ARL2BP, a protein linked to retinitis pigmentosa, is needed for normal photoreceptor cilia doublets and outer segment structure

Abigail R Moye, Ratnesh Singh, Victoria A Kimler, Tanya L Dilan, Daniella Munezero, Thamaraiselvi Saravanan, Andrew F X Goldberg, Visvanathan Ramamurthy | Molecular Biology of the Cell | 2018 Jul 1 | Vol 29(13) | pages 1590–1598 | doi:10.1091/mbc.E18-01-0040

Abstract

The outer segment (OS) of photoreceptor cells is an elaboration of a primary cilium with organized stacks of membranous disks that contain the proteins needed for phototransduction and vision. Though ciliary formation and function has been well characterized, little is known about the role of cilia in the development of photoreceptor OS. Nevertheless, progress has been made by studying mutations in ciliary proteins, which often result in malformed OSs and lead to blinding diseases. To investigate how ciliary proteins contribute to OS formation, we generated a knockout (KO) mouse model for ARL2BP, a ciliary protein linked to retinitis pigmentosa. The KO mice display an early and progressive reduction in visual response. Before photoreceptor degeneration, we observed disorganization of the photoreceptor OS, with vertically aligned disks and shortened axonemes. Interestingly, ciliary doublet microtubule (MT) structure was also impaired, displaying open B-tubule doublets, paired with loss of singlet MTs. On the basis of results from this study, we conclude that ARL2BP is necessary for photoreceptor ciliary doublet formation and axoneme elongation, which is required for OS morphogenesis and vision.

Introduction

Photoreceptors are ciliated neurons that absorb photons and convert light into electrical signals. These neurons are compartmentalized with outer and inner segments (OS and IS) bridged by a narrow connecting cilium (CC, corresponds to the ciliary transition zone) with an extended axoneme (Pearring et al., 2013; Goldberg et al., 2016; May-Simera et al., 2017). The OS contains stacked membranous disks that anchor proteins that participate in phototransduction (Molday and Moritz, 2015). Remarkably, photoreceptors shed 10% of their disks every day (Young, 1967; Goldberg et al., 2016). To maintain the OSs, photoreceptors need to ensure continuous transport of proteins and membranes from their site of synthesis in the IS through the CC to the OS (Young, 1967). In addition to facilitating protein movement, the photoreceptor axoneme is thought to play a structural role in the formation and continual replacement of OS disks (Liu et al., 2004).

Though the photoreceptor cilium is highly modified in function, the basic structure is consistent with immotile primary cilia seen in other tissues, containing 9 + 0 MT morphology that undergoes a switch from doublet microtubules (DMTs) to singlet MTs approximately one-third of the way up the axoneme (Brown et al., 1963; Steinberg and Wood, 1975; Knabe and Kuhn, 1997; Insinna et al., 2008; Wensel et al., 2016). The axonemal DMTs consist of an A-tubule containing 13 tubulin protofilaments joined to a B-tubule containing 10 tubulin protofilaments, with an 11th nontubulin complex linking the inner junction of A and B tubules (see Figure 7A later in this article) (Linck and Stephens, 2007; Nicastro et al., 2011; Pigino et al., 2012; Linck et al., 2014; Ichikawa et al., 2017). Defects in the structural integrity of photoreceptor CC/axoneme lead to retinal degenerative diseases such as retinitis pigmentosa (RP), Lebers congenital amaurosis, and multiple ciliopathies (Pierce et al., 1999; Ramamurthy and Cayouette, 2009; Omori et al., 2010; Boldt et al., 2011; Bujakowska et al., 2017). For example, mice lacking retinitis pigmentosa 1 (RP1), a protein that links the OS disks to the axoneme, displayed shortened axonemes and disordered OS disk structure (Liu et al., 2003, 2004).

Conversely, the absence of male germ-cell associated kinase (MAK) in murine retina resulted in extended axonemes, as well as disorganized OS disks (Omori et al., 2010). Despite the importance of the ciliary axoneme in photoreceptor structure and function, relatively little is known of the mechanism and players behind ciliogenesis and disk organization in the OS.

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