Local Cell Populations Research Articles

Darwinian Dynamics of Intratumoral Heterogeneity: Not Solely Random Mutations but Also Variable Environmental Selection Forces.

Spatial heterogeneity in tumors is generally thought to result from branching clonal evolution driven by random mutations that accumulate during tumor development. However, this concept rests on the implicit assumption that cancer cells never evolve to a fitness maximum because they can always acquire mutations that increase proliferative capacity. In this study, we investigated the validity of this assumption. Using evolutionary game theory, we demonstrate that local cancer cell populations will rapidly converge to the fittest phenotype given a stable environment. In such settings, cellular spatial heterogeneity in a tumor will be largely governed by regional variations in environmental conditions, for example, alterations in blood flow. Model simulations specifically predict a common spatial pattern in which cancer cells at the tumor-host interface exhibit invasion-promoting, rapidly proliferating phenotypic properties, whereas cells in the tumor core maximize their population density by promoting supportive tissue infrastructures, for example, to promote angiogenesis. We tested model predictions through detailed quantitative image analysis of phenotypic spatial distribution in histologic sections of 10 patients with stage 2 invasive breast cancers. CAIX, GLUT1, and Ki67 were upregulated in the tumor edge, consistent with an acid-producing invasive, proliferative phenotype. Cells in the tumor core were 20% denser than the edge, exhibiting upregulation of CAXII, HIF-1α, and cleaved caspase-3, consistent with a more static and less proliferative phenotype. Similarly, vascularity was consistently lower in the tumor center compared with the tumor edges. Lymphocytic immune responses to tumor antigens also trended to higher level in the tumor edge, although this effect did not reach statistical significance. Like invasive species in nature, cancer cells at the leading edge of the tumor possess a different phenotype from cells in the tumor core. Our results suggest that at least some of the molecular heterogeneity in cancer cells in tumors is governed by predictable regional variations in environmental selection forces, arguing against the assumption that cancer cells can evolve toward a local fitness maximum by random accumulation of mutations. Cancer Res; 76(11); 3136-44. ©2016 AACR.

Increased uNK Progenitor Cells in Women With Endometriosis and Infertility are Associated With Low Levels of Endometrial Stem Cell Factor.

Uterine natural killer (uNK) cells play a significant role in successful human pregnancy. Having previously demonstrated uNK cell progenitors in human endometrium, we hypothesized that abnormal uNK cell maturation contributes to infertility in women with endometriosis. We aimed to characterize uNK cells at different developmental stages in women with and without endometriosis and to investigate possible mechanisms to explain any differences. We characterized uNK cell development in women with and without endometriosis using flow cytometry, protein array and in vitro experiments. We found increased proportions of uNK cells at developmental stages 1 and 2 in endometrium from women with endometriosis (n = 36; mean = 21.2%) when compared with healthy fertile women (n = 9; mean = 7.0%). Protein array analysis revealed significantly lower levels of stem cell factor (SCF) in the eutopic endometrium of women with endometriosis when compared to healthy women. Addition of SCF to endometrial progenitor cells in vitro restored uNK cell maturation. We have shown that women with endometriosis have low levels of endometrial SCF, which we hypothesize contributes to abnormal maturation of local uNK cell populations. This defect may also compromise embryo implantation and hence contribute to endometriosis-associated infertility. SCF replacement may be a new therapeutic approach.

Role of Electrochemically Activated Solution in Asepsis in Osteoblasts and Chondrocytes in vitro

