Functions and mechanisms of the GPCR adaptor protein Norbin

Norbin is a highly conserved adaptor protein first identified in 1997 [1]. Early on, Norbin was considered almost exclusively neuronal and was mainly studied for its important roles in neuronal morphology and synaptic plasticity, which were reviewed previously [2]. However, over recent years, Norbin was also found in myeloid cells, acting as an immune suppressor. We review here the physiological and pathophysiological roles of Norbin, and the mechanisms through which it acts, with particular emphasis on recent findings.

Neurite-outgrowth-related protein from the rat brain (Norbin/Neurochondrin/NCDN) is an essential protein encoded by the NCDN gene, conserved from invertebrates to vertebrates, with high (98%) sequence identity among human, mouse and rat, and 47% similarity between human and Drosophila melanogaster (Figure 1). Our bioinformatic analysis shows Norbin expression even in plants, e.g. with 44% similarity between human and Arabidopsis thaliana (AT4G32050) (Figures 2 and 3). Norbin has never been studied in plants, except for a genome-wide association study in the Asian cotton (Gossypium arboretum) identifying an association between ncdn SNPs and salt tolerance [3]. It appears that Norbin was lost from worms, as it is absent from Caenorhabditis elegans.

Norbin is a 79 kDa protein without catalytic activity, homologies to other proteins, or known protein domains. Of note, InterPro designated a ‘neurochondrin domain’ spanning the length of the protein and describing the protein rather than a functional domain. Norbin is enriched in leucines (∼16%) and has interspersed low-complexity regions [4]. A structure has not been solved, although gel filtration indicated it is non-globular, and circular dichroism suggested an α-helical structure (∼65%) [5]. These observations are supported by in silico structures in AlphaFold (alphafold.ebi.ac.uk) predicting a curved structure like a boomerang, comprised of α-helices (∼67% of the protein) organising into units resembling HEAT repeats, flexible arrays of amphiphilic α-helices connected via a short linker sequence and arranged in an antiparallel conformation concealing their hydrophobic core (Figure 3) [2,6,7]. The curved conformation is thought important in protein–protein interactions, facilitating binding to structural features such as hydrophobic regions [8], and suggests Norbin may only adopt a fixed conformation in complex with other interactors.

Two isoforms of Norbin were identified, the canonical full-length 729 aa isoform and a 712 aa isoform, N-terminally truncated through alternative splicing [4,9]. These isoforms are conserved between humans and mice, although only single bands presumed to be full-length were observed by northern and western blots [4,9]. To evaluate the abundance of Norbin variants, we analysed ENCODE RNAseq datasets of brain regions of newborn mice (Figure 4). This identified two transcripts (Ncdn-201 and Ncdn-202) which give rise to the 729 aa full-length protein (UniProt: Q9Z0E0-1) and one (Ncdn-203) encoding the 712 aa splice-variant (UniProt: Q9Z0E0-2). Furthermore, we found two previously undescribed transcripts that also encode the 712 aa splice variant and one that might encode a shorter hypothetical protein for which there is no experimental evidence. Ncdn-203 was predominant throughout the brain, particularly in the fore- and midbrain. Of the two RNAs encoding full-length Norbin, Ncdn-201 was most common. Extrapolating protein levels from transcript abundance, we propose the 712 aa splice variant may be the predominant Norbin protein in neonatal mouse brain, the full-length protein reaching comparable levels only in hindbrain. Isoform-specific antibodies or proteomics need to confirm this expression in future. Potentially, the two variants have different functions. Notably, the 712 aa splice-variant lacks cysteines C3 and C4, whose palmitoylation localises Norbin to early endosomes in hippocampal neurons [10].

Little is known regarding Norbin regulation. A mechanism of intramolecular inhibition was proposed, as the isolated aa 1–100 of Norbin stimulated more neurite outgrowth in N2a cells than the full-length protein did, and bound Norbin aa 100–729 more robustly [11]. However, structural predictions do not support intramolecular interactions between the N- and C-termini. It was suggested that Norbin might dimerise [3]. Perhaps dimerisation, rather than intramolecular inhibition, serves to sequester inactive Norbin. Whether Norbin exists as a dimer, and if signalling alters its conformation or dimerisation state, remains to be demonstrated. The only other tangible evidence for Norbin regulation is related to subcellular localisation, through binding other proteins and lipids (see ‘Subcellular localisation’).

