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J. Biol. Chem., Vol. 279, Issue 9, 8029-8037, February 27, 2004

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Role of Amphiphysin II in Somatostatin Receptor Trafficking in Neuroendocrine Cells^*

Philippe Sarret, M. James Esdaile, Peter S. McPherson, Agnes Schonbrunn¶, Hans-Jürgen Kreienkamp||, and Alain Beaudet**

From the Department of Neurology and Neurosurgery, Montreal Neurological Institute, McGill University, Montreal, Quebec H3A 2B4, Canada, the ^¶Department of Pharmacology, University of Texas Medical School, Houston, Texas 77225, the ^||Institut für Zellbiochemie und Klinische Neurobiologie, Universität Hamburg, Hamburg D-20246, Germany

Amphiphysins are SH3 domain-containing proteins thought to function in clathrin-mediated endocytosis. To investigate the potential role of amphiphysin II in cellular trafficking of G protein-coupled somatostatin (SRIF) receptors, we generated an AtT-20 cell line stably overexpressing amphiphysin IIb, a splice variant that does not bind clathrin. Endocytosis of ¹²⁵I-[D-Trp⁸]SRIF was not affected by amphiphysin IIb overexpression. However, the maximal binding capacity (B_max) of the ligand on intact cells was significantly lower in amphiphysin IIb overexpressing than in non-transfected cells. This difference was no longer apparent when the experiments were performed on crude cell homogenates, suggesting that amphiphysin IIb overexpression interferes with SRIF receptor targeting to the cell surface and not with receptor synthesis. Accordingly, immunofluorescence experiments demonstrated that, in amphiphysin overexpressing cells, sst_2A and sst₅ receptors were segregated in a juxtanuclear compartment identified as the trans-Golgi network. Amphiphysin IIb overexpression had no effect on corticotrophin-releasing factor 41-stimulated adrenocorticotropic hormone secretion, suggesting that it is not involved in the regulated secretory pathway. Taken together, these results suggest that amphiphysin II is not necessary for SRIF receptor endocytosis but is critical for its constitutive targeting to the plasma membrane. Therefore, amphiphysin IIb may be an important component of the constitutive secretory pathway.

Received for publication, September 30, 2003 , and in revised form, December 2, 2003.

^* This work was supported in part by Canadian Institutes of Health Research Grants MT-7366 (to A. B.) and Deutsche Forschungsgemeinschaft Grant SFB 545/B7 (to H.-J. K). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Recipient of Ligue Nationale Contre le Cancer Research and Fonds de la Recherche en Santé du Québec (FRSQ) fellowships.

^** To whom correspondence should be addressed: Dept. of Neurology and Neurosurgery, Montreal Neurological Institute, 3801 University St., Montreal, Quebec H3A 2B4, Canada. Tel.: 514-398-1913; Fax: 514-398-5871; E-mail: alain.beaudet{at}mcgill.ca.

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