Name: rodent estrous cycle monitoring utilizing vaginal lavage: no such thing as a normal cycle
Uploaded: 2021-08-30T15:00:00
Description: Watch this Scientific Journal Video about Rodent Estrous Cycle Monitoring Utilizing Vaginal Lavage: No Such Thing As a Normal Cycle at JoVE.com

This study was conducted through an NIH-funded collaboration between the University of California Los Angeles Brain Injury Research Center (BIRC).

All handling and procedure methods outlined in this protocol align with National Institutes of Health (NIH) animal care and use guidelines and have been approved by the Institutional Animal Care and Use Committee (IACUC) of Pepperdine University and The UCLA Chancellor&#39;s Animal Research Committee (ARC).
1. Animal care and use
<ol>
	<li>Acquire female rats, in numbers according to power analysis, and male rats to promote the Whitten effect or more consistent cycling46</s...

Key steps and important considerations 
Certain critical steps in the provided protocol require emphasis, especially within the collection of vaginal cells. During the vaginal fluid extraction, ensuring the proper angle and depth of syringe insertion is key to producing satisfactory results and ultimately preventing irritation, injury, or cervical stimulation to the animal. The stimulation of the cervix can be one source of pseudopregnancy induction, indicated by 12-14 days of a leukocyte-only vagin...

