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Synaptopodin: an actin-associated protein in telencephalic dendrites and renal podocytes.

Mundel P, Heid HW, Mundel TM, Krüger M, Reiser J, Kriz W - J. Cell Biol. (1997)

Bottom Line: In particular, synaptopodin does not contain functional domains found in receptor-clustering PSD proteins.The exclusive synaptopodin synthesis in the telencephalon has been confirmed by in situ hybridization, where synaptopodin mRNA is only found in perikarya of the olfactory bulb, cerebral cortex, striatum, and hippocampus, i.e., the expression is restricted to areas of high synaptic plasticity.From these results and experiments with cultured cells we conclude that synaptopodin represents a novel kind of proline-rich, actin-associated protein that may play a role in modulating actin-based shape and motility of dendritic spines and podocyte foot processes.

View Article: PubMed Central - PubMed

Affiliation: Department of Anatomy and Cell Biology, University of Heidelberg, Germany. peter.mundel@urz.uni-heidelberg.de

ABSTRACT
Synaptopodin is an actin-associated protein of differentiated podocytes that also occurs as part of the actin cytoskeleton of postsynaptic densities (PSD) and associated dendritic spines in a subpopulation of exclusively telencephalic synapses. Amino acid sequences determined in purified rat kidney and forebrain synaptopodin and derived from human and mouse brain cDNA clones show no significant homology to any known protein. In particular, synaptopodin does not contain functional domains found in receptor-clustering PSD proteins. The open reading frame of synaptopodin encodes a polypeptide with a calculated Mr of 73.7 kD (human)/74.0 kD (mouse) and an isoelectric point of 9.38 (human)/9. 27 (mouse). Synaptopodin contains a high amount of proline ( approximately 20%) equally distributed along the protein, thus virtually excluding the formation of any globular domain. Sequence comparison between human and mouse synaptopodin revealed 84% identity at the protein level. In both brain and kidney, in vivo and in vitro, synaptopodin gene expression is differentiation dependent. During postnatal maturation of rat brain, synaptopodin is first detected by Western blot analysis at day 15 and reaches maximum expression in the adult animal. The exclusive synaptopodin synthesis in the telencephalon has been confirmed by in situ hybridization, where synaptopodin mRNA is only found in perikarya of the olfactory bulb, cerebral cortex, striatum, and hippocampus, i.e., the expression is restricted to areas of high synaptic plasticity. From these results and experiments with cultured cells we conclude that synaptopodin represents a novel kind of proline-rich, actin-associated protein that may play a role in modulating actin-based shape and motility of dendritic spines and podocyte foot processes.

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Immunofluorescence microcopy of synaptopodin in striatum. This micrograph shows the transition between the telencephalic  putamen (P) and the diencephalic globus pallidum (G). (a) Synaptopodin staining is restricted to the putamen and ends at the border to  the diencephalon (arrows). (b) Staining with anti-synaptophysin to detect all synapses. Bar, 1.5 μm.
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Figure 5: Immunofluorescence microcopy of synaptopodin in striatum. This micrograph shows the transition between the telencephalic putamen (P) and the diencephalic globus pallidum (G). (a) Synaptopodin staining is restricted to the putamen and ends at the border to the diencephalon (arrows). (b) Staining with anti-synaptophysin to detect all synapses. Bar, 1.5 μm.

Mentions: To determine the distribution of synaptopodin in the brain, we performed immunofluorescence stainings of sagittal and coronal serial sections of the adult rat brain. Synaptopodin was found in the olfactory bulb, the cerebral cortex, the striatum, and the hippocampus (Fig. 4). The immunoreactivity was observed in the dendritic layers, whereas the perikarya were free of labeling. No other regions of the central nervous system (CNS) showed any expression of synaptopodin as confirmed by additional immunohistochemical mapping of serial coronal sections from a complete rat brain using the preembedding peroxidase technique (data not shown). Applying the polyclonal antisera NT-61 and 26-1E, the same pattern of immunoreactivity was observed as with the original mAb G1 (data not shown). Within the striatum, synaptopodin expression was restricted to the telencephalic part, i.e., the putamen, whereas the globus pallidum, which belongs to the diencephalon, was not reactive with anti-synaptopodin antibodies (Fig. 5). To reveal the subcellular localization of synaptopodin, we performed preembedding PAP labeling experiments of telencephalic regions that had been found to express synaptopodin. The electron-dense reaction product was restricted to postsynaptic densites and associated dendritic spines of a subset of synapses (Fig. 6). While the PSD itself was homogeneously decorated with the DAB reaction product (Fig. 6, a and b), in the adjacent dendritic shaft a clustered arrangement of the immunoreactivity was observed (Fig. 6, c and d). The restriction of synaptopodin expression to a subset of synapses was confirmed by a double immunofluorescence labeling approach with mAb G1 for synaptopodin and polyclonal anti-synaptophysin antiserum as a marker of all synapses (Fig. 7).


