Evaluation of sample preparation methods for label-free quantitative profiling of salivary proteome

Salivary Proteome Insights: Evaluation of Saliva Preparation Methods in Mucopolysaccharidoses Research

Background: This research aimed to compare the traditional in-solution digestion (inSol) and solid-phase-enhanced sample preparation (SP3) methods for salivary proteomics, with a focus on identifying mucopolysaccharidosis (MPS)-relevant proteins. Methods: Saliva samples were processed under multiple analytical conditions, including two precipitation methods (methanol or incubation with trichloroacetic acid), paired with either Rapigest or 8M urea/2M thiourea (UT) solubilization buffers. Additionally, the SP3 method was directly applied to raw saliva without pre-processing. Proteome coverage, reproducibility, digestion efficiency, and gene function were assessed. Results: The inSol method consistently provided superior proteome coverage, with trichloroacetic acid precipitation and Rapigest buffer yielding 74 MPS-relevant proteins, compared to 40 with SP3 MeOH UT. Both methods showed high digestion efficiency, particularly with Rapigest buffer, achieving over 80% full cleavage across conditions. Functional analysis revealed broad similarities, with protocol-specific impacts on protein classes and cellular components. Conclusions: This study is the first to compare SP3 and in-solution digestion for salivary proteomics, emphasizing the importance of method selection to address matrix-specific challenges. The results highlight the robustness of inSol for comprehensive proteome profiling and SP3's potential for streamlined clinical workflows, offering valuable insights into optimizing salivary proteomics for biomarker discovery in MPS and other diseases.

Benchmarking low- and high-throughput protein cleanup and digestion methods for human fecal metaproteomics

The application of fecal metaproteomics to large-scale studies of the gut microbiota requires high-throughput analysis and standardized experimental protocols. Although high-throughput protein cleanup and digestion methods are increasingly used in shotgun proteomics, no studies have yet critically compared such protocols using human fecal samples. In this study, human fecal protein extracts were processed using several different protocols based on three main approaches: filter-aided sample preparation (FASP), solid-phase-enhanced sample preparation (SP3), and suspension trapping (S-Trap). These protocols were applied in both low-throughput (i.e., microtube-based) and high-throughput (i.e., microplate-based) formats, and the final peptide mixtures were analyzed by liquid chromatography coupled to high-resolution tandem mass spectrometry. The FASP-based methods and the combination of SP3 with in-StageTips (iST) yielded the best results in terms of the number of peptides identified through a database search against gut microbiome and human sequences. The efficiency of protein digestion, the ability to preserve hydrophobic peptides and high molecular weight proteins, and the reproducibility of the methods were also evaluated for the different protocols. Other relevant variables, including interindividual variability of stool, duration of protocols, and total costs, were considered and discussed. In conclusion, the data presented here can significantly contribute to the optimization and standardization of sample preparation protocols in human fecal metaproteomics. Furthermore, the promising results obtained with the high-throughput methods are expected to encourage the development of automated workflows and their application to large-scale gut microbiome studies.IMPORTANCEFecal metaproteomics is an experimental approach that allows the investigation of gut microbial functions, which are involved in many different physiological and pathological processes. Standardization and automation of sample preparation protocols in fecal metaproteomics are essential for its application in large-scale studies. Here, we comparatively evaluated different methods, available also in a high-throughput format, enabling two key steps of the metaproteomics analytical workflow (namely, protein cleanup and digestion). The results of our study provide critical information that may be useful for the optimization of metaproteomics experimental pipelines and their implementation in laboratory automation systems.

Proteome profiling of salivary small extracellular vesicles in glioblastoma patients

BACKGROUND

Extracellular vesicles (EVs) play a critical role in intercellular communication under physiological and pathological conditions, including cancer. EVs cargo reflects their cell of origin, suggesting their utility as biomarkers. EVs are detected in several biofluids, and their ability to cross the blood-brain barrier has highlighted their potential as prognostic and diagnostic biomarkers in gliomas, including glioblastoma (GBM). Studies have demonstrated the potential clinical utility of plasma-derived EVs in glioma. However, little is known about the clinical utility of saliva-derived EVs in GBM.

