Background: Osteoclasts are terminally differentiated cells of the monocyte/macrophage lineage that resorb bone matrix. Bone destruction in rheumatoid arthritis is mainly attributable to the abnormal activation of these cells. We have previously shown that environmental arginine is a strict requirement for osteoclastogenesis [1].

Objectives: Whether availability of other semi- or non-essential amino acids is also important for the differentiation process is unknown. This study aims to describe the importance of the availability of the amino acid serine for osteoclast differentiation.

Methods: Bone-marrow-derived macrophages were stimulated with M-CSF and RANKL in the presence or absence of the respective amino acids in the medium. This in vitro approach was used for multiple applications including RNA sequencing, stable isotope tracing and CRISPR/Cas9 mediated knockout of genes. K/BxN serum transfer arthritis was induced in C57BL/6WT that were maintained on a serine/glycine deficient or control diet and treated with a phosphoglycerate dehydrogenase inhibitor or vehicle only. Serum amino acid levels were analysed using mass spectrometry. TRAP staining was performed on tissue slides from the inflamed paws.

Results: A conserved transcriptional signature of Receptor activator of nuclear factor kappa-Β ligand stimulated preosteoclasts starved from four different amino acids shows that the serine synthesis pathway is among the top induced biosynthetic pathways. Osteoclast precursors critically depend on the availability of serine in order to differentiate into mature osteoclasts. The source of serine is either environmental or this amino acid is produced endogenously by a highly efficient serine synthesis pathways in precursor cells if extracellular serine is limited. Pharmacological inhibition or CRISPR/Cas9-mediated gene knockout of phosphoglycerate dehydrogenase (PHGDH), the enzyme that catalyzes the first reaction of the serine synthesis pathway, completely abolishes the ability of preosteoclasts to differentiate into osteoclasts when environmental serine is limited. This pathway is also functional in vivo, as a serine and glycine free diet combined with pharmacological inhibition of PHGDH alleviates the clinical course in an arthritis model by specifically reducing osteoclast generation in inflamed joints and consequently ameliorates inflammation-induced joint destruction.

Conclusion: The PHGDH mediated serine synthesis pathway represents an efficient compensatory metabolic pathway in osteoclast generation in vitro and in vivo when serine availability is limited, which might be an interesting target for therapeutic intervention in osteoclast mediated pathologies such as inflammatory arthritis.

REFERENCES: [1] Brunner, J.S., Vulliard, L., Hofmann, M. et al. Environmental arginine controls multinuclear giant cell metabolism and formation. Nat Commun 11, 431 (2020).

Acknowledgements: NIL.

Disclosure of Interests: None declared.

© The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.