Drug Discovery Opinion

Repairing the Damage

SR — Fri, 19 Mar 2010 08:30:54 +0000

Salamander photo: Flickr - brian.gratwicke; Mouse photo: Wikimedia - Ilmari Karonen

Although some species of amphibians – such as salamanders and newts – are able to regenerate lost or damaged tissue, the capacity for regeneration diminished as vertebrates evolved and adult mammals generally have limited regenerative capacity. Over a decade ago, it was discovered that the Murphy Roths Large (MRL) mouse strain was able to regenerate new epidermis, new hair follicles, and new cartilage to repair punch holes in the ears, unlike other strains of mice that close the holes with scar tissue. The MRL mice were later shown to have the ability to repair damaged heart muscle and spinal cord with restoration of normal structure and function and to have some limited capacity for digit re-growth. Scientists have been trying to identify the gene or genes that are responsible for the increased capacity for regeneration in MRL mice and researchers at the Wistar Institute and Washington University have now shown that the p21 gene is involved in regulating the regeneration process.

The team showed that p21, a cell cycle regulator, was found to be consistently inactive in cells from MRL mice. When they looked at p21 knockout mice, they found that, unlike normal mice which heal wounds by forming a scar, mice that lack p21 begin by forming a blastema, a mass of cells capable of rapid growth and de-differentiation which behave more like embryonic stem cells than adult mammalian cells. The p21 knockout mice were able to replace missing or damaged tissue with healthy tissue that showed no signs of scarring.

Since the cyclin-dependent kinase p21 is one of the best characterized downstream targets of the tumour suppressor p53, knockout of p21 might be expected to increase the incidence of cancer and other studies have suggested that p21-deficient mice develop tumours at an earlier age than their wild-type counterparts and are more susceptible to the effects of some carcinogens. Although increased DNA damage was observed in the present study, there was also an increased incidence of apoptosis and no net increase in the incidence of cancer. If MRL mice and p21 knockout mice are a good model for tissue regeneration in humans, temporary inactivation of the p21 gene could eventually be used to speed up wound healing in people.

The study is published in PNAS.

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Controlling Fat Distribution

WH — Thu, 18 Mar 2010 08:30:10 +0000

Image: Patrick Hoesly

Extreme accumulation of fat in muscle tissue is associated with cardiovascular disease and is a contributory factor in insulin resistance and type II diabetes. It is therefore important to understand the mechanisms by which fat is taken up from the bloodstream and metabolised by tissues. Surprisingly, the role of blood vessels themselves in the transport of lipids has not been clearly established. Researchers at the Karolinska Institutet have now identified a role for vascular endothelial growth factor-B (VEGF-B) in endothelial targeting of lipids to peripheral tissues.

The VEGFs and their receptors are major regulators of angiogenesis and pharmacological intervention, for example with bevacizumab (a monoclonal antibody specific for VEGF-A), has been successfully exploited in oncology. This latest study has shown that VEGF-B, in mice, controls endothelial uptake of fatty acids via transcriptional regulation of vascular fatty acid transport proteins. Mice that were deficient in VEGF-B (Vegfb^-/-) showed reduced uptake and accumulation of lipids in muscle, heart and brown adipose tissue. Instead, the Vegfb^-/- mice preferentially transported lipids to white adipose tissue, resulting in a small weight increase. This regulation was mediated by VEGF receptor 1 and neuropilin 1 expressed by the endothelium.

The authors of the study, published in Nature, propose that this new role for VEGF-B could potentially lead to novel strategies to modulate pathological lipid accumulation in diabetes, obesity and cardiovascular diseases.

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Don’t Ignore the Neighbours

SR — Wed, 17 Mar 2010 08:30:26 +0000

Image: Flickr - cliff1066™

Out of every hundred investigational cancer drugs that enter clinical trials, only eight prove to be sufficiently safe and effective for FDA approval. Scientists at the Dana-Faber Cancer Institute believe that those odds can be shortened by taking into account neighbouring cells in the tumour microenvironment in preclinical testing.

