Targeting PRSS23 with tipranavir induces gastric cancer stem cell apoptosis and inhibits growth of gastric cancer via the MKK3/p38 MAPK-IL24 pathway.

Gastric cancer stem cells (GCSCs) contribute to the refractory features of gastric cancer (GC) and are responsible for metastasis, relapse, and drug resistance. The key factors drive GCSC function and affect the clinical outcome of GC patients remain poorly understood. PRSS23 is a novel serine protease that is significantly up-regulated in several types of cancers and cancer stem cells, and related to tumor progression and drug resistance. In this study, we investigated the role of PRSS23 in GCSCs as well as the mechanism by which PRSS23 regulated the GCSC functions. We demonstrated that PRSS23 was critical for sustaining GCSC survival. By screening a collection of human immunodeficiency virus (HIV) protease inhibitors (PIs), we identified tipranavir as a PRSS23-targeting drug, which effectively killed both GCSC and GC cell lines (its IC50 values were 4.7 and 6.4 µM in GCSC1 cells and GCSC2 cells, respectively). Administration of tipranavir (25 mg·kg-1·d-1, i.p., for 8 days) in GCSC-derived xenograft mice markedly inhibited the growth of subcutaneous GCSC tumors without apparent toxicity. In contrast, combined treatment with 5-FU plus cisplatin did not affect the tumor growth but causing significant weight loss. Furthermore, we revealed that tipranavir induced GCSC cell apoptosis by suppressing PRSS23 expression, releasing MKK3 from the PRSS23/MKK3 complex to activate p38 MAPK, and thereby activating the IL24-mediated Bax/Bak mitochondrial apoptotic pathway. In addition, tipranavir was found to kill other types of cancer cell lines and drug-resistant cell lines. Collectively, this study demonstrates that by targeting both GCSCs and GC cells, tipranavir is a promising anti-cancer drug, and the clinical development of tipranavir or other drugs specifically targeting the PRSS23/MKK3/p38MAPK-IL24 mitochondrial apoptotic pathway may offer an effective approach to combat gastric and other cancers.

Small Heat Shock Proteins in Cancers: Functions and Therapeutic Potential for Cancer Therapy.

Small heat shock proteins (sHSPs) are ubiquitous ATP-independent chaperones that play essential roles in response to cellular stresses and protein homeostasis. Investigations of sHSPs reveal that sHSPs are ubiquitously expressed in numerous types of tumors, and their expression is closely associated with cancer progression. sHSPs have been suggested to control a diverse range of cancer functions, including tumorigenesis, cell growth, apoptosis, metastasis, and chemoresistance, as well as regulation of cancer stem cell properties. Recent advances in the field indicate that some sHSPs have been validated as a powerful target in cancer therapy. In this review, we present and highlight current understanding, recent progress, and future challenges of sHSPs in cancer development and therapy.

Targeting LGSN restores sensitivity to chemotherapy in gastric cancer stem cells by triggering pyroptosis.

Gastric cancer (GC) is notoriously resistant to current therapies due to tumor heterogeneity. Cancer stem cells (CSCs) possess infinite self-renewal potential and contribute to the inherent heterogeneity of GC. Despite its crucial role in chemoresistance, the mechanism of stemness maintenance of gastric cancer stem cells (GCSCs) remains largely unknown. Here, we present evidence that lengsin, lens protein with glutamine synthetase domain (LGSN), a vital cell fate determinant, is overexpressed in GCSCs and is highly correlated with malignant progression and poor survival in GC patients. Ectopic overexpression of LGSN in GCSC-derived differentiated cells facilitated their dedifferentiation and treatment resistance by interacting with vimentin and inducing an epithelial-to-mesenchymal transition. Notably, genetic interference of LGSN effectively suppressed tumor formation by inhibiting GCSC stemness maintenance and provoking gasdermin-D-mediated pyroptosis through vimentin degradation/NLRP3 signaling. Depletion of LGSN combined with the chemo-drugs 5-fluorouracil and oxaliplatin could offer a unique and promising approach to synergistically rendering this deadly cancer eradicable in vivo. Our data place focus on the role of LGSN in GCSC regeneration and emphasize the critical importance of pyroptosis in battling GCSC.

Insight into the molecular mechanisms of gastric cancer stem cell in drug resistance of gastric cancer.

