Analysis of Rheb in the cellular slime mold Dictyostelium discoideum: cellular localization, spatial expression and overexpression

The Multifaceted Interactions of Dictyostelium Atg1 with Mitochondrial Function, Endocytosis, Growth, and Development

Autophagy is a degradative recycling process central to the maintenance of homeostasis in all eukaryotes. By ensuring the degradation of damaged mitochondria, it plays a key role in maintaining mitochondrial health and function. Of the highly conserved autophagy proteins, autophagy-related protein 1 (Atg1) is essential to the process. The involvement of these proteins in intracellular signalling pathways, including those involving mitochondrial function, are still being elucidated. Here the role of Atg1 was investigated in the simple model organism Dictyostelium discoideum using an atg1 null mutant and mutants overexpressing or antisense-inhibiting atg1. When evaluated against the well-characterised outcomes of mitochondrial dysfunction in this model, altered atg1 expression resulted in an unconventional set of phenotypic outcomes in growth, endocytosis, multicellular development, and mitochondrial homeostasis. The findings here show that Atg1 is involved in a tightly regulated signal transduction pathway coordinating energy-consuming processes such as cell growth and multicellular development, along with nutrient status and energy production. Furthermore, Atg1's effects on energy homeostasis indicate a peripheral ancillary role in the mitochondrial signalling network, with effects on energy balance rather than direct effects on electron transport chain function. Further research is required to tease out these complex networks. Nevertheless, this study adds further evidence to the theory that autophagy and mitochondrial signalling are not opposing but rather linked, yet strictly controlled, homeostatic mechanisms.

Deletion of Htt cause alterations in cAMP signaling and spatial patterning in Dictyostelium discoideum

In the present study, we have analyzed in detail the functions of Htt during growth and development of the protist, Dictyostelium discoideum by creating mutants (both overexpressor and knockout). The mRNA was present at all stages of growth and development. Overexpression of htt did not show any major anomaly, while deletion resulted in delayed aggregation territory formation and showed asynchronous development especially after slug stage. The slugs formed by htt ^- cells showed aberration in anterior-posterior boundary, showing increased prestalk region. DdHtt regulates STAT transcription factors in the tip organizer region that help maintain patterning and culmination. In chimeras with the wild-type, htt ^- cells preferentially localized to the tip of the slug and basal disc regions of the fruiting body showing prestalk/stalk bias, while the overexpressing cells majorly populated the prespore/spore region showing spore bias. These differences could be attributed to protein kinase A (PKA)-regulated cyclic adenosine monophosphate (cAMP) signaling.

Sestrin-like protein from Dictyostelium discoideum is involved in autophagy under starvation stress

Sestrins are highly conserved; stress inducible proteins that help maintain metabolic homeostasis and protect cells under stress conditions. They are up-regulated during stress and influence AMPK and mTOR pathways. Our objective was to find the role of Sestrin protein from Dictyostelium discoideum (Dd), a lower eukaryote where starvation stress initiates multicellular development. The single DdSesn-like gene was expressed and its endogenous functions were characterized. Both, the knockout and constitutively expressing strains were made and their involvement in starvation-induced autophagy was analyzed. Autophagic fluxes and ROS levels were also monitored. Additionally, overexpression of DdSesn decreased cell growth and showed a longer lag phase. Upon starvation both DdSesn and ROS levels increased. Sesn^OE showed reduced ROS levels while sesn^- showed increased ROS levels when compared to the wild type. Therefore, we suggest that increased sesn expression may be beneficial in reducing ROS levels during starvation. Deletion of sesn showed reduced autophagic flux and increased p4EBP1 levels. We show that DdSesn promotes autophagy in D. discoideum upon starvation.

Recent progress in the study of the Rheb family GTPases

In this review we highlight recent progress in the study of Rheb family GTPases. Structural studies using X-ray crystallography and NMR have given us insight into unique features of this GTPase. Combined with mutagenesis studies, these works have expanded our understanding of residues that affect Rheb GTP/GDP bound ratios, effector protein interactions, and stimulation of mTORC1 signaling. Analysis of cancer genome databases has revealed that several human carcinomas contain activating mutations of the protein. Rheb's role in activating mTORC1 signaling at the lysosome in response to stimuli has been further elucidated. Rheb has also been suggested to play roles in other cellular pathways including mitophagy and peroxisomal ROS response. A number of studies in mice have demonstrated the importance of Rheb in development, as well as in a variety of functions including cardiac protection and myelination. We conclude with a discussion of future prospects in the study of Rheb family GTPases.