Tetracyclines were discovered in the 1940s and exhibited activity against a wide range of microorganisms including gram-positive and gram-negative bacteria, chlamydiae, mycoplasmas, rickettsiae, and protozoan parasites. They are inexpensive antibiotics, which have been used extensively in the prophlylaxis and therapy of human and animal infections and also at subtherapeutic levels in animal feed as growth promoters. The first tetracycline-resistant bacterium, Shigella dysenteriae, was isolated in 1953. Tetracycline resistance now occurs in an increasing number of pathogenic, opportunistic, and commensal bacteria. The presence of tetracycline-resistant pathogens limits the use of these agents in treatment of disease. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Many of these genes are associated with mobile plasmids or transposons and can be distinguished from each other using molecular methods including DNA-DNA hybridization with oligonucleotide probes and DNA sequencing. A limited number of bacteria acquire resistance by mutations, which alter the permeability of the outer membrane porins and/or lipopolysaccharides in the outer membrane, change the regulation of innate efflux systems, or alter the 16S rRNA. New tetracycline derivatives are being examined, although their role in treatment is not clear. Changing the use of tetracyclines in human and animal health as well as in food production is needed if we are to continue to use this class of broad-spectrum antimicrobials through the present century.

Improving diet and lifestyle is a critical component of the American Heart Association's strategy for cardiovascular disease risk reduction in the general population. This document presents recommendations designed to meet this objective. Specific goals are to consume an overall healthy diet; aim for a healthy body weight; aim for recommended levels of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides; aim for normal blood pressure; aim for a normal blood glucose level; be physically active; and avoid use of and exposure to tobacco products. The recommendations are to balance caloric intake and physical activity to achieve and maintain a healthy body weight; consume a diet rich in vegetables and fruits; choose whole-grain, high-fiber foods; consume fish, especially oily fish, at least twice a week; limit intake of saturated fat to <7% of energy, trans fat to <1% of energy, and cholesterol to <300 mg/day by choosing lean meats and vegetable alternatives, fat-free (skim) or low-fat (1% fat) dairy products and minimize intake of partially hydrogenated fats; minimize intake of beverages and foods with added sugars; choose and prepare foods with little or no salt; if you consume alcohol, do so in moderation; and when you eat food prepared outside of the home, follow these Diet and Lifestyle Recommendations. By adhering to these diet and lifestyle recommendations, Americans can substantially reduce their risk of developing cardiovascular disease, which remains the leading cause of morbidity and mortality in the United States.

The innate immune system constitutes the first line of defense against invading microbial pathogens and relies on a large family of pattern recognition receptors (PRRs), which detect distinct evolutionarily conserved structures on pathogens, termed pathogen-associated molecular patterns (PAMPs). Among the PRRs, the Toll-like receptors have been studied most extensively. Upon PAMP engagement, PRRs trigger intracellular signaling cascades ultimately culminating in the expression of a variety of proinflammatory molecules, which together orchestrate the early host response to infection, and also is a prerequisite for the subsequent activation and shaping of adaptive immunity. In order to avoid immunopathology, this system is tightly regulated by a number of endogenous molecules that limit the magnitude and duration of the inflammatory response. Moreover, pathogenic microbes have developed sophisticated molecular strategies to subvert host defenses by interfering with molecules involved in inflammatory signaling. This review presents current knowledge on pathogen recognition through different families of PRRs and the increasingly complex signaling pathways responsible for activation of an inflammatory and antimicrobial response. Moreover, medical implications are discussed, including the role of PRRs in primary immunodeficiencies and in the pathogenesis of infectious and autoimmune diseases, as well as the possibilities for translation into clinical and therapeutic applications.

The aim of the Task Force was to derive continuous prediction equations and their lower limits of normal for spirometric indices, which are applicable globally. Over 160,000 data points from 72 centres in 33 countries were shared with the European Respiratory Society Global Lung Function Initiative. Eliminating data that could not be used (mostly missing ethnic group, some outliers) left 97,759 records of healthy nonsmokers (55.3% females) aged 2.5-95 yrs. Lung function data were collated and prediction equations derived using the LMS method, which allows simultaneous modelling of the mean (mu), the coefficient of variation (sigma) and skewness (lambda) of a distribution family. After discarding 23,572 records, mostly because they could not be combined with other ethnic or geographic groups, reference equations were derived for healthy individuals aged 3-95 yrs for Caucasians (n=57,395), African-Americans (n=3,545), and North (n=4,992) and South East Asians (n=8,255). Forced expiratory value in 1 s (FEV(1)) and forced vital capacity (FVC) between ethnic groups differed proportionally from that in Caucasians, such that FEV(1)/FVC remained virtually independent of ethnic group. For individuals not represented by these four groups, or of mixed ethnic origins, a composite equation taken as the average of the above equations is provided to facilitate interpretation until a more appropriate solution is developed. Spirometric prediction equations for the 3-95-age range are now available that include appropriate age-dependent lower limits of normal. They can be applied globally to different ethnic groups. Additional data from the Indian subcontinent and Arabic, Polynesian and Latin American countries, as well as Africa will further improve these equations in the future.

