Internalization of ferritin-concanavalin A by the lactating mammary cell in vivo

Abstract

Ferritin-concanavalin A (Fer-Con A) was used to label the apical plasma membrane of the lactating cell to determine whether membrane internalization takes place. Rat glands were infused in vivo via the teat with 0.2 mg of Fer-Con A in 0.2 ml tris buffer (pH 7.0) containing 0.1% trypan blue, the latter acting as a marker of the infusate. Tissues were obtained from separate animals 5, 10 and 60 min postinfusion. Fer-Con A was seen in alveolar lumina bound to the outer surfaces of apical plasma membrane, microvilli and milk fat globules. It was observed within lactating cells on the inner membrane surfaces of endocytotic vesicles, Golgi cisternae, and secretory vesicles containing casein micelles, and in multivesicular bodies and lysosomes. Internalization of the ferritin-lectin conjugate into casein-containing secretory vesicles was detectable in the 5-min postinfusion tissue. Lysosomes were the only structures in control tissue that contained particles bearing some resemblance to Fer-Con A. The data provide evidence that apical plasma membrane is internalized and distributed to a number of intracellular compartments.

The distribution of MUC1, an apical membrane glycoprotein, in mammary epithelial cells at the resolution of the electron microscope: implications for the mechanism of milk secretion

“…Ferritin was detected in secretory vesicles that may have derived from small ferritin-labelled vesicles observed in close proximity to, and apparently fusing with mature secretory vesicles (see, for example, Fig. 3b of Welsch et al 1984). This observation raises the intriguing possibility that the MUC1-containing vesicles seen in close association with secretory vesicles in this study (Fig.…”

“…However, much of this vesicle-associated MUC1 may have recycled from the cell surface. Evidence for this latter possibility is provided by the earlier studies of Welsch et al (1984) who showed that concanavalin A/ferritin conjugates, infused via the nipple into the mammary glands of lactating rats, bound to the apical plasma membrane and were then internalised in vesicles. Ferritin was detected in secretory vesicles that may have derived from small ferritin-labelled vesicles observed in close proximity to, and apparently fusing with mature secretory vesicles (see, for example, Fig.…”

“…In mammary cells, membrane may be retrieved by endocytosis or shed as fragments into milk (Plantz and Patton 1973;Welsch et al 1984). Membrane is also removed during the budding and secretion of milk-fat globules from the apical surface (Bargmann and Knoop 1959;Bargmann et al 1961;discussed in Mather and Keenan 1998).…”

Endocytic prolactin routes to the secretory pathway in lactating mammary epithelial cells

“…An endosomal recycling pathway has been demonstrated by the accumulation of endocytosed fluorescent transferrin in a vesicular compartment near the basal membrane of the alveolar cells in MEC from lactating mice incubated in vivo with a bath solution (Zhang et al, 2000). Moreover, a subapical endosomal compartment, closely related to the endocytic-secretory pathway, has been demonstrated in lactating rat MEC (Welsch et al, 1984). Lastly, the transfer of a lipoproteincontaining particle, from the serum, across the mammary epithelium into the milk of lactating mice, has been demonstrated (Monks et al, 2001).…”

Lipid Transport in the Lactating Mammary Gland

“…1) [13, 52, 53]. The mechanisms of transcytosis involve a series of complex sorting events in the basolateral early endosome and the common endosome recycling (CER) compartments that intersect with Golgi and secretory vesicle pathways (pathway10) [54–56]. Consequently, transcytosis provides mechanisms for transferring basolateral membrane phospholipids, which are distinct from apical membrane phospholipids and enriched in PC [5], to multiple internal membrane compartments as well as to the apical plasma membrane.…”