α-Phenylalanyl tRNA synthetase competes with Notch signaling through its N-terminal domain

Abstract

The alpha subunit of the cytoplasmic Phenylalanyl tRNA synthetase (α-PheRS, FARSA in humans) displays cell growth and proliferation activities and its elevated levels can induce cell fate changes and tumor-like phenotypes that are neither dependent on the canonical function of charging tRNAPhe with phenylalanine nor on stimulating general translation. In intestinal stem cells of Drosophila midguts, α-PheRS levels are naturally slightly elevated and human FARSA mRNA levels are elevated in multiple cancers. In the Drosophila midgut model, elevated α-PheRS levels caused the accumulation of many additional proliferating cells resembling intestinal stem cells (ISCs) and enteroblasts (EBs). This phenotype partially resembles the tumor-like phenotype described as Notch RNAi phenotype for the same cells. Genetic interactions between α-PheRS and Notch suggest that their activities neutralize each other and that elevated α-PheRS levels attenuate Notch signaling when Notch induces differentiation into enterocytes, type II neuroblast stem cell proliferation, or transcription of a Notch reporter. These non-canonical functions all map to the N-terminal part of α-PheRS which accumulates naturally in the intestine. This truncated version of α-PheRS (α-S) also localizes to nuclei and displays weak sequence similarity to the Notch intracellular domain (NICD), suggesting that α-S might compete with the NICD for binding to a common target. Supporting this hypothesis, the tryptophan (W) residue reported to be key for the interaction between the NICD and the Su(H) BTD domain is not only conserved in α-PheRS and α-S, but also essential for attenuating Notch signaling.

Molecular regulation after mucosal injury and regeneration in ulcerative colitis

“…The conditional deletion of RBP-J and gamma-secretase inhibitor treatment to block NICD release resulted in the differentiation of crypt ISCs into secretory cells ( Won et al, 2022 ). Similar results were observed in young mice deficient in Hes1, a Notch target gene, which led to an increase in the number of secretory cells and a decrease in the number of absorptive cells ( Ho et al, 2022 ). However, due to early death of the mice, the effect of Hes1 deficiency on adult mice is uncertain ( Tsai et al, 2022 ).…”

Production of β-PheRS fragments correlates with food avoidance and slow growth, and is suppressed by the appetite-inducing hormone CCHa2

“…Overexpression of α-/β-PheRS causes only a mild hyperaccumulation in most cell types (Fig. 4; Ho et al, 2022; Lu 2013), because most cells actively control the PheRS levels and cleave and degrade excessive PheRS. Because the B5 mutations cause additional instability of β-PheRS B5a/b (Table 6, Suppl Table S3, Suppl Fig.…”

“…S7), the mutant β-PheRS B5a/b seems to become fragmented even more. The cleavage of the two subunits is accompanied by the formation of stable proteolytic fragments (Ho et al, 2022; Suppl. Fig.…”

“…There is precedent for aaRS fragments performing non-canonical activities (Smirnova et al 2012, Park et al 2005 b, Wakasugi and Schimmel 1999, Wakasugi et al 2002, Jung 2015). Particularly relevant for this work is that the α-S fragment of the Drosophila α-PheRS subunit induces a proliferation phenotype and represses Notch activity (Ho et al, 2022).…”

“…An active site mutant form of α-PheRS can accelerate growth and proliferation. Furthermore, an α-PheRS fragment that is present in some tissues counteracts Notch signaling in situations where this signal affects tissue homeostasis by either promoting stem cell fate or differentiation (Ho et al, 2022).…”