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite "sensor" that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Reliability refers to the reproducibility of values of a test, assay or other measurement in repeated trials on the same individuals. Better reliability implies better precision of single measurements and better tracking of changes in measurements in research or practical settings. The main measures of reliability are within-subject random variation, systematic change in the mean, and retest correlation. A simple, adaptable form of within-subject variation is the typical (standard) error of measurement: the standard deviation of an individual's repeated measurements. For many measurements in sports medicine and science, the typical error is best expressed as a coefficient of variation (percentage of the mean). A biased, more limited form of within-subject variation is the limits of agreement: the 95% likely range of change of an individual's measurements between 2 trials. Systematic changes in the mean of a measure between consecutive trials represent such effects as learning, motivation or fatigue; these changes need to be eliminated from estimates of within-subject variation. Retest correlation is difficult to interpret, mainly because its value is sensitive to the heterogeneity of the sample of participants. Uses of reliability include decision-making when monitoring individuals, comparison of tests or equipment, estimation of sample size in experiments and estimation of the magnitude of individual differences in the response to a treatment. Reasonable precision for estimates of reliability requires approximately 50 study participants and at least 3 trials. Studies aimed at assessing variation in reliability between tests or equipment require complex designs and analyses that researchers seldom perform correctly. A wider understanding of reliability and adoption of the typical error as the standard measure of reliability would improve the assessment of tests and equipment in our disciplines.

Solid cancers are spatially and temporally heterogeneous. This limits the use of invasive biopsy based molecular assays but gives huge potential for medical imaging, which has the ability to capture intra-tumoural heterogeneity in a non-invasive way. During the past decades, medical imaging innovations with new hardware, new imaging agents and standardised protocols, allows the field to move towards quantitative imaging. Therefore, also the development of automated and reproducible analysis methodologies to extract more information from image-based features is a requirement. Radiomics--the high-throughput extraction of large amounts of image features from radiographic images--addresses this problem and is one of the approaches that hold great promises but need further validation in multi-centric settings and in the laboratory.

Moving from cell monolayers to three-dimensional (3D) cultures is motivated by the need to work with cellular models that mimic the functions of living tissues. Essential cellular functions that are present in tissues are missed by 'petri dish'-based cell cultures. This limits their potential to predict the cellular responses of real organisms. However, establishing 3D cultures as a mainstream approach requires the development of standard protocols, new cell lines and quantitative analysis methods, which include well-suited three-dimensional imaging techniques. We believe that 3D cultures will have a strong impact on drug screening and will also decrease the use of laboratory animals, for example, in the context of toxicity assays.

