Cilia interactome with predicted protein–protein interactions reveals connections to Alzheimer’s disease, aging and other neuropsychiatric processes

Cilia are dynamic microtubule-based organelles present on the surface of many eukaryotic cell types and can be motile or non-motile primary cilia. Cilia defects underlie a growing list of human disorders, collectively called ciliopathies, with overlapping phenotypes such as developmental delays and cognitive and memory deficits. Consistent with this, cilia play an important role in brain development, particularly in neurogenesis and neuronal migration. These findings suggest that a deeper systems-level understanding of how ciliary proteins function together may provide new mechanistic insights into the molecular etiologies of nervous system defects. Towards this end, we performed a protein–protein interaction (PPI) network analysis of known intraflagellar transport, BBSome, transition zone, ciliary membrane and motile cilia proteins. Known PPIs of ciliary proteins were assembled from online databases. Novel PPIs were predicted for each ciliary protein using a computational method we developed, called High-precision PPI Prediction (HiPPIP) model. The resulting cilia “interactome” consists of 165 ciliary proteins, 1,011 known PPIs, and 765 novel PPIs. The cilia interactome revealed interconnections between ciliary proteins, and their relation to several pathways related to neuropsychiatric processes, and to drug targets. Approximately 184 genes in the cilia interactome are targeted by 548 currently approved drugs, of which 103 are used to treat various diseases of nervous system origin. Taken together, the cilia interactome presented here provides novel insights into the relationship between ciliary protein dysfunction and neuropsychiatric disorders, for e.g. interconnections of Alzheimer’s disease, aging and cilia genes. These results provide the framework for the rational design of new therapeutic agents for treatment of ciliopathies and neuropsychiatric disorders.

and proteins restricted to motile cilia. Known PPIs were collected from Human Protein Reference Database (HPRD) 39 and Biological General Repository for Interaction Datasets (BioGRID) 40 . Gene-drug associations and ATC classifications were collected from DrugBank 41 , while neuropsychiatric gene-disease associations were collected from the GWAS catalog (www.ebi.ac.uk/gwas/). Random gene sets used in shortest path comparisons were sampled from about twenty thousand human proteins listed in the Ensembl database (www.ensem bl.org).
Novel PPIs were predicted using the HiPPIP model that we developed 42 . Each ciliary protein (say C 1 ) was paired with each of the other human genes say, (G 1 , G 2 , … G n ), and each pair was evaluated with the HiPPIP model. The predicted interactions of each of the cilia genes were extracted, which resulted in 620 newly discovered PPIs of cilia genes. The average shortest path distance was computed using the Networkx package in python. Pathway associations were computed using Ingenuity Pathway Analysis suite. GO term enrichment was carried out using BinGO 43 ; for each C 1 , a list of its known and predicted interacting partners (i.e. B 1 , B 2 , … B n ) are given as input to BinGO, which extracts the GO terms of all these genes and finds which of the GO terms are statistically enriched in comparison to the background distribution of GO terms of all human proteins. All statistically significant terms are assigned as network-based enriched GO terms of C 1 .
