Cyclical heat stress during lactation influences the microstructure of the bovine mammary gland

This study aimed to evaluate the impact of heat stress on mammary epithelial cell (MEC) losses into milk, secretory mammary tissue structure, and mammary epithelial cell activity. Sixteen multiparous Holstein cows (632 ± 12 kg BW) approximately 100 d in milk housed in climate-controlled rooms were paired by body weight and randomly allocated to one of 2 treatments, heat stress (HS) or pair feeding thermoneutral (PFTN) using 2 cohorts. Each cohort was subjected to 2 periods of 4 d each. In period 1, both treatments had ad libitum access to a common total mixed ration and were exposed to a controlled daily temperature-humidity index (THI) of 64. In period 2, HS cows were exposed to controlled cyclical heat stress (THI: 74 to 80), while PFTN cows remained at 64 THI and daily dry matter intake was matched to HS. Cows were milked twice daily, and milk yield was recorded at each milking. Individual milk samples on the last day of each period were used to quantify MEC losses by flow cytometry using butyrophilin as a cell surface marker. On the final day of period 2, individual bovine mammary tissue samples were obtained for histomorphology analysis, assessment of protein abundance, and evaluation of gene expression of targets associated with cellular capacity for milk and milk component synthesis, heat response, cellular proliferation, and autophagy. Statistical analysis was performed using the GLIMMIX procedure of SAS. Milk yield was reduced by 4.3 kg by HS (n = 7) compared with PFTN (n = 8). Independent of treatment, MEC in milk averaged 174 cells/mL (2.9% of total cells). There was no difference between HS vs. PFTN cows for MEC shed or concentration in milk. Alveolar area was reduced 25% by HS, and HS had 4.1 more alveoli than PFTN. Total number of nucleated MEC per area were greater in HS (389 ± 1.05) compared with PFTN (321 ± 1.05); however, cell number per alveolus was similar between groups (25 ± 1.5 vs. 26 ± 1.4). There were no differences in relative fold expression for GLUT1, GLUT8, CSN2, CSN3, LALBA, FASN, HSPA5, and HSPA8 in HS compared with PFTN. Immunoblotting analyses showed a decrease abundance for phosphorylated STAT5 and S6K1, and an increase in LC3 II in HS compared with PFTN. These results suggest that even if milk yield differences and histological changes occur in the bovine mammary gland after 4 d of heat exposure, MEC loss into milk, nucleated MEC number per alveolus, and gene expression of nutrient transport, milk component synthesis, and heat stress related targets are unaffected. In contrast, the abundance of proteins related to protein synthesis and cell survival decreased significantly, while an upregulation of proteins associated with autophagy in HS compared with PFTN.

LPS-induced SOCS3 antagonizes the JAK2-STAT5 pathway and inhibits β-casein synthesis in bovine mammary epithelial cells

Bovine mammary epithelial cells (BMECs) are essential for lactation in the dairy cow mammary gland, and are often used as a cellular model to study changes in inflammatory responses and lactation functions with exogenous stimuli. Prolactin (PRL) promotes milk protein synthesis by continuously activating the Janus kinase 2 and signal transducer and activator of transcription 5 (JAK2-STAT5) pathway. Lipopolysaccharides (LPS) activates inflammatory responses in cells and inhibits casein synthesis, but the exact mechanism is still unclear. Suppressor of cytokine signaling-3 (SOCS3) is a negative regulator of the JAK-STATs signaling pathway, and regulates a variety of inflammatory responses by inhibiting STAT3. Previous studies also suggested that SOCS3 plays a role in the development and involution of bovine mammary glands. The purpose of this study was to investigate whether LPS activated SOCS3, and whether SOCS3 resisted the regulation of casein synthesis by PRL in a JAK2-STAT5-dependent manner. We treated in vitro BMECs with 125 ng/mL PRL, 10 μg/mL LPS, SOCS3 siRNA (silencing), a SOCS3-GFP adenovirus overexpression vector, or combinations, to determine β-casein expression. We demonstrated that PRL up-regulated phospho-JAK2, phsopho-STAT5 and β-casein expression, whereas LPS caused the opposite effects, and activated SOCS3. SOCS3 overexpression interrupted the JAK2-STAT5 pathway in BMECs. With SOCS3 was silenced, LPS could not activate the JAK2-STAT5 pathway, and no inhibition of β-casein expression was observed. In conclusion, we showed that LPS activated SOCS3 in BMECs, antagonized the JAK2-STAT5 pathway via SOCS3 regulation, and ultimately reduced β-casein expression in these cells.