Gastric cancer (GC) is one of the most common causes of cancer-related death worldwide, and gastric cancer stem cells (GCSCs) are considered as the major factor for resistance to conventional radio- and chemotherapy. Accumulating evidence in recent years implies that GCSCs regulate the drug resistance in GC through multiple mechanisms, including dormancy, drug trafficking, drug metabolism and targeting, apoptosis, DNA damage, epithelial-mesenchymal transition, and tumor microenvironment. In this review, we summarize current advancements regarding the relationship between GCSCs and drug resistance and evaluate the molecular bases of GCSCs in drug resistance.

MAPK10 Expression as a Prognostic Marker of the Immunosuppressive Tumor Microenvironment in Human Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) remains a devastating malignancy worldwide due to lack of effective therapy. The immune-rich contexture of HCC tumor microenvironment (TME) makes this tumor an appealing target for immune-based therapies; however, the immunosuppressive TME is still a major challenge for more efficient immunotherapy in HCC. Using bioinformatics analysis based on the TCGA database, here we found that MAPK10 is frequently down-regulated in HCC tumors and significantly correlates with poor survival of HCC patients. HCC patients with low MAPK10 expression have lower expression scores of tumor infiltration lymphocytes (TILs) and stromal cells in the TME and increased scores of tumor cells than those with high MAPK10 expression. Further transcriptomic analyses revealed that the immune activity in the TME of HCC was markedly reduced in the low-MAPK10 group of HCC patients compared to the high-MAPK10 group. Additionally, we identified 495 differentially expressed immune-associated genes (DIGs), with 482 genes down-regulated and 13 genes up-regulated in parallel with the decrease of MAPK10 expression. GO enrichment and KEGG pathway analyses indicated that the biological functions of these DIGs included cell chemotaxis, leukocyte migration and positive regulation of the response to cytokine-cytokine receptor interaction, T cell receptor activation and MAPK signaling pathway. Protein-protein interaction (PPI) analyses of the 495 DIGs revealed five potential downstream hub genes of MAPK10, including SYK, CBL, VAV1, LCK, and CD3G. Several hub genes such as SYK, LCK, and VAV1 could respond to the immunological costimulatory signaling mediated by the transmembrane protein ICAM1, which was identified as a down-regulated DIG associated with low-MAPK10 expression. Moreover, ectopic overexpression or knock-down of MAPK10 could up-regulate or down-regulate ICAM1 expression via phosphorylation of c-jun at Ser63 in HCC cell lines, respectively. Collectively, our results demonstrated that MAPK10 down-regulation likely contributes to the immunosuppressive TME of HCC, and this gene might serve as a potential immunotherapeutic target and a prognostic factor for HCC patients.

Evaluation of two cell surface modification methods for proteomic analysis of plasma membrane from isolated mouse hepatocytes.

The hepatocyte is a highly polarized cell with a heterogeneous distribution of plasma-membrane (PM) proteins. To reduce the complexity of the proteome of liver tissue and give a comprehensive profile of hepatocyte PM, two PM purification methods based on cell surface modification, named the biotin-avidin (BA) and cationic silica-polyanion (CSP) strategies were evaluated and compared with the traditional cell fractionation method to prepare highly enriched PM from freshly isolated C57 mouse hepatocytes. Employing different principles for PM modification, both methods were effective in the isolation of general and purified PM fraction. The CSP strategy showed better yield for the PM purification from freshly isolated hepatocytes. 189 non-redundant proteins were identified, including 49 from the BA method and 185 from CSP strategy. Many known and novel PM-associated proteins were also found. Our evaluation here should give implications for PM preparation from other freshly isolated tissue-derived cells. The hepatocyte PM proteins identified here should be taken as a references for the PM-related functional and diseases research.

Gel absorption-based sample preparation for the analysis of membrane proteome by mass spectrometry.

A gel absorption-based sample preparation method for shotgun analysis of membrane proteome has been developed. In this new method, membrane proteins solubilized in a starting buffer containing a high concentration of sodium dodecyl sulfate (SDS) were directly entrapped and immobilized into gel matrix when the membrane protein solution was absorbed by the vacuum-dried polyacrylamide gel. After the detergent and other salts were removed by washing, the proteins were subjected to in-gel digestion and the tryptic peptides were extracted and analyzed by capillary liquid chromatography coupled with tandem mass spectrometry (CapLC-MS/MS). The results showed that the newly developed method not only avoided the protein loss and the adverse protein modifications during gel embedment but also improved the subsequent in-gel digestion and the recovery of tryptic peptides, particularly the hydrophobic peptides, thereby facilitating the identification of membrane proteins, especially the integral membrane proteins. Compared with the conventional tube-gel digestion method, the newly developed method increased the numbers of identified membrane proteins and integral membrane proteins by 25.0% and 30.2%, respectively, demonstrating that the method is of broad practicability in gel-based shotgun analysis of membrane proteome.

