Targeting of endogenous sulfated glycoprotein‐1 and ‐2 to lysosomes within nonciliated cells of the efferent ducts during postnatal development of the rat
Authors: Hermo L. , Rosenthal A. L. , Igdoura S. A. , Morales C. R. 
Published: February 2005

Sulfated glycoprotein (SGP) ‐1 and ‐2, secretory products of Sertoli cells, are secreted into the lumen of seminiferous tubules where they bind to late spermatids. Once released, the spermatozoa traverse the efferent ducts where these proteins detach from their surface and are endocytosed by the nonciliated cells. In adult animals, SGP‐1 and SGP‐2 are also synthesized by nonciliated cells and targeted from the Golgi apparatus to lysosomes. The purpose of the present study was to determine the pattern of expression of SGP‐1 and SGP‐2 within nonciliated cells during postnatal development. The efferent ducts of animals at different postnatal ages were prepared for an electron microscopic immunocytochemical quantitative analysis as well as for Northern blot analysis. The data expressed as labeling content (no. gold particles/μm2 and taking into account the volume of the endocytic or‐ganelles and the cell) revealed that anti‐SGP‐1 labeling in endosomes of nonciliated cells was minimal at 15, 21, and 29 days of age. On the other hand, the lysosomal labeling content showed a significant increase by day 29 compared to 15 and 21‐day‐old animals indicating that an endogenous form of SGP‐1 was being synthesized by nonciliated cells and targeted to lysosomes. By day 39 a significant increase in endosomal labeling occurred; this was attributed to the endocytosis of Sertoli‐derived SGP‐1 which coincided with the entry of spermatozoa into the lumen of these ducts at this age. Lysosomal labeling showed further significant increases at days 39, 49, and then again at day 90. Northern blot analysis detected SGP‐1 mRNA transcripts at all postnatal ages examined. While decreases or increases in transcripts could not be determined due to the greater amount of tissue present with increasing age, these data taken together support the idea of an endogenous form of SGP‐1 being synthesized by nonciliated cells and targeted to lysosomes during postnatal development.

In the case of SGP‐2, endosomal labeling was minimal at 15, 21, and 29 days of age but was significantly increased by day 39, with similar values at all subsequent ages. The high value at day 39 was attributed to the endocytosis of SGP‐2 which coincided with the entry of spermatozoa into the lumen at this age. Lysosomal labeling, on the other hand, was low at days 15 and 21 but peaked at day 29 at a time when endosomal labeling was minimal. These results suggested the synthesis of an endogenous form of SGP‐2 which was being targeted to lysosomes. Similar values for SGP‐2 lysosomal labeling comparable to that at day 29 were obtained at all other ages. Since SGP‐2 endosomal labeling was significantly increased at day 39 and maintained thereafter, it is suggested that labeling in lysosomes at this and subsequent ages could also be due to the endocytosis of Sertoli‐derived SGP‐2. However, Northern blot analysis confirmed the presence of mRNA transcripts for SGP‐2 at all postnatal ages examined, although increases or decreases in their amount were not determined. These results thus consolidate the hypothesis of an endogenous form of SGP‐2 being synthesized by nonciliated cells and targeted to lysosomes. Finally, since the amounts of endogenous SGP‐1 and SGP‐2 peak at different ages, it is suggested that different factors are involved in regulation of these two proteins during postnatal development. © 1995 Wiley‐Liss, Inc.