Gene expression datasets in Gene Expression Omnibus were used to compute the overlap of the cilia interactome with genes differentially expressed in various neuropsychiatric disorders: major depressive disorder (GSE53987 44 ), schizophrenia (GSE17612 45 ), bipolar disorder (GSE12679 46 ), autism spectrum disorder (GSE18123 47 ), Alzheimer's disease (GSE29378 48 and GSE28146 49 ), Parkinson's disease (GSE28894) and nonsyndromic intellectual disability (GSE39326 50 ). Genes with fold change > 2 or < ½ were considered as significantly overexpressed and underexpressed respectively at p value < 0.05. A gene with transcripts per million ≥ 2 was considered to be 'expressed' while analyzing the overlap of the interactome with genes expressed in the amygdala, anterior cingulate cortex, caudate, cerebellum, frontal cortex, hippocampus, hypothalamus, nucleus accumbens, putamen, spinal cord and substantia nigra extracted from GTEx 51 . Time-dependent gene expression variation in the hippocampal region was extracted from BrainSpan Atlas containing RNA-Seq data from post-conceptional weeks to middle adulthood 52 . 78 genes associated with Alzheimer's disease were extracted from DisGeNET 53 (with score > 0.2 to include only expert-curated disease-gene associations). Then, to construct the Alzheimer's disease interactome, whose overlap was to be checked with the cilia interactome, 4,742 known PPIs extracted from HPRD 54 and BioGRID 55 , and 490 computationally predicted PPIs of these 78 genes were assembled. The biological validity of the interactome was shown by the fact that 676 genes out of the 3,944 genes in the AD interactome are differentially expressed in CA1 hippocampal gray matter from patients with severe Alzheimer's disease versus healthy controls (GSE28146 49 ), out of which 71 were novel interactors (p value = 1.138e −20).

Results
We assembled a list of 165 genes encoding proteins known to be associated with primary and/or motile cilia, including IFT, BBS, TZ, and ciliary membrane proteins, as well as proteins restricted to motile cilia. Known PPIs of ciliary proteins were assembled from HPRD and BioGRID 40,56 . Novel PPIs were predicted for each of the cilia genes using our High-precision Protein-Protein Interaction Prediction (HiPPIP) model 42 . In this manner, a ciliary protein interactome was assembled comprising 165 ciliary proteins (red color square shaped nodes) with 1,011 known PPIs (blue edges) and 765 novel PPIs (red edges) that connect to 800 previously known interactors (light blue nodes) and 705 novel interactors (red nodes) ( Fig. 1 and Table 1). We predicted 216 new interactions for 50 out of the 56 cilia genes that had no known PPIs. For example, GPR83 has 12 novel PPIs, LRRC48 has 10, PKD1L1 has 10, and SPEF has 10 novel PPIs. The number of known and novel PPIs of cilia genes are given in Supplementary File 1, and the lists of all genes and PPIs is given in Supplementary File 2.
For each of the ciliary proteins, we computed the enrichment of gene ontology (GO) terms among its interacting partners in order to aid in the discovery of its function using BinGO (Biological Networks Gene Ontology tool) 43 . This information is especially useful for those ciliary proteins that have either no known or very few known GO biological process terms. For example, there are 11 genes that have no known GO terms, and we predicted new GO terms for each of those genes, for e.g. 27 novel GO terms for ARMC4, 11 for CCDC63, and 30 for DNAAF2.