Combined Conditional Knockdown and Adapted Sphere Formation Assay to Study a Stemness-Associated Gene of Patient-derived Gastric Cancer Stem Cells.

Cancer stem cells (CSCs) are implicated in tumor initiation, development and recurrence after treatment, and have become the center of attention of many studies in the last decades. Therefore, it is important to develop methods to investigate the role of key genes involved in cancer cell stemness. Gastric cancer (GC) is one of the most common and mortal types of cancers. Gastric cancer stem cells (GCSCs) are thought to be the root of gastric cancer relapse, metastasis and drug resistance. Understanding GCSCs biology is needed to advance the development of targeted therapies and eventually to reduce mortality among patients. In this protocol, we present an experimental design using a conditional knockdown system and an adapted sphere formation assay to study the effect of clusterin on the stemness of patient-derived GCSCs. The protocol can be easily adapted to study both in vitro and in vivo function of stemness-associated genes in different types of CSCs.

Enrichment and proteomic analysis of plasma membrane from rat dorsal root ganglions.

BACKGROUND: Dorsal root ganglion (DRG) neurons are primary sensory neurons that conduct neuronal impulses related to pain, touch and temperature senses. Plasma membrane (PM) of DRG cells plays important roles in their functions. PM proteins are main performers of the functions. However, mainly due to the very low amount of DRG that leads to the difficulties in PM sample collection, few proteomic analyses on the PM have been reported and it is a subject that demands further investigation. RESULTS: By using aqueous polymer two-phase partition in combination with high salt and high pH washing, PMs were efficiently enriched, demonstrated by western blot analysis. A total of 954 non-redundant proteins were identified from the plasma membrane-enriched preparation with CapLC-MS/MS analysis subsequent to protein separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) or shotgun digestion. 205 (21.5%) of the identified proteins were unambiguously assigned as PM proteins, including a large number of signal proteins, receptors, ion channel and transporters. CONCLUSION: The aqueous polymer two-phase partition is a simple, rapid and relatively inexpensive method. It is well suitable for the purification of PMs from small amount of tissues. Therefore, it is reasonable for the DRG PM to be enriched by using aqueous two-phase partition as a preferred method. Proteomic analysis showed that DRG PM was rich in proteins involved in the fundamental biological processes including material exchange, energy transformation and information transmission, etc. These data would help to our further understanding of the fundamental DRG functions.

Verteporfin blocks Clusterin which is required for survival of gastric cancer stem cell by modulating HSP90 function.

Gastric cancer stem cell (GCSC) is implicated in gastric cancer relapse, metastasis and drug resistance. However, the key molecule(s) involved in GCSC survival and the targeting drugs are poorly understood. We discovered increased secreted clusterin (S-Clu) protein expression during the sphere-forming growth of GCSC via mass spectrometry. Overexpression of clusterin was detected in 69/90 (77%) of primary GC tissues and significantly associated with T stage, lymph node metastasis and TNM stage. Depletion of clusterin (Clu, the full-length intracellular clusterin) led to the declustering of GCSC tumorspheres and apoptosis of GCSC. Subsequently, we found clusterin was in complex with heat shock protein 90 beta (HSP90) and involved in regulating the cellular level of HSP90 client proteins. Furthermore, by screening a collection of drugs/inhibitors, we found that verteporfin (VP), a phototherapy drug, blocked clusterin gene expression, decreased the HSP90 client proteins and caused cell death of GCSC. VP treatment is more effective in eradicating GCSCs than in killing GC cells. Both clusterin silencing or VP treatment deterred tumor growth in human GCSC xenografts. These findings collectively suggest that GC patients can promptly benefit from clusterin-targeted therapy as well as VP treatment in combination with or subsequent to conventional chemotherapy for reducing mortality of GC.

Proteomic analysis of circulating monocytes in Chinese premenopausal females with extremely discordant bone mineral density.

