Investigation of a potential role for angiopoietin-like protein 2 in the migration of leukocytes during hibernation in thirteen-lined ground squirrels

Abstract

Thirteen-lined ground squirrels enter a state of torpor during the winter months, characterized by weight loss, decreased breathing rate, and decreased immune function. Upon entrance into torpor, the squirrels? white blood cells (WBCs) almost completely disappear from the blood stream, and most levels quickly return to normal within 2 hours post arousal. It is hypothesized that these immune cells are stored in a peripheral organ or tissue during torpor; however the exact mechanism and location remain largely unknown. Angiopoietin-like protein 2 (Angptl2) is a protein primarily secreted by adipose tissue, which results in inflammation and the migration of WBCs to the adipose tissue. Angptl2 secretion is increased by conditions that are present during torpor, oxidative stress, and an increase in adipose tissue, so it was hypothesized that Angptl2 might play a role in the behavior of immune cells during torpor. A chemiluminescent western blot was used to measure and compare Angptl2 levels in plasma of non-hibernating, entering hibernation, torpid, interbout arousal and post arousal ground squirrels. Angptl2 levels were significantly higher in torpid than interbout arousal or post arousal animals. Immunohistochemical staining of adipose tissue found macrophages were approximately twice as abundant in torpid as non-hibernating tissue.