. 2022 Apr 1;11(4):547. doi: 10.3390/biology11040547

Table 6.

Characteristics of the most relevant candidate genes in RW chickens, associated with adaptation to cold environments.

Gene	GGA	Function
SOCS3	18	In the adipocytokine signaling pathway, SOCS3 affects adipogenesis by inhibiting the expression of LERP (lipoforin receptor) and IRS1 (insulin receptor), participating in the adipocytokine signaling pathway. IGF2BP3 affects the insulin signaling pathway by regulating intramuscular fat deposition [43]. In mammals, the immune system takes the most active part in the transformation of one adipose tissue into another. White adipose tissue macrophages force fat cells to turn into brown adipose tissue cells when the temperature drops. During physical exercise and when the ambient temperature decreases, meteorin-like hormone is released from the muscles, which, through the immune signaling proteins interleukins, acts on macrophages located in adipose tissue. Then, under the action of special signaling proteins, macrophages compel adipose tissue to burn its reserves. SOCS proteins also regulate innate immunity cells [44]. Exposure to cold causes hyperphagia to counteract fat loss associated with lipid mobilization and thermogenic activation. In experiments, chronic cold stress increased the expression of SOCS3 gene mRNA in the hypothalamus and peripheral mononuclear blood cells in rats and ferrets [45]. The selected ROH island overlapped with QTL for metabolic traits: the level of total protein in the blood (QTL:165737) [46], as well as with QTL for immune traits: bursa mass (QTL: 101247) [47].
RYR2	3	High-conductivity Ca²⁺ release channels, known as ryanodine receptors (RyR), mediate the release of Ca²⁺ from the endo/sarcoplasmic reticulum. Prolonged exposure of birds to cold causes the development of non-shivering thermogenesis (NST) of muscular origin. NST is characterized by increased heat release, which can be achieved by increasing the ATP-dependent Ca²⁺ cycle between the sarcoplasmic reticulum (SR) and cytosolic compartments in the muscles. Studies on the effect of prolonged cold exposure on SR function in ducklings have established that the activity of SR Ca²⁺-ATPase, and the proportion of vesicles containing a Ca²⁺ release channel sensitive to ryanodine in the muscle gastrocnemius of ducklings, increased by 30–50% in response to prolonged exposure to cold. These results show that the content of two components directly involved in the Ca²⁺ cycle by SR increases with acclimatization to cold, and this is due to NST [48]. The region of RYR2’s location found as affected by selection in WC chickens overlaps with several QTL associated with the percentage of intramuscular fat (QTL:24481) [49], as well as with the pH of the pectoral muscle (QTL:157164), which reflects the glycogen content in chicken muscles and is associated with diseases of glycogen accumulation in humans [50].
CAMK2G	6	This gene participates in the transport of Ca²⁺ in the sarcoplasmic reticulum in skeletal muscles. In slow-twitch muscles, it participates in the regulation of Ca²⁺ transport in the sarcoplasmic reticulum (SR), and in fast-twitch muscles, it participates in the control of Ca²⁺ release from SR by phosphorylation of the ryanodine receptor. The studied region of the genome overlaps with several QTL associated with the fatty acid composition of meat: QTL for the content of stearic acid (QTL:193105), oleic acid (QTL:193106) and linoleic acid (QTL:193113) [51]. The skeletal muscles of birds have a very high ability to absorb circulating fatty acids. Endurance muscle work in birds can actually be provided with up to 95% of the energy derived from lipids. The use of this high-calorie fuel allows birds to successfully activate the mechanisms of NST under the impact of cold stress [52].
NDUFA4	2	This gene is a component of cytochrome-c-oxidase, the last enzyme in the mitochondrial electron transport chain that controls oxidative phosphorylation. Cytochrome-c-oxidase is a component of the respiratory chain that catalyzes the reduction of oxygen to water [53]. According to literature data, in skeletal muscles of cold-adapted chickens, there is an increase in the content of slow-twitch muscle fibers that contain many mitochondria and use oxidative phosphorylation to produce ATP [54,55].
METTL21C	1	This gene is closely related to the development of chicken muscles. METTL21C performs regular processing of the protein structure and is also a heat shock protein that acts as an ATP-dependent molecular chaperone. METTL21C can regulate calcium homeostasis in bones and muscles and promote differentiation of myoblasts into muscle tubes, as well as reducing osteocyte apoptosis and regulating intracellular homeostasis [56]. The studied RW-specific ROH island with the METTL21C gene located is also overlapped by the well-known QTL for the pH of the pectoral muscle (QTL:157161), reflecting the glycogen content in the muscles, which, in turn, affects the processes of shivering thermogenesis.
