Despite using extensive epitope unmasking that readily revealed RS protein throughout the retina, we did not see RS immunoreactivity in Müller cells or processes, although this has been reported by others.
13 14 RS localized with bipolar cell axons that run tightly parallel and immediately adjacent to Müller processes
(Fig. 9) , which may have led to the prior interpretation of immunohistochemical data. The interpretation of the present data is that RS is secreted by nearly all retina neurons and acts locally, with no evidence of Müller cell expression or transport through Müller cells of RS from outer retinal photoreceptors. However, XLRS disease historically has been attributed to primary pathology in the Müller cells, based in part on the characteristic b-wave absence in ERG responses of affected males.
16 18 Although the mammalian ERG b-wave was thought to originate from Müller cell activity,
53 54 recent evidence implicates the bipolar cells directly in b-wave generation.
37 Further, our ERG modeling of human XLRS responses implicates abnormal signaling by both the cone depolarizing and hyperpolarizing bipolar circuits.
17 This feature of the ERG had not been suspected of involving hyperpolarizing bipolar cells. The present data confirm RS involvement in both these cell classes and their synapses. Müller cells may, however, participate secondarily in XLRS. XLRS human histopathology shows strong Müller cell staining for the intermediate filament proteins vimentin and glial fibrillary acidic protein (GFAP) across the retina and in the schisis cavities.
55 56 Müller cell hypertrophy and vimentin and GFAP expression are nonspecific findings in many retinal diseases.
57 One can surmise that the outer retinal reticular gliotic appearance that is frequently seen clinically in older XLRS-affected males
16 may result from Müller cell hypertrophy. The relatively unaffected Müller cells in the recently engineered RS mouse,
RS1h −/Y, confirms that this effect is most likely secondary.
58 Although the heavy expression by photoreceptors has been proposed to couple with Müller glial transport of RS into the proximal retina, there is no compelling rationale for this in the face of local production by RGCs and essentially all other developing and adult retinal neurons.