Distinct expression patterns of aquaporin 3 and 5 in ductal and alveolar epithelial cells in mouse mammary glands before and after parturition

Culture Models to Investigate Mechanisms of Milk Production and Blood-Milk Barrier in Mammary Epithelial Cells: a Review and a Protocol

Mammary epithelial cells (MECs) are the only cell type that produces milk during lactation. MECs also form less-permeable tight junctions (TJs) to prevent the leakage of milk and blood components through the paracellular pathway (blood-milk barrier). Multiple factors that include hormones, cytokines, nutrition, and temperature regulate milk production and TJ formation in MECs. Multiple intracellular signaling pathways that positively and negatively regulate milk production and TJ formation have been reported. However, their regulatory mechanisms have not been fully elucidated. In addition, unidentified components that regulate milk production in MECs likely exist in foods, for example plants. Culture models of functional MECs that recapitulate milk production and TJs are useful tools for their study. Such models enable the elimination of indirect effects via cells other than MECs and allows for more detailed experimental conditions. However, culture models of MECs with inappropriate functionality may result in unphysiological reactions that never occur in lactating mammary glands in vivo. Here, I briefly review the physiological functions of alveolar MECs during lactation in vivo and culture models of MECs that feature milk production and less-permeable TJs, together with a protocol for establishment of MEC culture with functional TJ barrier and milk production capability using cell culture inserts.

Sweet taste receptor subunit T1R3 regulates casein secretion and phosphorylation of STAT5 in mammary epithelial cells

During lactation, mammary epithelial cells (MECs) on the apical membrane are in contact with lactose in milk, while MECs on the basolateral membrane are in contact with glucose in blood. Both glucose and lactose are sweeteners that are sensed by a sweet taste receptor. Previously, we have shown that lactose exposure on the basolateral membrane, but not the apical membrane, inhibits casein production and phosphorylation of STAT5 in MECs. However, it remains unclear whether MECs have a sweet taste receptor. In this study, we confirmed that the sweet taste receptor subunit T1R3 existed in both the apical and basolateral membranes of MECs. Subsequently, we investigated the influence of apical and basolateral sucralose as a ligand for the sweet taste receptor using a cell culture model. In this model, upper and lower media were separated by the MEC layer with less-permeable tight junctions. The results showed in the absence of glucose, both apical and basolateral sucralose induced phosphorylation of STAT5, which is a positive transcriptional factor for milk production. In contrast, the T1R3 inhibitor basolateral lactisole reducing phosphorylated STAT5 and secreted caseins in the presence of glucose. Furthermore, exposure of the apical membrane to sucralose in the presence of glucose inhibited the phosphorylation of STAT5. Simultaneously, GLUT1 was partially translocated from the basolateral membrane to the cytoplasm in MECs. These results suggest that T1R3 functions as a sweet receptor and is closely involved in casein production in MECs.

SIM2s directed Parkin-mediated mitophagy promotes mammary epithelial cell differentiation

The functionally differentiated mammary gland adapts to extreme levels of stress from increased demand for energy by activating specific protective mechanisms to support neonatal health. Here, we identify the breast tumor suppressor gene, single-minded 2 s (SIM2s) as a novel regulator of mitophagy, a key component of this stress response. Using tissue-specific mouse models, we found that loss of Sim2 reduced lactation performance, whereas gain (overexpression) of Sim2s enhanced and extended lactation performance and survival of mammary epithelial cells (MECs). Using an in vitro model of MEC differentiation, we observed SIM2s is required for Parkin-mediated mitophagy, which we have previously shown as necessary for functional differentiation. Mechanistically, SIM2s localizes to mitochondria to directly mediate Parkin mitochondrial loading. Together, our data suggest that SIM2s regulates the rapid recycling of mitochondria via mitophagy, enhancing the function and survival of differentiated MECs.

Using QTL to Identify Genes and Pathways Underlying the Regulation and Production of Milk Components in Cattle

Milk is a complex liquid, and the concentrations of many of its components are under genetic control. Many genes and pathways are known to regulate milk composition, and the purpose of this review is to highlight how the discoveries of quantitative trait loci (QTL) for milk phenotypes can elucidate these pathways. The main body of this review focuses primarily on QTL discovered in cattle (Bos taurus) as a model species for the biology of lactation, and there are occasional references to sheep genetics. The following section describes a range of techniques that can be used to help identify the causative genes underlying QTL when the underlying mechanism involves the regulation of gene expression. As genotype and phenotype databases continue to grow and diversify, new QTL will continue to be discovered, and although proving the causality of underlying genes and variants remains difficult, these new data sets will further enhance our understanding of the biology of lactation.

