Impaired Presynaptic Function Contributes Significantly to the Pathology of Glycine Receptor Autoantibodies.

BACKGROUND AND

OBJECTIVES

Autoantibodies (aAbs) against glycine receptors (GlyRs) are mainly associated with the rare neurologic diseases stiff person syndrome (SPS) and progressive encephalomyelitis with rigidity and myoclonus (PERM). GlyR aAbs are also found in other neurologic diseases such as epilepsy. The aAbs bind to different GlyR α-subunits and, more rarely, also to the GlyR β-subunit. So far, studies on the pathogenic effects of the aAbs have focused on postsynaptic, heteromeric GlyRs, reporting a loss of ion channel function and receptor internalization upon aAb binding. We asked whether the aAbs also affect expression and functionality of presynaptic homomeric GlyRs.

METHODS

We established interneuron cultures from mouse embryonic spinal cord neurons and used protein biochemistry and super-resolution microscopy to determine aAb binding to presynaptic GlyRs in a uniform neuronal subpopulation. Brainstem slice recordings were used to detect functional alterations.

RESULTS

Several days-long exposure of spinal cord cultures with GlyR aAbs did not change expression levels of proteins building a functional glycinergic synapse. A notable exception was the enhanced expression of presynaptic glycine transporter 2 (GlyT2), possibly reflecting an adaptation to altered synaptic properties. Super-resolution microscopy revealed rather similar binding of patient-derived aAbs to postsynaptic vs presynaptic sites with individual binding preferences. Although characterization of interneurons showed absence of GlyRα1 in some interneuron subpopulations, GlyRα2 and patient serum signals exhibited a significantly higher colocalization in samples with presynaptic preference. This finding identifies GlyRα2 as the hitherto unknown predominant presynaptic GlyR subunit in the spinal cord and a target of patient aAbs. Whole-cell recordings from glycinergic neurons in mouse brainstem slices underscored the functional relevance of presynaptic aAb binding demonstrated by a significant reduction in the frequency of spontaneous and miniature inhibitory postsynaptic potentials.

DISCUSSION: In summary, our study is the first to implicate presynaptic defects in the pathophysiology of autoimmune diseases such as SPS and PERM, which are associated with GlyR aAbs. Individually tuned binding preferences for presynaptic and postsynaptic targets thus underlie the rather diverse appearance of clinical symptoms and different therapeutic responses in patients suffering from GlyR autoimmunity.