Tulp1 deficiency causes early-onset retinal degeneration through affecting ciliogenesis and activating ferroptosis in zebrafish

Abstract

Mutations in TUB-like protein 1 (TULP1) are associated with severe early-onset retinal degeneration in humans. However, the pathogenesis remains largely unknown. There are two homologous genes of TULP1 in zebrafish, namely tulp1a and tulp1b. Here, we generated the single knockout (tulp1a−/− and tulp1b−/−) and double knockout (tulp1-dKO) models in zebrafish. Knockout of tulp1a resulted in the mislocalization of UV cone opsins and the degeneration of UV cones specifically, while knockout of tulp1b resulted in mislocalization of rod opsins and rod-cone degeneration. In the tulp1-dKO zebrafish, mislocalization of opsins was present in all types of photoreceptors, and severe degeneration was observed at a very early age, mimicking the clinical manifestations of TULP1 patients. Photoreceptor cilium length was significantly reduced in the tulp1-dKO retinas. RNA-seq analysis showed that the expression of tektin2 (tekt2), a ciliary and flagellar microtubule structural component, was downregulated in the tulp1-dKO zebrafish. Dual-luciferase reporter assay suggested that Tulp1a and Tulp1b transcriptionally activate the promoter of tekt2. In addition, ferroptosis might be activated in the tulp1-dKO zebrafish, as suggested by the up-regulation of genes related to the ferroptosis pathway, the shrinkage of mitochondria, reduction or disappearance of mitochondria cristae, and the iron and lipid droplet deposition in the retina of tulp1-dKO zebrafish. In conclusion, our study establishes an appropriate zebrafish model for TULP1-associated retinal degeneration and proposes that loss of TULP1 causes defects in cilia structure and opsin trafficking through the downregulation of tekt2, which further increases the death of photoreceptors via ferroptosis. These findings offer insight into the pathogenesis and clinical treatment of early-onset retinal degeneration.

SRRM2 splicing factor modulates cell fate in early development

“…ETV4 expression can impair ectodermal commitment ( Akagi et al, 2015 ), and also undergoes an exon-skipping event in SRRM2 -depleted differentiated human cells ( Cui et al, 2023 ). TULP1 is important for neuron and photoreceptor cell development ( Jia et al, 2022 ), and Meg3 transcripts sponge the microRNA miR-423-5p that in turn downregulates Srf expression ( Cheng et al, 2020 ), which is upregulated in Srrm2 +/− cells.…”

FKBP5 genetic variants are associated with respiratory- and sleep-related parameters in Chinese patients with obstructive sleep apnea

“…However, there has been no relevant report available on the relationship between the two genes and sleep disorders thus far. Tubby-like protein 1 ( TULP1 ) is located approximately 60 kb downstream from FKBP5 and has been reported to be important for vesicular trafficking of photoreceptor proteins and associated with early-onset retinal degeneration ( Remez et al, 2020 ; Jia et al, 2022 ). Armadillo repeat containing 12 ( ARMC12 ) is located 3 kb upstream of FKBP5 and has been reported to promote neuroblastoma progression and play crucial roles in spermiogenesis ( Li et al, 2018 ; Shimada et al, 2021 ).…”

Male germ cell-associated kinase (MAK) is required for axoneme formation during ciliogenesis in zebrafish photoreceptors

“…Recently, it was reported that photoreceptor degeneration in ciliogenesis-defective zebrafish Tulp1 mutants is associated with pathological features of ferroptosis (Jia et al, 2022), which is a new form of cell death linked to iron-dependent accumulation of lipid peroxides (Yu et al, 2017), suggesting that ciliogenesis defects may trigger non-apoptotic cell death mechanisms. However, we found that overexpression of anti-apoptotic protein, Bcl2, effectively suppresses degeneration in both rods and cones, suggesting that in mak mutants, both depend on Bax-mediated apoptosis.…”

“…Interestingly, early pyknotic nuclei and later progressive shrinkage of the photoreceptor layer was also observed in zebrafish ift88 and traf3ip1 mutants, both of which show defects in axoneme formation (Bahadori et al, 2003; Doerre and Malicki, 2002; Sukumaran and Perkins, 2009; Wang et al, 2022). Recently, it was reported that photoreceptor degeneration in ciliogenesis-defective zebrafish Tulp1 mutants is associated with pathological features of ferroptosis (Jia et al, 2022), which is a new form of cell death linked to accumulation of lipid peroxides (Yu et al, 2017). Although both rod and cone degeneration in mak mutants depend on apoptosis, it is interesting to examine why cones show progressive shrinkage in the absence of axoneme formation.…”