Syntaxin 3 is essential for photoreceptor outer segment protein trafficking and survival

Abstract

Trafficking of photoreceptor membrane proteins from their site of synthesis in the inner segment (IS) to the outer segment (OS) is critical for photoreceptor function and vision. Here we evaluate the role of syntaxin 3 (STX3), in trafficking of OS membrane proteins such as peripherin 2 (PRPH2) and rhodopsin. Photoreceptor-specific Stx3 knockouts [Stx3f/f(iCre75) and Stx3f/f(CRX-Cre)] exhibited rapid, early-onset photoreceptor degeneration and functional decline characterized by structural defects in IS, OS, and synaptic terminals. Critically, in the absence of STX3, OS proteins such as PRPH2, the PRPH2 binding partner, rod outer segment membrane protein 1 (ROM1), and rhodopsin were mislocalized along the microtubules to the IS, cell body, and synaptic region. We find that the PRPH2 C-terminal domain interacts with STX3 as well as other photoreceptor SNAREs, and our findings indicate that STX3 is an essential part of the trafficking pathway for both disc (rhodopsin) and rim (PRPH2/ROM1) components of the OS.

Pathogenic STX3 variants affecting the retinal and intestinal transcripts cause an early-onset severe retinal dystrophy in microvillus inclusion disease subjects

“…We show that STX3B rather than STX3A is highly expressed in the human retina, where the protein is found in the inner and outer segments of rod and cone photoreceptors, and in both plexiform layers, where the synaptic endings of photoreceptors and bipolar cells reside. Inactivation of Stx3 in murine rod photoreceptors resulted in their rapid degeneration as shown here and in a very recent study (Kakakhel et al 2020 ). The non-cell autonomous loss of cones in our knockout animals is likely attributable to a bystander effect, as seen in a number of retinal disorders (Brockerhoff and Fadool 2011 ; Lewis et al 2010 ).…”

“…We demonstrate that STX3B is highly expressed in human retina and that the protein is enriched in the inner and outer segments of photoreceptors and in ribbon synapses of the human retina. Finally, we show that the inactivation of Stx3 in murine rod photoreceptors leads to a progressive degeneration of photoreceptors, corroborating a recently published study that used a different Stx3 knockout mouse line (Kakakhel et al 2020 ). Our study demonstrates that STX3 is essential for the function of the mammalian retina and that human variants affecting STX3B are associated with retinal dysfunction.…”

“…Given the high level of STX3 expression in the murine and non-mammalian retina and roles for Stx3 in protein trafficking in photoreceptors (Chuang et al 2007 ; Kakakhel et al 2020 ; Mazelova et al 2009 ; Zulliger et al 2015 ), and in neurotransmitter release at the first two synapses in the visual throughput pathway (Curtis et al 2008 , 2010 ; Datta et al 2017 ; Liu et al 2014 ) we asked whether MVID subjects with STX3 variants might have visual impairment. To address this question, we studied five newly identified MVID subjects in which we found homozygous STX3 variants; in addition, we investigated five previously described MVID subjects with STX3 variants (Alsaleem et al 2017 ; Julia et al 2019 ; Maddirevula et al 2019 ; Wiegerinck et al 2014 ) for the presence or absence of symptoms of marked visual impairment.…”

“…At age 5 weeks, however, rhodopsin was present in the outer segments and partly mislocalized to the OPL, indicating that proper rhodopsin trafficking is affected in the absence of Stx3. Similarly, a recent study using conditional knockout mouse lines that had Stx3 inactivated in rods as well as cones during early development, showed that rhodopsin, PRPH2 and ROM1 can get transported to the outer segments in the absence of Stx3 (Kakakhel et al 2020 ). However, this study also found a significant amount of mislocalized rhodopsin, PRPH2 and ROM1 in photoreceptors that lack Stx3.…”

Ciliary Proteins Repurposed by the Synaptic Ribbon: Trafficking Myristoylated Proteins at Rod Photoreceptor Synapses

“…Thus, the photoreceptor synaptic ribbon appears to have several components in common with the primary cilium, raising the possibility that common functional mechanisms could also prevail at these two compartments. In agreement with this proposal, the t-SNARE protein Syntaxin-3 is essential for vesicle fusion both at the photoreceptor cilium as well as at the synaptic ribbon [61][62][63][64]. Future analyses might reveal further molecular and functional similarities between the synaptic ribbon and primary cilia.…”