Although both non-malignant cells and secreted proteins from tumour and stromal cells are recognised to be active participants in cancer progression, early laboratory testing is typically carried out using homogeneous populations of tumour cells. The Dana-Faber group have now developed a tumour cell-specific in vitro bioluminescence imaging (CS-BLI) assay that is suitable for high throughput screening. Cell viability can be measured in tumour cells stably expressing luciferase in the presence or absence of non-malignant accessory cells (for example, stromal cells) or drug treatment.

Unlike conventional screening methods, CS-BLI was able to identify compounds with increased activity against tumour cells interacting with stroma. For example, reversine showed more potent activity in an orthotopic model of diffuse myeloma bone lesions than in conventional subcutaneous xenografts. CS-BLI was also able to identify stroma-induced chemo-resistance such as imatinib resistance in leukaemia cells. The use of CS-BLI should help to identify preclinical candidates that overcome stroma-mediated drug resistance as well as those that can act in a synthetic lethal manner in the context of tumour-stroma interactions, thereby increasing the chances of clinical success.

The study is published in Nature Methods.

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Role for Tau in ‘Microtentacles’ and Metastasis

SR — Tue, 16 Mar 2010 08:39:00 +0000

Image: Flickr – quinn.anya

Tau proteins interact with tubulin to stabilise microtubules and are abundant in neurons but less common in non-neuronal cells. Recently, tau proteins have received a bad press because of the association between tangles of hyperphosphorylated tau and Alzheimer’s disease and now researchers at the University of Maryland have reported that tau proteins may also play a role in tumour metastasis. Tau was found to promote formation of extensions of the plasma membrane or ‘microtentacles’ on breast cancer cells which break away from the primary tumour and circulate in the bloodstream. The microtentacles then increase the ability of the cells to attach to the walls of capillaries in the lung and seed new tumours. Tau protein has previously been associated with chemotherapy-resistant breast cancers and a poor prognosis but this is the first time that it has been implicated in metastasis. In the present study involving 102 breast cancer patients, 52% showed tau expression in metastases and 26% showed significantly increased tau expression as the disease progressed.

The team hope that drugs might be identified which will prevent the growth of microtentacles and inhibit tumour metastasis. Although current breast cancer treatments are often effective in treating the primary tumour, they are less effective in treating metastatic cancer which can develop years after the primary tumour is discovered and is the leading cause of death in cancer patients.

The study is published in the journal Oncogene.

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Dicing with Death

SR — Mon, 15 Mar 2010 08:40:19 +0000

Image: Wikimedia - Maximaximax

Unlike necrosis, apoptosis (programmed cell death) is generally beneficial to an organism and loss of apoptotic pathways can lead to uncontrolled cell proliferation. Apoptosis takes place, for example, when a cell is irreparably damaged or infected by a virus and the signal for apoptosis can come from the cell itself, from surrounding tissue, or from immune cells. Although a variety of signals and pathways can trigger apoptosis, it is ultimately the action of caspases that degrades cellular organelles and proteins. Chromosome fragmentation is a hallmark of apoptosis and, in mammals, caspases activate apoptotic chromosome fragmentation by cleaving and inactivating an apoptotic nuclease inhibitor.

Working with C. elegans, a team led by researchers at the University of Colorado at Boulder have now shown that the Dicer ribonuclease (DCR-1), instead of being inactivated by caspases, undergoes a change of function. DCR-1 normally processes small RNAs but cleavage by the CED-3 caspase produces a C-terminal fragment which shows deoxyribonuclease activity and produces 3′ hydroxyl DNA breaks on chromosomes and promotes apoptosis. Although there are many enzymes that cleave either RNA or DNA, this is the first demonstration that proteolysis of a ribonuclease can generate a deoxyribonuclease.

The researchers are now investigating whether the function of DCR-1 can be altered in the same way in human cells – if so, the authors hope that their work may lead to new ways to treat diseases caused by abnormal apoptosis such as cancer and autoimmune diseases.

The study is published in the journal Science.

