This study is the first to report on the pattern of protein adsorption to CMD membranes following implantation in human brain tissue. It is also the first to compare this pattern to that of CSF and microdialysate; two compartments commonly sampled for biomarker discovery. Our results offer insights into which proteins adsorb to the membranes and thus may not be present in the microdialysate or CSF samples.
In recent years CMD has been used as part of NCC monitoring and to study biomarkers of various neurological diseases in particular traumatic brain injury(18–20), gliomas(21, 22), ischemic stroke(1), intracerebral hemorrhage(23) and subarachnoid hemorrhage(24, 25). CMD sampling for subsequent proteomic biomarker analysis has advantages, such as capture of proteins near the site of origin in the extracellular space, and no effect of dilution as is the case in CSF or plasma(18). But there are well known challenges to the microdialysis method including its focal resolution, limited time resolution, small sample volume(26), a tendency to elicit an inflammatory response(27) and variations in relative recovery(28, 29), which is in part due to protein adsorption. Such protein adsorption on CMD membranes implanted in patients has been poorly described to date.
Ultrafiltration is another technical issue to consider when using high-molecular-weight cut-off CMD catheters (100 kDa). Ultrafiltration means that fluid escapes from the perfusate into the sampling tissue thereby changing the environment immediately surrounding the CMD catheter(2, 26, 30). To counteract ultrafiltration colloids such as albumin can be added to the perfusate as was clinical routine in our department during the study period. However, albumin is known to adsorb to membrane surfaces and may in turn affect the recovery of other proteins. This was indirectly shown in a previous study using CMD in a NCC setting where the addition of albumin to the perfusate was found to be necessary in order to extract amyloid-β (Aβ) in the microdialysate. The authors argue that this was presumably due to Aβ adhering to the CMD membrane and tubing in the absence of albumin(31). It is plausible that the addition of albumin to the perfusate also in the present study has affected the pattern of protein adsorption to the CMD membrane, and use of a different perfusate could result in a different pattern. Therefore the pattern of protein adsorption to CMD membranes cannot be assumed to be the same in different study protocols as protein adsorption will be affected by a number of factors, of which perfusate composition is one. This is why we recommend analyzing membrane adsorbed proteins when sampling macromolecules using CMD.
Modifications can be made to either the membrane or the perfusate in order to increase relative recovery of macro molecules. Modifications to the perfusate include the addition of antibodies with high affinity for a specific compound of interest which can increase its relative recovery as has been done with cytokines(32). This method, however, is not relevant when the aim is to sample all proteins present. Previous in vitro and preclinical studies show decreased protein adhesion to both CMD membrane and tubing following catheter modifications using triblock copolymers such as Pluronics 127®(6, 33). However, to date these modifications have only been used in vitro or in preclinical studies. Certain proteins of interest may display particular tendencies to adhere to surface materials, such as for example Aβ protein and modifications of membrane and tubing may be useful in studies focused on such particularly ‘sticky’ proteins. As this present study shows proteins have differing tendency to adsorb to the membrane. An awareness of this is recommended when using CMD for sampling macromolecules.
It has been suggested that adsorption of proteins to the membrane surface is the first step in a potential foreign body reaction elicited by catheter implantation(34). Such a foreign body reaction has not been fully characterized following implantation in human brain, however, several preclinical studies have shown an immediate traumatic response in brain tissue following CMD catheter implantation(12). Furthermore, one study using scanning electron microscopy demonstrated buildup of cellular debris on CMD membranes implanted in human brain tissue(30). This cellular debris is presumably preceded by protein adsorption to the membrane surface, and characterization of such protein adsorption can plausibly give more insights also into the foreign body reaction.
In a recent study of ICH patients, using a similar study design we found a difference in proteins expressed in the perihemorrhagic zone (PHZ) compared to seemingly normal cortex (SNX)(13). In light of the results of this present study it is likely that several proteins evaded detection by adsorbing to the CMD membranes. Plausibly, a paired catheter design involves a similar pattern of protein adsorption to both catheters, but this cannot be guaranteed. Factors such as pH, rate of diffusion, and brain tissue edema may differ locally in the tissue and thus protein adsorption to CMD membranes could be different in two catheters placed in different regions of the brain. This can only be adequately characterized by analyzing the membranes.
Limitations of the present study include the small sample size, which precludes any comparisons or correlation with clinical parameters such as outcome or medical or surgical complications. Furthermore, the present study cannot determine the effect of time on protein adsorption, and future studies should aim to elucidate the time course of protein adsorption to CMD membranes in brain tissue as this may cause relative recovery to change over time.
The present study is the first to characterize protein adsorption to CMD membranes after implantation in human brain tissue. Previous studies have described protein adhesion to membranes in animal studies or in vitro studies(7, 11). One such in vitro study described protein adsorption to two microdialysis membranes after dialysis of a sample matrix of ventricular CSF. The study showed that 50% of proteins found on the CMD membrane were not identified in the sample matrix pointing to methodological limitations. This study used on-surface tryptic digestion of proteins on the membrane prior to denaturation, followed by a bottom-up approach for protein identification using LC-MS/MS. Technical issues may explain the low overlap of proteins between the CMD membrane and the sample matrix. To avoid failure of proteins to desorb from the CMD membrane in the present study we employed a protocol previously determined by our group, used on membranes implanted in human muscle tissue (unpublished data), and calibrated for optimal protein elution.
By showing particular differences in the proteome profile of the three compartments, CMD membrane, microdialysate and CSF, our results emphasize the need to analyze protein adsorption to CMD membranes in addition to microdialysate protein content and highlight the need to interpret analysis results of proteins sampled by CMD with caution.