, KASHGARI, LIN, NGUYEN, DOAN and ANDERSEN2017). KLF7 plays an important role in the regulation of mammalian lipid metabolism. It has been shown that its mRNA expression in the abdominal fat of chickens is significantly associated with fat content. Its expression in abdominal fat of lean chickens has been demonstrated to be significantly greater than that in obese chickens (Zhang et al., Reference ZHANG, WANG, SUN, LI and WANG2013). KFL7 overexpression in chicken preadipocytes led to a decrease in the activity of LPL, C/EBPα and FAS promoters, thus inhibiting the differentiation of preadipocytes into adipocytes. KLF7 gene knockdown led to significant enhancement in FABP4, PPARγ and C/EBPα promoter activity. These findings showed that KLF7 inhibits the differentiation of chicken adipocytes. Its overexpression and knockdown experiments confirmed that it directly affects the activity of C/EBPα; therefore, KLF7 affects chicken preadipocyte proliferation and adipogenic differentiation by regulating the downstream target gene, C/EBPα.
KLF2 is known to regulate the differentiation of adipocytes. It has been shown that the expression level of KLF2 in chicken abdominal fat was the highest one week after birth, and then expression decreased over time. However, KLF2 expression did not significantly correlate with fat deposition in different strains of chickens. It is known that the expression of KLF2 gradually decreases throughout the process of preadipocyte differentiation. KLF2 overexpression in chicken preadipocytes decreased the expression of PPAR, C/EBPα and FABP4 and ultimately inhibited the differentiation of adipocytes. Such results showed that KLF2 inhibits the differentiation of chicken adipocytes by inhibiting the expression of PPARγ and C/EBPα (Zhang et al., Reference ZHANG, RONG, SHI, WU, SUN, WANG, WANG and LI2014a).
Further studies have demonstrated that KLF2 overexpression has different effects on the transcriptional activity of chicken PPARγ and C/EBPα promoters, because it activates two PPARγ promoters (of different lengths) and three C/EBPα promoters. These results suggested that the effect of KLF2 on the activity of both promoters is associated with the length of the promoter (Chen et al., Reference CHEN, WU and ZHANG2017).
KLF3 is highly expressed in chicken adipose tissue. KLF3 overexpression led to a significant decrease in C/EBPα, FABP4, FAS and LPL promoter activity in chicken preadipocytes, and PPARγ activity improved. It has been speculated that this gene regulates the differentiation of chicken adipocytes by affecting C/EBPα transcription activity. Site-directed mutagenesis of KLF3 demonstrated that the ASP mutation at the PVDLT site was significantly associated with the activity of KLF3 (Zhang et al., Reference ZHANG, WU, LI and WANG2014b).
KLF5 expression is highest in chicken spleen, while its expression in abdominal fat is low. During chicken preadipocyte differentiation, the expression level of KLF5 was drastically reduced, and a significant increase in expression occurred 12 h after differentiation. KLF5 is highly expressed in the chicken liver and in abdominal adipose tissue. During preadipocyte differentiation, expression gradually increased (Matsubara et al., Reference MATSUBARA, AOKI, ENDO and SATO2013). Currently, the specific mechanisms of the KLF family members in poultry adipose differentiation are not very clear and require further investigation.
Other genes
FABP4 is a genetic marker of the middle and late stages of adipocyte differentiation. It promotes the transport and diffusion of aliphatic acids and participates in the biochemical cycle for regulating the formation and dissolution of lipids. Therefore, FABP4 plays an important role in the transport and metabolism of aliphatic acids. Its expression is regulated by PPARγ, and as one of the target genes of PPARγ, FABP4 has the highest expression level in the late stage of cell differentiation. FABP4 provides aliphatic acids for triglyceride formation, thereby promoting adipocyte hypertrophy and inducing adipogenesis. When preadipocyte differentiation was induced, FABP4 overexpression resulted in increased release of free fatty acids, promoting lipolysis and significantly increasing fat deposition simultaneously. Further studies have shown that FABP4 overexpression increased the expression of PPARγ at different stages of differentiation. It has been speculated that FABP4 regulates the differentiation of chicken adipocytes by affecting PPARγ (Shi et al., Reference SHI, WANG, WANG, LENG, ZHANG, SHANG and LI2010). When the FABP4 expression was knocked down and chicken preadipocyte differentiation was induced under the same conditions, no evident changes were observed in adipocyte differentiation and adipolysis. However, the specific mechanisms need to be further investigated. The FABP4 gene is highly expressed in duck adipose tissue, while it is low in other tissues. At a cellular level, oleic acid significantly promotes the expression of FABP4. When oleic acid was used to induce preadipocyte differentiation in ducks, FABP4 gene knock-down led to a significant reduction in the expression of genes related to adipose differentiation, such as FAS, LPL and PPARγ, leading to inhibition of adipogenic differentiation. This indicated that FABP4 plays an important role in the regulation of duck adipocyte differentiation (He et al., Reference HE, TIAN, LI, SHEN, TAO, FU, NIU and LU2012).
