Abstract
BackgroundGlycogen and trehalose are storage carbohydrates and their levels in microorganisms vary according to environmental conditions. In Neurospora crassa, alkaline pH stress highly influences glycogen levels, and in Saccharomyces cerevisiae, the response to pH stress also involves the calcineurin signaling pathway mediated by the Crz1 transcription factor. Recently, in yeast, pH stress response genes were identified as targets of Crz1 including genes involved in glycogen and trehalose metabolism. In this work, we present evidence that in N. crassa the glycogen and trehalose metabolism is modulated by alkaline pH and calcium stresses.ResultsWe demonstrated that the pH signaling pathway in N. crassa controls the accumulation of the reserve carbohydrates glycogen and trehalose via the PAC-3 transcription factor, which is the central regulator of the signaling pathway. The protein binds to the promoters of most of the genes encoding enzymes of glycogen and trehalose metabolism and regulates their expression. We also demonstrated that the reserve carbohydrate levels and gene expression are both modulated under calcium stress and that the response to calcium stress may involve the concerted action of PAC-3. Calcium activates growth of the Δpac-3 strain and influences its glycogen and trehalose accumulation. In addition, calcium stress differently regulates glycogen and trehalose metabolism in the mutant strain compared to the wild-type strain. While glycogen levels are decreased in both strains, the trehalose levels are significantly increased in the wild-type strain and not affected by calcium in the mutant strain when compared to mycelium not exposed to calcium.ConclusionsWe previously reported the role of PAC-3 as a transcription factor involved in glycogen metabolism regulation by controlling the expression of the gsn gene, which encodes an enzyme of glycogen synthesis. In this work, we extended the investigation by studying in greater detail the effects of pH on the metabolism of the reserve carbohydrate glycogen and trehalose. We also demonstrated that calcium stress affects the reserve carbohydrate levels and the response to calcium stress may require PAC-3. Considering that the reserve carbohydrate metabolism may be subjected to different signaling pathways control, our data contribute to the understanding of the N. crassa responses under pH and calcium stresses.
Highlights
Glycogen and trehalose are storage carbohydrates and their levels in microorganisms vary according to environmental conditions
We investigated the regulation of glycogen and trehalose metabolism under alkaline pH and calcium stresses in N. crassa, and we demonstrated that the accumulation of both reserve carbohydrates is modulated under both conditions
The pH-signaling pathway controls accumulation of the reserve carbohydrate glycogen and trehalose in Neurospora crassa We previously identified the PAC-3 transcription factor as a putative regulator of glycogen metabolism [15], and later we showed that PAC-3 is required for proper glycogen accumulation by down-regulating the expression of the gene encoding glycogen synthase, the regulatory enzyme in glycogen synthesis [12]
Summary
Glycogen and trehalose are storage carbohydrates and their levels in microorganisms vary according to environmental conditions. In Neurospora crassa, alkaline pH stress highly influences glycogen levels, and in Saccharomyces cerevisiae, the response to pH stress involves the calcineurin signaling pathway mediated by the Crz transcription factor. In yeast, pH stress response genes were identified as targets of Crz including genes involved in glycogen and trehalose metabolism. We present evidence that in N. crassa the glycogen and trehalose metabolism is modulated by alkaline pH and calcium stresses. Glycogen and trehalose are storage carbohydrates found in many microorganisms and their contents vary dynamically, in response to changes in environmental conditions, and throughout their life cycle. These two enzymes are regulated by allosterism, such that glucose-6-phosphate and adenosine monophosphate modulate glycogen synthase and glycogen phosphorylase, respectively
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