We computed the pathway associations of genes in the interactome, using the Ingenuity Pathway Analysis (IPA) suite (Ingenuity Systems, www.ingen uity.com). This showed a significant overlap of neuronal pathways with the cilia interactome (see selected pathways in Table 2). The complete list of all pathways, their p values and the genes from the interactome that are associated with these pathways, are given in Supplementary File 3. We also extracted information about drugs targeting the genes in the interactome. This analysis showed that there are several genes that are targets to drugs belonging to the Anatomic category of "nervous system", highlighting the connection between cilia and the nervous system as shown in Fig. 2 and Supplementary File 4. experimental Validation of novel cilia ppis in independent studies. Four of the novels PPIs that we predicted for cilia genes were independently recovered by other groups. TMEM237-SFXN5 and DYNLL2-c17orf47 were recovered by yeast two-hybrid experiments in the recent release of the human protein interactome map 57 . We also predicted two PPIs of IFT140 that were discovered as part of the CPLANE interactome using affinity purification-mass spectrometry, but were not deposited in BioGRID or HPRD: IFT140-TELO2 and IFT140-TRAP1 36 . It is also worth noting that 8 novel interactors in the interactome appeared among the proteins isolated from the primary cilia of mouse kidney cells using a method called Mud-PIT (multidimensional protein identification technology) 33 : ABCE1, CCDC47, CCT5, G3BP1, GBF1, RAB10, RAN and USP14. 94 genes in the cilia interactome, including 44 cilia genes, 36 known and 14 novel interactors, were also recovered as regulators of the ciliary sonic hedgehog pathway in a CRISPR genetic screen (p  58 . The interactome was also significantly enriched with genes differentially expressed in bronchial biopsies of primary ciliary dyskinesia patients (p value = 2.64e−02) 59 .   PKD1  24  9 NTHL1, NADSYN1, PDPK1, OR1F1, DDX58, NTN3, RPL3L, SOX17, PPL   PKD1L1  0 10 C7orf69, ABCC3, C7orf65, ACAA2, C7orf57, MGAT2, ABCA13, RPL6, PLCB2, PSME3   PKD2  14  6 KIF11, ADH1C, MTAP, ADH1A, PPP3CA, UCHL3   PKD2L1  3  6 MYOF, HNRNPA2B1, LGI1, CAB39, GBF1, PRPS2   PKHD1  1  7 ORM1, CSTF2T, ILK, ATXN2,

Discussion
We developed the interactome of ciliary proteins that included IFT, BBS, TZ, ciliary membrane proteins and proteins in motile cilia. The interactome includes novel computationally predicted PPIs for multiple proteins, including proteins with few or no previously known PPIs.  www.nature.com/scientificreports/ Both analysis of individual novel PPIs and the cilia interactome as a whole has the potential to highlight connections to specific neurological disorders and lead to biologically insightful and clinically translatable results. We interpreted the functions of individual novel PPIs using literature-based evidence and top pathways obtained from IPA (See Supplementary File 5 for testable hypotheses on novel PPIs involved in neuropsychiatric disorders, primary ciliary dyskinesia, hydrocephalus and in biological processes such as ciliogenesis and trafficking of membrane receptors in cilia). The following is a demonstrative example of a systems-level analysis. cilia, Alzheimer's disease and aging. Alzheimer's disease (AD) is a progressive neurodegenerative disease with an estimated prevalence of 10-30% in the population aged 65 years and more, characterized by memory loss (dementia), behavioral changes, impaired cognition and language 61 . Around two-thirds of dementia cases is attributed to AD 61 . Hippocampus, a region in the brain critical to memory and learning, exhibits signs of neurodegeneration in the early stages of AD 62 . It has been speculated that memory and learning deficits in AD may be associated with aging and reduced neurogenesis in the hippocampus [62][63][64] . It is interesting to note that primary cilia have been shown to mediate sonic hedgehog signaling (Shh) to regulate hippocampal neurogenesis 65,66 . So, we explored interconnections of AD, aging and cilia in the PPI network (the 'interactome'), while asking the following questions: Are genes associated with AD, aging and cilia closely connected in the interactome? Will such a network also include genes involved in Shh signaling and neurogenesis, and genes expressed in the hippocampus? What specific biological processes may underlie the connections of AD to aging, and will they interact with the Shh pathway?
