Microdialysis of large molecules

Interstitial fluid-a reflection of the tumor cell microenvironment and secretome

The interstitium or interstitial space describes the space outside the blood and lymphatic vessels. It contains two phases; the interstitial fluid (IF) and the extracellular matrix. In this review we focus on the interstitial fluid phase, which is the physical and biochemical microenvironment of the cells, and more specifically that of tumors. IF is created by transcapillary filtration and cleared by lymphatic vessels, and contains substances that are either produced and secreted locally, thus denoted secretome, or brought to the organ by the circulation. The structure of the interstitium is discussed briefly and moreover techniques for IF isolation focusing on those that are relevant for studies of the secretome. Accumulated data show that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma, and that there are gradients between IF and plasma giving information on where substances are produced and thereby reflecting the local microenvironment. We review recent data on the origin of tissue specific substances, challenges related to isolating a representative secretome and the use of this as a substrate for biomarker identification. Finally we perform a comparative analysis across human tumor types and techniques and show that there is great variation in the results obtained that may at least partially be due to the isolation method used. We conclude that when care is taken in isolation of substrate, analysis of the secretome may give valuable biological insight and result in identification of biomarker candidates. This article is part of a Special Issue entitled: An Updated Secretome.

Identification of proteins from interstitium of trapezius muscle in women with chronic myalgia using microdialysis in combination with proteomics

Background

Microdialysis (MD) of the trapezius muscle has been an attractive technique to investigating small molecules and metabolites in chronic musculoskeletal pain in human. Large biomolecules such as proteins also cross the dialysis membrane of the catheters. In this study we have applied in vivo MD in combination with two dimensional gel electrophoresis (2-DE) and mass spectrometry to identify proteins in the extracellular fluid of the trapezius muscle.

Materials and Methods

Dialysate from women with chronic trapezius myalgia (TM; n = 37), women with chronic wide spread pain (CWP; n = 18) and healthy controls (CON; n = 22) was collected from the trapezius muscle using a catheter with a cut-off point of 100 kDa. Proteins were separated by two-dimensional gel electrophoresis and visualized by silver staining. Detected proteins were identified by nano liquid chromatography in combination with tandem mass spectrometry.

Results

Ninety-seven protein spots were identified from the interstitial fluid of the trapezius muscle; 48 proteins in TM and 30 proteins in CWP had concentrations at least two-fold higher or lower than in CON. The identified proteins pertain to several functional classes, e.g., proteins involved in inflammatory responses. Several of the identified proteins are known to be involved in processes of pain such as: creatine kinase, nerve growth factor, carbonic anhydrase, myoglobin, fatty acid binding protein and actin aortic smooth muscle.

Conclusions

In this study, by using in vivo microdialysis in combination with proteomics a large number of proteins in muscle interstitium have been identified. Several of the identified proteins were at least two-fold higher or lower in chronic pain patients. The applied techniques open up for the possibility of investigating protein changes associated with nociceptive processes of chronic myalgia.

Relative recovery over time - an in vivo microdialysis study of human skeletal muscle

BACKGROUND

The microdialysis technique is a method for sampling endogenous molecules from the interstitial compartments of varying tissues and relies on diffusion of molecules between the tissue and a perfusate via a membrane. Such samples do not allow determination of the true interstitial concentration but only a certain percentage. This gives rise to one of the most crucial parameter that needs to be considered for a dependable microdialysis; the relative recovery. Relative recovery states the efficiency of which an analyte is extracted from its external medium. Aim. To investigate the relative recovery of small molecules (< 20 kDa) such as lactate, fluid recovery and the reproducibility of the relative recovery at group and individual level of the microdialysis technique applied in muscle.

MATERIALS AND METHODS

Using in vivo microdialysis of the trapezius muscle of 65 women from two separate occasions 4-6 months apart. Relative recovery of small molecules was measured from samples collected every 20 min during a period of 220 min.

RESULTS

Good reproducibility at group level of catheters with cut-offs 100 and 20kDa were found. Furthermore, there was a high and steady relative recovery with an overall good fluid recovery. Poor reproducibility was found at the individual level for both catheters.

CONCLUSIONS

This study demonstrates that when using microdialysis in skeletal muscle relative recovery is stable over time and is not affected by low-force exercise. Although there is a good reproducibility at group level this is not the case at the individual level. Thus in vivo, the relative recovery should be determined for each test subject and at each test occasion.