Novel contributions 
The rodent estrous cycle&#160;has been identified as an essential indicator of wellness. However, unconscious biases of investigators and inaccurate interpretations regarding the female body hinder the scientific community. The very etymology of the word &#34;estrous&#34; implies&#160;a sense of inferiority and negativity. Euripides used the term to describe a &#34;frenzy&#34; or madness, Homer to describe panic, and Plato to describe an irrational drive. This study highlights how these primeval perspectives influence the current scientific community and addresses these concerns through a novel mosaic paradigm&#8212;an updated combination of previously studied methods, expanded in scope for a more comprehensive approach.
The study and use of this technique are necessary, first, as there is no standardized and comprehensive monitoring technique, and data interpretation practices can be unclear. Second, although estrous cycle characteristics are dependent on individual rats being studied, they are often universalized. Third, while hormonal cycles are routine and beneficial processes, they are surrounded by&#160;hazardous stigma explored in the &#39;Translation to Humans&#39; section. This study aims to address these three issues in three ways&#8212;(A)&#160;by describing an in-depth estrous cycle monitoring technique and clarifying how the results can be interpreted, (B) by outlining methods that maintain the integrity and individuality of each cycle, and (C) by calling attention to misconceptions that perpetuate unsubstantiated practices.
This study is also unique in its focus on adolescent rats, a period marked by crucial developmental changes that shed light on various behavioral, anatomical, and physiological manifestations in adulthood1. Building a standardized experimental design to monitor hormonal cycles in an under-researched population while deconstructing common biases will allow for the development of reliable and valid hormonal correlations2,3,4 and the determination of condition-dependent cycle disruptions5,6,7,8,9,10. Ultimately, these novelties serve to expand diagnostic criteria, treatments, and interventions of various wellness concerns.
Fundamental definitions and uses 
The estrous cycle is a collection of dynamic physiological processes that occur in response to the three oscillating female sex steroid hormones: estradiol, leuteinizing hormone (LH), and progesterone (Figure 1A,B). Interactions between the endocrine and central nervous system regulate the cycle, which most often persists for 4-5 days and recurs from the onset of sexual maturation until reproductive senescence and/or cessation. It is divided into separate categories based on hormone levels&#8212;most commonly into the 4 stages of diestrus (DIE), proestrus (PRO), estrus (EST), and metestrus (MET), which progress in a circular fashion. The number of divisions can range from 3 stages11 to 13 stages12, depending on the nature of the study13. The lower number of divisions often excludes MET as a stage and classifies it as a short-duration transitionary period. The higher number typically includes subsections that allow for a closer inspection of phenomena such as tumor development or spontaneous pseudopregnancy, the physiological state of pregnancy without embryonic implantation12,14,15.
In this study, the stages were identified through components of the vaginal canal, named the 3 categorizing determinants-cell type(s) present, cell arrangement, and cell quantity (Figure 2A-D). While the condition of the vaginal fluid was not monitored in this study, it is recommended to include it as a fourth categorizing component. Further information on examining the vaginal fluid can be found in the reference list16. The categorizing components can be examined by extracting cells via vaginal lavage, the primary technique recommended in modern-day estrous cycle monitoring. While the in-depth physiological processes within each stage are outside the scope of this study, more information can be found in the literature17.
The use and continued development of this estrous cycle monitoring technique is rooted in the connections between sex steroid hormones and the function of bodily systems such as the cardiovascular system18, endocrine system8, and central nervous system19,20,21. At the same time, estrous cycle monitoring may not always be necessary when female rodents are involved22,23,24,25. Rather, it is important first to consider if sex differences have been reported in the specific area of study, which can be further explored in published reviews22,23. Though estrous cycle monitoring is vital in a broad spectrum of research investigations, it should not be seen as an obstacle to including female rodents in experiments. While this technique may appear complex and time-consuming, the procedure itself can take less than 15 min to complete, depending on the investigator, and is cost-effective. Overall, the inclusion of female rodents in scientific studies is advantageous to the understanding of bodily systems, various conditions and pathologies, and general wellness, as these developments have been mainly based on the male body template.
Universal parameters and natural variabilities in the rodent 
Establishing ranges for aspects seen as &#34;typical&#34; is necessary to define standard cycle patterns, set parameters for comparative and analytic purposes, and detecting abnormalities and outliers. At the same time, it is also important to recognize that each rat&#39;s cycle is unique, and deviations based on animal strain, physiological processes, and environmental conditions are expected. In fact, one of the most &#34;normal&#34; aspects of the estrous cycle is variability. This is seen in the total cycle length, with a range of 3-38 days26,27; the age of sexual maturation that can range from 32-34 days to multiple weeks28,29,30; what is considered acyclical11, and the categorizing determinant patterns11,13. Overall, there is no universal template for the estrous cycle, and translating that to both the scientific community and the general public is an important part of the experimental process.
Experimental timepoints and developmental age 
Recognizing this principle of variability assists in building a reliable and valid experimental design. For example, the start of estrous cycling monitoring relies on the rats&#39; anatomical and physiological development, which varies based on environmental and physiological factors. Monitoring cannot begin until the development of the vaginal opening (VO), which is the external vaginal orifice surrounded by the vulva that leads to the interior portion of the vaginal canal (Figure 3A-D). While the VO often fully develops between the ages of 32 and 34 days, it remains individualized to each subject, and much about the process remains unknown. This opening has been used to identify the onset of sexual maturation, which has been linked to the increase of estradiol31, the maturation of the hypothalamic-pituitary-ovarian axis32, and the first ovulation in rats17, 33,34,35. However, recent publications have found that it is only an indirect marker of reproductive development, as it can become uncoupled from hormonal and developmental occurrences in unfavorable environments31 and may represent changes in estradiol levels rather than sexual maturation33. Therefore, it is recommended to not rely solely on the VO to determine developmental age and as a qualifier for estrous cycle monitoring36 but to also utilize the appearance of the first EST stage and cornification of the epithelial cells30 to mark the onset of sexual maturation.
Body weight is notably correlated with developmental age during the adolescent period in rodents30,37 and can therefore also assist in determining developmental age in this period. Proposed mechanisms related to this phenomenon include the stimulation of hormones necessary for reproductive development, such as growth hormone, and the inhibition of the hypothalamic-pituitary adrenal (HPA) axis by the appetite regulator, leptin30. However, it is not recommended to utilize this measure as the sole indicator of developmental age due to the large variance seen between rats across species and vendor providers38. The variability seen in the development of the VO and body weight exemplify the importance of the concept in the overall experimental process.
Translation to humans: cultural and scientific contexts 
The translational relationship of animal-to-human reproductive studies is bidirectional. The results from animal-based studies influence how the human processes are assessed, approached, and analyzed39. The perception of the human reproductive system and its related processes influence how animals are studied. In fact, one of the loudest indications for further research in this area stems from biased sociocultural beliefs related to hormonal cycles that influence the scientific process. Many of these conventions are derived from a general cultural aversion to discussing menstruation, which has led to a data gap in well-substantiated knowledge40,41. This has a spectrum of consequences that span from minor to lethal&#8212;from shelving height and smartphone size to police body armor fitting and missed cancer diagnoses42.
The description of menstruation as unsanitary, destructive, and toxic&#8212;seen in revered texts, media, dictionaries, and medical teachings&#8212;is conserved by scientific publications. This occurs through inaccurate and biased descriptions of hormonal cycles, the isolation of the reproductive system from its neuroendocrine counterparts and environmental influences, and the reductionist perspective of the completion of a cycle as a &#39;failure to conceive&#39;43,44. This leads to the creation of unsound experimental practices, such as the omission of external variables that influence hormonal cycles, determining start and endpoints based solely on anatomical developments, and measuring cycle advancement in a linear rather than circular fashion. Despite the direct correlation between sociocultural factors and biological consequences, it is not often considered in scientific literature. Through the inspection of more holistic publications43,44,45, researchers can deconstruct these stigmas and create more reliable and valid experimental designs.

