Alterations of peripheral nerve excitability in an experimental autoimmune encephalomyelitis mouse model for multiple sclerosis

Abstract

Background: Experimental autoimmune encephalomyelitis (EAE) is the most commonly used and clinically relevant murine model for human multiple sclerosis (MS), a demyelinating autoimmune disease characterized by mononuclear cell infiltration into the central nervous system (CNS). The aim of the present study was to appraise the alterations, poorly documented in the literature, which may occur at the peripheral nervous system (PNS) level.

Methods: To this purpose, a multiple evaluation of peripheral nerve excitability was undertaken, by means of a minimally invasive electrophysiological method, in EAE mice immunized with the myelin oligodendrocyte glycoprotein (MOG) 35-55 peptide, an experimental model for MS that reproduces, in animals, the anatomical and behavioral alterations observed in humans with MS, including CNS inflammation, demyelination of neurons, and motor abnormalities. Additionally, the myelin sheath thickness of mouse sciatic nerves was evaluated using transmission electronic microscopy.

Results: As expected, the mean clinical score of mice, daily determined to describe the symptoms associated to the EAE progression, increased within about 18 days after immunization for EAE mice while it remained null for all control animals. The multiple evaluation of peripheral nerve excitability, performed in vivo 2 and 4 weeks after immunization, reveals that the main modifications of EAE mice, compared to control animals, are a decrease of the maximal compound action potential (CAP) amplitude and of the stimulation intensity necessary to generate a CAP with a 50% maximum amplitude. In addition, and in contrast to control mice, at least 2 CAPs were recorded following a single stimulation in EAE animals, reflecting various populations of sensory and motor nerve fibers having different CAP conduction speeds, as expected if a demyelinating process occurred in the PNS of these animals. In contrast, single CAPs were always recorded from the sensory and motor nerve fibers of control mice having more homogeneous CAP conduction speeds. Finally, the myelin sheath thickness of sciatic nerves of EAE mice was decreased 4 weeks after immunization when compared to control animals.

Conclusions: In conclusion, the loss of immunological self-tolerance to MOG in EAE mice or in MS patients may not be only attributed to the restricted expression of this antigen in the immunologically privileged environment of the CNS but also of the PNS.

Keywords: Electrophysiology; Experimental autoimmune encephalomyelitis; Mouse; Multiple sclerosis; Myelin oligodendrocyte glycoprotein; Neuromuscular junction; Peripheral nervous system.

Fig. 1

Traces of compound nerve action potential (CNAP) and compound muscle action potential (CMAP) for sensory and motor nerve recordings, respectively, evoked by nerve stimulation (upper), and corresponding locations of stimulation and detection (red arrows) electrodes (lower)

Fig. 2

Stimulus-response curves (i.e., CNAP in response to nerve stimulations of increasing intensity) and derived excitability parameters (left). Relationships between the CNAP amplitude and the stimulation intensity delivered manually (in pink) and then automatically (in green) by the program (right)

Fig. 3

Individual clinical scores of 6 EAE mice (MOG-M1 to MOG-6) and control mice (CFA-M1 to CFA-6) as a function of time after immunization at day 0. The red arrows indicate the days of electrophysiological recordings (i.e., 2 and 4 weeks) after immunization

Fig. 4

Body weight (left) and temperature (right) of EAE (MOG) mice, compared to control (CFA) animals, 2 (n = 6) and 4 (n = 5–6) after immunization. *P = 0.027 and **P = 0.001

Fig. 5

Maximal CNAP amplitude (CNAPmax), stimulation intensity necessary to generate a CNAP with an amplitude equal to 50% of its maximum value (SI-50%), slope of the stimulus-response curve (Slope), and time between the stimulus onset and the first peak amplitude of CNAP (Latency) in EAE (MOG) mice, compared to control (CFA) animals, 2 (n = 3–5) and 4 (n = 5–6) weeks after immunization. *P ≤ 0.035, **P ≤ 0.008, and ***P ≤ 0.0005

Fig. 6

Example of increased Latency and decreased CNAPmax, Slope, and SI-50% in an EAE (MOG) mouse, compared to a control (CFA) animal, 4 weeks after immunization

Fig. 7

Low propagation velocity of CNAP (increased Latency) and sensory nerve hyperexcitability (decreased SI-50%) in EAE (MOG) mice, compared to control (CFA) animals, are exemplified by CNAP recordings showing (1) “control-like” (i.e., unaffected), (2) “slower”, (3) “very slower”, and (4) “very very slower” conducting nerve fibers in a MOG-mouse, compared to a CFA-animal, 2 weeks after immunization

Fig. 8

Traces of CMAP recorded from the tail and plantar muscles of a control (CFA) mouse and an EAE (MOG) animal, 4 weeks after immunization

Fig. 9

Expression of CMAP amplitude as a percentage of the first CMAP amplitude in individual EAE mice (MOG-M1 to MOG-6), 2 and 4 weeks after immunization (a and b), and relationship between the second CMAP amplitude and the EAE mouse clinical score (c)

Fig. 10

Peripheral nerve myelin sheath of EAE (MOG) mice, compared to control (CFA) animals, 2 and 4 weeks after immunization. Data represent the mean ± S.D. of 48 randomly chosen fibers per group, each constituted of 3 mice (i.e., 16 fibers per mouse). *P < 0.0001