Molecular mechanisms of caseins exocytosis: role of the SNARE proteins

The mammary gland (MG) is a complex tissue which function is to ensure the offspring’s feeding by producing and secreting the milk components during lactation. In addition to stimulate the synthesis of the main milk proteins, the caseins, prolactin (PRL) also induces an increase of arachidonic acid (AA) in mammary epithelial cells (MECs). This lipid second messenger may accelerate the transport and possibly the exocytosis of caseins (secretagogue effect of PRL, [1]). SNARE (Soluble NSF Attachment Receptor) proteins are able to form stable ternary complexes (SNARE complexes) which promote the fusion of two membranous compartments [2, 3]. Indeed, SNAREs are involved in intracellular traffic and exocytosis processes in many cell types. SNAREs have recently been described as target for AA, thus promoting exocytosis in neuroendocrine cells [4, 5]. Milk products secretion is known in general terms but the molecular mechanisms underlying caseins secretion have not been described to date. One intriguing hypothesis is that the AA produced in response to PRL may regulate the transport and the exocytosis of caseins by targeting the SNARE proteins in MECs during lactation. We show for the first time that mRNAs encoding most of the SNARE proteins are present in the mouse MG and MECs at different developmental stages. Moreover, quantitative Western blots show that levels of some SNARE proteins are regulated during the development of the MG. The localization of the SNARE proteins was investigated in lactating mouse MG by indirect immunofluorescence and electron microscopy. We found that both the developmental stage of the MG and the suckling induce important changes in the localization of certain SNARE proteins. The association of some SNARE proteins with the lipid droplets and the vesicles containing caseins was observed in lactating MECs. We also noticed the presence of gold particles corresponding to SNAP-23 at the interface between the lipid droplets and the caseins vesicles membranes. Immunoprecipitation experiments were performed to identify the SNARE complex involved in caseins exocytosis in murine MECs. The identification of the proteins engaged in SNARE complexes was investigated by Western Blot and mass spectrometry. The expression of some regulatory proteins of the SNARE complex such as synaptotagmins, Munc13 and 18, Rab3, NSF and alpha-SNAP was also investigated in murine MECs by RT-PCR. [1] Ollivier-Bousquet et al. J Nutr, 1993. 123(12):2090-100. [2] Rothman & Orci. Nature, 1992. 355(6359):409-15. [3] Sollner et al. Nature, 1993. 362(6418):318-24. [4] Latham et al. J Neuroche, 2007. 100(6):1543-54. [5] Darios & Davletov. Nature, 2006. 440(7085):813-7.