Osteoporosis (OP) is a major public health problem, mainly characterized by low bone mineral density (BMD). Circulating monocytes (CMCs) may serve as progenitors of osteoclasts and produce a wide variety of factors important to bone metabolism. However, the specific action mechanism of CMCs in the pathogenesis of OP is far from clear. We performed a comparative protein expression profiling study of CMCs in Chinese premenopausal females with extremely discordant BMD, identified a total of 38 differentially expressed proteins, and confirmed with Western blotting five proteins: ras suppressor protein1 (RSU1), gelsolin (GSN), manganese-containing superoxide dismutase (SOD2), glutathione peroxidase 1(GPX1), and prolyl 4-hydroxylase beta subunit (P4HB). These proteins might affect CMCs' trans-endothelium, differentiation, and/or downstream osteoclast functions, thus contribute to differential osteoclastogenesis and finally lead to BMD variation. The findings promote our understanding of the role of CMCs in BMD determination, and provide an insight into the pathogenesis of human OP.

A proteomic study reveals the diversified distribution of plasma membrane-associated proteins in rat hepatocytes.

To investigate the heterogeneous protein composition of highly polarized hepatocyte plasma membrane (PM), three PM-associated subfractions were obtained from freshly isolated rat hepatocytes using density gradient centrifugation. The origins of the three subfractions were determined by morphological analysis and western blotting. The proteins were subjected to either one-dimensional (1-D) SDS-PAGE or two-dimensional (2-D) benzyldimethyl-n-hexadecylammonium chloride (BAC)/SDS-PAGE before nano-Liquid Chromatography-Electrospray Ionization--tandem mass spectrometry analysis (LC-ESI-MS/MS). A total of 613 non-redundant proteins were identified, among which 371 (60.5%) proteins were classified as PM or membrane-associated proteins according to GO annotations and the literatures and 32.4% had transmembrane domains. PM proteins from microsomal portion possessed the highest percentage of transmembrane domain, about 46.5% of them containing at least one transmembrane domain. In addition to proteins known to be located at polarized liver PM regions, such as asialoglycoprotein receptor 2, desmoplakin and bile salt export pump, several proteins which had the potential to become novel subfraction-specific proteins were also identified, such as annexin a6, pannexin and radixin. Our analysis also evaluated the application of 1-D SDS-PAGE and 2-D 16-BAC/SDS-PAGE on the separation of integral membrane proteins.

Dataset of the plasma membrane proteome of nasopharyngeal carcinoma cell line HNE1 for uncovering protein function.

Nasopharyngeal carcinoma (NPC) is a commonly occurring tumor in southern China and Southeast Asia. The current study focused on developing an extensive analysis method for the peripheral and integral proteins of NPC cell line HNE1. The peripheral membrane proteins were extracted by biotinylated enrichment, 0.1 M Na2CO3, and H2O. Integral or total plasma membrane fractions were prepared using 30% Percoll density grade centrifugation with or without 0.1 M Na2CO3 treatment and evaluated by Western blot analysis. The proteins were subjected to two-dimensional electrophoresis combined with tandem mass spectrometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis combined with tandem mass spectrometry, and shotgun analysis. We identified 371, 180, and 702 proteins from peripheral, integral, and total plasma membrane fractions, respectively. In all, 848 non-redundant proteins (534 groups) were identified. Binding, catalytic, and structural molecules were the major classes. In addition to the known cell surface markers of NPC cells, the analysis revealed 311 proteins involved in multiple cell-signaling pathways and 25 proteins in disease pathways that are characteristic of cancer cells. By searching the Differentially Expressed Protein Database (http://protchem.hunnu.edu.cn/depd/index.jsp), 199 proteins were found to be differentially expressed in previous cancer proteome research. A 671 protein-protein interaction network was obtained, including 178 identified proteins in this work. The plasma membrane localization of five proteins was confirmed by immunological techniques, validating this proteomic strategy. Our study could offer some help for understanding the molecular mechanism of NPC.

Epigenetic alterations of a novel antioxidant gene SLC22A3 predispose susceptible individuals to increased risk of esophageal cancer.