Significant overlap was found between cilia and the AD interactomes (p value = 0.022). The AD interactome was highly significantly enriched in 'human aging-related genes' (p value = 1.77e−37), compiled from the GenAge database 67 . 51 aging genes co-occurred in AD and cilia interactomes. The subnetwork of these 51 genes and their AD and cilia interactors is shown in Fig. 3. In this subnetwork, aging genes connected cilia genes with/without Shh involvement to AD genes (Fig. 3).The next question we asked was: do any of the 51 genes directly interact with a ciliary gene involved in the Shh pathway? 15 cilia genes in the network were also recovered as regulators of the Shh pathway in a CRISPR genetic screen: ARL13B, BBS1-2, BBS4-5, BBS7, CBY1, DYNLL1, IFT140, IFT20, IFT52, IFT81, PTCH1, STUB1 and TRAF3IP1 58 . These 15 genes had direct interactions with 14 aging genes, 6 AD genes and 2 cilia genes. This included 13 novel predicted interactions connecting aging genes to cilia genes including 4 Shh genes (in italics): BAK1-BBS1, CDKN2A-DNAI1, TRAP1-IFT140, PDPK1-PKD1, SOD1-DNAH3, CCNA2-DRD2, TERT-IFT57, HTT-IFT57, FOS-DYNLT3, EP300-MCHR1, SHC1-DNAH7, PRKCA-CDK3 and RICTOR-IFT20. The network was significantly enriched in the GO term 'neurogenesis' (p value = 5.66e−12) and in genes expressed in the hippocampus (transcripts per million ≥ 2) (p value = 2.54e−09). The cilia genes DYNLT1 and PKD1 were associated with neurogenesis, and IFT20, IFT140, PTCH1 and BBS4 were Shh regulators also associated with neurogenesis. Reduced size of hippocampus was noted in mutant mouse models of 5 cilia genes, namely BBS1, BBS2, BBS4, BBS7 and PDCD6IP (Mammalian Phenotype Ontology term: small hippocampus) [68][69][70] . We next identified the biological processes that may be specifically affected in AD in relation to its links with aging. 75 genes in the network were differentially expressed in the hippocampus of AD patients compared with non-AD subjects (GSE48350 71 , GSE36980 72 , GSE1297 73 , GSE28146 49 , GSE29378 48 ). We then examined the fold change in the normal expression of these 75 genes in the hippocampus at 40 years compared with 8 post-conceptional weeks. For this, we used the ' developmental transcriptome' from the Brain-Span Atlas containing RNA-Seq data of up to 16 brain regions from post-conceptional weeks (number of weeks elapsed from the first day of the last menstrual period and the day of the delivery) to middle adulthood (up to 40 years) 52 . The genes were grouped based on the specific direction in which their expression varied in AD versus aging (i.e. fold change in same/opposite directions in AD versus non-AD hippocampal samples compared with expression at 40 years versus 8 post-conceptional weeks) (Fig. 4). 42 genes showed an expression change in the opposite direction in AD versus aging. Out of this, 18 genes were underexpressed in AD but overexpressed in aging; they were enriched in the GO term 'calcium-mediated signaling' (p value = 8.72e−09). It has been postulated that calcium signaling pathways involved in cognition may be remodeled by an activated amyloidogenic pathway in AD, resulting in elevated levels of calcium and a constant erasure of new memories through enhancement of mechanisms involved in long term depression 74 . It is also worth noting that Shh signaling requires calcium mobilization 75 . The 18 genes included the cilia genes DYNLL1, DYNLT3, PKD1 and MCHR1, and the ciliary Shh regulator BBS7. 24 genes were overexpressed in AD but underexpressed in aging; they were enriched in 'circulatory system development' (p value = 3.04e−07). Loss of hippocampal blood vessel density accompanied by ultrastructural changes in the blood vessels have been observed in a senescence-accelerated rat model of AD 76 . It is interesting to note that circulatory system processes were found to be upregulated in early stages of AD-like pathology in this model, while they were found to be downregulated with age, similar to our observations 76 . It is also interesting to note that neovascularization requires Shh signaling 77 . The 24 genes included the cilia genes CCDC40, SPAG6, ZMYND10, DNALI1 and SPAG1, BBS2 and CBY1 which are ciliary Shh regulators, DYNLT1, a cilia gene involved in neurogenesis and PTCH1 which is an Shh ligand also involved in neurogenesis. 25 genes showed an expression change in the same direction (either under/overexpression) in AD versus aging including the cilia genes VPS4B, CCNA1, DYNLRB2, NPHP1, DNAH7 and the ciliary Shh regulator BBS5; 'negative regulation of cell death' was enriched in this group (p value = 1.59e−09). Shh maintains neural stem cells in the hippocampus by inhibiting cell death 78 .
