DRiPs also contribute to formation of

DRiPs also contribute to formation of protein Cytidine 5-triphosphate mg that result from dysregulation of proteostasis, during which the proteolytic machinery is unable to process and degrade defective proteins (Tyedmers et al., 2010). Aggregation of poly-ubiquitinated proteins has been observed in different cell types submitted to proteasome or autophagy inhibition (Wenger et al., 2012). Importantly, in addition to poly-ubiquitinated proteins, protein aggregates contain various autophagosome-associated receptors from the LC3 family, as well as specific molecular adaptors necessary for receptor mediated autophagy (Johansen and Lamark, 2011). Generally, these adaptors, such as p62 (also known as sequestrosome 1, SQTM1), NBR1, NDP52, TAX1BP1 or optineurin, contain an LC3-interacting region (LIR) and ubiquitin-interacting domains (UBA) (Deretic et al., 2013). Activation of DCs by microbes stimulates the mTORC1 pathway that augments protein synthesis, reduces autophagic flux (Terawaki et al., 2015), and causes accumulation of newly synthesized poly-ubiquitinated proteins in dendritic cell aggresome-like induced structures (DALIS) (Lelouard et al., 2004, 2002). DALIS require p62 and NBR1 for their induction, accumulation of DRiPs within 5 min after synthesis, and stabilization for several hours in activated DCs (Lelouard et al., 2004; Pierre, 2005). In HeLa cells, DRiPs contribute directly to aggresome-like-induced structures (ALIS) formation upon artificial inhibition of autophagy. p62-positive ALIS do not recruit the chaperone HSPA8 (HSC70), which distinguishes them from other aggregates formed, for example, after proteasome inhibition. Importantly, presentation of model antigens on MHC I increases by 30% after inhibition of autophagy by Atg5, PLEKHM1 or NBR1 depletion. This suggests that autophagy degrades DRiPs using NBR1 as LC3 targeting adaptor, and thus normally segregates these antigens away from the proteasome degradation pathway (McEwan et al., 2015; Wenger et al., 2012). Alternative sorting of DRiPs to different processing pathways seems to be mediated in part by members of the Bcl-2 associated athanogene (BAG) family, which promote the ADP exchange by ATP in HSP70 chaperones and facilitate recognition of proteins requiring stabilization or folding. Each BAG/HSP70 complex binds proteins with specific biochemical features. BAG-1 seems to be important for degradation of long-lived obsolete proteins and can be found in DALIS, although its depletion has little effect on degradation of DRiPs and associated MHC I presentation (Kettern et al., 2011; Wenger et al., 2012). BAG-6 is important for degradation of DRiPs by the proteasome, and its inactivation profoundly decreases MHC I presentation (Kettern et al., 2011; Wenger et al., 2012), although its function seems dispensable for presentation of lymphocytic choriomeningitis virus (LCMV)-derived antigens (Bitzer et al., 2016). BAG-3 has been proposed to shift protein turnover from proteasome- to autophagy-mediated degradation, and an increased BAG-3/BAG-1 ratio was correlated with lower autophagy flux in ageing cells (Gamerdinger et al., 2009). In agreement with these results, BAG-3 silencing in HeLa cells was observed to increase presentation of model antigens on MHC I (Wenger et al., 2012), similarly to aforementioned Atg5 or NBR1 inactivation. Autophagy-deficient cells seem to compensate for the loss of proteolysis by re-routing autophagy targeted DRiPs to ALIS, allowing their biochemical re-programming through alternative ubiquitination and subsequent targeting to proteasome degradation (Wenger et al., 2012). In most cells, autophagy reduction therefore leads to the global increase of MHC I-restricted presentation. In LPS-activated DCs, inhibition of autophagy leads to DALIS formation. This alters the capacity of endogenous antigens to be presented both on MHC I by the standard pathway and on MHC II by CP2, which might have consequences for immunogenicity and tolerance at the T cells level (Fig. 2) (Terawaki et al., 2015).