ABSTRACTPurpose: Implant infections are severe complications in orthopedic surgery. Treatment using antibiotics is often unsuccessful without removing the implant due to biofilm formation. In this context, antiseptics may be a potential option. These solutions have a wide antimicrobial spectrum but often cause damage to the local cell populations. In these cases, electrochemically activated solutions (ECAS) may be an alternative.Methods: Staphylococcus aureus was cultured overnight on polyethylene platelets under different growing conditions (standard-/-different biofilm inducing conditions). The samples were treated with an ECAS (0%–30%) and analyzed with regard to their antimicrobial efficacy and their effect on biofilm structures by determining the living bacterial count and scanning electron microscopy.Osteoblast and chondrocyte cell lines (MG-63, and CAL-78, respectively) were cultured to confluence and treated with an ECAS. Total cell count, metabolic activity using FDA staining, and proliferation using Ki67 staining were analyzed.Results: ECAS showed antimicrobial activity in concentrations <1%. In cases of biofilm formation, bacterial toxicity could be detected at concentrations from 3.75 –7.5%. At these same concentrations, a visible change in biofilm structure was observed. Regarding biocompatibility rates, up to 7.5% (of osteoblasts) and 3.75% (of chondrocytes) had little influence on metabolic activity and proliferation.Conclusion: ECAS provide good antimicrobial efficacy while maintaining high biocompatibility. These results suggest that these solutions may provide promising alternatives in the treatment of implant-associated infections.

Envelope gene evolution and HIV-1 neuropathogenesis.

In the era of combined antiretroviral therapy (cART), HIV-associated neurocognitive disorders (HAND) account for 40 to 56% of all HIV+ cases. During the acute stage of HIV-1 infection (<6 months), the virus invades and replicates within the central nervous system (CNS). Compared to peripheral tissues, the local CNS cell population expresses distinct levels of chemokine receptors, which levels exert selective pressure on the invading virus. HIV-1 envelope (env) sequences recovered from the brains and cerebrospinal fluid (CSF) of neurocognitively impaired HIV+ subjects often display higher nucleotide variability as compared to non-impaired HIV+ subjects. Specifically, env evolution provides HIV-1 with the strategies to evade host immune response, to reduce chemokine receptor dependence, to increase co-receptor binding efficiency, and to potentiate neurotoxicity. The evolution of env within the CNS leads to changes that may result in the emergence of novel isolates with neurotoxic and neurovirulent features. However, whether specific factors of HIV-1 evolution lead to the emergence of neurovirulent and neurotropic isolates remains ill-defined. HIV-1 env evolution is an ongoing phenomenon that occurs independently of neurological and neurocognitive disease severity; thus HIV env evolution may play a pivotal and reciprocal role in the etiology of HAND. Despite the use of cART, the reactivation of latent viral reservoirs represents a clinical challenge because of the replenishment of the viral pool that may subsequently lead to persistent infection. Therefore, gaining a more complete understanding of how HIV-1 env evolves over the course of the disease should be considered for the development of future therapies aimed at controlling CNS burden, diminishing persistent viremia, and eradicating viral reservoirs. Here we review the current literature on the role of HIV-1 env evolution in the setting of HAND disease progression and on the impact of cART on the dynamics of viral evolution.

Basophil-Derived Amphiregulin Is Essential for UVB Irradiation–Induced Immune Suppression

UVB irradiation (290-320 nm) is used to treat skin diseases like psoriasis and atopic dermatitis, and is known to suppress contact hypersensitivity (CHS) reactions in mouse models. Regulatory T cells (Treg cells) have been shown to be responsible for this UVB-induced suppression of CHS. The epidermal growth factor (EGF)-like growth factor amphiregulin (AREG) engages EGFR on Treg cells and, in different disease models, it was shown that mast cell-derived AREG is essential for optimal Treg cell function in vivo. Here we determined whether AREG has a role in UVB-induced, Treg cell-mediated suppression of CHS reactions in the skin. Our data show that AREG is essential for UVB-induced CHS suppression. In contrast to the general assumption, however, mast cells were dispensable for UVB-induced immune suppression, whereas basophil-derived AREG was essential. These data reveal, to our knowledge, a previously unreported function for basophils in the homeostasis of immune responses in the skin. Basophils thus fulfill a dual function: they contribute to the initiation of effective type 2 immune responses and, by enhancing the suppressive capacity of local Treg cell populations, also to local immune regulation in the skin.

