Inactivation of HAP4 Accelerates RTG-Dependent Osmoadaptation in Saccharomyces cerevisiae

Abstract

Mitochondrial RTG (an acronym for ReTroGrade) signaling plays a cytoprotective role under various intracellular or environmental stresses. We have previously shown its contribution to osmoadaptation and capacity to sustain mitochondrial respiration in yeast. Here, we studied the interplay between RTG2, the main positive regulator of the RTG pathway, and HAP4, encoding the catalytic subunit of the Hap2-5 complex required for the expression of many mitochondrial proteins that function in the tricarboxylic acid (TCA) cycle and electron transport, upon osmotic stress. Cell growth features, mitochondrial respiratory competence, retrograde signaling activation, and TCA cycle gene expression were comparatively evaluated in wild type and mutant cells in the presence and in the absence of salt stress. We showed that the inactivation of HAP4 improved the kinetics of osmoadaptation by eliciting both the activation of retrograde signaling and the upregulation of three TCA cycle genes: citrate synthase 1 (CIT1), aconitase 1 (ACO1), and isocitrate dehydrogenase 1 (IDH1). Interestingly, their increased expression was mostly dependent on RTG2. Impaired respiratory competence in the HAP4 mutant does not affect its faster adaptive response to stress. These findings indicate that the involvement of the RTG pathway in osmostress is fostered in a cellular context of constitutively reduced respiratory capacity. Moreover, it is evident that the RTG pathway mediates peroxisomes–mitochondria communication by modulating the metabolic function of mitochondria in osmoadaptation.

RetroGREAT signaling: The lessons we learn from yeast

“…The TCA cycle genes encoding enzymes acting downstream of isocitrate dehydrogenase are not dependent on the RTG pathway. 1,12,14,15…”

“…The RTG pathway exhibits crosstalk with the target of rapamycin (TOR) pathway and the HOG (high osmolarity glycerol) pathway, therefore nutritional starvation or osmostress may also activate the metabolic retrograde response. 14,[18][19][20]…”

“…The common feature of this regulation is the control of stress-response gene expression to ensure mitochondrial function. 14,19 It was shown that the RTG pathway is involved in oxidative stress resistance, by induction of CTT1 (cytosolic catalase T) transcription and thereby has a protective role against acetic acid toxicity. This activation depends on the HOG pathway, particularly on the interplay between RTG2 with HOG1, but the exact mechanism of this interaction was not described.…”

“…In respiratory deficient cells, the retrograde transcription factors Rtg1/3 take over the control of CIT2 , ACO1 , and IDH1/2 expression. The TCA cycle genes encoding enzymes acting downstream of isocitrate dehydrogenase are not dependent on the RTG pathway 1,12,14,15 …”

“…It has also been shown that the mitochondrial retrograde pathway is involved in different types of stress including oxidative stress, and mitochondria act as signal transducers to maintain cell homeostasis. The RTG pathway exhibits crosstalk with the target of rapamycin (TOR) pathway and the HOG (high osmolarity glycerol) pathway, therefore nutritional starvation or osmostress may also activate the metabolic retrograde response 14,18–20 …”