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rodent estrous cycle monitoring utilizing vaginal lavage: no such thing as a normal cycle

The current methodology establishes a reproducible, standardized, and cost-effective approach to monitoring the estrous cycle of female Sprague Dawley (SD) adolescent rats. This study demonstrates the complexity of hormonal cycles and the broad spectrum of understanding required to construct a reliable and valid monitoring technique. Through an in-depth examination of principal experimental design and procedural elements,&#160;this description of the cycle and its fundamental principles provides a framework for further understanding and deconstructs misconceptions for future replication.
Along with an outline of the sample collection process employing vaginal lavage, the procedure describes the mechanism of data categorization into the four-stage model of proestrus, estrus, metestrus, and diestrus. These stages are characterized by a new proposed approach, utilizing the 4 categorizing determinants of vaginal fluid condition, cell type(s) present, cell arrangement, and cell quantity at the time of collection. Variations of each stage, favorable and unfavorable samples, the distinction between cyclicity and acyclicity, and graphic depictions of the collected categorizing components are presented alongside effective interpretive and organizational practices of the data. Overall, these tools allow for the publication of quantifiable data ranges for the first time, leading to the standardization of categorization factors upon replication.

The current data reflect that of female adolescent SD International Genetic Standardization Program (IGS) in the presence of male SD rats. These animals were located at both Pepperdine University and UCLA laboratories as part of a collaborative study. Figure 5 presents multiple variations of the 4 cycle stages. Figure 5A1 was identified as a diestrus sample with several cell types present. This example demonstrates that samples with a larger num...

Watch this Scientific Journal Video about Rodent Estrous Cycle Monitoring Utilizing Vaginal Lavage: No Such Thing As a Normal Cycle at JoVE.com

Rodent Estrous Cycle Monitoring Utilizing Vaginal Lavage: No Such Thing As a Normal Cycle

This study details the crucial factors to consider in experimental designs involving female rats. In a larger sense, these data serve to decrease stigma and assist in the development of more inclusive diagnostic and intervention tools.

The current methodology establishes a reproducible, standardized, and cost-effective approach to monitoring the estrous cycle of female Sprague Dawley ...

This protocol is significant in that it provides both a comprehensive and low cost time effective technique. It answers key elements of translational research and categorizing component data on both individual and group levels. This technique is advantageous due to the total amount of information that can be extracted regarding the overall profile of the animal.This includes the categorizing components and the subsequent conclusions. This technique could provide insight into a broad range of research areas, including cardiology, neurology, and pharmacology. This technique is not solely applicable to reproductive studies.Begin by handling the rat with an age appropriate hold, such as the compression style hold for adolescent aged rats, by placing the subject against the upper chest region with its nose pointed down at the ground. Expose the vaginal canal of the subject by gently flexing the tail before inserting the syringe. While holding the syringe steady with the fore finger and middle finger insert the tip of the syringe into the vaginal canal at an angle parallel to it, slowly expel the sodium chloride into the canal by pushing the plunger inwards with the thumb.Extract the sodium chloride from the vaginal canal by pulling the syringe's plunger away from the epithelial lining. Use the criteria for each cell type to characterize the cells present in the sample and record the findings. A nucleated keratinized epithelial or quantified epithelial cells will appear jagged or angular edged.Despite the lack of nuclei, it may show light round areas within the cell representing where a nucleus was once present called ghost nuclei. The large nucleated epithelial, or LNE cells, are typically round to polygonal shaped cells encased by irregular jagged or angular borders. Their nuclei can range from intact to degenerate or pyknotic relating to the irreversible condensation of chromatin in the nucleus of a cell undergoing death or deterioration.The nuclei take up less space than the cytoplasm within the cell, with a lower nuclear to cytoplasmic ratio than the small epithelial cells. Leukocytes are compact spherical cells. The smallest when compared to all other described cell types.SNE cells are non-keratinized cells with round nuclei that take up a larger space than the cytoplasm inside the cell, creating a higher nucleus to cytoplasm ratio relative to large epithelial cells. Use the lower objectification, such as four X to get a representative view of the overall cell arrangement. Record whether the cells are clumped together, evenly dispersed, or randomly dispersed, and note the specific organization of each cell type.Then visually estimate and record the total cell quantity, as a smidge, moderate, or numerous, and the individual cell quantities as a percentage of each cell type present. With these categorizing components combined, determine which estrous cycle stage is being presented. Within the diestrus stage, it can be seen that leukocytes are the dominant or the only cell type.These are typically arranged in a clumped manner at the beginning of, but more dispersed in late stages. The total quantity of cells may be comparatively low, most often in the later stages of the diestrus period on the second or third day. Concentrated strands of leukocytes may be observed due to the high amount of mucus that may be present in this stage.Small clumps of cellular strands of SNE cells accompanying the leukocytes may be seen during late phases in the transition to proestrus. The vaginal fluid, when transitioning into, fully transitioned into, and transitioning out of diestrus, appears viscous and opaque. During the proestrus stage, SNE cells are the dominant cells while leukocytes, LNE, and AKE cells may be found in low numbers.SNE cells are typically arranged in clusters, sheets, or strands during this stage and have a granular appearance that can be observed at higher magnification. The vaginal fluid becomes non viscous and transparent once the subject fully transitions into the proestrus stage. The AKE cells are dominant during the estrous stage, with a decrease in the SNE cells, and an overall increase in the number and size of cells as estrous continues.The AKE cells are often clustered in the form of keratin bars. They may also contain ghost nuclei that can become more dispersed in the transition from proestrus to metestrus. The vaginal fluid is non viscous and transparent as the rats are transitioning into, fully transition into, and transition out of estrous.As the rats transition into metestrus, look for all cell types, though there is often a higher amount of SNE and LNE cells compared to the LEUs. A greater amount of debris is observed during metestrus and the transitions due to the epithelial cell decay following estrous. During this stage, there is a lack of consistent arrangement as all cell types are seen and in various amounts.However, the leukocytes that are packed in proximity to the epithelial cells in the beginning stages may return to the clumped arrangement when transitioning into diestrus. The vaginal fluid appears non viscous and transparent in the stage. And changes to a more viscous and opaque appearance while moving into diestrus.Within each stage, many variations can be seen, for example within the diestrus stage, both epithelial cells and leukocytes were observed. In the arrangements, clumped mucus strands, composed of leukocytes, and initial clumping of leukocytes, and a later random or even disbursement were also seen. When collecting the vaginal fluid sample, there were non-represented collections that warranted repeat lavages, which included the extraction of squamous cells from the vaginal canal wall due to improper syringe insertion.A collection of solely debris, a cell count that was too low to categorize, and those that were out of focus. It is common to graph the stage progressions on bar graphs, which allows researchers to examine the overall cycle pattern and identify a-cyclisity. And the samples can then be analyzed by the cycle length and stage progression pattern.Estrous cycle profiles can be created using a stacked bar graph, either tracking the stage progression per rat or per cohort. Where the categorizing components for each day are graphed either per day or per stage. The categorizing component data can also be organized numerically to create standardized ranges and collect statistics.Practice the technique so it is minimally invasive and scan the microscope slide for representative samples to decrease selection bias. Overall, remember, there is no such thing as a normal cycle. Following this procedure, experimental variables could be introduced, such as exercise and behavioral paradigms.This would allow research to establish baseline cycling of animals beforehand and to detect hormonal responses to specific interventions afterwards. This method allows researchers to explore the connection between sex steroid hormones and other bodily systems. An example of this is how cardiac receptors for sex steroid hormones relates to system functionality.

