Distinct neurochemical influences on fMRI response polarity in the striatum

The striatum, known as the input nucleus of the basal ganglia, is extensively studied for its diverse behavioral roles. However, the relationship between its neuronal and vascular activity, vital for interpreting functional magnetic resonance imaging (fMRI) signals, has not received comprehensive examination within the striatum. Here, we demonstrate that optogenetic stimulation of dorsal striatal neurons or their afferents from various cortical and subcortical regions induces negative striatal fMRI responses in rats, manifesting as vasoconstriction. These responses occur even with heightened striatal neuronal activity, confirmed by electrophysiology and fiber-photometry. In parallel, midbrain dopaminergic neuron optogenetic modulation, coupled with electrochemical measurements, establishes a link between striatal vasodilation and dopamine release. Intriguingly, in vivo intra-striatal pharmacological manipulations during optogenetic stimulation highlight a critical role of opioidergic signaling in generating striatal vasoconstriction. This observation is substantiated by detecting striatal vasoconstriction in brain slices after synthetic opioid application. In humans, manipulations aimed at increasing striatal neuronal activity likewise elicit negative striatal fMRI responses. Our results emphasize the necessity of considering vasoactive neurotransmission alongside neuronal activity when interpreting fMRI signal.

APOE4 expression confers a mild, persistent reduction in neurovascular function in the visual cortex and hippocampus of awake mice

Vascular factors are known to be early and important players in Alzheimer's disease (AD) development, however the role of the ε4 allele of the Apolipoprotein (APOE) gene (a risk factor for developing AD) remains unclear. APOE4 genotype is associated with early and severe neocortical vascular deficits in anaesthetised mice, but in humans, vascular and cognitive dysfunction are focused on the hippocampal formation and appear later. How APOE4 might interact with the vasculature to confer AD risk during the preclinical phase represents a gap in existing knowledge. To avoid potential confounds of anaesthesia and to explore regions most relevant for human disease, we studied the visual cortex and hippocampus of awake APOE3 and APOE4-TR mice using 2-photon microscopy of neurons and blood vessels. We found mild vascular deficits: vascular density and functional hyperaemia were unaffected in APOE4 mice, and neuronal or vascular function did not decrease up to late middle-age. Instead, vascular responsiveness was lower, arteriole vasomotion was reduced and neuronal calcium signals during visual stimulation were increased. This suggests that, alone, APOE4 expression is not catastrophic but stably alters neurovascular physiology. We suggest this state makes APOE4 carriers more sensitive to subsequent insults such as injury or beta amyloid accumulation.

Long COVID: mechanisms, risk factors and recovery

NEW FINDINGS

What is the topic of this review? The emerging condition of long COVID, its epidemiology, pathophysiological impacts on patients of different backgrounds, physiological mechanisms emerging as explanations of the condition, and treatment strategies being trialled. The review leads from a Physiological Society online conference on this topic. What advances does it highlight? Progress in understanding the pathophysiology and cellular mechanisms underlying Long COVID and potential therapeutic and management strategies.

ABSTRACT

Long COVID, the prolonged illness and fatigue suffered by a small proportion of those infected with SARS-CoV-2, is placing an increasing burden on individuals and society. A Physiological Society virtual meeting in February 2022 brought clinicians and researchers together to discuss the current understanding of long COVID mechanisms, risk factors and recovery. This review highlights the themes arising from that meeting. It considers the nature of long COVID, exploring its links with other post-viral illnesses such as myalgic encephalomyelitis/chronic fatigue syndrome, and highlights how long COVID research can help us better support those suffering from all post-viral syndromes. Long COVID research started particularly swiftly in populations routinely monitoring their physical performance - namely the military and elite athletes. The review highlights how the high degree of diagnosis, intervention and monitoring of success in these active populations can suggest management strategies for the wider population. We then consider how a key component of performance monitoring in active populations, cardiopulmonary exercise training, has revealed long COVID-related changes in physiology - including alterations in peripheral muscle function, ventilatory inefficiency and autonomic dysfunction. The nature and impact of dysautonomia are further discussed in relation to postural orthostatic tachycardia syndrome, fatigue and treatment strategies that aim to combat sympathetic overactivation by stimulating the vagus nerve. We then interrogate the mechanisms that underlie long COVID symptoms, with a focus on impaired oxygen delivery due to micro-clotting and disruption of cellular energy metabolism, before considering treatment strategies that indirectly or directly tackle these mechanisms. These include remote inspiratory muscle training and integrated care pathways that combine rehabilitation and drug interventions with research into long COVID healthcare access across different populations. Overall, this review showcases how physiological research reveals the changes that occur in long COVID and how different therapeutic strategies are being developed and tested to combat this condition.

SARS-CoV-2 variants of concern alpha, beta, gamma and delta have extended ACE2 receptor host ranges

Following the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in PR China in late 2019 a number of variants have emerged, with two of these - alpha and delta - subsequently growing to global prevalence. One characteristic of these variants are changes within the spike protein, in particular the receptor-binding domain (RBD). From a public health perspective, these changes have important implications for increased transmissibility and immune escape; however, their presence could also modify the intrinsic host range of the virus. Using viral pseudotyping, we examined whether the variants of concern (VOCs) alpha, beta, gamma and delta have differing host angiotensin-converting enzyme 2 (ACE2) receptor usage patterns, focusing on a range of relevant mammalian ACE2 proteins. All four VOCs were able to overcome a previous restriction for mouse ACE2, with demonstrable differences also seen for individual VOCs with rat, ferret or civet ACE2 receptors, changes that we subsequently attributed to N501Y and E484K substitutions within the spike RBD.