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Vive la Différence

SR — Fri, 12 Mar 2010 08:38:09 +0000

Image: Flickr - Andresmoschini

A year ago, when researchers at Purdue University argued that environmental standardisation in laboratory experiments involving mice was likely to lead to more, rather than less, variation between different laboratories, they met with some resistance since it was not clear what factors should be varied to improve reproducibility. Following an analysis of data from behavioural tests commonly used in drug discovery studies, they have now shown that introducing only two controlled environmental variables to preclinical studies using mice can greatly reduce false positives and the number of animals needed for testing. The tests, which compared behaviours between two inbred strains of mice, were repeated in four different model laboratories that varied in details such as background noise, the age of the mice, environmental enrichment, familiarity with handler, lighting levels and cage size. In each laboratory, one group of mice (standardised) were treated identically whilst the other group (heterogenised) were tested under four different sets of conditions produced by varying two environmental factors in a controlled manner. Mice of the same strain would have been expected to show the same behaviours in each laboratory but, in 33 out of 36 behavioural characteristics such as fear and curiosity, the standardised group showed as much as five times more variation between laboratories compared with the heterogenised group.

When conditions are highly standardised, the variation in data produced within a particular laboratory will be very low, but variations between laboratories will be large and unpredictable. The researchers believe that tests in mice using a heterogeneous test design more closely resemble human clinical trials and should reduce both the number of animals needed for preclinical studies and the number of false positives. A reduction in false positives could have important implications for reducing the number of compounds that fail in expensive downstream clinical trials.

The study is published in Nature Methods.

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Eating for How Many?

SR — Thu, 11 Mar 2010 08:30:06 +0000

Image: Flickr - Beneneuman

Soon after birth, the human body is colonised by bacteria. Trillions of bacteria take up residence in the gut and perform a range of useful functions such as helping with digestion and absorption of nutrients, producing vitamins, preventing growth of pathogenic bacteria, and developing the immune system. In 2006, it was shown that the proportion of Bacteroidetes relative to Firmicutes was reduced in the guts of obese people compared with lean individuals and also in the guts of genetically obese mice compared with lean littermates. Researchers at Emory University have now shown that mice engineered to lack toll-like receptor 5 (TLR5) – a component of the innate immune system that is expressed in the gut mucosa and that helps defend against infection – are 20% heavier than normal mice and have elevated triglycerides, cholesterol and blood pressure as well as slightly elevated blood sugar and a decreased response to insulin. TLR5-deficient mice consume about 10% more food than wild type mice and, although they lose weight when food is restricted, they still show insulin resistance. On a high fat diet, TLR5-deficient mice gain more weight than normal mice and develop full-blown diabetes and fatty liver disease, mimicking “metabolic syndrome” which increases the risk of developing heart disease and diabetes in humans.

Treating TLR5-deficient mice with antibiotics to kill most of the bacteria in the intestine reduced their metabolic abnormalities and, conversely, transfer of intestinal bacteria from TLR5-deficient mice to germ-free wild type mice transferred many of the characteristics of metabolic syndrome, including increased appetite, obesity, elevated blood sugar, and insulin resistance. Although earlier studies had shown that greater numbers of Firmicutes bacteria lead to more calories being extracted from the diet, the TLR5-deficient mice had normal proportions of Firmicutes and Bacteroidetes but differed in the composition of bacterial species in the two families. The new study shows that, as well as influencing how well energy is absorbed from food, gut flora can also influence appetite and may contribute to human obesity and metabolic disease.

The study is published in Science Express.

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Ritalin® Circuits Revealed

SR — Wed, 10 Mar 2010 08:30:50 +0000

Image: Flickr - Quapan

The dopamine reuptake inhibitor, Ritalin® (methylphenidate) has been used for almost 50 years to treat children with attention-deficit hyperactivity disorder (ADHD) and, more recently – and controversially, has been used by students to enhance academic performance and as a recreational drug. Although Ritalin® has been prescribed for millions of children, the mechanisms by which it modifies behavioural performance remain poorly understood. Researchers at the University of California, San Francisco have now shown, in animals at least, that Ritalin® improves ability to focus on tasks and directly enhances speed of learning by distinct dopamine receptor-mediated mechanisms.

