Recombinant Rat Interleukin-1 receptor accessory protein-like 1 (Il1rapl1)

What is IL1RAPL1 and where is it primarily expressed?

IL1RAPL1 (Interleukin-1 Receptor Accessory Protein-Like 1) is a member of the interleukin 1 receptor family, sharing approximately 52% homology with the IL-1 receptor accessory protein (IL1RAcP) . The protein contains three extracellular Immunoglobulin (Ig)-like domains, a single transmembrane domain, an intracellular Toll/IL-1R (TIR) domain, and a C-terminal tail of approximately 150 amino acids that is not shared with other family members . IL1RAPL1 is predominantly expressed in the brain, with particularly high expression in the hippocampus . Within neurons, IL1RAPL1 is localized to excitatory synapses with significant enrichment at the postsynaptic compartment .

How do IL1RAPL1 knockout models demonstrate its functional significance?

Knockout mouse models have been instrumental in demonstrating IL1RAPL1's functional significance in vivo. IL1RAPL1 knockout mice display several notable phenotypic changes including:

• Significant reduction in spine density of cortical neurons

• Mild impairments in spatial reference and working memories

• Deficits in remote fear memory consolidation or retrieval

• Reduced behavioral flexibility as observed in T-maze tests

• Surprisingly enhanced performance in rotarod tests compared to wild-type mice

• Consistently elevated locomotor activity across various behavioral tasks

• Decreased open-space and height anxiety-like behaviors

• Disturbed neuronal physiology leading to inhibitory-excitatory imbalance

• Altered dendrite morphology

These findings collectively suggest that IL1RAPL1 not only affects learning and memory processes but also influences behavioral flexibility, locomotor activity, and anxiety-related behaviors, highlighting its multifaceted role in brain function .

What technical challenges are commonly encountered when producing and purifying recombinant rat IL1RAPL1?

Producing recombinant IL1RAPL1 presents several technical challenges that researchers should anticipate. The protein contains multiple Ig-like domains in its extracellular region, which require proper disulfide bond formation for correct folding. The C31R mutation identified in patients with intellectual disability is located in the first Ig-like domain and affects a cysteine residue potentially involved in disulfide bonding, highlighting the importance of these structural features .

When expressing recombinant IL1RAPL1, researchers have observed decreased protein stability with certain mutations. For example, the Δex6 and C31R mutations lead to approximately 75% and 60% reduction in protein expression respectively compared to wild-type IL1RAPL1 when expressed in HEK293 cells . This indicates that maintaining proper protein folding and stability during recombinant production is a significant challenge.

For optimal production, mammalian expression systems are recommended over bacterial systems since post-translational modifications appear important for IL1RAPL1 function. Employing fusion tags that enhance solubility while minimizing interference with protein function is advisable. Careful optimization of purification protocols is necessary to preserve the protein's functional domains and interaction surfaces.

How should researchers evaluate IL1RAPL1 functionality in experimental systems?

Evaluating recombinant IL1RAPL1 functionality requires multiple complementary approaches:

• Protein localization assays: Since proper subcellular targeting is essential for IL1RAPL1 function, immunofluorescence assays measuring membrane localization and dendritic distribution are critical. While the Δex6 mutant shows severely reduced expression in neurons, C31R mutant localizes similarly to wild-type despite functional deficits .

• Synaptogenic activity assessment: Overexpression of IL1RAPL1 in cultured hippocampal neurons increases excitatory synapse formation, which can be quantified using pre- and post-synaptic markers. Wild-type IL1RAPL1 transfected neurons show significant increases in pre-synaptic markers like synaptophysin and VGLUT1, while mutants like Δex6 and C31R fail to induce similar effects .

• Electrophysiological recordings: Measure spontaneous excitatory postsynaptic currents (sEPSCs) to assess functional synapse formation. This provides direct evidence of IL1RAPL1's effect on synaptic transmission.

