Microglial recruitment and mechanisms involved in the disruption of afferent synaptic terminals on spinal cord motor neurons after acute peripheral nerve injury

Abstract

Peripheral nerve section with subsequent disconnection of motor neuron (MN) cell bodies from their skeletal muscle targets leads to a rapid reactive response involving the recruitment and activation of microglia. In addition, the loss of afferent synapses on MNs occurs in concomitance with microglial reaction by a process described as synaptic stripping. However, the way in which postaxotomy‐activated microglia adjacent to MNs are involved in synaptic removal is less defined. Here, we used confocal and electron microscopy to examine interactions between recruited microglial cells and presynaptic terminals in axotomized MNs between 1 and 15 days after sciatic nerve transection in mice. We did not observe any bulk engulfment of synaptic boutons by microglia. Instead, microglial cells internalized small membranous‐vesicular fragments which originated from the acute disruption of synaptic terminals involving the activation of the necroptotic pathway. The presence of abundant extracellular vesicles in the perineuronal space after axotomy, together with the increased expression of phospho‐mixed lineage kinase domain‐like protein and, later, of extracellular vesicle markers, such as CD9, CD63, and flotillin, indicate that the vesicles mainly originated in synapses and were transferred to microglia. The upregulation of Rab7 and Rab10 in microglia interacting with injured MNs, indicated the activation of endocytosis. As activated microglia and synaptic boutons displayed positive C1q immunoreactivity, a complement‐mediated opsonization may also contribute to microglial‐mediated synaptic disruption. In addition to the relevance of our data in the context of neuroinflammation and MN disease, they should also be taken into account for understanding functional recovery after peripheral nerve injury.

Accumulation of misfolded SOD1 outlines distinct patterns of motor neuron pathology and death during disease progression in a SOD1G93A mouse model of amyotrophic lateral sclerosis

“…The ultrastructure of presynaptic boutons in advanced stages of the disease displayed extreme degenerative changes and presynaptic membrane disruption with the release of vesicular structures into the extracellular space (Figure S6D). Similar changes have been reported to occur in acute degeneration of synaptic terminals in axotomized MNs and involve activation of the necroptotic pathway [24].…”

“…Nerve axotomy performed at later stages, such as P60 or P90, also entailed a worsening in the MN phenotype score (Figure 11B ). It is well known that peripheral nerve lesions induce a plethora of changes in axotomized MN cell bodies and in adjacent synaptic afferents and glial cells [ 24 , 115 ]. We found here that the strong neuroinflammatory response induced prematurely by axotomy in SOD1 G93A animals acts as a synergistic mechanism in promoting mfSOD1 expression (Figure 11C,D ).…”

“…In addition, it seems that before NM degeneration, a process of a motor axon dying back determines an early disconnection of MNs from their muscle targets [ 11 , 21 ]. The interruption of nerve‐muscle interactions also occurs in healthy animals after peripheral nerve transection (axotomy), resulting in a CNS response that includes glial activation around axotomized MNs and a loss of synaptic afferents contacting MN cell bodies [ 22 , 23 , 24 ]. Thus, the reactive response observed in axotomized MNs may share some common pathogenic mechanisms with ALS.…”

“…Instead, axotomy triggers the expression of “regeneration‐associated genes”, shifting neurons from “transmitting” to a “growth” mode, which provides MNs with the structural and molecular machinery for restoration of neuromuscular connectivity [ 22 ]. Although it may be deleterious in some respects [ 24 , 27 ], the glial neuroinflammatory response in the vicinity of axotomized MNs is also considered neuroprotective [ 28 , 29 ]. Therefore, analyzing afferent synapses and the glial response in axotomized MNs in WT and mutant SOD1 genetic backgrounds may provide new clues for understanding the mechanisms of MN degeneration in ALS.…”

Complement and Coagulation System Crosstalk in Synaptic and Neural Conduction in the Central and Peripheral Nervous Systems

“…Hirudin, a specific thrombin inhibitor, and protease nexin 1 (PN1), an endogenous thrombin inhibitor, can block thrombin-induced synaptic loss at the NMJ [ 119 ]. After acute peripheral nerve injury, activated microglia and synaptic boutons display positive C1q immunoreactivity occurring near motor neurons which may be involved in synaptic disruption [ 120 ]. Modifications of complement and coagulation factors and inhibitors at the NMJ may imply a common role in NMJ pathology.…”