The neurodegenerative disease Friedreich's ataxia is caused by lower than normal levels of frataxin, an important protein involved in iron–sulfur (Fe-S) cluster biogenesis. An important step in designing strategies to treat this disease is to understand whether increasing the frataxin levels by gene therapy would simply be beneficial or detrimental, because previous studies, mostly based on animal models, have reported conflicting results. Here, we have exploited an inducible model, which we developed using the CRISPR/Cas9 methodology, to study the effects of frataxin overexpression in human cells and monitor how the system recovers after overexpression. Using new tools, which range from high-throughput microscopy to in cell infrared, we prove that overexpression of the frataxin gene affects the cellular metabolism. It also leads to a significant increase of oxidative stress and labile iron pool levels. These cellular alterations are similar to those observed when the gene is partly silenced, as occurs in Friedreich's ataxia patients. Our data suggest that the levels of frataxin must be tightly regulated and fine-tuned, with any imbalance leading to oxidative stress and toxicity.

Cell-to-cell communication it is a fundamental mechanism by which unicellular and multicellular organisms maintain relevant functions as development or homeostasis. Tunneling nanotubes (TNTs) are a type of contact-mediated cell-to-cell communication defined by being membranous structures based on actin that allow the exchange of different cellular material. TNTs have been shown to have unique structural features compared with other cellular protrusions and to contain the cell adhesion molecule N-Cadherin. Here, we investigated the possible role of N-Cadherin and of its primary linker to the actin cytoskeleton, alpha-Catenin in regulating the formation and transfer function of TNTs. Our data indicate that N-Cadherin through its downstream effector alpha-Catenin is a major regulator of TNT formation, ultrastructure, as well as of their ability to transfer material to other cells.

Tunneling nanotubes (TNTs) are open actin- and membrane-based channels, connecting remote cells and allowing direct transfer of cellular material (e.g. vesicles, mRNAs, protein aggregates) from cytoplasm to cytoplasm. Although they are important especially in pathological conditions (e.g., cancers, neurodegenerative diseases), their precise composition and their regulation were still poorly described. Here, using a biochemical approach allowing to separate TNTs from cell bodies and from extracellular vesicles and particles (EVPs), we obtained the full composition of TNTs compared to EVPs. We then focused to two major components of our proteomic data, the CD9 and CD81 tetraspanins, and further investigated their specific roles in TNT formation and function. We show that these two tetraspanins have distinct functions: CD9 participates in the initiation of TNTs, whereas CD81 expression is required to allow the functional transfer of vesicle in the newly formed TNTs, possibly by regulating fusion with the opposing cell.