Lamina‐specific distribution of synaptopodin, an actin‐associated molecule essential for the spine apparatus, in identified principal cell dendrites of the mouse hippocampus

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Abstract
Synaptopodin is an actin‐associated molecule found in a subset of telencephalic spines. It is an essential component of the spine apparatus, a Ca2+‐storing organelle and has been implicated in synaptic plasticity (Deller et al. [2003] Proc Natl Acad Sci U S A 100:10494–10499). In the rodent hippocampus, Synaptopodin is distributed in a characteristic region‐ and lamina‐specific manner. To learn more about the cellular basis underlying this distribution, the regional, laminar, and cellular localization of Synaptopodin and its mRNA were analyzed in mouse hippocampus. First, Synaptopodin puncta densities were quantified after immunofluorescent labeling using confocal microscopy. Second, the dendritic distribution of Synaptopodin‐positive puncta was studied using three‐dimensional confocal reconstructions of Synaptopodin‐immunostained and enhanced green fluorescence protein (EGFP)‐labeled principal neurons. Synaptopodin puncta located within dendrites of principal neurons were primarily found in spines (>95%). Analysis of dendritic segments located in different layers revealed lamina‐specific differences in the percentage of Synaptopodin‐positive spines. Densities ranged between 37% (outer molecular layer) and 14% (stratum oriens; CA1). Finally, synaptopodin mRNA expression was studied using in situ hybridization, laser microdissection, and quantitative reverse transcriptase‐polymerase chain reaction. Expression levels were comparable between all regions. These data demonstrate a lamina‐specific distribution of Synaptopodin within dendritic segments of identified neurons. Within dendrites, the majority of Synaptopodin‐positive puncta were located in spines where they represent spine apparatuses. We conclude, that this organelle is distributed in a region‐ and layer‐specific manner in the mouse hippocampus and suggest that differences in the activity of afferent fiber systems could determine its distribution. J. Comp. Neurol. 487:227–239, 2005.