17,187 patients entering 417 hospitals up to 24 hours (median 5 hours) after the onset of suspected acute myocardial infarction were randomised, with placebo control, between: (i) a 1-hour intravenous infusion of 1.5 MU of streptokinase; (ii) one month of 160 mg/day enteric-coated aspirin; (iii) both active treatments; or (iv) neither. Streptokinase alone and aspirin alone each produced a highly significant reduction in 5-week vascular mortality: 791/8592 (9.2%) among patients allocated streptokinase infusion vs 1029/8595 (12.0%) among those allocated placebo infusion (odds reduction: 25% SD 4; 2p less than 0.00001); 804/8587 (9.4%) vascular deaths among patients allocated aspirin tablets vs 1016/8600 (11.8%) among those allocated placebo tablets (odds reduction: 23% SD 4; 2p less than 0.00001). The combination of streptokinase and aspirin was significantly (2p less than 0.0001) better than either agent alone. Their separate effects on vascular deaths appeared to be additive: 343/4292 (8.0%) among patients allocated both active agents vs 568/4300 (13.2%) among those allocated neither (odds reduction: 42% SD 5; 95% confidence limits 34-50%). There was evidence of benefit from each agent even for patients treated late after pain onset (odds reductions at 0-4, 5-12, and 13-24 hours: 35% SD 6, 16% SD 7, and 21% SD 12 for streptokinase alone; 25% SD 7, 21% SD 7, and 21% SD 12 for aspirin alone; and 53% SD 8, 32% SD 9, and 38% SD 15 for the combination of streptokinase and aspirin). Streptokinase was associated with an excess of bleeds requiring transfusion (0.5% vs 0.2%) and of confirmed cerebral haemorrhage (0.1% vs 0.0%), but with fewer other strokes (0.6% vs 0.8%). These "other" strokes may have included a few undiagnosed cerebral haemorrhages, but still there was no increase in total strokes (0.7% streptokinase vs 0.8% placebo infusion). Aspirin significantly reduced non-fatal reinfarction (1.0% vs 2.0%) and non-fatal stroke (0.3% vs 0.6%), and was not associated with any significant increase in cerebral haemorrhage or in bleeds requiring transfusion. An excess of non-fatal reinfarction was reported when streptokinase was used alone, but this appeared to be entirely avoided by the addition of aspirin. Those allocated the combination of streptokinase and aspirin had significantly fewer reinfarctions (1.8% vs 2.9%), strokes (0.6% vs 1.1%), and deaths (8.0% vs 13.2%) than those allocated neither. The differences in vascular and in all-cause mortality produced by streptokinase and by aspirin remain highly significant (2p less than 0.001 for each) after the median of 15 months of follow-up thus far available.

Gene dosage variations occur in many diseases. In cancer, deletions and copy number increases contribute to alterations in the expression of tumour-suppressor genes and oncogenes, respectively. Developmental abnormalities, such as Down, Prader Willi, Angelman and Cri du Chat syndromes, result from gain or loss of one copy of a chromosome or chromosomal region. Thus, detection and mapping of copy number abnormalities provide an approach for associating aberrations with disease phenotype and for localizing critical genes. Comparative genomic hybridization (CGH) was developed for genome-wide analysis of DNA sequence copy number in a single experiment. In CGH, differentially labelled total genomic DNA from a 'test' and a 'reference' cell population are cohybridized to normal metaphase chromosomes, using blocking DNA to suppress signals from repetitive sequences. The resulting ratio of the fluorescence intensities at a location on the 'cytogenetic map', provided by the chromosomes, is approximately proportional to the ratio of the copy numbers of the corresponding DNA sequences in the test and reference genomes. CGH has been broadly applied to human and mouse malignancies. The use of metaphase chromosomes, however, limits detection of events involving small regions (of less than 20 Mb) of the genome, resolution of closely spaced aberrations and linking ratio changes to genomic/genetic markers. Therefore, more laborious locus-by-locus techniques have been required for higher resolution studies. Hybridization to an array of mapped sequences instead of metaphase chromosomes could overcome the limitations of conventional CGH (ref. 6) if adequate performance could be achieved. Copy number would be related to the test/reference fluorescence ratio on the array targets, and genomic resolution could be determined by the map distance between the targets, or by the length of the cloned DNA segments. We describe here our implementation of array CGH. We demonstrate its ability to measure copy number with high precision in the human genome, and to analyse clinical specimens by obtaining new information on chromosome 20 aberrations in breast cancer.

It has been established that cancer can be promoted and/or exacerbated by inflammation and infections. Indeed, chronic inflammation orchestrates a tumor-supporting microenvironment that is an indispensable participant in the neoplastic process. The mechanisms that link infection, innate immunity, inflammation, and cancer are being unraveled at a fast pace. Important components in this linkage are the cytokines produced by activated innate immune cells that stimulate tumor growth and progression. In addition, soluble mediators produced by cancer cells recruit and activate inflammatory cells, which further stimulate tumor progression. However, inflammatory cells also produce cytokines that can limit tumor growth. Here we provide an overview of the current understanding of the role of inflammation-induced cytokines in tumor initiation, promotion, and progression.

Coupled nonlinear differential equations are derived for the dynamics of spatially localized populations containing both excitatory and inhibitory model neurons. Phase plane methods and numerical solutions are then used to investigate population responses to various types of stimuli. The results obtained show simple and multiple hysteresis phenomena and limit cycle activity. The latter is particularly interesting since the frequency of the limit cycle oscillation is found to be a monotonic function of stimulus intensity. Finally, it is proved that the existence of limit cycle dynamics in response to one class of stimuli implies the existence of multiple stable states and hysteresis in response to a different class of stimuli. The relation between these findings and a number of experiments is discussed.