Norbin was first identified as a gene up-regulated upon induction of long-term potentiation (LTP) in rat hippocampus [1]. This spurred extensive characterisation in the brain where Norbin is strongly expressed, particularly the hippocampus, cerebral cortex, cerebellum and amygdala [4,9,12,13]. Norbin is found in multiple types of neurons, localising to somatodendritic regions, but is absent from glial cells, as identified by the lack of expression in cells staining positive for glial fibrillary acidic protein [1,10,13–17]. Its neuronal expression is regulated by transcription factor FoxO3a [18].

Norbin expression is more widespread than first thought, detectable in the peripheral nervous system (sciatic nerve, brachial plexus), testes, ovaries, heart, lung, kidney and skeletal muscle [4,12,13]. In Drosophila, Norbin is expressed in fibrillar skeletal muscle, limiting sarcomere branching during myofibril formation, is required for the ability to fly [19], and is up-regulated during hyperactivity-induced myopathy [20]. Norbin is also expressed in bones (nucleus pulposus, osteoblasts, osteocytes, chondrocytes) and linked with resorption [9,21–23]. Recently, Norbin was shown to play an important role in the immune system, where it is expressed in the myeloid lineage (neutrophils and macrophages), as well as in the thymus (see below) [12,24].

Norbin is mostly cytosolic, in a range of cell types [10,12,13,15]. Co-expression with the guanine-nucleotide exchange factor (GEF) P-Rex1 in endothelial cells translocates both Norbin and P-Rex1 to the plasma membrane [12]. Furthermore, Norbin associates with G protein-coupled receptors (GPCRs) (see below), which also promotes plasma membrane localisation [25]. Norbin remains attached to the GPCR even after prolonged agonist stimulation, and potentially during receptor endocytosis [25]. Accordingly, Norbin associates with Rab5-positive endosomes in cortical and hippocampal neurons, and co-purifies with Rab5 upon co-expression in SH-SY5Y human neuroblastoma cells [10,15].

Norbin can be palmitoylated at cysteines C3 and C4, by palmitoyl-transferases DHHC 1, 3 and 10, its endosomal localisation depending on this palmitoylation [8]. Palmitoylation is a reversible post-translational modification, which can confer plasma-membrane association and trafficking between membrane compartments, sometimes in response to signalling [26]. However, regulation and consequences of Norbin palmitoylation remain unknown.

Finally, Norbin interacts with phosphatidic acid (PA) [27], a phospholipid second messenger generated by phospholipase D and diacylglycerol kinases [28,29]. Using immobilised phosphatidic acid, we show that both full-length Norbin and the 712 aa splice variant bind PA (Figure 5A–C). Mutagenesis revealed that PA binds mainly to the middle portion, but also the N- and C-termini (Figure 5C–E). In contrast, Norbin does not bind phosphatidylinositol (4,5)-bisphosphate (PIP₂) (Figure 5A,E). Hence, Norbin might associate with cell membranes in response to PA-generating signalling pathways.

Several studies proposed that Norbin may be secreted. Norbin was found in the conditioned medium of BW5147 metastatic T cell lymphoma cells [9], and in the extracellular uterine luminal fluid in cattle [30]. The frequent occurrence of Norbin autoantibodies in autoimmune-diseases (see below) may add further evidence. However, Norbin is largely cytosolic, any interactions with lipids and membrane proteins place it on the cytoplasmic face of membranes, and it has no signal sequences which would destine it for secretion. Rather, Norbin might escape cells upon their death, and as it is highly abundant in neurons, an appreciable amount could be found extracellularly upon neuronal death.

