Neuregulin 1-ErbB module in C-bouton synapses on somatic motor neurons: molecular compartmentation and response to peripheral nerve injury

Abstract

The electric activity of lower motor neurons (MNs) appears to play a role in determining cell-vulnerability in MN diseases. MN excitability is modulated by cholinergic inputs through C-type synaptic boutons, which display an endoplasmic reticulum-related subsurface cistern (SSC) adjacent to the postsynaptic membrane. Besides cholinergic molecules, a constellation of proteins involved in different signal-transduction pathways are clustered at C-type synaptic sites (M2 muscarinic receptors, Kv2.1 potassium channels, Ca2+ activated K+ [SK] channels, and sigma-1 receptors [S1R]), but their collective functional significance so far remains unknown. We have previously suggested that neuregulin-1 (NRG1)/ErbBs-based retrograde signalling occurs at this synapse. To better understand signalling through C-boutons, we performed an analysis of the distribution of C-bouton-associated signalling proteins. We show that within SSC, S1R, Kv2.1 and NRG1 are clustered in highly specific, non-overlapping, microdomains, whereas ErbB2 and ErbB4 are present in the adjacent presynaptic compartment. This organization may define highly ordered and spatially restricted sites for different signal-transduction pathways. SSC associated proteins are disrupted in axotomised MNs together with the activation of microglia, which display a positive chemotactism to C-bouton sites. This indicates that C-bouton associated molecules are also involved in neuroinflammatory signalling in diseased MNs, emerging as new potential therapeutic targets.

Localization and dynamic changes of neuregulin‐1 at C‐type synaptic boutons in association with motor neuron injury and repair

“…We next asked if axonal injury-induced changes in MNs and their synaptic and glial environment (42) also affect SSC-associated NRG1 clusters. We recently showed that microglial processes are rapidly attracted to C-boutons of axotomized MNs (11). Here, we extended these findings by performing a long-term analysis in both reversible (crush) and irreversible (axotomy) axonal interruption paradigms ( Fig.…”

“…Nevertheless, a more detailed examination revealed that although S1R and NRG1 were closely associated, they did not occupy identical microdomains (Fig. 7C, D), very similar to their configuration in C-boutons from WT mice (11). In contrast, a close apposition between NRG1 and Kv2.1 was observed (Pearson's r .…”

“…These include the voltage-gated K + channel 2.1 (Kv2.1) (7), Ca 2+ -activated K + channels (8), and s-1 receptors (S1Rs) (9). Most of these proteins are clustered in nonoverlapping microdomains in the PM and SSC (10,11), but their exact roles and functional organization are largely unknown. One novel, intriguing aspect of the molecular assembly at C-boutons is the SSC accumulation of neuregulin (NRG) 1 (11,12).…”

“…Most of these proteins are clustered in nonoverlapping microdomains in the PM and SSC (10,11), but their exact roles and functional organization are largely unknown. One novel, intriguing aspect of the molecular assembly at C-boutons is the SSC accumulation of neuregulin (NRG) 1 (11,12). The NRG1 family of pleiotropic signaling proteins serve as epidermal growth factor (EGF)-like ligands for transmembrane tyrosine kinase receptors of the ErbB family and regulate a variety of neurodevelopmental processes, including myelination and synaptogenesis (13,14).…”

“…Following axotomy, activated microglia appear to displace synaptic terminals from the soma and dendrites, a phenomenon recognized as "synaptic striping" (22). Thus, a partial loss of C-boutons in response to axotomy (23) may result from recruitment of microglial processes to Cbouton sites that have been observed in axotomized MNs (11). Intrathecal treatment with NRG1 causes microglial proliferation in the spinal cord and has been linked to increased pain sensitivity after nerve injury (24).…”