The ability of transcription factors to directly reprogram the identity of cell types is usually restricted and is defined by cellular context. We show here that through ectopic expression of single C. elegans transcription factors, the identity of mitotic germ cells can be directly converted into that of specific neuron types (glutamatergic, cholinergic or GABAergic). This reprogramming event requires the removal of the histone chaperone LIN-53/RbAp48, a component of several histone remodeling and modifying complexes, and this removal can be mimicked by chemical inhibition of histone deacetylases. Our findings illustrate the ability of germ cells to be directly converted into individual, terminally differentiated neuron types and demonstrate that a specific chromatin factor provides a barrier for cellular reprogramming.

Engineering fluorescent proteins into large genomic clones, contained within BACs or fosmid vectors, is a tool to visualize and study spatiotemporal gene expression patterns in transgenic animals. Because these reporters cover large genomic regions, they most likely capture all cis-regulatory information and can therefore be expected to recapitulate all aspects of endogenous gene expression. Inserting tags at the target gene locus contained within genomic clones by homologous recombination (“recombineering”) represents the most straightforward method to generate these reporters. In this methodology paper, we describe a simple and robust pipeline for recombineering of fosmids, which we apply to generate reporter constructs in the nematode C. elegans, whose genome is almost entirely covered in an available fosmid library. We have generated a toolkit that allows for insertion of fluorescent proteins (GFP, YFP, CFP, VENUS, mCherry) and affinity tags at specific target sites within fosmid clones in a virtually seamless manner. Our new pipeline is less complex and, in our hands, works more robustly than previously described recombineering strategies to generate reporter fusions for C. elegans expression studies. Furthermore, our toolkit provides a novel recombineering cassette which inserts a SL2-spliced intercistronic region between the gene of interest and the fluorescent protein, thus creating a reporter controlled by all 5′ and 3′ cis-acting regulatory elements of the examined gene without the direct translational fusion between the two. With this configuration, the onset of expression and tissue specificity of secreted, sub-cellular compartmentalized or short-lived gene products can be easily detected. We describe other applications of fosmid recombineering as well. The simplicity, speed and robustness of the recombineering pipeline described here should prompt the routine use of this strategy for expression studies in C. elegans.

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How specific cell types can be directly converted into other distinct cell types is a matter of intense investigation with wide-ranging basic and biomedical implications. We show here that the removal of the histone 3 lysine 27 (H3K27) methyltransferase complex PRC2 (“Polycomb Repressor Complex 2”) permits ectopically expressed, neuron-type-specific transcription factors (“terminal selectors”) to convert C. elegans germ cells directly into specific neuron types. Terminal selector-induced germ cell-to-neuron conversion can not only be observed upon genome-wide loss of H3K27 methylation in PRC2(−) animals, but also upon genome-wide redistribution of H3K27 methylation patterns in animals which lack the H3K36 methyltransferase MES-4. Manipulation of the H3K27 methylation status not only permits conversion of germ cells into neurons, but also permits hlh-1/MyoD-dependent conversion of germ cells into muscle cells, indicating the PRC2 protects the germline from the aberrant execution of multiple distinct somatic differentiation programs. Taken together, our findings demonstrate that the normally multi-step process of development from a germ cell via a zygote to a terminally differentiated somatic cell type can be shortcut by providing an appropriate terminal selector transcription factor and manipulating histone methylation patterns.

LIM kinase 1 (LIMK1) controls important cellular functions such as morphogenesis, cell motility, tumor cell metastasis, development of neuronal projections, and growth cone actin dynamics. We have investigated the role of the RING finger protein Rnf6 during neuronal development and detected high Rnf6 protein levels in developing axonal projections of motor and DRG neurons during mouse embryogenesis as well as cultured hippocampal neurons. RNAi-mediated knock-down experiments in primary hippocampal neurons identified Rnf6 as a regulator of axon outgrowth. Consistent with a role in axonal growth, we found that Rnf6 binds to, polyubiquitinates, and targets LIMK1 for proteasomal degradation in growth cones of primary hippocampal neurons. Rnf6 is functionally linked to LIMK1 during the development of axons, as the changes in axon outgrowth induced by up-or down-regulation of Rnf6 levels can be restored by modulation of LIMK1 expression. Thus, these results assign a specific role for Rnf6 in the control of cellular LIMK1 concentrations and indicate a new function for the ubiquitin/proteasome system in regulating local growth cone actin dynamics.

SummaryThe chromatin regulator FACT (facilitates chromatin transcription) is essential for ensuring stable gene expression by promoting transcription. In a genetic screen using Caenorhabditis elegans, we identified that FACT maintains cell identities and acts as a barrier for transcription factor-mediated cell fate reprogramming. Strikingly, FACT’s role as a barrier to cell fate conversion is conserved in humans as we show that FACT depletion enhances reprogramming of fibroblasts. Such activity is unexpected because FACT is known as a positive regulator of gene expression, and previously described reprogramming barriers typically repress gene expression. While FACT depletion in human fibroblasts results in decreased expression of many genes, a number of FACT-occupied genes, including reprogramming-promoting factors, show increased expression upon FACT depletion, suggesting a repressive function of FACT. Our findings identify FACT as a cellular reprogramming barrier in C. elegans and humans, revealing an evolutionarily conserved mechanism for cell fate protection.

Autophagy is a ubiquitous catabolic process that causes cellular bulk degradation of cytoplasmic components and is generally associated with positive effects on health and longevity. Inactivation of autophagy has been linked with detrimental effects on cells and organisms. The antagonistic pleiotropy theory postulates that some fitness-promoting genes during youth are harmful during aging. On this basis, we examined genes mediating post-reproductive longevity using an RNAi screen. From this screen, we identified 30 novel regulators of post-reproductive longevity, including Through downstream analysis of, we identified that the inactivation of genes governing the early stages of autophagy up until the stage of vesicle nucleation, such as , strongly extend both life span and health span. Furthermore, our data demonstrate that the improvements in health and longevity are mediated through the neurons, resulting in reduced neurodegeneration and sarcopenia. We propose that autophagy switches from advantageous to harmful in the context of an age-associated dysfunction.

Mammary oncogenesis is profoundly influenced by signaling pathways controlled by estrogen receptor A (ERA). Although it is known that ERA exerts its oncogenic effect by stimulating the proliferation of many human breast cancers through the activation of target genes, our knowledge of the underlying transcriptional mechanisms remains limited. Our published work has shown that the in vivo activity of LIM homeodomain transcription factors (LIM-HD) is critically regulated by cofactors of LIM-HD proteins (CLIM) and the ubiquitin ligase RING finger LIM domain-interacting protein (RLIM). Here, we identify CLIM and RLIM as novel ERA cofactors that colocalize and interact with ERA in primary human breast tumors. We show that both cofactors associate with estrogen-responsive promoters and regulate the expression of endogenous ERA target genes in breast cancer cells. Surprisingly, our results indicate opposing functions of LIM cofactors for ERA and LIM-HDs: whereas CLIM enhances transcriptional activity of LIM-HDs, it inhibits transcriptional activation mediated by ERA on most target genes in vivo. In turn, the ubiquitin ligase RLIM inhibits transcriptional activity of LIM-HDs but enhances transcriptional activation of endogenous ERA target genes. Results from a human breast cancer tissue microarray of 1,335 patients revealed a highly significant correlation of elevated CLIM levels to ER/progesterone receptor positivity and poor differentiation of tumors. Combined, these results indicate that LIM cofactors CLIM and RLIM regulate the biological activity of ERA during the development of human breast cancer.