Recombinant Mouse Cholesterol 25-hydroxylase (Ch25h)

What is the basic structure of mouse Ch25h protein?

Mouse Ch25h is a polytopic membrane protein consisting of 298 amino acids. Unlike most sterol hydroxylases, Ch25h is not a cytochrome P450 enzyme but belongs to a small family of enzymes that utilize diiron cofactors to catalyze hydroxylation of hydrophobic substrates. The protein contains clusters of histidine residues that are essential for its catalytic activity. The Ch25h gene is intronless, which is relatively uncommon in mammalian genomes, and is expressed at low levels across multiple tissues .

How does Ch25h enzymatically convert cholesterol to 25-hydroxycholesterol?

Ch25h catalyzes the formation of 25-hydroxycholesterol (25HC) from cholesterol through a hydroxylation reaction. The enzyme uses cholesterol and molecular oxygen as substrates and requires NADPH as a cofactor . The reaction involves the introduction of a hydroxyl group specifically at the 25-position of the cholesterol molecule. This conversion is crucial as 25HC serves as a potent regulatory oxysterol involved in numerous biological processes including cholesterol homeostasis and immune modulation.

What are the established techniques for measuring Ch25h expression and activity?

For Ch25h expression analysis, quantitative PCR (qPCR) remains the most widely used approach due to the relatively low expression levels of Ch25h in most tissues. For activity measurements, gas chromatography-mass spectrometry (GC-MS) analysis is the gold standard to detect and quantify 25HC production in cellular systems. In research settings with Ch25h-expressing cells, culture supernatants can be analyzed for 25HC release, which typically begins 2-3 days after stimulation with inducers like IL-27 and can reach concentrations of approximately 100 nM . Sequential sampling over 5 days provides comprehensive data on the kinetics of 25HC production.

What factors regulate Ch25h expression in different cell types?

Ch25h expression is regulated by several factors:

• IL-27 strongly induces Ch25h in CD4+ T cells, and this effect is enhanced by TGF-β but antagonized by T-bet

• Type I interferons (IFNs) induce Ch25h expression through STAT signaling pathways

• Lipopolysaccharide (LPS) causes a significant, transient upregulation of Ch25h in macrophages (up to 35-fold increase at 2 hours post-treatment)

• Toll-like receptor 4 (TLR4) signaling is required for LPS-induced Ch25h expression, but this occurs independently of MyD88 signaling

• Various pattern recognition receptor (PRR) ligands and intact viruses can upregulate Ch25h expression in immune cells

How does tissue-specific expression of Ch25h differ in mouse models?

Ch25h demonstrates variable expression across different tissues. In Chinese tongue sole, a marine teleost model, Ch25h is highly expressed in gonads, followed by skin and muscle, but shows lower expression in intestine, spleen, kidney, and liver . In mice, Ch25h is expressed at low levels across multiple tissues under normal conditions but can be dramatically upregulated during inflammatory responses . The expression of Ch25h also increases in specific tissues during disease conditions, such as in lungs during influenza infection and in the brain during neurodegenerative processes . This tissue-specific expression pattern suggests specialized functions in different physiological contexts.

How does Ch25h-produced 25HC affect T cell function and survival?

Ch25h-produced 25HC has profound effects on T cell function:

• 25HC impairs the viability and proliferation of TCR/IL-2-stimulated T cells by impeding cellular cholesterol biosynthesis

• The inhibitory effect of 25HC on cell viability is most evident during early T cell activation (<48 hours) but diminishes if exposure is delayed

• Interestingly, T cells expressing Ch25h themselves become refractory to the inhibitory effects of autocrine 25HC, while exerting paracrine regulatory effects on bystander T cells

• 25HC suppresses cholesterol biosynthesis in T cells, inducing nutrient-deprivation cell death without releasing high-mobility group box-1 (HMGB-1)

• The inhibitory effect can be reversed when extracellular cholesterol is replenished, indicating that 25HC primarily acts by disrupting cholesterol metabolism

What is the role of Ch25h in inflammatory responses?

Ch25h plays a complex role in inflammatory responses:

• It participates in a transcriptional feed-forward loop that amplifies the production of proinflammatory mediators following infection

• Loss of Ch25h attenuates transcriptional responses to various inflammatory stimuli, while treatment with 25HC magnifies these responses

• The amplification of inflammatory signaling by 25HC appears to be mediated, at least partially, by recruitment or retention of AP-1 transcription factors at the promoters of pattern recognition receptor (PRR)-induced target genes

• In macrophages, LPS-induced Ch25h expression leads to increased 25HC production, which can further stimulate the release of inflammatory chemokines like CCL5 in a dose-dependent manner

How can researchers effectively study Ch25h's role in infectious disease models?