• Protein-protein interaction assays: Cell aggregation assays and co-immunoprecipitation can evaluate interactions with key binding partners like PTPδ, which is essential for IL1RAPL1's synaptogenic function .

• Signaling pathway activation: Assess JNK pathway activation, which remains functional even with certain mutations that disrupt synaptogenic activity .

This multi-faceted approach provides a comprehensive evaluation of recombinant IL1RAPL1 functionality across structural, localization, interaction, and signaling domains.

What are the optimal experimental conditions for studying IL1RAPL1 interactions with its synaptic partners?

Studying IL1RAPL1 interactions with its synaptic partners requires carefully optimized experimental conditions. For investigating interactions with PTPδ, which occurs through the extracellular domain and is essential for synaptogenesis, cell aggregation assays using HEK293 cells expressing the respective proteins have proven effective . These assays should be performed in calcium-containing media as these interactions may be calcium-dependent.

For studying intracellular domain interactions with partners like PSD-95, NCS-1, RhoGAP2, and Mcf2l, co-immunoprecipitation assays under non-denaturing conditions are recommended. Mild detergents that preserve protein complexes should be used during cell lysis. Including phosphatase inhibitors is critical when studying interactions influenced by phosphorylation states, such as with PSD-95.

Proximity ligation assays in cultured neurons provide valuable information about endogenous protein interactions in their native cellular context. For quantitative binding kinetics, surface plasmon resonance or isothermal titration calorimetry using purified protein domains offers precise measurement of binding affinities and thermodynamic parameters.

When working with recombinant proteins, it's important to verify that fusion tags do not interfere with the interaction interfaces, particularly for IL1RAPL1's C-terminal domain which mediates many of its protein interactions.

How do different IL1RAPL1 mutations affect protein stability and function?

Different mutations in IL1RAPL1 can affect protein stability and function through distinct mechanisms, providing valuable insights for researchers working with recombinant variants. Based on studies of naturally occurring mutations:

• Exon 6 deletion (Δex6): This in-frame deletion found in patients with intellectual disability results in severe protein instability. When expressed in HEK293 cells, Δex6 shows ~75% reduction in protein expression compared to wild-type . In neurons, Δex6 protein expression is almost completely abolished to background levels . While the protein can still reach the cell membrane, its severe instability prevents normal function.

• Point mutation C31R: Located in exon 3, this mutation leads to a cysteine-to-arginine substitution. In HEK293 cells, C31R shows ~60% reduction in protein expression compared to wild-type . Interestingly, in neurons, C31R demonstrates similar expression levels to wild-type protein and correct membrane targeting . Despite normal localization, C31R fails to induce pre- and post-synaptic differentiation.

• I643V variant: This variant shows higher expression levels than wild-type protein but maintains normal function across all parameters tested, suggesting it is a non-pathogenic polymorphism .

These findings highlight that mutations can disrupt IL1RAPL1 function through different mechanisms: severe protein instability (Δex6), maintained expression but functional impairment (C31R), or no significant effect (I643V). When working with recombinant IL1RAPL1, researchers should carefully consider how specific mutations might impact protein production, stability, and functional assays.

What molecular mechanisms underlie the synaptic defects caused by IL1RAPL1 mutations?

The synaptic defects caused by IL1RAPL1 mutations involve multiple molecular mechanisms that disrupt normal neuronal connectivity. Research has revealed several key pathways:

• Disrupted interaction with PTPδ: Both Δex6 and C31R mutations reduce the interaction between IL1RAPL1 and PTPδ as demonstrated by cell aggregation and immunoprecipitation assays . This trans-synaptic interaction is essential for IL1RAPL1-induced pre- and post-synaptic differentiation, and its disruption directly impacts synapse formation.

• Impaired excitatory synapse formation: Wild-type IL1RAPL1 overexpression in hippocampal neurons increases the excitatory pre-synaptic marker VGLUT1, while Δex6 and C31R mutants fail to induce this effect . Importantly, the inhibitory pre-synaptic marker VGAT remains unaffected by both wild-type and mutant IL1RAPL1, confirming its specific role in excitatory synapse development .