Feeding level regulates the expression of some genes involved with programed cell death and remodeling in goat and sheep mammary tissue

Mammary tissue (MT) turnover is characterized by programed cell death and remodeling which might be affected by both feeding level and animal species. Thus, twenty-four dairy goats and the same number of sheep were assigned to three homogenous sub-groups per animal species and fed the same diet in quantities which met 70% (FL70), 100% (FL100) and 130% (FL130) of their daily energy and crude protein requirements. Individual MT samples were taken by biopsy from the animals on the 30th and 60th experimental day. The results showed, in the first sampling time, a significant reduction in the mRNA abundance for selected genes involved in programed cell death in both FL 70 fed goats (STAT3 and BECN1) and sheep (CASPASE8 and BECN1) compared with the respective FL100 groups. The FL130, in comparison with the FL100, caused a significant increase in transcripts accumulation of STAT3 gene in both sampling times and CASPASE8 gene in the second sampling time in goat MT, while the opposite happened for the mRNA expression of CASPASE8 and BECN1 genes in sheep MT, but only in the first sampling time. Moreover, a significant up regulation in the mRNA levels of MMP2 gene in MT of FL130 fed sheep was observed. The FL130, in comparison with the FL70, caused an enhancement in the mRNA expression levels of BECN1, CASPASE8, BAX and STAT3 genes in goat MT only. It was also shown that apoptosis and autophagy can be affected simultaneously by the feeding level. Overfeeding affects MT programed cell death and remodeling by a completely different way in goats than sheep. In conclusion, feeding level and animal species have strong effects on both MT programed cell death (apoptosis and autophagy) and remodeling but the molecular mechanisms need further investigation.

Suppression of prolactin and reduction of milk secretion by effect of cabergoline in lactating dairy ewes

The effects of cabergoline, an ergot derivative and dopamine receptor agonist, were investigated in 30 ewes of 2 dairy breeds (Manchega; MN, n = 15; Lacaune; LC, n = 15). Ewes were in a similar late-lactation stage, but differed in milk yield according to breed (MN vs. LC, 1.02 ± 0.03 vs. 2.27 ± 0.05 kg/d). Treatments consisted of a single intramuscular injection of cabergoline at different doses per ewe. Cabergoline doses (per ewe) were: low (0.56 mg), high (1.12 mg), and control (CON; 0 mg; 1 mL of saline). Milk yield was recorded daily (d -14 to 25), milk and blood were sampled, and udder traits were measured from d -2 to 14 after injection. No local reaction at the injection site, nor behavior and metabolic indicators of the ewes were detected after the cabergoline injection, but milk yield fell rapidly in both breeds (MN vs. LC, -54% vs. -27%) when compared with CON ewes. Cabergoline effects progressively disappeared after d 5, and no milk yield differences between treatments were detected from d 8 to 25 after injection. Milk fat and protein contents increased similarly (22% and 23%; respectively) in both breeds and at both cabergoline doses until d 5, and the effects disappeared thereafter. Plasma prolactin (PRL) decreased dramatically in the low- and high-treated ewes the day after injection when compared with the CON ewes, and reached values below the detection limit of the assay between d 1 and 5, increasing similarly thereafter. On d 14, PRL values were 58% greater in the low- and high-treated than in the CON ewes, showing that PRL concentrations rebounded when the cabergoline effects ceased. Total udder volume correlated with milk accumulated in the udder (r = 0.77) of all groups of ewes throughout the experiment, suggesting its use as a noninvasive method for the estimation of milk stored in the udder. Udder volume was similar for the low and high ewes, but both values were lower than those of the CON ewes from d 1 to 14 after injection. No other effects on udder size were detected. Cabergoline dramatically inhibited PRL secretion and decreased milk yield and udder volume of lactating dairy ewes. The low dose of cabergoline was as effective as the high dose in the 2 breeds of dairy ewes. These results suggest the use of cabergoline to facilitate the decrease of milk production in dairy ewes (e.g., dry-off, illness care), although further research in pregnant dairy ewes and during the following lactation is still needed.

