ATP6AP2 Human
Description
Introduction
ATP6AP2 (ATPase H+ Transporting Accessory Protein 2), also known as the (pro)renin receptor (PRR), is a multifunctional transmembrane protein encoded by the ATP6AP2 gene on the X chromosome. It plays critical roles in vacuolar H+-ATPase (V-ATPase) assembly, cellular pH homeostasis, autophagy, and signaling pathways such as planar cell polarity (PCP). Mutations in ATP6AP2 are linked to multisystem disorders, including glycosylation defects, immunodeficiency, and neurodevelopmental impairments .
Gene Overview
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Protein Domains:
Table 1: Key Features of ATP6AP2
V-ATPase Assembly and Acidification
ATP6AP2 is essential for V-ATPase biogenesis, a proton pump critical for lysosomal acidification and intracellular trafficking. Key findings:
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Yeast Rescue: Drosophila ATP6AP2 restores V-ATPase activity in yeast lacking Voa1 (a V-ATPase assembly factor), particularly when co-expressed with ATP6AP1 .
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Autophagy Regulation: ATP6AP2 deficiency disrupts lysosomal acidification, impairing autophagic flux and causing lipid accumulation in hepatocytes .
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Cardiomyocyte Survival: Cardiac-specific ATP6AP2 knockout in mice leads to lethal heart failure due to defective autophagosome-lysosome fusion .
Planar Cell Polarity (PCP)
In Drosophila, ATP6AP2 localizes asymmetrically in wing epithelia and interacts with PCP core proteins (e.g., Flamingo). Dysregulation induces ER stress and disrupts hair orientation, linking V-ATPase dysfunction to PCP defects .
Organ-Specific Functions
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Pancreatic β Cells: Robustly expressed in human insulinoma and normal β cells, regulating insulin secretion .
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Liver: Hepatic ATP6AP2 knockdown causes hypoglycosylation of serum proteins and steatohepatitis .
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Immune System: Mutations impair T-cell function and antibody production, leading to recurrent infections .
Genetic Disorders
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X-linked Parkinsonism with Spasticity: Linked to ATP6AP2 mutations causing neurodevelopmental delays .
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Congenital Disorder of Glycosylation Type IIr: Characterized by hepatosteatosis and immunodeficiency .
Cardiovascular and Metabolic Disease
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Heart Failure: ATP6AP2 knockdown exacerbates pressure overload-induced cardiac dysfunction and fibrosis .
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Autophagic Liver Disease: Mutations induce ER stress, block autophagic degradation, and promote lipid accumulation .
Table 2: Clinical Phenotypes of ATP6AP2 Deficiency
Research Models and Experimental Findings
Product Specs
Q&A
What is ATP6AP2 and what are its primary cellular functions?
ATP6AP2, also known as (pro)renin receptor, is an essential accessory component of the vacuolar H+ ATPase (V-ATPase). It functions as a transmembrane protein involved in several critical cellular processes including V-ATPase assembly and activity . Originally identified as a receptor for renin and prorenin, ATP6AP2 can activate the renin-angiotensin system and trigger intracellular signaling pathways such as MAPK independently of angiotensin II production . Additionally, ATP6AP2 participates in Wnt signaling pathways and plays crucial roles in cell division, cell cycle progression, and mitosis .
What is the molecular structure of ATP6AP2?
ATP6AP2 is a type I transmembrane protein that can be cleaved in the Golgi apparatus into two fragments: a lumenal or extracellular N-terminal fragment (NTF) and a membrane-bound C-terminal fragment (CTF) that contains an ER retrieval motif . The full-length protein includes domains for interaction with renin/prorenin, V-ATPase components, and various signaling molecules . Structurally, the C-terminal ER retrieval motif (KKXX) is essential for its function in V-ATPase assembly, as demonstrated by rescue experiments showing that a full-length version lacking this motif (ATP6AP2 ΔKKXX) fails to rescue V-ATPase assembly defects .
Where is ATP6AP2 expressed in human tissues?
ATP6AP2 demonstrates a broad expression pattern across multiple tissue types. In the brain, high expression levels are observed in the cortex, hippocampus, medial habenular nucleus, cerebellum, medulla, and olfactory bulb . At the cellular level, brain ATP6AP2 is predominantly expressed in neurons, including both glutamatergic (Camk2a-positive) and GABAergic (GAD67-positive) neurons, while expression in astrocytes (GFAP-positive cells) appears minimal or below detection limits of standard immunohistochemistry . ATP6AP2 is also robustly expressed in pancreatic β cells and insulinoma cells . This widespread expression pattern reflects its essential role in fundamental cellular processes.
How does ATP6AP2 subcellular localization change during cellular processes?
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Prophase, metaphase, and anaphase: Localizes to mitotic spindle poles
This dynamic relocalization during mitosis suggests direct involvement in spindle organization and cell division. ATP6AP2 knockdown results in markedly deformed spindles, confirming the functional significance of this mitotic redistribution .