• Selective pathway disruption: Interestingly, not all IL1RAPL1-mediated signaling is affected by mutations. The JNK pathway activation remains functional even with mutants that disrupt synaptogenic activity . This selective impact on downstream pathways suggests different functional domains of IL1RAPL1 operate semi-independently.

• Inhibitory-excitatory imbalance: In IL1RAPL1 knockout mice, disturbed neuronal physiology leads to inhibitory-excitatory imbalance , which may contribute to the cognitive and behavioral phenotypes observed.

• Altered dendrite morphology: IL1RAPL1 deficiency affects dendrite complexity and neuronal maturation , potentially through disrupted interactions with regulators of Rho GTPases like RhoGAP2 and Mcf2l that are normally required for IL1RAPL1-induced dendritic spine formation .

Understanding these mechanisms provides critical context for researchers working with recombinant IL1RAPL1 to design functional assays that can detect subtle but important differences in protein activity.

How do Il1rapl1 mutations contribute to intellectual disability and neurodevelopmental disorders?

IL1RAPL1 mutations contribute to intellectual disability and neurodevelopmental disorders through several interrelated mechanisms that disrupt normal brain development and function. Current research indicates:

• Synapse formation deficits: The primary cellular mechanism involves impaired excitatory synapse formation and function. IL1RAPL1 mutants fail to induce pre- and post-synaptic differentiation , resulting in reduced connectivity between neurons. Knockout mice show decreased spine density in cortical neurons , supporting the critical role of IL1RAPL1 in establishing proper neuronal networks.

• Learning and memory impairments: IL1RAPL1 knockout mice exhibit mild impairments in spatial reference and working memories, as well as deficits in remote fear memory . These cognitive deficits align with the intellectual disability observed in patients with IL1RAPL1 mutations.

• Behavioral flexibility deficits: Reduced behavioral flexibility, as demonstrated in T-maze tests in knockout mice , suggests IL1RAPL1 plays a role in cognitive adaptability, which is often impaired in neurodevelopmental disorders.

• Altered excitatory-inhibitory balance: IL1RAPL1 deficiency leads to disturbed neuronal physiology causing inhibitory-excitatory imbalance , a common feature in many neurodevelopmental disorders including autism and intellectual disability.

• Expanded clinical spectrum: Beyond intellectual disability, IL1RAPL1 mutations have been implicated in anxiety disorders, language delay, infantile-onset seizures, developmental delay, autism with behavioral problems, depression, oppositional behavior, and impulsivity . Even schizophrenia has been potentially linked to IL1RAPL1 variants .

• Sex-specific manifestation: As an X-linked gene, most cases occur in males, though female cases have been reported, possibly due to unfavorable X-linked inactivation patterns or polygenic modifiers .

The diverse roles of IL1RAPL1 in synapse formation, neuronal maturation, and circuit function explain its broad impact on neurodevelopment when mutated, making it an important target for understanding and potentially treating these disorders.

What are the most effective cellular models for studying recombinant IL1RAPL1 function?

Multiple cellular models offer complementary advantages for studying recombinant IL1RAPL1 function, each appropriate for specific research questions:

• Primary hippocampal neurons: These represent the gold standard for studying IL1RAPL1's role in synapse formation and function. Primary neurons from rat or mouse hippocampus (typically cultured for 18 days in vitro) provide the most physiologically relevant environment for assessing IL1RAPL1's effects on excitatory synapse formation, dendritic spine morphology, and synaptic transmission . Co-transfection with GFP and HA-tagged IL1RAPL1 constructs allows visualization of transfected neurons and protein localization.

• HEK293 cells: While lacking neuronal properties, HEK293 cells are valuable for biochemical studies of IL1RAPL1, including protein expression, stability assessment, and protein-protein interactions . They're particularly useful for cell aggregation assays to study trans-synaptic interactions between IL1RAPL1 and partners like PTPδ.