Interstitial Fluid and Lymph Formation and Transport: Physiological Regulation and Roles in Inflammation and Cancer

The interstitium describes the fluid, proteins, solutes, and the extracellular matrix (ECM) that comprise the cellular microenvironment in tissues. Its alterations are fundamental to changes in cell function in inflammation, pathogenesis, and cancer. Interstitial fluid (IF) is created by transcapillary filtration and cleared by lymphatic vessels. Herein we discuss the biophysical, biomechanical, and functional implications of IF in normal and pathological tissue states from both fluid balance and cell function perspectives. We also discuss analysis methods to access IF, which enables quantification of the cellular microenvironment; such methods have demonstrated, for example, that there can be dramatic gradients from tissue to plasma during inflammation and that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma. Accumulated recent data show that IF and its convection through the interstitium and delivery to the lymph nodes have many and diverse biological effects, including in ECM reorganization, cell migration, and capillary morphogenesis as well as in immunity and peripheral tolerance. This review integrates the biophysical, biomechanical, and biological aspects of interstitial and lymph fluid and its transport in tissue physiology, pathophysiology, and immune regulation.

Cutaneous microdialysis: cytokine evidence for altered innate reactivity in the skin of psoriasis patients?

Cutaneous microdialysis demonstrates cytokine production in living human skin. In the present study, microdialysis samples taken from uninvolved and lesional skin in three test subjects with psoriasis over 24 h have been investigated for cytokine content with a bead-based multiplex immunoassay from Luminex. Concentration curves for a set of Th1/Th2 and pro-inflammatory cytokines measured differed from a reference group of ten subjects without psoriasis. The time to return to near baseline values after innate insertion reactivity is between 9 and 16 h. Post-equilibration levels (17-24 h) for the three main cytokines elevated in the reference group were differentially elevated outside the range of the reference group for interleukin-1β (IL1β) and IL8 but not so for IL6. Two further cytokines, granulocyte-macrophage colony-stimulating factor and tumor necrosis factor-α not generally elevated in the reference group, showed elevated values in the test subjects. Multivariate time series analysis (chemometry) showed that cytokine patterns for the individual test subjects often fell outside the 99% confidence intervals of a model generated from the reference group. In a clinical research situation, cutaneous microdialysis is feasible, gives generally higher cytokine levels than in the blood and generates interpretable data on an individual's reactivity compared with a reference group. This may well prove useful in delineation of pathogenetic issues, selection of appropriate therapy and monitoring of subsequent response in inflammatory dermatoses such as psoriasis.

Iontophoresis of a 13 kDa protein monitored by subcutaneous microdialysis in vivo

Background: The purpose of this study was to optimize parameters pertaining to microdialysis technique so as to make this method feasible for evaluating transdermal transport of macromolecules. Results: Microdialysis experiments were performed in vivo using hairless rats with daniplestim as the model protein. Two perfusion fluids – phosphate-buffered saline (PBS) and 3% dextran in PBS – were evaluated with respect to their effect on sample volume retrieval and recovery of the target protein from the microdialysis probe. Incorporation of dextran-60 in the perfusion fluid reduced fluid loss to 10% as opposed to 34% in the absence of dextran-60. Improvement in daniplestim recovery was also seen with dextran-PBS (56.5 ± 10.3%) as the perfusion fluid than with PBS alone (26.7±4.5%). Conclusion: Subcutaneous levels of daniplestim were measured following iontophoresis after improving recovery and minimizing fluid loss from the microdialysis probe.

A new method for isolation of interstitial fluid from human solid tumors applied to proteomic analysis of ovarian carcinoma tissue

Major efforts have been invested in the identification of cancer biomarkers in plasma, but the extraordinary dynamic range in protein composition, and the dilution of disease specific proteins make discovery in plasma challenging. Focus is shifting towards using proximal fluids for biomarker discovery, but methods to verify the isolated sample's origin are missing. We therefore aimed to develop a technique to search for potential candidate proteins in the proximal proteome, i.e. in the tumor interstitial fluid, since the biomarkers are likely to be excreted or derive from the tumor microenvironment. Since tumor interstitial fluid is not readily accessible, we applied a centrifugation method developed in experimental animals and asked whether interstitial fluid from human tissue could be isolated, using ovarian carcinoma as a model. Exposure of extirpated tissue to 106 g enabled tumor fluid isolation. The fluid was verified as interstitial by an isolated fluid:plasma ratio not significantly different from 1.0 for both creatinine and Na(+), two substances predominantly present in interstitial fluid. The isolated fluid had a colloid osmotic pressure 79% of that in plasma, suggesting that there was some sieving of proteins at the capillary wall. Using a proteomic approach we detected 769 proteins in the isolated interstitial fluid, sixfold higher than in patient plasma. We conclude that the isolated fluid represents undiluted interstitial fluid and thus a subproteome with high concentration of locally secreted proteins that may be detected in plasma for diagnostic, therapeutic and prognostic monitoring by targeted methods.