In summary, our analysis demonstrates that aging and AD genes directly interact with ciliary Shh regulators in the interactome. This network is enriched in genes associated with neurogenesis and expressed in the hippocampus. Genes involved in calcium-mediated signaling and circulatory system development are differentially expressed in the opposite direction in AD versus aging, whereas genes involved in regulation of cell death are differentially expressed in the same direction.  19 , suggesting that these novel cilia interacting partners may have a role in neurotransmission. Dopamine signaling, eNOS signaling, and synaptic long term potentiation pathways are also known to be associated with neuropsychiatric disorders such as schizophrenia 79,80 . The identification of Huntington's disease (HD) pathway in the cilia interactome is also notable given that the protein huntingtin (HTT) localizes to the centrosome and plays an important role in ciliogenesis. The HD mutant mouse model exhibits abnormal cilia motility and cerebrospinal fluid flow 23 . Recovery of Wnt signaling thought to be involved in schizophrenia etiology is also of interest 81,82 . Analysis of the known and novel PPIs and GO term associations identified a role for cilia in neuronal disease pathogenesis. While consistent with the known role of cilia in several key processes in the nervous system such as the neuronal signaling and development, these findings reveal novel connections between cilia and www.nature.com/scientificreports/ these functional modules. The defects in neuronal migration and differentiation are the underlying cause of abnormal neural circuitry in psychiatric disorders 12 . This is further supported by the reported linkage of neuropsychiatric risk genes to cilia 15,19 and the finding of neuropsychiatric phenotypes and brain abnormalities in ciliopathies 5,12 . Our interactome analysis shows that TCTN2, cilia gene with known role in neuronal development and migration 12 has 3 novel interactors and neuronal GO terms such as initiation of neural tube closure, midbrain morphogenesis and mid brain development are enriched among the interacting partners. The GO terms that are enriched for interacting partners of ARMC4 include sympathetic neuron projection guidance, axonogenesis, axon extension, and axon fasciculation. Dynein gene, DNAAF2 has only one known but 4 predicted interactions. Two of those novel interactors, ATL1 and TRIM9 are shown to be associated with cognitive performance and psychosis respectively through GWAS. The GO terms such as axonogenesis, neuron maturation, synaptic growth at neuromuscular junction are enriched among the interacting partners. Ciliary membrane genes DRD1 and DRD2 that are implicated in neurotransmission and linked to mental illnesses such as schizophrenia 83 were identified with 4 and 12 novel interactors, respectively; the associated GO terms were neuronal action potential and synaptic plasticity regulation. We also observed 4 novel interactors for the cilia protein TMEM67, including two proteins associated with cilia assembly, LAPTM4B and NDUFAF6, with NDUFAF6 also known to be associated with Alzheimer's disease 84 . Both ATG7, a novel interactor of the ciliary protein PDCD6IP, and SPR, a novel interactor of GPR83 have been associated with Parkinson's disease 85,86 . GIT1, a novel interactor of B9D1 is associated with attention deficit hyperactivity disorder and MME, a novel interactor of SPAG1with Alzheimer's disease 87 .
On inspecting mammalian phenotype ontology (MPO) terms (www.infor matic s.jax.org/), 42 novel interactors were found to be associated with various morphological or physiological aspects of brain in mice. For example, the novel interactor ITSN1 was associated with decreased brain size, abnormal corpus callosum, hippocampal fimbria, hippocampal fornix, brain white matter and anterior commissure morphology. These findings support the role of these novel interactions and the GO terms in understanding the crucial role played by cilia biology in neuropsychiatric disorders.

overlap of cilia and neuropsychiatric disorder interactomes.
To examine the connection between cilia and neuropsychiatric disorders, we computed the overlap between their interactomes. We considered 7 neuropsychiatric disorders (NPDs), namely Attention Deficit Hyperactivity Disorder (ADHD), Major Depressive Disorder (MDD), schizophrenia, bipolar disorder, autism spectrum disorder, Alzheimer's disease and Parkinson's disease. We extracted the genes associated with each disorder from the GWAS catalog (www.ebi.ac.uk/ gwas/) and then assembled disorder-specific interactomes with known PPIs from HPRD and BioGRID. We then computed how closely connected the cilia genes are to NPD genes by computing how many genes or interactors were shared between the cilia interactome and each NPD interactome. This analysis showed the overlap to be statistically significant (Table 3). For example, cilia interactome has an overlap of 88 genes with ADHD interactome (p value = 1.2E−16) of which 17 are novel interactors of cilia. Similar comparisons with other NPDs also showed overlaps as shown in Table 3.