Gradual Not Sudden Change: Multiple Sites of Functional Transition Across the Microvascular Bed

In understanding the role of the neurovascular unit as both a biomarker and target for disease interventions, it is vital to appreciate how the function of different components of this unit change along the vascular tree. The cells of the neurovascular unit together perform an array of vital functions, protecting the brain from circulating toxins and infection, while providing nutrients and clearing away waste products. To do so, the brain's microvasculature dilates to direct energy substrates to active neurons, regulates access to circulating immune cells, and promotes angiogenesis in response to decreased blood supply, as well as pulsating to help clear waste products and maintain the oxygen supply. Different parts of the cerebrovascular tree contribute differently to various aspects of these functions, and previously, it has been assumed that there are discrete types of vessel along the vascular network that mediate different functions. Another option, however, is that the multiple transitions in function that occur across the vascular network do so at many locations, such that vascular function changes gradually, rather than in sharp steps between clearly distinct vessel types. Here, by reference to new data as well as by reviewing historical and recent literature, we argue that this latter scenario is likely the case and that vascular function gradually changes across the network without clear transition points between arteriole, precapillary arteriole and capillary. This is because classically localized functions are in fact performed by wide swathes of the vasculature, and different functional markers start and stop being expressed at different points along the vascular tree. Furthermore, vascular branch points show alterations in their mural cell morphology that suggest functional specializations irrespective of their position within the network. Together this work emphasizes the need for studies to consider where transitions of different functions occur, and the importance of defining these locations, in order to better understand the vascular network and how to target it to treat disease.

An open-source pipeline for analysing changes in microglial morphology

Changes in microglial morphology are powerful indicators of the inflammatory state of the brain. Here, we provide an open-source microglia morphology analysis pipeline that first cleans and registers images of microglia, before extracting 62 parameters describing microglial morphology. It then compares control and 'inflammation' training data and uses dimensionality reduction to generate a single metric of morphological change (an 'inflammation index'). This index can then be calculated for test data to assess inflammation, as we demonstrate by investigating the effect of short-term high-fat diet consumption in heterozygous Cx3CR1-GFP mice, finding no significant effects of diet. Our pipeline represents the first open-source microglia morphology pipeline combining semi-automated image processing and dimensionality reduction. It uses free software (ImageJ and R) and can be applied to a wide variety of experimental paradigms. We anticipate it will enable others to more easily take advantage of the powerful insights microglial morphology analysis provides.

Choice of method of place cell classification determines the population of cells identified

Place cells, spatially responsive hippocampal cells, provide the neural substrate supporting navigation and spatial memory. Historically most studies of these neurons have used electrophysiological recordings from implanted electrodes but optical methods, measuring intracellular calcium, are becoming increasingly common. Several methods have been proposed as a means to identify place cells based on their calcium activity but there is no common standard and it is unclear how reliable different approaches are. Here we tested four methods that have previously been applied to two-photon hippocampal imaging or electrophysiological data, using both model datasets and real imaging data. These methods use different parameters to identify place cells, including the peak activity in the place field, compared to other locations (the Peak method); the stability of cells' activity over repeated traversals of an environment (Stability method); a combination of these parameters with the size of the place field (Combination method); and the spatial information held by the cells (Information method). The methods performed differently from each other on both model and real data. In real datasets, vastly different numbers of place cells were identified using the four methods, with little overlap between the populations identified as place cells. Therefore, choice of place cell detection method dramatically affects the number and properties of identified cells. Ultimately, we recommend the Peak method be used in future studies to identify place cell populations, as this method is robust to moderate variations in place field within a session, and makes no inherent assumptions about the spatial information in place fields, unless there is an explicit theoretical reason for detecting cells with more narrowly defined properties.

Neurovascular coupling and oxygenation are decreased in hippocampus compared to neocortex because of microvascular differences

The hippocampus is essential for spatial and episodic memory but is damaged early in Alzheimer's disease and is very sensitive to hypoxia. Understanding how it regulates its oxygen supply is therefore key for designing interventions to preserve its function. However, studies of neurovascular function in the hippocampus in vivo have been limited by its relative inaccessibility. Here we compared hippocampal and visual cortical neurovascular function in awake mice, using two photon imaging of individual neurons and vessels and measures of regional blood flow and haemoglobin oxygenation. We show that blood flow, blood oxygenation and neurovascular coupling were decreased in the hippocampus compared to neocortex, because of differences in both the vascular network and pericyte and endothelial cell function. Modelling oxygen diffusion indicates that these features of the hippocampal vasculature may restrict oxygen availability and could explain its sensitivity to damage during neurological conditions, including Alzheimer's disease, where the brain's energy supply is decreased.

More than just summed neuronal activity: how multiple cell types shape the BOLD response

Functional neuroimaging techniques are widely applied to investigations of human cognition and disease. The most commonly used among these is blood oxygen level-dependent (BOLD) functional magnetic resonance imaging. The BOLD signal occurs because neural activity induces an increase in local blood supply to support the increased metabolism that occurs during activity. This supply usually outmatches demand, resulting in an increase in oxygenated blood in an active brain region, and a corresponding decrease in deoxygenated blood, which generates the BOLD signal. Hence, the BOLD response is shaped by an integration of local oxygen use, through metabolism, and supply, in the blood. To understand what information is carried in BOLD signals, we must understand how several cell types in the brain-local excitatory neurons, inhibitory neurons, astrocytes and vascular cells (pericytes, vascular smooth muscle and endothelial cells), and their modulation by ascending projection neurons-contribute to both metabolism and haemodynamic changes. Here, we review the contributions of each cell type to the regulation of cerebral blood flow and metabolism, and discuss situations where a simplified interpretation of the BOLD response as reporting local excitatory activity may misrepresent important biological phenomena, for example with regards to arousal states, ageing and neurological disease. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity

Functional neuroimaging using MRI relies on measurements of blood oxygen level-dependent (BOLD) signals from which inferences are made about the underlying neuronal activity. This is possible because neuronal activity elicits increases in blood flow via neurovascular coupling, which gives rise to the BOLD signal. Hence, an accurate interpretation of what BOLD signals mean in terms of neural activity depends on a full understanding of the mechanisms that underlie the measured signal, including neurovascular and neurometabolic coupling, the contribution of different cell types to local signalling, and regional differences in these mechanisms. Furthermore, the contributions of systemic functions to cerebral blood flow may vary with ageing, disease and arousal states, with regard to both neuronal and vascular function. In addition, recent developments in non-invasive imaging technology, such as high-field fMRI, and comparative inter-species analysis, allow connections between non-invasive data and mechanistic knowledge gained from invasive cellular-level studies. Considered together, these factors have immense potential to improve BOLD signal interpretation and bring us closer to the ultimate purpose of decoding the mechanisms of human cognition. This theme issue covers a range of recent advances in these topics, providing a multidisciplinary scientific and technical framework for future work in the neurovascular and cognitive sciences. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

