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Developmental biology - Retinal Disease|
Neurons have antennae?
Previous work in the Chen lab discovered the SPATA7 gene is expressed in all primary cilia in the body. But surprisingly, when this gene mutates, only the primary cilia in photoreceptors are affected. They lose their ability to function — causing visual impairment.
"We began our investigation by determining which proteins interact with the SPATA7 protein. We identified a number of SPATA7-binding proteins and others that do not bind to SPATA7. Both types localize in photoreceptors' connecting cilium, a specialized form of transition zone in all cilia connecting them with the body of the neuron," explains Dharmat. "The connecting cilium are very small, about 1.5 microns long. A human hair can be between 17 and 180 microns thick. This is the first time researchers have looked at proteins within this very small region and, in particular, proteins interacting with SPATA7."
High-resolution and Perseverance Uncover Biological Mystery
Using state-of-the-art super-resolution microscopy (STORM) coupled with cryo-electron tomography and genetic models, researchers discovered that when SPATA7 is present, SPATA7-binding proteins localize throughout the connecting cilium. But in the absence of SPATA7, the binding proteins concentrate at the base of the cilium in a region closest to the body of the neuron — researchers call it the proximal region, leaving a distal region empty of SPATA7-binding proteins. Chen, Dharmat and their colleagues also observed that proteins not binding to SPATA7 always localized in the proximal region, both when SPATA7 was present and when absent.
This suggests a new concept; that two distinct regions exist within the connecting cilium of photoreceptors. One being the proximal zone – where all ciliary proteins reside. The other being the distal region, where SPATA7-binding proteins are localized when SPATA7 is available. Researchers believe that SPATA7 either delivers proteins to the distal region or maintains them there by via a long, elaborate connecting cilium structure unique to photoreceptors.
Additional experiments revealed that without SPATA7, proteins disrupt localization and stability in microtubules, specifically in the distal region of the connecting cilium, which would otherwise provide structural integrity to the cilia.
"Cilia in other cell types also have SPATA7, but these cilia do not have a distal region in the transition zone like the one we discovered in cilia of photoreceptors, therefore they are not affected when SPATA7 is mutated," Dharmat said.
"The presence of this unique distal zone exclusively in the cilia of photoreceptors also explains the biological mystery of non-syndromic blindness seen in both patients and mouse models, that loss of certain transition zone proteins only causes degeneration of cilia in photoreceptors without affecting the cilia on other cell types," Dharmat said.
This research brings to light a novel sub-cellular structure in the cilia of photoreceptors, their biochemical components and how they impact on cilia function – all contributing to a better understanding of cilia and their genetic disorders.
Photoreceptor-specific ciliopathies often affect a structure that is considered functionally homologous to the ciliary transition zone (TZ) called the connecting cilium (CC). However, it is unclear how mutations in certain ciliary genes disrupt the photoreceptor CC without impacting the primary cilia systemically. By applying stochastic optical reconstruction microscopy technology in different genetic models, we show that the CC can be partitioned into two regions: the proximal CC (PCC), which is homologous to the TZ of primary cilia, and the distal CC (DCC), a photoreceptor-specific extension of the ciliary TZ. This specialized distal zone of the CC in photoreceptors is maintained by SPATA7, which interacts with other photoreceptor-specific ciliary proteins such as RPGR and RPGRIP1. The absence of Spata7 results in the mislocalization of DCC proteins without affecting the PCC protein complexes. This collapse results in destabilization of the axonemal microtubules, which consequently results in photoreceptor degeneration. These data provide a novel mechanism to explain how genetic disruption of ubiquitously present ciliary proteins exerts tissue-specific ciliopathy phenotypes.
Authors: Rachayata Dharmat, Aiden Eblimit, Michael A. Robichaux, Zhixian Zhang, Thanh-Minh T. Nguyen, Sung Yun Jung, Feng He, Antrix Jain, Yumei Li, Jun Qin, Paul Overbeek, Ronald Roepman, Graeme Mardon, and Theodore G. Wensel.
This project was funded by the Retina Research Foundation, National Eye Institute R01EY022356, R01EY020540, R01-EY026545, R01-EY07981 and F32EY027171. Suport was also provided by the European Community's Seventh Framework Programmes FP7/2009 under grant agreement no: 241955 684 (SYSCILIA) and by the Netherlands Organization for Scientific Research 685 (NWO Vici-865.12.005).
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3D Cross-section of the architecture of DCC (distal connecting cilium) and PCC (proximal connecting cilium) of Spata7 mutant photoreceptor CC (connecting cilium). Continuous isodense surfaces are colored green (A-microtubule) and purple (B-microtubule). The absence of Spata7 results in mislocalization of DCC proteins without affecting the PCC protein complexes.
Image credit: King's College London.