Norbin was recognised early on for its functions in the nervous system. Whole-mouse knock-out is embryonic lethal [31], and conditional knock-out in parts of the nervous system results in spatial learning defects, epileptic seizures, impaired cognitive functions, and depression- or schizophrenia-like behaviours [31–33]. In neuronal cell lines, neurite outgrowth depends on Norbin [11,13,18], although mice with nervous system-wide deletion show no obviously defective neuronal morphology [32]. In mice with Norbin deficiency in postnatal forebrain, hippocampal synaptic plasticity is impaired [34], and hippocampal Norbin deficiency affects adult neurogenesis [33]. Neuronal Norbin expression can respond to extracellular cues. For example, Norbin was up-regulated in the amygdala upon exposure of rats to cat odours to induce fear [35], and in the brain of bees after olfactory training exercises [36].

Recent work from our laboratory identified Norbin as a suppressor of innate immunity. Mice with Norbin deficiency in the myeloid lineage (Ncdn^Δmye) had 10-fold elevated immunity during pulmonary pneumococcal infection and septic peritonitis [24]. Using immune-cell depletion, we showed that Norbin deficiency in neutrophils was responsible for this elevated immunity, whereas Norbin deficiency in macrophages had no effect [24]. During pneumococcal infection, overall leukocyte recruitment was normal, but neutrophils migrated more rapidly into the infected alveolar airspace [24]. The responsiveness of isolated Ncdn^Δmye neutrophils to a range of stimuli was increased, with enhanced degranulation, phagocytosis, production of reactive oxygen species (ROS) and neutrophil extracellular traps (NETs), and ROS-dependent killing of bacteria [24]. Neutrophils are tightly regulated, their excessive responses can exacerbate inflammatory diseases such as rheumatoid arthritis [37], but despite Norbin functioning as an immune-suppressor, autoinflammation was not detected in Ncdn^Δmye mice [24]. Of note, our recent evaluation of the neutrophil proteomes from young and old mice showed no age-related difference in Norbin expression, suggesting no obvious role in inflamm-aging, a state of chronic, low-grade sterile inflammation associated with aging [38]. Furthermore, in lymphoma-derived MSB-1 cells, Norbin was shown to indirectly affect the phosphorylation of nuclear factor of activated T cells (NFAT), and may thus play a role in transcription [39]. It would be interesting to assess Norbin in chronic inflammation and in different types of immune cells.

The mechanisms through which Norbin carries out its functions are incompletely understood. The best-described mechanism is through its constitutive, direct interaction with GPCRs, which regulates GPCR trafficking and/or signalling. Other mechanisms include the regulation of Rac GEFs and protein kinase A (PKA).

Norbin was identified as a GPCR adapter protein in a yeast two-hybrid screen for interactors of class A GPCR melanin-concentrating hormone receptor 1 (MCH₁) [25]. Norbin was subsequently screened against other GPCRs and shown to interact with the C-terminal portions of 35 out of 55 GPCRs tested, including class C metabotropic glutamate receptors (mGluRs) [25,34,40]. The binding was mapped to membrane-proximal portions of the MCH₁ and mGlu_5a tails and the Norbin C-terminus (Figure 6). Two adjacent Norbin-binding regions were identified in mGlu_5a [34], and recently an ostensibly innocuous A687G Norbin point mutation was shown to block mGlu₅ binding [14] (Figure 6). However, Norbin binding to GPCRs is unpredictable, as there is no sequence homology ([34,40]; and unpublished observations), so Norbin is presumed to confer binding through its secondary structure.

Commonly, Norbin regulates the steady-state cell surface levels of GPCRs it interacts with. For example, mGlu₅ cell surface localisation was elevated upon Norbin expression in N2a cells, except with Norbin-binding deficient mGlu₅ mutants [34]. Knock-down of Norbin in rodent neurons reduced the cell surface localisation of mGlu₅ and mGlu₁ [14,34]. Similarly, more MCH₁ localised at the surface of HEK293 cells expressing Norbin [25]. In contrast, the surface localisation of several class A GPCRs, including CXCR4 and C5a receptor (C5a₁), was increased in Ncdn^Δmye mouse neutrophils, whereas CXCR1, which does not interact, localised normally [24,40]. The total cellular levels of GPCRs are unaffected ([34,40]; and unpublished observations)), so it is unlikely that Norbin regulates GPCR biosynthesis or degradation.