Extinction of cue-evoked food-seeking recruits a GABAergic interneuron ensemble in the dorsal medial prefrontal cortex of mice

Animals must quickly adapt food-seeking strategies to locate nutrient sources in dynamically changing environments. Learned associations between food and environmental cues that predict its availability promote food-seeking behaviors. However, when such cues cease to predict food availability, animals undergo "extinction" learning, resulting in the inhibition of food-seeking responses. Repeatedly activated sets of neurons, or "neuronal ensembles," in the dorsal medial prefrontal cortex (dmPFC) are recruited following appetitive conditioning and undergo physiological adaptations thought to encode cue-reward associations. However, little is known about how the recruitment and intrinsic excitability of such dmPFC ensembles are modulated by extinction learning. Here, we used in vivo 2-Photon imaging in male Fos-GFP mice that express green fluorescent protein (GFP) in recently behaviorally activated neurons to determine the recruitment of activated pyramidal and GABAergic interneuron dmPFC ensembles during extinction. During extinction, we revealed a persistent activation of a subset of interneurons which emerged from a wider population of interneurons activated during the initial extinction session. This activation pattern was not observed in pyramidal cells, and extinction learning did not modulate the excitability properties of activated pyramidal cells. Moreover, extinction learning reduced the likelihood of reactivation of pyramidal cells activated during the initial extinction session. Our findings illuminate novel neuronal activation patterns in the dmPFC underlying extinction of food-seeking, and in particular, highlight an important role for interneuron ensembles in this inhibitory form of learning.

Small vessels, dementia and chronic diseases - molecular mechanisms and pathophysiology

Cerebral small vessel disease (SVD) is a major contributor to stroke, cognitive impairment and dementia with limited therapeutic interventions. There is a critical need to provide mechanistic insight and improve translation between pre-clinical research and the clinic. A 2-day workshop was held which brought together experts from several disciplines in cerebrovascular disease, dementia and cardiovascular biology, to highlight current advances in these fields, explore synergies and scope for development. These proceedings provide a summary of key talks at the workshop with a particular focus on animal models of cerebral vascular disease and dementia, mechanisms and approaches to improve translation. The outcomes of discussion groups on related themes to identify the gaps in knowledge and requirements to advance knowledge are summarized.

Interpreting BOLD: towards a dialogue between cognitive and cellular neuroscience

Cognitive neuroscience depends on the use of blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to probe brain function. Although commonly used as a surrogate measure of neuronal activity, BOLD signals actually reflect changes in brain blood oxygenation. Understanding the mechanisms linking neuronal activity to vascular perfusion is, therefore, critical in interpreting BOLD. Advances in cellular neuroscience demonstrating differences in this neurovascular relationship in different brain regions, conditions or pathologies are often not accounted for when interpreting BOLD. Meanwhile, within cognitive neuroscience, the increasing use of high magnetic field strengths and the development of model-based tasks and analyses have broadened the capability of BOLD signals to inform us about the underlying neuronal activity, but these methods are less well understood by cellular neuroscientists. In 2016, a Royal Society Theo Murphy Meeting brought scientists from the two communities together to discuss these issues. Here, we consolidate the main conclusions arising from that meeting. We discuss areas of consensus about what BOLD fMRI can tell us about underlying neuronal activity, and how advanced modelling techniques have improved our ability to use and interpret BOLD. We also highlight areas of controversy in understanding BOLD and suggest research directions required to resolve these issues.This article is part of the themed issue 'Interpreting BOLD: a dialogue between cognitive and cellular neuroscience'.

Non-signalling energy use in the developing rat brain

Energy use in the brain constrains its information processing power, but only about half the brain's energy consumption is directly related to information processing. Evidence for which non-signalling processes consume the rest of the brain's energy has been scarce. For the first time, we investigated the energy use of the brain's main non-signalling tasks with a single method. After blocking each non-signalling process, we measured oxygen level changes in juvenile rat brain slices with an oxygen-sensing microelectrode and calculated changes in oxygen consumption throughout the slice using a modified diffusion equation. We found that the turnover of the actin and microtubule cytoskeleton, followed by lipid synthesis, are significant energy drains, contributing 25%, 22% and 18%, respectively, to the rate of oxygen consumption. In contrast, protein synthesis is energetically inexpensive. We assess how these estimates of energy expenditure relate to brain energy use in vivo, and how they might differ in the mature brain.

What is a pericyte?

Pericytes, spatially isolated contractile cells on capillaries, have been reported to control cerebral blood flow physiologically, and to limit blood flow after ischaemia by constricting capillaries and then dying. Paradoxically, a recent paper dismisses the idea of pericytes controlling cerebral blood flow, despite confirming earlier data showing a role for pericytes. We show that these discrepancies are apparent rather than real, and depend on the new paper defining pericytes differently from previous reports. An objective definition of different sub-classes of pericyte along the capillary bed is needed to develop novel therapeutic approaches for stroke and disorders caused by pericyte malfunction.

Capillary pericytes regulate cerebral blood flow in health and disease

Increases in brain blood flow, evoked by neuronal activity, power neural computation and form the basis of BOLD (blood-oxygen-level-dependent) functional imaging. Whether blood flow is controlled solely by arteriole smooth muscle, or also by capillary pericytes, is controversial. We demonstrate that neuronal activity and the neurotransmitter glutamate evoke the release of messengers that dilate capillaries by actively relaxing pericytes. Dilation is mediated by prostaglandin E2, but requires nitric oxide release to suppress vasoconstricting 20-HETE synthesis. In vivo, when sensory input increases blood flow, capillaries dilate before arterioles and are estimated to produce 84% of the blood flow increase. In pathology, ischaemia evokes capillary constriction by pericytes. We show that this is followed by pericyte death in rigor, which may irreversibly constrict capillaries and damage the blood-brain barrier. Thus, pericytes are major regulators of cerebral blood flow and initiators of functional imaging signals. Prevention of pericyte constriction and death may reduce the long-lasting blood flow decrease that damages neurons after stroke.

