Summary In vertebrates, mechano-electrical transduction of sound is accomplished by sensory hair cells. While mammalian hair cells are not replaced when lost, in fish they constantly renew and regenerate after injury. In vivo tracking and cell fate analyses of all dividing cells during lateral line hair cell regeneration revealed that support and hair cell progenitors localize to distinct tissue compartments. Importantly, we find that the balance between self-renewal and differentiation in these compartments is controlled by spatially restricted Notch signaling and its inhibition of Wnt-induced proliferation. The ability to simultaneously study and manipulate individual cell behaviors and multiple pathways in vivo, transforms the lateral line into a powerful paradigm to mechanistically dissect sensory organ regeneration. The striking similarities to other vertebrate stem cell compartments uniquely place zebrafish to help elucidate why mammals possess such low capacity to regenerate hair cells.

Deafness caused by the terminal loss of inner ear hair cells is one of the most common sensory diseases. However, nonmammalian animals (e.g., birds, amphibians, and fish) regenerate damaged hair cells. To understand better the reasons underpinning such disparities in regeneration among vertebrates, we set out to define at high resolution the changes in gene expression associated with the regeneration of hair cells in the zebrafish lateral line. We performed RNA-Seq analyses on regenerating support cells purified by FACS. The resulting expression data were subjected to pathway enrichment analyses, and the differentially expressed genes were validated in vivo via whole-mount in situ hybridizations. We discovered that cell cycle regulators are expressed hours before the activation of Wnt/β-catenin signaling following hair cell death. We propose that Wnt/β-catenin signaling is not involved in regulating the onset of proliferation but governs proliferation at later stages of regeneration. In addition, and in marked contrast to mammals, our data clearly indicate that the Notch pathway is significantly down-regulated shortly after injury, thus uncovering a key difference between the zebrafish and mammalian responses to hair cell injury. Taken together, our findings lay the foundation for identifying differences in signaling pathway regulation that could be exploited as potential therapeutic targets to promote either sensory epithelium or hair cell regeneration in mammals.signaling pathway analysis | RNA sequencing | neuromast | Jak/Stat3 | cdkn1b

RA-inducible gene I (RIG-I/DDX58) has been shown to activate IFN-β promoter stimulator 1 (IPS-1) on recognizing cytoplasmic viral RNAs. It is unclear how RIG-I functions within the IFN and/or RA signaling process in acute myeloid leukemia (AML) cells, however, where obvious RIG-I induction is observed. Here, we show that the RIG-I induction functionally contributes to IFN-α plus RA-triggered growth inhibition of AML cells. Interestingly, although RIG-I induction itself is under the regulation of STAT1, a major IFN intracellular signal mediator, under circumstances in which it does not stimulate IPS-1, it conversely augments STAT1 activation to induce IFN-stimulatory gene expression and inhibit leukemia cell proliferation. Thus, our results unveil a previously undescribed RIG-I activity in regulating the cellular proliferation of leukemia cells via STAT1, which is independent of its classic role of sensing viral invasion to trigger type I IFN transcription.

RIG-I has been implicated in innate immunity by sensing intracellular viral RNAs and inducing type I IFN production. However, we have found a significant RIG-I induction in a biological setting without active viral infection-namely, during RA-induced terminal granulocytic differentiation of acute myeloid leukemias. Here, we present evidence that a significant Rig-I induction also occurs during normal myelopoiesis and that the disruption of the Rig-I gene in mice leads to the development of a progressive myeloproliferative disorder. The initiation of progressive myeloproliferative disorder is mainly due to an intrinsic defect of Rig-I ؊/؊ myeloid cells, which are characterized by a reduced expression of IFN consensus sequence binding protein, a major regulator of myeloid differentiation. Thus, our study reveals a critical regulatory role of Rig-I in modulating the generation and differentiation of granulocytes. knockout mice ͉ myelopoiesis ͉ RA-inducible gene I R A-induced granulocytic differentiation of acute promyelocytic leukemia (APL) represents a successful application of tumor differentiation-inducing therapy in the treatment of human malignancies. It is also an excellent model for studying the cellular and molecular mechanisms controlling RA-induced, and probably physiological cues-regulated myelopoiesis. Previously, we reported the isolation of a number of genes whose mRNA levels were highly up-regulated, along with all-trans-RA (ATRA)-induced terminal granulocytic differentiation of APL cell line NB4 cells in vitro. Among the up-regulated genes, a particularly interesting one was RIG-I, encoding a putative DExD/H box-containing RNA helicase (1). Recently, RIG-I has also been identified in the screening experiment for the molecules that mediate viral RNA-induced type I IFN generation (2). Upon the occupancy of its DExD/H box-containing carboxyl terminal region by foreign invading RNAs, or even RNase L-cleaved small self-RNAs (3), the amino-terminal double-CARD motif of RIG-I binds onto IPS-1/MAVS/VISA/Cardif, resulting in the recruitment and phosphorylation of certain downstream cytoplasmic factors, including IKK␥ and IKKrelated kinases TBK1/IKK. These immediate events, in turn, relay signals via phosphorylation cascade and nuclear translocation of transcription factors such as NF-B and IRF3 to coordinate the formation of IFN-␤ promoter enhancesome within the nucleus (4, 5). As expected, melanoma differentiation antigen 5 (MDA-5), a close structural and functional analogue of RIG-I, was also found to mediate a viral infection-induced type I IFN transcription through the IPS-1 pathway (6, 7). Notably, MDA-5 activity has been implicated in a biological setting without any signs of active viral infection. The overexpression of full-length of MDA-5, but not that of the CARD domain-or ATPase domain-deleted mutant, induced melanoma cells to apoptosis (8). Now that RIG-I expression can be highly induced by regulatory cues other than type I IFN, such as RA and IFN-␥, we postulated that additional biological funct...