The cannabinoid receptors, CB1 and CB2, were first identified as the receptors for the active components of Cannabis sativa. The endogenous ligands for the cannabinoid receptors, anandamide (from ananda, a Sanskrit word meaning ‘bliss’) and 2-arachidonoylglycerol (2-AG), exert most of their analgesic affects through binding to the main cannabinoid receptor in the brain, CB1. Attempts to prepare CB1 agonists that mimic the analgesic effects of the main component of cannabis, Δ9-tetrahydrocannabinol (THC) but have reduced potential for memory impairment, locomotor dysfunction, and possibly addiction have met with limited success and a recent alternative approach has been to devise ways of preventing the breakdown of anandamide and 2-AG. Signalling by the endocannabinoids is terminated by enzymatic hydrolysis which, for anandamide, is mediated by fatty acid amide hydrolase (FAAH) and, for 2-arachidonoylglycerol, by monoacylglycerol lipase (MAGL).
Inhibitors of both FAAH and MAGL produce analgesic effects but scientists at the Scripps Research Institute, Virginia Commonwealth University and the Medical College of Wisconsin have now shown that the effects of blocking MAGL are short-lived compared with the effects of blocking FAAH. Although a single injection of the MAGL inhibitor, JZL184, reduced pain in the mouse, after six consecutive daily injections, the effect disappeared. The chronically treated animals also became less sensitive to THC or the synthetic CB1 agonist, WIN55,212-2, and showed characteristic drug withdrawal symptoms when treated with the CB1 blocker, rimonabant.
Further tests showed that CB1 receptors were down-regulated in some brain areas of mice treated for 6 days with JZL184. Genetic disruption of MAGL also resulted in elevated levels of 2-AG and desensitised the CB1 signalling system, suggesting that chronic inhibition of MAGL may not provide effective analgesia. In contrast, chronic treatment with a FAAH inhibitor to boost anandamide levels did not lead to desensitisation of the CB1 system.
Although the team do not yet understand why chronic administration of FAAH inhibitors and MAGL inhibitors should produce such strikingly different results, the study, which is published in Nature Neuroscience, suggests that complete blockade of MAGL may not provide effective analgesia over the longer term.
The main symptoms of Parkinson’s disease are tremor, rigidity and involuntary movement, caused by loss of dopaminergic neurons in the brain. Leucine-rich repeat protein kinase-2 (LRRK2) is mutated in a significant number of Parkinson’s disease cases, both familial and sporadic late-onset. A common mutation in which a glycine residue in the active site is altered to serine enhances catalytic activity of the kinase, suggesting that LRRK2 inhibitors might be useful for the treatment of Parkinson’s disease, although it is not entirely clear why enhanced LRRK2 activity causes loss of dopamine-producing neurons. Scientists led by a team at the Johns Hopkins University School of Medicine have now shown that inhibitors of the G2019S variant of LRRK2 can protect the nerve cells of mice genetically modified to produce the mutated kinase. Three weeks twice daily injections of GW5074 provided almost complete protection against loss of dopaminergic neurons compared with placebo treatment.
Although GW5074 is not an especially potent inhibitor of wild type or G2019S LRRK2 (IC50 0.2µM – 1.0µM depending on substrate) and is not selective (IC50 vs cRaf ca 10nM), the study, which is published in Nature Medicine, provides encouragement that a more potent and selective inhibitor could lead to a new disease-modifying treatment for Parkinson’s disease. The John Hopkins team are collaborating with researchers at Southern Methodist University to design more selective inhibitors and many other groups in both industry and academia are engaged in the search for potent and selective LRRK2 inhibitors.
Epidemiological studies have suggested that either rheumatoid arthritis itself – or the anti-inflammatory drugs used to control it – are associated with a reduced risk of developing Alzheimer’s disease. Recent clinical trials with non-steroidal anti-inflammatory drugs (NSAIDs) have failed to show a benefit in patients with Alzheimer’s disease and researchers at the University of South Florida have now shown that it may be the disease itself that affords protection.
In a mouse model of Alzheimer’s disease, one of the cytokines that is elevated in patients with rheumatoid arthritis, granulocyte-macrophage colony-stimulating factor (GM-CSF), was shown to improve working memory and learning and to lead to an apparent increase in neural cell connections in the animals’ brains. GM-CSF also led to an accumulation of microglia in the brains of treated animals which was associated with a greater than 50% reduction in β-amyloid peptides. Recombinant human GM-CSF is currently approved to stimulate the production of white blood cells in some cancer patients and the new study, which is published in the Journal of Alzheimer’s Disease, suggests that GM-CSF could also be of benefit to Alzheimer’s patients.
The USF Health Byrd Alzheimer’s Institute plans to begin a pilot clinical trial later this year to investigate recombinant human GM-CSF in patients with mild or moderate Alzheimer’s disease.
