Recombinant Human Probable palmitoyltransferase ZDHHC21 (ZDHHC21)

What is the primary function of ZDHHC21 in cellular physiology?

ZDHHC21 functions as a palmitoyltransferase that catalyzes the addition of palmitate (a 16-carbon fatty acid) to specific cysteine residues of target proteins through a thioester bond. This post-translational modification, called protein palmitoylation, affects protein localization, stability, and function. ZDHHC21 has been identified as a key regulator of oxidative phosphorylation (OXPHOS) in acute myeloid leukemia (AML) cells, making it the first reported palmitoyltransferase to serve in this capacity . Additionally, ZDHHC21 plays crucial roles in regulating endothelial barrier function during inflammation and intestinal epithelial permeability following injury .

How does ZDHHC21 expression differ between normal and pathological states?

Research indicates that ZDHHC21 expression varies significantly between normal and diseased tissues. In AML, ZDHHC21 is one of the three most upregulated ZDHHC family members compared to normal cells . Among all cancer types studied, ZDHHC21 shows the highest expression specifically in AML . Furthermore, high ZDHHC21 expression correlates with poor prognosis in AML patients, with notable upregulation in patients with minimal residual disease and high-risk profiles . In inflammatory conditions, ZDHHC21 activity increases and contributes to endothelial dysfunction, while in thermal injury models, elevated ZDHHC21 function mediates gut epithelial hyperpermeability .

What experimental models are available to study ZDHHC21 deficiency?

The primary genetic model used in ZDHHC21 research is the Zdhhc21dep/dep mouse strain, which contains a spontaneous 3-base pair deletion in the coding region of the Zdhhc21 gene that renders the enzyme functionally deficient . These mice display characteristic phenotypes including depilation and heavily pigmented greasy skin but have normal gestational rates and no obvious abnormalities in basal cardiopulmonary or microcirculatory function . They show attenuated α1 adrenergic-dependent vasomotor reactivity and transient hypotension but maintain normal endothelial-dependent vasodilation, making them particularly valuable for studying barrier-specific regulation without hemodynamic confounding factors . For cellular models, siRNA and shRNA approaches targeting ZDHHC21 have been successfully employed across multiple cell lines and primary cells .

What techniques are effective for measuring ZDHHC21-mediated palmitoylation?

To assess ZDHHC21-mediated palmitoylation activity, researchers can employ several complementary approaches:

• Palmitoylation assays: Using 2-bromopalmitate (2BP), a general inhibitor of palmitoyl acyltransferases, as a control to verify ZDHHC21-specific effects.

• Target protein identification: In silico analysis using the CSS-palm palmitoylation algorithm can predict potential palmitoylation sites, which can then be confirmed experimentally . For example, this approach identified cysteine residue 17 of PLCβ1 as a likely palmitoylation target of ZDHHC21 .

• Site-directed mutagenesis: Creating mutants by replacing specific cysteine residues with serine (e.g., C17S mutation in PLCβ1) to disable palmitoylation at predicted sites and confirm functional relevance .

• Functional readouts: Using cellular phenotypes as indirect measures of palmitoylation activity, such as ATP levels, oxygen consumption rate (OCR), or transendothelial electric resistance (TER) measurements in cells with modified ZDHHC21 expression .

How can ZDHHC21 knockdown or inhibition be achieved in experimental settings?

Several effective approaches for ZDHHC21 inhibition have been documented:

• Genetic approaches:

  • siRNA-mediated knockdown: Used successfully to screen all 23 ZDHHC family members in AML cells

  • shRNA-mediated knockdown: Demonstrated in multiple AML cell lines with successful reduction of ZDHHC21 expression

  • Genetically modified mice: Zdhhc21dep/dep mice with functionally deficient ZDHHC21

• Pharmacological inhibition:

  • 2-bromopalmitate (2BP): While not specific to ZDHHC21, it effectively inhibits palmitoyltransferase activity and has been used as a control in ZDHHC21 studies

• Combination approaches:

  • Researchers have used both genetic and pharmacological methods in the same study to validate findings and distinguish between enzyme-specific and non-specific effects

What are reliable methods to assess the impact of ZDHHC21 on cellular function?

