Studies on functional differentiation of xpr1a and xpr1b genes in zebrafish

Xenotropic and polytropic retrovirus receptor 1 (XPR1) is known to be involved in various biological processes, including phosphate homeostasis, cellular signaling, brain and vascular mineralization, whereas its specific contribution to bone development remains incompletely characterized. Due to genome duplication in teleosts, zebrafish Danio rerio possess two paralogous genes of XPR1 namely xpr1a and xpr1b, whose functional divergence remains unclear. The amino acid sequence similarity between zebrafish xpr1a and xpr1b was 83.26%. In situ hybridization demonstrated overlapping localization in the head and spinal cord at 24-48 hpf, while diverged by 72 hpf, with xpr1a becoming restricted to the head while xpr1b persisted in both regions. CRISPR/Cas9 was used to generate xpr1a and xpr1b mutants. The xpr1a mutants are comparatively healthy, viable but with mild growth reduction, whereas the xpr1b mutants display high mortality, reduced body length and severe vertebral deformities. Interestingly, all the double mutants died at the embryonic stage. Moreover, to further investigate the molecular and regulatory mechanisms, we conducted comparative transcriptome analysis on bone and brain tissues from xpr1b⁺/⁺ and xpr1b^-/^- zebrafish. In bone tissue, 6749 DEGs were identified, comprising 3846 upregulated and 2903 downregulated genes. These DEGs were mainly enriched in the MAPK signaling pathway, Wnt signaling pathway, cysteine and methionine metabolism, and ECM-receptor interaction. RT-qPCR validated results showed that seven osteogenesis-related genes (col1a1a, sp7, runx2b, col1a2, col1a1b, alp1 and entpd5), and two phosphate homeostasis related genes (slc20a2 and pdgfba), which are essential for skeletal mineralization and phosphate homeostasis, exhibited significantly downregulated expression in bone tissue of xpr1b mutant zebrafish. These results highlight the pivotal role of xpr1b in regulating skeletal mineralization and phosphate metabolism, thereby elucidating the functional specialization of XPR1 paralogs while providing a theoretical basis for understanding bone developmental mechanism in teleost vertebrates.