Imaging pericytes and capillary diameter in brain slices and isolated retinae

The cerebral circulation is highly specialized, both structurally and functionally, and it provides a fine-tuned supply of oxygen and nutrients to active regions of the brain. Our understanding of blood flow regulation by cerebral arterioles has evolved rapidly. Recent work has opened new avenues in microvascular research; for example, it has been demonstrated that contractile pericytes found on capillary walls induce capillary diameter changes in response to neurotransmitters, suggesting that pericytes could have a role in neurovascular coupling. This concept is at odds with traditional models of brain blood flow regulation, which assume that only arterioles control cerebral blood flow. The investigation of mechanisms underlying neurovascular coupling at the capillary level requires a range of approaches, which involve unique technical challenges. Here we provide detailed protocols for the successful physiological and immunohistochemical study of pericytes and capillaries in brain slices and isolated retinae, allowing investigators to probe the role of capillaries in neurovascular coupling. This protocol can be completed within 6-8 h; however, immunohistochemical experiments may take 3-6 d.

Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease

Because regional blood flow increases in association with the increased metabolic demand generated by localized increases in neural activity, functional imaging researchers often assume that changes in blood flow are an accurate read-out of changes in underlying neural activity. An understanding of the mechanisms that link changes in neural activity to changes in blood flow is crucial for assessing the validity of this assumption, and for understanding the processes that can go wrong during disease states such as ischaemic stroke. Many studies have investigated the mechanisms of neurovascular regulation in arterioles but other evidence suggests that blood flow regulation can also occur in capillaries, because of the presence of contractile cells, pericytes, on the capillary wall. Here we review the evidence that pericytes can modulate capillary diameter in response to neuronal activity and assess the likely importance of neurovascular regulation at the capillary level for functional imaging experiments. We also discuss evidence suggesting that pericytes are particularly sensitive to damage during pathological insults such as ischaemia, Alzheimer's disease and diabetic retinopathy, and consider the potential impact that pericyte dysfunction might have on the development of therapeutic interventions and on the interpretation of functional imaging data in these disorders.

What is the real physiological NO concentration in vivo?

Clarity about the nitric oxide (NO) concentrations existing physiologically is essential for developing a quantitative understanding of NO signalling, for performing experiments with NO that emulate reality, and for knowing whether or not NO concentrations become abnormal in disease states. A decade ago, a value of about 1 μM seemed reasonable based on early electrode measurements and a provisional estimate of the potency of NO for its guanylyl cyclase-coupled receptors, which mediate physiological NO signal transduction. Since then, numerous efforts to measure NO concentrations directly using electrodes in cells and tissues have yielded an irreconcilably large spread of values. In compensation, data from several alternative approaches have now converged to provide a more coherent picture. These approaches include the quantitative analysis of NO-activated guanylyl cyclase, computer modelling based on the type, activity and amount of NO synthase enzyme contained in cells, the use of novel biosensors to monitor NO release from single endothelial cells and neurones, and the use of guanylyl cyclase as an endogenous NO biosensor in tissue subjected to a variety of challenges. All these independent lines of evidence suggest the physiological NO concentration range to be 100 pM (or below) up to ∼5 nM, orders of magnitude lower than was once thought.

Dietary variables associated with DNA N7-methylguanine levels and O6-alkylguanine DNA-alkyltransferase activity in human colorectal mucosa

Components of human diets may influence the incidence of colorectal adenomas, by modifying exposure or susceptibility to DNA-damaging alkylating agents. To examine this hypothesis, a food frequency questionnaire was used to assess the diet of patients recruited for a case-referent study where biopsies of normal colorectal mucosa were collected during colonoscopy and subsequently analysed for DNA N7-methylguanine (N7-MeG) levels, as an indicator of exposure, and activity of the DNA repair protein O6-alkylguanine DNA-alkyltransferase (MGMT), as an indicator of potential susceptibility. Cases with histologically proven colorectal adenomas (n = 38) were compared with referents (n = 35) free of gastrointestinal neoplasia. The case group consumed significantly more red meat (4.5 versus 3.4 servings/week, P < 0.05), processed meats, (4.7 versus 3.2 servings/week, P < 0.05) and % food energy as fat (34.9 versus 30.7%, P < 0.001). N7-MeG [mean: 95% confidence interval (CI)] levels were significantly lower in the group that consumed the highest proportion of dietary fibre/1000 kcal in comparison with the group with the lowest intake (0.61; 0.35-0.86 versus 1.88; 0.88-2.64 micromol/mol dG, P < 0.05). N7-MeG levels were also inversely associated with folate consumption (P < 0.05). MGMT activity (mean; 95% CI) was significantly higher in the group with the lowest consumption of vegetables than in the group with the greatest vegetable consumption (7.02; 5.70-8.33 versus 4.93; 3.95-5.91 fmol/microg DNA, P < 0.05). Our results are consistent with the hypothesis that dietary factors may modify exposure or susceptibility, respectively, to DNA damage by alkylating agents.

Assessing the physiological concentration and targets of nitric oxide in brain tissue

Low nanomolar concentrations of nitric oxide activate guanylyl cyclase to produce cGMP, which has diverse physiological effects. Higher concentrations inhibit mitochondrial respiration at cytochrome c oxidase and this has been proposed to be important physiologically, increasing oxygen permeation into tissue (by reducing the oxygen use of cells near blood vessels), activating AMP kinase, and regulating the relationship between cerebral blood flow and oxygen use. It is unclear, however, whether nitric oxide can accumulate physiologically to concentrations at which inhibition of respiration occurs. In rat cerebellar slices, we activated nitric oxide production from each isoform of nitric oxide synthase. Only activation of inducible nitric oxide synthase, which is expressed pathologically, caused any significant inhibition of respiration. Modelling oxygen and nitric oxide concentrations predicted that, in vivo, physiological nitric oxide levels are too low to affect respiration. Even pathologically, the nitric oxide concentration may only rise to 2.5 nm, producing a 1.5% inhibition of respiration. Thus, under physiological conditions, nitric oxide signals do not inhibit respiration but are well-tuned to the dynamic range of guanylyl cyclase activation.

