When exposed to external antigens, T cells are rapidly activated to proliferate and differentiate. A genetic screen identified a mutation called elektra that causes immunodeficiency in mice through a single loss-of-function missense mutation in the Schlafen 2 (Slfn2) gene (1). Slfn2 mutation was associated with impaired T cell activation. However, whether Slfn2 regulates T cell function directly, and how, was unclear. On page 703 of this issue, Yue et al. (2) report that SLFN2 safeguards T cells from excessive stress during activation and thus facilitates the necessary up-regulation of protein translation. SLFN2 binds and shields transfer RNAs (tRNAs), essential adaptor molecules in translation of messenger RNAs (mRNAs), from stress-activated fragmentation. Without SLFN2, excessive tRNA fragmentation lowers global translation and specifically decreases the translation of key cytokine receptor proteins important for T cell activation. This study expands the role of tRNA fragmentation and implicates SLFN2 in preventing fragmentation to enable immune function.
During T cell activation, quiescent T cells turn on cellular machineries to ramp up metabolism, which demands an increase in protein synthesis. SLFN2 is a member of the Schlafen gene family, mostly present in mammals. The family was first identified in 1998 through the screening of regulators for thymocyte development (3). Owing to a lack of homology with other protein families, the functions and mechanisms of action of the Schlafen proteins are still mostly elusive. Yue et al. found that signaling in response to the cytokine interleukin-2 (IL-2) was impaired by Slfn2 deficiency in T cells. IL-2 receptors (IL-2Rs) are expressed on activated T cells, and binding to IL-2 induces gene expression and metabolic programs that are important for T cell activation (4, 5). Expression of IL-2Rβ and IL-2Rγ, but not the IL-2Rα subunit, is decreased, most likely as a result of decreased translation, when Slfn2 is absent. A global proteomics study showed that translation is highly dynamic during T cell activation, with both rapid turnover of select proteins and idling ribosomes poised for new synthesis (6). A similar analysis will clarify which proteins, besides IL-2Rβ and IL-2Rγ, are specifically protected for translation by SLFN2 during T cell activation.
In addition to specific IL-2R proteins, global translation was lowered by Slfn2 deficiency (2). This decrease was observed both in quiescent T cells and after T cell activation. The connection between SLFN2 and translation is perhaps not completely unexpected. Other Schlafen family members, human SLFN11 and SLFN13, can recognize and cleave tRNAs during HIV infection (7, 8). Yue et al. find that SLFN2 also binds tRNA. However, unlike SLFN11 and SLFN13, SLFN2 does not cleave tRNAs because of two missing acidic residues important for the ribonuclease (RNase) activity, but instead shields the tRNAs from cleavage into 30- to 40-nucleotide RNA fragments (tRNAs are 79 to 90 nucleotides).
tRNA fragmentation is known to repress global protein translation, repress translation of specific target mRNAs by microRNA-like action, and regulate mRNA stability. tRNA-derived fragments (tRFs), particularly the subclass that is nearly half a tRNA in size, are often generated upon stress and form stress granules, cytoplasmic RNA-protein complexes that repress global translation by sequestering translation initiation factors (9). What is causing tRNA cleavage during T cell activation? Paradoxically, rapid T cell proliferation is also accompanied by increased concentrations of reactive oxygen species (ROS). ROS arise as by-products from amplified mitochondrial metabolism and activate important transcription factors during T cell activation (10, 11). However, ROS have other downstream effects during T cell activation: tRNA cleavage, translation inhibition, and stress-granule formation. These stress responses are hyperactivated in Slfn2-deficient T cells, despite similar ROS concentrations, suggesting that Slfn2 deficiency exacerbates ROS sensing. The increased stress response and defective proliferation in Slfn2-deficient T cells are counteracted by antioxidant (N-acetylcysteine) treatment. Together, this suggests that increased ROS trigger excessive tRNA cleavage and suppress translation in the absence of SLFN2. Thus, SLFN2 sets the threshold for determining whether T cells should proliferate or be inactivated by the stress response when the cells recognize antigen (see the figure).
T cell activation is accompanied by increased amounts of reactive oxygen species (ROS). Schlafen 2 (SLFN2) protects transfer RNAs (tRNAs) from ROS-induced cleavage to maintain the increased translation that is necessary for T cell activation. When SLFN2 is absent, this tRNA protection is removed, and T cells are more sensitive to ROS amounts and thus more likely to undergo cell death.
GRAPHIC: KELLIE HOLOSKI/SCIENCE
Angiogenin (ANG) is a potent RNase that cleaves the tRNA anticodon loop to produce the half-molecule type of tRFs (9). Yue et al. found that SLFN2 protects tRNAs from ANG cleavage, and lower ANG expression restored tRNA amounts, protein translation, and cell proliferation of Slfn2-deficient T cells. Ang mRNA and protein expression are upregulated by T cell activation, which can be reversed by antioxidant. This represents a newly identified mechanism to trigger tRNA cleavage by directly regulating RNase expression. Further investigation is needed to understand whether the tRNA-protective function of SLFN2 also exists in other cell types.
The study by Yue et al. reveals an intricate regulation of ROS sensing and translation by SLFN2 during T cell activation. This mechanism may be relevant in autoimmunity because SLFN2 is important in a mouse model of multiple sclerosis (2). Notably, Schlafen family functions are not completely understood (12). This study opens several future directions in Schlafen family research. Is the regulation of RNA metabolism and translation a common theme of Schlafen function? It is unknown whether Schlafen proteins can bind to (and protect or cleave) other cellular RNAs or viral RNAs. Additionally, whether there is redundancy of function between members of the family awaits further research.
Growing evidence shows that tRNA fragments are not random degradation products, but rather they have biological functions (9). Because tRNA fragmentation can repress translation by decreasing the pool of tRNAs, or through direct inhibitory effects of tRNA fragments on the translation machinery through stress-granule formation or microRNA-like action, it will be interesting to determine which mechanism, if not all, are contributing to translation repression in SLFN2-deficient T cells. Additionally, RNases other than ANG mediate tRNA fragmentation (9), and in other contexts, tRNA fragments are generated efficiently even after Ang deletion (13). This suggests that although Yue et al. characterize ANG as the major tRNase during T cell activation, the mechanism may be generalizable to other RNases, whose activities could be regulated by other SLFN2-like tRNA-protective proteins in other biological contexts or cell lineages.
Acknowledgments: Work in the authors' laboratory is supported by R01 AR067712 from the National Institutes of Health.
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