ATP6AP2 in Disease Pathogenesis
ATP6AP2 mutations disrupt multiple cellular processes that contribute to disease pathogenesis:
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Impaired V-ATPase assembly and function, leading to defective acidification of cellular compartments
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Disrupted protein glycosylation resembling deficiencies found in other V-ATPase assembly factors
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Altered Wnt, NOTCH, and mTOR signaling pathways that require proper endosome-lysosomal acidification
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Defective cell division and mitotic spindle formation resulting in aberrant proliferation
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Deregulated ciliogenesis affecting cell differentiation and specialized signaling pathways
The tissue-specific manifestations likely result from differential sensitivity to these disruptions and varying requirements for ATP6AP2's functions across tissues.
What genetic approaches are most effective for studying ATP6AP2 function?
Several complementary genetic approaches have proven effective for studying ATP6AP2 function:
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RNA Interference: siRNA-mediated knockdown enables transient reduction of ATP6AP2 levels. Example protocol:
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Conditional knockout models: Cell type-specific deletion using Cre-loxP systems. Examples include:
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Domain-specific constructs: Expression of specific ATP6AP2 fragments (NTF vs. CTF) or mutated constructs to identify domain-specific functions
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CRISPR/Cas9 genome editing: For introducing specific patient mutations or creating model systems
Each approach offers advantages for addressing different research questions, with conditional systems being particularly valuable given the embryonic lethality of complete ATP6AP2 ablation .
How can researchers distinguish between ATP6AP2's V-ATPase-dependent and independent functions?
Distinguishing between ATP6AP2's V-ATPase-dependent and independent functions requires sophisticated experimental designs:
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Comparative phenotype analysis: Compare phenotypes between ATP6AP2 depletion and specific V-ATPase subunit depletion or pharmacological inhibition with bafilomycin A
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Domain-specific rescue experiments: Express specific ATP6AP2 fragments in ATP6AP2-deficient backgrounds to determine which domains rescue which functions
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V-ATPase activity measurement: Assess lysosomal/endosomal pH using pH-sensitive fluorescent probes while manipulating ATP6AP2 levels
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Localization studies: Examine subcellular localization of ATP6AP2 relative to V-ATPase components during different cellular processes
Key evidence for distinct functions comes from observations that bafilomycin A (V-ATPase inhibitor) treatment does not replicate all phenotypes of ATP6AP2 knockdown, particularly regarding ciliogenesis, where "bafilomycin A neither influenced the ciliary expression pattern nor the percentage of ciliated cells" .
How does ATP6AP2 participate in Wnt signaling pathways?
ATP6AP2 plays context-dependent roles in both canonical and non-canonical Wnt signaling pathways:
Canonical Wnt/β-catenin pathway:
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Functions as an adaptor protein between V-ATPase and the Wnt receptor complex in acidic endosomal compartments
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Maintains proliferation in adult neuronal stem cells through this pathway
Non-canonical Wnt pathways (PCP, Wnt/Ca2+):
The developmental context determines which pathway ATP6AP2 functions in: "in adult neuronal stem cells, ATP6AP2 maintains proliferation as a component of the canonical Wnt pathway, while in differentiating cells, it becomes a component of the non-canonical Wnt/PCP pathway" .
What is the relationship between ATP6AP2 and the renin-angiotensin system?
ATP6AP2 was originally identified as a receptor for renin and prorenin ((P)RR), though this function remains controversial . When renin or prorenin binds to ATP6AP2's extracellular domain, two main effects occur:
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RAS-dependent effects: Activation of the renin-angiotensin system, leading to increased production of angiotensin II
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RAS-independent effects: Triggering of intracellular signaling cascades, particularly the mitogen-activated protein kinase (MAPK) pathway
The controversy regarding ATP6AP2's role as a functional renin receptor stems partly from evolutionary considerations: ATP6AP2 is also found in invertebrates that lack the renin-angiotensin system , suggesting its primary functions may be independent of renin binding.
How does ATP6AP2 regulate cell proliferation and the cell cycle?
ATP6AP2 promotes cell proliferation through multiple mechanisms:
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Cell cycle progression: Promotes G1 to S phase transition, likely through canonical Wnt pathway activation
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Mitotic spindle organization: Translocates to spindle poles during mitosis and is essential for proper spindle formation
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Ciliogenesis inhibition: Prevents primary cilia formation, thereby promoting proliferation over differentiation
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Cell survival: Knockdown in rat insulinoma cells (INS-1) decreases cell viability and increases apoptosis
The multi-faceted nature of ATP6AP2's proliferative functions is evident in studies of ATP6AP2-deficient cells, which show markedly deformed spindles and cell cycle arrest .
What is the role of ATP6AP2 in ciliogenesis and how does this affect cellular differentiation?
