TNAP and PHOSPHO1 function synergistically to afford critical control over the mineralisation of the postnatal murine skeleton

Abstract Biomineralisation is essential for skeletal integrity, yet the synergistic roles of tissue non-specific alkaline phosphatase (TNAP) and PHOSPHO1 in postnatal bone mineralisation remain poorly defined. To decipher this, we generated a novel murine model in which Alpl was deleted in Prx1- expressing cells (AlplPrx1/Prx1) in mice with a global Phospho1-/- deficiency to overcome the perinatal lethality that arises upon dual global deletion. Using a multi-modal approach to spatially phenotype the limbs of these animals, we reveal mice lacking both TNAP and PHOSPHO1 exhibit a distinct lack of mineralisation and altered anatomical structure at postnatal day 1 (PN1) and 3-weeks of age. Although viable, these mice did not thrive due to their reduced size, thus further investigations were conducted on mice with a heterozygous deletion of TNAP (Alplwt/Prx1;Phospho1-/-). Although smaller than wild-types at PN1 and 3 weeks old, these mice did not display the gross limb deformations observed in the homozygous animals and the single, functioning Alpl allele rescued the loss of biomineralisation observed following dual phosphatase deletion. At 6-weeks of age, compromised epiphyses and metaphyses were only seen in AlplPrx1/Prx1 animals. Further, we found that tibial geometry and porosity was significantly altered by Phospho1 deletion (Phospho1-/-), which was compounded in the Alplwt/Prx1;Phospho1-/- mice and linked to alterations in collagen configuration, matrix mineralisation and growth plate deformities. Together, our findings establish the mechanistic framework for TNAP and PHOSPHO1 in permissive biomineralisation, providing critical insights into this fundamental process. Significance Statement Biomineralisation is essential for skeletal development and is critically dependent on phosphatases that release inorganic phosphate for hydroxyapatite formation. Our study investigates the dual role of PHOSPHO1 and TNAP in this process, using a novel murine knockout model. Deletion of both enzymes results in complete loss of bone mineralisation, demonstrating their critical synergistic function. Further, we show that PHOSPHO1 and TNAP exhibit distinct, spatially-restricted functions in the tibia and thus enhances our understanding of the fundamentals processes underpinning biomineralisation. These findings also have clinical relevance as they have the potential to inform on treatment strategies for hypo- and hyper-mineralised pathologies. Competing Interest Statement The authors have declared no competing interest. Footnotes Competing Interest Statement: The authors declare no conflicts of interest.