A qualitative and quantitative proteomic study of human microdialysate and the cutaneous response to injury

The extracellular fluid space is the site of intercellular communication and represents an important source of mediators that can shed light on the parenchymal environment. Sampling of this compartment using continuous microdialysis allows assessment of the temporal changes in extracellular mediators involved in tissue homeostasis and disease processes. However, novel biomarker identification is limited by the current need to utilize specific, targeted molecular assays. The aim of our study was to explore the use of qualitative and quantitative proteomic approaches to define the protein content of dermal dialysate. Timed dermal dialysate samples were collected from healthy human volunteers for 5 h following probe insertion, using a 3,000-kDa MWCO membrane perfused at a rate of 3 μl/min. Dialysate proteins were identified using GeLC-MS/MS and iTRAQ approaches and functions assigned according to the Gene Ontology classification system. More than 80 proteins (size range 11-516 kDa) originating from both extracellular and intracellular fluid space were identified using the qualitative approach of GeLC-MS/MS. Quantitative iTRAQ data were obtained for 27 proteins with relative change ratios between consecutive timed samples showing changes of >1.5-fold. Interstitial proteins can be identified and measured using shotgun proteomic techniques and changes detected during the acute inflammatory response. Our findings provide a platform from which to explore novel protein biomarkers and their modulation in health and disease.

Are cutaneous microdialysis cytokine findings supported by end point biopsy immunohistochemistry findings?

Insertion of a cutaneous microdialysis catheter into normal dermis has been shown to induce the production of IL1b, IL6 and IL8 in an innate response to minimal trauma. In the present study, skin biopsy for immunohistochemistry has been performed at the site of the microdialysis catheter to compare the findings with that of the microdialysis findings 24 h after insertion. Of the three named cytokines, concordance between the two investigated technologies was highest for IL8 (100%) followed by IL6 (70%) and IL1b (50%). For seven other pro-inflammatory and T cell-relevant cytokines studied, concordance ranged between 50% and 80%. The total number of positive (microdialysis or immunofluorescence) findings was similar between the two methodologies. Technical and biological phenomenon can explain the differences. We conclude that both methodologies illustrate important features of tissue biology and that a combination of the two methods in clinical research can provide the chronology of soluble mediator participation and the more classic, but also more invasive, biopsy-based methodology at a point which constitutes the end of the observation period. We conclude further that at the 24-h time period here studied, microdialysis catheters are still functional and thus capable of producing relevant data which can be corroborated and extended by the "end point biopsy".

Early osteoarthritis and microdialysis: a novel in vivo approach for measurements of biochemical markers in the perisynovium and intraarticularly

The microdialysis technique was evaluated as a possible method to obtain local measurements of biochemical markers from knee joints with degenerative changes. Seven patients scheduled for arthroscopy of the knee due to minor to moderate degenerative changes had microdialysis catheters inserted under ultrasonographic guidance, intraarticularly and in the synovium-close tissue. Catheters were perfused at a rate of 2 μl/min for approximately 100 min with a Ringer solution containing radioactively labeled glucose, and the positions of the catheters were later visualized during arthroscopy. All intraarticular catheters and 6/7 subsynovial catheters were positioned correctly. Relative recovery (RR) was intraarticularly 0.64 ± 0.02 (mean ± SEM) and synovium-close 0.54 ± 0.06. Mean values of cartilage oligomeric matrix protein (COMP), aggrecan and glucosyl-galactosyl-pyridinoline in the intraarticular dialysates were 18.1 ± 7.0 U/l, 243.6 ± 108.6 ng/ml and 108.0 ± 29.0 pmol/ml, respectively. COMP and glucosyl-galactosyl-pyridinoline concentrations were significantly higher intraarticularly compared to perisynovial tissue (P < 0.05), whereas for aggrecan, no significant difference was found (P = 0.06). The microdialysis method can be used for intraarticular and subsynovial determination of metabolites in human knees at these sites. The present methodology displays a potential for future studies of simultaneous biochemical changes within and around joints.

