Fig. 1

MERTK expression in relation to RAMP1, CGRP and CASPR. A: MER proto-oncogene tyrosine kinase (MERTK) immunoreactivity was observed mainly in satellite glial cells (SGCs) and, to a lesser extent, in Schwann cells. Interestingly, MERTK was observed to have a striking expression in the region of the Schwann cell overhanging the nodes of Ranvier of Aδ-fibres (Arrow). This was confirmed by double staining with contactin associated protein 1 (CASPR), which is explicitly expressed in the para- and juxtaparanodal regions of myelinated fibres (Arrowheads). Insert-A: Close-up on an Aδ-fibre with para- and juxtaparanodal regions stained by CASPR (Arrowheads). Notably, a weak expression of MERTK can be seen in the outer layer of the Schwann cell, which is strikingly pronounced where the Schwann cells border the node of Ranvier (Arrow). B: To confirm the expression of MERTK in Schwann cells and SGCs, a double-staining was performed with a well-established antibody, RAMP1. MERTK can be seen expressed in SGCs enveloping a RAMP1 positive neuron (Arrowhead), and a striking immunoreactivity of MERTK can be seen in Schwann cell outer membranes proximal to the nodes of Ranvier (Arrow). Insert-B: Close-up on an Aδ-fibre immunoreactive for RAMP1 (Arrowhead). Flanking the axon, its associated Schwann cell indicates immunoreactivity for MERTK with a strong expression near the node of Ranvier (Arrow). C: The expression of MERTK in relation to calcitonin gene-related peptide (CGRP). CGRP was observed in C-fibre boutons (Arrowhead) and associated neuron cell bodies (Asterix). MERTK did not co-localize with CGRP but could be observed in SGCs surrounding CGRP-positive neurons, and in Schwann cell membranes (Arrow). Insert-C: A C-fibre displaying immunoreactivity for CGRP with a bouton (Arrowhead) in close proximation to the MERTK-positive Schwann cell membranes flanking the node of Ranvier of an Aδ-fibre (Arrow). Inserts are separate images from the lower magnification images A, B and C