14Metastasis is the main cause of death in cancer patients but remains a poorly understood 15 process. Small cell lung cancer (SCLC) is one of the most lethal and most metastatic types of 16 human cancer. SCLC cells normally express neuroendocrine and neuronal gene programs but 17 accumulating evidence indicates that these cancer cells become relatively more neuronal and less 18 neuroendocrine as they gain the ability to metastasize. Here we show that mouse and human 19 SCLC cells in culture and in vivo can grow cellular protrusions that resemble axons. The 20 formation of these protrusions is controlled by multiple neuronal factors implicated in 21 axonogenesis, axon guidance, and neuroblast migration. Disruption of these axon-like 22 protrusions impairs cell migration in culture and inhibits metastatic ability in vivo. The co-option 23 of developmental neuronal programs is a novel molecular and cellular mechanism that 24 contributes to the high metastatic ability of SCLC. 25 65 cells that have transitioned to a more neuronal cell state. 66 5 68 To investigate SCLC migration, we developed an assay in which SCLC cells, which 69 classically grow in culture as floating spheres or aggregates, are grown as a monolayer under 70 Matrigel ((Denny et al., 2016) and Methods). Unexpectedly, we noticed that cells from some 71 SCLC cell lines (N2N1G, 16T, 6PF) derived from the Rb f/f ;p53 f/f (DKO) and Rb f/f ;p53 f/f ;p130 f/f 72 (TKO) genetically engineered mouse models form long cellular protrusions into cell-free spaces 73 ( Figure 1A-B). To determine whether these structures specifically project into cell-free areas or 74 they also exist within monolayers, we cultured a minor fraction of fluorescently-labeled,
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SCLC cells can form long cellular protrusions in culture and in vivo
75GFP positive SCLC cells with control SCLC cells. We found that SCLC cells also form protrusions 76 when they are in close contact with surrounding cancer cells (Figure S1A). Similar mixing 77 experiments performed in subcutaneous allografts also documented the growth of protrusions by 78 SCLC cells in vivo (Figure 1C-D). Finally, similar structures also extend from SCLC micro-79 metastases in the liver in the autochthonous TKO mouse model and after intravenous 80 transplantations of SCLC cells (Figure S1B-C).
81Human SCLC patient-derived xenografts (PDXs) recapitulate many important features of the 82 human disease (e.g. (Gardner et al., 2017, Saunders et al., 2015). To label rare cancer cells 83 within human SCLC PDXs and identify whether they had protrusions in unperturbed tumors, we 84 used DiI tracing. DiI is a lipophilic dye that diffuses within cell membranes and has been widely 85 employed to label projections from individual neurons (Heilingoetter and Jensen, 2016, Mufson 86 et al., 1990). Protrusions from SCLC cells were easily identifiable in two out of three PDX 87 models ( Figure 1E-F). In the 2D monolayer culture system, not all human SCLC cell lines 88 formed protrusions, but NCI-H446 cells formed long protru...