Microdialysis for assessing intratumoral drug disposition in brain cancers: a tool for rational drug development

IMPORTANCE OF THE FIELD

many promising targeted agents and combination therapies are being investigated for brain cancer. However, the results from recent clinical trials have been disappointing. A better understanding of the disposition of drug in the brain early in drug development would facilitate appropriate channeling of new drugs into brain cancer clinical trials.

AREAS COVERED IN THIS REVIEW

barriers to successful drug activity against brain cancer and issues affecting intratumoral drug concentrations are reviewed. The use of the microdialysis technique for extracellular fluid (ECF) sampling and its application to drug distribution studies in brain are reviewed using published literature from 1995 to the present. The benefits and limitations of microdialysis for performing neuorpharmacokinetic (nPK) and neuropharmacodynamic (nPD) studies are discussed.

WHAT THE READER WILL GAIN

the reader will gain an appreciation of the challenges involved in identifying agents likely to have efficacy in brain cancer, an understanding of the general principles of microdialysis, and the power and limitations of using this technique in early drug development for brain cancer therapies.

TAKE HOME MESSAGE

a major factor preventing efficacy of anti-brain cancer drugs is limited access to tumor. Intracerebral microdialysis allows sampling of drug in the brain ECF. The resulting nPK/nPD data can aid in the rational selection of drugs for investigation in brain tumor clinical trials.

Methodological aspects on microdialysis protein sampling and quantification in biological fluids: an in vitro study on human ventricular CSF

There is growing interest in sampling of protein biomarkers from the interstitial compartment of the brain and other organs using high molecular cutoff membrane microdialysis (MD) catheters. However, recent data suggest that protein sampling across such MD membranes is a highly complex process that needs to be further studied. Here, we report three major improvements for microdialysis sampling of proteins in complex biological matrixes. The improvements in this in vitro study using human ventricular cerebrospinal fluid as the sample matrix include increased fluid recovery control, decreased protein adsorption on the microdialysis membrane and materials, and novel quantitative mass spectrometry analysis. Dextrans in different concentrations and sizes were added to the perfusion fluid. It was found that dextrans with molecular mass 250 and 500 kDa provided a fluid recovery close to 100%. An improved fluid recovery precision could be obtained by self-assembly triblock polymer surface modification of the MD catheters. The modified catheters also delivered a significantly increased extraction efficiency for some of the investigated proteins. The final improvement was to analyze the dialysates with isobaric tagged (iTRAQ) proteomics, followed by tandem mass spectrometric analysis. By using this technique, 48 proteins could be quantified and analyzed with respect to their extraction efficiencies. The novel aspects of microdialysis protein sampling, detection, and quantification in biological fluids presented in this study should be considered as a first step toward better understanding and handling of the challenges associated with microdialysis sampling of proteins. The next step is to optimize the developed methodology in vivo.

Interstitial fluid: the overlooked component of the tumor microenvironment?

Background

The interstitium, situated between the blood and lymph vessels and the cells, consists of a solid or matrix phase and a fluid phase, together constituting the tissue microenvironment. Here we focus on the interstitial fluid phase of tumors, i.e., the fluid bathing the tumor and stromal cells. Novel knowledge on this compartment may provide important insight into how tumors develop and how they respond to therapy.

Results

We discuss available techniques for interstitial fluid isolation and implications of recent findings with respect to transcapillary fluid balance and uptake of macromolecular therapeutic agents. By the development of new methods it is emerging that local gradients exist in signaling substances from neoplastic tissue to plasma. Such gradients may provide new insight into the biology of tumors and mechanistic aspects linked to therapy. The emergence of sensitive proteomic technologies has made the interstitial fluid compartment in general and that of tumors in particular a highly valuable source for tissue-specific proteins that may serve as biomarker candidates. Potential biomarkers will appear locally at high concentrations in the tissue of interest and will eventually appear in the plasma, where they are diluted.

Conclusions

Access to fluid that reliably reflects the local microenvironment enables us to identify substances that can be used in early detection and monitoring of disease.