TXNRD2	15	Thioredoxins are crucial for the redox regulation of protein function and signal transmission through the redox control of thiols. Mammalian cytosolic thioredoxin performs many functions to protect against oxidative stress and controls growth and apoptosis, but it is also secreted and has co-cytokine and chemokine activities [57].
IGFBP1,3	2	This gene can regulate the expression of genes related to the metabolism of fatty acids and promote the proliferation and differentiation of adipocytes [58].
STK 25	9	This gene is crucial for regulating glucose and insulin homeostasis in the body and accumulation of ectopic lipids. STK25 interacts with GOLPH3 and mediates glycolysis through GOLPH3-regulated mTOR signaling [59]. Partial depletion of STK25 increases the expression of uncoupling protein 3 (Ucp3), which is accompanied by increased lipid oxidation in myoblasts. Additionally, a reduced level of STK25 indirectly improves insulin-stimulated glucose uptake by muscle cells [60].
ADIPOQ	9	This gene is involved in the differentiation of adipocytes in mammals and may play a similar role in chickens. The structures of the adiponectin protein domain among the homologs of chickens and mammals are highly conserved [61]. Adipose tissue can secrete hormones called adipokines, which play an important role in regulating metabolic and reproductive processes at both the central and peripheral levels. The metabolic system of chickens is closely related to that of mammals. Glucose is stored in tissues as glycogen and is used for energy production through glycolysis. However, chickens are constantly hyperglycemic and insulin-resistant, which mimics the condition of type 2 diabetes in mammals. Glycemic levels depend on the line, age and sex of birds, and increased obesity in chickens is associated with lower fasting plasma glucose levels, which contrasts with the situation in mammals [62]. Adiponectin is an important regulator of thermogenesis, and it is necessary to maintain body temperature when exposed to cold [63]. Adiponectin is a circulating hormone that is predominantly produced by adipose tissue. Several pharmacological studies have shown that this protein has powerful antidiabetic, antiatherogenic and anti-inflammatory properties. ADIPOQ plays a moderate regulatory role in mitophagy caused by oxidative stress and suppresses apoptosis. These data demonstrate the antioxidant potential of ADIPOQ in skeletal muscle diseases associated with oxidative stress [64].
GCGR	18	A glucagon-like peptide (GCGL) encoded by a glucagon-like gene identified in chickens and other lower vertebrates is probably a hypophysiotropic factor in non-mammalian vertebrates. Hypothalamic GCGL is a thyroid-stimulating hormone release factor active in chickens and participates in the control of the pituitary–thyroid axis [65].
TRPM2	9	This class of ion channels, which belongs to the superfamily of transient receptor potentials (TRP) and is present in specialized neurons that are projected onto the skin, evolved as temperature sensors. TRP thermal channels are polymodal receptors. That is, they can be activated by temperature, voltage, pH and chemical agents [66]. TRPM2 initiates a “warm” signal that controls the coolness search behavior. In this regard, it should be noted that the behavioral reactions of neonatal RW chicks under standard conditions of rearing (+30–32 °C) differ markedly from the reaction of chicks of other breeds: they tend to look for a zone with a lower temperature. In the absence of such an opportunity, they try to cool off by bathing in a drinking bowl. TRPM2 is involved in stress-related inflammatory, vascular and neurodegenerative conditions [67]. TRPM2 is a member of the melastatin-related transient potential ion channel receptor subfamily associated with inflammatory and neuropathic pain perception. The main endogenous activator of the TRPM2 channel is ADP-ribose, which has been identified in TRPM2 as a high-temperature sensor, which plays a central role in preventing overheating when the temperature rises. In addition, some reactive oxygen species are positive regulators of TRPM2 activation; thus, this ion channel plays an important role as a sensor of the oxidative response in some cellular systems [68]. Additionally, TRPM2 plays a key role in the development of pathological pain. Thus, in mice deprived of TRF2 expression, the pain response decreased [69]. This gene is supposed to have an effect on reducing the sensitivity of central thermoreceptive structures to low temperature in RW chickens, which contributes to the adaptation of the body in cold environments.