Cashmere goat is a unique animal husbandry resource in China (Cai et al., 2020; Jin et al., 2020), which has made outstanding contributions to the needs of the livestock textile industry and human society (Yang et al., 2020). The characteristics of cashmere show significant differences due to different goat species and regions (Jin et al., 2017). Cashmere goat has a double coat, with the primary hair follicles producing coarse hair and the secondary hair follicles producing fine hair (Dai et al., 2019). The cashmere thickness produced by the secondary hair follicles varies among individuals. Cashmere fiber is famous for being slender and soft. To improve the economic value of cashmere, the fineness of its fibers needs to be reduced, making cultivating high-yield and high-quality cashmere goat varieties the core element to improve the economic value of cashmere fiber. Cashmere fineness is a quantitative characteristic, which is determined by micro effective genes. Molecular breeding research in domestic and foreign studies have proved that the single nucleotide polymorphisms (SNPs) of KAP (Jin et al., 2011), RPL (Mahata et al., 2012), FGF (Liu et al., 2009), KRT (Hui, 2020), PROP (Zeng et al., 2011), MAF70 (Arranz et al., 2001), and KIFI (Li, 2009) genes are correlated with cashmere fineness. Many studies have identified some genes related to goat wool fiber growth characteristics, such as DSG1, IGF-IR, KRTAPs, ILK, and KRTAP genes (Rufaut et al., 1999; Jia et al., 2006; Yu et al., 2011; Yang et al., 2012; Liu et al., 2015). Full transcriptomics also proved that NFKBIA, TCHH, COL1A1, CXCL8, and LTBP2 genes are closely related to cashmere fineness, and researchers have been looking for ways to improve the quantity and quality of cashmere (Bai et al., 2016; Zhang et al., 2018), as well as analyzing the key genes, signal pathways, and expression regulation level under different cashmere state diameters (Fu et al., 2020). Despite the efforts, all the genes regulating cashmere fineness from the translation level remain unknown. Due to the gap between the transcriptome and proteome in omics, only 20–40% of proteins in mammalian cells are determined at the transcription level (Tian et al., 2004; Cox et al., 2005). Current translatomics can make up the gap between the two. In the study of cashmere fineness, translatomics can be used to identify genes that are not translated into proteins at the transcriptional level. Translatomics provides new insights into gene expression (Courtes et al., 2013).

Translatomics is the sequencing and analysis of translated RNA molecules, which can accurately quantify the genes being translated, and compare the translation amount of different samples of genes under different physiological and pathological conditions or different treatments. At the same time, translation control is the key determinant of protein abundance, which in turn determines cell state (Sendoel et al., 2017). The gene expression process reveals that there are specific phenotypic characteristics among species, but their evolutionary process is uncertain outside the transcriptome. There are studies on coevolution at the level of mammalian transcriptome and translatome. Ribosome analysis and RNA sequencing are used to analyze the three organs of five mammals (human, macaque, mouse, opossum, and platypus) and birds (chicken), coevolutionary analysis (brain, liver, and testis) shows that translation regulation widely exists in different organs, especially in spermatogenic cell types of testis, and some genes evolve faster at the translatome level (Wang et al., 2020). Ribosomal analysis can also evaluate translation efficiency on a genome-wide scale, which has been previously proved in yeast (McManus et al., 2014), nematodes (Stadler and Fire, 2013), primate (Wang et al., 2018) cells, and hybrid mouse cells, it was also found that translation efficiency was a momentous predictor of protein level in mouse fibroblasts (Hou et al., 2015). These studies provide preliminary insights into the coevolution model of the transcriptome and translatome. Therefore, protein abundance seems to be mainly regulated by ribosomes, highlighting the importance of translation control (Gebauer and Hentze, 2004; Sonenberg and Hinnebusch, 2009). Using microRNA (miR-430) in zebrafish to investigate its translational repression and mRNA decay, we found that translation repression occurs before mRNA decay, which is induced by reducing the translation initiation rate, and that mRNA decay is induced by deadenylation. Besides, microRNA has been proposed to affect protein translation by reducing the rate of translation initiation (Bazzini et al., 2012). On the other hand, because the translatome studies RNA molecules being translated, which includes the RNA molecules that are traditionally considered noncoding, it can provide direct translation evidence for the study of these new translatable molecules (Gerashchenko et al., 2012). In addition, the lack of a strict direct correlation between gene and protein levels limits translation studies by combining the transcriptome and proteome. Considering the high cost associated with protein synthesis, the dominant role of translation regulation is meaningful. Therefore, it promotes the progress of translatomics technology (King et al., 1996; Kirkpatrick et al., 2005).

