Secretion of Milk Fat Globules

By virtue of their static nature, electron micrographs provide no information on the kinetics of secretion. Just the process of fixation may provoke a temporary decrease in secretory events and a consequent accumulation of lipid droplets and secretory vesicles in cells before they succumb to the fixative agent. Such crowding in the apical cytoplasm could well lead to promiscuous associations and interactions that are unrelated to normal secretory processes (Mather and Keenan, 1998). 

That other proteins are associated with the MFGM coat is probable, particularly proteins associated with the surface of intracellular lipid droplets. However, several of the proteins identified as being associated with intracellular lipid droplets (Wu et al., 2000) have yet to be identified as constituents of the MFGM coat. Two proteins associated with intracellular lipid droplets, protein disulfide isomerase (Ghosal et al., 1994) and the nuclear coactivator protein plOO (Keenan et al., 2000) are absent from MFGM preparations. Thus, there apparently is some selectivity in which of the proteins associated with intracellular lipid droplets are secreted. 

Huston and Patton (1990) suggested that an abnormality in the protein coat along the cytoplasmic face of the apical plasma membrane may be responsible for the phenomenon of crescent formation. Inadequate amounts of proteins forming this coat or abnormalities in the distribution of the coat complex may interfere with adhesion of the membrane to the droplet. 

Gaining an understanding of the molecular mechanism of crescent formation would advance our understanding of the process of milk fat globule secretion. Crescents are a route by which cellular constituents can 

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Heid, H.W., Keenan, T.W. 2005. Intracellular origin and secretion of milk fat globules. Eur. J. Cell Biol. 84, 245-258. 

Couvreur, S., C. Hurtaud, RG. Mamet, P. Faverdin and J.L. Peyraud, 2007. Composition of ilk fat from cows selected for milk fat globule size and offered either fresh pasture or a com silage- based diet. J Dairy Sci. 90, 392-403. Heid, H.W and T. W. Keenan, 2005. Intracellilar origin and secretion of milk fat globules. Fur J Cell Biol. 84, 245-258. Pfeuffer, M. and J. Schrezenmeir, 2006. Milk and the metaholic syndrom. Obesity Rev 8, 109-118. 

Baumrucker, C. R. and Keenan, T. W. 1973. Membranes of mammary gland. VII. Stability of milk fat globule membrane in secreted milk. J. Dairy Sci. 56, 1092-1094. 

One major obstacle to molecular studies in the area of milk-fat globule formation and secretion is the lack of a cell line that secretes, or can be induced to secrete, milk fat globules, although some progress in this area has been reported (Rohlfs et al, 1993). Development of such a cell line would be a major advance, in and of itself, and would be an invaluable aid in fostering further research in this area. 

Patton, S., Stemberger, B.FT, Knudson, C.M. 1977. The suppression of milk fat globule secretion by colchicine an effect coupled to inhibition of exocytosis. Biochim. Biophys. Acta 499, 404 110. 

PAS 6/7 has no known role in milk-fat globule secretion. The human homologue, lactadherin, binds to rotaviruses and may protect the gastrointestinal tract of the suckling young from infection (Peterson et al., 2001). 

Jack, L.J.W., Mather, I.H. 1990. Cloning and analysis of cDNA encoding bovine butyrophilin, an apical glycoprotein expressed in mammary tissue and secreted in association with the milk fat globule membrane during lactation. 1. Biol. Chem. 265, 14482-14486. 

In some milk fat globules, small aqueous compartments are located beneath the membrane bilayer, which have been termed cytoplasmic crescents (Huston and Patton, 1990). Whether or not this cytoplasmic inclusion provides some benefit is unknown. Yet, as this aqueous compartment is protected from the bulk serum phase by the MFGM, constituents located therein are presumably afforded some protection, at least initially, from gastric hydrolysis. Huston and Patton (1990) found crescents in all samples of milk they examined, and they were more prevalent in human (7.3% of globules), than in bovine (1% of globules) milk. Furthermore, there was considerable individual and diurnal variation. The structure of a cytoplasmic inclusion, surrounded by an intact plasma membrane on one side and a fat globule surface on the other, may allow certain labile constituents to be protected until they reach their proper site of bioactivity. At this point it is not known whether the crescents have a purpose or are simply the result of inefficiencies in the secretion process. As it is possible to isolate milk preparations enriched in cytoplasmic crescents, there is an opportunity to determine the nature of the materials found within. This unusual biocompartment may prove to be a model of food structure for biodelivery. 

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