Nuclear versus cytosolic activity of the yeast Hog1 MAP kinase in response to osmotic and tunicamycin-induced ER stress

Osmotic Stress Responses, Cell Wall Integrity, and Conidiation Are Regulated by a Histidine Kinase Sensor in Trichoderma atroviride

Trichoderma atroviride responds to various environmental stressors through the mitogen-activated protein kinase (MAPK) Tmk3 and MAPK-kinase Pbs2 signaling pathways. In fungi, orthologues to Tmk3 are regulated by a histidine kinase (HK) sensor. However, the role of T. atroviride HKs remains unknown. In this regard, the function of the T. atroviride HK Nik1 was analyzed in response to stressors regulated by Tmk3. The growth of the Δnik1 mutant strains was compromised under hyperosmotic stress; mycelia were less resistant to lysing enzymes than the WT strain, while conidia of Δnik1 were more sensitive to Congo red; however, ∆pbs2 and ∆tmk3 strains showed a more drastic defect in cell wall stability. Light-regulated blu1 and grg2 gene expression was induced upon an osmotic shock through Pbs2-Tmk3 but was independent of Nik1. The encoding chitin synthases chs1 and chs2 genes were downregulated after an osmotic shock in the WT, but chs1 and chs3 expression were enhanced in ∆nik1, ∆pbs2, and ∆tmk3. The vegetative growth and conidiation by light decreased in ∆nik1, although Nik1 was unrequired to activate the light-responsive genes by Tmk3. Altogether, Nik1 regulates responses related to the Pbs2-Tmk3 pathway and suggests the participation of additional HKs to respond to stress.

The high osmolarity glycerol (HOG) pathway in fungi†

While fungi are widely occupying nature, many species are responsible for devastating mycosis in humans. Such niche diversity explains how quick fungal adaptation is necessary to endow the capacity of withstanding fluctuating environments and to cope with host-imposed conditions. Among all the molecular mechanisms evolved by fungi, the most studied one is the activation of the phosphorelay signalling pathways, of which the high osmolarity glycerol (HOG) pathway constitutes one of the key molecular apparatus underpinning fungal adaptation and virulence. In this review, we summarize the seminal knowledge of the HOG pathway with its more recent developments. We specifically described the HOG-mediated stress adaptation, with a particular focus on osmotic and oxidative stress, and point out some lags in our understanding of its involvement in the virulence of pathogenic species including, the medically important fungi Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus, compared to the model yeast Saccharomyces cerevisiae. Finally, we also highlighted some possible applications of the HOG pathway modifications to improve the fungal-based production of natural products in the industry.

Loss of Smi1, a protein involved in cell wall synthesis, extends replicative life span by enhancing rDNA stability in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, replicative life span (RLS) is primarily affected by the stability of ribosomal DNA (rDNA). The stability of the highly repetitive rDNA array is maintained through transcriptional silencing by the NAD⁺-dependent histone deacetylase Sir2. Recently, the loss of Smi1, a protein of unknown molecular function that has been proposed to be involved in cell wall synthesis, has been demonstrated to extend RLS in S. cerevisiae, but the mechanism by which Smi1 regulates RLS has not been elucidated. In this study, we determined that the loss of Smi1 extends RLS in a Sir2-dependent manner. We observed that the smi1Δ mutation enhances transcriptional silencing at the rDNA locus and promotes rDNA stability. In the absence of Smi1, the stress-responsive transcription factor Msn2 translocates from the cytoplasm to the nucleus, and nuclear-accumulated Msn2 stimulates the expression of nicotinamidase Pnc1, which serves as an activator of Sir2. In addition, we observed that the MAP kinase Hog1 is activated in smi1Δ cells and that the activation of Hog1 induces the translocation of Msn2 into the nucleus. Taken together, our findings suggest that the loss of Smi1 leads to the nuclear accumulation of Msn2 and stimulates the expression of Pnc1, thereby enhancing Sir2-mediated rDNA stability and extending RLS in S. cerevisiae.

Tunicamycin Sensitivity-Suppression by High Gene Dosage Reveals New Functions of the Yeast Hog1 MAP Kinase

In the yeast Saccharomyces cerevisiae, components of the High Osmolarity Glycerol (HOG) pathway are important for the response to diverse stresses including response to endoplasmic reticulum stress (ER stress), which is produced by the accumulation of unfolded proteins in the lumen of this organelle. Accumulation of unfolded proteins may be due to the inhibition of protein N-glycosylation, which can be achieved by treatment with the antibiotic tunicamycin (Tn). In this work we were interested in finding proteins involved in the ER stress response regulated by Hog1, the mitogen activated protein kinase (MAPK) of the HOG pathway. A high gene dosage suppression screening allowed us to identify genes that suppressed the sensitivity to Tn shown by a hog1Δ mutant. The suppressors participate in a limited number of cellular processes, including lipid/carbohydrate biosynthesis and protein glycosylation, vesicle-mediated transport and exocytosis, cell wall organization and biogenesis, and cell detoxification processes. The finding of suppressors Rer2 and Srt1, which participate in the dolichol biosynthesis pathway revealed that the hog1Δ strain has a defective polyprenol metabolism. This work uncovers new genetic and functional interactors of Hog1 and contributes to a better understanding of the participation of this MAPK in the ER stress response.

Total flavones of Abelmoschus manihot improve diabetic nephropathy by inhibiting the iRhom2/TACE signalling pathway activity in rats

CONTEXT

Total flavones extracted from Abelmoschus manihot L. (Malvaceae) medic (TFA) have been proven clinically effective at improving renal inflammation and glomerular injury in chronic kidney disease (CKD).