• PC12 cells: These cells have been used to study IL1RAPL1's interaction with the calcium sensor NCS-1 and its regulation of N-type voltage-gated calcium channel activity , offering insights into IL1RAPL1's role in calcium signaling.

• Neuron-HEK293 co-culture systems: These hybrid systems allow assessment of IL1RAPL1's ability to induce presynaptic differentiation when expressed in HEK293 cells and co-cultured with neurons, providing a controlled environment to study specific trans-synaptic interactions.

• Cortical neurons: While hippocampal neurons are most commonly used, cortical neurons provide an alternative model system, particularly relevant since IL1RAPL1 knockout mice show decreased spine density in cortical neurons .

Each model system has specific advantages depending on the research question, with primary hippocampal neurons offering the most comprehensive platform for functional studies, while HEK293 cells provide a simpler system for biochemical and interaction studies.

What imaging techniques are most suitable for visualizing IL1RAPL1 localization and function in neurons?

Visualizing IL1RAPL1 localization and function in neurons requires specialized imaging techniques that provide both spatial resolution and functional insights:

• Confocal microscopy with immunofluorescence: This fundamental approach allows visualization of IL1RAPL1 distribution within neuronal compartments. Using HA-tagged IL1RAPL1 constructs co-transfected with GFP enables identification of transfected neurons and assessment of protein localization . Multi-channel imaging can simultaneously visualize IL1RAPL1 along with synaptic markers like VGLUT1 (excitatory presynaptic), VGAT (inhibitory presynaptic), or PSD-95 (postsynaptic) .

• Super-resolution microscopy: Techniques like STED (Stimulated Emission Depletion) or STORM (Stochastic Optical Reconstruction Microscopy) overcome the diffraction limit of conventional microscopy, providing nanoscale resolution of IL1RAPL1 localization within synaptic structures. This is particularly valuable for studying precise positioning relative to the synaptic cleft.

• Live cell imaging: For studying dynamic aspects of IL1RAPL1 trafficking, fusion constructs with fluorescent proteins (rather than fixed immunofluorescence) enable real-time visualization in living neurons. This approach can reveal transport mechanisms and activity-dependent relocalization.

• Surface labeling techniques: Since IL1RAPL1 is a transmembrane protein, distinguishing surface-expressed protein from intracellular pools is important. This can be achieved by live-labeling surface proteins using antibodies against extracellular epitopes (like the HA tag in recombinant constructs) before fixation and permeabilization .

• Proximity ligation assay (PLA): This technique enables visualization of protein-protein interactions with spatial resolution, allowing researchers to detect IL1RAPL1 interactions with binding partners like PTPδ or PSD-95 in situ.

• FRET/FLIM imaging: For studying protein-protein interactions with high spatial resolution, Förster Resonance Energy Transfer (FRET) combined with Fluorescence Lifetime Imaging Microscopy (FLIM) can reveal interactions between IL1RAPL1 and its binding partners at the nanometer scale.

These complementary approaches provide researchers with a comprehensive toolkit for characterizing IL1RAPL1's localization, trafficking, and functional interactions within the complex architecture of neurons.

What electrophysiological approaches are most informative for assessing IL1RAPL1's impact on synaptic function?

Electrophysiological approaches provide direct functional assessment of how IL1RAPL1 influences synaptic transmission and neuronal excitability. The most informative techniques include:

• Spontaneous excitatory postsynaptic current (sEPSC) recordings: This approach directly measures the functional outcome of IL1RAPL1-induced synaptogenesis. In cultured hippocampal neurons, overexpression of wild-type IL1RAPL1 increases both the frequency and amplitude of sEPSCs, reflecting enhanced excitatory synapse formation and function . Comparing these parameters between neurons expressing wild-type versus mutant IL1RAPL1 (e.g., Δex6 or C31R) provides functional evidence of how mutations impact synaptic transmission.