Research has established several reliable functional assays to evaluate ZDHHC21's impact:

• For metabolic effects in cancer cells:

  • ATP level measurement: Directly correlates with ZDHHC21 activity in AML cells

  • Oxygen consumption rate (OCR): Decreases significantly upon ZDHHC21 knockdown

  • Extracellular acidification rate (ECAR): Less affected by ZDHHC21 knockdown compared to OCR

  • Mitochondrial membrane potential: Decreased by shZDHHC21

• For endothelial barrier function:

  • Transendothelial electric resistance (TER): Measures real-time changes in endothelial barrier integrity

  • FITC-albumin permeability assays: Quantifies macromolecular passage across endothelial monolayers

  • Evans Blue extravasation: Assesses vascular leakage in vivo

• For epithelial barrier function:

  • Intestinal permeability assays: Measures gut barrier integrity following thermal injury

How does ZDHHC21 contribute to the pathogenesis of acute myeloid leukemia?

ZDHHC21 plays multiple critical roles in AML pathogenesis:

• Regulation of oxidative phosphorylation: ZDHHC21 selectively enhances ATP production and OXPHOS activity in AML cells but not in healthy hematopoietic stem cells (HSCs) . Knockdown of ZDHHC21 dramatically decreases oxygen consumption rate (OCR) and mitochondrial membrane potential .

• Myeloid differentiation block: Gene expression analysis reveals that high ZDHHC21 expression correlates with genes typically upregulated in HSCs and downregulated during myeloid cell development . ZDHHC21 expression decreases gradually during normal HSC differentiation but remains continuously overexpressed in AML cells . Consequently, ZDHHC21 knockdown increases expression of various differentiation markers in AML cells .

• Leukemia stem cell (LSC) maintenance: ZDHHC21 is significantly upregulated in patients with high LSC17 scores (an indicator of leukemia stemness) . It is particularly elevated in patients with the FLT3-ITD mutation, which correlates with increased stemness and drug resistance . ZDHHC21 inhibition suppresses proliferation and colony formation of CD34+ LSCs from AML specimens without affecting normal HSCs .

• Chemotherapy resistance: AML samples with high OXPHOS show poorer sensitivity to standard chemotherapy drugs including cytarabine and doxorubicin . ZDHHC21-mediated high OXPHOS activity may contribute to this chemoresistance .

What is the role of ZDHHC21 in vascular inflammatory responses?

ZDHHC21 mediates several key aspects of endothelial dysfunction during inflammation:

• Endothelial barrier disruption: ZDHHC21 deficiency (in Zdhhc21dep/dep mice) attenuates inflammation-induced vascular leakage, as evidenced by reduced protein extravasation in lung microvessels during systemic inflammatory response syndrome (SIRS) . In vitro, ZDHHC21 knockdown or inhibition reduces endothelial hyperpermeability to FITC-albumin and attenuates transendothelial electric resistance (TER) changes .

• Leukocyte-endothelial interactions: In SIRS models, Zdhhc21dep/dep mice show significantly reduced leukocyte rolling, adhesion, and transmigration compared to wild-type controls . Importantly, while endothelial ZDHHC21 deficiency significantly inhibits leukocyte adhesion, leukocyte ZDHHC21 deficiency does not affect this process, highlighting the endothelial-specific role of ZDHHC21 .

• Mechanistic pathway: ZDHHC21 mediates endothelial dysfunction through palmitoylation of PLCβ1, specifically at cysteine residue 17 . Overexpression of wild-type PLCβ1 in wild-type endothelial cells augments thrombin-induced barrier dysfunction, while a C17S mutant PLCβ1 (that cannot be palmitoylated) or wild-type PLCβ1 in ZDHHC21-deficient cells fails to enhance this response .