Human colorectal mucosal O6-alkylguanine DNA-alkyltransferase activity and DNA-N7-methylguanine levels in colorectal adenoma cases and matched referents

BACKGROUND AND AIMS

O(6)-alkylguanine DNA-alkyltransferase (MGMT) provides protection against alkylating agent-induced GC-->AT transition mutations. Such mutations are frequently seen in the KRAS oncogene of large colorectal adenomas, but whether adenoma or mutational risk in humans is influenced by MGMT activity and alkylating agent exposure is unclear. Hence, MGMT activity and, as an indicator of alkylating agent exposure, DNA-N7-methylguanine (N7-MeG) levels were determined in the normal tissue of patients with and without adenomas.

METHODS

Biopsy specimens of normal colorectal mucosa were collected during colonoscopy from 85 patients with histologically proved colorectal adenomas (cases) and from 85 patients free of gastrointestinal neoplasia (referents) matched by age, sex and biopsy location. MGMT activity and N7-MeG levels were measured in colorectal tissue extracts and DNA, respectively.

RESULTS

MGMT activity was higher in the normal mucosa of cases than in referents (6.65+/-3.03 vs 5.61+/-2.74 fmol/micro g DNA, p = 0.01). On stratification of cases, MGMT activity was found to be considerably greater in the normal mucosa of cases with large adenomas (p = 0.003) and slightly higher in cases with a GC-->AT transition mutation in the K-ras gene (p = 0.03). Elevated MGMT levels were associated with an increased risk of adenoma (OR 1.17, 95% CI 1.03 to 1.33 per unit increase in activity). Detectable levels of N7-MeG were found in DNA from 89% of cases and 93% of referents, with levels ranging from <0.1 to 7.7 micro mol/mol dG. Cases and referents had similar DNA-N7-MeG levels.

CONCLUSIONS

Human exposure to methylating agents is widespread. MGMT activity is increased in the normal mucosa of patients with adenomas.

Inactivation of nitric oxide by rat cerebellar slices

Nitric oxide (NO) functions as an intercellular messenger throughout the brain. For this role to be performed efficiently, there must be a mechanism for neutralizing NO, but whether an active biological process exists, or whether NO is lost mainly through diffusion is unclear. To investigate this issue, rat cerebellar slices were exposed to constant levels of NO and the cGMP generated within the slice used as an indicator of NO concentrations therein. NO was about 1000-fold less potent in slices (EC50, 1 microM) than in separated cells from the same tissue (EC50, 1.6 nM), consistent with access of NO to the slice interior being greatly hindered by inactivation. Supporting this interpretation, immunohistochemical analysis indicated a marked concentration gradient of cGMP across the thickness of slices exposed to subsaturating NO concentrations, signifying a marked NO gradient. Several known NO-degrading processes, including reaction with lipid peroxyl radicals, erythrocytes and superoxide ions, were eliminated as contributing factors, indicating a novel mechanism. A diffusion-inactivation model was used to estimate the kinetics of NO consumption by the slices. The best fits to experimental data indicated a Michaelis-Menten-type reaction having a Vmax of 1-2 microM s-1 and a Km of around 10 nM. The rates predict that inactivation would impose a very short half-life (<10 ms) on NO in physiological concentrations (up to 10 nM) and that it would play an important role in shaping the NO concentration profiles when it is synthesized by multiple nearby sites.

Longitudinal variation in O(6)-alkylguanine DNA-alkyltransferase activity in the human colon and rectum

In a systematic study of O(6)-alkylguanine DNA-alkyltransferase activity in the human colon and rectum, tumours were found to occur in regions of low activity. These results are consistent with the hypothesis that O(6)-alkylguanine DNA-alkyltransferase levels and alkylating agent exposure may be important determinants of large bowel tumorigenesis.

Kirsten ras mutations in patients with colorectal cancer: the 'RASCAL II' study

Researchers worldwide with information about the Kirsten ras (Ki-ras) tumour genotype and outcome of patients with colorectal cancer were invited to provide that data in a schematized format for inclusion in a collaborative database called RASCAL (The Kirsten ras in-colorectal-cancer collaborative group). Our results from 2721 such patients have been presented previously and for the first time in any common cancer, showed conclusively that different gene mutations have different impacts on outcome, even when the mutations occur at the same site on the genome. To explore the effect of Ki-ras mutations at different stages of colorectal cancer, more patients were recruited to the database, which was reanalysed when information on 4268 patients from 42 centres in 21 countries had been entered. After predetermined exclusion criteria were applied, data on 3439 patients were entered into a multivariate analysis. This found that of the 12 possible mutations on codons 12 and 13 of Kirsten ras, only one mutation on codon 12, glycine to valine, found in 8.6% of all patients, had a statistically significant impact on failure-free survival (P = 0.004, HR 1.3) and overall survival (P = 0.008, HR 1.29). This mutation appeared to have a greater impact on outcome in Dukes' C cancers (failure-free survival, P = 0.008, HR 1.5; overall survival P = 0.02, HR 1.45) than in Dukes' B tumours (failure-free survival, P = 0.46, HR 1.12; overall survival P = 0.36, HR 1.15). Ki-ras mutations may occur early in the development of pre-cancerous adenomas in the colon and rectum. However, this collaborative study suggests that not only is the presence of a codon 12 glycine to valine mutation important for cancer progression but also that it may predispose to more aggressive biological behaviour in patients with advanced colorectal cancer.

Host determinants of DNA alkylation and DNA repair activity in human colorectal tissue: O(6)-methylguanine levels are associated with GSTT1 genotype and O(6)-alkylguanine-DNA alkyltransferase activity with CYP2D6 genotype