• Evoked excitatory postsynaptic potentials/currents (EPSPs/EPSCs): By stimulating presynaptic neurons and recording from postsynaptic neurons expressing recombinant IL1RAPL1 variants, researchers can assess synaptic strength, release probability, and short-term plasticity. This approach is particularly valuable in paired recordings where both pre- and postsynaptic cells are identified.

• Miniature excitatory postsynaptic currents (mEPSCs): Recorded in the presence of tetrodotoxin to block action potentials, mEPSCs reflect spontaneous release of single synaptic vesicles. Changes in mEPSC frequency indicate alterations in synapse number or release probability, while amplitude changes suggest modified postsynaptic receptor content.

• Long-term potentiation/depression (LTP/LTD): Since IL1RAPL1 knockout mice show impaired learning and memory , examining how IL1RAPL1 affects synaptic plasticity provides insights into its role in learning mechanisms. LTP/LTD protocols in hippocampal slices from wild-type versus knockout mice, or in neurons with acute manipulation of IL1RAPL1 expression, can reveal its contribution to activity-dependent synaptic modifications.

• Paired-pulse ratio analysis: This protocol assesses presynaptic release probability by delivering two stimuli in quick succession and measuring the ratio of the second response to the first. Given IL1RAPL1's involvement in presynaptic differentiation , this approach can reveal its impact on presynaptic function.

• In vivo recordings: For more integrative understanding, in vivo recordings from IL1RAPL1 knockout mice during behavioral tasks can link cellular electrophysiological changes to behavioral phenotypes like impaired spatial memory or reduced behavioral flexibility .

These electrophysiological approaches provide complementary information about how IL1RAPL1 influences synapse formation, function, and plasticity at both cellular and circuit levels.

How might recombinant IL1RAPL1 be utilized as a therapeutic target for neurodevelopmental disorders?

Recombinant IL1RAPL1 offers several promising therapeutic avenues for neurodevelopmental disorders based on our understanding of its molecular function:

• Protein replacement therapy: For loss-of-function mutations like deletions or unstable mutants (e.g., Δex6) , delivering functional recombinant IL1RAPL1 could potentially rescue synaptogenic deficits. This approach would require development of methods to effectively deliver proteins across the blood-brain barrier and into neurons.

• Small molecule enhancers: For mutations that reduce but don't eliminate protein expression or function (like C31R) , small molecule stabilizers or functional enhancers could potentially boost residual activity to therapeutic levels. High-throughput screening using recombinant IL1RAPL1 variants could identify such compounds.

• Targeting downstream pathways: Since IL1RAPL1 regulates several signaling pathways including JNK activation and PSD-95 phosphorylation , modulating these downstream effectors could potentially bypass the need for functional IL1RAPL1. This approach might be particularly relevant for complete loss-of-function scenarios.

• Peptide mimetics: Synthetic peptides mimicking critical interaction interfaces of IL1RAPL1 (particularly those mediating trans-synaptic interactions with PTPδ ) could potentially substitute for the full protein in promoting synapse formation.

• Gene therapy approaches: For genetic disorders resulting from IL1RAPL1 mutations, gene therapy offers a potential avenue for delivering functional genes to affected neurons. The X-linked nature of many IL1RAPL1-related disorders makes them particularly amenable to gene therapy approaches.

• Early intervention strategies: Given IL1RAPL1's role in synapse formation and neuronal maturation , earlier therapeutic intervention during critical developmental windows might yield better outcomes than later treatment. Animal models could help determine optimal timing for such interventions.

These approaches are still in early research stages but represent promising directions for translating our understanding of IL1RAPL1 function into therapeutic strategies for associated neurodevelopmental disorders.

What are the most promising approaches for resolving contradictory findings in IL1RAPL1 research?

Resolving contradictory findings in IL1RAPL1 research requires systematic approaches that address experimental, biological, and analytical variables:

• Standardized experimental systems: Establishing consistent cellular models, expression systems, and functional assays would reduce variability across studies. For recombinant IL1RAPL1, standardizing protein tagging strategies (e.g., position and type of epitope tags) and expression levels is particularly important, as these can influence protein localization and function.