Effect of atorvastatin and fenofibric acid on adipokine release from visceral and subcutaneous adipose tissue of patients with mixed dyslipidemia and normolipidemic subjects

Because of methodological limitations and conflicting results of studies conducted thus far, the possible involvement of human adipose tissue in pleiotropic effects of statins and fibrates requires better understanding. Samples of visceral and subcutaneous adipose tissue obtained from 23 mixed dyslipidemic patients and 23 normolipidemic subjects were treated in vitro for 48 h with atorvastatin, fenofibric acid or both these agents. Visceral and subcutaneous fat of mixed dyslipidemic patients released more leptin, resistin, interleukin-6, tumor necrosis factor alpha (TNFalpha and plasminogen activator inhibitor-1 (PAI-1), and less adiponectin than respective adipose tissue of patients without lipid abnormalities. In both groups of patients, visceral and subcutaneous tissue varied in the amount of secreted adipokines. In dyslipidemic patients both drugs administered alone affected adipose tissue adiponectin and resistin secretion. Additionally, atorvastatin decreased PAI-1 while fenofibric acid reduced leptin release. A combined administration of atorvastatin and fenofibric acid changed the release of all studied markers by visceral fat but did not affect interleukin-6 and TNFalpha release by subcutaneous tissue. In normolipidemic subjects the effect on adipokine release was more pronounced in visceral fat, in which it was strongest if the drugs were given together. Adipose tissue hormonal activity differs between mixed dyslipidemic and normolipidemic patients and between visceral and subcutaneous adipose tissue. Atorvastatin and fenofibrate exhibit their pleiotropic effects in part by changing the adipokine release by human adipose tissue, regardless of its origin. These effects are stronger in patients with mixed dyslipidemia and are particularly pronounced if atorvastatin and fenofibric acid are given together.

Proteomics of brain extracellular fluid (ECF) and cerebrospinal fluid (CSF)

Mass spectrometry has become the gold standard for the identification of proteins in proteomics. In this review, I will discuss the available literature on proteomic experiments that analyze human cerebrospinal fluid (CSF) and brain extracellular fluid (ECF), mostly obtained by cerebral microdialysis. Both materials are of high diagnostic value in clinical neurology, for example, in cerebrovascular disorders like stroke, neurodegenerative diseases like Alzheimer's Disease, Parkinson's Disease, amyotrophic lateral sclerosis (ALS), traumatic brain injury and cerebral infectious and inflammatory disease, such as multiple sclerosis. Moreover, there are standard procedures for sampling. In a number of studies in recent years, biomarkers have been proposed in CSF and ECF for improved diagnosis or to control therapy, based on proteomics and mass spectrometry. I will also discuss the needs for a transition of research-based experimental screening with mass spectrometry to fast and reliable diagnostic instrumentation for clinical use.

Affinity-based microdialysis sampling using heparin for in vitro collection of human cytokines

Microdialysis sampling is a widely used method to sample from complex biological matrices. Cytokines are important signaling proteins that are typically recovered with low relative recovery values during microdialysis sampling. Heparin was included in the microdialysis perfusion fluid as an affinity agent to increase in vitro recovery of different cytokines through polyethersulfone (PES) microdialysis membranes with 100 kDa molecular weight cutoff. No change in fluid volumes collected from the microdialysis probes occurred when heparin was included in the perfusion fluid up to concentrations of 10 microM. The loss of heparin (10 microM) across the dialysis membrane was minimal (2.7+/-0.9%, n=3). Additionally, heparin at these concentrations did not interfere with the cytokine immunoassays. The control and heparin-enhanced relative recoveries for five human cytokines using 0.1 microM heparin in the microdialysis perfusion fluid flowing at 0.5 microL min(-1) were (n=3): interleukin-4 (IL-4), 4.2+/-0.5% and 7.2+/-3.1%; interleukin-6 (IL-6), 1.4+/-0.8% and 3.6+/-1.3%; interleukin-7 (IL-7), 1.3+/-0.8% and 4.8+/-1.8%; monocyte chemoattractant protein-1 (MCP-1), 9.0+/-1.6% and 19.5+/-2.7%; and tumor necrosis factor-alpha (TNF-alpha), 7.4+/-1.3% and 16.9+/-1.6%, respectively. Heparin increased the microdialysis sampling relative recovery of several human cytokines in vitro.

Improving the recovery of S100B protein in cerebral microdialysis: implications for multimodal monitoring in neurocritical care

INTRODUCTION

Cerebral microdialysis is an established research tool that is used by an increasing number of neurocritical care units as a component of bedside multimodality monitoring. Body fluid biomarkers are an emerging tool for the assessment of brain injury. The correct interpretation of body fluid biomarker levels depends on the degree of recovery, i.e. relative recovery and the accuracy of the analytical technique.

METHODS

In vitro recovery experiments were performed on 100mL volumes of cerebrospinal fluid and solutions of S100B, glucose, lactate and pyruvate comparing relative recoveries using commercially available 20 kDa (CMA70) and 100 kDa (CMA71) microdialysis catheters. We also compared the CMA 600 microdialysis analyzer with a YSI 2003 STAT Plus analyzer for glucose and lactate to determine its reliability.

