Leveraging the power of mass spectrometry to unravel complex brain pathologies

The human brain is generally considered to be the most complex organ in the human body, and among the most difficult to study due to its (i) immensely complex construction, (ii) protected location in the skull, (iii) delicate nature, and (iv) essential function. Nevertheless, neuroscience has made tremendous advances in the discovery of various molecular features critical to brain function using classical analytical techniques, but also, more recently, with rapidly developing imaging technologies. Mass spectrometry is playing an increasing role in basic to translational research as well as, the ultimate goal, patient care. Accordingly, this special issue on the brain contains papers on a broad selection of topics, ranging from forward-leaning studies on neurological diseases, to applications targeting direct patient care, including metabolic diseases and analysis of psychoactive drugs.

Since the brain and nervous system are involved, and likewise affected, in many pathogenic processes, there is a need to create better tools for sampling and analysis of neurological tissue. As an extra challenge, the targeted molecules are often present in extremely low concentrations (ng-pg/mL or picomol/L) and only in selected regions or organelles. In my research group, we have been struggling to unravel the unexplored mysteries within a number of complex, hard-to-reach and hard-to-collect brain- and nerve-related tissues and fluids, such as the proteomes of human cerebrospinal fluid (CSF), the perilymph and the aqueous humor of the human eye. We are currently focused on neurodegenerative processes in Alzheimer’s disease (AD), Parkinson’s disease (PD) and Amyotrophic Lateral Sclerosis (ALS), while we are exploring neuroinflammatory processes in Multiple Sclerosis (MS) and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS), as well as the degenerative processes in muscle wasting disorders, Traumatic Brain Injury (TBI) and Pseudo Exfoliation Syndrome (PEX) in the eye [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Mass spectrometry – most often in combination of high resolution liquid-based separation and high resolution tandem mass spectrometry (i.e., LC–MSMS), – has given us the opportunity to push these clinical projects forward and develop our understanding of the complex pathophysiological processes involved.

It has been a great pleasure to be part of the process to collect papers for this special issue of Clinical Mass Spectrometry in brain-related applications. This collection clearly demonstrates the width of subject areas that can be facilitated and explored by the help of mass spectrometry. A clear majority of the included papers are forward-looking papers on neurodegenerative processes and the specific detection of peptides and proteins as biomarkers. In the paper “Association of PTHrP levels in CSF with Alzheimer’s disease biomarkers” by Mark M. Kushnir and colleagues [26], the authors describe a mass spectrometric method for the detection of the parathyroid hormone-related protein (PTHrP) in CSF and they evaluate the potential association between CSF concentrations of PTHrP with the core CSF biomarkers (total tau (T-tau), phospho-tau (P-tau) and amyloid-beta 1–42 (Aβ42)) of Alzheimer’s disease (AD). PTHrP is involved in intracellular calcium regulation, neural cell proliferation and synaptic transmission and could potentially act as a biomarker of brain pathophysiology. The data show that the concentrations of PTHrP were positively correlated with concentrations of T-tau and P-tau, suggesting an association with neuronal secretion and function, which is known to be reduced upon progression to AD pathology. Their data also suggest the potential utility of the Aβ42/PTHrP ratio in assessment of AD progression – something of potentially high clinical relevance.

In our own paper “A simplified and sensitive immunoprecipitation mass spectrometry protocol for the analysis of amyloid-beta peptides in brain tissue” by Bernhard C. Richard and colleagues [27], we report a simplified and detailed protocol for robust immunoprecipitation of Aβ in brain tissue prior to mass spectrometric detection using MALDI-TOF MS. Preclinical trials commonly employ amyloid-beta (Aβ) peptide signatures as a read-out to evaluate new therapeutics, but often employ rather complicated and not always robust methodologies. By using transgenic mice models we have demonstrated that a fairly simple and straightforward approach can provide accurate data with good precision. Due to the simplicity of the demonstrated method we think that both clinical diagnostic applications and further biomedical research could benefit from this approach.

In the paper “Use of the tau protein-to-peptide ratio in CSF to improve diagnostic classification of Alzheimer’s disease” by Karl Hansson and colleagues [28], the authors describes the use of a liquid chromatography-mass spectrometry (LC–MS) method to study an endogenous tau peptide, 175–194 (phosphorylated and non-phosphorylated), in CSF in two independent clinical cohorts. Their data show that the diagnostic value of Tau proteins can be improved by the normalization of the Tau ELISA measurements to the concentrations of the endogenous peptides, possibly because these endogenous tau peptides serve to normalize for physiological, and disease-independent, secretion of tau from neurons to the extracellular space and the CSF. This could become very useful in the biochemical-based diagnosis of AD.

