Altered expression of necroptotic, angiogenic and spermatogenic proteins in degenerate testes from aged stallions

Abstract

We utilized RNA sequencing to compare the testis transcriptomes of fertile, adult stallions to those of adult stallions with age-related testicular degeneration (TD). Large-scale gene ontology analysis identified pathway-wide upregulation of inflammation and alterations in apoptotic pathways that likely play roles in the disease process (Woodward et al., Eq Vet J. 2022; 1-14). In humans and mice, inflammation, together with necroptosis, and alterations in capillary density, all contribute to age-related degenerative changes (Li et. al, ELife. 2017; 6:e27692 DOI: 10.7554/eLife.27692; Das et al, Cell. 2018;(173)74-89). One objective of the current study was to identify differentially expressed genes (DEGs) that may contribute similarly to TD in the horse. We also searched for DEGs that play roles in spermatogenesis. A second objective was to use immunohistochemistry (IHC) to determine if the protein products of these genes are differentially expressed between normal and diseased testes and to determine their patterns of expression. The hypothesis is that aberrations in necroptotic, angiogenic, and spermatogenic signaling pathways contribute to the pathophysiology of TD in stallions. RNA isolation and sequencing were performed on testicular tissue samples from 3 fertile adult stallions and from 3 adult stallions with confirmed TD as previously described (Woodward et al., Eq Vet J. 2022; 1-14). Transcript expression was compared between the 2 populations using either EdgeA or DESeq2, and a minimum of >1.5x fold change and Padj<0.05 asthe cutoffs for significance. Protein expression from selected DEGs was compared between normal adult and TD tissue using IHC. In degenerate testes, significant increases in gene expression were found in testicular-origin growth factors (BMP4 (9x increase), FGF2 (9x increase), and CSF1 (7x increase)). IHC confirmed increased expression of BMP4 and FGF2 proteins in germ cells from degenerate testes. Expression of CSF1R protein also was altered, with loss of expression of the protein in early germ cells, and increased expression in later stages of spermatogenesis. Upregulation of RIPK3 (3x increase) and VEGF (17x increase) transcripts also were confirmed in degenerate testes. These genes are central to necroptotic and angiogenic pathways, respectively. IHC confirmed diffuse upregulation of the RIPK3 protein in germ cells from degenerate testes. IHC for the VEGF protein is pending. In conclusion, alterations in expression of proteins responsible for stimulation of mitosis and meiosis in male germ cells were identified. Additionally, we provide evidence that chronic inflammation, necroptosis and aberrant angiogenesis may cause age-related degenerative changes in equine testes. In mice, inhibitors of the RIPK3 pathway and stimulation of angiogenesis can ameliorate clinical signs of aging in testes and muscle respectively (Li et. al, ELife. 2017; Das et al, Cell. 2018). Financial support: Grayson Jockey Club Research Foundation.