Passive and active immunization models of MuSK-Ab positive myasthenia: Electrophysiological evidence for pre and postsynaptic defects

Abstract

Antibodies directed against the post-synaptic neuromuscular junction protein, muscle specific kinase (MuSK) are found in a small proportion of generalized myasthenia gravis (MuSK-MG) patients. MuSK is a receptor tyrosine kinase which is essential for clustering of the acetylcholine receptors (AChRs) at the neuromuscular junction, but the mechanisms by which MuSK antibodies (MuSK-Abs) affect neuromuscular transmission are not clear. Experimental models of MuSK-MG have been described but there have been no detailed electrophysiological studies and no comparisons between the MuSK-MG and the typical form with AChR-Abs (AChR-MG). Here we studied the electrophysiology of neuromuscular transmission after immunization against MuSK compared with immunization against AChR, and also after passive transfer of IgG from MuSK-MG or AChR-MG patients. Overt clinical weakness was observed in 6/10 MuSK-immunized and 3/9 AChR-immunized mice but not in those injected with patients' IgG. Miniature endplate potentials (MEPPS) were reduced in all weak mice consistent with the reduction in postsynaptic AChRs that was found. However, whereas there was an increase in the quantal release of acetylcholine (ACh) in the weak AChR-immunized mice, no such increase was found in the weak MuSK-immunized mice. Similar trends were found after the passive transfer of purified IgG antibodies from MuSK-MG or AChR-MG patients. Preliminary results showed that MuSK expression was considerably higher at the neuromuscular junctions of the masseter (facial) than in the gastrocnemius (leg) with no reduction in MuSK immunostaining at the neuromuscular junctions. Overall, these results suggest that MuSK antibodies act in at least two ways. Firstly by indirectly affecting MuSK's ability to maintain the high density of AChRs and secondly by interfering with a compensatory presynaptic mechanism that regulates quantal release and helps to preserve neuromuscular function. These results raise questions about how MuSK is involved in retrograde signaling, and the combination of post-synaptic defects with lack of presynaptic compensation may begin to explain the more severe disease in MuSK-MG patients.

The Neuromuscular Junction: Measuring Synapse Size, Fragmentation and Changes in Synaptic Protein Density Using Confocal Fluorescence Microscopy

“…Importantly the rate of decline was sufficient to explain comparable declines in the amplitude of synaptic potentials, failure of synaptic transmission and muscle weakness 17,18 . Qualitatively similar findings were reported by other research groups 10,19 . The same NMJ measurement methods have since been used to assess the impact of three drugs for treating anti-MuSK myasthenia gravis in this mouse model 20,21 .…”

MuSK IgG4 autoantibodies cause myasthenia gravis by inhibiting binding between MuSK and Lrp4

“…An increase in quantal content, however, is not evident in MuSK MG (12,16,45). These findings suggest that MuSK plays an important role in this homeostatic response.…”

Antibodies against low-density lipoprotein receptor–related protein 4 induce myasthenia gravis

“…with these reports, presynaptic deficits were observed in EAMG models of AChR and MuSK, proteins that are expressed specifically in skeletal muscles (9,40,72). Moreover, antibody interaction with muscle LRP4 may stimulate immune responses such as complement activation, which could damage nearby presynaptic terminals.…”