The role of hepatic immune regulation in systemic immunity to viral infection

The liver has particular immune functions attributed by its unique microenvironment and its liver-resident cell populations. During autoimmunity and viral hepatitis, the liver serves as target for effector responses of immune cells. However, skewing of effector T cell functions through tolerogenic liver-resident antigen-presenting cells and through the immune regulatory hepatic microenvironment. Importantly, the liver also participates in shaping systemic antigen-specific immunity. Local antigen-presenting cell populations, in particular liver sinusoidal endothelial cells (LSECs), cross-present soluble, circulating or hepatocyte-derived antigens to naïve CD8 T cells. Upon priming by cross-presenting LSECs, naïve CD8 T cells develop into a unique population of antigen-experienced memory-like T cell population that can be reactivated in an inflammatory context to protect against infection with viruses or bacteria. Furthermore, upon prolonged inflammatory TNF-dependent signaling, the induction of intrahepatic myeloid cell aggregates for T cell population expansion (iMATEs) is observed in liver tissue. iMATEs are formed by inflammatory monocytes developing into dendritic cells and function to attract recently activated CD8 T cells. Those CD8 T cells located within the cocoon-like iMATE structure show strong proliferation initiated by co-stimulatory signaling. Locally expanded CD8 T cells are key to control acute and chronic viral infections. The mechanistic understanding of local hepatic T cell priming and local expansion of effector CD8 T cells will help to develop novel therapeutic vaccination strategies.

Early Carcinogenesis Involves the Establishment of Immune Privilege via Intrinsic and Extrinsic Regulation of Indoleamine 2,3-dioxygenase-1: Translational Implications in Cancer Immunotherapy.

Although prolonged genetic pressure has been conjectured to be necessary for the eventual development of tumor immune evasion mechanisms, recent work is demonstrating that early genetic mutations are capable of moonlighting as both intrinsic and extrinsic modulators of the tumor immune microenvironment. The indoleamine 2,3-dioxygenase-1 (IDO) immunoregulatory enzyme is emerging as a key player in tumor-mediated immune tolerance. While loss of the tumor suppressor, BIN-1, and the over-expression of cyclooxygenase-2 have been implicated in intrinsic regulation of IDO, recent findings have demonstrated the loss of TβRIII and the upregulation of Wnt5a by developing cancers to play a role in the extrinsic control of IDO activity by local dendritic cell populations residing within tumor and tumor-draining lymph node tissues. Together, these genetic changes are capable of modulating paracrine signaling pathways in the early stages of carcinogenesis to establish a site of immune privilege by promoting the differentiation and activation of local regulatory T cells. Additional investigation of these immune evasion pathways promises to provide opportunities for the development of novel strategies to synergistically enhance the efficacy of the evolving class of T cell-targeted “checkpoint” inhibitors.

Regulation and function of adult neurogenesis: from genes to cognition.

Adult neurogenesis in the hippocampus is a notable process due not only to its uniqueness and potential impact on cognition but also to its localized vertical integration of different scales of neuroscience, ranging from molecular and cellular biology to behavior. This review summarizes the recent research regarding the process of adult neurogenesis from these different perspectives, with particular emphasis on the differentiation and development of new neurons, the regulation of the process by extrinsic and intrinsic factors, and their ultimate function in the hippocampus circuit. Arising from a local neural stem cell population, new neurons progress through several stages of maturation, ultimately integrating into the adult dentate gyrus network. The increased appreciation of the full neurogenesis process, from genes and cells to behavior and cognition, makes neurogenesis both a unique case study for how scales in neuroscience can link together and suggests neurogenesis as a potential target for therapeutic intervention for a number of disorders.

