Pioneer transcription factors (TFs) such as OCT4 can target silent genes embedded in nucleosome-dense regions. How nucleosome interaction enables TFs to target chromatin and determine cell identity remains elusive. Here, we systematically dissect OCT4 to show that nucleosome binding is encoded within the DNA-binding domain and yet can be uncoupled from free DNA binding. Furthermore, accelerating the binding kinetics of OCT4 to DNA enhances nucleosome binding. In cells, uncoupling nucleosome binding diminishes the ability of OCT4 to individually access closed chromatin, while more dynamic nucleosome binding results in expansive genome scanning within closed chromatin. However, both uncoupling and enhancing nucleosome binding are detrimental to inducing pluripotency from differentiated cells. Remarkably, stable interactions between OCT4 and nucleosomes are continuously required for maintaining the accessibility of pluripotency enhancers in stem cells. Our findings reveal how the affinity and residence time of OCT4-nucleosome complexes modulate chromatin accessibility during cell fate changes and maintenance. 3 MAIN To maintain cell identity, TFs are often associated with accessible enhancers and promoters of active genes. However, during cell fate changes, certain TFs target silent genes within closed chromatin, acting as "pioneer factors" 1 . For instance, Oct4, Sox2, Klf4 and c-Myc (OSKM) predominantly occupy open chromatin to maintain pluripotency in embryonic stem cells (ESCs) 2-4 . Yet, during early reprogramming of fibroblasts to induced pluripotent stem cells (iPSCs), OSK, but not c-Myc, act as pioneer factors 5-9 .Nonetheless, binding of OSK to open chromatin has also been suggested to be important for reprogramming 10,11 . To date, it has not been possible to separate the conventional open chromatin binding from closed chromatin targeting, limiting the ability to directly examine the role of pioneer activity in pluripotency.We have previously shown that OSK can directly interact with nucleosomes like the paradigm pioneer factor FoxA, supporting a link between pioneer activity and nucleosome binding [12][13][14][15][16][17] . A systematic evolution of ligands by exponential enrichment (SELEX) study has revealed that a wide variety of DNA-binding domains (DBDs) can bind nucleosomes in vitro 18 . Commonly, DBDs containing short anchoring α helices have been shown to interact most strongly with nucleosomes in vitro 19 . However, FoxA-DBD requires an extra helical region to bind nucleosomes and open chromatin, suggesting that DBDs may not contain the full pioneer capacity 15,20 . Recently, the cryo-EM structure of OCT4-DBD co-bound with SOX2-DBD to an engineered nucleosome has shown a potential OCT4-SOX2 nucleosome readout that involves the OCT4 POU-specific domain (POUS) and SOX2 high mobility group (HMG), but not the POU-homeodomain (POUHD) of OCT4-DBD 21 . The cooperative interaction between OCT4 and SOX2 is critical for pluripotency

The highly restrictive blood-brain barrier (BBB) plays a critically important role in maintaining brain homeostasis and is pivotal for proper neuronal function. The BBB is currently considered the main limiting factor restricting the passage of large (up to 200 nm) intravenously administered nanoparticles to the brain. Breakdown of the barrier occurs as a consequence of cerebrovascular diseases and traumatic brain injury. In this article, we report that remote injuries in the CNS are also associated with BBB dysfunction. In particular, we show that a focal partial transection of the optic nerve triggers a previously unknown transient opening of the mammalian BBB that occurs in the visual centres. Importantly, we demonstrate that this transient BBB breakdown results in a dramatic change in the biodistribution of intravenously administered large polymeric nanoparticles which were previously deemed as BBB-impermeable.

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