Examining the effect of stress during lactation on marine mammals

Elephant seals are deep-diving marine mammals that feed in the open ocean, but give birth on land. During foraging, elephant seals build up large fat stores, which provide them with energy while hauled out on land. Female elephant seals give birth to a single pup, which they nurse with milk that is up to 60% fat for 4 weeks. However, unlike most other mammals, they are capital breeders so they do so while fasting for the entire lactation period. In contrast, income breeders like sea lions forage for food during the lactation period. We are interested in determining whether maternal stress experienced during lactation in capital breeding mammals affects their ability to provide offspring with high-quality milk. Cortisol plays an important role in lipolysis, and its elevation during stress responses may impact the fat stores of lactating females and the amount of fat they can provide in the milk. Stress may also impact the composition of the milk proteome, through alterations in the protein expression. Milk from the mother not only supplies nutrients, but also proteins that serve a variety of roles from aiding digestion and absorption of the milk to passive immunity in the neonate. To simulate a stress response, ACTH was injected in a group of female elephant seals during the third week of lactation to stimulate cortisol synthesis and release by the adrenal glands. Milk samples were obtained from females ~24 hours after ACTH administration. Whole milk was first centrifuged and proteins were extracted from the skim milk fraction using methanol and chloroform. We then denatured, reduced, alkylated, and digested proteins into peptides. Then we desalted and quantified peptides to prepare them for LC-MS/MS. We identified a total of 149 proteins in the elephant seal milk proteome, some were also found in blood plasma of the same females and their pups in a previous study. These proteins are associated with cholesterol transport, lipid digestion, lipid absorption, lipid transportation, Thrombogenesis, Fibrinolysis, Immunity, cell migration, cell signaling, fatty acid intracellular transport in heart, cortisol transport, cortisol metabolism, iron uptake into cells, bone remodeling, and protection from oxidative damage. Currently we are working on comparing milk proteome data between ACTH-treated and control females. The study helps gain a better understanding of the effects that maternal stress has on lactation of capital breeding mammals, which in turn may affect the development and stress adaptation of their offspring.