METHODS

Small EVs were isolated from whole mouth saliva of GBM patients pre- and postoperatively. Isolation was performed using differential centrifugation and/or ultracentrifugation. EVs were characterized by concentration, size, morphology, and EVs cell-surface protein markers. Protein cargo in EVs was profiled using mass spectrometry.

RESULTS

There were no statistically significant differences in size and concentration of EVs derived from pre- and post GBM patients' saliva samples. A higher number of proteins were detected in preoperative samples compared to postoperative samples. The authors found four highly abundant proteins (aldolase A, 14-3-3 protein ε, enoyl CoA hydratase 1, and transmembrane protease serine 11B) in preoperative saliva samples from GBM patients with poor outcomes. Functional enrichment analysis of pre- and postoperative saliva samples showed significant enrichment of several pathways, including those related to the immune system, cell cycle and programmed cell death.

CONCLUSIONS

This study, for the first time, demonstrates the feasibility of isolating and characterizing small EVs from pre- and postoperative saliva samples from GBM patients. Preliminary findings encourage further large cohort validation studies on salivary small EVs to evaluate prognosis in GBM.

Influence of different sample preparation strategies on hypothesis-driven shotgun proteomic analysis of human saliva

This research aimed to find an efficient and repeatable bottom-up proteolytic strategy to process the unstimulated human saliva. The focus is on monitoring immune system activation via the cytokine and interleukin signaling pathways. Carbohydrate metabolism is also being studied as a possible trigger of inflammation and joint damage in the context of the diagnostic procedure of temporomandibular joint disorder. The preparation of clean peptide mixtures for liquid chromatography–mass spectrometry analysis was performed considering different aspects of sample preparation: the filter-aided sample preparation (FASP) with different loadings of salivary proteins, the unfractionated saliva, amylase-depleted, and amylase-enriched salivary fractions. To optimize the efficiency of the FASP method, the protocols with the digestion in the presence of 80% acetonitrile and one-step digestion in the presence of 80% acetonitrile were used, omitting protein reduction and alkylation. The digestion procedures were repeated in the standard in-solution mode. Alternatively, the temperature of 24 and 37°C was examined during the trypsin digestion. DyNet analysis of the hierarchical networks of Gene Ontology terms corresponding to each sample preparation method for the bottom-up assay revealed the wide variability in protein properties. The method can easily be tailored to the specific samples and groups of proteins to be examined.

96DRA-urine: A high throughput sample preparation method for urinary proteome analysis

Mass spectrometry (MS)-based urinary proteomics is increasingly used for clinical research. A critical step in urinary proteomic analysis comprises the implementation of a reliable sample preparation method with high yields of peptides and proteins. In this study, we developed a urinary sample preparation method, DRA-Urine (Direct reduction/alkylation in urine), which urinary proteins were directly reduced/alkylated in urine, and then precipitated by acetone, washed and digestion on an ultrafilter unit. The qualitative and quantitative comparison of different urinary sample preparation methods (in-solution methods and ultrafilter-assisted methods) showed that DRA-Urine could achieve better results. Adapting DRA-Urine protocol to a 96-well format, namely 96DRA-Urine, shortened the time for buffer change and improved sample preparation throughput. The results showed that 96DRA-Urine displayed similar proteomic performance to DRA-Urine. Finally, the 96DRA-Urine method was used in a label-free, small pilot biomarker discovery analysis for differential urinary proteome analysis of bladder cancer urine. The results showed that urinary proteins could differentiate bladder cancer (BCa) patients from healthy controls and distinguish high-grade BCa from low-grade BCa with area under the curve (AUC) values of 0.972 and 0.847, respectively. Consequently, the 96DRA-Urine method might be a high-throughput method for preparing body fluid samples used in clinical research but needs to be further verified.

Proteomics analysis in the kidney of mice following oral feeding Realgar

ETHNOPHARMACOLOGICAL RELEVANCE

Realgar, a famous traditional Chinese mineral medicine, has been toxic to the renal system. However, the underlying mechanism of Realgar nephrotoxicity is still unclear.