Effects of micronutrient supplementation on performance and epigenetic status in dairy cows

The postpartum period is crucial in dairy cows and is marked by major physiological and metabolic changes that affect milk production, immune response and fertility. Nutrition remains the most important lever for limiting the negative energy balance and its consequences on general health status in highly selected dairy cows. In order to analyze the effect of a commercial micronutrient on intrinsic parameters, performances and the epigenome of dairy cows, 2 groups of 12 Holstein cows were used: 1 fed a standard diet (mainly composed of corn silage, soybean meal and non-mineral supplement) and the other 1 fed the same diet supplemented with the commercial micronutrient (µ-nutrient supplementation) for 4 weeks before calving and 8 weeks thereafter. Milk production and composition, BW, body condition score (BCS), DM intake (DMI) and health (calving score, metritis and mastitis) were recorded over the study period. Milk samples were collected on D15 and D60 post-calving for analyses of casein, Na+ and K+ contents and metalloprotease activity. Milk leukocytes and milk mammary epithelial cells (mMECs) were purified and counted. The viability of mMECs was assessed, together with their activity, through an analysis of gene expression. At the same time points, peripheral blood mononuclear cells (PBMCs) were purified and counted. Using genomic DNA extracted from PBMCs, mMECs and milk leukocytes, we assessed global DNA methylation (Me-CCGG) to evaluate the epigenetic imprinting associated with the µ-nutrient-supplemented diet. The µ-nutrient supplementation increased BCS and BW without modifying DMI or milk yield and composition. It also improved calving condition, reducing the time interval between calving and first service. Each easily collectable cell type displayed a specific pattern of Me-CCGG with only subtle changes associated with lactation stages in PBMCs. In conclusion, the response to the µ-nutrient supplementation improved the body condition without alteration of global epigenetic status in dairy cows.

Prolactin-Responsive Circular RNA circHIPK3 Promotes Proliferation of Mammary Epithelial Cells from Dairy Cow

The highly expressed circHIPK3 is a circular RNA that has been previously reported to regulate the growth of human cells. In this study, we found an increased expression of circHIPK3 in bovine mammary epithelial cells treated with prolactin (PRL) in high-throughput sequencing data. Thus, we further investigated the effect of circHIPK3 on the proliferation and differentiation of mammary epithelial cells. We used qRT-PCR/Cell Counting Kit-8 (CCK-8) and a Western blotting analysis to evaluate the effects on cell proliferation. We found that circHIPK3 promotes the proliferation of mammary epithelial cells. The STAT5 signaling pathway was previously associated with the prolactin response and when the STAT5 was suppressed, the expression of circHIPK3 decreased. The results suggest that the response to prolactin and the associated STAT5 signaling pathway affect the expression of circHIPK3, which subsequently affects the proliferation of mammary epithelial cells in dairy cows.

Review: the cellular mechanisms underlying mammary tissue plasticity during lactation in ruminants

The mammary tissue is characterized by its capacity to adapt in response to a wide variety of changing conditions. This adaptation capacity is referred to as the plasticity of mammary tissue. In dairy ruminants, lactation is challenged by modifications that can either be induced on purpose, such as by modifying management practices, or occur involuntarily, when adverse environmental constraints arise. These modifications can elicit both immediate changes in milk yield and composition and carryover effects that persist after the end of the challenge. This review focuses on the current knowledge concerning the cellular mechanisms underlying mammary tissue plasticity. The main mechanisms contributing to this phenomenon are changes in the activity and number of mammary epithelial cells (MECs). Changes in the number of these cells result from variations in the rates of cell proliferation and death as well as changes in the rate MEC exfoliation. The number of MECs also depends on the number of resident adult mammary stem cells and their progenitors, which can regenerate the pools of the various mammary cells. Several challenges, including changes in milking frequency, changes in level of feed supply and hormonal manipulations, have been shown to modulate milk yield together with changes in mammary cell activity, turnover and exfoliation. Epigenetic changes may be an additional mechanism of adaptation. Indeed, changes in DNA methylation and reductions in milk yield have been observed during once-daily milking and during mastitis in dairy cows and may affect cell activity persistently. In contrast to what has been assumed for a long time, no carryover effect on milk yield were observed after feed supply challenges in dairy cows and modification of milking frequency in dairy goats, even though the number of mammary cells was affected. In addition, mammary tissue plasticity has been shown to be influenced by the stage of lactation, health status and genetic factors. In conclusion, the cellular mechanisms underlying mammary tissue plasticity are diverse, and the mammary tissue either does or does not show elastic properties (with no permanent deformation), in response to environmental changes.

