Like mitochondria, peroxisomes can multiply by division of pre-existing ones (Figure ). However, so far no evidence has been obtained that peroxisomes fuse. The initial stage of peroxisome fission is organelle elongation mediated by the peroxisomal membrane protein Pex11p. Dynamin-related proteins (DRPs) are responsible for the final scission event. In
S. cerevisiae the DRPs Vps1p and Dnm1p are involved in peroxisome fission (Hoepfner et al.,
2001; Kuravi et al.,
2006), whereas in the yeast
Hansenula polymorpha peroxisome fission entirely depends on Dnm1p (Nagotu et al.,
2008). Recruitment of Dnm1p to the peroxisomal membrane is mediated by the peroxisomal membrane protein Fis1p. Interestingly, Fis1p also recruits Dnm1p to mitochondria for mitochondrial fission (Mozdy et al.,
2000). Similarly, mammalian Fis1 and Drp1 are both involved in peroxisome and mitochondrial fission. Hence, peroxisomes and mitochondria share key components of their fission machineries.
In two fungal model systems for aging, the filamentous ascomycete
Podospora anserina and baker’s yeast down-regulation of mitochondrial fission by deletion of the
DNM1 gene leads to a robust increase in replicative lifespan (Scheckhuber et al.,
2007,
2008). Moreover, deletion of
DNM1 also has a positive effect on chronological aging in baker’s yeast (Palermo et al.,
2007). These beneficial effects might be based on improved content mixing of mitochondria so that molecular damage to proteins, lipids, and mtDNA can be ameliorated more efficiently. However, it has to be stressed that the effect of
DNM1 deletion on peroxisome fission was not investigated in these studies. Hence, the observed effects may also be partially due to defects in peroxisomal fission.
Several data suggest that peroxisomes divide asymmetrically, resulting in larger mature organelles and small, nascent ones (Koch et al.,
2003,
2010; Cepinska et al.,
2011; Huber et al.,
2012). As a consequence cells contain a heterologous population of peroxisomes, ranging from relatively young and vital nascent organelles to relatively old, mature ones, in which dysfunctional components accumulate in time due to damage caused by products of peroxisomal metabolism.
In addition to fission, peroxisomes can also be formed
de novo, a process that is most prominent in yeast
pex3 or
pex mutants that lack pre-existing peroxisomes, upon reintroduction of the corresponding genes. Several data suggested that these new organelles originate from the endoplasmic reticulum (ER; Hoepfner et al.,
2005; Zipor et al.,
2009).
Data in yeast, data in yeast indicate that the major pathway of peroxisome proliferation is fission. However, possibly in other species the formation of peroxisomes from the ER is a more prominent process (Geuze et al.,
2003; Tabak et al.,
2003; Kim et al.,
2006; Yonekawa et al.,
2011).
Autophagy can result in a reduction in the number of peroxisomes per cell. Autophagy is the pivotal cellular housekeeping process that can eliminate redundant or unwanted components or entire organelles from the cell. Selective degradation of peroxisomes by autophagy is designated “pexophagy,” a phenomenon mainly studied in the methylotrophic yeast species H. polymorpha and Pichia pastoris.
In
H. polymorpha, macropexophagy is induced when methanol-grown cells are shifted to glucose (Figure ). Under these conditions, the key peroxisomal enzymes of methanol metabolism become redundant for growth. The surplus in organelles is then selectively degraded by macropexophagy (Monastryska et al.,
2004), a process that involves sequestration of individual peroxisomes from the cytosol and subsequent fusion with the vacuole for degradation (Veenhuis et al.,
1983).
When methanol-grown cells of
P. pastoris are shifted to glucose, peroxisomes are degraded by micropexophagy, which involves the formation of finger-like protrusions by the vacuole and subsequent engulfment of clusters of peroxisomes from the cytosol with ultimate degradation in the vacuole. Before this takes place, a double membrane flattened sac-like structure termed micropexophagy-specific membrane apparatus (MIPA) is synthesized at the peroxisome surface to complete the sequestration process (Tuttle and Dunn,
1995; Mukaiyama et al.,
2004).
In addition, data obtained with
H. polymorpha revealed a constant removal of peroxisomes by autophagy (Aksam et al.,
2007). This process most likely prevents the accumulation of damaged peroxisomal components, which would be potentially hazardous for the cells. Hence, at peroxisome-inducing conditions, organelle proliferation and degradation occur simultaneously to maintain tightly controlled organelle homeostasis. This mechanism is consistent with the view that timely rejuvenation of peroxisomes is vital for cell viability and survival. Most likely the oldest, mature organelles are most susceptible for autophagic degradation. A central question that offers interesting perspectives for future research is how these peroxisomes are recognized by the pexophagy machinery. The peroxisomal membrane proteins Pex3p and Pex14p have been shown to play a role in the early stage of pexophagy (Bellu et al.,
2001,
2002; van Zutphen et al.,
2008) and may play a role in this recognition process. Indeed Pex3p was shown to interact with Atg30p, a phosphoprotein that is necessary for the presentation of the compromised peroxisome to the vacuole for its subsequent degradation (Farre et al.,
2008).