• Survival outcomes: ZDHHC21 deficiency dramatically improves survival in both burn-induced and LPS-induced SIRS models .

How does ZDHHC21 affect intestinal barrier function following injury?

Research shows that ZDHHC21 mediates gut epithelial hyperpermeability following severe burn injury . This barrier disruption is a major complication in burn patients and contributes to systemic inflammation and multi-organ dysfunction. Pharmacological inhibition of palmitoyl acyltransferases and genetic ablation of ZDHHC21 both significantly attenuate the hyperpermeability response in experimental models of thermal injury . These findings identify ZDHHC21 as a potential therapeutic target for treating burn-induced intestinal barrier dysfunction .

What evidence supports ZDHHC21 as a therapeutic target in AML?

Multiple lines of evidence support ZDHHC21 as a promising therapeutic target in AML:

• Differential expression and prognostic value: ZDHHC21 is highly expressed in AML compared to normal cells and other cancer types . High ZDHHC21 expression correlates with poor prognosis, especially when co-expressed with AK2 .

• Selective effects on malignant cells: ZDHHC21 inhibition selectively reduces ATP levels, OXPHOS activity, proliferation, and colony formation in AML cells and leukemia stem cells while sparing healthy HSCs .

• Differentiation induction: ZDHHC21 knockdown induces myeloid differentiation in AML cells, with FLT3-ITD mutated cells showing increased sensitivity to this effect .

• Enhanced chemosensitivity: Targeting ZDHHC21 may overcome chemoresistance in AML, as high OXPHOS activity correlates with poor response to standard chemotherapy .

• Preclinical efficacy: ZDHHC21 inhibition significantly prolongs survival in patient-derived xenograft (PDX) AML models .

• Broad applicability: ZDHHC21 inhibition shows efficacy across AML cells with multiple genotypes, suggesting potential as a broadly applicable therapeutic strategy .

What approaches can be used to validate ZDHHC21 as a therapeutic target in inflammatory conditions?

To validate ZDHHC21 as a therapeutic target in inflammatory conditions, researchers have employed:

• Genetic validation models:

  • Zdhhc21dep/dep mice with functionally deficient ZDHHC21 show resistance to SIRS-induced organ injury and improved survival in both burn-induced and LPS-induced SIRS models

  • Cell-specific effects were distinguished using co-culture models with wild-type or ZDHHC21-deficient endothelial cells and leukocytes

• Pharmacological validation:

  • 2-bromopalmitate treatment mimics the protective effects of genetic ZDHHC21 deficiency in vascular barrier models

• Target restoration experiments:

  • Overexpression of wild-type ZDHHC21 in ZDHHC21-deficient endothelial cells restores inflammatory barrier dysfunction, confirming specificity

• Downstream effector validation:

  • Identifying and manipulating PLCβ1 palmitoylation (a downstream effect of ZDHHC21) confirms the specific mechanistic pathway

• Multiple disease models:

  • Consistent findings across different inflammatory conditions (endothelial dysfunction, intestinal barrier disruption) strengthen the case for ZDHHC21 as a therapeutic target

What are the considerations for developing selective ZDHHC21 inhibitors?

Development of selective ZDHHC21 inhibitors should consider:

• Target specificity: Current inhibitors like 2-bromopalmitate are non-selective and affect multiple ZDHHC family members . Selective inhibitors would need to target unique structural features of ZDHHC21 while sparing other ZDHHC enzymes.

• Cell type selectivity: Given that ZDHHC21 has different effects in different cell types, tissue-specific delivery may be important. ZDHHC21 inhibition selectively affects AML cells over normal HSCs and has endothelial-specific effects in inflammatory models .

• Substrate specificity: ZDHHC21 palmitoylates specific substrates like PLCβ1 at Cys17 . Structure-based drug design could target the substrate recognition domains of ZDHHC21.