OBJECTIVE

This study evaluated the function of TFA as an inhibitor of iRhom2/TACE (tumour necrosis factor-α converting enzyme) signalling and investigated its anti-DN (diabetic nephropathy) effects in a DN rat model.

MATERIALS AND METHODS

In vitro, cells were treated with 200 μg/mL advanced glycation end products (AGEs), and then co-cultured with 20 μg/mL TFA for 24 h. Real time PCR, western blotting and co-immunoprecipitation assays were performed. In vivo, DN was induced in 8 week old male Sprague-Dawley rats via unilateral nephrectomy and intraperitoneal injection of streptozotocin, then TFA were administered to rats by gavage for 12 weeks at three different doses (300, 135 and 75 mg/kg/d). 4-Phenylbutanoic acid (2.5 mg/kg/d) was used as a positive control.

RESULTS

IC₅₀ of TFA is 35.6 μM in HK2 and 39.6 μM in HRMC. TFA treatment (20 μM) inhibited the activation of iRhom2/TACE signalling in cultured cells induced by AGEs. LD₅₀>26 g/kg and ED₅₀=67 mg/kg of TFA in rat by gavage, TFA dose-dependently downregulated the expression of proinflammatory cytokines and exerted anti-inflammatory effects significantly though inhibiting the activation of iRhom2/TACE signalling.

DISCUSSION AND CONCLUSIONS

Our results show that TFA could dose-dependently ameliorate renal inflammation by inhibiting the activation of iRhom2/TACE signalling and attenuating ER stress. These results suggest that TFA has potential therapeutic value for the treatment of DN in humans.

Stress-induced nuclear accumulation is dispensable for Hog1-dependent gene expression and virulence in a fungal pathogen

Stress-activated protein kinase (SAPK) pathways are evolutionarily conserved eukaryotic signalling modules that are essential for the virulence of human pathogenic fungi. The Hog1 SAPK in Candida albicans is robustly phosphorylated in response to a number of host-imposed stresses, and is essential for virulence. The current dogma is that stress-induced phosphorylation activates the SAPK, and promotes its nuclear accumulation that is necessary for the expression of SAPK-dependent stress-protective genes. Here we challenge this dogma. C. albicans strains were constructed in which Hog1 was either tethered to the plasma membrane or constitutively nuclear. Strikingly, tethering Hog1 to the plasma membrane did not abrogate stress resistance or stress-induced gene expression. Furthermore, preventing the nuclear accumulation of Hog1 had no impact on C. albicans virulence in two distinct models of systemic infection. However, tethering Hog1 to the plasma membrane did impact on signal fidelity, and on the magnitude and kinetics of the stress-induced phosphorylation of this SAPK. Taken together, these findings challenge the dogma that nuclear accumulation of SAPKs is a pre-requisite for SAPK-dependent gene expression, and reveal that stress-induced nuclear accumulation of Hog1 is dispensable for the virulence of a major human fungal pathogen.

The Biology of Pichia membranifaciens Killer Toxins

The killer phenomenon is defined as the ability of some yeast to secrete toxins that are lethal to other sensitive yeasts and filamentous fungi. Since the discovery of strains of Saccharomyces cerevisiae capable of secreting killer toxins, much information has been gained regarding killer toxins and this fact has substantially contributed knowledge on fundamental aspects of cell biology and yeast genetics. The killer phenomenon has been studied in Pichia membranifaciens for several years, during which two toxins have been described. PMKT and PMKT2 are proteins of low molecular mass that bind to primary receptors located in the cell wall structure of sensitive yeast cells, linear (1→6)-β-d-glucans and mannoproteins for PMKT and PMKT2, respectively. Cwp2p also acts as a secondary receptor for PMKT. Killing of sensitive cells by PMKT is characterized by ionic movements across plasma membrane and an acidification of the intracellular pH triggering an activation of the High Osmolarity Glycerol (HOG) pathway. On the contrary, our investigations showed a mechanism of killing in which cells are arrested at an early S-phase by high concentrations of PMKT2. However, we concluded that induced mortality at low PMKT2 doses and also PMKT is indeed of an apoptotic nature. Killer yeasts and their toxins have found potential applications in several fields: in food and beverage production, as biocontrol agents, in yeast bio-typing, and as novel antimycotic agents. Accordingly, several applications have been found for P. membranifaciens killer toxins, ranging from pre- and post-harvest biocontrol of plant pathogens to applications during wine fermentation and ageing (inhibition of Botrytis cinerea, Brettanomyces bruxellensis, etc.).

Snapshot: implications for melatonin in endoplasmic reticulum homeostasis

The endoplasmic reticulum (ER) is an important intracellular membranous organelle. Previous studies have demonstrated that the ER is responsible for protein folding and trafficking, lipid synthesis and the maintenance of calcium homeostasis. Interestingly, the morphology and structure of the ER were recently found to be important. Melatonin is a hormone that anticipates the daily onset of darkness in mammals, and it is well known that melatonin acts as an antioxidant by scavenging free radicals and increasing the activity of antioxidant enzymes in the body. Notably, the existing evidence demonstrates that melatonin is involved in ER homeostasis, particularly in the morphology of the ER, indicating a potential protective role of melatonin. This review discusses the existing knowledge regarding the implications for the involvement of melatonin in ER homeostasis.