Overlap of cilia interactome with genes differentially expressed in neuropsychiatric disorders. 965 genes in the cilia interactome were found to be expressed (transcripts per million ≥ 2) in several brain regions including amygdala, anterior cingulate cortex, caudate, cerebellum, frontal cortex, hippocampus, hypothalamus, nucleus accumbens, putamen, spinal cord and substantia nigra, from GTEx RNA-Seq data 51 (p value = 3.93E−58). Novel interactors of cilia genes were found to be highly statistically enriched among these genes expressed in the human brain (p value = 8.14E−09). www.nature.com/scientificreports/ We then computed the overlap of genes differentially expressed in neuropsychiatric disorders with the genes in the cilia interactome. We analyzed gene expression datasets of MDD (GSE53987) 44 , schizophrenia (GSE17612) 45 , bipolar disorder (GSE12679) 46 , autism spectrum disorder (GSE18123) 47 , Alzheimer's disease (GSE29378) 48 , Parkinson's disease (GSE28894) and non-syndromic intellectual disability (GSE39326) 50 . The analysis showed the overlap to be statistically significant (Table 3). For example, the cilia interactome has an overlap of 106 genes with genes differentially in the Alzheimer's disease dataset (p value = 4.7E−05) of which 46 are novel interactors of cilia. cilia and nervous system drug targets. Given the strong connection between the cilia interactome and neuronal pathways, we tested the possibility of repurposing drugs targeting proteins in the cilia interactome for treating neurological disorders. Identifying new uses for drugs shortens the time of drug discovery and approval 88 . For example, the drug amantadine which is used to treat influenza infection was successfully repurposed to treat dyskinesia and Parkinson's disease 88 . This analysis identified 548 drugs targeting 184 genes in the cilia interactome. These fall into 3 major Anatomic Therapeutic Chemical (ATC) classification system categories, nervous system with 99 drugs, 102 drugs in the respiratory system, and 98 drugs in the cardiovascular system (Fig. 2, Supplementary File 4). This finding points at therapeutics targeting the cilia proteins which may provide a novel strategy for treating neurological disorders.
Overall, 76 nervous system drugs targeted 7 novel interactors: HRH1, SLC6A2, CHRNA9, NQO2, ORM1, CACNA1I and CACNA1G. 57 drugs targeting 22 genes in the interactome are used in the treatment of at least one among the following neurological disorders-Parkinson's disease, Alzheimer's disease, attention deficit hyperactivity disorder (ADHD), major depressive disorder (MDD), autism spectrum disorder, schizophrenia and bipolar disorder-out of which 35 drugs target 6 novel interactors, namely CACNA1G, CACNA1I, CHRNA9, HRH1, SLC6A2 and ORM1. 10 out of these 57 drugs targeted cilia genes as well as known and novel interactors of cilia genes: asenapine, chlorpromazine, clozapine, loxapine and paliperidone are schizophrenia drugs, olanzapine is used in the treatment of Alzheimer's disease and schizophrenia, amphetamine in ADHD, imipramine in ADHD and MDD, mirtazapine in MDD and nortriptyline in schizophrenia, ADHD, MDD and bipolar disorder.
Among other novel interactors targeted by nervous system drugs is SLC6A2 which is involved in neurotransmission and is associated with ADHD 89,90 . SLC6A2 interacts with RPGRIP1L, a ciliary protein known to cause Joubert syndrome, MKS and bipolar disorder 91,92 . The novel interactors CACNA1I and CACNA1G targeted by nervous system drugs are calcium channels that are known to be associated with Alzheimer's disease and schizophrenia, respectively 93,94 . These novel interactors which are drug targets may have significant impact on the nervous system, and the pathogenesis of neurological disorders.
