The Disease Protein Tulp1 Is Essential for Periactive Zone Endocytosis in Photoreceptor Ribbon Synapses

Abstract

Mutations in the Tulp1 gene cause severe, early-onset retinitis pigmentosa (RP14) in humans. In the retina, Tulp1 is mainly expressed in photoreceptors that use ribbon synapses to communicate with the inner retina. In the present study, we demonstrate that Tulp1 is highly enriched in the periactive zone of photoreceptor presynaptic terminals where Tulp1 colocalizes with major endocytic proteins close to the synaptic ribbon. Analyses of Tulp1 knock-out mice demonstrate that Tulp1 is essential to keep endocytic proteins enriched at the periactive zone and to maintain high levels of endocytic activity close to the synaptic ribbon. Moreover, we have discovered a novel interaction between Tulp1 and the synaptic ribbon protein RIBEYE, which is important to maintain synaptic ribbon integrity. The current findings suggest a new model for Tulp1-mediated localization of the endocytic machinery at the periactive zone of ribbon synapses and offer a new rationale and mechanism for vision loss associated with genetic defects in Tulp1.

Early auto‐immune targeting of photoreceptor ribbon synapses in mouse models of multiple sclerosis

“…In addition to the labeling of the synaptic ribbon complex, we also found immunolabeling of the presynaptic plasma membrane of the active zone (arrows in A1, A2) and of the postsynaptic plasma membrane of horizontal cells in direct apposition to the active zone (dashed arrows in A2, B2, B3). Arrowhead in (B3) shows CASPR1 immunosignal on a vesicle in the peri‐active zone, a site of endocytic vesicle retrieval (Wahl et al , , ). The arrowhead in (B2) denotes a presynaptic plasma membrane‐associated CASPR1 immunosignal.…”

“…The large extracellular portions of CASPR1/CNTN1 are exposed to the extracellular space and are thus accessible for binding to auto‐antibodies. CASPR1/CNTN1 bound to ribbon‐associated synaptic vesicles could be targeted to the ribbon via endocytosis that is known to be enriched at the peri‐active zone (Wahl et al , , ). In agreement with this proposal, we found CASPR1‐positive vesicles in close vicinity to the peri‐active zone of rod photoreceptor synapses (Fig EV1B3).…”

“…MOG/CFA‐ and CFA‐injected SypHy reporter mice were sacrificed on day 7, day 8, or day 9 after injection and their retinas were isolated within 5 min of post‐mortem as described in Wahl et al (). Briefly, enucleated eyes were punctured at anterior side using thin syringe needle and bisected at the equatorial plane.…”

“…SDS–PAGE and Western blotting experiments were performed as previously described (Schmitz et al , ; Wahl et al , ; Eich et al , ). Protein determination of retinal samples dissolved in Laemmli SDS sample buffer: Mouse retinas were isolated 5 min post‐mortem and dissolved in 100 μl hot (96°C) Laemmli sample buffer.…”

The Calcineurin-Binding, Activity-Dependent Splice Variant Dynamin1xb Is Highly Enriched in Synapses in Various Regions of the Central Nervous System

“…For rapid freezing, small tissue blocks (about 1 mm 3 ) in volume were dissected and plunge-frozen, as previously described (Wahl et al, 2013, 2016; Dembla et al, 2014). …”