WNT vampirization by glioblastoma leads to tumor growth and neurodegeneration

Abstract

22Glioblastoma (GB) is the most lethal brain tumor due to its high proliferation, 23 aggressiveness, infiltration capacity and resilience to current treatments. Activation of 24 the Wingless-related-integration-site (WNT) pathway is associated with a bad 25 prognosis. Using Drosophila and primary xenograft models of human GB, we describe 26 a mechanism that leads to the activation of WNT signaling [Wingless (Wg) in 27 Drosophila] in tumor cells. GB cells display a network of tumor microtubes (TMs) which 28 enwraps neurons, accumulates Wg receptor Frizzled1 (Fz1), and, thereby, actively 29 depletes Wg from the neurons. Consequently, GB cells proliferate due to β-catenin 30 activation, and neurons degenerate due to Wg signaling extinction. This novel view 31 explains both neuron-dependent tumor progression, and also the neural decay 32 associated with GB. 33 Keywords 34Neuron, Glia, Cancer, wingless, Frizzled, Glioblastoma, 35 Cytoneme, Wg-depletion. 36 37The WNT canonical pathway is activated upon the ligand "Wingless-related integration 48 site" (WNT) binding to specific receptors (LRPs and FZD) in the plasma membrane. As 49 a consequence, the destruction complex (APC and Axin) is inactivated and β-catenin 50 (armadillo in Drosophila) is released. Further, β-catenin translocates into the cell 51 nucleus where it promotes the expression of target genes (i.e. Cyclin D1 and Myc) 7-8 . 52The WNT pathway is conserved through metazoans and it plays a central role in brain 53 development 9 , adult neuronal physiology 10 and synaptogenesis 11 . Perturbations in 54WNT signaling are associated with neural deficits, Alzheimer´s disease and brain 55 cancer, most notably GB 12 . WNT and FZD signaling can be deregulated in 56 glioblastoma 13-14 (reviewed in 15 ). In particular, one of the hallmarks of bad prognosis is 57 the accumulation of ß-catenin in tumoral cells [16][17] , indicating an activation of WNT/FZD 58 pathway 18 . 59 GB cells extend ultra-long membrane protrusions that interconnect tumor cells 19 . 60These tumor microtubes (TMs) are associated with the worst prognosis in molecular 61 subtypes of human gliomas. TMs contribute to invasion and proliferation, resulting in 62 effective brain colonization by GB cells. Moreover, TMs constitute a multicellular 63 network that connects GB cells over long distances, a feature that likely provides 64 resistance against radiotherapy, chemotherapy and surgery [19][20] . Considering the many 65 cytological similarities of TMs and tunneling nanotubes (TNTs) 21 , it seems that TMs in 66 aggressive gliomas are the in vivo correlate of TNTs described in cell culture. In 67 addition, TMs seem akin to a basic mechanism of cell-cell connection and molecular 68 communication called "cytonemes" in Drosophila 22 . Growth Associated Protein-43 69 (GAP43) is essential for the development of TMs and, thus, the tumor cell network 70 which is associated with GB progression 19 . However, many aspects of this 71 paradigmatic finding in glioma biology are still unexplored, including its i...

Cytonemes, Their Formation, Regulation, and Roles in Signaling and Communication in Tumorigenesis

“…TMs share many characteristics with cytonemes, they are actin-based projections and they are marked by several cytoneme markers, including Ihog, LifeActin, GMA, GPI, myosin light chain (MLC), and the nonmuscle type 2 myosin, spaghetti squash (sqh). Moreover, this study [86] showed in a Drosophila glioma model that the glioma network is dependent on the fly GAP43-like gene ( igloo , igl ), as has been described in human tumor cells. The glioma network does not develop upon igl silencing.…”

“…Various laboratories described the existence and function of cytonemes in the trachea, myoblast, ovary, and brain cells of Drosophila . The functions of filopodia in neurons are related to path finding [9] and synapse formation [8], chemotaxis in Dictyostelium discoideum [81], cell migration and adhesion [82,83], cell signaling [22,40,52,55,84], and cancer progression and metastasis [83,85,86,87].…”

“…TMs stability in GB is sensitive to GAP43 expression in tumoral cells. Moreover, downregulating Nrg ( Nrg-RNAi ), which is known to prevent epithelial cytoneme formation, resulted in a reduction of the TM-like processes in GB [86]. Moreover, TMs accumulate Frizzled1 receptor (Fz1) that mediates Wg signaling (Figure 3) [86].…”

“…Moreover, downregulating Nrg ( Nrg-RNAi ), which is known to prevent epithelial cytoneme formation, resulted in a reduction of the TM-like processes in GB [86]. Moreover, TMs accumulate Frizzled1 receptor (Fz1) that mediates Wg signaling (Figure 3) [86]. Thus, there are molecular and functional similarities between cytonemes and TMs; however, the term cytoneme is used for physiological situations, and TMs is restricted to the tumoral condition.…”