rodent-estrous-cycle-monitoring-utilizing-vaginal-lavage-no-such

Research

JoVE Journal

Biology

Monitoring Kinase and Phosphatase Activities Through the Cell Cycle by Ratiometric FRET

F&ouml;rster resonance energy transfer (FRET)-based reporters1 allow the assessment of endogenous kinase and phosphatase activities in living cells. Such probes typically consist of variants of CFP and YFP, intervened by a phosphorylatable sequence and a phospho-binding domain. Upon phosphorylation, the probe changes conformation, which results in a change of the distance or orientation between CFP and YFP, leading to a change in FRET efficiency (Fig 1). Several probes have been published during the last decade, monitoring the activity balance of multiple kinases and phosphatases, including reporters of PKA2, PKB3, PKC4, PKD5, ERK6, JNK7, Cdk18, Aurora B9 and Plk19. Given the modular design, additional probes are likely to emerge in the near future10. 
Progression through the cell cycle is affected by stress signaling pathways 11. Notably, the cell cycle is regulated differently during unperturbed growth compared to when cells are recovering from stress12.Time-lapse imaging of cells through the cell cycle therefore requires particular caution. This becomes a problem particularly when employing ratiometric imaging, since two images with a high signal to noise ratio are required to correctly interpret the results. Ratiometric FRET imaging of cell cycle dependent changes in kinase and phosphatase activities has predominately been restricted to sub-sections of the cell cycle8,9,13,14.
Here, we discuss a method to monitor FRET-based probes using ratiometric imaging throughout the human cell cycle. The method relies on equipment that is available to many researchers in life sciences and does not require expert knowledge of microscopy or image processing.

FRET-based reporters are increasingly used to monitor kinase and phosphatase activities in live cells. Here we describe a method on how to use FRET-based reporters to assess cell cycle-dependent changes in target phosphorylation.

F&ouml;rster resonance energy transfer (FRET)-based reporters1 allow the assessment of endogenous kinase and phosphatase activities in living cells. Such ...

