The spine apparatus organelle

The spine apparatus (arrow) is an enigmatic cellular organelle composed of stacked smooth endoplasmic reticulum (Gray 1959; Spacek and Harris 1997; Deller et al., 2000), which is found in a subset of dendritic spines of telencephalic neurons. Although it has been suggested that the spine apparatus could play a role in the regulation of intracellular calcium levels (Fifkova et al., 1983) as well as local protein synthesis and trafficking (Pierce et al., 2000), its function remains not well understood. The discovery of the actin-binding molecule synaptopodin (Mundel et al., 1997), which is a marker and essential component of the spine apparatus (Deller et al., 2000; 2003) facilitated the research in this field during the past years. Synaptopodin-deficient mice do not form spine apparatus organelles and show deficits in LTP and spatial learning (Deller et al., 2003; Jedlicka et al., 2009). Synaptopodin is expressed postnatally (Czarnecki et al., 2005) and distributed in the hippocampus in a layer-specific manner (Deller et al., 2000; Bas Orth et al., 2005). Its expression/distribution can change under various experimental conditions (e.g., Yamazaki et al., 2001; Fukazawa et al., 2003; Deller et al., 2006; Vlachos et al., 2008; Fester et al., 2009; Vlachos et al., 2009). Recent work has linked synaptopodin to structural remodeling of dendritic spines (Okubo-Suzuki et al., 2008; Vlachos et al., 2009) and has revealed an important role for synaptopodin-associated intracellular calcium surges (Vlachos et al., 2009; Korkotian and Segal 2011) in regulating the accumulation of AMPA-receptors at excitatory postsynapses (Holbro et al., 2009; Vlachos et al., 2009). For reviews on synaptopodin and the spine apparatus see Deller et al., 2000; Deller et al. 2007, Jedlicka et al., 2008; Segal et al., 2010; Vlachos 2012. Electron micrograph from a 3-weeks-old entorhino-hippocampal slice culture (outer molecular layer of the dentate gyrus). Scale bar: 500nm.