S100A7 Human

What is the molecular structure of human S100A7 and how does it differ from murine S100A7?

Human S100A7 (hS100A7) is an 11.7-kDa EF-hand calcium-binding protein that exists as a homodimer with the classic S100 protein architecture. Crystallographic studies reveal significant structural differences between human and murine S100A7 (mS100A7), with a relatively high RMSD of 1.68 Å over all Cα atoms - larger than expected for typical homologs .

The most notable structural differences include:

• Human S100A7 has a truncated N-terminal Ca²⁺-binding loop (12 residues: Tyr19-Ser30) compared to the standard loop in murine S100A7 (14 residues: Ala21-Glu34)

• The human protein lacks the strictly conserved bidentate Glu residue at the C-terminus of this loop, which affects Ca²⁺ binding affinity

• The linker between EF-hands in mS100A7 (Val43–Ala53) is four residues shorter than in hS100A7 (Phe39-Tyr53) and packed differently

• Murine S100A7 exhibits larger interhelical angles, indicating a more open conformation

These structural differences help explain the functional divergence between species, particularly in host-pathogen interactions.

What are the metal-binding properties of human S100A7?

Human S100A7 binds both calcium and zinc with high affinity, properties crucial to its biological functions:

• Calcium binding: hS100A7 exhibits conformational changes upon Ca²⁺ binding, unlike mS100A7 which shows no significant structural change

• Zinc binding: Both human and murine S100A7 bind Zn²⁺ with similar high affinity - apparent Kd values for hS100A7 are 0.70 ± 0.11 nM in the absence of Ca²⁺ and 0.31 ± 0.07 nM in the presence of Ca²⁺

• Calcium-zinc interplay: Calcium binding modestly increases zinc affinity in hS100A7 (from Kd = 0.70 nM to 0.31 nM)

The similar zinc binding affinities between human and murine S100A7 suggest that differences in metal binding alone do not explain the species-specific effects in pathogen interactions.

What are the recommended techniques for detecting and quantifying S100A7 expression?

Several complementary approaches can be used to detect and quantify S100A7 expression:

Protein Detection Methods:

• Western blotting: Effective for semi-quantitative analysis of S100A7 expression in cell/tissue lysates; samples should be separated by SDS-PAGE under reducing or non-reducing conditions

• Immunohistochemistry (IHC): Single and dual staining protocols allow visualization of S100A7 expression patterns in FFPE tissues, particularly useful for cancer studies

• Surface-enhanced laser desorption ionization-mass spectrometry (SELDI-MS): Effective for identification and purification of S100A7 from complex biological samples like CSF

Transcriptional Analysis:

• Real-time PCR: Allows quantification of S100A7 mRNA levels using the 2^(-ΔΔCT) method

• Microarray transcriptomics: Useful for examining S100A7 expression changes alongside global transcriptional profiles

Proteomics Approaches:

• LC-MS/MS: Provides comprehensive protein profiling that can detect S100A7 alongside other differentially expressed proteins

• SAGE (Serial Analysis Gene Expression): Has been used successfully to identify S100A7 upregulation in tissues such as larynx tumor

How can recombinant human S100A7 be expressed and purified for experimental studies?

For functional studies of human S100A7, producing pure, correctly folded recombinant protein is essential:

Expression Systems:

• Bacterial expression: E. coli-based systems are commonly used for producing recombinant hS100A7, generally providing good yield

• Adenoviral expression systems: Effective for studying S100A7 function in neuronal cultures, as demonstrated in Tg2576 transgenic embryo studies

Purification Protocol:

• Lyse cells in RIPA buffer containing protease inhibitors (complete protease cocktail inhibitor, PMSF) and phosphatase inhibitors (sodium orthovanadate, β-glycerophosphate, sodium fluoride)

• Sonicate lysates on ice and centrifuge at 14,000 rpm at 4°C

• Collect supernatant containing total protein

• Quantify protein concentration using BCA protein assay

• For structural studies, additional chromatography steps may be necessary to achieve high purity

Quality Control:

• Verify protein identity by mass spectrometry

• Assess structural integrity using circular dichroism

• Confirm functional activity through metal binding assays

What is the role of S100A7 in Alzheimer's disease pathology?

S100A7 has emerged as a potential biomarker and therapeutic target in Alzheimer's disease (AD):

• Biomarker potential: S100A7 levels are increased in the cerebrospinal fluid (CSF) and brain tissue of AD dementia subjects as a function of clinical dementia

• Amyloid pathology: Expression of exogenous S100A7 in primary cortico-hippocampal neuron cultures from Tg2576 transgenic embryos inhibits the generation of β-amyloid (Aβ) 1–42 and Aβ 1–40 peptides

• Mechanistic pathway: S100A7 selectively promotes "non-amyloidogenic" α-secretase activity via ADAM (a disintegrin and metalloproteinase)-10

• In vivo effects: Selective expression of human S100A7 in the brain of transgenic mice results in significant promotion of α-secretase activity

These findings suggest S100A7 may have neuroprotective properties in AD by modulating amyloid processing pathways, offering potential as both a diagnostic biomarker and therapeutic target.

