SAM50, dal nucleo al mitocondrio: ha un ruolo nella progressione tumorale?

Mitochondria play essential important roles in cells fate as key regulatory hubs for cell metabolism, energy conversion, calcium homeostasis, redox homeostasis, cell signaling, cell differentiation, aging and cell death. Therefore, it is not surprising that they are also critical for tumor malignancy. The mitochondria can contribute to these many essential functions because of the complexity, the diversity and the plasticity of their proteome. Remarkably, the vast majority of the mitochondrial proteome is encoded in the nuclear genome, translated in the cytoplasm and imported into mitochondria by means of anterograde protein import signaling [9][10]. This is accomplished using five distinct sophisticated protein import machineries that regulate this anterograde signaling, which for the essential rely on targeting sequences expressed by the mitochondrial pre-proteins. These five anterograde protein import pathways strongly depend on the translocase of the outer mitochondrial membrane (TOM) complex, making TOM40, the central channel subunit of this complex, the mitochondrial entry gate. However, we recently characterized a novel mitochondrial protein import pathway that is only dependent on SAM50 channel localized at the outer mitochondrial membrane. We, indeed, demonstrated that Granzyme A (GzmA), Granzyme B (GzmB), Granzyme M (GzmM) and caspase 3 strictly use this novel mitochondrial route and interestingly, their mitochondrial entry is not dependent on target sequences. The work presented in this Ph.D. thesis aims at studying the mechanisms that regulate SAM50 translocase activity. Since post-translational modifications (PTM) such as phosphorylation and dephosphorylation can affect protein conformation and activity, and are profoundly dysregulated in cancer, we, therefore, aim to understand whether this novel pathway has a gain of function in cancer. I first, checked the phosphorylation status of SAM50 in tumoral cells, using the aggressive glioblastoma multiforme tumor model, and in normal astrocytes as control. I found that SAM50 is more phosphorylated in glioma cancer cells compared to control normal astrocytes, and moreover, SAM50 shows a different pattern of phosphorylation between the two counterparts of glioblastoma (GDCs and GSCs). Remarkably, we also found that SAM50-dependent mitochondrial import is potentiated by its phosphorylation, showing that SAM50 translocase activity is dependent on its phosphorylation status. In yeast, SAM50 interacts with SAM35/Metaxin 2 and SAM37/Metaxin 1 to form the SAM complex, where the later subunits cap the opening of SAM50 channel on the cytosolic side of the mitochondrial outer membrane. Unexpectedly, I found that in human glioblastoma cells, SAM50 co-migrates only with Metaxin 2, indicating that SAM complex is different between mammal and yeast. Compellingly, I also showed that the translocase activity of SAM50 disrupts the SAM complex. Collectively, we found that SAM50 channel, central to this novel mitochondrial import pathway, is more phosphorylated in cancer cells compared to normal cells. SAM50 phosphorylation is correlated with an increased anterograde signaling that destabilizes the SAM complex. Under this condition, SAM50-dependent anterograde signaling is likely to have a gain of function in cancer and could be a putative therapeutic target to eliminate glioblastoma multiform.