• Temporal considerations: The dynamic regulation of ZDHHC21 during processes like differentiation suggests that timing of inhibition may be critical for therapeutic efficacy .

• Combination therapy potential: In AML, combining ZDHHC21 inhibitors with standard chemotherapy may overcome resistance , while in inflammatory conditions, combining with other barrier-protective agents may provide synergistic effects .

What is the relationship between ZDHHC21 and oxidative phosphorylation in cancer cells?

ZDHHC21 has been identified as a key regulator of oxidative phosphorylation (OXPHOS) in AML cells:

• Selective regulation: Among 23 ZDHHC family members screened, only ZDHHC21 knockdown selectively inhibited ATP levels in AML cells without affecting normal HSCs .

• Metabolic reprogramming: ZDHHC21 knockdown dramatically decreases oxygen consumption rate (OCR) rather than extracellular acidification rate (ECAR), indicating a specific effect on oxidative metabolism rather than glycolysis .

• Mitochondrial function: ZDHHC21 depletion reduces mitochondrial membrane potential generated by OXPHOS .

• Clinical correlation: High OXPHOS activity correlates with poor chemosensitivity in primary AML samples .

• Differentiation link: OXPHOS-related genes gradually decrease during normal myeloid differentiation, while ZDHHC21 overexpression in AML maintains high OXPHOS and blocks differentiation .

The exact molecular mechanisms by which ZDHHC21 regulates OXPHOS remain to be fully elucidated, but may involve palmitoylation of mitochondrial proteins or regulators of mitochondrial function.

How does the substrate specificity of ZDHHC21 contribute to its diverse cellular effects?

The diverse effects of ZDHHC21 across different cellular contexts likely reflect its substrate specificity:

• Endothelial cells: In these cells, PLCβ1 has been identified as a critical ZDHHC21 substrate, with palmitoylation at Cys17 mediating inflammatory barrier dysfunction .

• Cancer cells: While specific substrates in AML cells are not fully characterized in the provided literature, ZDHHC21 clearly affects mitochondrial function and differentiation pathways .

• Epithelial cells: In intestinal epithelial cells, ZDHHC21 regulates barrier function, though the specific substrates remain to be identified .

Substrate specificity of ZDHHC21 may be determined by:

• Recognition sequences around target cysteines

• Protein-protein interactions that bring ZDHHC21 into proximity with specific substrates

• Subcellular localization of ZDHHC21 in different cell types

• Cell-type specific expression of potential substrate proteins

Future research using proteomics approaches to systematically identify ZDHHC21 substrates across different cell types will be crucial for understanding its diverse functions.

What are the technical challenges in working with recombinant ZDHHC21 for structural and functional studies?

Working with recombinant ZDHHC21 presents several technical challenges:

• Membrane protein expression: As a palmitoyltransferase, ZDHHC21 is a membrane-associated protein with multiple transmembrane domains, making expression and purification of functional protein technically challenging.

• Enzymatic activity assays: Developing reliable in vitro assays for ZDHHC21 activity requires appropriate substrates, lipid environments, and detection methods for protein palmitoylation.

• Structural studies: Obtaining high-resolution structures of membrane proteins like ZDHHC21 for structure-based drug design is difficult and may require specialized approaches like lipid cubic phase crystallization or cryo-electron microscopy.

• Substrate identification: Comprehensive identification of physiological ZDHHC21 substrates requires sophisticated proteomics approaches to detect palmitoylated proteins and distinguish ZDHHC21-specific substrates from those modified by other ZDHHC family members.

• Physiological relevance: Ensuring that recombinant ZDHHC21 maintains native activity and substrate specificity outside its natural cellular environment presents additional challenges.

Researchers addressing these challenges might consider using cell-free expression systems optimized for membrane proteins, nanodiscs or liposomes to maintain proper lipid environments, and click chemistry approaches to detect palmitoylation events.