Esophageal squamous cell carcinoma (ESCC) occurs with the highest frequency in China, especially in the high-risk Northern Chinese. Recent studies have reported that SLC22A3 is significantly downregulated in non-tumor (NT) esophageal tissues from familial ESCC patients compared with those from sporadic ESCC. However, the mechanism of how SLC22A3 regulates familial ESCC remains unknown. In this study, post hoc genome-wide association studies (GWAS) in 496 cases with a family history of upper gastrointestinal tract cancers and 1056 controls were performed and the results revealed that SLC22A3 is a novel susceptibility gene for familial ESCC. Reduced expression of SLC22A3 in NT esophageal tissues from familial ESCC patients significantly correlates with its promoter hypermethylation. Moreover, case-control study of Chinese descendants from different risk areas of China revealed that the methylation of the SLC22A3 gene in peripheral blood leukocyte (PBL) DNA samples could be a risk factor for developing ESCC in this high-risk population. Functional studies showed that SLC22A3 is a novel antioxidant gene, and deregulation of SLC22A3 facilitates heat stress-induced oxidative DNA damage and formation of γ-H2AX foci in normal esophageal epithelial cells. Collectively, we show that epigenetic alterations of SLC22A3 predispose susceptible individuals to increased risk of esophageal cancer.

Evaluation of strategy for analyzing mouse liver plasma membrane proteome.

Plasma membrane (PM) proteome is one of the major subproteomes present in the cell, and is very important in liver function. In the present work, C57 mouse liver PM was purified by density-gradient centrifugation. The purified PM was verified by electron microscope analysis and Western blotting. The results showed that the PM was enriched by more than 20-fold and the contamination of mitochondria was reduced by 2-fold compared with the homogenization fraction. Proteins were separated by 2DE and 1DE, trypsin-digested and submitted to ESI-Q-TOF and MALDI-TOF-TOF mass spectrometry or directly digested in solution and analyzed by LC-ESI ion trap mass spectrometry. In all, 547 non-redundant mouse liver PM proteins were identified, of which 34% contributed to plasma membrane or plasma membrane-related proteins. This study optimized and evaluated the HLPP plasma membrane proteome analysis method and made a systematic analysis on PM proteome.

High Rab25 expression associates with Ki67/TP53/CD133/VEGFR expression predicts poor prognosis in gastric cancer.

Rab25 belongs to Rab GTPases which regulating vesicle trafficking of various extracellular and intracellular resources. Aberrant high Rab25 expression is closely linked to cancer development including gastric cancer. However, the underlying mechanism of Ras25 in gastric cancer is still unclear. In this study, we determined to investigate the potential association between Rab25 and four tumor markers, including Ki67 (a well-known hallmarker of tumor proliferation), TP53 (tumor p53, a master tumor regulator associated with cell apoptosis), CD133 (a common cancer stem cell marker) and VEGFR (Vascular endothelial growth factor receptor, an efficient therapy target for gastric cancer). The results indicated that Rab25 expression in both cytoplasm and nucleus was significantly higher in gastric cancer tissues than para-carcinoma tissues. High Rab25 nucleus expression was positively associated with distant metastasis (M stage) and clinical (cTNM) stage, while Rab25 nucleus expression correlated with M stage, cTNM stage and regional lymph metastasis stage (N stage). Survival analysis revealed that high Rab25 cytoplasm/nucleus expression predicted shorter overall survival time of patients with gastric cancer. Rab25 expression was significantly associated with Ki67 expression, TP53 expression, CD133 expressionand VEGFR expression in gastric cancer. In conclusion, our results indicated that Rab25 behaved as an oncogene in gastric cancer related to Ki67/TP53/CD133/VEGFR expression and suggested Rab25 to be a prognostic marker.

Shotgun proteomics and network analysis between plasma membrane and extracellular matrix proteins from rat olfactory ensheathing cells.

Olfactory ensheathing cells (OECs) are a special type of glial cells that have characteristics of both astrocytes and Schwann cells. Evidence suggests that the regenerative capacity of OECs is induced by soluble, secreted factors that influence their microenvironment. These factors may regulate OECs self-renewal and/or induce their capacity to augment spinal cord regeneration. Profiling of plasma membrane and extracellular matrix through a high-throughput expression proteomics approach was undertaken to identify plasma membrane and extracellular matrix proteins of OECs under serum-free conditions. 1D-shotgun proteomics followed with gene ontology (GO) analysis was used to screen proteins from primary culture rat OECs. Four hundred and seventy nonredundant plasma membrane proteins and 168 extracellular matrix proteins were identified, the majority of which were never before reported to be produced by OECs. Furthermore, plasma membrane and extracellular proteins were classified based on their protein-protein interaction predicted by STRING quantitatively integrates interaction data. The proteomic profiling of the OECs plasma membrane proteins and their connection with the secretome in serum-free culture conditions provides new insights into the nature of their in vivo microenvironmental niche. Proteomic analysis for the discovery of clinical biomarkers of OECs mechanism warrants further study.

