The majority of the characterized NETs bind
The majority of the characterized NETs bind lamins and/or chromatin binding proteins (Harr et al., 2016; Kind et al., 2013; Wong et al., 2014). Chromatin proteins in pluripotent stem ache inhibitors display hyperdynamic plasticity (Chen and Dent, 2014; Meshorer et al., 2006). During differentiation, when stem cells lose their pluripotency, chromatin becomes more static and organized (Ricci et al., 2015). Cells tether tightly packed heterochromatin to the nuclear periphery, in a Lamin A/C dependent (A-tether) and/or Lamin B receptor dependent (B-tether) manner (Solovei et al., 2013). Chromatin is first tethered by the B-tether and then during differentiation by the A-tether. The B-tether has a generally silencing effect and is made up by Lamin B and Lamin B receptor. Whereas the A-tether is important for differentiation, and involves Lamin A/C and tissue-specific inner nuclear membrane (INM) proteins, including chromatin binding LEM-domain proteins. Disruption of these tethering proteins affects gene expression and cellular differentiation, for example in muscle differentiation (myogenesis) (Solovei et al., 2013). The nuclear envelope is highly specialized in each specific cell type, with most NETs expressed in a tissue-specific manner, comparing three tissues (muscle, liver and leukocytes) only 16% of the NETs were shared (Worman and Schirmer, 2015). Many, not yet identified, proteins might be involved in this tethering process (Czapiewski et al., 2016) and could have their own unique function in the diversity of tissue-specific differentiation.
In this paper we used human iPSCs (hereon referred to as just iPSCs) as a model for stem cell differentiation, to study if the INM protein Samp1 (spindle associated membrane protein 1) could have a role in the differentiation process. Samp1 (Buch et al., 2009) (also called NET5 in rat liver (Schirmer et al., 2003) and Tmem201) binds directly to Emerin (Gudise et al., 2011; Jafferali et al., 2014) and is functionally associated to proteins of the A-type lamina network (Borrego-Pinto et al., 2012; Jafferali et al., 2014). Accordingly, we asked whether Samp1 could act as an A-tether component and promote cell differentiation. In this report we find that Samp1 levels increased in the nuclear periphery during differentiation in parallel with expression of the early differentiation marker Lamin A/C, and was able to induce a rapid differentiation of iPSCs despite pluripotent culturing conditions.
Results and discussion
Acknowledgement We would like to thank Dr. Katherine Wilson, Johns Hopkins University School of Medicine, Baltimore for sharing plasmid encoding YFP-Emerin. This work was supported by grants from the Swedish Research Council #621-2010-448, Cancer Foundation #110590 and Olle Engkvists Minne.
Introduction Inner ear hair cells (HCs), primary transducers for perception of sound and balance, are not regenerated in mammals once they are lost (Hawkins et al., 1976; Schacht, 1986), thus replacement using various medical strategies, such as gene or cell therapy, is required to improve hearing ability (de Felipe et al., 2011; Qi et al., 2014; Hu and Ulfendahl, 2013; Okano and Kelley, 2012). Forced expression of the transcription factor Math1 (also known as Atoh1) via viral infection has been reported to generate new HCs in vivo (Kawamoto et al., 2003; Zheng and Gao, 2000; Staecker et al., 2014; Husseman and Raphael, 2009), while induction of HC-like cells from various stem cell sources has also been achieved using novel methods (McLean et al., 2016; Elbana et al., 2015; Hartman et al., 2015; Bramhall et al., 2014) and applied as translational therapy for individuals with hearing loss (Li et al., 2004; Oiticica et al., 2010; Xu et al., 2016; Barboza et al., 2016; Jongkamonwiwat et al., 2010). Among several candidate cell sources, including neural stem cells (NSCs) from adult brain tissues (Ito et al., 2001), mesenchymal stem cells from bone marrow (Jeon et al., 2007), and olfactory precursor cells (Doyle et al., 2007), embryonic stem (ES) (Rathjen and Rathjen, 2001) and induced-pluripotent stem (iPS) cells (Takahashi and Yamanaka, 2006) are considered to be particularly promising because of their ability for self-renewal and pluripotency. Induction of inner ear HCs from ES and iPS cells was first reported by Oshima et al. (2010), though their efficiency for induction of HCs was found to be low, accounting for only around 1% to 2% of the total cell population. Recently, an elegant protocol for generation of inner ear organoids from mouse ES cells using a 3D culture method was reported, in which chemically defined culture medium with conditions mimicking the developmental environment of the inner ear was used under a 3D setting (Koehler et al., 2013; Liu et al., 2016; DeJonge et al., 2016). Although this 3D culture method does not require use of exogenous tissue or undefined medium components, several steps are needed and the main products are vestibular sensory organ-like structures.