Oral Presentation 30th Australian and New Zealand Bone and Mineral Society Annual Scientific Meeting 2020

Single-cell mapping of bone identifies key intercellular interactions within the endosteal bone microenvironment (#8)

James T Smith 1 2 , Ryan C Chai 1 2 , Alexander P Corr 1 2 , Matthew A Summers 1 2 , Weng Hua Khoo 1 , John A Eisman 1 2 3 , Peter I Croucher 1 2 , Paul A Baldock 1 2 3
  1. Healthy Aging Theme, Bone Biology Laboratory, Garvan Institute of Medical Research, Darlinghurst, NSW, Australia
  2. Faculty of Medicine, St Vincent's Clinical School, UNSW Australia, Sydney, NSW, Australia
  3. School of Medicine Sydney, University of Notre Dame Australia, Sydney, NSW, 2010

Bone remodelling is enacted by osteoclasts and osteoblasts, however, the endosteal surface, a major site for bone remodelling, is populated by a complex and incompletely defined cellular milieu. Consequently, our understanding of the cell types and signalling pathways that influence bone remodelling remains incomplete. We hypothesised that ligand-receptor mapping at the single-cell resolution can identify which cell types within the endosteal microenvironment may regulate bone remodelling.

Single-cell RNA sequencing was performed on 134,000 cells from the endosteal bone surface and bone marrow of 25x male C57BL/6 mice via the 10X Chromium platform to identify cell types present on the endosteal surface. Ligand-receptor screening was then used to identify putative intercellular interactions.

34 separate cell types were identified by their distinct expression patterns. Ligand-receptor screening determined that cells of the osteoblast lineage exhibit the greatest potential for intercellular interactions, displaying significant interactions with endothelial cells (73 ligand-receptor pairs), NK cells (29 pairs) and multiple populations of monocytes/macrophages (11 populations, 6-21 pairs).

Initial focus upon osteoblastic and monocyte/macrophage lineages identified 6 differentiation stages of osteoblasts and 14 monocyte/macrophage sub-clusters. Osteoblast/macrophage interactions were most enriched between Cxcl12+ mesenchymal stem cells (MSCs) and a population of Vcam1+ macrophages found only on the endosteal surface.  All ligand-receptor pairs involved MSC ligands and macrophage receptors, with Gas6-Axl and Cxcl12-Sdc4 the top-ranking pairs. In contrast, interactions involving mature osteoblasts/macrophages were entirely dependent on ECM genes (e.g. Col1a1/2). No significant interactions were identified involving early-stage monocytes or their progenitors.

This combination of transcriptomic techniques and intercellular ligand-receptor screening has characterised key interaction partners of osteoblastic cells, implicating a subset of bone surface macrophages as potential key regulators of bone remodelling via their interactions with osteoblast precursors. This dataset could revolutionise our understanding of intercellular communications within the bone microenvironment, with numerous interactions still to be explored.