ABSTRACTThe remarkable plasticity of Schwann cells (SCs) enables the acquisition of repair-specific functions essential for peripheral nerve regeneration. We hypothesized that this plastic potential is manifested in stromal SCs found within mostly benign-behaving peripheral neuroblastic tumors. To shed light on the cellular state and impact of stromal SCs, we combined transcriptome and proteome profiling of human ganglioneuromas and neuroblastomas, rich and poor in SC-stroma, respectively, as well as human injured nerve explants, rich in repair SCs. The results revealed a nerve repair-characteristic gene expression signature of stromal SCs. In turn, primary repair SCs had a pro-differentiating and anti-proliferative effect on aggressive neuroblastoma cell lines after direct and trans-well co-culture. Within the pool of secreted stromal/repair SC factors, we identified EGFL8, a matricellular protein with so far undescribed function, to induce neuronal differentiation of neuroblastoma cell lines. This study indicates that human SCs undergo a similar adaptive response in two patho-physiologically distinct situations, peripheral nerve injury and tumor development. This response is mediated by EGFL8 and other SC derived factors, which might be of therapeutic value for neuroblastic tumors and nerve regeneration.SYNOPSISIn order to investigate the nature of stromal Schwann cells in benign peripheral neuroblastic tumors (ganglioneuromas), we compared the cellular state of stromal Schwann cells with repair-associated Schwann cells emerging in peripheral nerves after injury.Stromal Schwann cells in ganglioneuromas and repair Schwann cells in injured nerves share the expression of nerve repair-associated genes.Neuroblastoma cell lines, derived from high-risk metastatic peripheral neuroblastic tumors (neuroblastomas), respond to primary repair Schwann cells and their secretome with increased neuronal differentiation and reduced proliferation.Stromal and repair Schwann cells express the matricellular protein EGFL8, which is capable to induce neuronal differentiation of neuroblastoma cell lines in recombinant form.THE PAPER EXPLAINEDProblemIn response to peripheral nerve damage, Schwann cells (SCs) are able to transform into specialized repair cells essential for nerve cell regeneration. Our previous studies indicated that this reactive/adaptive potential of human SCs is not restricted to injured nerve cells but also emerges in response to peripheral neuroblastic tumor cells. The usually benign subtypes of peripheral neuroblastic tumors, i.e. ganglioneuroblastomas and ganglioneuromas, contain neuronal differentiating tumor cells and are pervaded by various portions of stromal SCs. Of note, the amount of stromal SCs correlates with a favorable tumor behavior and increased patient survival, whereas aggressive subtypes of peripheral neuroblastic tumors, i.e. neuroblastomas, usually lack stromal SCs and have bad prognosis. This enigma prompted us to investigate the molecular wiring and functional state of stromal SCs versus injury-associated repair SCs and how SC signals could be leveraged as therapeutics.ResultOur study revealed that the cellular state of stromal SCs in ganglioneuromas is in many aspects very similar to human repair SCs in injured nerves as both, stromal SCs and repair SCs, are equipped with distinct nerve repair-associated functions. Hence, we exposed different cell lines, derived from high-risk metastatic neuroblastomas, to primary repair SCs or their secretome. The results demonstrated that repair SCs had a pro-differentiating and anti-proliferative effect of on neuroblastoma cell lines upon direct and/or indirect contact. Searching for secreted anti-tumor factors by transcriptome and proteome analyses identified that the matricellular protein EGFL8 was highly expressed in injured nerves and ganglioneuromas. EGFL8 gene expression in peripheral neuroblastic tumors further correlated with increased patient survival. Indeed, treatment of neuroblastoma cell lines with recombinant EGFL8 promoted neuronal differentiation and present EGFL8 as a novel neuritogen.ImpactThese findings demonstrate that stromal SCs are equipped with the tools to exert nerve repair-associated functions on peripheral neuroblastic tumor cells and the tumor microenvironment. We further show that the pool of secreted stromal/repair SC molecules contains yet uncharacterized factors with a therapeutic potential for aggressive neuroblastomas. We conclude that the inherent plasticity (reactive/adaptive potential) of SCs is responsible for the development of usually benign ganglioneuroblastomas and ganglioneuromas and, thus, is of utmost interest to be exploited in future treatment approaches for aggressive neuroblastoma subtypes.
Schwann cell plasticity regulates neuroblastic tumor cell differentiation via epidermal growth factor-like protein 8