Basolateral Localization of SCF Provides Ligand to Responsive Cells Located at the Basal Aspect

of Epithelia

Regardless of the mechanism of localization, we suggest that altered localization of SCF mutant protein negatively affects the survival of melanocyte precursors during the stages at which they require SCF protein released from the epidermal layer. The basolateral secretion of SCF by epider-mal keratinocyte may serve both as a survival and as an invasion signal for c-kit-dependent melanocyte precursors prior to their migration into the epidermis. Similarly, male germ cells require SCF during the proliferation and differ-entiation of spermatogonia while they reside in the baso-lateral compartment of the Sertoli cell layer (Yoshinaga et al., 1991; Manova et al., 1993; for review see Morrison-Graham and Takahashi, 1993). A lack of delivery of SCF to the basolateral compartment in Sertoli cells could be re-sponsible for the absence of spermatogonial proliferation during the time of testicular maturation of Sl^17H mutant males (Brannan et al., 1992). During sexual maturation (about the third and fourth postnatal weeks), the blood–

testis barrier, which prevents diffusion of proteins across the tight junctions within the Sertoli cell layer is estab-lished (Vitale et al., 1973). This barrier would prevent the diffusion of apical Sl^17H mutant membrane-bound SCF to the spermatogonia located in the basolateral compartment.

In both males and females, the numbers of spermatogonia and oogonia, respectively, are equally reduced due to the lack of primordial germ-cell proliferation in the genital ridges (Brannan et al., 1992). In females, oocytes that arise from persistent oogonia can undergo successful maturation stimulated by granulosa cell-derived Sl^17Hmutant SCF. In contrast, continuously proliferating spermatogonia appear to be more sensitive to reduced levels of SCF activity (Yoshinaga et al., 1991). Moreover, since oocytes associate with the apical aspect of the SCF-producing granulosa cells (Manova et al., 1993; Packer et al., 1994), the targeting defect in Sl^17Hmutant SCF does not further reduce access of oocytes to SCF and therefore would be expected to allow maturation of oocytes and hence fertility in females.

In conclusion, we propose that a general reduction of SCF mutant protein expressed on the cell surface accounts for the reduced number of melanocyte precursors and primor-dial germ cells on the lateral pathway and genital ridges, respectively. However, the main cause for the loss of melanocyte precursors, hence the Sl^17Hpigmentation defect, as well as the sterility in males appears to be the inability of Sl^17Hmutant SCF to be targeted to the basolateral compart-ment in SCF-producing epidermal keratinocytes and Sertoli cells. As a consequence, SCF-dependent cell types do not encounter the appropriately localized SCF signals for epi-dermal invasion and proliferation. We propose, therefore, that not only the amount of growth factor expression is critical for development, but also its appropriate presenta-tion toward responsive cells.

ACKNOWLEDGMENTS

We are grateful to Monique Wehrle-Haller and Dr. Beat Imhof for their critical comments on the manuscript. We also thank Drs.

John Flanagan and Carolyn Brannan for generously providing cDNA plasmids. Special thanks to Marie-Claude Jacquier, Sheree Harrison, Adam Hadley, Zigmus Kingis, Victoria Robinson, Do-minique Wohlwend, and Dr. Kathleen- Morrison-Graham for tech-nical assistance and Drs. Yoshiko Takahashi, Yoshio Wakamatsu, Linda Hansen, and Ruth Bremiller for advice on in situ hybridiza-tion and immunohistochemistry. We thank Drs. Mary Bedell and Shin-Ishi Nishikawa for insightful discussions and for sharing unpublished results. We are especially grateful to Rick Gosswiler for his careful animal husbandry. Our work has been supported by Grant DE-04316 from the USPHS to J.A.W. B.W.-H. has been supported by a grant from the Swiss Foundation for Medical and Biological Fellowships and from the Swiss National Science Foun-dation (31-52727.97).

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Received for publication October 13, 1998 Revised March 1, 1999 Accepted March 2, 1999

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2.7. A mono-leucine, acidic cluster associated basolateral targeting domain

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* This work was supported by the Swiss National Science Foundation Grants 31-52727-97 (to B. W.-H.) and 31-49241-96 (to B. A. I.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “adver-tisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

‡ To whom correspondence should be addressed. Tel.: 41-22-702-5735; Fax: 41-22-702-5746; E-mail: Bernhard.Wehrle-Haller@

medecine.unige.ch.

1The abbreviations used are: SCF, stem cell factor; CSF-1, colony-stimulating factor-1; TGN,trans-Golgi network; GFP, green fluorescent protein; EGFP, enhanced green fluorescent protein; Tac, interleukin-2 receptor␣-chain; MDCK, Madin-Darby canine kidney; PACS, phospho-furin acidic cluster-sorting.

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