Functions of Lysosomes in Mammalian Female Reproductive System.

Since the identification of lysosomes in 1955, tremendous progress has been made with respect to understanding the functions and mechanisms of lysosomes in eukaryotic cells. Novel functions and mechanisms of lysosomes are continuously being uncovered. Lysosomes have a spatiotemporal presence in the female reproductive system, as well as in the HPO axis that regulates the functions of the female reproductive system. Relative to the progresses in understanding the lysosome in some other systems, such as the nervous system, the progresses in understanding the lysosome in the female reproductive system are lagging behind. The importance of lysosomes can be reflected in lysosomal storage disorders, which comprise a group of at least 50 distinct genetic diseases that affect about 1 in 7,000 of newborns. [ 100 ] Genetically modified animal models that mimic lysosomal storage disorders could provide invaluable insights into the functions of lysosomes in the female reproductive system. Since the acidic environment in the lysosome is maintained via V-ATPase and counter ion channels for proper lysosomal functions, and lysosomal enzymes are main contributors for lysosomal functions, pharmacological approaches modifying the activities of V-ATPase, counter ion channels, and/or lysosomal enzymes could be employed for studying the functions of lysosomes in the female reproductive system. Because the female reproductive system is under the control of HPO axis, it is important that any potential functions of lysosomes in the hypothalamus and pituitary are also considered when the function (s) of lysosomes in the female reproductive system are being investigated. Cellular functions of lysosomes are being revealed in different cells of the female reproductive system, while the molecular mechanisms involved are emerging. These are a few directions for studying functions and mechanisms of lysosomes in the female reproductive system.

In female reproduction, the ultimate role of mammary glands is to produce milk for nursing the offspring. The mammary gland undergoes cyclic changes including development, lactation and involution. At puberty, estrogen induces mammary gland development. During pregnancy, estrogen and progesterone coordinately promote the continuous development of mammary alveoli to prepare for lactation. After parturition, prolactin and oxytocin regulate milk synthesis and secretion from mammary alveoli. Upon weaning of the offspring and cessation of lactation, the mammary glands undergo involution. [ 92 ] Early studies in rodents suggested potential roles of lysosomes in mammary glands. Although there were discrepancies in different studies, a relative consist observation was peaked lysosomal enzyme activities during mammary gland involution ( [ 93 , 94 ] and their citations). An electron microscopy study revealed shifts in lysosomal populations (primary, secondary, casein-positive, and dense body) in the mammary epithelial cells during differentiation/pregnancy, secretion/lactation, and involution, suggesting active roles of lysosomes in mammary glands throughout different stages. [ 94 ] Recent studies have demonstrated functions of lysosomes in mammary gland development, producing milk components and mammary gland involution [ Figures 3B and 4E ]. V-ATPase is essential for regulating lysosomal pH. Mouse mammary glands deficient of the a2 subunit (a2V) of V-ATPase had impaired development, accompanied with disrupted endolysosomal route in Notch and TGF signaling, which plays a major role in mammary gland development. [ 95 ] Although not in lactation per se , lysosomes function in producing milk components, such as cholesterol, an important lipid component in milk. Lysosomes have a fundamental role in regulating cellular cholesterol homeostasis. [ 42 ] Lysosomal cholesteryl ester hydrolase activity was dramatically increased in the lactating rat mammary gland compared to the virgin counterpart. [ 96 ] Impaired lactation in a2V-deficient mouse mammary gland might be a secondary effect of impaired mammary gland development. [ 95 ] Lysosome-dependent cell death [ Figure 3 ] is a hallmark of mammary gland involution. It is activated by upregulation of lysosome biogenesis and acidification, a process involving Zinc transporter 2 (ZnT2 [SLC30A2]), [ 97 ] proceeds with lysosomal membrane permeabilization and leakage of intralysosomal components (e.g., cathepsins). Mouse models have revealed signal transducer and activator of transcription 3 (STAT3) [ 98 ] and calpains [ 99 ] in regulating this process. The trigger for STAT3 involvement is the uptake of milk fat globules, which are delivered to lysosomes to cause lysosomal membrane permeabilization; the involvement of STAT3 may also be contributed by its upregulation of cathepsins B and L, which will be among leaked cathepsin proteases. [ 98 ] Calpains, on the other hand, proteolyze lysosomal membrane proteins to cause lysosomal membrane permeabilization. [ 99 ]

