Adhesion molecules and synapse remodeling during motoneuron regeneration

Abstract

Spinal motoneurons integrate a vast synaptic input and are the final conveyors of motor commands to skeletal muscle. Motoneuron regeneration includes altered contacts between the motoneuron and several other cell types. The severed axon elongates along Schwann cells in the nerve, while other types of glia interact closely with lesioned motoneurons in the spinal cord. Simultaneously, cell contacts are disrupted when synapses are lost from lesioned motoneurons in a process called synaptic stripping.

The aim of this thesis project was to identify potential mediators of cell interactions during motoneuron regeneration, with special emphasis on molecules that could be of importance for synaptic remodeling. A reduced expression of cell adhesion molecules has been speculated to be involved in synaptic stripping of motoneurons. Over the last years, several such molecules have been described which influence formation and maintenance of synapses, and also mediate adhesion in other events that occur during motoneuron regeneration, such as axon guidance and myelination. We demonstrate that peripheral axotomy of sciatic motoneurons results in altered expression of several cell adhesion molecules, many of which are previously unstudied in this context. Some of these changes indicate possible involvement of the molecules in synapse elimination, whereas other molecules may be involved in other regenerative events.

Specifically, the expression of nectin-1 and -3 increased in lesioned motoneurons, as did the expression of nectin-3-binding necl-5. Nectin proteins did not localize to synapses on spinal motoneurons, but instead to neuronal processes and glia, both within the spinal cord and in the lesioned sciatic nerve. Immunoreactivity for N-cadherin localized to synapses on the surface of motoneurons and was reduced after sciatic nerve transection (SNT). Motoneuron expression of mRNA encoding N-cadherin was not altered after axotomy, and immunoreactivity for the molecule increased in the severed nerve. Axotomy also resulted in altered expression of SynCAM3/necl-1 and SynCAM4/necl-4 in the nerve, which indicates possible involvement of these molecules in remyelination. SynCAM1/necl-2, SynCAM2/necl-3, and neuroligin (NLG) -2 and -3 were expressed by unlesioned motoneurons, and SynCAM immunoreactivity localized to synapses on motoneuron cell bodies. In vitro these molecules have synapse-inducing properties, and following SNT, expression of SynCAM1 and NLG2 and -3 decreased rapidly, prior to loss of staining for synaptophysin in the motoneuron pool. SynCAM1 expression correlated to loss and return of synapses in regeneration after SNT. NLG expression decreased to a smaller degree after sciatic nerve crush than after SNT, although the loss of synapses was similar in both lesion models.

Finally, while this work was ongoing, complement-tagging of CNS synapses for removal was demonstrated to occur in the visual system. We investigated whether complement could be involved also in synapse removal from axotomized motoneurons. Complement C3-/- mice displayed reduced synaptic stripping after lesion, a larger upregulation of growth-associated protein 43 in motoneurons, and a more rapid restoration of motor function.

We conclude that the motoneuron response to axotomy involves downregulation of several synaptic adhesion molecules. Expression of SynCAM1 correlates closely to the loss and return of synapses but the magnitude of the downregulation of NLGs does not seem to reflect the magnitude of the loss of synapses. Contact with the distal nerve stump may stimulate expression of NLGs, but does not seem to influence that of SynCAM1. Expression of NLGs and SynCAM1 does not seem to be the sole determinant of the elimination of synapses, since the expression pattern of these molecules was similar in mice with altered synaptic stripping and wild type mice. We also conclude that complement C3 is required for normal synapse elimination. Thus, complement may be a potential target in therapeutic attempts to preserve synaptic circuits.