Ilbay, Orkan et al. (2013) International Worm Meeting "Ascaroside signals suppress heterochronic phenotypes of the daf-12(rh61) mutant."

Schroeder, Frank C., Ilbay, Orkan, Ren, Zhiji, Srinivasan, Jagan, Ambros, Victor

[International Worm Meeting, 2013]

During animal development, cells divide and adopt specific fates and progressively differentiate into distinct cell types. The relative timing of a series of developmental events determines morphological endpoints and may affect the physiology of an organism. In Caenorhabditis elegans development, there are four larval stages (L1-L4), and all cell division and differentiation events at all stages have been mapped. Heterochronic genes, whose products include developmentally-expressed microRNAs and transcription factors, control the temporal patterning of developmental events, and contribute to developmental robustness. One of the components of the heterochronic pathway, DAF-12, promotes L2-to-L3 cell fate transitions by positively regulating the transcription of certain let-7 family microRNAs. Under unfavorable conditions, such as crowding, scant food supplies, or elevated temperature, C. elegans L2 larvae can commit to developmental arrest as the stress-resistant dauer larva. In such unfavorable conditions, DAF-12 negatively regulates progression to L3 cell fates, in part by repressing expression of let-7 family microRNAs, and promotes dauer formation. Several daf-12 mutations have been characterized as causing defective dauer regulation or heterochronic phenotypes, or both. In particular daf-12(rh61) results in expression of a truncated form of DAF-12. daf-12(rh61) mutant animals are dauer-defective and have penetrant heterochronic phenotypes. Dauer formation is induced by ascarosides, signaling molecules produced by C. elegans that serve as measures of population status for individual worms. Here, we report that a combination of three dauer-inducing ascarosides, namely ascr#2, ascr#3, and ascr#5, suppresses heterochronic phenotype of daf-12(rh61). Interestingly, ascr#5 alone can suppress the rh61 heterochronic phenotype, while only moderately inducing dauer formation in N2 animals. This suggests that while signaling from all three ascarosides contributes to the dauer program, suppression of rh61 heterochronic phenotype is mainly regulated by ascr#5 signaling.

Bulut, Reyyan et al. (2021) International Worm Meeting "Positioning sea-2 and lin-66 in the heterochronic pathway in the context of continuous and L2d-interrupted development"

Ilbay, Orkan, Bulut, Reyyan, Ambros, Victor

[International Worm Meeting, 2021]

Caenorhabditis elegans larvae are born with 10 hypodermal stem cells (seam cells) along each side of their body. Seam cells execute an asymmetric (self-renewal) division before each molt, and in addition, at the L1 molt, certain seam cells divide symmetrically (doubling cell number in the corresponding lineage). The L2 stage symmetric seam cell divisions are specified by the heterochronic gene regulatory pathway and the transcription factor Hunchback-like-1 (HBL-1) is the most proximal heterochronic regulator of this L2-specific developmental event. Mutations that disrupt the timely down-regulation of HBL-1 result in abnormal seam cell numbers - too many seam cells if HBL-1 is over-expressed, or too few seam cells if HBL-1 is precociously down regulated. Previous research described two distinct and parallel HBL-1 down-regulation mechanisms: On the one hand, post-transcriptional silencing via let-7 family and lin-4 microRNA complementary sites in the 3'UTR of hbl-1 transcript and, on the other hand, post-translational regulation mediated by the lin-28/lin-46 axis of the heterochronic pathway. Importantly, which of these HBL-1 regulatory mechanisms predominates depends upon the larva's life history (and hence dictated by the environmental conditions): during the rapid and continuous development associated with replete culture conditions, microRNA-mediated post-transcriptional regulation of HBL-1 is the primary mode, whilst LIN-46-mediated post-translational regulation dominates during L2d interrupted development. Previous genetic screens identified additional factors that affect seam cell numbers, including sea-2, which encodes a zinc finger protein [1] and lin-66, which encodes a protein of unknown function [2]. sea-2 or lin-66 loss-of-function mutants cause increased seam cell numbers during continuous development, and molecular analysis suggested that these genes act as negative regulators of LIN-28 expression [1, 2]. Here we employed epistasis analysis and sensitized genetic backgrounds to investigate the functional relationship of sea-2 and lin-66 to other genes in the heterochronic pathway, particularly with respect to lin-28 and hbl-1, and during both continuous and L2d-interrupted development. Our results indicate that sea-2 and lin-66 (especially sea-2) are far more critical for the regulation of seam cell number during L2d-interrupted development than during continuous development. Also, our results suggest that sea-2 may function by opposing a previously undefined lin-28/hbl-1 axis that is independent of lin-46, and that lin-66 may regulate hbl-1 downstream of lin-28 and lin-46. Our findings suggest additional regulation mechanisms for a developmentally critical transcription factor and expands our understanding of how robustness of animal development is achieved against adverse growth conditions. [1] PMID: 21471153 [2] PMID: 17139256