Mammary Epithelial Cell Lineage Changes During Cow's Life

Milk production is highly dependent on the optimal development of the mammary epithelium. It is therefore essential to better understand mammary epithelial cell growth and maintenance from the related epithelial lineage during the animal life. Here, we characterized the epithelial lineage at puberty, lactation and dry-off in bovine using the cell surface markers CD49_f, CD24, and CD10. The pubertal period was characterized by a high proportion of CD49_f^pos cells corresponding to various epithelial subpopulations, notably the CD24^pos subpopulations. The proportion of CD49_f^pos cells was weaker during lactation and dry-off, and CD24^pos cells were relatively few. Of note, the (sub)population profile at dry-off appeared close to that during lactation. Using a targeted gene approach, we associated specific genes with epithelial subpopulations, their expression level varying, or not, according to physiological stages. Caseins were only expressed in the CD49_f^medCD24^neg subpopulation. Basal marker genes (keratin(KRT)5, KRT14 and αSMA) were found in the CD49_f^highCD24^neg subpopulations. Luminal gene markers (KRT7, KRT8 and KRT19, CDH1 and the PRLR) were expressed in the CD49_f^lowCD24^neg subpopulation. The CD49_f^lowCD24^pos subpopulation, only abundant at puberty, expressed luminal gene markers and KI67 at high level. In contrast to others, the CD49_f^highCD24^pos cells accounted for a small proportion of total cells, decreasing from puberty to dry-off. They were characterized by expression of luminal and basal gene markers and low KI67 level. Interestingly, this subpopulation showed a remarkable stability of gene expression profile throughout physiological stages and bear the hallmark of quiescence that designate them as the potential bovine mammary stem cells.

Milk yield loss in response to feed restriction is associated with mammary epithelial cell exfoliation in dairy cows

In dairy cows, feed restriction is known to decrease milk yield by reducing the number of mammary epithelial cells (MEC) in the udder through a shift in the MEC proliferation-apoptosis balance, by reducing the metabolic activity of MEC, or both. The exfoliation of MEC from the mammary epithelium into milk is another process that may participate in regulating the number of MEC during feed restriction. The aim of the present study was to clarify the mechanisms that underlie the milk yield loss induced by feed restriction. Nineteen Holstein dairy cows producing 40.0 ± 0.7 kg/d at 77 ± 5 d in milk were divided into a control group (n = 9) and a feed-restricted group (n = 10). Ad libitum dry matter intake (DMI) was recorded during a pre-experimental period of 2 wk. For 29 d (period 1), cows were fed either 100 (control) or 80% (feed-restricted) of their ad libitum DMI measured during the pre-experimental period. Then, all cows were fed ad libitum for 35 d (period 2). Milk production and DMI were recorded daily. Blood and milk samples were collected once during the pre-experimental period; on d 5, 9, and 27 of period 1; and on d 5, 9, and 30 of period 2. Mammary epithelial cells were purified from milk using an immunomagnetic method to determine the rate of MEC exfoliation. Mammary tissue samples were collected by biopsy at the end of each period to analyze the rates of cell proliferation and apoptosis and the expression of genes involved in synthesizing constituents of milk. Feed restriction decreased milk yield by 3 kg/d but had no effect on rates of proliferation and apoptosis in the mammary tissue or on the expression of genes involved in milk synthesis. The daily MEC exfoliation rate was 65% greater in feed-restricted cows than in control cows. These effects in feed-restricted cows were associated with reduced insulin-like growth factor-1 and cortisol plasma concentrations. When all cows returned to ad libitum feeding, no significant difference on milk yield or MEC exfoliation rate was observed between feed-restricted and control cows, but refeeding increased prolactin release during milking. These results show that the exfoliation process may play a role in regulating the number of MEC in the udders of dairy cows during feed restriction without any carryover effect on their milk production.

