Direct conversion or transdifferentiation of specific cell types to a new, distinct cell identity has implications in basic biology as well as biomedicine. However, transcription factors (TFs) required for induction of specific cell differentiation programs during development are often inefficient in imposing such programs on other differentiated cells. It has recently been shown that inhibitory mechanisms could be involved in preventing these TFs from driving their differentiation programs in other cell types (Tursun et al. 2011, Patel et al. 2012). Such inhibition may be due to safeguarding mechanisms that protect and maintain cell identity. In order to identify factors contributing to safeguarding cells against ectopic fates we performed RNAi screens. We used transgenic animals expressing CHE-1, the ASE neuron fate-inducing TF, under the heat shock promoter. Transgenic animals also contain the ASE neuron-specific reporter gcy-5::gfp, expressed in only one ASE neuron under non-heat-shocked conditions. After inducing ubiquitous che-1 mis-expression, gcy-5::gfp is visible in a few other head neurons but no broad induction in other tissues is detectable. However, upon depletion of members of the histone chaperone complex FACT: hmg-3, hmg-4 and F55A3.3, a homolog of human SUPT16H which we name spt-16, gcy-5::gfp can be induced in various tissues such as gut and germline. Likewise, depletion of FACT network members such as transcription elongation factors emb-5 and spt-5, and the chromatin remodeling factor isw-1 phenocopy FACT RNAi. Removal of hmg-3 or isw-1 by RNAi permits CHE-1 to convert germ cells directly into ASE neuron-like cells. Moreover, we identified that mutants of a previously uncharacterized shorter variant of spt-16, F55A3.7 which we term sspt-16 (short spt-16), allow direct germ cell conversion into neuron-like cells. This suggests a hitherto unknown role in maintaining and protecting germ cell identity. Notably, depletion of FACT subunits leads to a loss of expression of several intestinal fate markers suggesting that FACT complex plays an important role in maintaining cell fate identity of gut cells. Furthermore, we revealed a new role for FACT complex as a gatekeeper of specific histone modifications such as H3Ser10ph in order to maintain a refractory epigenetic signature toward induction of ectopic cell fates. This is additionally supported by ubiquitous drug-induced modulation of the phosphorylation level in the cells.
Our study suggests that the histone chaperone complex FACT plays a crucial role in maintaining cell identity and protecting germ cells from being directly converted into neuron-like cells.
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