Norbin was assessed by several groups for its involvement in the agonist-induced internalisation of GPCRs. Initial reports suggested no role in the MCH-stimulated internalisation of MCH₁ in HEK293 cells, with normal agonist-concentration dependence and kinetics [25]. However, Ojha et al. [14] showed that Norbin depletion in cortical and hippocampal neurons abrogates the agonist-induced internalisation of mGlu₁ and mGlu₅, rescued by shRNA-resistant Norbin. How Norbin regulates agonist-induced GPCR internalisation is under investigation, possibly by scaffolding the internalisation machinery or by inhibiting negative regulators. Norbin competes with periplakin (another promiscuous GPCR adapter) for binding to MCH₁ [40], and it competes with calmodulin (involved in the agonist-induced internalisation of certain GPCRs) [41,42] for binding to mGlu₅. [34]. In any event, Norbin likely uses distinct mechanisms to influence steady-state and agonist-induced GPCR trafficking. Norbin might regulate the constitutive internalisation of GPCRs, which often occurs at significant level [43–46], or the recycling of GPCRs from endosomes back to the plasma membrane. These processes warrant future investigation.

Norbin regulates GPCR signalling through the associated heterotrimeric G protein alpha subunits (Gα). Norbin expression in HEK293 cells reduces the activation (GTP-loading) of Gα_o and Gα₁₁ upon stimulation of MCH₁ with MCH [25]. It is unknown whether Norbin can affect GPCR signalling also through the Gβγ subunits, or independently of the heterotrimeric G protein altogether. However, it is clear that Norbin influences several pathways downstream of GPCRs, positively or negatively. Furthermore, Norbin can control GPCR signalling separately from GPCR trafficking. Firstly, many of its effects on GPCR signalling pathways are measurable within seconds, too fast for trafficking events. Secondly, for MCH₁, Norbin was shown to control downstream Ca²⁺ signalling without obviously affecting MCH₁ trafficking [25].

One common pathway downstream of GPCRs is the activation of phospholipase C, which generates inositol 1,4,5-trisphosphate (IP₃), leading to Ca²⁺ release from intracellular stores. Ca²⁺ mobilisation in response to MCH stimulation of MCH₁ was reduced upon Norbin expression in HEK293 cells [25]. Similarly, stimulation of orexin-1 receptor (OX₁) or thromboxane receptor (TP), which bind Norbin, also reduced Ca²⁺ release upon expression of Norbin [25,40]. In contrast, HEK293 cells expressing Norbin showed raised IP₃ accumulation upon mGlu₅ stimulation with the group 1 mGluR agonist l-quisqualic acid [34]. Stimulation with mGlu₅ agonist 3,5-dihydroxyphenylglycine (DHPG) also caused significantly more Ca²⁺ oscillations in the presence of Norbin, except when mGlu₅ mutants defective in Norbin binding were used [34]. Therefore, regulation of Ca²⁺ signalling requires direct interaction between Norbin and GPCR. Accordingly, Ca²⁺ levels were normal upon stimulation of the histamine H1 receptor (H1), which does not interact with Norbin [40].

Ca²⁺ binds numerous target proteins, including calmodulin, which results in the activation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). CaMKII can signal downstream of mGlu₅, among other receptors, and normal CamKII activity is essential for spatial learning [47–49]. Mice with neuronal Norbin deficiency showed increased CaMKII activity in hippocampal lysates and impaired spatial learning [32]. It remains to be investigated if Norbin regulation of CaMKII occurs downstream of GPCR signalling.

Stimulation of GPCRs commonly leads to ERK1/2 activation. However, Norbin did not affect ERK activity upon stimulation of MCH₁ or histamine receptor H1, despite regulating Gα activity and Ca²⁺ mobilisation in these contexts [40]. In contrast, ERK activation upon DHPG stimulation of mGlu₅ was dependent on Norbin in HEK293 cells [34] and SH-SY5Y cells [50]. Interestingly, ERK activation was lost when two Norbin variants with SNPs near or within the mGlu₅-binding region, identified in patients with intellectual disability, were expressed rather than wild-type Norbin [50], suggesting that Norbin promotes ERK signalling downstream of mGlu₅. This is another example of separate roles of Norbin in GPCR trafficking and signalling. ERK activity downstream of mGlu₅ is increased by Norbin, despite Norbin promoting the agonist-stimulated internalisation of mGlu₅ and less receptor remaining on the cell surface. There are also examples of Norbin inhibiting ERK signalling downstream of GPCRs. Stimulation of Norbin-deficient mouse neutrophils with f-Met-Leu-Phe (fMLP), ligand of the GPCRs formyl peptide receptor (FPR) 1 and 2, caused elevated ERK activation compared with wild-type, suggesting that Norbin limits FPR-dependent ERK activity [24].

