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© The Rockefeller University Press, 0021-9525/1997//193 $5.00
The Journal of Cell Biology, Volume 139, Number 1, , 1997 193-204

Article

Synaptopodin: An Actin-associated Protein in Telencephalic Dendrites and Renal Podocytes



Peter Mundel*, Hans W. Heid{ddagger}, Thomas M. Mundel*, Meike Krüger*, Jochen Reiser*, and Wilhelm Kriz*

* Department of Anatomy and Cell Biology, University of Heidelberg, D-69120 Heidelberg, Germany; and {ddagger} Division of Cell Biology, German Cancer Research Center, D-69120 Heidelberg, Germany

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 (~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.

Abbreviations used in this paper: aa, amino acid; CNS, central nervous system; MAP, microtubule-associated protein; ORF, open reading frame; PSD, postsynaptic densities.

Address all correspondence to Dr. Peter Mundel, Department of Anatomy and Cell Biology, University of Heidelberg, Im Neuenheimer Feld 307, D-69120 Heidelberg, Germany. Tel.: (49) 6221-548687. Fax: (49) 6221-544951. e-mail: peter.mundel{at}urz.uni-heidelberg.de

The conditionally immortal podocyte cell line was established in collaboration with Rolf Zeller (European Molecular Biological Laboratory, Heidelberg, Germany). We gratefully acknowledge the cooperation of Liane Meyn and Carlos Dotti (European Molecular Biological Laboratory) on the studies with cultured hippocampal neurons and of Peter Seeburg (Max Planck Institute for Medical Research, Heidelberg, Germany) on the in situ hybridization studies. We are thankful to H. Spring (German Cancer Research Center) for performing laser scanning confocal microscopy; H.-R. Rackwitz for synthesizing peptides for antibody production; as well as Alexandra Zeller, Stefanie Winter-Simanowski, Andreas Hunziker, Hiltraud Hosser, and Bruni Hähnel for expert technical assistance. We also thank Rolf Nonnenmacher and Ingrid Ertl for skillful artwork and photographical work.


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