Abstract
The phenotype of the spontaneous mutant mouse hop-sterile (hop) is characterized by a hopping gait, polydactyly, hydrocephalus, and male sterility. Previous analyses of the hop mouse revealed a deficiency of inner dynein arms in motile cilia and a lack of sperm flagella, potentially accounting for the hydrocephalus and male sterility. The etiology of the other phenotypes and the location of the hop mutation remained unexplored. Here we show that the hop mutation is located in the Ttc26 gene and impairs Hedgehog (Hh) signaling. Expression analysis showed that this mutation led to dramatically reduced levels of the Ttc26 protein, and protein-protein interaction assays demonstrated that wild-type Ttc26 binds directly to the Ift46 subunit of Intraflagellar Transport (IFT) complex B. Although IFT is required for ciliogenesis, the Ttc26 defect did not result in a decrease in the number or length of primary cilia. Nevertheless, Hh signaling was reduced in the hop mouse, as revealed by impaired activation of Gli transcription factors in embryonic fibroblasts and abnormal patterning of the neural tube. Unlike the previously characterized mutations that affect IFT complex B, hop did not interfere with Hh-induced accumulation of Gli at the tip of the primary cilium, but rather with the subsequent dissociation of Gli from its negative regulator, Sufu. Our analysis of the hop mouse line provides novel insights into Hh signaling, demonstrating that Ttc26 is necessary for efficient coupling between the accumulation of Gli at the ciliary tip and its dissociation from Sufu.
Highlights
The Hedgehog (Hh) signaling pathway plays critical roles in embryonic development, wound healing, and tumorigenesis [1,2,3,4]
We used positional cloning to localize the hop mutation to the Ttc26 gene, which encodes a component [52] of Intraflagellar Transport (IFT) complex B
We hypothesized that the breeding history of the hop mouse line could facilitate further genetic mapping because the hop mutation had been transferred from an undefined genetic background onto the BALB/c background at The Jackson Laboratory, through repeated backcrosses
Summary
The Hedgehog (Hh) signaling pathway plays critical roles in embryonic development, wound healing, and tumorigenesis [1,2,3,4]. It is activated when the receptor protein Patched-1 (Ptch1) binds to one of the secreted Hh lipoproteins, Sonic Hh (Shh), Indian Hh, or Desert Hh [5,6]. Gli has no repressor form; it is regulated transcriptionally through activation of the other two Gli proteins [11] Hh signaling activates both Gli2-F and Gli3-F and blocks their processing into repressors [9,10]. Data on the full range and importance of various posttranscriptional modifications of Gli are still emerging [12,13,14,15,16], it is clear that a crucial step in the activation of Gli2-F and Gli3-F is their dissociation from Sufu [17,18]
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