How is S100A7 involved in cancer progression?

S100A7 exhibits context-dependent roles in various cancers:

Head and Neck Squamous Cell Carcinoma (HNSCC):

• S100A7 is upregulated in HNSCC compared to normal tissues

• Nuclear S100A7 expression is associated with poor prognosis in head and neck cancer

• Expression of S100A7 correlates with CD82 (tetraspanin) in oral squamous cell carcinoma (OSCC)

Breast Cancer:

• Differential expression of S100A7 mRNA has been observed between in situ and invasive human breast carcinoma

• May serve as a marker for tumor progression

Regulatory Mechanisms:

• CD82 may regulate S100A7 expression and influence cell migration in cancer models

• Reciprocal negative regulation exists between S100A7/psoriasin and β-catenin signaling, playing an important role in tumor progression of squamous cell carcinoma of the oral cavity

Despite its potential as a cancer biomarker, S100A7 expression in some normal tissues may limit its specificity as a standalone diagnostic marker .

What is the significance of S100A7 in host-pathogen interactions?

S100A7 plays a critical role in host-pathogen interactions, particularly through zinc sequestration:

• Nutritional immunity: S100A7 normally contributes to host defense by sequestering essential metal ions (nutritional immunity)

• Pathogen evasion: Neisseria gonorrhoeae can exploit human S100A7 (but not murine S100A7) to evade host nutritional immunity

• Species specificity: This exploitation is species-specific, with N. gonorrhoeae utilizing human but not murine S100A7, which has implications for research models

• Molecular basis: The species specificity appears related to substantial differences in the C-terminal region of S100A7, which mediates protein-protein interactions

This research highlights limitations of mouse models for studying human bacterial infections and demonstrates how pathogens can co-opt host defense mechanisms.

What are the key differences between human and murine S100A7 and their implications for experimental models?

Despite being classified as homologs, human and murine S100A7 exhibit significant differences that impact their use in experimental models:

Sequence Divergence:

• Only 31.7% sequence identity (approximately 50% conservation) between human and murine S100A7

• Substantial difference in the C-terminal region, critical for protein-protein interactions

Structural Differences:

• Different responses to calcium binding: hS100A7 shows conformational changes upon Ca²⁺ binding while mS100A7 does not

• RMSD of 1.68 Å between structures is larger than expected for homologs (compare to human vs. bovine S100B with RMSD of 0.34-0.47 Å)

Functional Implications:

• Inability of murine S100A7 to mediate zinc piracy by Neisseria gonorrhoeae

• Different protein-protein interaction profiles likely due to C-terminal differences

Research Considerations:

• Mouse models may have limited utility for studying certain S100A7-dependent processes

• Transgenic mice expressing human S100A7 may provide more relevant disease models

• Detailed sequence analysis supports the proposal that human and murine S100A7 are distinct orthologs

This evidence suggests researchers should exercise caution when extrapolating results from murine models to human S100A7 function.

How does the interaction between S100A7 and CD82 influence cellular processes?

Recent research has identified a significant relationship between S100A7 and tetraspanin CD82:

Expression Correlation:

• CD82 expression correlates with and may regulate S100A7 expression in oral squamous cell carcinoma (OSCC)

• Knockout of CD82 affects S100A7 expression levels, suggesting a regulatory relationship

Functional Consequences:

• CD82 knockout inhibits cell migration in cancer cell models

• CD82-S100A7 interaction may influence tumor invasion and metastasis pathways

Experimental Evidence:

• Transcriptomic and proteomic data demonstrate correlation between CD82 and S100 family members

• Real-time PCR and Western blotting confirm that CD82 expression correlates with S100 proteins in multiple cell lines (CAL 27, SCC-25, S-G)

• Immunohistochemical analysis shows correlation between CD82 and S100A7 expression in OSCC tissue

Further research into this interaction may reveal new therapeutic targets for controlling cancer cell migration and invasion.

How might S100A7 be developed as a therapeutic target or biomarker?

S100A7 shows promise as both a biomarker and therapeutic target in several disease contexts:

Alzheimer's Disease Applications:

• Potential use as a CSF biomarker for early AD diagnosis

• Possible development as a surrogate index of therapeutic efficacy in drug trials

• Therapeutic strategy of promoting S100A7 expression to enhance α-secretase activity and reduce amyloidogenic peptide generation

Cancer Applications:

• Potential prognostic biomarker in HNSCC, with nuclear S100A7 associated with poor prognosis

• Possible inclusion in risk assessment for malignant transformation in oral lesions

• Target for developing RNA-mediated interference through NF-κB-mediated pathways

Research Needs:

• Validation of S100A7 sensitivity to therapeutic interventions in clinical settings

• Development of specific modulators of S100A7 expression or activity

• Further characterization of S100A7's role in specific disease mechanisms

The dual potential of S100A7 as both biomarker and therapeutic target makes it a compelling focus for translational research across multiple disease areas.