There is increasing evidence that alkylating agent exposure may increase large bowel cancer risk and factors which either alter such exposure or its effects may modify risk. Hence, in a cross-sectional study of 78 patients with colorectal disease, we have examined whether (i) metabolic genotypes (GSTT1, GSTM1, CYP2D6, CYP2E1) are associated with O(6)-methyldeoxyguanosine (O(6)-MedG) levels, O(6)-alkylguanine-DNA alkyltransferase (ATase) activity or K-ras mutations, and (ii) there was an association between ATase activity and O(6)-MedG levels. Patients with colon tumours and who were homozygous GSTT1(*)2 genotype carriers were more likely than patients who expressed GSTT1 to have their DNA alkylated (83 versus 32%, P=0.03) and to have higher O(6)-MedG levels (0.178+/-0.374 versus 0.016+/-0.023 micromol O(6)-MedG/mol dG, P=0.04) in normal, but not tumour, DNA. No such association was observed between the GSTT1 genotype and the frequency of DNA alkylation or O(6)-MedG levels in patients with benign colon disease or rectal tumours. Patients with colon tumours or benign colon disease who were CYP2D6-poor metabolisers had higher ATase activity in normal tissue than patients who were CYP2D6 extensive metabolisers or CYP2D6 heterozygotes. Patients with the CYP2E1 Dra cd genotype were less likely to have a K-ras mutation: of 55 patients with the wild-type CYP2E1 genotype (dd), 23 had K-ras mutations, whereas none of the 7 individuals with cd genotype had a K-ras mutation (P=0.04). No other associations were observed between GSTT1, GSTM1, CYP2D6 and CYP2E1 Pst genotypes and adduct levels, ATase activity or mutational status. O(6)-MedG levels were not associated with ATase activity in either normal or tumour tissue. However, in 15 patients for whom both normal and tumour DNA contained detectable O(6)-MedG levels, there was a strong positive association between the normal DNA/tumour DNA adduct ratio and the normal tissue/tumour tissue ATase ratio (r(2)=0.66, P=0.001). These results indicate that host factors can affect levels both of the biologically effective dose arising from methylating agent exposure and of a susceptibility factor, the DNA repair phenotype.

Development and application of a sensitive and rapid immunoassay for the quantitation of N7-methyldeoxyguanosine in DNA samples

N7-Methyldeoxyguanosine (N7-MedG) in DNA is a biomarker of exposure to environmental and endogenous methylating agents and may be of use in epidemiological studies. To quantitate N7-MedG in human samples, a sensitive assay system that uses only small quantities of DNA (<10 microg) is required. To this end, polyclonal antibodies against the imidazole ring-opened form of N7-MedG have been used to develop a highly sensitive immunoslot blot (ISB) assay. The limit of detection of the assay is 0.10 micromol of N7-MedG/mol of deoxyguanosine (dG) using 1 microg of DNA per analysis. The method was optimized using in vitro-methylated calf thymus DNA and then applied to a study of DNA methylation in liver and brain tissues of mice following a single iv dose of the antitumor agent Temozolomide. The amount of N7-MedG in both tissues was strictly proportional to dose over a range of 10-200 mg of Temozolomide/kg of body weight. The ISB assay was then validated using pyloric DNA of rats treated with N-methyl-N'-nitro-N-nitrosoguanidine and DNA samples from human bladder tumors, for both of which N7-MedG levels had already been quantitated by an HPLC/(32)P-postlabeling method previously described. The results showed a high degree of correlation (r = 0.98) between the two assays. The ISB assay was then applied to a range of human samples. A series of peripheral blood mononuclear cell DNA samples from cancer patients following treatment with Temozolomide had levels of N7-MedG ranging from 0.22 to 320 micromol/mol of dG. DNA samples from colon carcinoma and normal colorectal mucosa from individuals not known to be exposed to methylating agents contained levels of 0.11-1.34 micromol of N7-MedG/mol of dG. The ISB assay offers the potential for the rapid and high-throughput analysis of DNA obtained from routine biopsies and blood samples, thus enabling the determination of the extent of human exposure to environmental and endogenous sources of methylating agents in large-scale biomonitoring studies.

Elevated levels of the pro-carcinogenic adduct, O(6)-methylguanine, in normal DNA from the cancer prone regions of the large bowel

BACKGROUND

The pro-mutagenic lesion O(6)-methyldeoxyguanosine (O(6)-MedG), a marker of exposure to many N-nitroso compounds (NOC), can be detected in normal and tumour DNA isolated from colorectal tissue. The biological significance of this exposure is, as yet, unknown but in situ NOC formation is bacterially catalysed suggesting that NOC formation and potentially DNA alkylation will vary throughout the large bowel.

AIMS

To determine if O(6)-MedG levels in colorectal DNA vary within the large bowel.

PATIENTS

We studied 62 men and women undergoing surgery for colorectal tumours in the north west of England.

METHODS

O(6)-MedG levels were measured in paired normal and tumour DNA samples. DNA was digested to nucleosides, fractionated by HPLC, and purified O(6)-MedG quantified by a radioimmunoassay.

RESULTS

O(6)-MedG was detected in 27 out of a total of 62 (43%) normal DNA samples and in 30 of 58 (52%) tumour DNA samples: it was present at concentrations of <0. 01-0.94 and <0.01-0.151 micromol O(6)-MedG/mol deoxyguanosine for normal and tumour DNA, respectively. Levels of O(6)-MedG in normal, but not tumour, DNA from the proximal colon were lower than those found in DNA from either the sigmoid colon (p=0.03) or rectum (p=0. 05). When the analysis was restricted to samples that contained O(6)-MedG, similar results were obtained in that O(6)-MedG levels in normal DNA were lower in the proximal colon than in the sigmoid colon (p=0.04) or rectum (p=0.03).

CONCLUSIONS

DNA alkylation varied within the large bowel possibly due to in situ NOC formation and was highest in areas of the colon and rectum where the highest incidence of large bowel tumours occurs, suggesting that DNA alkylation may play a role in the aetiology of colorectal cancer.

Determinants of O(6)-alkylguanine-DNA alkyltransferase activity in normal and tumour tissue from human colon and rectum

O(6)-Alkylguanine-DNA-alkyltransferase (ATase) is an important modulator of alkylating agent-induced toxicity and carcinogenicity, but those factors which influence the expression of this repair protein in human tissues are poorly characterised. In this study, we have determined ATase levels in macroscopically normal and tumour tissues from 76 individuals with benign or malignant colorectal disease. All tissue samples had detectable ATase activity, with values ranging from 35 to 451 fmol/mg protein. ATase activity in normal rectal tissue was significantly higher than that in normal tissue from the sigmoid colon (148 +/- 76 vs. 100 +/- 40 fmol/mg protein, p = 0.01), whereas ATase levels within different regions of the colon (proximal vs. sigmoid colon) were similar. In normal tissue, inter-individual variation in ATase activity was 4-fold in the colon and 6-fold in the rectum, whereas in tumour tissue the corresponding figures were approx. 13.0- and 7-fold, respectively. There was no detectable difference in normal tissue ATase activity between individuals with benign or malignant disease of the colon. Normal and tumour tissue ATase activities were strongly correlated in the sigmoid colon (r = 0.80) and rectum (r = 0.59) but not the caecum (r = -0.03). In a multivariate analysis, ATase activity in normal colon tissue increased with age (p = 0.01) and current smoking (p = 0.06), whereas tumour ATase activity increased only with use of anti-histamines (p = 0.05). In rectal tumour tissue, activity decreased with age (p = 0.05) and use of anti-muscarinic medications (p = 0.01): in normal rectal tissue, no modulating factors were identified.