AIM OF THE STUDY

This study aimed to investigate the potential mechanism of Realgar-induced nephrotoxicity by using a label-free quantitative proteomic method.

MATERIALS AND METHODS

36 mice were randomly divided into four groups: Control group, 0.5-, 1.0, and 2.0 g/kg Realgar group. After one week, serum biochemical parameters and renal histopathological examination were performed. Label-free quantitative proteomics was used to identify differentially expressed proteins which were subsequently analyzed with bioinformatics methods. Western blot was utilized to verify the six representative protein expressions.

RESULTS

The results showed that 2.0 g/kg Realgar significantly increased blood urea nitrogen and induced the formation of tube cast of renal tubules, while the lower-dose of 0.5 g/kg and 1.0 g/kg Realgar showed no changes. Label-free proteomic analysis identified 3138 proteins, and 272 of those proteins were screened for significant changes in a dose-dependent manner. Functional enrichment analysis suggested that these proteins could affect the apoptotic process and oxidative stress. Representative proteins in the 2.0 g/kg Realgar group, including Cat, Bad, Cycs, Nqo1, Podxl, and Hmox1, were verified by western blot.

CONCLUSIONS

The results in this study suggest that apoptosis and oxidative stress might be related to the Realgar-induced nephrotoxicity in mice. Moreover, the strategy of proteomics could contribute to the understanding of the mechanisms of nephrotoxicity in mice exposed to Realgar.

Proteomic Alterations in Salivary Exosomes Derived from Human Papillomavirus-Driven Oropharyngeal Cancer

BACKGROUND

Increasing evidence supports the notion that human papillomavirus (HPV) DNA integration onto the human genome can influence and alter the molecular cargo in the exosomes derived from head and neck cancer cells. However, the molecular cargo of salivary exosomes derived from HPV-driven oropharyngeal cancer (HPV-driven OPC) remains unelucidated.

METHODS AND MATERIALS

Salivary exosomes morphology and molecular characterizations were examined using the nanoparticle tracking (NTA), western blot analysis, transmission electron microscopy (TEM) and mass spectrometry analysis.

RESULTS

We report that HPV16 DNA was detected (80%) in isolated salivary exosomes of HPV-driven OPC patients. Importantly, we demonstrate elevated protein levels of six main glycolytic enzymes [i.e., aldolase (ALDOA), glyceraldehye-3-phosphate dehydrogenase (GAPDH), lactate dehydrogenase A/B (LDHA and LDHB), phosphoglycerate kinase 1 (PGK1) and pyruvate kinase M1/2 (PKM)] in isolated salivary exosomes of HPV-driven OPC patients, suggesting a novel mechanism underlying the potential role of salivary exosomes in mediating the reciprocal interplay between glucose metabolism and HPV-driven OPC.

CONCLUSION

Our data demonstrate the potential diagnostic value of HPV16 DNA and glycolytic enzymes in salivary exosomes in discriminating healthy controls from HPV-driven OPC patients, thereby opening new avenues in the future for clinical translation studies.

From fuzziness to precision medicine: on the rapidly evolving proteomics with implications in mitochondrial connectivity to rare human disease

Mitochondrial (mt) dysfunction is linked to rare diseases (RDs) such as respiratory chain complex (RCC) deficiency, MELAS, and ARSACS. Yet, how altered mt protein networks contribute to these ailments remains understudied. In this perspective article, we identified 21 mt proteins from public repositories that associate with RCC deficiency, MELAS, or ARSACS, engaging in a relatively small number of protein-protein interactions (PPIs), underscoring the need for advanced proteomic and interactomic platforms to uncover the complete scope of mt connectivity to RDs. Accordingly, we discuss innovative untargeted label-free proteomics in identifying RD-specific mt or other macromolecular assemblies and mapping of protein networks in complex tissue, organoid, and stem cell-differentiated neurons. Furthermore, tag- and label-based proteomics, genealogical proteomics, and combinatorial affinity purification-mass spectrometry, along with advancements in detecting and integrating transient PPIs with single-cell proteomics and transcriptomics, collectively offer seminal follow-ups to enrich for RD-relevant networks, with implications in RD precision medicine.