The lysosome was first described and named by the Belgian cytologist and biochemist De Duve et al . in 1955. [ 1 ] It is the most acidic membrane-bound intracellular organelle, with a lumen pH of ~4.5–5.0. This environment is optimal for >60 different hydrolytic enzymes in the lysosome, such as proteases (e.g., cathepsins), lipases, nucleases, glycosidases and phosphatases, that can break down biomolecules within intracellular and extracellular origins. Lysosomal acidity is primarily maintained by vacuolar H + -ATPase (V-ATPase) to pump H + into the lysosomal lumen and counter ion channels to dissipate the transmembrane voltage built up by V-ATPase. [ 2 ] In addition to its digestive role, the lysosome is also important for intracellular trafficking, cellular homeostasis, metabolic signaling, cholesterol transport, lipid metabolism, immune response, and hormonal signaling, etc. [ 2 – 6 ] Mutations in lysosomal genes could potentially lead to lysosomal dysfunction. Disrupted lysosomal functions in degradation, export, or trafficking can lead to abnormal accumulation of lysosomal materials, resulting in >50 rare inherited metabolic disorders in humans, collectively termed lysosomal storage diseases. [ 7 ] The lysosome participates in essential cellular processes such as endocytosis and exocytosis [ Figure 1 ], autophagy [ Figure 2 ], and cell death [ Figure 3 ]. Endocytosis transports extracellular cargo molecules to the lysosome for processing. There are four main types of endocytosis [ Figure 1A – 1D ]: clathrin-mediated endocytosis, [ 8 ] clathrin-independent endocytosis, [ 9 ] pinocytosis, [ 10 ] and phagocytosis. [ 11 ] The involvement of lysosomes in exocytosis is relatively less studied and the mechanisms involved are still under investigation. Lysosomes and lysosome-related organelles can be transported to and fused with the plasma membrane to release the lysosomal contents into the extracellular space. Increased intracellular calcium level and depletion of cholesterol can both trigger lysosomal exocytosis, which is a tightly regulated process involving microtubules and actins, Rab GTPases, and SNAREs, [ 12 ] [ Figure 1F ]. Autophagy delivers unwanted intracellular cargo molecules, which could be of extracellular origin, to the lysosome for degradation and nutrient cycling. [ 13 ] There are three main types of autophagy [ Figure 2A – 2C ]: macroautophagy, chaperone-mediated autophagy, and microautophagy. The primary type of autophagy is macroautophagy. It requires the formation of autophagosomes [ Figure 2A ] that involves LC3/MAP1LC3 (microtubule-associated proteins 1A/1B light chain 3B, cytosolic form LC3-I, membrane form LC3-II). Autophagosomes fuse with lysosomes to form autolysosomes for cargo degradation by lysosomal hydrolases. Many genes, such as autophagy-related (ATG) genes, have been identified to play essential roles in the autophagic pathways and implicated in the functions of the female reproductive tract. [ 13 ] There are generally two types of cell death, regulated cell death and accidental cell death. Lysosome-dependent cell death [ Figure 3 ] is a type of regulated cell death, [ 14 ] in which lysosomal membrane permeabilization causes selective release of cathepsins or massive release of lysosomal enzymes leading to cell death. It could also trigger other types of regulated cell death, such as apoptosis and pyroptosis, which is a highly inflammatory form of programmed cell death in which cathepsins are involved. [ 15 ] The mammalian female reproductive system generally consists of two ovaries, two fallopian tubes/oviducts, a uterus/two uterine horns, a placenta/placentas, a cervix, a vagina, and mammary glands. During the years following the discovery of the lysosome, studies dating back to 1973 revealed the presence of lysosomes and/or the activities of lysosomal enzymes in the ovary, uterus, cervix, and vagina using electron microscopy and biochemical approaches. Many of these studies were previously reviewed. [ 16 , 17 ] Since then, more studies in different species have implicated lysosomes in the female reproductive system, but have not been comprehensively reviewed. This review updates the research on the functions and mechanisms of lysosomes in the female reproductive system. It will be organized roughly based on the order of female reproductive organs involved in the pregnancy process: ovary, oviduct, uterus, placenta, parturition, cervix and vagina, and mammary gland [ Figure 4 ].