ERK is best known for controlling proliferation. Norbin is required for the proliferation and maturation of neuronal precursors during adult neurogenesis [33], and we have evidence that Norbin is required for the growth, survival and cell cycle progression of PC12 cells (manuscript under revision). It is conceivable that Norbin dysregulation contributes to diseases of deregulated proliferation, such as cancer, through its role in ERK signalling. Furthermore, ERK activity is also required for ROS production, and we showed that the elevated ROS production in Ncdn^Δmye neutrophils, which underlies their enhanced capacity to kill bacteria, can be reversed by ERK inhibitors [24], so the control of ERK activity also appears important for the immune functions of Norbin.

We showed that Norbin binds the Rac-GEF P-Rex1 directly, at the PH domain, and that interaction between P-Rex1 and Norbin also occurs in cells [12]. Norbin and P-Rex1 promote each other's localisation at the plasma membrane, where P-Rex1 needs to be localised to activate Rac [12]. Norbin also directly stimulates the Rac-GEF activity of P-Rex1 in vitro, enhancing the limited constitutive Rac-GEF activity of P-Rex1, as well as its PIP₃- and Gβγ-stimulated activities [12]. Norbin expression in HEK293 cells led to increased P-Rex1-dependent Rac activation in response to lysophosphatidic acid stimulation [12]. These results suggested Norbin positively regulates Rac-GEF function. Rac activity is required for changes in cell morphology, adhesion and migration, through the control of actomyosin cytoskeletal dynamics. Indeed, we showed Norbin promotes Rac-dependent endothelial cell morphologies upon stimulation with lysophosphatidic acid [12]. Others showed that Norbin controls the morphology and synaptic plasticity of neuronal cells [11,13,18,34], which are also Rac-dependent. Hence, it is conceivable that dysregulation of Rac-GEFs and Rac activity underlie Norbin-associated neurological disorders (see below).

Norbin can also act as a suppressor of Rac activity, as Ncdn^Δmye neutrophils have elevated Rac1 and Rac2 activities when stimulated with fMLP, independently of P-Rex1 [12,24]. Use of Rac inhibitors showed this Rac activity is required for the elevated ROS production in Ncdn^Δmye neutrophils which confers the increased capacity to kill bacteria [24]. Furthermore, we identified Vav1 as another Rac-GEF target of Norbin, as Vav1 activity was increased in Norbin-deficient neutrophils [24]. Thus, Norbin can both promote and limit Rac-GEF function and Rac activity, depending on context, and at least in part downstream of GPCR signalling.

Protein kinase A (PKA) is a cAMP-dependent kinase regulated by multiple mechanisms, including binding of A-kinase anchoring proteins (AKAPs) [51]. AKAPs are scaffolds which confine PKA signalling to specific membrane microdomains or organelles through interactions with other proteins [51]. Norbin acts as an atypical AKAP by binding to the PKA regulatory subunit RIIα with nanomolar affinity [5]. Two separate locations on Norbin, aa 65–84 and 277–291, play a role in this interaction [5]. Association of Norbin with PKA within the cell presumably occurs at the plasma membrane, as it is required for the agonist-induced internalisation of mGlu₅ in hippocampal neurons [14], for the recruitment of PKA to AMPA receptors (AMPARs), and for the mGluR-mediated endocytosis of AMPARs, important steps in group I mGluR-dependent synaptic plasticity [14]. Hence, the regulation of PKA by Norbin appears closely linked to GPCR trafficking, but it remains to be seen if it affects PKA activity and PKA signalling downstream of GPCRs.

