Table Of Content

AutophAgic punctum AutophAgic punctum Autophagy 9:4, 609–611; April 2013; © 2013 Landes Bioscience Sulfide as a signaling molecule in autophagy Cecilia Gotor,* Irene García, José L. Crespo and Luis C. Romero Instituto de Bioquímica Vegetal y Fotosíntesis; Consejo Superior de Investigaciones Científicas and Universidad de Sevilla; Sevilla, Spain Hydrogen sulfide is already recog- is involved in the degradation of cysteine nized as an important signaling and the concomitant generation of H S 2 molecule in mammalian systems, and in this cellular compartment. Mutations emerging data suggest that H S is a sig- of the DES1 gene impede H S generation 2 2 naling molecule just as important as in the cytosol and strongly affect plant nitric oxide (NO) and H O in plants. metabolism. The detailed characterization 2 2 Although sulfide is generated in chloro- of null mutants has provided insight into plasts and mitochondria, it is present pre- the role of sulfide generated from Cys in dominantly in the charged HS- form due the cytosol as a signaling molecule regulat- to the basic pH inside both organelles, ing the process of autophagy. thus requiring an active transporter, DES1 deficiency leads to the accumu- which is yet to be identified, to be released. lation and lipidation of ATG8 isoforms In Arabidopsis, we found that the cyto- in Arabidopsis, a landmark of auto- solic L-cysteine desulfhydrase DES1 is phagy activation in different systems. In involved in the degradation of cysteine, addition, transcriptomic data confirm and therefore responsible for the genera- the induction of autophagy in des1 null tion of H S in this cellular compartment. mutants. Because mutation of the DES1 2 DES1 deficiency leads to the induction gene disrupts H S generation in the 2 of autophagy. Moreover, we have dem- Arabidopsis cytosol, restoring the capac- onstrated that sulfide in particular exerts ity of H S generation eliminates the phe- 2 a general effect on autophagy through notypic differences of the null mutants negative regulation, in a way unrelated from wild-type plants. Exogenous sulfide to nutrient deficiency. The mechanisms prevents ATG8 protein accumulation and of H S action and its molecular targets lipidation in des1 mutant plants, indicat- 2 are largely unknown, although in ani- ing that intracellular H S regulates auto- 2 mal systems, protein S-sulfhydration phagy. Interestingly, exogenous sulfide is Keywords: Arabidopsis, ATG8 lipida- has been proposed as a mechanism for also able to suppress autophagy induction tion, carbon starvation, cysteine desulf- sulfide-mediated signaling. caused by carbon starvation in wild-type hydrase, cytosolic sulfide, senescence plants, whereas exogenous ammonium, a Submitted: 12/13/12 Hydrogen sulfide has increasingly been second product of DES1 activity, has no Revised: 12/27/12 recognized as an important signaling mol- effect on autophagy. Therefore, sulfide, in ecule of comparable importance to NO and particular, exerts a general effect on auto- Accepted: 01/02/13 carbon monoxide in mammalian systems. phagy through negative regulation, and http://dx.doi.org/10.4161/auto.23460 Similarly, emerging data over recent years in a way unrelated to nutrient limitation *Correspondence to: Cecilia Gotor; Email: gotor@ suggest that H S may be a new signaling stress, as we have demonstrated in our 2 ibvf.csic.es molecule equally important to plants as plant system. NO and H O . It has been implicated, for There are three subcellular compart- 2 2 Punctum to: Álvarez C, García I, Moreno I, example, in the protection against metal ments where sulfide is generated in the Pérez-Pérez ME, Crespo JL, Romero LC, et al. stress or the regulation of photosynthe- plant cell (Fig. 1). In the chloroplast, sul- Cysteine-generated sulfide in the cytosol nega- sis and stomata movement. We recently fide is produced by photosynthetic sulfate tively regulates autophagy and modulates the transcriptional profile in Arabidopsis. Plant Cell investigated DES1 from Arabidopsis thali- reduction to incorporate it into organic 2012; 24:4621–34; PMID:23144183; http://dx.doi. ana, which is the only identified L-cysteine metabolites and cellular components. org/10.1105/tpc.112.105403 desulfhydrase located in the cytosol, and Although it has been proposed that H S 2 www.landesbioscience.com Autophagy 609 controlling programmed cell death asso- ciated with microbial effector-triggered immunity, although its exact role is still unclear, since autophagy acts with a pro- survival or prodeath role. However, it is accepted that autophagy suppresses the spread of pathogen-induced cell death to uninfected tissue from the infection site that is the hypersensitive response (HR) in addition to the activation of systemic acquired resistance (SAR). Thus again, the des1 null mutants behave as constitu- tive SAR mutants upon pathogen infection, suggesting an inverse correlation between endogenous sulfide content and autophagy. The knowledge about the mechanisms of