Takahashi Kei, Kwok Jennifer C, Sato Yu, Aguirre Gustavo D, Miyadera Keiko
Division of Experimental Retinal Therapies, Department of Clinical Sciences & Advanced Medicine, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA, United States.
Front Cell Neurosci. 2023 Aug 15;17:1226603. doi: 10.3389/fncel.2023.1226603. eCollection 2023.
Photoreceptors possess a highly specialized primary cilium containing expanded ciliary membrane discs called the outer segment. The photoreceptor cilium is essential for the maintenance of the outer segment, and pathogenic variants in more than 50 cilia-related genes have been identified as causing non-syndromic inherited retinal diseases in patients. The retinitis pigmentosa GTPase regulator interacting protein 1 (RPGRIP1) is a structural protein localized to the photoreceptor cilium and biallelic variants have been associated with non-syndromic human inherited retinal diseases. In a canine cone-rod dystrophy model, a naturally occurring 44-bp exonic insertion in () is the primary disease locus while an additional homozygous variant in (microtubule associated protein 9) () acts as a modifier associated with early disease onset. MAP9 was originally identified as a microtubule-binding protein stabilizing microtubule structure during both mitosis and interphase in human cell lines. However, the roles of MAP9 in primary cilia, including photoreceptor neurosensory cilia, have not been well understood. Hence, we characterized the pathogenic phenotypes associated with homozygous variant, and investigated the molecular function of MAP9 in primary cilia using the -associated oligogenic canine cone-rod dystrophy model as well as cultured cells. Both functionally and structurally, the retina exhibited progressive cone photoreceptor degeneration starting earlier than the retina affected by alone. Based on immunostaining of canine retinal sections and cultured cells, we found that MAP9 is prominently localized in the basal body of primary cilia and played an important role in maintaining the structure of ciliary microtubule axoneme. These findings suggest that the affected MAP9, together with mutant RPGRIP1, is deprived of critical roles in cilia organization and maintenance resulting in altered cilia structure and function giving rise to early onset and accelerated disease progression in the double homozygote cone-rod dystrophy canine model.
光感受器拥有一种高度特化的初级纤毛，其包含被称为外段的扩展的纤毛膜盘。光感受器纤毛对于外段的维持至关重要，并且已鉴定出50多个与纤毛相关的基因中的致病变体可导致患者出现非综合征性遗传性视网膜疾病。视网膜色素变性GTP酶调节因子相互作用蛋白1（RPGRIP1）是一种定位于光感受器纤毛的结构蛋白，双等位基因变体已与非综合征性人类遗传性视网膜疾病相关联。在犬类视锥 - 视杆营养不良模型中，（）中自然发生的44个碱基对的外显子插入是主要疾病位点，而（微管相关蛋白9）（）中的另一个纯合变体作为与疾病早期发作相关的修饰因子起作用。MAP9最初被鉴定为一种微管结合蛋白，在人类细胞系的有丝分裂和间期都能稳定微管结构。然而，MAP9在初级纤毛（包括光感受器神经感觉纤毛）中的作用尚未得到充分了解。因此，我们表征了与纯合变体相关的致病表型，并使用与相关的寡基因犬类视锥 - 视杆营养不良模型以及培养细胞研究了MAP9在初级纤毛中的分子功能。在功能和结构上，视网膜均表现出比单独受影响的视网膜更早开始的进行性视锥光感受器退化。基于犬类视网膜切片和培养细胞的免疫染色，我们发现MAP9主要定位于初级纤毛的基体，并在维持纤毛微管轴丝的结构中发挥重要作用。这些发现表明，受影响的MAP9与突变的RPGRIP1一起，在纤毛组织和维持中被剥夺了关键作用，导致纤毛结构和功能改变，从而在双纯合子视锥 - 视杆营养不良犬模型中引发早期发作和疾病进展加速。