RESULTS

Significantly, we demonstrate the improved recovery of the protein S100B using a larger molecular weight (MW) cut-off catheter (20 kDa range: 0.1-9%; 100 kDa range: 1.7-18.3%) while maintaining comparable performance for the conventional markers glucose, lactate and pyruvate. Additionally we found that the CMA 600 analyzer may be prone to overestimation of lactate readings at higher concentration with implications for clinical decision-making.

CONCLUSION

Our data demonstrates that the 100 kDa MW cut-off catheter allows for the improved recovery of macromolecules in cerebral microdialysis research while maintaining the value of existing MD data for routine clinical use.

Discovering new bioactive neuropeptides in the striatum secretome using in vivo microdialysis and versatile proteomics

The striatum, a major component of the brain basal nuclei, is central for planning and executing voluntary movements and undergoes lesions in neurodegenerative disorders such as Huntington disease. To perform highly integrated tasks, the striatum relies on a complex network of communication within and between brain regions with a key role devoted to secreted molecules. To characterize the rat striatum secretome, we combined in vivo microdialysis together with proteomics analysis of trypsin digests and peptidomics studies of native fragments. This versatile approach, carried out using different microdialysis probes and mass spectrometer devices, allowed evidencing with high confidence the expression of 88 proteins and 100 processed peptides. Their secretory pathways were predicted by in silico analysis. Whereas high molecular weight proteins were mainly secreted by the classical mode (94%), low molecular weight proteins equally used classical and non-classical modes (53 and 47%, respectively). In addition, our results suggested alternative secretion mechanisms not predicted by bioinformatics tools. Based on spectrum counting, we performed a relative quantification of secreted proteins and peptides in both basal and neuronal depolarization conditions. This allowed detecting a series of neuropeptide precursors and a 6-fold increase for neurosecretory protein VGF and proenkephalin (PENK) levels. A focused investigation and a long peptide experiment led to the identification of new secreted non-opioid PENK peptides, referred to as PENK 114-133, PENK 239-260, and PENK 143-185. Moreover we showed that injecting synthetic PENK 114-133 and PENK 239-260 into the striatum robustly increased glutamate release in this region. Thus, the combination of microdialysis and versatile proteomics methods shed new light on the secreted protein repertoire and evidenced novel neuropeptide transmitters.

Cytokines are produced locally by myocytes in rat skeletal muscle during endotoxemia

Cytokines act as chemical mediators during the inflammatory process. Measurements of cytokine levels in tissue have previously been performed in homogenized tissue, but the true concentrations in native interstitial fluid (ISF), i.e., the compartment where cytokines exert their biologically active role, have remained unknown. The role of skeletal muscle myocytes as a source for cytokines during endotoxemia was explored by collecting muscle ISF using a wick method, and the levels of 14 cytokines in ISF and plasma were related to the corresponding changes in mRNA levels to reveal any potential discrepancies between gene expression and protein release of cytokines to ISF. The majority of investigated cytokines were elevated in muscle ISF during endotoxemia, and an analysis of cytokine mRNA levels revealed consistency between gene expression and protein release. The elevated cytokine level in ISF, in addition to elevated gene expression in muscle, indicated a significant local production and release of several proinflammatory cytokines and chemokines within skeletal muscle tissue during endotoxemia. Immunohistochemistry revealed that myocytes constituted a significant source of IL-1beta and TNF-alpha production during endotoxemia, whereas the contribution from inflammatory cells i.e., leukocytes, was found to be less significant. Muscle cells apparently constitute an important source of several different cytokines during endotoxemia, governing the level in the muscle microenvironment, and are likely to contribute significantly to cytokine levels in plasma.

Proteomics of human cerebral microdialysate: From detection of biomarkers to clinical application

Cerebral microdialysis is applied in clinical neurology and neurosurgery as monitoring tool in patients to evaluate the progression of severe diseases, such as stroke or trauma. Besides small molecules, e.g. metabolites and neurotransmitters, also the macromolecules, such as proteins and larger chemical compounds cross the dialysis membrane of the catheters implanted into the human brain parenchyma. Microdialysis can be used to extract molecules from the extracellular space of the brain in vivo, but additionally to deliver drugs, since the exchange is dependent on concentration gradients. Cerebral microdialysis may also be useful in the prediction of the clinical onset of symptoms, based on changes in the composition of pre-symptomatic microdialysate. For example, symptomatic vasospasm, which is a complication after subarachnoid hemorrhage, may be predicted by the combination of cerebral microdialysis and a proteomics approach. We will introduce the basic concepts of cerebral microdialysis, discuss possible clinical applications, and evaluate the application of proteomic approaches. With regard to technological aspects, we describe two-dimensional gel electrophoresis, high-pressure liquid chromatography, and mass spectrometry. With regard to clinical aspects, we discuss ethics, feasibility, time-course, and therapeutic options. In conclusion, proteomics of cerebral microdialysate may be used for diagnosis, disease monitoring, and therapeutic intervention of neurological patients.

