Heat stress is one of the most important environmental stressors for the poultry industry in the world. The effects of heat stress on meat quality have been extensively studied and they are multifaceted and involve changes in pH, color, drip loss, cooking loss, shear force, and other parameters [7, 14, 15]. At the molecular level, some genes related to heat stress were also identified. Hsp70 has been found to effectively improve the meat quality of transported broilers by improving the energy status, inhibiting glycolytic influx, and restoring redox homeostasis [16]. Yao et al. [17] found that heat stress caused mitochondrial injury in the chicken primary myocardial cells, leading to oxidative stress and calcium overload. However, Hsp90 activation was found to prevent these effects, thus protecting the mitochondria and reducing cellular damage. Another research found that the expression of Hsp90b1 mRNA significantly increased during heat stress and decreased during recovery, with higher expression observed in Huainan chickens [18].
Currently, research on heat stress-related genes in chickens remains limited, primarily focusing on genes encoding heat shock proteins (HSPs) proteins [19]. This study subjected chickens of breeds JF and TM to acute heat stress stimulation, followed by the collection of breast muscle samples for transcriptome sequencing, aiming to identify heat stress-resistant genes and their functional pathways in the breast muscles.
GO results showed that the most significantly enriched GO term of DEGs in JFN-vs-JFT was phospholipid homeostasis. When cells were exposed to heat stress, the composition and organization of phospholipids in the plasma membrane can be disrupted, affecting the overall integrity and function of the membrane. This can lead to changes in the fluidity, permeability, and signaling properties of the membrane, impacting the cell's ability to maintain homeostasis [20, 21]. Aggrephagy was a selective autophagy process that targets the removal of protein aggregates, which were often formed under conditions of cellular stress, such as heat stress [22]. Additionally, some GO terms related to skeletal muscle development were also enriched in the JFN-vs-JFT, indicating that heat stress indeed affects skeletal muscle development. In the top20 terms of TMN-vs-TMT, catabolism-related BP terms were mainly enriched. Heat stress can accelerate cellular metabolism, leading to increased catabolic processes to generate the energy needed for cellular repair, adaptation, and survival under elevated temperatures. Therefore, catabolism is closely related to the cellular response to heat stress, as it provides the necessary energy for cells to maintain homeostasis and function effectively in challenging thermal environments [23, 24].
MAPK signaling pathway with high connectivity in the top20 pathways was found both in JFN-vs-JFT and TMN-vs-TMT. The MAPK (Mitogen-Activated Protein Kinase) signaling pathway plays a crucial role in the response of cells to external stimuli, including heat stress [25]. Research has shown that heat stress can activate the MAPK pathway, leading to the phosphorylation and activation of MAPK proteins such as Hog1 in yeast or ERK, JNK, and p38 in mammalian cells. This activation triggers a series of downstream events that help cells cope with the stress induced by high temperatures [25]. The study found that Hsp70 modulates stress-activated signaling and acts protectively via the MAPK pathway. Inhibition of Hsp70 by quercetin exacerbated intestinal injury and affected the activation of MAPKs and caspase-3 [26].
Additionally, KEGG pathways (Metabolic pathways, FoxO signaling pathway, TGF-beta signaling pathway, AMPK signaling pathway) with higher connectivity in group JFN-vs-JFT may all be associated with heat stress resistance. When organisms are exposed to high temperatures, their metabolic processes undergo significant alterations to cope with the stress and these changes often involve shifts in energy production, allocation of resources, and synthesis of protective molecules [27, 28]. The FoxO signaling pathway would be activated in response to heat stress and a study showed that heat stress treatment led to a significant increase in FoxO3a phosphorylation (specifically at Ser253) in rat skeletal muscle of heat-stressed legs after 24 hours [29]. The TGF-beta signaling pathway is implicated in the regulation of cortisol synthesis and secretion, neuroactive ligand-receptor interactions, and the JAK/STAT signaling pathway in response to temperature changes [30]. The AMPK signaling pathway plays a crucial role in regulating cellular energy homeostasis and lipid metabolism [31]. Research has shown that heat stress (HS) inhibits the AMPK signaling pathway, leading to the promotion of lipid accumulation in 3T3-L1 preadipocytes [32].
In comparison group TMN-vs-TMT, we also have identified high connectivity pathways Ubiquitin mediated proteolysis, Autophagy-animal and Regulation of actin cytoskeleton. Ubiquitin-mediated proteolysis plays a significant role in cellular stress responses, including heat stress. When cells are exposed to heat stress, proteins like HuR can be targeted for degradation through ubiquitin-mediated proteolysis [33]. Autophagy is a cellular process that involves the degradation and recycling of cellular components, and it has been linked to increased tolerance to a range of abiotic stressors, including heat stress, in various ectothermic organisms. Flies exposed to heat stress exhibit protein aggregation, triggering an autophagy-related response, while rapamycin feeding induces local autophagy in the gut and enhances heat-stress tolerance [34]. sHsps (Small heat shock proteins) belong to the HSPs family, with a molecular weight of 15–40 kDa. They are known to interact with actin and play a significant role in protecting the actin cytoskeleton from disruption induced by stressful conditions. Some sHsps act as capping proteins, inhibiting actin polymerization, while others protect the actin cytoskeleton by coating microfilaments with small oligomers of phosphorylated sHsps [35].
The GO and KEGG pathways discussed above, along with their enriched genes, may be involved in skeletal muscle heat stress resistance. Most terms and their genes were enriched independently in either group JFN-vs-JFT or TMN-vs-TMT, with only one pathway, the MAPK signaling pathway, enriched universally. However, analysis revealed that the enriched DEGs within it were distinct. Therefore, we further analyzed the 24 co-differentially expressed genes and constructed a PPI network with them. The results revealed only 13 genes exhibiting interaction, with KLF9 and ASB2 having the highest degree, followed closely by TMEM164 and ARRDC2.
Krüppel-like factor 9 (Klf9) is a transcription factor that plays a crucial role in regulating various environmental and stress response pathways. It plays a critical role in mediating the cellular responses to stress, environmental cues, and developmental processes through its interactions with various signaling pathways [36]. Study showed that Klf9 demonstrates bimodal, reciprocal activity by activating the Fgfr1 promoter in proliferating myoblasts and repressing the same promoter via the same DNA-binding site in differentiated myotubes [37]. Asb2 encodes a protein engaged in cellular processes via its ankyrin repeat and SOCS box domains, regulating signaling pathways and protein degradation. Li et al. [38] found that the expression of the Asb2 gene in the breast muscle of Silkie fowl exposed to heat stress significantly increased by 1.74 times, and it might participate in the regulation of heat stress through the ubiquitin-proteasome pathways. The gene Tmem164, also known as Transmembrane Protein 164, has been identified as a new determinant of autophagy-dependent ferroptosis, a type of iron-dependent regulated cell death [39]. The resistance to heat stress is closely related to autophagy, and we have discussed this in the preceding text. Arrdc2 is a protein-coding gene that belongs to the arrestin family of proteins. Studies have found that stimuli suppressing growth such as testosterone depletion or acute aerobic exercise would increase Arrdc3 expression in skeletal muscle [40]. Another study found that Arrdc2 overexpression was sufficient to lower myotube diameter in C2C12 cells in part by altering the transcriptome favoring muscle atrophy [41].