Effect of the concentration of circulating prolactin on dairy cows' responsiveness to domperidone injection

The objective of this study was to determine whether the responsiveness of the mammary gland to prolactin (PRL) is affected by the concentration of the hormone. After 1 pre-experimental week (d -7 to -1), 18 Holstein cows in mid to late lactation were injected intramuscularly twice daily with either 0.5 mg of quinagolide (QN) or 2 mL of water (control) for 2 wk (d 1 to 14; treatment period). After the treatment period, all cows received daily subcutaneous injections of 300 mg of domperidone (DOMP) for 3 wk (d 15 to 35; DOMP period). The cows were monitored for an additional 2 wk as a posttreatment period (d 36 to 49). Blood and milk samples were collected 3 times per week. Additionally, blood samples were collected during the a.m. milking on d -4, 14, and 35. Milk production was not affected by QN during the treatment period but was increased during the DOMP and posttreatment periods in the QN cows. With respect to milk composition, the treatments affected only the protein content, which was greater in the QN cows during the treatment period. Blood PRL concentration declined during QN injections and was lower in the QN cows than in the control cows between d 5 and 14. The basal concentration of PRL was increased by DOMP injections during the DOMP and posttreatment periods but was not affected by previous QN injections. Prolactin concentration in milk was not affected by the QN treatments but was increased by DOMP injections during the DOMP and posttreatment periods. Milking-induced PRL release was decreased by QN on d 14. On d 35, milking did not induce a significant release of PRL above the baseline for both treatments. In conclusion, the results of this experiment support the contention that the mammary gland's responsiveness to PRL is modulated by the previous level of the hormone.

Mammary epithelium disruption and mammary epithelial cell exfoliation during milking in dairy cows

The presence of mammary epithelial cells (MEC) in the milk of ruminants indicates that some MEC are shed from the mammary epithelium; however, the mechanisms that regulate the MEC exfoliation process are not known. Through the release of oxytocin, prolactin, and cortisol and through oxytocin-induced mechanical forces on the mammary epithelium, milking could participate in regulating the MEC exfoliation process. The aims of the present study were to determine the rate of MEC exfoliation throughout milking and to investigate its relationship to mammary epithelium integrity and milking-induced hormone release. Milk samples from 9 Holstein dairy cows producing 40.6 ± 1.36 kg of milk/d were collected at the beginning (after 1 and 2 min), in the middle, and at the end of milking. Milk MEC were purified using an immunomagnetic method. Blood samples were collected before, during, and after milking, and the oxytocin, prolactin, and cortisol concentrations in the samples were measured. Tight junction opening was assessed by plasma lactose concentration and the Na⁺:K⁺ ratio in milk. The somatic cell count in milk varied during the course of milking; it decreased at the beginning of milking and then increased, reaching the highest values at the end of milking. Exfoliated MEC were present in all milk samples collected. The presence of MEC in the milk sample collected during min 1 of milking, likely corresponding to the cisternal milk fraction, suggests that MEC were exfoliated between milkings. The observed increase in the Na⁺:K⁺ ratio in milk and in the plasma concentration of lactose indicated that disruption of mammary epithelium integrity occurred during milking. The MEC exfoliation rate at milking was not correlated with the variables describing milking-induced prolactin release but was negatively correlated with cortisol release, suggesting that cortisol may play a role in limiting exfoliation. In conclusion, milking induced a disruption of the mammary epithelial barrier. Mammary epithelial cells may be continuously exfoliated between milkings or exfoliated during milking as a consequence of the oxytocin-induced mechanical forces and the disruption of mammary epithelium integrity.