Analysis of Cell Cycle Position in Mammalian Cells

The regulation of cell proliferation is central to tissue morphogenesis during the development of multicellular organisms. Furthermore, loss of control of cell proliferation underlies the pathology of diseases like cancer. As such there is great need to be able to investigate cell proliferation and quantitate the proportion of cells in each phase of the cell cycle. It is also of vital importance to indistinguishably identify cells that are replicating their DNA within a larger population. Since a cell&prime;s decision to proliferate is made in the G1 phase immediately before initiating DNA synthesis and progressing through the rest of the cell cycle, detection of DNA synthesis at this stage allows for an unambiguous determination of the status of growth regulation in cell culture experiments.
DNA content in cells can be readily quantitated by flow cytometry of cells stained with propidium iodide, a fluorescent DNA intercalating dye. Similarly, active DNA synthesis can be quantitated by culturing cells in the presence of radioactive thymidine, harvesting the cells, and measuring the incorporation of radioactivity into an acid insoluble fraction. We have considerable expertise with cell cycle analysis and recommend a different approach. We Investigate cell proliferation using bromodeoxyuridine/fluorodeoxyuridine (abbreviated simply as BrdU) staining that detects the incorporation of these thymine analogs into recently synthesized DNA. Labeling and staining cells with BrdU, combined with total DNA staining by propidium iodide and analysis by flow cytometry1 offers the most accurate measure of cells in the various stages of the cell cycle. It is our preferred method because it combines the detection of active DNA synthesis, through antibody based staining of BrdU, with total DNA content from propidium iodide. This allows for the clear separation of cells in G1 from early S phase, or late S phase from G2/M. Furthermore, this approach can be utilized to investigate the effects of many different cell stimuli and pharmacologic agents on the regulation of progression through these different cell cycle phases.
In this report we describe methods for labeling and staining cultured cells, as well as their analysis by flow cytometry. We also include experimental examples of how this method can be used to measure the effects of growth inhibiting signals from cytokines such as TGF-&beta;1, and proliferative inhibitors such as the cyclin dependent kinase inhibitor, p27KIP1. We also include an alternate protocol that allows for the analysis of cell cycle position in a sub-population of cells within a larger culture5. In this case, we demonstrate how to detect a cell cycle arrest in cells transfected with the retinoblastoma gene even when greatly outnumbered by untransfected cells in the same culture. These examples illustrate the many ways that DNA staining and flow cytometry can be utilized and adapted to investigate fundamental questions of mammalian cell cycle control.

Determining the cell cycle position of a population of cells, or understanding how signals affect proliferation, can be readily measured by flow cytometry using this protocol. We report a simple experimental approach to staining cells and quantifying their position in the cell cycle.

The regulation of cell proliferation is central to tissue morphogenesis during the development of multicellular organisms. Furthermore, loss of control of ...

Measuring Cell Cycle Progression Kinetics with Metabolic Labeling and Flow Cytometry

Precise control of the initiation and subsequent progression through the various phases of the cell cycle are of paramount importance in proliferating cells. Cell cycle division is an integral part of growth and reproduction and deregulation of key cell cycle components have been implicated in the precipitating events of carcinogenesis 1,2. Molecular agents in anti-cancer therapies frequently target biological pathways responsible for the regulation and coordination of cell cycle division 3. Although cell cycle kinetics tend to vary according to cell type, the distribution of cells amongst the four stages of the cell cycle is rather consistent within a particular cell line due to the consistent pattern of mitogen and growth factor expression. Genotoxic events and other cellular stressors can result in a temporary block of cell cycle progression, resulting in arrest or a temporary pause in a particular cell cycle phase to allow for instigation of the appropriate response mechanism. 
The ability to experimentally observe the behavior of a cell population with reference to their cell cycle progression stage is an important advance in cell biology. Common procedures such as mitotic shake off, differential centrifugation or flow cytometry-based sorting are used to isolate cells at specific stages of the cell cycle 4-6. These fractionated, cell cycle phase-enriched populations are then subjected to experimental treatments. Yield, purity and viability of the separated fractions can often be compromised using these physical separation methods. As well, the time lapse between separation of the cell populations and the start of experimental treatment, whereby the fractionated cells can progress from the selected cell cycle stage, can pose significant challenges in the successful implementation and interpretation of these experiments.
Other approaches to study cell cycle stages include the use of chemicals to synchronize cells. Treatment of cells with chemical inhibitors of key metabolic processes for each cell cycle stage are useful in blocking the progression of the cell cycle to the next stage. For example, the ribonucleotide reductase inhibitor hydroxyurea halts cells at the G1/S juncture by limiting the supply of deoxynucleotides, the building blocks of DNA. Other notable chemicals include treatment with aphidicolin, a polymerase alpha inhibitor for G1 arrest, treatment with colchicine and nocodazole, both of which interfere with mitotic spindle formation to halt cells in M phase and finally, treatment with the DNA chain terminator 5-fluorodeoxyridine to initiate S phase arrest 7-9. Treatment with these chemicals is an effective means of synchronizing an entire population of cells at a particular phase. With removal of the chemical, cells rejoin the cell cycle in unison. Treatment of the test agent following release from the cell cycle blocking chemical ensures that the drug response elicited is from a uniform, cell cycle stage-specific population. However, since many of the chemical synchronizers are known genotoxic compounds, teasing apart the participation of various response pathways (to the synchronizers vs. the test agents) is challenging. 
Here we describe a metabolic labeling method for following a subpopulation of actively cycling cells through their progression from the DNA replication phase, through to the division and separation of their daughter cells. Coupled with flow cytometry quantification, this protocol enables for measurement of kinetic progression of the cell cycle in the absence of either mechanically- or chemically- induced cellular stresses commonly associated with other cell cycle synchronization methodologies 10. In the following sections we will discuss the methodology, as well as some of its applications in biomedical research.