Synchronous and metachronous gastric adenocarcinoma: case report and literature review

Whilst synchronous adenocarcinoma of the stomach is well documented, metachronous primary disease is exceedingly rare. We report a man with a family history of colonic and gastric cancer, who underwent a resection of a Duke's C adenocarcinoma of the rectum, aged 56 years, and a proximal partial gastrectomy for synchronous stage 1 gastric adenocarcinomas of the lesser curve, aged 61 years. Nine years later, a metachronous gastric primary was discovered in the gastric remnant, necessitating total gastrectomy. Total gastrectomy is the operation of choice for synchronous gastric primaries as it ensures clearance and prevents metachronous growth. However, it may not be appropriate for all gastric cancer as operative morbidity and mortality are increased, and because synchronicity and metachronicity of gastric cancer are uncommon. Moreover, there are no consistent data to demonstrate a survival advantage for total compared with partial gastrectomy for operable gastric cancer. If, after partial gastrectomy, synchronous disease is detected in the resected specimen (as in this reported case), endoscopic surveillance for metachronous disease is advised, since this may be amenable to surgical cure.

Low O6-alkylguanine DNA-alkyltransferase activity in normal colorectal tissue is associated with colorectal tumours containing a GC-->AT transition in the K-ras oncogene

O6-alkylguanine DNA-alkyltransferase (ATase) provides protection against the toxic, mutagenic and carcinogenic effects of alkylating agents, principally by removing the promutagenic lesion O6-alkylguanine from DNA. Differences in ATase activity in human tissue may thus determine mutational susceptibility. As GC-->AT transitions, which can be induced by O6-alkylguanine in DNA, are commonly observed in the K-ras oncogene of alkylating agent induced animal tumours and in human colorectal tumours, we have examined whether differences in ATase activity may affect the risk of K-ras mutations in humans with colorectal tumours. NTase activity in normal tissue from individuals with a K-ras mutation in colorectal tissue and more specifically a GC-->AT transition (but not a transversion mutation) was significantly lower than that in individuals without a mutation (P < 0.01). Thus, individuals with low ATase activity in normal tissue (i.e. below the median) were at increased risk of having a transition (OR 10.1; 95% CI 1.9-99.0), but not a transversion mutation (OR 1.7; 95% CI 0.3-12.2). There were no significant differences in tumour ATase activity in individuals with or without a mutation. These results suggest that ATase can protect colorectal tissue against the mutagenic effects of alkylating agents and furthermore, that alkylating agent exposure plays a role in the aetiology of colorectal tumours containing a GC-->AT transition in the K-ras oncogene.

Frequency of Ki-ras mutations and DNA alkylation in colorectal tissue from individuals living in Manchester

Most human colorectal cancers arise through the accumulation of a series of genetic alterations such as point mutations within the Ki-ras and p53 genes, but the chemical carcinogens that may be implicated in these events are still unidentified. In a previous study, we showed that DNA from human colorectal tissue contained O6-methyldeoxyguanosine (O6-MedG), a promutagenic lesion arising from exposure to as yet unidentified methylating agents. To address whether such exposure may result in oncogene activation in human colorectal tumors, we examined another series of paired normal and tumor DNA samples from the lower intestinal tract for the presence of O6-MedG in DNA (as a marker of exposure) and for mutations within the Ki-ras gene. After isolation by high pressure liquid chromatography, O6-MedG was quantified by a radioimmunoassay with a limit of detection of 0.01 mumol O6-MedG/mol dG. The frequencies of methylation were 33%, 52%, and 48% for normal DNA and 58%, 32%, and 63% for tumor DNA isolated from the cecum, sigmoid colon, and rectum, respectively. Overall, 35% of the individuals had no detectable O6-MedG in the DNA from both their tumor and normal tissue. Ki-ras mutations were initially identified by a restriction site mutation assay and then sequenced to ascertain the mutations thus detected. The frequencies of mutations in tumor DNA isolated from the cecum, sigmoid colon, and rectum were 28%, 29%, and 42%, respectively. DNA isolated from macroscopically normal tissue was found to contain Ki-ras mutations in 14% of sigmoid colon samples and 12% of rectal samples. Most base mutations were in codon 12 (72%), and 64% were GC-->AT transitions: 28% and 8% were GC-->TA and CG-->CG transversions, respectively. All mutations were at the second base of either codon 12 or codon 13 except for a single GC-->TA transversion at the first base of codon 13 in a rectal tumor sample. There was no association between the presence of O6-MedG in DNA from either normal or tumor tissue or both normal and tumor tissue and the incidence of Ki-ras mutations or GC-->AT transitions in mutated Ki-ras genes. It remains to be determined, however, whether there is a relationship between methylating-agent exposure and Ki-ras mutations, as (i) the presence of O6-MedG in colorectal DNA in these samples may not represent the exposure when Ki-ras mutational activation was occurring (i.e., at some unknown time in the past), (ii) interindividual differences in repair-enzyme activity may alter susceptibility to a mutational event after exposure, (iii) the predominant mutagen in the colon and rectum may not be a methylating agent (e.g., nitric oxide), and (iv) exposure to methylating agents need not result in oncogene activation in human tissues but may perhaps promote the emergence of the mutator phenotype.

Biliary-cutaneous fistula: an uncommon complication of retained gallstones following laparoscopic cholecystectomy

A biliary-cutaneous fistula occurring as a late complication of laparoscopic cholecystectomy is described. The fistula was associated with retained gallstone fragments following intra-operative spillage and attempted retrieval. The fate of intra-peritoneal gallstones is reviewed and preventative measures discussed.