Multimodality monitoring in neurocritical care

Multimodality monitoring of cerebral physiology encompasses the application of different monitoring techniques and integration of several measured physiologic and biochemical variables into assessment of brain metabolism, structure, perfusion, and oxygenation status. Novel monitoring techniques include transcranial Doppler ultrasonography, neuroimaging, intracranial pressure, cerebral perfusion, and cerebral blood flow monitors, brain tissue oxygen tension monitoring, microdialysis, evoked potentials, and continuous electroencephalogram. Multimodality monitoring enables immediate detection and prevention of acute neurologic injury as well as appropriate intervention based on patients' individual disease states in the neurocritical care unit. Real-time analysis of cerebral physiologic, metabolic, and cardiovascular parameters simultaneously has broadened knowledge about complex brain pathophysiology and cerebral hemodynamics. Integration of this information allows for more precise diagnosis and optimization of management of patients with brain injury.

Estimation of proinflammatory biomarkers of skin irritation by dermal microdialysis following exposure with irritant chemicals

The aim of the present study was to quantify the release of proinflammatory biomarkers by dermal microdialysis after topical exposure with irritant chemicals, Jet fuel (JP-8) and xylene in rat skin. Occlusive dermal exposure (2h) was carried out with 230microl of JP-8 or xylene using Hill top chambers((R)). Linear microdialysis probes (10mm) were inserted in the dermis under urethane anesthesia. The dialysis fluid was pumped at a flow rate of 2microl/min and the dialysate was collected for 7h following probe insertion. The expression of substance P (SP), calcitonin-gene related peptide (CGRP) and prostaglandin E(2) (PGE(2)) in the dialysate following microdialysis was measured by enzyme immunoassay (EIA). The effect of pretreatment with an SP antagonist (SR-140333) and a PGE(2) inhibitor (celecoxib), 6 and 18h before the application of JP-8 was also assessed to further establish the sensitivity of the microdialysis set up. On similar lines, untreated and capsaicin treated control experiments were performed to compare with the SP release following JP-8 treatment. Further, we also investigated the SP release following topical application of xylene. The mean concentrations of SP after the application of JP-8 (90.01+/-3.31) and 3h after its removal (58.66+/-9.36) indicated that JP-8 induced significantly higher release of SP as compared to the baseline value (P<0.05). The release of SP following JP-8 treatment (58.66+/-9.36pg/ml) was comparable to capsaicin (58.18+/-11.29pg/ml). JP-8 exposure resulted in a significant increase (P<0.001) in PGE(2) levels over the baseline control at the end of 1 and 2h of exposure. JP-8 treatment also produced significant increase (P<0.001) in PGE(2) levels as compared to the untreated control during occlusion and 1h following its removal. There was a significant drop (P<0.05) in the PGE(2) levels by the end of 3h following exposure. Pretreatment with SR-140333 and celecoxib significantly reduced (P<0.05) SP and PGE(2) release induced by JP-8. The mean concentrations of SP following xylene exposure (25.50+/-8.80pg/ml) and 3h after its removal (34.37+/-5.61pg/ml) indicated its skin irritation potential. Unlike JP-8, xylene produced a significant increase in SP release only after the removal of occlusion. Pretreatment with SR-140333 significantly blocked the xylene induced SP release. CGRP was not detected in any of the samples. This study demonstrates that dermal microdialysis can be used to quantify skin irritation potential of JP-8 and related irritant chemicals.

What can microdialysis tell us about the temporal and spatial generation of cytokines in allergen-induced responses in human skin in vivo?

