Pharmacologic inhibition of mTORC1 mimics dietary protein restriction in a mouse model of lactation

Exploring the contribution of mammary-derived serotonin on liver and pancreas metabolism during lactation

During lactation, the murine mammary gland is responsible for a significant increase in circulating serotonin. However, the role of mammary-derived serotonin in energy homeostasis during lactation is unclear. To investigate this, we utilized C57/BL6J mice with a lactation and mammary-specific deletion of the gene coding for the rate-limiting enzyme in serotonin synthesis (TPH1, Wap-Cre x TPH1FL/FL) to understand the metabolic contributions of mammary-derived serotonin during lactation. Circulating serotonin was reduced by approximately 50% throughout lactation in Wap-Cre x TPH1FL/FL mice compared to wild-type mice (TPH1FL/FL), with mammary gland and liver serotonin content reduced on L21. The Wap-Cre x TPH1FL/FL mice had less serotonin and insulin immunostaining in the pancreatic islets on L21, resulting in reduced circulating insulin but no changes in glucose. The mammary glands of Wap-Cre x TPH1FL/FL mice had larger mammary alveolar areas, with fewer and smaller intra-lobular adipocytes, and increased expression of milk protein genes (e.g., WAP, CSN2, LALBA) compared to TPH1FL/FL mice. No changes in feed intake, body composition, or estimated milk yield were observed between groups. Taken together, mammary-derived serotonin appears to contribute to the pancreas-mammary cross-talk during lactation with potential implications in the regulation of insulin homeostasis.

Genetics, environmental stress, and amino acid supplementation affect lactational performance via mTOR signaling pathway in bovine mammary epithelial cells

Mammary glands are known for their ability to convert nutrients present in the blood into milk contents. In cows, milk synthesis and the proliferation of cow mammary epithelial cells (CMECs) are regulated by various factors, including nutrients such as amino acids and glucose, hormones, and environmental stress. Amino acids, in particular, play a crucial role in regulating cell proliferation and casein synthesis in mammalian epithelial cells, apart from being building blocks for protein synthesis. Studies have shown that environmental factors, particularly heat stress, can negatively impact milk production performance in dairy cattle. The mammalian target of rapamycin complex 1 (mTORC1) pathway is considered the primary signaling pathway involved in regulating cell proliferation and milk protein and fat synthesis in cow mammary epithelial cells in response to amino acids and heat stress. Given the significant role played by the mTORC signaling pathway in milk synthesis and cell proliferation, this article briefly discusses the main regulatory genes, the impact of amino acids and heat stress on milk production performance, and the regulation of mTORC signaling pathway in cow mammary epithelial cells.

Effects of methionine, leucine, and insulin on circulating concentrations and mammary extraction of energy substrates and amino acids in lactating dairy cows

The aim of this experiment was to test whether insulin potentiates the effects of two abomasally infused amino acids (AA), leucine and methionine (LM), on mammary extraction efficiency of energetic and nitrogenous nutrients. Six lactating Holstein cows (155 ± 9 DIM) were ruminally-cannulated and had the right carotid artery subcutaneously transposed. Cows were fed a 20% metabolizable protein-restricted diet and abomasally infused with water (8 L/d) or AA (Met 26 g/d, Leu 70 g/d) for 8 h/d, for 7 days. On the last day of each period, cows were intravenously infused with saline (0.9% NaCl, 110 mL/h) or subjected to 8 h hyperinsulinemic clamp (IC) alongside abomasal infusions. For IC, insulin was infused at 1 µg/kg/h. Normoglycemia was maintained by varying glucose (50% w/v in water) infusion rate based on coccygeal vein glucose concentration. Carotid arterial and subcutaneous abdominal (mammary) vein blood samples were collected at 0, 1, 2, 4, and 6 h from the start of infusions. Milk weights and samples for baseline measurements of production were taken on day 5 PM, day 6 AM and PM, and day 7 AM of the experimental period. A final milk weight and sample was taken immediately after abomasal and intravenous infusions on day 7 PM for assessing the interaction between insulin and the infused AA. The experiment had an incompletely replicated Latin square design with a 2 × 2 factorial arrangement of treatments (abomasal and intravenous infusion). Baseline milk production when cows were only receiving abomasal infusions was largely unaffected by LM, but milk protein yield tended to be decreased. On day 7, LM tended to positively increase milk fat and de novo fatty acid content, and IC tended to decrease milk protein content. Both milk urea nitrogen and plasma urea nitrogen were decreased by IC. Circulating AA concentrations in plasma were decreased by both LM and IC, but mammary extraction efficiency was affected by neither. Infusion of LM had no effect on any energy metabolite analyzed. Circulating non-esterified fatty acid concentration was decreased by IC, with no effect on mammary extraction efficiency. Mammary extraction efficiency of both acetate and β-hydroxybutyrate were decreased by IC. Overall, while both circulating concentrations of energy metabolites and amino acids were decreased in response to treatments, this was not due to improved mammary extraction efficiency.

