Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Abstract

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Keywords: Ia afferent synapses; astrocytes; axotomy; microglia; motoneuron; regeneration; sensorimotor integration; synaptic plasticity.

Figure 1

Different phases of synaptic and glia plasticity around cell bodies of axotomized motoneurons. Axotomy induces chromatolysis and rapid changes in gene expression due to positive injury signals arriving at the cell body and negative signals due to lost trophic support from muscle. During an early phase (2), adhesion in non-junctional areas is reduced and activated microglia migrate towards the cell body replacing membrane regions previously covered by synaptic bouton. In the periphery, the cut distal segment undergoes Wallerian degeneration (2, 3) and Schwann cells modify phenotype to orchestrate the removal of axon debris and upregulate trophic factors and adhesion proteins to promote regeneration. This phase is followed by reduced motoneuron expression of synaptic genes, including GlyRs and GluRs, as well as, the synaptic organizers PSD95 and gephyrin (3). This results in full detachment of many excitatory synapses and some inhibitory synapses. Finally, the motoneuron is covered by astrocytic lamellae replacing microglia (4). All changes on the surface of the motoneurons revert after the motor axon successfully reinnervates muscle: astrocyte coverage disappears, inhibitory synapses enlarge and recuperate gephyrin and GlyRs, excitatory synapses are re-established and KCC2 expression recovers resulting in the return of normal excitatory and inhibitory synaptic activity on the cell body.

Figure 2

Working model for the molecular and structural reorganization of inhibitory synapses on the cell surface of axotomized motoneurons. Top: normal organization of mixed GABA/glycine synapses on adult motoneurons. Glycine content is higher than GABA in mature synapses on the cell body of motoneurons. Each synaptic bouton forms multiple synaptic complexes (usually >6) and each has a postsynaptic density (PSD) formed by a gephyrin scaffold that co-clusters GABA_A and glycine receptors, with glycine receptors at higher density in mature synapses. Neuroligin-2 interacts with gephyrin and binds presynaptic neurexin located adjacent to the release site or presynaptic active zone (PAZ). The direction of the ionic synaptic current is defined by the chloride concentration gradient maintained low intracellularly by the activity of KCC2. Bottom: after axotomy, neuroligin, gephyrin, and KCC2 are removed. Glycine receptors increase their mobility and internalization is unmatched by new membrane insertion since glycine receptor expression strongly decreases. In contrast, the expression of GABA_A receptors with α2 and β2/3 subunits is maintained. In the presynaptic bouton, there is an increase in GABA synthesis. Bouton size is also reduced and contains fewer synaptic complexes and these now operate with a significant GABAergic component (see references in the text).

Figure 3

Fate of the Ia afferents synapses after crush or transection neve injuries. After nerve crush, the continuity of endoneurial tubes is preserved and guides sensory and motor axons during regeneration to the original targets. Centrally, Ia afferent synapses undergo normal synaptic stripping after nerve crush and then recover in coincidence with axon regeneration and muscle reinnervation in the periphery. After nerve transection, continuity is lost and the sensory or motor axon (or both) can be routed to the wrong muscles. Also, Ia sensory axons targeting the correct muscle can be misguided to the wrong muscle sensory receptor and/or fail to reinnervate any muscle spindle. After nerve transection, Ia axon terminal collaterals are removed from the ventral horn causing massive and permanent denervation of motoneuron cell bodies and proximal and mid-distance dendrites where the majority of Ia synapses reside. These synapses are not recovered after muscle reinnervation resulting in an enduring loss of connectivity between Ia afferents and motoneurons.