Several other proteins bind Norbin, including Dia1, a Rho-GTPase effector crucial for actin cytoskeleton dynamics. Norbin was isolated with GST-Dia1 from mouse brain lysate, and interacts with Dia1 through its N-terminal 100 aa, binding to the FH3 domain of Dia1. Co-expression of Norbin aa 1–100 and Dia1 resulted in elevated neurite outgrowth, independently of the actin-polymerising activity of Dia1 [11]. Additionally, Norbin interacts with Semaphorin 4C (Sema4C), a transmembrane receptor for plexin in the central nervous system. Norbin was isolated with GST-Sema4C from mouse brain, and the proteins interact in vitro (Figure 6), as well as upon co-expression in HEK293 cells [52]. The functional consequences of this interaction have yet to be elucidated. Finally, Norbin was recently identified as an interactor of the lysosomal enzyme iduronate-2-sulfatase in mouse brain lysates using an affinity chromatography approach, but again any functional consequences remain unknown [53].

Norbin dysregulation is linked to neurological conditions. One recent study identified six patients displaying developmental delay, intellectual disability, and epilepsy, with four separate inherited or de novo missense variants in NCDN (Figure 6) [50]. In contrast to wild-type Norbin, these variants were unable to restore neurite-outgrowth defects in Norbin-deficient SH-SY5Y cells [50]. Two of the Norbin mutations fell within or adjacent to the mGlu₅-binding region (Figure 6), and displayed defective mGlu₅-dependent ERK signalling, whereas two variants with mutations more distant to the mGlu₅-binding region did not [50].

Nicolas et al. [54] identified a de novo nonsense mutation in a patient with frontotemporal dementia, corresponding to a ∼30% reduction in Norbin expression in brain, and thus suspected haploinsufficiency (Figure 6). The disease subtype is associated with fused in sarcoma (FUS)-positive protein aggregates, and shRNA Norbin-depleted rat cortical neurons contained fewer but larger FUS-containing granules. Interestingly FUS depletion reduced Norbin expression, indicating a co-dependence speculated to contribute to disease pathophysiology [54].

Post-mortem examination of dorsolateral prefrontal cortex samples from schizophrenia patients showed significantly reduced Norbin expression [55]. To control for possible effects of drugs commonly prescribed for schizophrenia, Norbin levels were evaluated in rats treated with these drugs, and were normal [55].

Deregulation of Norbin is associated with bone and cartilage defects. In nucleus pulposus cells in the disks between vertebrae, loss of Norbin occurs during normal aging. Norbin-rich juvenile cells were proposed as future disk repair strategies [56]. Furthermore, a SNP in NCDN is linked to osteochondrosis in horses [57], and mice with heterozygous Norbin deletion show cartilage abnormalities reminiscent of chondrocyte proliferation and differentiation abnormalities [31].

Norbin autoantibodies are linked to various neurological conditions, including ataxia, dystonia, encephalopathy, cerebellar degeneration and Alzheimer's disease [58–65]. It is debated whether these autoantibodies contribute to the disease pathology, instead they were proposed to be symptomatic, as the result of a T-cell driven autoimmune response [62]. Accordingly, patients with cerebellar degeneration did not respond to antibody-depletion therapy, whereas long-term immunosuppressive treatment stabilised or improved their condition, despite Norbin antibody levels remaining unaltered [63]. Norbin protein was detected in patient plasma after ischemic stroke [66], suggesting that neuronal cell death associated with neurological conditions may cause cytosolic Norbin becoming available as an extracellular antigen. As mentioned above, several reports detected Norbin extracellularly, but it remains to be seen if this is relevant in autoantibody generation.

Spinal muscular atrophy (SMA) is caused by decreasing levels of survival motor neuron (SMN), a protein associated with mRNA transport. Norbin forms a complex with SMN, and SH-SY5Y cells depleted of SMN have fewer Norbin-containing cytoplasmic foci, whereas Norbin depletion increased SMN foci [15]. The authors speculated that the reduced SMN level seen in SMA may compromise Norbin localisation and function.