Tracking subtle changes in the progression and kinetics of cell cycle stages can be accomplished by use of a combination of metabolic labeling of nucleic acids with BrdU and total genomic DNA staining via Propidium Iodide. This method avoids the need of chemical synchronization of cycling cells, thereby preventing the introduction of non-specific DNA damage, which in turn affects cell cycle progression.

Precise control of the initiation and subsequent progression through the various phases of the cell cycle are of paramount importance in proliferating ...

Performing Vaginal Lavage, Crystal Violet Staining, and Vaginal Cytological Evaluation for Mouse Estrous Cycle Staging Identification

A rapid means of assessing reproductive status in rodents is useful not only in the study of reproductive dysfunction but is also required for the production of new mouse models of disease and investigations into the hormonal regulation of tissue degeneration (or regeneration) following pathological challenge. The murine reproductive (or estrous) cycle is divided into 4 stages: proestrus, estrus, metestrus, and diestrus. Defined fluctuations in circulating levels of the ovarian steroids 17-&beta;-estradiol and progesterone, the gonadotropins luteinizing and follicle stimulating hormones, and the luteotropic hormone prolactin signal transition through these reproductive stages. Changes in cell typology within the murine vaginal canal reflect these underlying endocrine events. Daily assessment of the relative ratio of nucleated epithelial cells, cornified squamous epithelial cells, and leukocytes present in vaginal smears can be used to identify murine estrous stages. The degree of invasiveness, however, employed in collecting these samples can alter reproductive status and elicit an inflammatory response that can confound cytological assessment of smears. Here, we describe a simple, non-invasive protocol that can be used to determine the stage of the estrous cycle of a female mouse without altering her reproductive cycle. We detail how to differentiate between the four stages of the estrous cycle by collection and analysis of predominant cell typology in vaginal smears and we show how these changes can be interpreted with respect to endocrine status.

Here, we describe how to identify the stage of the murine reproductive (proestrus, estrus, metestrus, or diestrus) by simple, non-invasive collection and cytological assessment of vaginal smear samples. We further describe how vaginal cytology reflects circulating hormonal levels underlying transition through the murine reproductive cycle.

A rapid means of assessing reproductive status in rodents is useful not only in the study of reproductive dysfunction but is also required for the ...

Synchronization of Caulobacter Crescentus for Investigation of the Bacterial Cell Cycle

The cell cycle is important for growth, genome replication, and development in all cells. In bacteria, studies of the cell cycle have focused largely on unsynchronized cells making it difficult to order the temporal events required for cell cycle progression, genome replication, and division. Caulobacter crescentus provides an excellent model system for the bacterial cell cycle whereby cells can be rapidly synchronized in a G0 state by density centrifugation. Cell cycle synchronization experiments have been used to establish the molecular events governing chromosome replication and segregation, to map a genetic regulatory network controlling cell cycle progression, and to identify the establishment of polar signaling complexes required for asymmetric cell division. Here we provide a detailed protocol for the rapid synchronization of Caulobacter NA1000 cells. Synchronization can be performed in a large-scale format for gene expression profiling and western blot assays, as well as a small-scale format for microscopy or FACS assays. The rapid synchronizability and high cell yields of Caulobacter make this organism a powerful model system for studies of the bacterial cell cycle.

Synchronization of Caulobacter Crescentus for Investigation of the Bacterial Cell Cycle

Synchronization of bacterial cells is essential for studies of the bacterial cell cycle and development. Caulobacter crescentus is synchronizable through density centrifugation allowing a rapid and powerful tool for studies of the bacterial cell cycle. Here we provide a detailed protocol for the synchronization of Caulobacter cells.

The cell cycle is important for growth, genome replication, and development in all cells.  In bacteria, studies of the cell cycle have focused largely on ...