Sedative medications are widely used in intensive care unit (ICU) patients. Structured assessment of sedation and agitation is useful to titrate sedative medications and to evaluate agitated behavior, yet existing sedation scales have limitations. We measured inter-rater reliability and validity of a new 10-level (+4 "combative" to -5 "unarousable") scale, the Richmond Agitation-Sedation Scale (RASS), in two phases. In phase 1, we demonstrated excellent (r = 0.956, lower 90% confidence limit = 0.948; kappa = 0.73, 95% confidence interval = 0.71, 0.75) inter-rater reliability among five investigators (two physicians, two nurses, and one pharmacist) in adult ICU patient encounters (n = 192). Robust inter-rater reliability (r = 0.922-0.983) (kappa = 0.64-0.82) was demonstrated for patients from medical, surgical, cardiac surgery, coronary, and neuroscience ICUs, patients with and without mechanical ventilation, and patients with and without sedative medications. In validity testing, RASS correlated highly (r = 0.93) with a visual analog scale anchored by "combative" and "unresponsive," including all patient subgroups (r = 0.84-0.98). In the second phase, after implementation of RASS in our medical ICU, inter-rater reliability between a nurse educator and 27 RASS-trained bedside nurses in 101 patient encounters was high (r = 0.964, lower 90% confidence limit = 0.950; kappa = 0.80, 95% confidence interval = 0.69, 0.90) and very good for all subgroups (r = 0.773-0.970, kappa = 0.66-0.89). Correlations between RASS and the Ramsay sedation scale (r = -0.78) and the Sedation Agitation Scale (r = 0.78) confirmed validity. Our nurses described RASS as logical, easy to administer, and readily recalled. RASS has high reliability and validity in medical and surgical, ventilated and nonventilated, and sedated and nonsedated adult ICU patients.

The fine spatial scales of the structures in the human brain represent an enormous challenge to the successful integration of information from different images for both within- and between-subject analysis. While many algorithms to register image pairs from the same subject exist, visual inspection shows that their accuracy and robustness to be suspect, particularly when there are strong intensity gradients and/or only part of the brain is imaged. This paper introduces a new algorithm called Boundary-Based Registration, or BBR. The novelty of BBR is that it treats the two images very differently. The reference image must be of sufficient resolution and quality to extract surfaces that separate tissue types. The input image is then aligned to the reference by maximizing the intensity gradient across tissue boundaries. Several lower quality images can be aligned through their alignment with the reference. Visual inspection and fMRI results show that BBR is more accurate than correlation ratio or normalized mutual information and is considerably more robust to even strong intensity inhomogeneities. BBR also excels at aligning partial-brain images to whole-brain images, a domain in which existing registration algorithms frequently fail. Even in the limit of registering a single slice, we show the BBR results to be robust and accurate.

Although iron is required to sustain life, its free concentration and metabolism have to be tightly regulated. This is achieved through a variety of iron-binding proteins including transferrin and ferritin. During infection, bacteria acquire much of their iron from the host by synthesizing siderophores that scavenge iron and transport it into the pathogen. We recently demonstrated that enterochelin, a bacterial catecholate siderophore, binds to the host protein lipocalin 2 (ref. 5). Here, we show that this event is pivotal in the innate immune response to bacterial infection. Upon encountering invading bacteria the Toll-like receptors on immune cells stimulate the transcription, translation and secretion of lipocalin 2; secreted lipocalin 2 then limits bacterial growth by sequestrating the iron-laden siderophore. Our finding represents a new component of the innate immune system and the acute phase response to infection.

The release of negative regulators of immune activation (immune checkpoints) that limit antitumor responses has resulted in unprecedented rates of long-lasting tumor responses in patients with a variety of cancers. This can be achieved by antibodies blocking the cytotoxic T lymphocyte-associated protein 4 (CTLA-4) or the programmed cell death 1 (PD-1) pathway, either alone or in combination. The main premise for inducing an immune response is the preexistence of antitumor T cells that were limited by specific immune checkpoints. Most patients who have tumor responses maintain long-lasting disease control, yet one-third of patients relapse. Mechanisms of acquired resistance are currently poorly understood, but evidence points to alterations that converge on the antigen presentation and interferon-γ signaling pathways. New-generation combinatorial therapies may overcome resistance mechanisms to immune checkpoint therapy.

