Peroxisome Maintenance Depends on De Novo Peroxisome Formation in Yeast Mutants Defective in Peroxisome Fission and Inheritance.

There is an ongoing debate on how peroxisomes form: by growth and fission of pre-existing peroxisomes or de novo from another membrane. It has been proposed that, in wild type yeast cells, peroxisome fission and careful segregation of the organelles over mother cells and buds is essential for organelle maintenance. Using live cell imaging we observed that cells of the yeast Hansenula polymorpha, lacking the peroxisome fission protein Pex11, still show peroxisome fission and inheritance. Also, in cells of mutants without the peroxisome inheritance protein Inp2 peroxisome segregation can still occur. In contrast, peroxisome fission and inheritance were not observed in cells of a pex11 inp2 double deletion strain. In buds of cells of this double mutant, new organelles likely appear de novo. Growth of pex11 inp2 cells on methanol, a growth substrate that requires functional peroxisomes, is retarded relative to the wild type control. Based on these observations we conclude that in H. polymorpha de novo peroxisome formation is a rescue mechanism, which is less efficient than organelle fission and inheritance to maintain functional peroxisomes.

FgPEX1 and FgPEX10 are required for the maintenance of Woronin bodies and full virulence of Fusarium graminearum.

Peroxisomes are ubiquitous single-membrane-bound organelles that perform a variety of biochemical functions in eukaryotic cells. Proteins involved in peroxisomal biogenesis are collectively called peroxins. Currently, functions of most peroxins in phytopathogenic fungi are poorly understood. Here, we report identification of PEX1 and PEX10 in the phytopathogenic fungus, Fusarium graminearum, namely FgPEX1 and FgPEX10, the orthologs of yeast ScPEX1 and ScPEX10. To functionally characterize FgPEX1 and FgPEX10, we constructed deletion mutants of FgPEX1 and FgPEX10 (ΔPEX1 and ΔPEX10) by targeting gene-replacement strategies. Our data demonstrate that both mutants displayed reduced mycelial growth, conidiation, and production of perithecia. Deletion of FgPEX1 and FgPEX10 resulted in a shortage of acetyl-CoA, which is an important reason for the reduced deoxynivalenol production and inhibited virulence of F. graminearum. Moreover, ΔPEX1 and ΔPEX10 showed an increased accumulation of lipid droplets and endogenous reactive oxygen species. In addition, FgPEX1 and FgPEX10 were found to be involved in the maintenance of cell wall integrity and Woronin bodies.

The peroxisome biogenesis factors Pex3 and Pex19: multitasking proteins with disputed functions.

The peroxisomal membrane protein (PMP) Pex3 and its cytosolic interaction partner Pex19 have been implicated in peroxisomal membrane biogenesis. Although these peroxins have been extensively studied, no consensus has been reached yet on how they operate. Here, we discuss two major models of their function, namely, in direct insertion of proteins into the peroxisomal membrane or in formation of PMP-containing vesicles from the endoplasmic reticulum (ER). Pex3 can also recruit other proteins to the peroxisomal membrane (e.g., Inp1, Atg30, Atg36), thereby fulfilling roles in other processes such as autophagy and organelle retention. Recent studies indicate that Pex3 and Pex19 can also facilitate sorting of certain membrane proteins to other cellular organelles, including the ER, lipid droplets, and mitochondria.

Pex16 is involved in peroxisome and Woronin body formation in the white koji fungus, Aspergillus luchuensis mut. kawachii.

We characterized Pex16 in Aspergillus luchuensis mut. kawachii to examine the role of peroxisomes on citric acid production during the shochu-fermentation process. Rice koji made using a Δpex16 strain exhibited no significant change in citric acid accumulation but a 1.4-fold increase in formic acid production. Microscopic observation of mRFP-SKL (a peroxisome protein marker) showed that pex16 disruption decreased the number of dot-like structures per hyphal cell to 5% of the control. Pex16-GFP exclusively co-localized with mRFP-SKL throughout the hyphae including the very close position to the septal pore. Moreover, the Δpex16 strain was hypersensitive to calcofluor white, which appeared to induce bursting of the hyphal tip and translocation of mRFP-SKL signals to the septal pore. These results indicate that Pex16 does not play a role in citric acid accumulation but is significantly involved in peroxisome and Woronin body formation in Aspergillus kawachii.

The Brain Network in a Model of Thalamocortical Dysrhythmia.

Sensory information processing and higher cognitive functions rely on the interactions between thalamus and cortex. Many types of neurological and psychiatric disorders are accompanied or driven by alterations in the brain connectivity. In this study, putative changes in functional and effective corticocortical (CC), thalamocortical (TC), and corticothalamic (CT) connectivity during wakefulness and slow-wave sleep (SWS) in a model of thalamocortical dysrhythmia, TRIP8b-/- mice, and in control (wild-type or WT) mice are described. Coherence and nonlinear Granger causality (GC) were calculated for twenty 10 s length epochs of SWS and active wakefulness (AW) of each animal. Coherence was reduced between 4 and ca 20 Hz in the cortex and between cortex and thalamus during SWS compared with AW in WT but not in TRIP8b-/- mice. Moreover, TRIP8b-/- mice showed lower CT coherence during AW compared with WT mice; these differences were no longer present during SWS. Unconditional GC analysis also showed sleep-related reductions in TC and CT couplings in WT mice, while TRIP8b-/- mice showed diminished wake and enhanced sleep CC coupling and rather strong CT-directed coupling during wake and sleep, although smaller during sleep. Conditional GC coupling analysis confirmed the diminished CC and enhanced CT coupling in TRIP8b-/- mice. Our findings indicate that altered properties of hyperpolarization-activated cyclic nucleotide-gated cation channels, characterizing TRIP8b-/- mice, have clear effects on CC, TC, and CT networks. A more complete understanding of the function of the altered communication within these networks awaits detailed phenotyping of TRIP8b-/- mice aimed at specifics of sensory and attentional processes.

The N-terminal amphipathic helix of Pex11p self-interacts to induce membrane remodelling during peroxisome fission.

Pex11p plays a crucial role in peroxisome fission. Previously, it was shown that a conserved N-terminal amphipathic helix in Pex11p, termed Pex11-Amph, was necessary for peroxisomal fission in vivo while in vitro studies revealed that this region alone was sufficient to bring about tubulation of liposomes with a lipid consistency resembling the peroxisomal membrane. However, molecular details of how Pex11-Amph remodels the peroxisomal membrane remain unknown. Here we have combined in silico, in vitro and in vivo approaches to gain insights into the molecular mechanisms underlying Pex11-Amph activity. Using molecular dynamics simulations, we observe that Pex11-Amph peptides form linear aggregates on a model membrane. Furthermore, we identify mutations that disrupted this aggregation in silico, which also abolished the peptide's ability to remodel liposomes in vitro, establishing that Pex11p oligomerisation plays a direct role in membrane remodelling. In vivo studies revealed that these mutations resulted in a strong reduction in Pex11 protein levels, indicating that these residues are important for Pex11p function. Taken together, our data demonstrate the power of combining in silico techniques with experimental approaches to investigate the molecular mechanisms underlying Pex11p-dependent membrane remodelling.