Inflammatory Mediators in Mastitis and Lactation Insufficiency

Mastitis is a common inflammatory disease during lactation that causes reduced milk supply. A growing body of evidence challenges the central role of pathogenic bacteria in mastitis, with disease severity associated with markers of inflammation rather than infection. Inflammation in the mammary gland may be triggered by microbe-associated molecular patterns (MAMPs) as well as danger-associated molecular patterns (DAMPs) binding to pattern recognition receptors such as the toll-like receptors (TLRs) on the surface of mammary epithelial cells and local immune cell populations. Activation of the TLR4 signalling pathway and downstream nuclear factor kappa B (NFkB) is critical to mediating local mammary gland inflammation and systemic immune responses in mouse models of mastitis. However, activation of NFkB also induces epithelial cell apoptosis and reduced milk protein synthesis, suggesting that inflammatory mediators activated during mastitis promote partial involution. Perturbed milk flow, maternal stress and genetic predisposition are significant risk factors for mastitis, and could lead to a heightened TLR4-mediated inflammatory response, resulting in increased susceptibility and severity of mastitis disease in the context of low MAMP abundance. Therefore, heightened host inflammatory signalling may act in concert with pathogenic or commensal bacterial species to cause both the inflammation associated with mastitis and lactation insufficiency. Here, we present an alternate paradigm to the widely held notion that breast inflammation is driven principally by infectious bacterial pathogens, and suggest there may be other therapeutic strategies, apart from the currently utilised antimicrobial agents, that could be employed to prevent and treat mastitis in women.

Kidney pericytes: roles in regeneration and fibrosis.

Renal pericytes have been neglected for many years, but recently they have become an intensively studied cell population in renal biology and pathophysiology. Pericytes are stromal cells that support vasculature, and a subset of pericytes are mesenchymal stem cells. In kidney, pericytes have been reported to play critical roles in angiogenesis, regulation of renal medullary and cortical blood flow, and serve as progenitors of interstitial myofibroblasts in renal fibrogenesis. They interact with endothelial cells through distinct signaling pathways and their activation and detachment from capillaries after acute or chronic kidney injury may be critical for driving chronic kidney disease progression. By contrast, during kidney homeostasis it is likely that pericytes serve as a local stem cell population that replenishes differentiated interstitial and vascular cells lost during aging. This review describes both the regenerative properties of pericytes as well as involvement in pathophysiologic conditions such as fibrogenesis.

Prospects for engineering neurons from local neocortical cell populations as cell-mediated therapy for neurological disorders.

There is little cell replacement following neurological injury, limiting the regenerative response of the CNS. Progress in understanding the biology of neural stem cells has raised interest in using stem cells for replacing neurons lost to injury or to disease. Stem cell therapy may also have a role in rebuilding deficient neural circuitry underlying mood disorders, epilepsy, and pain modulation among other roles. In vitro expansion of stem cells with directed differentiation prior to transplantation is one approach to stem cell therapy. Emerging evidence suggests that it may be possible to convert in vivo endogenous neural cells to a neuronal fate directly, providing an alternative strategy for stem cell therapy to the CNS. This review assesses the evidence for engineering a subtype-specific neuronal fate of endogenous neural cells in the cerebral cortex as a function of initial cell lineage, reactive response to injury, conversion factors, and environmental context. We conclude with a discussion of some of the challenges that must be overcome to move this alternative in vivo engineered conversion process toward becoming a viable therapeutic option.

Epicardium-Derived Heart Repair.

In the last decade, cell replacement therapy has emerged as a potential approach to treat patients suffering from myocardial infarction (MI). The transplantation or local stimulation of progenitor cells with the ability to form new cardiac tissue provides a novel strategy to overcome the massive loss of myocardium after MI. In this regard the epicardium, the outer layer of the heart, is a tractable local progenitor cell population for therapeutic pursuit. The epicardium has a crucial role in formation of the embryonic heart. After activation and migration into the developing myocardium, epicardial cells differentiate into several cardiac cells types. Additionally, the epicardium provides instructive signals for the growth of the myocardium and coronary angiogenesis. In the adult heart, the epicardium is quiescent, but recent evidence suggests that it becomes reactivated upon damage and recapitulates at least part of its embryonic functions. In this review we provide an update on the current knowledge regarding the contribution of epicardial cells to the adult mammalian heart during the injury response.

