Claudia AM Wheeler-Kingshott, Ph.D.
University Reader in Magnetic Resonance Physics, Department of Neuroinflammation, UCL Institute of Neurology, London, UK
Abstract
Detecting neuronal activity in vivo non-invasively is possible with a number of techniques. Amongst these, in 1990 functional magnetic resonance imaging (fMRI) was proposed as a technique that has a great ability to spatially map brain activity by exploiting the blood oxygenation level dependent (BOLD) contrast mechanism [1, 2]. In fact, neuronal activation triggers a demand for oxygen and induces a localised increase in blood flow and blood volume, which actually exceeds the metabolic needs. This in turns causes an increase of oxyhaemoglobin in the venous compartment, which is a transient phenomenon and is accompanied by a transient change (decrease) in the concentration of deoxyhaemoglobin. Due to its paramagnetic properties, the amount of deoxyhaemoglobin present in the venous blood affects the local magnetic field seen by the spins (protons) and determines the local properties of the MR signal. A decrease in deoxyhaemoglobin during neuronal activity, therefore, induces local variations of this magnetic field that increases the average transverse relaxation time of tissue, measured via the T2* parameter [3]. This means that there is an increase of the MR signal (of the order of a few %, typically <5%) linked to metabolic changes happening during brain function. Activation can be inferred at different brain locations by performing tasks while acquiring the MR signal and comparing periods of rest to periods of activity.
The macroscopic changes of the BOLD signal are well characterised, while the reason for the increased blood supply, exceeding demands, needs further thoughts. Here we will discuss two approaches for explaining the BOLD phenomenon, one that links it to adenosine triphosphate production [4] and enzyme saturation, the other that relates it to the very slow diffusion of oxygen through the blood-brain-barrier with a consequent compensatory high demand of oxygen [5]. Some evidence of restricted oxygen diffusion has been shown by means of hypercapnia [6], although it is not excluded that both mechanisms may be present.
Overall, the BOLD signal changes theory and its physiological basis will be presented and discussed.
References
- Ogawa, S., et al., Brain magnetic resonance imaging with contrast dependent on blood oxygenation. Proc Natl Acad Sci U S A, 1990. 87(24): p. 9868-72.
- Kwong, K.K., et al., Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. Proc Natl Acad Sci U S A, 1992. 89(12): p. 5675-9.
- Bandettini PA, et al. Spin-echo and gradient-echo EPI of human brain activation using BOLD contrast: a comparative study at 1.5 T. NMR Biomed. 1994 Mar;7(1-2):12-20
- Fox, P.T., et al., Nonoxidative glucose consumption during focal physiologic neural activity. Science, 1988. 241(4864): p. 462-4.
- Gjedde, A., et al. Reduction of functional capillary density in human brain after stroke. J Cereb Blood Flow Metab, 1990. 10(3): p. 317-26.
- Hoge, R.D., et al., Linear coupling between cerebral blood flow and oxygen consumption in activated human cortex. Proc Natl Acad Sci U S A, 1999. 96(16): p. 9403-8.
Gillian Murphy, Ph.D.
Professor, Department of Oncology, University of Cambridge, UK
A novel strategy for targeting metalloproteinases in cancer
Epithelial tumours evolve in a multi-step manner, involving both inflammatory and mesenchymal cells. Although intrinsic factors drive malignant progression, the influence of the micro-environment of neoplastic cells is a major feature of tumorigenesis. Extracellular proteinases, notably the metalloproteinases, are key players in the regulation of this cellular environment, acting as major effectors of both cell-cell and cell-extracellular matrix (ECM) interactions. They are involved in modifying ECM integrity, growth factor availability and the function of cell surface signalling systems, with consequent effects on cellular differentiation, proliferation and apoptosis.This has made metalloproteinases important targets for therapeutic interventions in cancer and small molecule inhibitors focussed on chelation of the active site zinc and binding within the immediate active site pocket were developed. These were not successful in early clinical trials due to the relative lack of specificity and precise knowledge of the target proteinase(s) in specific cancers. We can now appreciate that it is essential that we understand the relative roles of the different enzymes (of which there are over 60) in terms of their pro and anti tumour activity and their precise sites of expression The next generations of metalloproteinase inhibitors need the added specificity that might be gained from an understanding of the structure of individual active sites and the role of extra catalytic domains in substrate binding and other aspects of their biology. We have prepared scFv antibodies to the extra catalytic domains of two membrane metalloproteinases, MMP-14 and ADAM17, that play key roles in the tumour microenvironment. Our rationale and experiences with these agents will be presented in more detail.
John Stanley, Satheesh Babu, Ramacahandran T and Bipin Nair
Pt-Pd decorated TiO2 nanotube array for the non-enzymatic determination of glucose in neutral medium
Rapidly expanding diabetic population and the complications associated with elevated glycemic levels necessitates the need for a highly sensitive, selective and stable blood glucose measurement strategy. The high sensitivity and selectivity of enzymatic sensors together with viable manufacturing technologies such as screen-printing have made a great social and economic impact. However, the intrinsic nature of the enzymes leads to lack of stability and consequently reduces shelf life and imposes the need for stringent storage conditions. As a result much effort has been directed towards the development of ‘enzyme-free’ glucose sensors (Park et al. 2006). In this paper, a non-enzymatic amperometric sensor for selective and sensitive direct electrooxidation of glucose in neutral medium was fabricated based on Platinum-Palladium (Pt–Pd) nanoparticle decorated titanium dioxide (TiO2) nanotube arrays. Highly ordered TiO2 nanotube arrays were obtained using a single step anodization process (Grimes C A and Mor G K 2009) over which Pt–Pd nanoparticles where electrochemically deposited. Scanning Electron Microscopy (SEM) analysis revealed the diameter of the TiO2 nanotubes to be approximately 40 nm. Elemental analysis after electrochemical deposition confirms the presence of Pt–Pd. Electrochemical characterization of the sensor was carried out using cyclic voltammetry technique (−1.0 to +1.0V) in phosphate buffer saline (PBS) pH 7.4. All further glucose oxidation studies were performed in PBS (pH 7.4). The sensor exhibited good linear response towards glucose for a concentration range of 1 μM to 20mM with a linear regression coefficient of R = 0.998. The electrodes are found to be selective in the presence of other commonly interfering molecules such as ascorbic acid, uric acid, dopamine and acetamidophenol. Thus a nonenzymatic sensor with good selectivity and sensitivity towards glucose in neutral medium has been developed.