Abstract
The balance between stem cell maintenance and differentiation has been proposed to depend on antagonizing ubiquitination and deubiquitination reactions of key stem cell transcription factors (SCTFs) mediated by pairs of E3 ubiquitin ligases and deubiquitinating enzymes. Accordingly, increased ubiquitination results in proteasomal degradation of the SCTF, thereby inducing cellular differentiation, whereas increased deubiquitination stabilizes the SCTF, leading to maintenance of the stem cell fate. In neural stem cells, one of the key SCTFs is c-Myc. Previously, it has been shown that c-Myc is ubiquitinated by the E3 ligase TRIM32, thereby targeting c-Myc for proteasomal degradation and inducing neuronal differentiation. Accordingly, TRIM32 becomes upregulated during adult neurogenesis. This upregulation is accompanied by subcellular translocation of TRIM32 from the cytoplasm of neuroblasts to the nucleus of neurons. However, we observed that a subpopulation of proliferative type C cells already contains nuclear TRIM32. As these cells do not undergo neuronal differentiation, despite containing TRIM32 in the nucleus, where it can ubiquitinate c-Myc, we hypothesize that antagonizing factors, specifically deubiquitinating enzymes, are present in these particular cells. Here we show that TRIM32 associates with the deubiquitination enzyme USP7, which previously has been implicated in neural stem cell maintenance. USP7 and TRIM32 were found to exhibit a dynamic and partially overlapping expression pattern during neuronal differentiation both in vitro and in vivo. Most importantly, we are able to demonstrate that USP7 deubiquitinates and thereby stabilizes c-Myc and that this function is required to maintain neural stem cell fate. Accordingly, we propose the balanced ubiquitination and deubiquitination of c-Myc by TRIM32 and USP7 as a novel mechanism for stem cell fate determination.
Highlights
We have shown that the cell-fate determinant TRIM32 becomes upregulated during neuronal differentiation of mouse Neural stem cells (NSCs) in vitro [13, 19] as well as during mouse embryonic [14] and adult neurogenesis in vivo [18]
We further observed a subpopulation of cells expressing low levels of TRIM32 in the nucleus (Fig. 1, yellow circles), which co-expressed Ki67 and Mash1 (Fig. 1)
Since type C cells are highly proliferative and do not yet undergo neuronal differentiation, we hypothesized that the ubiquitination function of TRIM32 towards c-Myc, which induces neuronal differentiation, has to be antagonized in these cells
Summary
One of the major forms of post-translational protein modifications in eukaryotes is ubiquitination, which serves as a key regulatory mechanism to control the stability and activity of proteins [2,3,4,5]. It has been previously proposed, that the fate specification of stem cells is controlled by antagonizing ubiquitination and deubiquitination of so-called stem cell transcription factors (SCTFs) [6, 7].
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