*5.2. UPRam*

The 'UPR activated by the mistargeting of proteins' (UPRam) is another major stress response pathway that responds to mitochondrial import defects via the TIM23 or MIA pathways [362]. It has been well characterised in yeast, and there is some evidence that suggests that it also takes place in mammalian cells [362,363]. In yeast, the trigger for this is not the lack of import of a sensor protein, like ATFS-1 in the UPRmt, but instead the accumulation of cytosolic precursor proteins [362]. This accumulation of cytosolic precursors leads to increased proteasome assembly, triggered by increased activity of proteasome assembly factors Irc25 and Poc4, and subsequent proteasomal degradation of the accumulated cytosolic precursor proteins [362]. This is accompanied by an inhibition of protein synthesis, which acts to prevent further accumulation of mistargeted proteins in the cytosol [362].

The UPRam pathway is in part identical to the UPRmt and is probably activated simultaneously alongside the UPRmt; however, they differ in that the UPRmt acts by regulating the abundance of mitochondrial chaperones and proteases, whilst the UPRam regulates the expression of all mitochondrial proteins, as well as activating the proteasome to clear aggregated proteins [214,364].

To the best of our knowledge, there have been no studies thus far directly implicating the UPRam pathway in neurodegeneration. However, proteasomal degradation via the ubiquitin–proteasome system is known to be downregulated in the affected neurons of many neurodegenerative diseases including AD, PD, HD, and ALS, and it is thought that this is mainly caused by the accumulation of cytotoxic protein aggregates [365–367]. For example, in AD, aggregated, ubiquitinated Tau can block entry of unfolded proteins to the 19S catalytic subunit of the proteasome by binding to the recognition site, resulting in impaired proteasomal degradation and enhancing the accumulation of precursor proteins [368].

#### *5.3. mPOS*

The mitochondrial precursor over-accumulation stress (mPOS) pathway is a mechanism of mitochondria mediated cell death, and has been characterised in yeast [369]. mPOS is usually triggered by any dysfunction that leads to over-accumulation of precursor

proteins in the cytoplasm. Usually, this accumulation would occur as a consequence of import dysfunctions, but it can also be related to other mitochondrial damage, particularly damage that alters IMM integrity such as misfolding of IMM proteins [369]. mPOS is thought to lead to cell degeneration due to the toxic cytosolic accumulation of misfolded proteins exceeding the cells' capacity to remove these proteins [369]. However, there is a large network of genes responsible for suppressing mPOS and thus promoting cell survival by means of modulating ribosomal biogenesis, translation of specific transcripts, increasing protein chaperones and turnover, and decapping mRNA [369]. Of these proteins in yeast, Gis2 and Nog2 are particularly important in encouraging cell survival. Gis2 is involved in promoting cap-independent translation whilst Nog2 inhibits the nuclear export of the 60S RNA subunit of the ribosome, promoting cell survival and attenuating mPOS [369–371]. Furthermore, the mPOS pathway can trigger additional stress response pathways within the cell, including the ISR, which restores cellular homeostasis by reducing global protein synthesis, triggered by phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) [372].

Though there have been no specific examples of mPOS in neurodegeneration as of yet, it may have extremely important implications, especially given the mutations in genes of the anti-degenerative network seen in some neurodegenerative diseases such as ALS [373] and PD [374], which have been implicated in suppressing mPOS. The potential association of mPOS in neurodegeneration has been discussed in detail in a recent review [375].
