The lysosomal calcium channel TRPML1, whose mutations cause the lysosomal storage disorder (LSD) mucolipidosis type IV (MLIV), contributes to upregulate autophagic genes by inducing the nuclear translocation of the transcription factor EB (TFEB). Here we show that TRPML1 activation also induces autophagic vesicle (AV) biogenesis through the generation of phosphatidylinositol 3-phosphate (PI3P) and the recruitment of essential PI3P-binding proteins to the nascent phagophore in a TFEB-independent manner. Thus, TRPML1 activation of phagophore formation requires the calcium-dependent kinase CaMKKβ and AMPK, which increase the activation of ULK1 and VPS34 autophagic protein complexes. Consistently, cells from MLIV patients show a reduced recruitment of PI3P-binding proteins to the phagophore during autophagy induction, suggesting that altered AV biogenesis is part of the pathological features of this disease. Together, we show that TRPML1 is a multistep regulator of autophagy that may be targeted for therapeutic purposes to treat LSDs and other autophagic disorders.

Selective removal of dysfunctional mitochondria via autophagy is crucial for the maintenance of cellular homeostasis. This event is initiated by the translocation of the E3 ubiquitin ligase Parkin to damaged mitochondria, and it requires the Serine/Threonine-protein kinase PINK1. In a coordinated set of events, PINK1 operates upstream of Parkin in a linear pathway that leads to the phosphorylation of Parkin, Ubiquitin, and Parkin mitochondrial substrates, to promote ubiquitination of outer mitochondrial membrane proteins. Ubiquitin-decorated mitochondria are selectively recruiting autophagy receptors, which are required to terminate the organelle via autophagy. In this work, we show a previously uncharacterized molecular pathway that correlates the activation of the Ca²⁺-dependent phosphatase Calcineurin to Parkin translocation and Parkin-dependent mitophagy. Calcineurin downregulation or genetic inhibition prevents Parkin translocation to CCCP-treated mitochondria and impairs stress-induced mitophagy, whereas Calcineurin activation promotes Parkin mitochondrial recruitment and basal mitophagy. Calcineurin interacts with Parkin, and promotes Parkin translocation in the absence of PINK1, but requires PINK1 expression to execute mitophagy in MEF cells. Genetic activation of Calcineurin in vivo boosts basal mitophagy in neurons and corrects locomotor dysfunction and mitochondrial respiratory defects of a Drosophila model of impaired mitochondrial functions. Our study identifies Calcineurin as a novel key player in the regulation of Parkin translocation and mitophagy.

Intracellular mono-ADP-ribosyltransferases (mono-ARTs) catalyze the covalent attachment of a single ADP-ribose molecule to protein substrates, thus regulating their functions. PARP10 is a soluble mono-ART involved in the modulation of intracellular signaling, metabolism and apoptosis. PARP10 also participates in the regulation of the G1- and S-phase of the cell cycle. However, the role of this enzyme in G2/M progression is not defined. In this study, we found that genetic ablation, protein depletion and pharmacological inhibition of PARP10 cause a delay in the G2/M transition of the cell cycle. Moreover, we found that the mitotic kinase Aurora-A, a previously identified PARP10 substrate, is actively mono-ADP-ribosylated (MARylated) during G2/M transition in a PARP10-dependent manner. Notably, we showed that PARP10-mediated MARylation of Aurora-A enhances the activity of the kinase in vitro. Consistent with an impairment in the endogenous activity of Aurora-A, cells lacking PARP10 show a decreased localization of the kinase on the centrosomes and mitotic spindle during G2/M progression. Taken together, our data provide the first evidence of a direct role played by PARP10 in the progression of G2 and mitosis, an event that is strictly correlated to the endogenous MARylation of Aurora-A, thus proposing a novel mechanism for the modulation of Aurora-A kinase activity.

ADP-ribosyl-transferases (ARTs) are versatile post-translational regulators. Mammalian ARTs include poly- and mono-ADP-ribosylating enzymes, which transfer ADP-ribose molecules deriving from β-NAD+ to their targets. Mono-ADP-ribosylation (MARylation), which is catalyzed by mono-ARTs such as PARP3, PARP6–PARP12 and PARP14–PARP16, tunes the activity of targets involved in fundamental cell processes and various signaling pathways, ranging from those regulating cell survival and proliferation to those modulating the cellular response to stress and viral infection. Recent advancements of techniques that enable the discovery of MARylation targets across cellular compartments have further expanded our knowledge about the physiological roles of these targets and the potential connection between MARylation and the onset of pathologies. Furthermore, increasing efforts in the development of specific drugs targeting the different MARylating PARP proteins are opening avenues for innovative pharmacological treatments. In this Review, we illustrate the cell cycle progression, intracellular membrane trafficking and cellular stress pathways regulated by mono-ART PARP proteins. We then describe what is known about the roles of MARylating PARP proteins in the context of viral infection and cancer. Finally, we discuss potential future directions towards mapping out the complex network of PARP targets and functions.

The GTPase Rab14 is localized at the trans-Golgi network and at the intermediate compartment associated to sorting/recycling endosomes-like structures of the transferrin-recycling pathway: as other Rab family members, it is involved in the regulation of intracellular vesicle trafficking, though its role and functional relationship with effector/endosomal proteins is still incomplete. We have analysed whether post-translational modifications could affect Rab14 activity: the results obtained define mono-ADP-ribosylation (MARylation) as the yet-unknown Rab14 modification, catalysed by the ADP-ribosyltransferase PARP12, which specifically modifies glutamic acid residues in position 159/162. This modification is essential for the Rab14-dependent endosome progression. Accordingly, recycling of the transferrin receptor is inhibited when MARylation of Rab14 is prevented by PARP12 knocking-down or inhibition, or by overexpression of Rab14 ADP-ribosylation-defective mutant. Under these conditions, Rab14 and transferrin receptors are withheld at the cell periphery at the level of the Rab4-RUFY1-positive sorting endosomes, indicating that the interaction of Rab14 with the dual effectors RUFY and then FIP1c (which specifically binds both Rab11 and Rab14) determines the progression between the Rab4-RUFY- and Rab11-FIP1c-specific vesicles. Therefore Rab14-MARylation determines the sequential binding of this GTPase to RUFY and FIP1c, thus controlling endosome progression ( i . e ., transferrin receptors recycling) through the Rab4-, Rab14- and Rab11-specific vesicles. This identifies a Rab14-specific compartment of the recycling pathway and a crucial enzymatic reaction amenable to pharmacological control.