Approximately half of the world’s population is infected with Helicobacter pylori, the bacterium that causes peptic ulcers and some forms of stomach cancer. Although ‘triple therapy’ with a proton pump inhibitor and two antibiotics – selected from a very limited number – can eradicate H.pylori, an increasing number of people are found to be infected with antibiotic-resistant bacteria. Scientists in Australia, New Zealand and France have now shown that H.pylori needs vitamin B6 to establish and maintain chronic infection, and have identified two genes in the vitamin B6 biosynthesis pathway as potential targets for new antibiotics.
The team used an established technique known as in vitro attenuation to create variants of a mouse-colonising strain of H.pylori with low infectivity and then compared the gene expression profiles of the attenuated bacteria with the original highly virulent strain. The most significant changes were found to be in the genes that encode homologues of the Escherichia coli vitamin B6 biosynthesis enzymes, PdxA and PdxJ, which catalyse sequential steps in the pathway. In vitro, H. pylori PdxA mutants could only be recovered when pyridoxal-5’-phosphate, the bioactive form of vitamin B6, was added to the growth medium whereas it was not possible to produce viable bacteria with mutated PdxJ. PdxA was also shown to be necessary for H. pylori to establish a chronic infection in mice.
Further studies showed that, in addition to its well known metabolic roles, vitamin B6 is needed for the synthesis of glycosylated flagella and for flagellum-based motility in H. pylori. The study, which is published in the new open access journal mBio™, suggests that Pdx enzymes, which are present in a number of human pathogens, but not in mammalian cells, may present attractive targets for new antibiotic medicines.
Debilitating muscle wasting or cachexia affects the majority of patients with advanced cancer but although the condition is believed to contribute to cancer-related deaths, the precise mechanisms by which cancer causes cachexia and those by which cachexia contributes to a poor prognosis are ill understood. There are currently limited treatment options for patients with cachexia, but scientists at Harvard Medical School and Amgen Research have now created a decoy receptor that can reverse cachexia in mice and increase survival, even though it has no effect on tumour growth.
ActRIIB is a high affinity activin type 2 receptor that mediates signalling by a subset of TGF-β family ligands, including myostatin, which inhibits muscle cell differentiation and growth, and activin, which is abundant in some cancer patients. Activation of ActRIIB initiates a signalling cascade that leads to increased degradation of myofibrillar proteins through the ubiquitin-proteasome pathway. In several mouse models of cachexia, administration of soluble ActRIIB (sActRIIB) was found not only to prevent further wasting but also to fully reverse both skeletal muscle loss and atrophy of the heart. Treatment with sActRIIB had no effect on fat mass or tumour growth and did not reduce elevated inflammatory cytokines, although it did stimulate feeding.
The study, which is published in the journal Cell, suggests that blocking the ActRIIB pathway has the potential to treat various muscle wasting diseases, particularly cancer cachexia, and if the results of the mouse studies translate to people, could also prolong the lives of cancer patients.
Methicillin-resistant Staphylococcus aureus (MRSA) infections are a particular problem in hospitals and other healthcare environments. MRSA can survive on normal surfaces and fabrics but researchers at Rensselaer Polytechnic Institute have now developed a coating that kills MRSA on contact. The coating contains lysostaphin linked by a short flexible polymer to carbon nanotubes and can be applied to surgical equipment, hospital walls, door handles and other surfaces. Lysostaphin is an example of a bacteriocin, a defensive bacterial antimicrobial agent that kills other, often closely related, bacteria. Lysostaphin, a cell wall-degrading enzyme, is produced by non-pathogenic strains of Staphylococcus bacteria and is very effective against Staphylococcus aureus, including MRSA, but completely harmless to humans and other organisms.
The lysostaphin-nanotube conjugate can be mixed with a wide range of surface finishes – in the present study, latex house paint was used. In tests, 100% of MRSA were killed within 20 minutes of contact with the paint. Treated surfaces can be washed repeatedly without losing their effectiveness and the team believe that the new coating is likely to prove superior to coatings that release biocides or those that ‘spear’ bacteria using amphipathic polycations and antimicrobial peptides. The team also believe that is unlikely that Staphylococcus aureus will be able to develop resistance to lysostaphin.
Nickel allergy is one of the most common causes of allergic contact dermatitis and a team led by researchers at the University of Giessen have now shown that the response is linked to activation of a single receptor, toll-like receptor 4 (TLR4).
The family of toll-like receptors normally recognizes structurally similar molecules derived from microbial pathogens and plays a key role in host defense. The team showed that nickel directly activated human, but not mouse, TLR4 and studies with mutant proteins showed that non-conserved histidine residues at positions 456 and 458 were necessary for activation of human TLR4. Wild type mice do not show an allergy to nickel but transgenic mice expressing human, rather than mouse, TLR4 could be efficiently sensitized to the metal. This is the first time that an inorganic substance has been shown to activate this innate immune pathway and, since histidines 456 and 458 are not essential for responses to microbial lipopolysaccharide, the authors suggest that site-specific inhibition of TLR4 could provide a potential strategy for treatment of nickel allergy, which would not compromise normal immune responses. Nickel allergy affects millions of people and is often associated with earrings and jewellery for other body piercings.