• Comprehensive mutation characterization: Different IL1RAPL1 mutations may affect distinct functional domains and molecular interactions. For example, the C31R mutation maintains near-normal expression levels in neurons while disrupting function, whereas the Δex6 mutation severely reduces protein expression . Comprehensive characterization of mutations across multiple functional assays would help resolve apparently contradictory findings.

• Cell-type specific effects: IL1RAPL1 function may vary across neuronal subtypes or brain regions. While many studies focus on hippocampal neurons , IL1RAPL1 is expressed throughout the brain with varying levels . Comparative studies across multiple neuronal populations would help clarify whether contradictory findings reflect genuine biological differences.

• Developmental timing considerations: IL1RAPL1's role in synapse formation and neuronal maturation suggests its function may change throughout development . Temporal analysis of IL1RAPL1 function at different developmental stages could reconcile findings that appear contradictory when developmental context is not considered.

• Integration of in vitro and in vivo approaches: Combining detailed mechanistic studies in cellular models with behavioral and physiological assessments in animal models provides complementary insights. For example, correlating synaptic defects observed in cultured neurons with specific behavioral phenotypes in IL1RAPL1 knockout mice creates a more coherent understanding.

• Meta-analysis of existing data: Systematic review and meta-analysis of published IL1RAPL1 studies could identify patterns and sources of variability across studies, highlighting which contradictions reflect biological complexity versus methodological differences.

By addressing these factors systematically, researchers can resolve apparent contradictions and develop a more nuanced understanding of IL1RAPL1's multifaceted roles in neuronal development and function.

What novel experimental tools would advance IL1RAPL1 research in the coming decade?

The advancement of IL1RAPL1 research in the coming decade will likely depend on developing several innovative experimental tools that address current limitations:

• Domain-specific conditional knockout models: Current knockout models eliminate the entire IL1RAPL1 protein , but domain-specific knockouts would enable dissection of the roles of individual protein domains (e.g., the extracellular Ig domains versus the intracellular TIR domain). Conditional knockout systems allowing temporal control would help distinguish developmental versus acute functions.

• High-resolution structural information: Despite its importance, the complete three-dimensional structure of IL1RAPL1 remains unresolved. Cryo-electron microscopy or X-ray crystallography of full-length IL1RAPL1 and its complexes with binding partners like PTPδ would provide crucial insights into interaction mechanisms and how mutations disrupt function.

• Single-cell multi-omics approaches: Combining single-cell transcriptomics, proteomics, and functional assays in IL1RAPL1-expressing neurons could reveal cell-specific functions and molecular networks, helping explain the selective vulnerability of certain neuronal populations in IL1RAPL1-related disorders.

• In vivo imaging of IL1RAPL1 dynamics: Development of genetic tools for visualization of endogenous IL1RAPL1 in live animals, such as knock-in fluorescent protein fusions compatible with two-photon imaging through cranial windows, would enable monitoring of IL1RAPL1 dynamics during development and in response to learning experiences.

• Nanobodies and intrabodies: Development of IL1RAPL1-specific nanobodies would enable acute manipulation of protein function without genetic modification. Intrabodies targeting specific domains could disrupt selected interactions while leaving others intact, providing more nuanced interventions than global knockout.

• Human neuronal models: Expanding research from rodent to human cellular models through induced pluripotent stem cell (iPSC)-derived neurons from patients with IL1RAPL1 mutations would better capture human-specific aspects of protein function and pathology.

• Optogenetic and chemogenetic tools: Developing tools for acute and spatially restricted manipulation of IL1RAPL1 function would help distinguish its roles in synapse formation versus maintenance and elucidate region-specific functions in complex behaviors.

These advanced tools would collectively enable a more sophisticated understanding of IL1RAPL1's multifaceted roles in brain development and function, potentially leading to targeted therapeutic approaches for associated neurodevelopmental disorders.