By co-administering Ritalin® with either the dopamine D₁ receptor antagonist, SCH-23390, or the dopamine D₂ receptor antagonist, raclopride, the team were able to show the well-known benefit of improved focus was mediated through D₂ receptor-dependent mechanisms whereas learning efficiency was enhanced through D₁ receptor-dependent mechanisms. The study also established that Ritalin® strengthens synapses and enhances neuroplasticity. A better understanding of the way that Ritalin® improves focus and enhances learning could lead to the development of more targeted drugs for ADHD and learning enhancement.

The study is published in Nature Neuroscience.

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Blocking Lipid Droplet Hydrolysis May Be Beneficial

WH — Tue, 09 Mar 2010 08:30:07 +0000

Image: Flickr - Fox Kiyo

Fatty acids can be stored as triacylglycerol in lipid droplets, typically within adipose tissue, and then later released by the action of triacylglycerol hydrolase (TGH, also known as carboxylesterase-3, Ces3). Under normal circumstances, the released fatty acids provide an energy source, but excessive accumulation of triacylglycerol in peripheral tissues is associated with obesity and is a risk factor for type II diabetes and cardiovascular disease.

Researchers at the University of Alberta, Canada, reasoned that blocking the action of TGH would lead to better blood lipid profiles, but might also result in accumulation of triacylglycerol in the liver. However, they have found that mice lacking TGH (tgh^-/-) display global metabolic benefits with no obvious down-side. In both fasted- and fed-states, the animals had reduced plasma triacylglycerol, apolipoprotein B, and fatty acid levels. Despite the attenuation of very low-density lipoprotein (VLDL) secretion, TGH deficiency did not increase hepatic triacylglycerol levels. The tgh^-/- mice exhibited increased food intake and energy expenditure without change in body weight, and these metabolic changes are accompanied by improved insulin sensitivity and glucose tolerance.

The authors of the study, published in Cell Metabolism, suggest that pharmacological inhibition of TGH could be a useful therapeutic target, although cautioning that further work is required. It may be desirable to target TGH in specific tissues (e.g. hepatic versus adipose) but those subtleties have yet to be established.

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Trauma: the Enemy Within

SR — Mon, 08 Mar 2010 08:47:45 +0000

Image: Flickr - Torley

Systemic inflammatory response syndrome (SIRS) is an exaggerated host inflammatory response to infection (sepsis) or to physical insults such as trauma. SIRS can lead to multiple organ dysfunction syndrome (MODS) and, amongst the under-35s, trauma is the leading cause of death in the United States. Although the pathways leading from infection to sepsis are relatively well understood, it has been much less clear why physical insults lead to SIRS. A study led by researchers at Beth Israel Deaconess Medical Center has now suggested a link between sepsis and SIRS that is caused by trauma. The team propose that mitochondria are released into the bloodstream after physical injury and, because mitochondria closely resemble the symbiotic bacteria from which they are believed to originate, they elicit a sepsis-like response.

Pathogen-associated molecular patterns (PAMPs) are molecules such as bacterial endotoxins which are recognised by pattern recognition receptors (PRRs) as non-self, and so trigger an innate immune response. Injured or necrotic tissue generates molecules known as damage-associated molecular pattern molecules (DAMPs) that can also initiate and perpetuate an immune response. Many DAMPs are molecules that are usually found exclusively within cells and, when released into the bloodstream, are not recognised as self and trigger an immune response. The team found blood samples from patients who had suffered multiple trauma contained high levels of mitochondrial DNA – often thousands-of-fold higher than normal levels – and that this DNA activates immune cells via toll-like receptor 9 which normally recognises bacterial or viral DNA. Mitochondrial peptides were also found to elicit an immune response via the formyl peptide receptor 1 (FPR1) which also plays a role in the immune response to bacterial infections. Mitochondrial DDA and peptides were found to act synergistically to activate neutrophils via downstream kinase pathways. In further experiments, injection of mitochondria into rats caused peritonitis and reproduced the pulmonary and hepatic inflammation typical of traumatic SIRS.

The study, which is published in the journal Nature, shows that trauma can initiate innate immune pathways identical to those activated in sepsis and may lead to new strategies for treating trauma patients as well as re-evaluation of patients believed to be suffering from sepsis.

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