Melatonin in testes of infertile men: evidence for anti‐proliferative and anti‐oxidant effects on local macrophage and mast cell populations

Melatonin acting through the hypothalamus and pituitary regulates testicular function. In addition, direct actions of melatonin at the testicular level have been recently suggested. We have described that melatonin inhibits androgen production in hamster Leydig cells via melatonin subtype 1a (mel1a) receptors and the local corticotrophin-releasing hormone (CRH) system. The initial events of the melatonin/CRH signalling pathway have also been established. Melatonin and all components of the melatonergic/CRH system were also detected in Leydig cells of infertile men. This study attempted to search for additional targets of melatonin in the human testis, and to investigate the effects of melatonin on proliferation and the oxidative state in these novel target cells. To this aim, evaluation of human testicular biopsies of patients suffering from hypospermatogenesis or Sertoli cell only syndrome and cell culture studies were performed. Melatonergic receptors were found in macrophages (MACs) and mast cells (MCs) of the human testis. In biopsies of patients suffering idiopathic infertility, melatonin testicular concentrations were negatively correlated with MAC number per mm(2) and TNFα, IL1β and COX2 expression, but positively correlated with the expression of the anti-oxidant enzymes SOD1, peroxiredoxin 1 and catalase. Melatonin inhibited proliferation and the expression of pro-inflammatory cytokines and cyclooxygenase 2 (COX2) in both the human non-testicular THP-1 MAC cell line and primary cell cultures of hamster testicular MACs. In the human HMC-1 MC line, melatonin increased the expression of anti-oxidant enzymes and decreased reactive oxygen species (ROS) generation. The results reveal new testicular targets of melatonin and describe anti-proliferative and anti-inflammatory effects of this hormone on testicular MACs. Furthermore, melatonin might provide protective effects against oxidative stress in testicular MCs.

Chemo-Immunotherapy with Oxaliplatin and Interleukin-7 Inhibits Colon Cancer Metastasis in Mice

Combination of immunotherapy and chemotherapy has shown promise for cancer. Interleukin-7 (IL-7) can potentially enhance immune responses against tumor, while oxaliplatin (OXP), a platinum-based drug, can promote a favorable immune microenvironment and stimulate anticancer immune responses. We evaluated the anti-tumor activity of IL-7 combining OXP against a murine colon carcinoma in vitro and in vivo and studied the tumor immune microenvironment to investigate whether the combined treatment affects on the local immune cell populations. Utilizing lung and abdomen metastasis models by inoculation of CT26 mice colon cancer cells, we evaluated the anti-tumor efficacy of combining IL-7 and OXP in mice models. Tumor immune microenvironment was evaluated by flow cytometric analysis and immunohistochemical staining. Our study showed that the in vivo administration of IL-7 combined with OXP markedly inhibited the growth of tumors in lung and abdomen metastasis models of colon cancer. IL-7 alone had no effect on tumor growth in mice and IL-7 did not alter cell sensitivity to OXP in culture. The antitumor effect of combining IL-7 and OXP correlated with a marked increase in the number of tumor-infiltrating activated CD8+ T cells and a marked decrease in the number of regulatory T (Treg) cells in spleen. Our data suggest that OXP plus IL-7 treatment inhibits tumor cell growth by immunoregulation rather than direct cytotoxicity. Our findings justify further evaluation of combining IL-7 and chemotherapy as a novel experimental cancer therapy.

Parenteral Adjuvant Effects of an Enterotoxigenic Escherichia coli Natural Heat-Labile Toxin Variant

