Elevated serum [Met5]-enkephalin levels correlate with improved clinical and behavioral outcomes in experimental autoimmune encephalomyelitis

Methionine enkephalin ([Met5]-enkephalin, Opioid growth factor (OGF)) is a small neuropeptide with growth-related as well as immunomodulatory properties. OGF is distributed widely throughout the body, is both autocrine and paracrine produced, and has a very short half-life in serum. In addition to its neurotransmitter functions, OGF inhibits cell replication of a wide variety of cells involved in the autoimmune process. In this preclinical study, mice were immunized with myelin oligodendrocytic glycoprotein (MOG35-55) to establish a chronic progressive form of autoimmune encephalomyelitis (EAE), and serum enkephalin levels were assessed throughout the disease as well as in response to OGF therapy in order to determine whether OGF may be a biological marker for EAE and multiple sclerosis. Immunized mice were randomly assigned to groups receiving daily 10 mg/kg OGF (n = 24) or saline (n = 25) beginning at the time of established disease and clinical behavior. Open field activity, rearing, forced swimming, and novel object tests were monitored. Serum levels of peptide were measured prior to immunization, before clinical symptoms were observed, and at the onset and peak period of disease. Spinal cord neuropathology was evaluated 40 days after immunization. EAE disease onset occurred on day 9 post immunization when the mean clinical score was 1.5. Peak disease scores for saline-injected EAE mice reached a mean of 5.7 on day 18, whereas mice receiving OGF had a peak clinical score of 2.5. Behavioral tests conducted 5 days post-immunization (and before clinical signs of EAE) revealed that EAE mice had reduced serum enkephalin levels related to elevated clinical disease scores. Serum levels of enkephalin collected at peak disease and after 40 days correlated with clinical scores. Disease status was associated with activity in the open field, rearing, time associating with a novel object, and pain sensitivity. Clinical signs of EAE correlated with levels of enkephalins such that as behavioral scores increased, serum [Met5]-enkephalin levels decreased. Thus, [Met5]-enkephalin is a novel biomarker that is associated with disease onset and progression, as well as response to therapy in a mouse model of EAE, and may provide new insight into MS.