In an independent study, we proposed that the drug acetazolamide which targets the genes CA2 and CA4, having known interactions with the cilia genes, DYNLL1 and CDK3 respectively, may be repurposed for schizophrenia based on negative correlation of drug-induced versus disease-associated gene expression profiles and other biological evidences 95 . Acetazolamide is currently under consideration for funding for clinical trial. Several cancer drugs with reported effects on ciliogenesis target known and novel interactors in the cilia interactome. Vinblastine targeting JUNN, a known interactor of BBS7 and TSG101, and TUBB, a known interactor of NPHP1 and DYNLL1, inhibits cilia regeneration in partially deciliated Tetrahymena (a unicellular ciliate) 96 . Valproic acid targeting HDAC9, a known interactor of PKD1, restores ciliogenesis in pancreatic ductal adenocarcinoma cells 97 . Gefitinib targeting EGFR, a known interactor of PDCD6IP, inhibits the smoking-induced loss of ciliated cells in the airway 98 . Gefitinib also increases the percentage of ciliated cells in human pancreatic cancer cell lines 99 . Geldanamycin targeting HSP90AB1, a novel interactor of CETN3, induces lengthening of cilia in 3T3-L1, a fibroblast cell line 100 . Intellectual disability n/a n/a n/a n/a 706 75 6.50E−03 32 Scientific RepoRtS | (2020) 10:15629 | https://doi.org/10.1038/s41598-020-72024-4 www.nature.com/scientificreports/ conclusion We identified novel PPIs of cilia proteins and their associated pathways, their enriched Gene Ontology term associations, and drugs that target the interactors. This cilia interactome analysis reveals a link between cilia function, neuronal function and neurological disorders. We also demonstrated the interconnections of Alzheimer's disease, cilia and aging genes. The predicted interactions will have to be validated at the level of network perturbations in the disease state by comparing neuropsychiatric patients with healthy controls. However, one has to be aware of a few caveats while studying the role of ciliary genes in neuropsychiatric disorders (NPDs). Association of a ciliary gene with a NPD can be unequivocally ascertained only if this association is discovered within the ciliary compartment in the context of the particular NPD, i.e. a mechanistic link between ciliary function and the disorder has to be demonstrated. It may not be a true association if a ciliary gene was shown to be associated with a NPD in a cellular context not connected with cilia; a protein may perform its function at different subcellular locations. Mapping the interactome of cilia genes would be useful in carrying out networkbased systems biology studies, which will help elucidate the contribution of these novel PPIs to nervous system disease pathology as well as to develop novel therapeutics for these disorders.

Data availability
We will make the cilia interactome publicly available on our web application Wiki-Pi 101 . Novel PPIs will be highlighted in yellow on the website. The number of novel and known PPIs of the cilia genes are given in Supplementary File 1. Interactome network diagram that is shown in Fig. 1 is also being made available in PDF format and in Cytoscape file format as Supplementary File 6 and Supplementary File 7 respectively. PDF file would be suitable for printing in high resolution and for electronically searching for specific genes, and Cytoscape would allow further processing and data analysis. Wiki-Pi allows users to search for interactions by specifying biomedical associations of one or both proteins involved. Thus, queries can be customized to include/exclude gene symbol, gene name, GO annotations, diseases, drugs, and/or pathways for either gene involved in an interaction. For example, researchers can search for interactions by giving at least one cilia gene and a pathway of interest, say "IFT20 interactions where the interactor is involved in immunity"; this query would match 5 PPIs out of a total of 19 PPIs of IFT20. Another example is the search "find interactions where one protein's annotation contains the word ciliary and the other protein's annotation contain the word neuronal". The search returns 353 PPIs, out of which 13 are novel PPIs.