Temporal Tracking of Cell Cycle Progression Using Flow Cytometry without the Need for Synchronization

This protocol describes a method to permit the tracking of cells through the cell cycle without requiring the cells to be synchronized. Achieving cell synchronization can be difficult for many cell systems. Standard practice is to block cell cycle progression at a specific stage and then release the accumulated cells producing a wave of cells progressing through the cycle in unison. However, some cell types find this block toxic resulting in abnormal cell cycling, or even mass death. Bromodeoxyuridine (BrdU) uptake can be used to track the cell cycle stage of individual cells. Cells incorporate this synthetic thymidine analog, while synthesizing new DNA during S phase. By providing BrdU for a brief period it is possible to mark a pool of cells that were in S phase while the BrdU was present. These cells can then be tracked through the remainder of the cell cycle and into the next round of replication, permitting the duration of the cell cycle phases to be determined without the need to induce a potentially toxic cell cycle block. It is also possible to determine and correlate the expression of both internal and external proteins during subsequent stages of the cell cycle. These can be used to further refine the assignment of cell cycle stage or assess effects on other cellular functions such as checkpoint activation or cell death.

This protocol describes the use of bromodeoxyuridine (BrdU) uptake to permit the temporal tracking of cells that were in S phase at a specific point in time. Addition of DNA dyes and antibody labeling facilitates detailed analysis of the fate of the S phase cells at later times.

This protocol describes a method to permit the tracking of cells through the cell cycle without requiring the cells to be synchronized. Achieving cell ...

Experimental Approaches to Study Mitochondrial Localization and Function of a Nuclear Cell Cycle Kinase, Cdk1

Although mitochondria possess their own transcriptional machinery, merely 1% of mitochondrial proteins are synthesized inside the organelle. The nuclear-encoded proteins are transported into mitochondria guided by their mitochondria targeting sequences (MTS); however, a majority of mitochondrial localized proteins lack an identifiable MTS. Nevertheless, the fact that MTS can instruct proteins to go into the mitochondria provides a valuable tool for studying mitochondrial functions of normally nuclear and/or cytoplasmic proteins. We have recently identified the cell cycle kinase CyclinB1/Cdk1 complex in the mitochondria. To specifically study the mitochondrial functions of this complex, mitochondrial overexpression and knock-down of this complex without interfering with its nuclear or cytoplasmic functions were essential. By tagging CyclinB1/Cdk1 with MTS, we were able to achieve mitochondrial overexpression of this complex to study its mitochondrial targets as well as functions. Via tagging dominant-negative Cdk1 with MTS, inhibition of Cdk1 activity was accomplished particularly in the mitochondria. Potential mitochondrial targets of CyclinB1/Cdk1 complex were identified using a gel-based proteomics approach. Unlike traditional 2D gel analysis, we employed 2-dimensional difference gel electrophoresis (2D-DIGE) technology followed by phosphoprotein staining to fluorescently label differentially phosphorylated proteins in mitochondrial Cdk1 expressing cells. Identification of phosphoprotein spots that were altered in wild type versus dominant negative Cdk1 bearing mitochondria revealed the identity of mitochondrial targets of Cdk1. Finally, to determine the effect of CyclinB1/Cdk1 mitochondrial localization in cell cycle progression, a cell proliferation assay using a synthetic thymidine analogue EdU (5-ethynyl-2&#8242;-deoxyuridine) was used to monitor the cells as they go through the cell cycle and replicate their DNA. Altogether, we demonstrated a variety of approaches available to study mitochondrial localization and activity of a cell cycle kinase. These are advanced, yet easy to follow methods that will be beneficial to many cell biology researchers.

Here, we outline how to study mitochondrial localization of a (cell cycle) kinase, and how to determine its sub-mitochondrial location as well as potential mitochondrial substrates/targets. Forced expression of proteins into the mitochondria provides a useful tool for studying the functional consequences of mitochondrial localization of a protein of interest.

Although mitochondria possess their own transcriptional machinery, merely 1% of mitochondrial proteins are synthesized inside the organelle. The ...

Combining Mitotic Cell Synchronization and High Resolution Confocal Microscopy to Study the Role of Multifunctional Cell Cycle Proteins During Mitosis

