Hierarchical reactivation of transcription during mitosis-to-G1 transition by Brn2 and Ascl1 in neural stem cells

During mitosis, chromatin condensation is accompanied by a global arrest of transcription. Recent studies suggest transcriptional reactivation upon mitotic exit occurs in temporally coordinated waves, but the underlying regulatory principles have yet to be elucidated. In particular, the contribution of sequence-specific transcription factors (TFs) remains poorly understood. Here we report that Brn2, an important regulator of neural stem cell identity, associates with condensed chromatin throughout cell division, as assessed by live-cell imaging of proliferating neural stem cells. In contrast, the neuronal fate determinant Ascl1 dissociates from mitotic chromosomes. ChIP-seq analysis reveals that Brn2 mitotic chromosome binding does not result in sequence-specific interactions prior to mitotic exit, relying mostly on electrostatic forces. Nevertheless, surveying active transcription using single-molecule RNA-FISH against immature transcripts reveals differential reactivation kinetics for key targets of Brn2 and Ascl1, with transcription onset detected in early (anaphase) versus late (early G1) phases, respectively. Moreover, by using a mitotic-specific dominant-negative approach, we show that competing with Brn2 binding during mitotic exit reduces the transcription of its target gene Nestin Our study shows an important role for differential binding of TFs to mitotic chromosomes, governed by their electrostatic properties, in defining the temporal order of transcriptional reactivation during mitosis-to-G1 transition.

Brn2 Alone Is Sufficient to Convert Astrocytes into Neural Progenitors and Neurons

Generating neurons or neural progenitor cells (NPCs) from astrocytes is a potential strategy for neurological repair by reprogramming. Previous study has showed that Brn2, by cooperating with other factors, participates in neurogenesis and neuronal reprogramming. However, it is still unclear whether the Brn2 alone can convert astrocytes into neurons or NPCs. Here, we explored the effect of Brn2 on reprogramming of astrocytes, and found that a single transcription factor Brn2 can convert mouse astrocytes into functional neurons. Furthermore, the Brn2-infected astrocytes can be induced into NPCs after changing culture condition. In addition, our study found that the reprogramming of astrocytes and the fate of transdifferentiated cells are closely associated with cell microenvironmental factors, such as the brain regions where the astrocytes come from, the proliferation ability of astrocytes, and culture condition of infected astrocytes. To sum up, for the first time, our results demonstrated that Brn2 alone is sufficient to convert astrocytes into neural progenitors and neurons, and the conversion is associated with cell microenvironments. This new conversion method will be a potential therapeutic approach to restore the injured diseased brain in regenerative medicine.

Effects of Brn2 overexpression on eccrine sweat gland development in the mouse paw

Eccrine sweat glands regulate body temperature by secreting water and electrolytes. In humans, eccrine sweat glands are ubiquitous in the skin, except in the lips and external genitalia. In mice, eccrine sweat glands are present only in the paw pad. Brn2 is a protein belonging to a large family of transcription factors. A few studies have examined Brn2 in melanoma cells and epidermal keratinocytes. This study investigated changes in the skin in the K5-Brn2 transgenic mouse, which overexpresses Brn2 and contains the keratin 5 promotor. Interestingly, the volume of eccrine sweat glands was reduced markedly in the K5-Brn2 transgenic mouse compared with the wild-type, while the expression of aquaporin 5, important molecule in sweat secretion, was increased in each sweat gland cell, probably to compensate for the reduction in gland development. However, sweating response to a pilocarpine injection in the hind paw was significantly decreased in the K5-Brn2 transgenic mouse compared with the wild-type. The paw epidermis was thicker in the K5-Brn2 transgenic mouse compared with the wild-type. Taken together, eccrine sweat gland development and sweat secretion were suppressed markedly in the K5-Brn2 transgenic mouse. These results may be associated with dominant development of the epidermis by Brn2 overexpression in the paw skin.

Mash1 efficiently reprograms rat astrocytes into neurons

To date, it remains poorly understood whether astrocytes can be easily reprogrammed into neurons. Mash1 and Brn2 have been previously shown to cooperate to reprogram fibroblasts into neurons. In this study, we examined astrocytes from 2-month-old Sprague-Dawley rats, and found that Brn2 was expressed, but Mash1 was not detectable. Thus, we hypothesized that Mash1 alone could be used to reprogram astrocytes into neurons. We transfected a recombinant MSCV-MASH1 plasmid into astrocytes for 72 hours, and saw that all cells expressed Mash1. One week later, we observed the changes in morphology of astrocytes, which showed typical neuronal characteristics. Moreover, β-tubulin expression levels were significantly higher in astrocytes expressing Mash1 than in control cells. These results indicate that Mash1 alone can reprogram astrocytes into neurons.

Direct conversion of human fibroblasts into neuronal restricted progenitors

Neuronal restricted progenitors (NRPs) represent a type of transitional intermediate cells that lie between multipotent neural progenitors and terminal differentiated neurons during neurogenesis. These NRPs have the ability to self-renew and differentiate into neurons, but not into glial cells, which is considered an advantage for cellular therapy of human neurodegenerative diseases. However, difficulty in the extraction of highly purified NRPs from normal nervous tissue prevents further studies and applications. In this study, we report the conversion of human fetal fibroblasts into human induced NRPs (hiNRPs) in 11 days by using just three defined factors: Sox2, c-Myc, and either Brn2 or Brn4. The hiNRPs exhibited distinct neuronal characteristics, including cell morphology, multiple neuronal marker expression, self-renewal capacity, and a genome-wide transcriptional profile. Moreover, hiNRPs were able to differentiate into various terminal neurons with functional membrane properties but not glial cells. Direct generation of hiNRPs from somatic cells will provide a new source of cells for cellular replacement therapy of human neurodegenerative diseases.