According to Tian Wenliang, the standard of cashmere fineness in China is: the diameter of coarse hairs: 16.0–18.5 μm and the diameter of fine hairs: 15.5–16.0 μm. In 2010, the cashmere of LCG was the thickest among all cashmere goats, and its diameter was 20.32 μm (Tian, 2015). It can be found that the main disadvantage of cashmere in LCG is that the cashmere is thicker. In this study, we found the key genes regulating cashmere fineness by using translatomics, and understood the regulatory relationship of related genes. We used Ribo-seq to test the fine skin samples and coarse skin samples of LCG, and found the key genes, differential genes, and co-expression genes related to cashmere fineness through GO function enrichment analysis and KEGG pathway enrichment analysis. These findings pave the way for the study of the regulation mechanism of cashmere fineness and the protection and cultivation of cashmere varieties.

LCG is unique in China, which produces a large number of high-quality cashmere fibers. Moreover, China is one of the largest cashmere producing countries in the world, which has made great contributions to the fiber industry and textile industry, and plays an indispensable role in the global cashmere industry (Zheng et al., 2020). But at present, the pursuit of cashmere fineness is increasing, and the cashmere fineness of LCG is still showing a relatively coarse trend, and quality cashmere products are still insufficient. Reducing cashmere fineness is an important issue (Zheng et al., 2019). Ribosome profiling is a mature method to identify translation regions by high-throughput sequencing, which fills the gap between RNA sequencing and proteomics, and has become a powerful tool for gene expression (Calviello and Ohler, 2017). Ribo-seq can not only measure the translation efficiency according to the relative abundance of ribosomes on transcripts, but also reveal the dynamic and local regulation of different translation stages according to the location information of footprints on transcripts (Li et al., 2020).

However, little is known about the issue of cashmere fineness of LCG by Ribo-seq sequencing in translatomics. In this study, we selected six adult female Liaoning Cashmere Goats (LCGs) with different cashmere fineness (divided into two groups). The coarse Liaoning Cashmere Goat sample group was the reference group, and the fine Liaoning Cashmere Goat sample group was the experimental group. The classification of groups was based on the phenotypic determination of cashmere fineness (cashmere fineness analyzer, sirolan technology, Australia). A total of 529 differentially expressed genes were identified by Ribo-seq sequencing, of which 343 were upregulated and 186 were downregulated. And the enrichment length of mRNA fragments was 22 nt.

COL6A5 (formerly known as COL29A1) is a member of the collagen superfamily. The gene is located on the long arm of chromosome 3 (Strafella et al., 2019), with a length of 8742. It is a protein-coding gene and is considered to be an extracellular matrix protein, accounting for 30% of the total mammalian protein (Haq et al., 2019). COL6A5 was found in the epithelial tissues of lung and gastrointestinal tract (Fitzgerald et al., 2008), but the highest expression was found below the dermal epidermal junction and around the reticular dermal vessels (Sabatelli et al., 2011). It was found that COL6A5 fibroblasts existed in atopic dermatitis skin, but not in healthy tissues (He et al., 2020). The COL6A5 gene is not only associated with skin inflammation, but also with cancer. It has been confirmed that the COL6A5 gene is significantly associated with the overall survival rate of patients with esophageal squamous cell carcinoma (ESCC). The overall survival rate of ESCC patients with low expression of the COL6A5 gene is poor, which may become a diagnostic marker of ESCC as a collagen gene (Li et al., 2019). Abundant type ⅵcollagen in lung tissue α5 (COL6A5), rs13062453, rs1497305, and rs77123808 of COL6A5 polymorphism are associated with lung cancer risk in Chinese Han population, and the overall survival rate of patients with low expression of the COL6A5 gene is poor (Duan et al., 2020). These studies can infer that the COL6A5 gene may play a role in changing the hair follicle and cashmere fineness of LCG.