Fatty acid transport protein 1 (FATP1) is a member of the fat transport protein family that facilitates the transport of long-chain free fatty acids into and out of cells. FATP1 influences aliphatic acid metabolism and fat deposition by regulating the acylation of aliphatic acid (Black et al., Reference BLACK, AHOWESSO, MONTEFUSCO, SAINI and DIRUSSO2016; Landrock et al., Reference LANDROCK, MILLIGAN, MARTIN, MCLNTOSH, LANDROCK, SCHROEDER and KIER2017). After transfecting chicken adipocytes with FATP1-specific siRNA, mRNA and protein expression levels of PPARγ, C/EBPα, FAS and other genes significantly decreased, and preadipocyte differentiation was inhibited. Furthermore, the factors Caspase-3 and Bax were activated, resulting in apoptosis. These results suggest that the FATP1 gene plays an important role in chicken preadipocyte growth and the induction of adipocyte differentiation (Qi et al., Reference QI, FENG, TAN, GAN, YAN and SUN2013).
Recent studies have demonstrated that long-chain non-coding RNA regulates adipose differentiation and plays an important role in the development and function of metabolic-related tissues. Ten differentially expressed genes for controlling fat deposition in ducks were screened through positive-selection and differential expression gene integration methods, but the specific mechanism has yet to be determined (Wang et al., Reference WANG, LI, MA, ZHANG and ZHANG2017b).
RNA sequencing was applied to determine the expression of long non-coding RNA and mRNA in the preadipocyte differentiation process; preadipocytes were isolated from chicken abdominal adipose tissue, and it has been demonstrated that the K2 clustering gene plays a key role in chicken adipocyte differentiation (Zhang et al., Reference ZHANG, ZHANG, HAN, ZHANG, WANG, XIE and XUE2017a). Furthermore, this research focused on preadipocytes in the muscle and adipose tissues of chickens. Several long-chain, non-coding RNAs associated with the differentiation of intramuscular adipocytes were also identified; 10 genes and nine long-chain non-coding RNAs were determined to be closely related to the differentiation of chicken intramuscular adipocytes. Thereby, researchers identified several classic signalling pathways associated with intramuscular adipocyte differentiation as well as several new signalling pathways (Zhang et al., Reference ZHANG, ZHANG, HAN, ZHANG, WANG, XIE, XUE and FAN2017b). These results provided a new and powerful basis for improving the understanding of adipocyte differentiation in different poultry tissues.
Conclusions
Adipocyte differentiation is a complex biological process that involves the regulation of multiple transcription factors. The molecular mechanisms associated with the regulation of mammalian adipose differentiation have been thoroughly studied, and the molecular regulatory network is now better understood. By contrast, research on poultry adipocyte differentiation is not well understood. Many studies have explored the regulatory network of poultry adipocyte differentiation. Several key transcription factors and genetic markers have been identified and confirmed, but there are still several aspects that require further research. In recent years, trials using the latest sequencing technology, new specific microRNAs and long-chain non-coding RNAs have been screened, and it has been preliminarily confirmed that they participate in the regulation of adipocyte differentiation. Future studies should consider an in-depth investigation of the mechanisms by which these specific RNAs affect adipocyte differentiation and comprehensively interpret the regulatory networks of adipocyte differentiation, to provide a scientific basis for livestock quality improvements and advances in animal genetic breeding technologies.
Acknowledgement
The authors would like to thank Michal Bell, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