H S action and its molecular targets is 2 Figure 1. cysteine-generated sulfide in the cytosol regulates autophagy. in the Arabidopsis largely deficient in animal systems. Protein plant cell there are different subcellular compartments where sulfide is generated: chloroplast, S-sulfhydration has been proposed as a mitochondria and cytosol. in the chloroplast, sulfide is produced by photosynthetic sulfate reduc- mechanism for sulfide-mediated signaling tion and its predominant charged hS- form requires an active transporter (unidentified to date) to in analogy with S-nitrosylation. For this cross the envelope. in the mitochondria, sulfide is recycled for cyanide detoxification and is also predominantly present in the charged hS- form. in the cytosol, DES1 is responsible for the genera- post-translational modification to occur, tion of sulfide. irrespective of nutrient conditions, the sulfide generated in the cytosol behaves as the cysteine residues should be in an oxi- a signaling molecule acting as a repressor of autophagy. dized state as sulfenic acid or disulfide, being then attacked by the hydrosulfide reaches the cytosol via diffusion through limitation. The mechanisms underlying anion to produce a persulfide product. It the chloroplast envelope membrane, this modulation and the specific condi- has been suggested that the thiol redox hydrogen sulfide is weakly acidic and dis- tions in which it takes place are still an state can profoundly influence auto- sociates in aqueous solution, and, at the open question. However, experimental phagy, especially during the initiation stromal pH of 8.5, sulfide is predomi- evidence can provide some clues about the and completion of the autophagosomes. nantly in the charged HS- form, which conditions in which sulfide acts as a sig- Proteins susceptible to modification by cannot permeate the chloroplast envelope naling molecule regulating autophagy. sulfhydration are the E1 and E2 enzymes and requires an active transporter that Autophagy is involved in normal plant involved in the ubiquitination processes, has yet to be identified in plant cells. In development processes even in nutrient- which are regulated through a revers- the mitochondrion, sulfide is produced in rich conditions. Thus, autophagy occurs ible S-thiolation. Another possible target the detoxification of cyanide, which is a before or during senescence irrespective of sulfide could be the cysteine protease potent inhibitor of cytochrome c oxidase. of the plant nutrient status, and sulfide ATG4 that has been characterized in But sulfide is also an inhibitor of this might have a signaling role in this process. detail in mammalian systems to be redox enzyme and is necessary to be consumed Accordingly, DES1 deficiency promotes a regulated, and a cysteine residue located to produce cysteine, which in turn, is significant reduction in endogenous sul- near the catalytic site is critical for this used to detoxify cyanide, thus generating fide together with premature leaf senes- regulation. Finally, autophagy is a process a cyclic pathway (Fig. 1). Similar to the cence and autophagy activation. Moreover, involved in the proteolytic degradation chloroplast, in metabolically active cells, increasing the level of endogenous sulfide of cellular macromolecules and requires the pH of the mitochondrial stroma is eliminates premature leaf senescence and the activity of proteases. The A. thaliana about 8, due to the pump of protons out of reverses autophagy induction. Autophagy genome has over 550 protease sequences, the matrix, and consequently the sulfide is has also been shown to play diverse roles and recent findings highlight the potential predominantly present in the charged HS- in response to abiotic and biotic stresses. of proteases to participate in many aspects form and its release out of the mitochon- Under abiotic stress conditions autophagy of plant growth and development. In par- dria is very limited. In the cytosol, DES1 is generally considered to be a response ticular, the C1A papain-like cysteine pro- is involved in the production of sulfide for cell survival, and, curiously, the des1 teases contain a catalytic Cys that can be a generated to act as a signaling molecule. null mutants with lower endogenous sul- target of S-sulfhydration. Therefore, in the plant cell DES1 should fide content and induced autophagy show be responsible for modulating the genera- an increased tolerance to Cd and H O Disclosure of Potential Conflicts of Interest 2 2 tion of sulfide for the repression of auto- stresses. Recently autophagy has been No potential conflicts of interest were phagy in a manner unrelated to nutrient implicated in plant immunity through disclosed. 610 Autophagy Volume 9 issue 4 AutophAgic punctum AutophAgic punctum Acknowledgments This work was funded by the European Regional Development Fund and the Ministerio de Economia y Competitividad, Spain (Grants BIO2010-15201, BFU2012- 35913 and CSD2007-00057). www.landesbioscience.com Autophagy 611