Because of that and because another gene from the same family, COL1A1, is known to have an impact on changing cashmere fineness (Wang et al., 2021), we hypothesize that COL6A5 is a candidate gene for future studies regarding cashmere fineness. Studies have shown that COL6A5 is associated with familial chronic neurotrophic itching (Martinelli-Boneschi et al., 2017). It has been found that COL6A5 expression in the papillary dermis and the surrounding dermis of the patients is reduced (Weisshaar and Dalgard, 2009; Ständer et al., 2010), and the incidence rate increases with age. This is the first time that the link between the COL6A5 gene and chronic pruritus has been revealed. Some studies have found a link between COL6A5 variants, reduced bone mass, dyspnea, and giant cell arteritis (Wang et al., 2016). In psoriasis (Ps) and psoriatic arthritis (PsA), bioinformatics analysis revealed that COL6A5 and COL8A1 participate in the altered proliferation and angiogenesis pathways in Ps/PsA, participate in inflammatory response together with miR-146a, and participate in the common and different biological pathways of Ps and PsA (Caputo et al., 2020).

The collagen gene may be closely related to the PI3K/Akt/mTOR pathway, p53 pathway, apoptosis, and cell cycle. COL1A1 and FGF10 genes are also enriched in the PI3K/Akt/mTOR pathway. COL1A1 can regulate the growth of alpaca wool fiber, and FGF10 can prolong the growth period of mouse hair follicles and promote hair growth (Maiese, 2015; Huang et al., 2017; Mendoza et al., 2019). Therefore, we take COL6A5 gene as a candidate gene for cashmere fineness research in the future.

In living organisms, ribosomes synthesize proteins in the process of translation, and translation regulation itself goes beyond the three processes of transcription, mRNA degradation, and protein degradation. Like other omics, translatomics analyzes all components in the translation process, and also includes the study of mRNAs, ribosomes, tRNAs, regulatory RNAs, and newborn peptide chains (Zhao et al., 2019). Meanwhile, the correlation between the transcriptome and proteome is usually very poor in omics data, because the phenomenon of post transcriptional regulation, translation control, and other factors such as frameset translation is common. Translational sequencing can accurately quantify the genes being translated, and indirectly detect the protein expression from the genomic level, indicating the real situation of gene transcription and expression in biological samples. In addition, by comparing the gene translation differences between different samples, we can reveal the molecular mechanism of related physiological and pathological differences. Translatomics is a bridge between transcriptomics and proteomics. The multi omics analysis of translatomics can better study the mechanism of translation regulation. By analyzing the correlation between translatomics and transcriptomics, we can study the change of translation rate within genes, compare the relationship between gene transcription and translation, and study the difference of gene translation efficiency under different physiological and pathological conditions by calculating gene translation efficiency, to explain its molecular mechanism. The association analysis of translatomics and proteomics can study the relationship between transient translation and protein accumulation, assist proteome identification, provide evidence of gene translation, and indirectly determine the proteins expressed in biological samples.

It is critical to accurately investigate the underlying mechanisms of miRNA translation inhibition, and analyze the effect of post transcriptional regulation and RNA modification on gene translation. It is generally believed that the gene that can encode a protein with a length of more than 100 amino acids is a protein coding gene, while other gene sequences are noncoding sequences. However, in recent years, more studies have shown that some RNA regions (including lncRNA, 5′UTR, 3′UTR, circRNA, etc.) that are traditionally considered not to encode proteins can translate some peptides with a length of less than 100 amino acids. These peptides, of less than 100 amino acids in length, also play a variety of important roles in organisms, including ontogeny, muscle contraction, and DNA repair. Because translatomics is used to sequence and quantify the RNA molecules being translated, it can provide direct translation evidence for these noncoding sequences, and help to find and identify new unknown peptides.

In conclusion, this study analyzed the process of screening cashmere fineness functional genes by translatomics through Ribo-seq sequencing, found that the COL6A5 gene may play an important role in cashmere fineness regulation, and provided some theoretical basis for future research on this gene in the field of cashmere fineness regulation.