Systemic Changes in Early Pregnancy in the Mare: An Integrated Proteomic Analysis of Blood Plasma, Histotroph, and Yolk Sac Fluid at Day 14 Post-Ovulation

Purpose: Embryo-maternal signaling during the establishment of pregnancy in horses remains one of the biggest mysteries in large animal physiology. Early pregnancy loss represents a major source of economic loss to the breeding industry. This study aimed to investigate the systemic changes associated with early pregnancy by mapping the proteome of blood plasma at 14 days in pregnant and non-pregnant mares. Experimental Design: Plasma proteomes were analysed in commercially bred pregnant (n = 17) and non-pregnant (n = 17) Thoroughbred mares at 14 days after ovulation, using high-resolution mass spectrometry. Day 14 histotroph and yolk sac fluid were also profiled and datasets were integrated through pathway analysis. Results: We identified 229 total protein IDs, with 12 increased and 10 decreased significantly in pregnant versus non-pregnant plasma. To gain functional insight, these data were aligned with proteomes of 14-day pregnant mare uterine fluid (n = 4; 1358 IDs) and conceptus fluid (soluble proteins within the yolk sac fluid; n = 4; 1152 IDs), and further interrogated using gene ontology databases and pathway analysis. Conclusions and Clinical Relevance: These analyses identified consistent systemic changes in the mare's proteome that indicate a profound and specific immune response to early pregnancy, which appears to precede the systemic endocrine response to pregnancy. Integrated pathway analysis suggests that embryo-maternal interactions in early pregnancy may mimic elements of the virus-host interaction to modulate the maternal immune response. Transthyretin (TTR) and uteroglobin (SCGB1A1) were respectively down- and upregulated in plasma while also present in uterine fluid, and are proposed to be key proteins in early pregnancy establishment. These findings contribute significantly to our knowledge of early pregnancy in the mare and identify potential new avenues for developing clinical approaches to reduce early embryo loss.