To study Ch25h in infectious disease models, researchers should:

• Utilize Ch25h knockout models alongside wild-type controls for comparative analyses during infection

• Employ tissue-specific Ch25h deletion to differentiate local versus systemic effects

• Use exogenous 25HC administration to determine if phenotypes can be rescued in Ch25h-deficient systems

• Monitor time-dependent changes in Ch25h expression and 25HC production during the course of infection

• Combine transcriptomic analyses with metabolic profiling to capture both inflammatory signatures and changes in cholesterol metabolism

For influenza models specifically, researchers observed that Ch25h is strongly upregulated in mouse lungs and human airway epithelial cells following infection with various influenza viruses, including the 2009 H1N1 pandemic strain . Despite 25HC's in vitro antiviral activity, Ch25h deficiency actually protected mice from influenza-induced morbidity, suggesting a more complex role in vivo where its proinflammatory effects may outweigh direct antiviral functions .

How does Ch25h and its product 25HC regulate cholesterol homeostasis?

Ch25h and 25HC regulate cholesterol homeostasis through several mechanisms:

• 25HC acts as a co-repressor that blocks sterol regulatory element binding protein (SREBP) processing, leading to inhibition of gene transcription involved in cholesterol biosynthesis

• 25HC serves as a ligand for liver X receptor (LXR), and this regulation of the LXR/SREBP signaling pathway reduces cholesterol synthesis while increasing its efflux and elimination

• Expression of Ch25h in transfected cells reduces the biosynthesis of cholesterol from acetate and suppresses the cleavage of SREBP-1 and SREBP-2

• 25HC can induce cholesterol efflux to ApoE particles from astrocytes through paracrine action, particularly when microglial cells are activated by IL-1β

What sex-specific differences exist in Ch25h expression and function?

Research has revealed notable sex-specific differences in Ch25h expression and function:

These sex-specific differences suggest that Ch25h may function differently between males and females, particularly in response to dietary arachidonic acid (ARA). The differential responses observed in various tissues (gonads, brain, and liver) indicate tissue-specific regulation patterns that vary by sex . These differences may be related to reproductive phases and could have implications for cholesterol metabolism during gonadal development and reproductive processes.

What evidence supports Ch25h involvement in neurodegenerative diseases?

Multiple lines of evidence implicate Ch25h in neurodegenerative diseases:

• Ch25h is overexpressed in brains of Alzheimer's disease (AD) patients as well as in mouse models of amyloid deposition and tau-mediated neurodegeneration

• 25HC levels are increased in cerebrospinal fluid of late-stage AD patients, as well as in AD brain tissue and mitochondria

• In PS19 mice expressing P301S mutant human tau, Ch25h deficiency strikingly reduced age-dependent neurodegeneration and neuroinflammation in the hippocampus and entorhinal/piriform cortex

• Transcriptomic analyses of bulk hippocampal tissue and single nuclei revealed that Ch25h deficiency in these mice strongly suppressed proinflammatory signaling in microglia

• 25HC treatments increase internalization of amyloid β peptides by neural cells and their accumulation in the endoplasmic reticulum, possibly because this oxysterol increases association of these peptides with membranes

• Direct treatment of neurons with 25HC decreases neurites, neuron viability, metabolism, and disrupts hippocampal synaptic transmission via N-methyl-D-aspartate receptor-mediated metaplasticity

How can researchers effectively study Ch25h in neuroinflammatory disease models?

For studying Ch25h in neuroinflammatory disease models, researchers should:

• Employ tissue-specific Ch25h knockout strategies to distinguish the roles of peripheral versus CNS-produced 25HC

• Use single-cell transcriptomics to identify cell type-specific responses to Ch25h/25HC in the CNS

• Combine genetic approaches (Ch25h knockout/overexpression) with pharmacological interventions (exogenous 25HC)

• Utilize both acute (stroke, infection) and chronic (neurodegeneration) disease models to understand context-dependent roles

• For multiple sclerosis research specifically, use experimental autoimmune encephalomyelitis (EAE) models and examine:

  • Levels of Ch25h, 25HC, and 7α,25-DHC in CNS tissues and blood-brain barrier endothelial cells

  • Trafficking of proinflammatory lymphocytes (EBI2 Th17-expressing cells) potentially mediated by 25HC and 7α,25-DHC

  • The role of Ch25h in controlling polymorphonuclear myeloid-derived suppressive cells expansion and infiltration across the blood-brain barrier

What methodological approaches can be used to determine if peripheral 25HC crosses the blood-brain barrier?

To determine if peripheral 25HC crosses the blood-brain barrier (BBB), researchers can employ several methodological approaches:

• Isotope-labeled 25HC administration coupled with mass spectrometry detection in CNS tissues

• Real-time in vivo imaging using fluorescently labeled 25HC analogues

• Transwell BBB models using human brain microvascular endothelial cells to assess 25HC permeability in vitro

• Analysis of cerebrospinal fluid following peripheral 25HC administration

• Comparison of CNS 25HC levels in wild-type versus liver-specific Ch25h knockout mice (to distinguish local production versus peripheral contribution)

• Examination of BBB integrity markers alongside 25HC measurements in neuroinflammatory conditions

This question remains particularly relevant as current evidence does not clearly establish whether peripheral 25HC can reach the CNS through the BBB or if CNS-specific production is primarily responsible for neurological effects .

What are the optimal conditions for expressing and purifying recombinant mouse Ch25h for in vitro studies?