Dry Period Heat Stress Impacts Mammary Protein Metabolism in the Subsequent Lactation

Simple Summary

Heat stress during the dry period of dairy cows reduces milk yield in the following lactation. Factors such as altered mammary metabolism could impact yields and alter milk composition, including milk protein. We sought to determine if exposure to dry period heat stress would influence mammary milk protein metabolism during the subsequent lactation. Objectives were to first determine the impact of dry period heat stress on milk protein yields and secondly characterize the amino acid and protein profiles in the mammary tissue, milk, and blood to elucidate potential carry-over impact of dry period heat stress on systems that participate directly in milk protein metabolism (i.e., mTOR). We found that heat stress during the dry period reduces milk yield, protein content, and protein yield in the subsequent lactation. The plasma amino acid profile and mammary amino acid transporters are altered in dry period heat-stressed cows, and mammary mTOR signaling proteins are differentially expressed as well. It appears that dry period heat stress impacts mammary metabolism with consequences on milk yield and protein content. The continuous production of high-quality and -quantity milk is vital as a sustainable source of protein in the face of rising global temperatures.

Abstract

Dry period heat stress impairs subsequent milk production, but its impact on milk protein content and yield is inconsistent. We hypothesize that dairy cow exposure to dry period heat stress will reduce milk protein synthesis in the next lactation, potentially through modified amino acid (AA) transport and compromised mTOR signaling in the mammary gland. Cows were enrolled into heat-stressed (dry-HT, n = 12) or cooled (dry-CL, n = 12) treatments for a 46-day dry period then cooled after calving. Milk yield and composition and dry matter intake were recorded, and milk, blood, and mammary tissue samples were collected at 14, 42, and 84 days in milk (DIM) to determine free AA concentrations, milk protein fractions, and mammary AA transporter and mTOR pathway gene and protein expression. Dry matter intake did not significantly differ between treatments pre- or postpartum. Compared with dry-CL cows, milk yield was decreased (32.3 vs. 37.7 ± 1.6 kg/day) and milk protein yield and content were reduced in dry-HT cows by 0.18 kg/day and 0.1%. Further, dry-HT cows had higher plasma concentrations of glutamic acid, phenylalanine, and taurine. Gene expression of key AA transporters was upregulated at 14 and 42 DIM in dry-HT cows. Despite minor changes in mTOR pathway gene expression, the protein 4E-BP1 was upregulated in dry-HT cows at 42 DIM whereas Akt and p70 S6K1 were downregulated. These results indicate major mammary metabolic adaptations during lactation after prior exposure to dry period heat stress.

Energy Metabolism Focused Analysis of Sexual Dimorphism in Biological Aging and Hypothesized Sex-specificity in Sirtuin Dependency

The process of biological aging or senescence refers to the gradual loss of homeostasis and subsequent loss of function - leading to higher chances of mortality. Many mechanisms and driving forces have been suggested to facilitate the evolution of a molecular circuit acting as a trade-off between survival and proliferation, resulting in senescence. A major observation on biological aging and longevity in humans and model organisms is the prevalence of significant sexual divergence in the onset, mechanisms and effects of aging associated processes. In the current account, we describe possible mechanisms by which aging, sex and reproduction are evolutionarily intertwined in order to maintain systemic energy homeostasis. We also interrogate existing literature on the sexual dimorphism of genetic, cellular, metabolic, endocrine and epigenetic processes driving cellular and systemic aging. Subsequently, based on available evidence, we propose a hypothetic model of sex-limited decoupling of female longevity from sirtuins, a major family of regulator proteins of the survival-proliferation trade-off. We also provide necessary considerations to be made in order to test the hypothesis and explore the physiological and therapeutic implications of this decoupling event in male and female longevity after reaching reproductive maturity. HYPOTHESIS STATEMENT: Sirtuins provide survival benefits in a sex-nonspecific manner but the dependency on sirtuins in driving metabolic networks after reaching reproductive maturity is evolutionarily decoupled from female longevity.

The market for amino acids: understanding supply and demand of substrate for more efficient milk protein synthesis

For dairy production systems, nitrogen is an expensive nutrient and potentially harmful waste product. With three quarters of fed nitrogen ending up in the manure, significant research efforts have focused on understanding and mitigating lactating dairy cows’ nitrogen losses. Recent changes proposed to the Nutrient Requirement System for Dairy Cattle in the US include variable efficiencies of absorbed essential AA for milk protein production. This first separation from a purely substrate-based system, standing on the old limiting AA theory, recognizes the ability of the cow to alter the metabolism of AA. In this review we summarize a compelling amount of evidence suggesting that AA requirements for milk protein synthesis are based on a demand-driven system. Milk protein synthesis is governed at mammary level by a set of transduction pathways, including the mechanistic target of rapamycin complex 1 (mTORC1), the integrated stress response (ISR), and the unfolded protein response (UPR). In tight coordination, these pathways not only control the rate of milk protein synthesis, setting the demand for AA, but also manipulate cellular AA transport and even blood flow to the mammary glands, securing the supply of those needed nutrients. These transduction pathways, specifically mTORC1, sense specific AA, as well as other physiological signals, including insulin, the canonical indicator of energy status. Insulin plays a key role on mTORC1 signaling, controlling its activation, once AA have determined mTORC1 localization to the lysosomal membrane. Based on this molecular model, AA and insulin signals need to be tightly coordinated to maximize milk protein synthesis rate. The evidence in lactating dairy cows supports this model, in which insulin and glucogenic energy potentiate the effect of AA on milk protein synthesis. Incorporating the effect of specific signaling AA and the differential role of energy sources on utilization of absorbed AA for milk protein synthesis seems like the evident following step in nutrient requirement systems to further improve N efficiency in lactating dairy cow rations.