Age-related macular degeneration (AMD) is the leading cause of blind registration in the developed world, and yet its pathogenesis remains poorly understood. Oxidative stress, which refers to cellular damage caused by reactive oxygen intermediates (ROI), has been implicated in many disease processes, especially age-related disorders. ROIs include free radicals, hydrogen peroxide, and singlet oxygen, and they are often the byproducts of oxygen metabolism. The retina is particularly susceptible to oxidative stress because of its high consumption of oxygen, its high proportion of polyunsaturated fatty acids, and its exposure to visible light. In vitro studies have consistently shown that photochemical retinal injury is attributable to oxidative stress and that the antioxidant vitamins A, C, and E protect against this type of injury. Furthermore, there is strong evidence suggesting that lipofuscin is derived, at least in part, from oxidatively damaged photoreceptor outer segments and that it is itself a photoreactive substance. However, the relationships between dietary and serum levels of the antioxidant vitamins and age-related macular disease are less clear, although a protective effect of high plasma concentrations of alpha-tocopherol has been convincingly demonstrated. Macular pigment is also believed to limit retinal oxidative damage by absorbing incoming blue light and/or quenching ROIs. Many putative risk-factors for AMD have been linked to a lack of macular pigment, including female gender, lens density, tobacco use, light iris color, and reduced visual sensitivity. Moreover, the Eye Disease Case-Control Study found that high plasma levels of lutein and zeaxanthin were associated with reduced risk of neovascular AMD. The concept that AMD can be attributed to cumulative oxidative stress is enticing, but remains unproven. With a view to reducing oxidative damage, the effect of nutritional antioxidant supplements on the onset and natural course of age-related macular disease is currently being evaluated.

The regulatory T (Treg) cells restrain immune responses through suppressor-function elaboration that is dependent upon expression of the transcription factor Foxp3. Despite a critical role for Treg cells in maintaining lympho-myeloid homeostasis, it remains unclear whether a single mechanism or multiple mechanisms of Treg cell-mediated suppression are operating in vivo and how redundant such mechanisms might be. Here we addressed these questions by examining the role of the immunomodulatory cytokine IL-10 in Treg cell-mediated suppression. Analyses of mice in which the Treg cell-specific ablation of a conditional IL-10 allele was induced by Cre recombinase knocked into the Foxp3 gene locus showed that although IL-10 production by Treg cells was not required for the control of systemic autoimmunity, it was essential for keeping immune responses in check at environmental interfaces such as the colon and lungs. Our study suggests that Treg cells utilize multiple means to limit immune responses. Furthermore, these mechanisms are likely to be nonredundant, in that a distinct suppressor mechanism most likely plays a prominent and identifiable role at a particular tissue and inflammatory setting.

The RNA-guided endonuclease Cas9 has emerged as a versatile genome-editing platform. However, the size of the commonly used Cas9 from Streptococcus pyogenes (SpCas9) limits its utility for basic research and therapeutic applications that use the highly versatile adeno-associated virus (AAV) delivery vehicle. Here, we characterize six smaller Cas9 orthologues and show that Cas9 from Staphylococcus aureus (SaCas9) can edit the genome with efficiencies similar to those of SpCas9, while being more than 1 kilobase shorter. We packaged SaCas9 and its single guide RNA expression cassette into a single AAV vector and targeted the cholesterol regulatory gene Pcsk9 in the mouse liver. Within one week of injection, we observed >40% gene modification, accompanied by significant reductions in serum Pcsk9 and total cholesterol levels. We further assess the genome-wide targeting specificity of SaCas9 and SpCas9 using BLESS, and demonstrate that SaCas9-mediated in vivo genome editing has the potential to be efficient and specific.