ATP6AP2 functions as an inhibitor of ciliogenesis, with significant implications for the balance between proliferation and differentiation:
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This inhibitory role promotes a proliferative cellular state while preventing differentiation
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Interestingly, this function appears to be V-ATPase-independent, as bafilomycin A treatment (which inhibits V-ATPase) does not affect the ciliary expression pattern or percentage of ciliated cells
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The regulation of ciliogenesis represents a potential switching mechanism between:
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Proliferation state: High ATP6AP2 → canonical Wnt signaling → cilia suppression → continued cell division
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Differentiation state: Altered ATP6AP2 activity → non-canonical Wnt/PCP signaling → cilia formation → cellular specialization
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This regulatory function may be particularly relevant in neuronal development and could contribute to the neurological phenotypes seen in patients with ATP6AP2 mutations .
What are key considerations when designing experiments to study ATP6AP2 patient mutations?
When investigating ATP6AP2 mutations identified in patients, researchers should consider:
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Mutation-specific effects: Different mutations affect different functions of ATP6AP2:
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Protein stability: Some mutations primarily affect protein stability rather than specific functions. Expression of mutant constructs in HEK293T cells shows reduced protein levels
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Temporal factors: Induced ablation of ATP6AP2 in adult mice results in rapid lethality with multi-organ effects , suggesting developmental compensation may mask certain phenotypes
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Tissue specificity: Effects vary across tissues – ATP6AP2 ablation affects colon crypt morphology, bone marrow, and metabolic parameters
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Model systems: Consider patient-derived iPSCs, CRISPR/Cas9 knock-in models with exact patient mutations, and conditional knockout models with tissue specificity
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Validation approaches: Recapitulate patient phenotypes in model systems (e.g., hypoglycosylation of serum proteins in mice with temporal downregulation of ATP6AP2 in the liver )
How can the multiple functions of ATP6AP2 be systematically investigated?
A systematic investigation of ATP6AP2's multiple functions requires a multi-dimensional experimental approach:
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Fragment-specific studies:
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Context-dependent function analysis:
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Compare ATP6AP2 function across different cell types and developmental stages
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Analyze ATP6AP2 interactome in proliferating versus differentiating cells
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Pathway dissection:
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Employ pathway-specific reporters and readouts (Wnt, mTOR, V-ATPase)
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Use epistasis experiments to position ATP6AP2 within signaling cascades
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Temporal dynamics:
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Track ATP6AP2 localization and modification throughout the cell cycle
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Employ time-controlled gene ablation systems
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Integration of multiple techniques:
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Combine genetic approaches with biochemical, cell biological, and in vivo studies
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Apply systems biology approaches (proteomics, transcriptomics) to capture the full spectrum of ATP6AP2's effects
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This systematic approach acknowledges ATP6AP2's multifunctional nature and can help reconcile seemingly contradictory observations about its diverse roles.
What are the most promising directions for future ATP6AP2 research?
The multifunctional nature of ATP6AP2 and its involvement in various disease processes suggest several promising research directions:
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Precise delineation of domain-specific functions to enable targeted therapeutic approaches
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Investigation of tissue-specific requirements for ATP6AP2 to explain the variable manifestations of mutations
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Exploration of the molecular mechanisms underlying ATP6AP2's dynamic subcellular relocalization during mitosis
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Development of conditional models that more precisely mimic human disease states
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Identification of potential compensatory mechanisms that might be therapeutically exploited
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Examination of ATP6AP2's roles in additional tissues and cellular contexts beyond those currently studied
Product Science Overview
ATP6AP2 is a transmembrane protein that plays a crucial role as an accessory component of the vacuolar ATPase (V-ATPase). V-ATPases are essential for the acidification of intracellular compartments, which is vital for various cellular processes, including protein degradation, receptor-mediated endocytosis, and autophagy . The protein has a calculated molecular mass of approximately 39 kDa and contains an N-terminal hydrophobic signal sequence, a C-terminal transmembrane region, and several putative phosphorylation sites .
- Lysosomal Degradation and Autophagy: ATP6AP2 is essential for the proper functioning of lysosomal degradative processes and autophagy. It helps maintain intracellular pH homeostasis by facilitating proton transport across lysosomal membranes .
- Renin-Angiotensin System: ATP6AP2 acts as a receptor for renin and prorenin, playing a role in the renin-angiotensin system, which regulates blood pressure and fluid balance .
- WNT Signaling: The protein is also involved in WNT signaling, a pathway crucial for cell proliferation, differentiation, and migration .
ATP6AP2 is expressed in various tissues, with the highest levels observed in the brain, heart, and placenta. Intermediate expression levels are found in the kidney and pancreas, while lower levels are present in the lung and skeletal muscle . In the kidney, ATP6AP2 localizes to glomeruli and vascular structures, whereas in the heart, it is found in coronary arteries .
Mutations or dysregulation of the ATP6AP2 gene have been associated with several disorders, including:
- Parkinsonism with Spasticity, X-Linked: A neurodegenerative disorder characterized by parkinsonian features and spasticity .
- Congenital Disorder of Glycosylation, Type IIr: A metabolic disorder affecting glycosylation processes .
- Intellectual Developmental Disorder, X-Linked Syndromic, Hedera Type: A condition involving intellectual disability and other syndromic features .
