Can asymmetric post-translational modifications regulate the behavior of STAT3 homodimers?

Asymmetric post-translational modifications regulate the nuclear translocation of STAT3 homodimers in response to leukemia inhibitory factor

STAT3 is a pleiotropic transcription factor overactivated in 70% of solid tumours. We have recently reported that inactivating mutations on residues susceptible to post-translational modifications (PTMs) in only one of the monomers (i.e. asymmetric) caused changes in the cellular distribution of STAT3 homodimers. Here, we used more controlled experimental conditions, i.e. without the interference of endogenous STAT3 (STAT3-/- HeLa cells) and in the presence of a defined cytokine stimulus (Leukemia Inhibitory Factor, LIF), to provide further evidence that asymmetric PTMs affect the nuclear translocation of STAT3 homodimers. Time-lapse microscopy for 20 min after LIF stimulation showed that S727 dephosphorylation (S727A) and K685 inactivation (K685R) slightly enhanced the nuclear translocation of STAT3 homodimers, while K49 inactivation (K49R) delayed STAT3 nuclear translocation. Our findings suggest that asymmetrically modified STAT3 homodimers could be a new level of STAT3 regulation and, therefore, a potential target for cancer therapy.

Sacsin Deletion Induces Aggregation of Glial Intermediate Filaments

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a neurodegenerative disorder commonly diagnosed in infants and characterized by progressive cerebellar ataxia, spasticity, motor sensory neuropathy and axonal demyelination. ARSACS is caused by mutations in the SACS gene that lead to truncated or defective forms of the 520 kDa multidomain protein, sacsin. Sacsin function is exclusively studied on neuronal cells, where it regulates mitochondrial network organization and facilitates the normal polymerization of neuronal intermediate filaments (i.e., neurofilaments and vimentin). Here, we show that sacsin is also highly expressed in astrocytes, C6 rat glioma cells and N9 mouse microglia. Sacsin knockout in C6 cells (C6Sacs-/-) induced the accumulation of the glial intermediate filaments glial fibrillary acidic protein (GFAP), nestin and vimentin in the juxtanuclear area, and a concomitant depletion of mitochondria. C6Sacs-/- cells showed impaired responses to oxidative challenges (Rotenone) and inflammatory stimuli (Interleukin-6). GFAP aggregation is also associated with other neurodegenerative conditions diagnosed in infants, such as Alexander disease or Giant Axonal Neuropathy. Our results, and the similarities between these disorders, reinforce the possible connection between ARSACS and intermediate filament-associated diseases and point to a potential role of glia in ARSACS pathology.

The role of understudied post-translational modifications for the behavior and function of Signal Transducer and Activator of Transcription 3

The Signal Transducer and Activator of Transcription (STAT) family of transcription factors is involved in inflammation, immunity, development, cancer, and response to injury, among other biological phenomena. Canonical STAT signaling is often represented as a 3-step pathway involving the sequential activation of a membrane receptor, an intermediate kinase, and a STAT transcription factor. The rate-limiting phosphorylation at a highly conserved C-terminal tyrosine residue determines the nuclear translocation and transcriptional activity of STATs. This apparent simplicity is actually misleading and can hardly explain the pleiotropic nature of STATs, the existence of various noncanonical STAT pathways, or the key role of the N-terminal domain in STAT functions. More than 80 post-translational modifications (PTMs) have been identified for STAT3, but their functions remain barely understood. Here, we provide a brief but comprehensive overview of these underexplored PTMs and their role on STAT3 canonical and noncanonical functions. A less tyrosine-centric point of view may be required to advance our understanding of STAT signaling.