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    • Here we have studied the fine ultrastructure and three dimen

    2018-11-08

    Here we have studied the fine ultrastructure and three-dimensional (3D) organization of these invaginations and show that they correspond to envelope-limited chromatin sheets (ELCS). These structures were originally described by Davies and Small (1968) in neutrophils, and named envelope-limited sheets (ELS). ELS have an unusual type of nuclear morphology characterized by the presence of a sheet of chromatin (∼30 nm thick) bound on two sides by the inner nuclear membrane (INM), creating a highly reproducible and regular “sandwich” of 40 nm thickness (Davies and Small, 1968). These structures, later called ELCS, are associated with the NE proteins Lamin B, Lamin B receptor (LBR), and Lap2 (Ghadially, 1997; Olins et al., 1998; Olins and Olins, 2009). Interestingly, ELCS have only been reported in certain lymphocytes and some cancer cells, including the CNS neuroectodermal tumor medulloblastoma (Tani et al., 1971). Furthermore, we show here that V-SVZ B1 cells with ELCS correspond to qNSCs in mice.
    Results
    Discussion In this study, we show that a subset of adult V-SVZ nebivolol hcl have NE-limited chromatin sheets or ELCS. Using molecular markers, 3H-Thy, and the antimitotic drug Ara-C, we found that B1 cells with ELCS correspond to qNSCs. TEM analysis revealed that nuclear ELCS start to appear in RGCs in the embryo around E14.5 and are present throughout adult life in a subpopulation of V-SVZ B cells. 3H-Thy birth dating suggests that quiescent B cells with ELCS have an embryonic origin. We also detected the expression of epigenetic markers associated with repression and telomeres within ELCS. This structure may represent a specific nuclear compartment associated with quiescent pre-B and B cells. Our work suggests that this unique compartment of the NE is associated with quiescence and, in particular, with the subpopulation of progenitors that are set apart during embryonic development to function in the juvenile and adult brain as NSCs. We found out that a subset of V-SVZ B cells display a nuclear structure, characterized by a thin chromatin layer bound to the inner and outer membranes. These distinct structures highly resemble the previously described ELCS in other cell types (Davies and Small, 1968; Olins et al., 2008). The ELCS we describe in post-natal qNSCs is similar to subtype 1-1 ELCS (a single chromatin sheet bound on two sides by cytoplasm) (Olins and Olins, 2009). In the CNS, ELCS have been observed in the developing human retina (Popoff and Ellsworth, 1969) and in the subcallosal zone cells during post-natal development (Wittmann et al., 2009). Outside the CNS the presence of ELCS has been mainly reported in myeloid, lymphoid (Davies and Small, 1968; Olins et al., 1998), and cancer cells (Mollo et al., 1969; Tani et al., 1971). The functional significance of ELCS remains elusive. It has been proposed that it could facilitate neutrophil functions (Rowat et al., 2013) and may also be a part of a developmental program to shut off gene activity during terminal differentiation (Sanchez and Wangh, 1999). Remarkably, tumor cells with ELCS survive to radiation and antimitotic treatments (Ahearn et al., 1967; Erenpreisa et al., 2002; Stalzer et al., 1965), suggesting that ELCS may be present in quiescent cancer cells. Further research is needed to determine the function of ELCS. Our observations are in line with the idea that ELCS are linked to genomic rearrangements associated with quiescence in NSCs. Adult V-SVZ NSCs are a heterogeneous population of primary progenitors that can exist in either quiescent or activated state (Codega et al., 2014; Llorens-Bobadilla et al., 2015; Morshead et al., 1994). We found that B cells with ELCS are GFAP+, BLBP+, Glast+, Nestin−, and EGFR−, do not incorporate 3H-Thy, and survive following Ara-C treatment. These data suggest that ELCS are present in qNSCs. In addition, 83% of B1 cells have nuclear ELCS in contrast to the rest present a more spherical nucleus devoid of ELCS. This is consistent with previous observations reporting that 11.4% ± 1.3% of B1 cells are EGFR+ and correspond to aNSCs (Codega et al., 2014) while about 8.6% are actively dividing (Ponti et al., 2013). Transcriptomic analyses have suggested that qNSCs or dormant cells enter a primed-quiescent state before activation (Llorens-Bobadilla et al., 2015; Shin et al., 2015). However, due to the dynamic nature of this process, this intermediate state has not yet been well characterized by molecular or morphological characteristics. At present we cannot conclude whether B cells with ELCS include dormant qNSCs or only primed-quiescent cells, or a subpopulation of these. Future studies might help to unravel this question.