Aquaporin-5 Dynamic Regulation

Aquaporin-5 (AQP5), belonging to the aquaporins (AQPs) family of transmembrane water channels, facilitates osmotically driven water flux across biological membranes and the movement of hydrogen peroxide and CO₂. Various mechanisms have been shown to dynamically regulate AQP5 expression, trafficking, and function. Besides fulfilling its primary water permeability function, AQP5 has been shown to regulate downstream effectors playing roles in various cellular processes. This review provides a comprehensive overview of the current knowledge of the upstream and downstream effectors of AQP5 to gain an in-depth understanding of the physiological and pathophysiological processes involving AQP5.

Aquaporins in Glandular Secretion

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.

Emerging roles for AQP in mammalian extracellular vesicles

Recent research in the aquaporin (AQP) field has identified a role for diverse AQPs in extracellular vesicles (EV). Though still in its infancy, there is a growing body of knowledge in the area; AQPs in EV have been suggested as biomarkers for disease, as drug targets and show potential as therapeutics. To advance further in this field, AQPs in EV must be better understood. Here we summarize current knowledge of the presence and function of AQPs in EV and hypothesise their roles in health and disease.

Role of changes in plasma prolactin concentrations on ram and buck sperm cryoresistance

Seasonal endocrine changes may modify sperm cryoresistance in certain small ruminant species. The present work examines the effect of prolactin (PRL) on ram and buck sperm cryoresistance. A dopamine agonist (bromocriptine [BCR] 60 mg i.m. twice per week from May 15 to June 15, that is, approaching the summer solstice) or antagonist (sulpiride [SLP] 100 mg s.c. daily from December 15 to January 15, that is, around the winter solstice) was administered under solstice-appropriate photoperiod conditions to modify PRL secretion. Control animals received the vehicle only. Compared to the corresponding controls, BCR reduced PRL secretion to basal levels in both the rams and bucks. In rams, the cryoresistance ratios for sperm curvilinear velocity (P < 0.05) and lateral head displacement (P < 0.01) were higher for the BCR-treated animals. In bucks, neither the characteristics of fresh nor frozen-thawed sperm were affected by BCR treatment. After the administration of SLP, PRL levels increased and remained high for more than 5 h in the rams though they immediately began to fall in the bucks. By 24 h, PRL had returned to basal concentrations in both species. In rams treated with SLP, the cryoresistance ratios for sperm progressive motility, straight line velocity, sperm mean path velocity, cross beat frequency, and the progression ratios linearity, straightness and oscillation, were all lower compared to the controls (P < 0.05), while the amplitude of lateral head displacement was higher (P < 0.01). In bucks, sperm cryoresistance was not affected by SLP administration. Together, these results suggest that high levels of PRL negatively affect the cryoresistance of ram sperm, while buck sperm seems unaffected.

Adverse effects of LPS on membrane proteins in lactating bovine mammary epithelial cells

Mastitis causes a decrease in milk yield and abnormalities in milk components from dairy cows. Escherichia coli and the E. coli lipopolysaccharide (LPS) cell wall component directly downregulate milk production in bovine mammary epithelial cells (BMECs). However, the detailed mechanism by which this occurs in BMECs remains unclear. Various membrane proteins, such as immune sensors (Toll-like receptors, TLR), nutrient transporters (glucose transporter and aquaporin), and tight junction proteins (claudin and occludin) are involved in the onset of mastitis or milk production in BMECs. In this study, we investigated the influence of LPS on membrane proteins using an in vitro culture model. This mastitis model demonstrated a loss of glucose transporter-1 and aquaporin-3 at lateral membranes and a decrease in milk production in response to LPS treatment. LPS disrupted the tight junction barrier and caused compositional changes in localization of claudin-3 and claudin-4, although tight junctions were maintained to separate the apical membrane domains and the basolateral membrane domains. LPS did not significantly affect the expression level and subcellular localization of epidermal growth factor receptor in lactating BMECs with no detectable changes in MEK1/2-ERK1/2 signaling. In contrast, NFκB was concurrently activated with temporal translocation of TLR-4 in the apical membranes, whereas TLR-2 was not significantly influenced by LPS treatment. These findings indicate the importance of investigating the subcellular localization of membrane proteins to understand the molecular mechanism of LPS in milk production in mastitis.