Study of the various regulatory events of the cell cycle in a phase-dependent manner provides a clear understanding about cell growth and division. The synchronization of cell populations at specific stages of the cell cycle has been found to be very useful in such experimental endeavors. Synchronization of cells by treatment with chemicals that are relatively less toxic can be advantageous over the use of pharmacological inhibitory drugs for the study of consequent cell cycle events and to obtain specific enrichment of selected mitotic stages. Here, we describe the protocol for synchronizing human cells at different stages of the cell cycle, including both in S phase and M phase with a double thymidine block and release procedure for studying the functionality of mitotic proteins in chromosome alignment and segregation. This protocol has been extremely useful for studying the mitotic roles of multifunctional proteins which possess established interphase functions. In our case, the mitotic role of Cdt1, a protein critical for replication origin licensing in G1 phase, can be studied effectively only when G2/M-specific Cdt1 can be depleted. We describe the detailed protocol for depletion of G2/M-specific Cdt1 using double thymidine synchronization. We also explain the protocol of cell fixation, and live cell imaging using high resolution confocal microscopy after thymidine release. The method is also useful for analyzing the function of mitotic proteins under both physiological and perturbed conditions such as for Hec1, a component of the Ndc80 complex, as it enables one to obtain large sample sizes of mitotic cells for fixed and live cell analysis as we show here.&#160;&#160;

We present a protocol for double thymidine synchronization of HeLa cells followed by analysis using high resolution confocal microscopy. This method is key to obtaining large number of cells that proceed synchronously from S phase to mitosis, enabling studies on mitotic roles of multifunctional proteins which also possess interphase functions.

Study of the various regulatory events of the cell cycle in a phase-dependent manner provides a clear understanding about cell growth and division. The ...

Live-Cell Imaging of the Life Cycle of Bacterial Predator Bdellovibrio bacteriovorus using Time-Lapse Fluorescence Microscopy

Bdellovibrio bacteriovorus is a small gram-negative, obligate predatory bacterium that kills other gram-negative bacteria, including harmful pathogens. Therefore, it is considered a living antibiotic. To apply B. bacteriovorus as a living antibiotic, it is first necessary to understand the major stages of its complex life cycle, particularly its proliferation inside prey. So far, it has been challenging to monitor successive stages of the predatory life cycle in real-time. Presented here is a comprehensive protocol for real-time imaging of the complete life cycle of B. bacteriovorus, especially during its growth inside the host. For this purpose, a system consisting of an agarose pad is used in combination with cell-imaging dishes, in which the predatory cells can move freely beneath the agarose pad while immobilized prey cells are able to form bdelloplasts. The application of a strain producing a fluorescently tagged &#946;-subunit of DNA polymerase III further allows chromosome replication to be monitored during the reproduction phase of the B. bacteriovorus life cycle.

Live-Cell Imaging of the Life Cycle of Bacterial Predator Bdellovibrio bacteriovorus using Time-Lapse Fluorescence Microscopy

Presented here is a protocol that describes monitoring of the complete life cycle of predatory bacterium Bdellovibrio bacteriovorus using time-lapse fluorescence microscopy in combination with an agarose pad and cell-imaging dishes.

Bdellovibrio bacteriovorus is a small gram-negative, obligate predatory bacterium that kills other gram-negative bacteria, including harmful pathogens. ...

Assessment of Global DNA Double-Strand End Resection using BrdU-DNA Labeling coupled with Cell Cycle Discrimination Imaging

The study of the DNA damage response (DDR) is a complex and essential field, which has only become more important due to the use of DDR-targeting drugs for cancer treatment. These targets are poly(ADP-ribose) polymerases (PARPs), which initiate various forms of DNA repair. Inhibiting these enzymes using PARP inhibitors (PARPi) achieves synthetic lethality by conferring a therapeutic vulnerability in homologous recombination (HR)-deficient cells due to mutations in breast cancer type 1 (BRCA1), BRCA2, or partner and localizer of BRCA2 (PALB2). 
Cells treated with PARPi accumulate DNA double-strand breaks (DSBs). These breaks are processed by the DNA end resection machinery, leading to the formation of single-stranded (ss) DNA and subsequent DNA repair. In a BRCA1-deficient context, reinvigorating DNA resection through mutations in DNA resection inhibitors, such as 53BP1 and DYNLL1, causes PARPi resistance. Therefore, being able to monitor DNA resection in cellulo is critical for a clearer understanding of the DNA repair pathways and the development of new strategies to overcome PARPi resistance. Immunofluorescence (IF)-based techniques allow for monitoring of global DNA resection after DNA damage. This strategy requires long-pulse genomic DNA labeling with 5-bromo-2&#8242;-deoxyuridine (BrdU). Following DNA damage and DNA end resection, the resulting single-stranded DNA is specifically detected by an anti-BrdU antibody under native conditions. Moreover, DNA resection can also be studied using cell cycle markers to differentiate between various phases of the cell cycle. Cells in the S/G2 phase allow the study of end resection within HR, whereas G1 cells can be used to study non-homologous end joining (NHEJ). A detailed protocol for this IF method coupled to cell cycle discrimination is described in this paper.

In the present protocol, we demonstrate how to visualize DNA double-strand end resection during S/G2 phase of the cell cycle using an immunofluorescence-based method.

The study of the DNA damage response (DDR) is a complex and essential field, which has only become more important due to the use of DDR-targeting drugs ...