Mesenchymal stroma drives axonogenesis and nerve-induced aggressiveness in osteosarcoma.

Background: Osteosarcoma (OS), the most common primary bone malignancy, is a leading cause of cancer-related mortality in children and adolescents. Besides genomic abnormalities, several features of tumour microenvironment (TME), including cancer-associated mesenchymal stromal cells (MSC), have been recognized to play a key role in OS progression. The pathogenetic function of de novo innervation in TME has been extensively studied in carcinomas but is still an unexplored area of investigation in sarcomas, including OS.

Methods: We evaluated nerve infiltration in tissue samples from a small cohort of human OS (n = 5) and from canine OS (n = 11), a translational model for the human disease, by βIII-tubulin immunostaining. We then analysed nerve-stroma-tumour crosstalk using direct and indirect co-cultures of dorsal root ganglion (DRG) neurons with OS/tumour-associated mesenchymal stromal cells (MSC and cancer-associated fibroblasts, CAF), both under standard and microfluidic conditions. In particular, we investigated the effects of tumour and stromal cells on axonal tropism and outgrowth by measuring neurite recruitment, length, and branches and, vice versa, the impact of neuron-derived secretome on OS cell proliferation and migration. Finally, we assessed the secretion of pro-neurotrophic mediators, including brain-derived neurotrophic factor (BDNF), interleukin-6 (IL-6), and nerve growth factor (NGF), by MSC, CAF, and OS cells. The functional roles of IL-6 and BDNF were also verified by the blocking antibody Tocilizumab (TCZ) and the neutralizing Anti-BDNF antibody.

Results: We provided evidence of OS innervation within and surrounding the tumour in association with mesenchymal stroma that also corresponded to the most proliferative area of the tumour (Ki-67+). In vitro, both MSC and, to a lesser extent, OS cells promoted axonal growth through cytokine (IL-6) and neuromodulator (BDNF) secretion. Extracellular acidosis - a hallmark of OS aggressiveness - amplified IL-6 release by stromal cells, and its pro-neurogenic effect was prevented by IL-6 blockade. In turn, tumour-associated innervation stimulated OS cell proliferation and migration, eventually driving tumour aggressiveness.

Conclusions: We showed, for the first time, that bone-associated nerves, fostered by the OS microenvironment, promote tumour aggressiveness. Interfering with the nerve-tumour axis, particularly with the signalling associated with mesenchymal stroma, offers novel opportunities for OS treatment.