Native type I heat-labile toxins (LTs) produced by enterotoxigenic Escherichia coli (ETEC) strains exert strong adjuvant effects on both antibody and T cell responses to soluble and particulate antigens following co-administration via mucosal routes. However, inherent enterotoxicity and neurotoxicity (following intra-nasal delivery) had reduced the interest in the use of these toxins as mucosal adjuvants. LTs can also behave as powerful and safe adjuvants following delivery via parenteral routes, particularly for activation of cytotoxic lymphocytes. In the present study, we evaluated the adjuvant effects of a new natural LT polymorphic form (LT2), after delivery via intradermal (i.d.) and subcutaneous (s.c.) routes, with regard to both antibody and T cell responses. A recombinant HIV-1 p24 protein was employed as a model antigen for determination of antigen-specific immune responses while the reference LT (LT1), produced by the ETEC H10407 strain, and a non-toxigenic LT form (LTK63) were employed as previously characterized LT types. LT-treated mice submitted to a four dose-base immunization regimen elicited similar p24-specific serum IgG responses and CD4+ T cell activation. Nonetheless, mice immunized with LT1 or LT2 induced higher numbers of antigen-specific CD8+ T cells and in vivo cytotoxic responses compared to mice immunized with the non-toxic LT derivative. These effects were correlated with stronger activation of local dendritic cell populations. In addition, mice immunized with LT1 and LT2, but not with LTK63, via s.c. or i.d. routes developed local inflammatory reactions. Altogether, the present results confirmed that the two most prevalent natural polymorphic LT variants (LT1 or LT2) display similar and strong adjuvant effects for subunit vaccines administered via i.d. or s.c. routes.

Computing local edge probability in natural scenes from a population of oriented simple cells

A key computation in visual cortex is the extraction of object contours, where the first stage of processing is commonly attributed to V1 simple cells. The standard model of a simple cell-an oriented linear filter followed by a divisive normalization-fits a wide variety of physiological data, but is a poor performing local edge detector when applied to natural images. The brain's ability to finely discriminate edges from nonedges therefore likely depends on information encoded by local simple cell populations. To gain insight into the corresponding decoding problem, we used Bayes's rule to calculate edge probability at a given location/orientation in an image based on a surrounding filter population. Beginning with a set of ∼ 100 filters, we culled out a subset that were maximally informative about edges, and minimally correlated to allow factorization of the joint on- and off-edge likelihood functions. Key features of our approach include a new, efficient method for ground-truth edge labeling, an emphasis on achieving filter independence, including a focus on filters in the region orthogonal rather than tangential to an edge, and the use of a customized parametric model to represent the individual filter likelihood functions. The resulting population-based edge detector has zero parameters, calculates edge probability based on a sum of surrounding filter influences, is much more sharply tuned than the underlying linear filters, and effectively captures fine-scale edge structure in natural scenes. Our findings predict nonmonotonic interactions between cells in visual cortex, wherein a cell may for certain stimuli excite and for other stimuli inhibit the same neighboring cell, depending on the two cells' relative offsets in position and orientation, and their relative activation levels.

Macromolecular prodrug of dexamethasone prevents particle-induced peri-implant osteolysis with reduced systemic side effects

Aseptic implant loosening related to implant wear particle-induced inflammation is the most common cause of failure after joint replacement. Modulation of the inflammatory reaction to the wear products represents a rational approach for preventing aseptic implant failure. Long-term treatment using anti-inflammatory agents, however, can be associated with significant systemic side effects due to the drugs' lack of tissue specificity. To address this issue, N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer–dexamethasone conjugate (P–Dex) was developed and evaluated for prevention of wear particle-induced osteolysis and the loss of fixation in a murine prosthesis failure model. Daily administration of free dexamethasone (Dex) was able to prevent wear particle-induced osteolysis, as assessed by micro-CT and histological analysis. Remarkably, monthly P–Dex administration (dose equivalent to free Dex treatment) was equally effective as free dexamethasone, but was not associated with systemic bone loss (a major adverse side effect of glucocorticoids). The reduced systemic toxicity of P–Dex is related to preferential targeting of the sites of wear particle-induced inflammation and its subcellular sequestration and retention by local inflammatory cell populations, resulting in sustained therapeutic action. These results demonstrate the feasibility of utilizing a macromolecular prodrug with reduced systemic toxicity to prevent wear particle-induced osteolysis.