Long-term beta blockade for perhaps a year or so following discharge after an MI is now of proven value, and for many such patients mortality reductions of about 25% can be achieved. No important differences are clearly apparent among the benefits of different beta blockers, although some are more convenient than others (or have slightly fewer side effects), and it appears that those with appreciable intrinsic sympathomimetic activity may confer less benefit. If monitored, the side effects of long-term therapy are not a major problem, as when they occur they are easily reversible by changing the beta blocker or by discontinuation of treatment. By contrast, although very early IV short-term beta blockade can definitely limit infarct size, more reliable information about the effects of such treatment on mortality will not be available until a large trial (ISIS) reports later this year, with data on some thousands of patients entered within less than 4 hours of the onset of pain. Our aim has been not only to review the 65-odd randomized beta blocker trials but also to demonstrate that when many randomized trials have all applied one general approach to treatment, it is often not appropriate to base inference on individual trial results. Although there will usually be important differences from one trial to another (in eligibility, treatment, end-point assessment, and so on), physicians who wish to decide whether to adopt a particular treatment policy should try to make their decision in the light of an overview of all these related randomized trials and not just a few particular trial results. Although most trials are too small to be individually reliable, this defect of size may be rectified by an overview of many trials, as long as appropriate statistical methods are used. Fortunately, robust statistical methods exist--based on direct, unweighted summation of one O-E value from each trial--that are simple for physicians to use and understand yet provide full statistical sensitivity. These methods allow combination of information from different trials while avoiding the unjustified direct comparison of patients in one trial with patients in another. (Moreover, they can be extended of such data that there is no real need for the introduction of any more complex statistical methods that might be more difficult for physicians to trust.) Their robustness, sensitivity, and avoidance of unnecessary complexity make these particular methods an important tool in trial overviews.

Evolutionary-biological reasoning suggests that individuals should be differentially susceptible to environmental influences, with some people being not just more vulnerable than others to the negative effects of adversity, as the prevailing diathesis-stress view of psychopathology (and of many environmental influences) maintains, but also disproportionately susceptible to the beneficial effects of supportive and enriching experiences (or just the absence of adversity). Evidence consistent with the proposition that individuals differ in plasticity is reviewed. The authors document multiple instances in which (a) phenotypic temperamental characteristics, (b) endophenotypic attributes, and (c) specific genes function less like "vulnerability factors" and more like "plasticity factors," thereby rendering some individuals more malleable or susceptible than others to both negative and positive environmental influences. Discussion focuses upon limits of the evidence, statistical criteria for distinguishing differential susceptibility from diathesis stress, potential mechanisms of influence, and unknowns in the differential-susceptibility equation.

In animals, cells are dependent on extracellular signals to prevent apoptosis. However, using growth factor-dependent cells from Bax/Bak-deficient mice, we demonstrate that apoptosis is not essential to limit cell autonomous survival. Following growth factor withdrawal, Bax-/-Bak-/- cells activate autophagy, undergo progressive atrophy, and ultimately succumb to death. These effects result from loss of the ability to take up sufficient nutrients to maintain cellular bioenergetics. Despite abundant extracellular nutrients, growth factor-deprived cells maintain ATP production from catabolism of intracellular substrates through autophagy. Autophagy is essential for maintaining cell survival following growth factor withdrawal and can sustain viability for several weeks. During this time, cells respond to growth factor readdition by rapid restoration of the ability to take up and metabolize glucose and by subsequent recovery of their original size and proliferative potential. Thus, growth factor signal transduction is required to direct the utilization of sufficient exogenous nutrients to maintain cell viability.

Adult mesenchymal stem cells (MSCs) can be isolated from bone marrow or marrow aspirates and because they are culture-dish adherent, they can be expanded in culture while maintaining their multipotency. The MSCs have been used in preclinical models for tissue engineering of bone, cartilage, muscle, marrow stroma, tendon, fat, and other connective tissues. These tissue-engineered materials show considerable promise for use in rebuilding damaged or diseased mesenchymal tissues. Unanticipated is the realization that the MSCs secrete a large spectrum of bioactive molecules. These molecules are immunosuppressive, especially for T-cells and, thus, allogeneic MSCs can be considered for therapeutic use. In this context, the secreted bioactive molecules provide a regenerative microenvironment for a variety of injured adult tissues to limit the area of damage and to mount a self-regulated regenerative response. This regenerative microenvironment is referred to as trophic activity and, therefore, MSCs appear to be valuable mediators for tissue repair and regeneration. The natural titers of MSCs that are drawn to sites of tissue injury can be augmented by allogeneic MSCs delivered via the bloodstream. Indeed, human clinical trials are now under way to use allogeneic MSCs for treatment of myocardial infarcts, graft-versus-host disease, Crohn's Disease, cartilage and meniscus repair, stroke, and spinal cord injury. This review summarizes the biological basis for the in vivo functioning of MSCs through development and aging.