In the paper “Detection and characterization of TDP-43 in human cells and tissues by multiple reaction monitoring mass spectrometry” by Taylor D. Pobran and colleagues [29], the focus is on the analysis of the transactive response DNA-binding protein 43 kDa (TDP-43) by a targeted multiplex mass spectrometric method. This protein is known to be a highly conserved and widely expressed protein in human tissues and it has been found to regulate nucleic acid processing. In frontotemporal dementia and amyotrophic lateral sclerosis, TDP-43 forms insoluble cell toxic aggregates in central nervous tissues and it is, therefore, important to have a method that is specific and sensitive enough for the characterisation of those molecular species in complex biological samples.

Over the years, mass spectrometry has also been widely applied in the diagnosis of inborn errors and neonatal screening to identify factors that can disturb the natural processes of neuronal development. In the paper “Mass spectrometric quantification of plasma glycosphingolipids in human GM3 ganglioside deficiency” by Kazuhiro Aoki and colleagues [30], a quantitative glycolipidomics mass spectrometric method has been developed. Among the Amish communities of North America, biallelic mutations in certain genes will eliminate the synthesis of GM3 and its derivative downstream a- and b-series gangliosides. Systemic ganglioside deficiency is associated with infantile onset psychomotor retardation, slow brain growth, intractable epilepsy, deafness, and cortical visual impairment. Their method represents a useful new strategy to diagnose and monitor GSL disorders in humans.

Yet another area where mass spectrometry has established itself as the gold standard for specific quantitative measurements is within drug analysis. Mass spectrometry can be applied to quality control, stability and bioavailability tests in drug development, as well as the determination of metabolic kinetics and compliancy. Screening for drugs of abuse and doping agents is nowadays, in a clear majority, relying on sensitive and selective mass spectrometric methods. In the paper “Detection of in utero exposure to cannabis in paired umbilical cord tissue and meconium by liquid chromatography-tandem mass spectrometry” by Triniti L. Jensen and colleagues [31], the authors have presented a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for the detection and quantification of four cannabinoid analytes in two neonatal matrices (umbilical cord tissue and meconium). Understanding levels of in utero drug exposure is important to properly customize the immediate, as well as ongoing, medical and social management needs of affected newborns, as well as studying correlations with clinical and social outcomes over a longer perspective.

In the paper “Compliance testing of patients in ADHD treatment with lisdexamphetamine (Elvanse) using oral fluid as specimen” by Michael Böttcher and colleagues [32], the authors present a UPLC-MS/MS method for general oral fluid drug testing, as exemplified by lisdexamphetamine and amphetamine quantification and chiral analysis of amphetamine. Pharmacological treatment of the attention-deficit/hyperactivity disorder (ADHD) includes use of the psychostimulant, amphetamine, and non-adherence to medication is a well-documented problem in ADHD treatment and a cause of treatment failure. The presented method facilitates compliancy testing for these patients.

In the paper “A semi-automated, isotope-dilution high-resolution mass spectrometry assay for therapeutic drug monitoring of antidepressants” by Johanna M. Lindner and colleagues [33], the authors present a method using high-resolution mass spectrometry (HRMS) to test the suitability of this approach for quantitative therapeutic drug monitoring (TDM) of antidepressants. Also in this case it is important to ensure compliance and to rule out pharmacokinetic abnormalities. The applicability of HRMS instruments to TDM, as an alternative to other mass spectrometers, was clearly demonstrated.

Finally, I would like to thank all of the contributors for sharing their latest research for this special issue, helping us to promote the usefulness of mass spectrometry in clinical applications. Warm thanks to the reviewers for taking the time to assess these contributions and provide helpful and insightful comments. I would also like to take the opportunity to thank the editorial board at Clinical Mass Spectrometry, especially Professor Alan Rockwood and Dr. Chris Herold, for giving me the opportunity to assemble this special issue with their great support. I hope that this special issue will encourage others to take any opportunity to explore how mass spectrometry could be a useful tool for, not only their research, but alsofor clinical applications – not only in neuroscience, but in any matter that needs in-depth qualitative and quantitative molecular understanding. Furthermore, I encourage you to submit high quality papers to Clinical Mass Spectrometry to help drive the continuous development of mass spectrometry in the clinical setting.

With best regards,

Jonas Bergquist