Exfoliation rate of mammary epithelial cells in milk on bovine mastitis caused by Staphylococcus aureus is associated with bacterial load

The exfoliation rate of mammary epithelial cells (MECs) in milk is affected by physiological, breeding and environmental factors. Little is known about the relationship between the MEC exfoliation into milk and mammary-infected Staphylococcus aureus (S. aureus) load on bovine mastitis caused by S. aureus. The aim of this study was to investigate the relationship between S. aureus load and the proportion of MEC exfoliation in milk using five substantial bovine mastitis models. In 64 randomly extracted milk samples from udders at 3-21 days after S. aureus infusion, there were various samples with different numbers of S. aureus counts and somatic cell counts. No significant correlations were found between the S. aureus counts and somatic cell count (r = 0.338). In contrast, a significant correlation was noted between S. aureus counts and the proportion of cytokeratin-positive cells in the milk from the infused udders (r = 0.734, P < 0.01). In conclusion, the increasing MEC exfoliation rate in milk from mastitis udders caused by S. aureus may contribute to reduced milk yield.

Effects of automatic cluster removal and feeding during milking on milking efficiency, milk yield and milk fat quality

In order to increase milking efficiency, the effects of two different cluster take-off levels (200 and 800 g/min) and feeding vs. not feeding during milking were tested in a Latin square design study including 32 cows. Milk yield, milking time, milk flow and milking interval were measured and milk samples were analysed for gross composition, sodium and potassium concentration, free fatty acid (FFA) content, milk fat globule (MFG) size, MFG membrane (MFGM) material and fatty acid composition. Residual milk was harvested to evaluate udder emptying. Increasing the take-off level from 200 to 800 g/min at the whole udder level decreased milking time and increased harvest flow. Udder emptying decreased slightly, but there were no effects on milk yield, FFA content or MFGM. There were interactive effects of take-off level and feeding during milking on content of fatty acids C4:0, C6:0, C16:0, C18:3(n-3) and C20:0. Feeding during milking increased milk yield per day and decreased milking interval. Sodium and potassium concentrations in milk were unaffected by treatments, indicating no loss of tight junction integrity. From these results, it is clear that feeding during milking should be used to increase milk yield and improve milking efficiency, regardless of take-off level used, and that the effect of feeding is more pronounced when a low take-off level is used. Feeding seemed to counteract the effects of the low take-off level on milking time and milking interval. Low take-off levels can therefore be used in combination with feeding.

Mammary epithelial cells isolated from milk are a valuable, non-invasive source of mammary transcripts

Milk is produced in the udder by mammary epithelial cells (MEC). Milk contains MEC, which are gradually exfoliated from the epithelium during lactation. Isolation of MEC from milk using immunomagnetic separation may be a useful non-invasive method to investigate transcriptional regulations in ruminants' udder. This review aims to describe the process of isolating MEC from milk, to provide an overview on the studies that use this method to analyze gene expression by qRT PCR and to evaluate the validity of this method by analyzing and comparing the results between studies. In several goat and cow studies, consistent reductions in alpha-lactalbumin mRNA levels during once-daily milking (ODM) and in SLC2A1 mRNA level during feed restriction are observed. The effect of ODM on alpha-lactalbumin mRNA level was similarly observed in milk isolated MEC and mammary biopsy. Moreover, we and others showed decreasing alpha-lactalbumin and increasing BAX mRNA levels with advanced stages of lactation in dairy cows and buffalo. The relevance of using the milk-isolated MEC method to analyze mammary gene expression is proven, as the transcript variations were also consistent with milk yield and composition variations under the effect of different factors such as prolactin inhibition or photoperiod. However, the RNA from milk-isolated MEC is particularly sensitive to degradation. This could explain the differences obtained between milk-isolated MEC and mammary biopsy in two studies where gene expression was compared using qRT-PCR or RNA Sequencing analyses. As a conclusion, when the RNA quality is conserved, MEC isolated from milk are a valuable, non-invasive source of mammary mRNA to study various factors that impact milk yield and composition (ODM, feeding level, endocrine status, photoperiod modulation, and stage of lactation).