although u6 is not known to be polyadenylated, recent studies have detected ma methylated u6 snrna in poly ( a ) rna in worms. while this may represent remnants left after poly ( a ) enrichment from total rna, it is also possible that a fraction of u6 snrna is polyadenylated in worms and other organisms, which could regulate u6 decay upon ma methylation similar to poly ( a ) mrnas via the ythdf2 - cnot1 interaction. it is possible that adenylation and / or polyadenylation cooperates with ma methylation of u6 snrna to regulate u6 snrna turnover. future investigation is warranted to elucidate the detailed mechanism by which ythdf2 and cnot1 regulate u6 snrna decay. nevertheless, our findings establish self u6 snrna as a previously unrecognized non - coding rna target of ythdf2. intriguingly, previous studies have also shown that loss of mett - 10 in c. elegans increases u6 snrna levels, while there is no ma reader protein like ythdf2 in worms. there are three possibilities: ( 1 ) ma methylation of u6 may affect other u6 snrna modifications that in turn regulate u6 snrna stability, ( 2 ) ma methylation of other mett - 10 target genes may regulate u6 snrna transcription, maturation, and / or stability, or ( 3 ) worms may express other proteins that recognize ma - methylated u6 snrna to mediate u6 snrna decay. future investigations are required to test these possibilities. in contrast, a previous report by warda and colleagues did not detect a change to u6 snrna levels upon mettl16 knockdown in hek293 cells. these findings, together with our own, suggest that mettl16 ’ s regulation of u6 snrna levels may be context - dependent, warranting further investigation to elucidate the molecular basis for these differing effects across cell types and organisms. previous reports have suggested that rna modifications, including ma methylation, of in vitro - transcribed synthetic rnas, inhibit activation of tlr3 and rig - i. accordingly, ma methylation has been shown to play critical roles in the immune recognition of circular rnas and non - coding rnas generated by back splicing, inhibiting their binding to rig - i and subsequent rig - i activation. u6 is best known for its role in splicing and functions in the nucleus. however, emerging evidence has shown that u6 can be localized in the cytoplasm and the nucleus. recently, several snrnas including u6 snrna have been shown to be glyco sylated and localized at the cell surface, suggesting emerging biological roles of cytoplasm and surface u6 snrna in rna biology. in addition, free u6 snrna has a double - stranded 5 ’ - short stem, 3 ’ - telestem, and internal stem - loop ( isl ) regions ( supplementary fig. s13b ), which may permit u6 binding to tlr3 as a self rna pathogen - associated molecular pattern ( pamp ). our findings demonstrate that endogenous u6 snrna is localized in endosomes and binds to tlr3 via the lrr21 motif, but not lrr20, the known motif that the synthetic ligand poly i: c binds, suggesting that tlr3 utilizes distinc t lrr motifs for different rna agonists. our data also demonstrates that tlr3 prefers to bind to ma u6 than u6 when ythdf2 is absent ( fig. 4o ), suggesting that the ma motif and / or methylation may be critical for tlr3 binding. intriguingly, we found that tlr3 preferentially binds to the u6 sequence containing the ma methylation site a43, which is also bound by ythdf2 ( fig. 4m ). thus, ythdf2 may compete with tlr3 in binding to ma u6, thereby inhibiting tlr3 activation. distinc t from the role of u1 snrna in uvb - induced inflammation, in vitro irradiation of u6 with uvb had no effect on cytokine expression, further supporting the critical role of the u6 ma methylation sequence, but not uvb - induced de novo damage lesions of u6, in tlr3 activation. under baseline conditions, mettl16 knockdown increases both total and endosomal u6 abundance and cytokine expression in mettl16 - low a431 cells ( fig. 4n ), suggesting that non - ma - modified u6 snrna, which is not bound by ythdf2, can also enter endosomes and be bound by tlr3 in a ythdf2 - independent manner. intriguingly we show that both ythdf2 and u6 are localized in endosomes, permitting ythdf2 binding to u6 in endosomes to regulate tlr3 activation. ythdf2 is known to localize in p - bodies to mediate the decay of ma - methylated mrnas, while u6 is mainly localized in the nucleus as well as cytoplasm. transporting rna from the nucleus to endosomes and other membrane locations requires rnas to cross a membrane at least once. the mammalian sidt1 and sidt2 genes, two homologs of the c. elegans sid - 1 gene, have been shown to mediate the trafficking of rnas across cellular or intracellular vesicle membranes such as lyso somes. recently sidt2 has been shown to be localized in endosomes. in addition, rna transport by the c. elegans sid - 1 protein is shown to be specific for dsrna. indeed, previous studies have shown that free u6 snrna contains double - stranded regions ( supplementary fig. s13b ), which may permit u6 to interact with sidt2 and thus be transported to endosomes. taken together, we show that both u6 snrna and ythdf2 are localized to endosomes to control tlr3 activation. specifically, our findings demonstrate that both u6 snrna and ythdf2 enter endosomes through sidt2 - and dynamin - dependent intracellular trafficking pathway, while ythdf2 is transported into endosome by the rna transporter sidt2 through binding to ma - methylated u6 snrna. this model is supported by the following key findings ( fig. 5, supplementary fig. s14 ): ( 1 ) sidt2 knockdown or dynamin inhibition reduces entry of both u6 snrna and ythdf2 to endosomes, ( 2 ) to our best knowledge, ythdf2 does not have an n - terminal peptide signal sequence that would enable ythdf2 access to the endosomal lumen, ( 3 ) u6 knockdown reduces ythdf2 levels in endosomes, ( 4 ) ythdf2 - n, a mutant lacking the m6a - rna binding domain, showed reduced endosomal level, ( 5 ) knockdown of ythdf2 or mettl16 increased u6 snrna levels in endosomes, indicating that neither ythdf2 nor ma methylation of u6 snrna is required for u6 snrna entry into endosomes, and ( 6 ) extracellular uptake for either u6 snrna or ythdf2 was not observed for endosome transportation. taken together, these findings support a model that u6 snrna is transported into endosomes at least in part by sidt2 in a dynamin - dependent, ma methylation - independent, and ythdf2 - independent manner, while ythdf2 entry into endosomes is dependent on u6 snrna. uvb radiation causes damage to biomol ecules including dna, resulting in genomic instability. besides genomic instability, uvb irradiation also induces inflammation, which manifests as sunburn, a critical enabling hallmark for tumorigenesis and aging. our findings demonstrate that ythdf2 inhibition enhances uvb - induced inflammation, leading to epidermal hyperplasia in mice. uv damage triggers a number of pathways such as the induction of cox - 2 and proinflammatory cytokines in epidermal keratinocytes. previously, we and others have demonstrated that cox - 2 in keratinocytes acts as a pro - inflammatory and tumor - promoting factor in skin tumorigenesis and is upregulated in human skin cancer. uv - induced cox - 2 upregulation can initiate the synthesis of the principle inflammation mediator pge2 and thus increase keratinocyte proliferation and hyperplasia in the epidermis. other cytokines such as tnf - α, il - 6, and il - 1β also play critical roles by contributing to the induction of cox - 2 and other genes involved in epidermal inflammation and hyperplasia. in this study, we found that ythdf2 inhibition increases uv - induced cox - 2 expression in primary keratinocytes and cultured human keratinocytes and epidermal hyperplasia, and that the cox - 2 inhibitor decreased cell proliferation in cells with ythdf2 knockdown, supporting a critical role of cox - 2 in increased cell proliferation caused by ythdf2 inhibition.