For optimal expression and purification of recombinant mouse Ch25h:

• Expression system: A mammalian expression system (HEK293 or CHO cells) is preferred over bacterial systems due to Ch25h being a membrane protein requiring proper folding and post-translational modifications

• Vector design: Use vectors containing a strong promoter (CMV) and incorporate tags (His-tag or FLAG-tag) at the C-terminus to preserve enzymatic activity

• Detergent solubilization: Since Ch25h is a membrane protein, gentle detergents like DDM (n-Dodecyl β-D-maltoside) or CHAPS at concentrations just above their critical micelle concentration are recommended for extraction

• Purification strategy: Two-step purification using affinity chromatography followed by size exclusion chromatography yields the purest protein

• Activity preservation: Maintain the diiron cofactor by including iron in buffers during purification and storage

• Storage conditions: Store purified Ch25h in 20% glycerol at -80°C in small aliquots to preserve enzymatic activity

How can researchers accurately measure 25HC production in experimental systems?

For accurate measurement of 25HC production:

• Gas chromatography-mass spectrometry (GC-MS): The gold standard method that provides high sensitivity and specificity for 25HC detection and quantification

• Liquid chromatography with tandem mass spectrometry (LC-MS/MS): Offers excellent sensitivity without requiring derivatization

• Sample preparation: For cell culture studies, extract lipids from both cells and culture media separately to account for secreted 25HC

• Internal standards: Use deuterated 25HC as an internal standard for accurate quantification

• Time course measurements: As shown in research, 25HC secretion follows specific kinetics, typically detected on day 2 post-stimulation, peaking at approximately 100 nM on day 3, and maintained before gradually decreasing on day 5

• Cellular fractionation: When studying subcellular distribution, separate membrane fractions from cytosolic components before extraction and analysis

What contradictory findings exist in Ch25h research and how might they be reconciled?

Several contradictory findings exist in Ch25h research:

These contradictions highlight the complexity of Ch25h biology and suggest context-dependent functions that require careful experimental design and interpretation.

What therapeutic potential does targeting Ch25h have in neuroinflammatory disorders?

Ch25h represents a promising therapeutic target for neuroinflammatory disorders based on several lines of evidence:

• In PS19 mice (P301S tau model), Ch25h deficiency leads to strikingly reduced age-dependent neurodegeneration and neuroinflammation in the hippocampus and entorhinal/piriform cortex

• Ch25h deficiency strongly suppresses proinflammatory signaling in microglia, suggesting that Ch25h inhibition could modulate the neuroinflammatory component of neurodegenerative diseases

• Ch25h is overexpressed in brains of AD patients and mouse models of amyloid deposition and tau-mediated neurodegeneration, making it a disease-relevant target

• In multiple sclerosis research, targeting Ch25h at the blood-brain barrier might dampen neuroinflammation

• Selective Ch25h inhibitors could potentially alleviate neuroinflammation without broadly suppressing immune function

The authors of a recent study concluded that "Ch25h may represent a novel therapeutic target for primary tauopathies, AD, and other neuroinflammatory diseases" , highlighting its translational potential.

How might Ch25h function differ between mouse models and human systems?

Important differences between mouse and human Ch25h systems include:

• Protein structure: Human Ch25h consists of 272 amino acids, while mouse Ch25h has 298 amino acids, though both are polytopic membrane proteins with critical histidine clusters required for catalytic activity

• Tissue expression patterns: While general expression patterns are similar, tissue-specific differences in Ch25h expression levels between mice and humans may exist

• Regulatory elements: The promoter regions and transcriptional regulation of Ch25h may differ between species, potentially affecting inducibility by cytokines and inflammatory stimuli

• Genetic variation: Human populations show genetic variation in CH25H that could affect enzyme activity or regulation, a factor not typically present in inbred mouse models

• Metabolic context: Differences in cholesterol metabolism between mice and humans may affect the physiological relevance of Ch25h function

When translating findings from mouse models to human systems, researchers should validate key observations in human cells and tissues, and consider species-specific differences in cholesterol metabolism and immune regulation.

What is the current understanding of Ch25h polymorphisms and their association with human diseases?

The current understanding of Ch25h polymorphisms and their disease associations is still emerging. While the search results don't provide specific information on Ch25h polymorphisms in humans, several important considerations for researchers in this area include:

• Genomic location: The human CH25H gene is located on chromosome 10q23, in an intronless genomic structure

• Expression variation: 25-HC levels are increased in plasma of multiple sclerosis patients in a 5-year follow-up study but decreased in relapsing-remitting MS patients compared to controls, suggesting disease state-specific variations

• Alzheimer's disease: CH25H is overexpressed in brains of AD patients, with increased 25HC levels in cerebrospinal fluid of late-stage AD patients, as well as in AD brain tissue and mitochondria

• Research approach: For studying CH25H polymorphisms, researchers should:

  • Perform case-control genetic association studies in relevant disease populations

  • Analyze functional consequences of identified variants using cell-based assays

  • Correlate genotypes with 25HC levels in relevant biological fluids

  • Consider interactions with established disease risk factors