There are four main cell types in the islets of Langerhans in the pancreas; α-cells which secrete glucagon, β-cells which secrete insulin, δ-cells which secrete somatostatin, and PP cells which secrete pancreatic polypeptide. Type I diabetes is an autoimmune disease in which the insulin-producing β-cells are destroyed and could potentially be treated by the creation of new β-cells, either from stem or stem-like cells or by conversion of another mature cell type. It has recently been shown that the transcription factor, Pax4, induces transdifferentiation of pancreatic α-cells into β-cells in adult mice and a team led by researchers at the Broad Institute of Harvard and MIT has now shown that a similar effect can be achieved with a small molecule.
Using a mouse α-cell line, the team screened over 30,000 compounds and found that one of them, BRD7389, induced insulin expression after 3 days treatment. Induction of insulin gene expression in α-cells peaked at 5 days (ca 50-fold at a BRD7389 concentration of 0.85µM) and the cells adopted a β-cell-like morphology. Insulin protein levels were also increased from a basal α-cell state, although were much lower than the levels produced by mature β-cells. BRD7389 also increased insulin secretion in primary human islet cells although since there are many different cells types in the human tissue samples, it is also not possible to attribute the increase in insulin levels exclusively to conversion of α-cells to β-like-cells.
Follow-up studies suggested that upregulation of insulin expression potentially involved inhibition of multiple members of the RSK family of protein kinases, but more experiments are needed to fully elucidate the mechanism of action of BRD7389. The study demonstrates, however, that a small molecule can induce insulin expression in α-cells and suggests that such a strategy could be used to increase β-cell mass by transdifferentiation in vivo. The team now want to identify other small molecules that could be used to enhance the effects of BRD7389, and boost insulin production in people with type I diabetes.
The ability to grow a replacement tail or limb, present in some species of amphibians such as salamanders and newts, has been lost in vertebrates. Earlier this year, scientists from the Wistar Institute and Washington University showed that mice lacking p21, a downstream target of the tumour suppressor p53, have a greater regenerative capacity than normal mice and now scientists at the Stanford University School of Medicine have shown the importance of other tumour suppression pathways in limiting regeneration in mammalian muscle cells.
Differentiated mammalian muscle cells are not normally able to divide but the team found that mouse myocytes can be induced to re-enter the cell cycle and begin proliferating by blocking the expression of two tumour suppressors, retinoblastoma protein (Rb) and ARF, a protein transcribed from an alternate reading frame of the INK4a locus. ARF is found in birds and mammals but not in lower vertebrates and, interestingly, is expressed at lower than normal levels in mammalian livers – the only organ with some regenerative capacity. When RNA interference was used to temporarily block expression of Rb and Arf in cultured mouse myocytes, the cells lost their differentiated properties, re-entered the cell cycle and began to proliferate. The cells are incorporated into existing muscle fibres when transplanted into mice, but only if Rb function was restored. Without functioning Rb, the new cells proliferated excessively and disrupted the structure of the muscle tissue.
Previous studies had shown that suppression of the Rb gene alone causes newt muscle cells, but not mammalian muscle cells, to re-enter the cell cycle.
Although knocking down tumour suppressor genes has obvious potential dangers, temporary silencing of gene expression may eventually allow regeneration of cardiac or pancreatic tissue if the technique is also successful in other cell types.
T-cell receptors are integral membrane proteins that recognise foreign antigens and initiate a series of intracellular signalling cascades that allow the immune system to fight infection. To avoid autoimmune diseases, T-cells must be able to discriminate between ‘self’ and ‘foreign’ antigens but this discrimination may also prevent the immune system from recognising and destroying tumour cells.
Researchers led by a team from the Max-Delbrück-Center for Molecular Medicine have now developed transgenic mice that produce T-cell receptors that recognise human cancer cell antigens and could potentially be introduced into the T cells of cancer patients. Using embryonic stem cells loaded with human DNA, the team generated transgenic mice that express the entire human T-cell repertoire. Negative selection normally removes maturing T-cells that are capable of binding strongly to ‘self’ antigens but the mouse does not recognise human cancer cell antigens as ‘self’ and T-cells expressing receptors to these antigens are allowed to survive. T-cells with such high affinity receptors for cancer cell antigens are not produced in humans and the researchers hope that introducing the high affinity receptors into the T-cells of cancer sufferers will boost the immune system’s ability to recognise and destroy tumour cells. A first clinical trial to evaluate the efficacy and tolerability of the methodology in cancer patients is planned.