AUTHOR=Takahashi Kei , Kwok Jennifer C. , Sato Yu , Aguirre Gustavo D. , Miyadera Keiko 

TITLE=Molecular characterization of MAP9 in the photoreceptor sensory cilia as a modifier in canine RPGRIP1-associated cone-rod dystrophy

JOURNAL=Frontiers in Cellular Neuroscience

VOLUME=17

YEAR=2023

URL=https://www.frontiersin.org/journals/cellular-neuroscience/articles/10.3389/fncel.2023.1226603

DOI=10.3389/fncel.2023.1226603

ISSN=1662-5102

ABSTRACT=<p>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 <italic>RPGRIP1</italic> 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 <italic>RPGRIP1</italic> (<italic>RPGRIP1</italic><sup><italic>ins</italic>44/<italic>ins</italic>44</sup>) is the primary disease locus while an additional homozygous variant in <italic>MAP9</italic> (microtubule associated protein 9) (<italic>MAP9<sup>aff/aff</sup></italic>) 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 <italic>MAP9</italic> variant, and investigated the molecular function of MAP9 in primary cilia using the <italic>RPGRIP1</italic>-associated oligogenic canine cone-rod dystrophy model as well as cultured cells. Both functionally and structurally, the <italic>RPGRIP1</italic><sup><italic>ins</italic>44/<italic>ins</italic>44</sup><italic>MAP9<sup>aff/aff</sup></italic> retina exhibited progressive cone photoreceptor degeneration starting earlier than the retina affected by <italic>RPGRIP1</italic><sup><italic>ins</italic>44/<italic>ins</italic>44</sup> 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 <italic>RPGRIP1</italic><sup><italic>ins</italic>44/<italic>ins</italic>44</sup><italic>MAP9<sup>aff/aff</sup></italic> double homozygote cone-rod dystrophy canine model.</p>