This study examined the suitability of microdialysis to assess the time course of cytokine generation from discrete sites within the skin following intradermal injection of allergen. Cytokines were recovered using two microdialysis probes, one close to the point of allergen injection and the other 1 cm away but within the area of the late-phase induration. Skin biopsies taken at both sites were stained immunocytochemically to investigate possible relationships between cytokine generation, expression of adhesion molecules, and recruitment of neutrophils and eosinophils during the late-phase allergic response. The cytokine response to probe insertion was assessed using a single probe in the opposite arm (control). At baseline, microdialysate contained low levels of IL-1alpha, IL-5, IL-8, IL-12, GM-CSF, and TNFalpha (n=27-33). At control sites, this was followed by increases in IL-6 and IL-8 at 3 and 6 hours. Allergen increased TNFalpha levels in 3/11 individuals within 30 minutes at the injection site. Levels of IL-6 and IL-8 rose rapidly and were significantly greater (P<0.05) than that of controls at 3 and 6 hours at both injection and distant sites. Adhesion molecule expression and leukocyte infiltration were elevated only at the allergen injection site, suggesting a complex relationship between cytokine generation and cellular events in allergic inflammation. In conclusion, microdialysis can be used to distinguish temporal and spatial changes in protein profiles in the skin. Furthermore, when used in conjunction with skin biopsies, it provides novel information about the mechanisms of dermal inflammation.

Microdialysis sampling membrane performance during in vitro macromolecule collection

Microdialysis sampling is well-established for sampling small molecules. Recently, there has been an increased interest toward collecting macromolecules using microdialysis sampling. In this work, fluorescein isothiocyanate-labeled dextrans (FITC-dextrans) with molecular weight between 10 and 70 kDa were chosen as representative molecules to study analyte mass transport properties during microdialysis sampling using different lengths (2 and 10 mm) of 100-kDa MWCO polyethersulfone membranes. Experiments were performed in both well-stirred and quiescent phosphate-buffered saline solutions as well as in a 0.3% agar solution. Different fundamental parameters affecting microdialysis sampling of macromolecules, including effective membrane diffusion coefficients, were evaluated. The applicability of the most-often-cited Bungay et al. mass transfer model was compared to experimental data for the FITC-dextrans. For the larger macromolecules, the membrane provides a significant mass transport resistance most likely caused by hindered diffusion. These experimental aspects that are critical to microdialysis sampling of macromolecules are presented.

Interleukin-6 Collection through Long-Term Implanted Microdialysis Sampling Probes in Rat Subcutaneous Space

Microdialysis sampling is a method that has promise for collection of important signaling proteins such as cytokines that are involved in every aspect of the immune response. The objective of this study was to determine the role of membrane and tissue alterations on the reduction of interleukin-6 (IL-6) relative recovery of microdialysis probes implanted for 3 and 7 days versus probes implanted on day 0 (acute implant or control probe). Lipopolysaccharide (LPS), a bacterial endotoxin, was used to elicit IL-6 production in the animals. Within the same animal, the recovery of IL-6 through control probes implanted the day of sample collection was compared to the 3- or 7-day implanted probes. Two hours post-LPS administration, the IL-6 concentrations obtained from either the 3-day or 7-day implanted probe were reduced by more than 8-fold when compared to the control probe. The IL-6 concentrations obtained for the 3-day versus control probes 2-h post-LPS injection were 730 +/- 310 and 6440 +/- 1550 pg/mL (mean +/- SD, n = 3), respectively. For the 7-day implant, the IL-6 concentration in the dialysis probe obtained at 2-h post-LPS injection was 990 +/- 590 versus 5520 +/- 1430 pg/mL (mean +/- SD, n = 3) for the control. In vitro recovery experiments and scanning electron microscopy images combined with the in vivo data suggest that the decreased IL-6 content in the dialysate was caused principally by tissue alterations or tissue encapsulation rather than membrane blockage with biological components (membrane biofouling).

Future of brain support: multimodality monitoring

Multimodality monitoring of cerebral physiology encompasses the application of different monitoring techniques and integration of several measured physiological and biochemical variables into the assessment of brain metabolism, structure, perfusion and oxygenation status, in addition to clinical evaluation. Novel monitoring techniques include transcranial Doppler ultrasonography, neuroimaging, intracranial pressure, cerebral perfusion and cerebral blood flow monitors, brain tissue oxygen tension monitoring, microdialysis, evoked potentials and continuous electroencephalography. Multimodality monitoring enables the immediate detection and prevention of acute neurological events, as well as appropriate intervention based on a patient’s individual disease state in the neurocritical care unit. Simultaneous real-time analysis of cerebral physiological, metabolic and cardiovascular parameters has broadened knowledge regarding complex brain pathophysiology and cerebral hemodynamics. Integration of this information allows for a more precise diagnosis and optimization of management of patients with brain injury.