Synaptopodin: an actin-associated protein in telencephalic dendrites and renal podocytes.

Mundel P, Heid HW, Mundel TM, Krüger M, Reiser J, Kriz W - J. Cell Biol. (1997)

Immunofluorescence microcopy of synaptopodin in striatum. This micrograph shows the transition between the telencephalic  putamen (P) and the diencephalic globus pallidum (G). (a) Synaptopodin staining is restricted to the putamen and ends at the border to  the diencephalon (arrows). (b) Staining with anti-synaptophysin to detect all synapses. Bar, 1.5 μm.
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Related In: Results  -  Collection

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Figure 5: Immunofluorescence microcopy of synaptopodin in striatum. This micrograph shows the transition between the telencephalic putamen (P) and the diencephalic globus pallidum (G). (a) Synaptopodin staining is restricted to the putamen and ends at the border to the diencephalon (arrows). (b) Staining with anti-synaptophysin to detect all synapses. Bar, 1.5 μm.
Mentions: To determine the distribution of synaptopodin in the brain, we performed immunofluorescence stainings of sagittal and coronal serial sections of the adult rat brain. Synaptopodin was found in the olfactory bulb, the cerebral cortex, the striatum, and the hippocampus (Fig. 4). The immunoreactivity was observed in the dendritic layers, whereas the perikarya were free of labeling. No other regions of the central nervous system (CNS) showed any expression of synaptopodin as confirmed by additional immunohistochemical mapping of serial coronal sections from a complete rat brain using the preembedding peroxidase technique (data not shown). Applying the polyclonal antisera NT-61 and 26-1E, the same pattern of immunoreactivity was observed as with the original mAb G1 (data not shown). Within the striatum, synaptopodin expression was restricted to the telencephalic part, i.e., the putamen, whereas the globus pallidum, which belongs to the diencephalon, was not reactive with anti-synaptopodin antibodies (Fig. 5). To reveal the subcellular localization of synaptopodin, we performed preembedding PAP labeling experiments of telencephalic regions that had been found to express synaptopodin. The electron-dense reaction product was restricted to postsynaptic densites and associated dendritic spines of a subset of synapses (Fig. 6). While the PSD itself was homogeneously decorated with the DAB reaction product (Fig. 6, a and b), in the adjacent dendritic shaft a clustered arrangement of the immunoreactivity was observed (Fig. 6, c and d). The restriction of synaptopodin expression to a subset of synapses was confirmed by a double immunofluorescence labeling approach with mAb G1 for synaptopodin and polyclonal anti-synaptophysin antiserum as a marker of all synapses (Fig. 7).

Bottom Line: In particular, synaptopodin does not contain functional domains found in receptor-clustering PSD proteins.The exclusive synaptopodin synthesis in the telencephalon has been confirmed by in situ hybridization, where synaptopodin mRNA is only found in perikarya of the olfactory bulb, cerebral cortex, striatum, and hippocampus, i.e., the expression is restricted to areas of high synaptic plasticity.From these results and experiments with cultured cells we conclude that synaptopodin represents a novel kind of proline-rich, actin-associated protein that may play a role in modulating actin-based shape and motility of dendritic spines and podocyte foot processes.

View Article: PubMed Central - PubMed

Affiliation: Department of Anatomy and Cell Biology, University of Heidelberg, Germany. peter.mundel@urz.uni-heidelberg.de

ABSTRACT
Synaptopodin is an actin-associated protein of differentiated podocytes that also occurs as part of the actin cytoskeleton of postsynaptic densities (PSD) and associated dendritic spines in a subpopulation of exclusively telencephalic synapses. Amino acid sequences determined in purified rat kidney and forebrain synaptopodin and derived from human and mouse brain cDNA clones show no significant homology to any known protein. In particular, synaptopodin does not contain functional domains found in receptor-clustering PSD proteins. The open reading frame of synaptopodin encodes a polypeptide with a calculated Mr of 73.7 kD (human)/74.0 kD (mouse) and an isoelectric point of 9.38 (human)/9. 27 (mouse). Synaptopodin contains a high amount of proline ( approximately 20%) equally distributed along the protein, thus virtually excluding the formation of any globular domain. Sequence comparison between human and mouse synaptopodin revealed 84% identity at the protein level. In both brain and kidney, in vivo and in vitro, synaptopodin gene expression is differentiation dependent. During postnatal maturation of rat brain, synaptopodin is first detected by Western blot analysis at day 15 and reaches maximum expression in the adult animal. The exclusive synaptopodin synthesis in the telencephalon has been confirmed by in situ hybridization, where synaptopodin mRNA is only found in perikarya of the olfactory bulb, cerebral cortex, striatum, and hippocampus, i.e., the expression is restricted to areas of high synaptic plasticity. From these results and experiments with cultured cells we conclude that synaptopodin represents a novel kind of proline-rich, actin-associated protein that may play a role in modulating actin-based shape and motility of dendritic spines and podocyte foot processes.

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