Since Norbin was first discovered, it has been clear that this protein plays crucial roles in the nervous system. However, recent research has shown that there is much more to Norbin. Of particular interest is its emerging role as an immune suppressor. Further investigation of Norbin in the immune system are required, especially to interrogate Norbin deregulation in immune diseases. Preliminary analysis of the NCBI GEO Profiles database (ncbi.nlm.nih.gov/geoprofiles) suggests altered Norbin levels in macrophages and monocytes during various infectious and inflammatory diseases.

Current efforts are concentrated on the molecular mechanisms through which Norbin controls cell responses, particularly its control of GPCR trafficking and signalling. For example, roles of Norbin in constitutive GPCR internalisation and recycling are being investigated. Generally, more mutational analysis would be beneficial for investigating the consequences of Norbin interactions with proteins or lipids. Ideally, one would generate mutants that affect Norbin-dependent GPCR signalling without affecting GPCR trafficking, so that these roles can be differentiated.

The involvement of Norbin in human neurological diseases means therapeutic control of Norbin is desirable. However, Norbin is not a good drug target, because it is an intracellular adaptor protein that works through interaction with other proteins, which is notoriously hard to tackle. Instead, future work could investigate avenues of targeting Norbin-effector GPCRs, to manipulate Norbin indirectly. In diseases where Norbin expression or its GPCR interactions are affected, GPCR levels at the cell surface will be altered, so drugs to target these GPCRs may be beneficial.

• Norbin is an essential protein which is conserved from plants to human, without any homologies to other proteins. It binds to numerous GPCRs, controlling their trafficking and signalling, through downstream pathways which remain incompletely understood but include Rac, ERK, and Ca²⁺.

• Norbin is well-known as an important regulator of neuronal function, but it was recently discovered that Norbin is also a suppressor of neutrophil-mediated innate immunity.

• Considering its critical roles and interesting modes of action, Norbin remains woefully understudied, so there is immense scope for important future discoveries. It would be of great interest to identify mechanisms of regulation, particularly any post-translational modifications that may acutely affect Norbin function.

The authors declare that there are no competing interests associated with the manuscript.

This review was funded by Institute Strategic Programme Grant BB/P013384/1 from the BBSRC to the Signalling Programme at the Babraham Institute. S.A.C. is the recipient of a targeted PhD studentship from the MRC.

S.A.C. has conducted some of the research reviewed in this article and has written the manuscript. C.D. has conducted some of the research presented in this article. S.I. and S.A. have performed the bioinformatics analysis. S.A. and N.T.K. have supervised some of the work presented and have reviewed the manuscript. H.C.E.W. has supervised the project and reviewed the manuscript.

We are grateful for interesting discussions with Anne Corcoran on the subject of autoantibodies and Arqum Anwar for proof-reading the manuscript.

AKAP

A-kinase anchoring protein

AMPAR

AMPA receptor

C5a1

C5a receptor C5a1

CaMKII

Ca²⁺/calmodulin-dependent protein kinase II

DHPG

3,5-dihydroxyphenylglycine

fMLP

f-Met-Leu-Phe

FPR

formyl peptide receptor

FUS

fused in sarcoma

G protein

guanine nucleotide-binding protein

GEF

guanine-nucleotide exchange factor

GPCR

G protein-coupled receptor

H1

histamine H1 receptor

IP₃

inositol 1,4,5- trisphosphate

LTD

long-term depression

LTP

long-term potentiation

MCH

melanin-concentrating hormone

MCH₁

melanin-concentrating hormone receptor 1

mGlu₁

metabotropic glutamate receptor 1

mGlu₅

metabotropic glutamate receptor 5

mGluR

metabotropic glutamate receptor

NET

neutrophil extracellular trap

NFAT

nuclear factor of activated T cells

Norbin

neurite-outgrowth-related protein from the rat brain (Neurochondrin, NCDN)

OX₁

orexin-1 receptor

PKA

protein kinase A

P-Rex1

PIP₃-dependent Rac exchanger 1

ROS

reactive oxygen species

Sema4C

Semaphorin 4C

SMA

spinal muscular atrophy

SMN

survival motor neuron

SNP

single nucleotide polymorphism

TP

thromboxane receptor