Diversity, heterogeneity and orientation‐dependent variation of spike count correlation in the cat visual cortex

Cortical neurons are known to be noisy encoders of information, showing large response variabilities with repeated presentations of identical stimuli. These spike count variabilities are correlated over the cell population and their neuronal mechanism and functional significance have not been well understood. Recently there has been much debate over the magnitude of the population mean of the correlation, ranging from 0.1 to 0.2 down to nearly zero. We performed multi-neuron recordings on the cat visual cortex and found that the population mean did not necessarily represent the nature of correlated variabilities because the spike count correlation showed significant diversity and heterogeneity. Although the population mean was relatively small (0.06), the correlations of individual unit pairs were distributed over a broad range, extending to both positive and negative values. In most of the recording sessions of local cell populations (83%), significantly positive correlations coexisted with significantly negative ones in different unit pairs. Furthermore, nearly 20% of the unit pairs showed significant variation in the spike count correlation for different stimulus orientations. Correlation analysis between the spike count correlation and the firing activity of the unit pair suggested that the orientation tuning properties of the two quantities were unlikely to have originated from a common neuronal mechanism. Diversity, heterogeneity and context-dependent variation suggests that the correlated spike count variabilities originate not from fixed anatomical connections but rather from the dynamic interaction of neuronal networks.

Mechanisms underlying the additive and redundant Qrr phenotypes in Vibrio harveyi and Vibrio cholerae

Vibrio harveyi and Vibrio cholerae regulate their virulence factors according to the local cell-population density in a regulatory system called quorum sensing. Their quorum sensing systems contain a small RNA (sRNA) circuit to regulate expression of a master transcriptional regulator via multiple quorum regulated RNA (Qrr) and a protein chaperon Hfq. Experiments and genetic analysis show that their respective quorum sensing networks are topologically equivalent and have homologous components, yet they respond differently to the same experimental conditions. In particular, V. harveyi Qrr are additive because all of its Qrr are required to maintain wild-type-like repression of its master transcriptional regulator. Conversely, V. cholerae Qrr are redundant because any of its Qrr is sufficient to repress its master transcriptional regulator. Given the striking similarities between their quorum sensing systems, experimentalists have been unable to identify conclusively the mechanisms behind these phenotypic differences. Nevertheless, the current hypothesis in the literature is that dosage compensation is the mechanism underlying redundancy.In this work, we identify the mechanisms underlying Qrr redundancy using a detailed mathematical model of the V. harveyi and V. cholerae sRNA circuits. We show that there are exactly two different cases underlying Qrr redundancy and that dosage compensation is unnecessary and insufficient to explain Qrr redundancy. Although V. harveyi Qrr are additive when the perturbations in Qrr are large, we predict that V. harveyi and V. cholerae Qrr are redundant when the perturbations in Qrr are small. We argue that the additive and redundant Qrr phenotypes can emerge from parametric differences in the sRNA circuit. In particular, we find that the affinity of Qrr and its expression relative to the master transcriptional regulator determine the level of redundancy in V. harveyi and V. cholerae. Furthermore, the additive and redundant Qrr phenotypes reflect differences in the concentration of Hfq-Qrr in V. harveyi and V. cholerae. We use our model to test the dosage compensation hypothesis and show that decreasing the expression of qrr, rather than removing dosage compensation, abolishes Qrr redundancy in V. cholerae. Further experimentation is needed to test our results and both Qrr redundancy hypotheses.

The Role of Mechanical Loading in Tendon Development, Maintenance, Injury, and Repair

Tendon injuries often result from excessive or insufficient mechanical loading, impairing the ability of the local tendon cell population to maintain normal tendon function. The resident cell population composing tendon tissue is mechanosensitive, given that the cells are able to alter the extracellular matrix in response to modifications of the local loading environment. Natural tendon healing is insufficient, characterized by improper collagen fibril diameter formation, collagen fibril distribution, and overall fibril misalignment. Current tendon repair rehabilitation protocols focus on implementing early, well-controlled eccentric loading exercises to improve repair outcome. Tissue engineers look toward incorporating mechanical loading regimens to precondition cell populations for the creation of improved biological augmentations for tendon repair.