The detection of alkylation damage in the DNA of human gastrointestinal tissues

Damage arising from putative environmental sources has been found in the DNA of the gastric and colorectal mucosae of patients presenting with gastrointestinal disorders from the South Manchester area. O6-Methylguanine (O6-MeG) in the range 0.010- greater than 0.300 mu moles mole-1 adenine was heterogeneously distributed both between and within individuals. The pattern of alkylation of tissue DNA appears to differ when comparison is made between gastric and colorectal samples. Most of the gastric tumour DNA samples were alkylated (5/6; 0.087 +/- 0.097), whereas the DNA of the associated mucosa was alkylated less frequently (2/7) and to a lesser extent; (0.017 +/- 0.030; P = 0.07). Conversely, colorectal tumour DNA was alkylated infrequently (1/7) and to a lower extent (0.003 +/- 0.007) than the DNA of the adjacent mucosa (8/10 samples alkylated with a mean of 0.083 +/- 0.106; P = less than 0.01), or indeed of any other tissue. Although increased levels of DNA damage in tissue associated with malignant disease have been indicated by independent studies of DNA damage at other cancer sites, significant differences were not observed in the present report, neither was there any suggestion of a relationship with smoking or alcohol consumption. The data provided by this report indicate that exposure to putative environmental alkylating agents occurs in the UK at levels comparable to those previously detected in areas of higher cancer risk. Although we cannot determine the extent to which this DNA damage is attributable to normal background exposures, it is evident that the alkylation of tissue DNA occurs and is not uniform. In conjunction with other reports, therefore these differences may begin to provide indications of mechanisms that could be of relevance in the aetiology of gastrointestinal cancers.

Increased risk of cancer at multiple sites after gastric surgery for peptic ulcer

Of 5018 patients who had undergone gastric surgery at St James Hospital, Balham, at least 25 years ago, death certificates have been received for 2768, whilst 1746 patients are still alive and are flagged (Office of Populations, Censuses and Surveys (OPCS) will notify us of their death and its cause) and only 504 could not be traced. Mortality from cancers of various organs has been determined using a 'years at risk' calculation in five year bands. There was no increase in mortality risk from any cancers during the first 15 postoperative years, but from 20 years after operation there was a significant excess risk not only of cancer of the stomach (4.5-fold), but also of the large bowel (1.6-fold), bronchus (3.9-fold), pancreas (4.0-fold), biliary tract (9.1-fold), oesophagus (2.3-fold), bladder (2.4-fold), breast (4.0-fold), and cancer of all sites (3.3-fold). These findings are consistent with the production in the operated-upon stomach of circulating carcinogens with a 20 year latency period.

Urinary N-nitrosoproline excretion: a further evaluation of the nitrosamine hypothesis of gastric carcinogenesis in precancerous conditions

Measurement of N-nitroso compounds in gastric juice by different methods has given conflicting results. In order to resolve this controversy, we have assessed endogenous nitrosation by the independent N-nitrosoproline excretion test in subjects who had previously undergone gastric juice analysis by one of these methods. Ten Polya gastrectomy, 10 pernicious anaemia and nine matched control subjects were fed 380 mg of nitrate in beetroot juice and 500 mg proline. N-nitrosoproline (N-Pro) synthesised intragastrically from these precursors, and quantitatively excreted by the kidneys, was measured in 24 hour urine samples (collection checked by creatinine clearance). N-Pro excretion (mean +/- SEM) was reduced (p less than 0.01) in pernicious anaemia (1.1 +/- 0.8 ng/day) compared with matched control (18.0 +/- 7.2 ng/day), and also tended to be lower (NS) in polya gastrectomy (3.2 +/- 2.3 ng/day). Twenty four hour intragastric pH was monitored on a separate occasion in 23 of the 29 subjects; 13 were hypoacidic (pH greater than 4 greater than 50% of 24 hours) and 10 were acidic. N-Pro yields were reduced (p less than 0.01) in the hypoacidic group (0.9 +/- 0.6 ng/day) compared with the acidic group (17.9 +/- 6.6 ng/day), and N-Pro was negatively associated with mean intragastric pH (tau = -0.53, p = 0.001). We conclude that endogenous synthesis of this specific N-nitroso compound is favoured by low rather than high pH. These results are concordant with those previously reported in gastric juice from the same subjects and suggest that nitrosation is chemically rather than bacterially mediated, contrary to the nitrosamine hypothesis of gastric carcinogenesis.

Evaluation of the nitrosamine hypothesis of gastric carcinogenesis in precancerous conditions

A 24 hour gastric aspiration study was carried out on nine Polya gastrectomy, eight pernicious anaemia, and nine matched control subjects. Intragastric pH, bacteria, nitrite, and N-nitroso compounds were assessed half hourly whilst ambulant and hourly when in bed. Both total and nitrate reducing bacterial counts were positively related to pH (chi 2 = 279.3; p less than 0.001), as was nitrite concentration (F = 19.1; p less than 0.0001). By contrast, total (F = 40.6; p less than 0.0001) and stable (F = 257.4; p less than 0.0001) N-nitroso compound concentrations were negatively related to pH. Clear differences in these gastric juice factors were not apparent between matched control and either pernicious anaemia, or Polya gastrectomy because the Polya gastrectomy and matched control groups were heterogeneous for gastric acidity. Thus, although eight of eight pernicious anaemia subjects were hypoacidic (defined as intragastric pH greater than 4 for greater than 50% of both daytime and night time periods), only five of nine Polya gastrectomy and two of nine matched control subjects were hypoacidic. When subjects were rearranged into hypoacidic (n = 15) and acidic (n = 11) groups, bacterial counts (p less than 0.01) and nitrite concentrations (p less than 0.01) were higher, whereas N-nitroso compounds tended to be lower (NS) in the hypoacidic group. These data suggest that, although hypoacidity predisposes to bacterial overgrowth and nitrite generation, it does not enhance nitrosation. Instead, this is maximal at low pH, suggesting chemical rather than bacterial nitrosation, contrary to the nitrosamine hypothesis of gastric carcinogenesis.