Proteomic and peptidomic characterization of the venom from the Chinese bird spider, Ornithoctonus huwena Wang.

The bird spider Ornithoctonus huwena Wang is a very venomous spider in China. Several compounds with different types of biological activities have been identified previously from the venom of this spider. In this study, we have performed a proteomic and peptidomic analysis of the venom. The venom was preseparated into two parts: the venom proteins with molecular weight (MW) higher than 10,000 and the venom peptides with MW lower than 10 000. Using one-dimensional gel electrophoresis (1-DE), two-dimensional gel electrophoresis (2-DE), and mass spectrometry, 90 proteins were identified, including some important enzymes, binding proteins, and some proteins with significant biological functions. For venom peptides, a combination of cation-exchange and reversed-phase chromatography was employed. More than 100 components were detected by mass spectrometry, and 47 peptides were sequenced by Edman degradation. The peptides display structural and pharmacological diversity and share little sequence similarity with peptides from other animal venoms, which indicates the venom of O. huwena Wang is unique. The venom peptides can be classified into several superfamilies. Also it is revealed that gene duplication and focal hypermutation have taken place during the evolution of the spider toxins.

Immunoaffinity purification of plasma membrane with secondary antibody superparamagnetic beads for proteomic analysis.

Plasma membrane (PM) has very important roles in cell-cell interaction and signal transduction, and it has been extensively targeted for drug design. A major prerequisite for the analysis of PM proteome is the preparation of PM with high purity. Density gradient centrifugation has been commonly employed to isolate PM, but it often occurred with contamination of internal membrane. Here we describe a method for plasma membrane purification using second antibody superparamagnetic beads that combines subcellular fractionation and immunoisolation strategies. Four methods of immunoaffinity were compared, and the variation of crude plasma membrane (CPM), superparamagnetic beads, and antibodies was studied. The optimized method and the number of CPM, beads, and antibodies suitable for proteome analysis were obtained. The PM of mouse liver was enriched 3-fold in comparison with the density gradient centrifugation method, and contamination from mitochondria was reduced 2-fold. The PM protein bands were extracted and trypsin-digested, and the resulting peptides were resolved and characterized by MALDI-TOF-TOF and ESI-Q-TOF, respectively. Mascot software was used to analyze the data against IPI-mouse protein database. Nonredundant proteins (248) were identified, of which 67% are PM or PM-related proteins. No endoplasmic reticulum (ER) or nuclear proteins were identified according to the GO annotation in the optimized method. Our protocol represents a simple, economic, and reproducible tool for the proteomic characterization of liver plasma membrane.

Proteomic analysis of rat hippocampal plasma membrane: characterization of potential neuronal-specific plasma membrane proteins.

The hippocampus is a distinct brain structure that is crucial in memory storage and retrieval. To identify comprehensively proteins of hippocampal plasma membrane (PM) and detect the neuronal-specific PM proteins, we performed a proteomic analysis of rat hippocampus PM using the following three technical strategies. First, proteins of the PM were purified by differential and density-gradient centrifugation from hippocampal tissue and separated by one-dimensional electophoresis, digested with trypsin and analyzed by electrospray ionization (ESI) quadrupole time-of-flight (Q-TOF) tandem mass spectrometry (MS/MS). Second, the tryptic peptide mixture from PMs purified from hippocampal tissue using the centrifugation method was analyzed by liquid chromatography ion-trap ESI-MS/MS. Finally, the PM proteins from primary hippocampal neurons purified by a biotin-directed affinity technique were separated by one-dimensional electrophoresis, digested with trypsin and analyzed by ESI-Q-TOF-MS/MS. A total of 345, 452 and 336 non-redundant proteins were identified by each technical procedure respectively. There was a total of 867 non-redundant protein entries, of which 64.9% are integral membrane or membrane-associated proteins. One hundred and eighty-one proteins were detected only in the primary neurons and could be regarded as neuronal PM marker candidates. We also found some hypothetical proteins with no functional annotations that were first found in the hippocampal PM. This work will pave the way for further elucidation of the mechanisms of hippocampal function.