Neurodevelopmental disease mechanisms, primary cilia, and endosomes converge on the BLOC-1 and BORC complexes

Abstract

The biogenesis of lysosome-related organelles complex-1 (BLOC-1) and the bloc-one-related complex (BORC) are the cytosolic protein complexes required for specialized membrane protein traffic along the endocytic route and the spatial distribution of endosome-derived compartments, respectively. BLOC-1 and BORC complex subunits and components of their interactomes have been associated with the risk and/or pathomechanisms of neurodevelopmental disorders. Thus, cellular processes requiring BLOC-1 and BORC interactomes have the potential to offer novel insight into mechanisms underlying behavioral defects. We focus on interactions between BLOC-1 or BORC subunits with the actin and microtubule cytoskeleton, membrane tethers, and SNAREs. These interactions highlight requirements for BLOC-1 and BORC in membrane movement by motors, control of actin polymerization, and targeting of membrane proteins to specialized cellular domains such as the nerve terminal and the primary cilium. We propose that the endosome-primary cilia pathway is an underappreciated hub in the genesis and mechanisms of neurodevelopmental disorders. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 311-330, 2018.

Keywords: BLOC-1; BORCS7; cilium; dysbindin; schizophrenia.

Figure 1. The BLOC-1 and BORC Interactomes

A. Diagram of the proposed structure of the BLOC-1 complex according to Lee et al. (Lee et al., 2012). The non-obligate component KXD1 has been included. B-C depict Interactomes of the BLOC-1 and BORC complexes in isolation and D presents all molecular interactions between the BLOC-1 and BORC complexes. Interactomes presented in B-D are derived from six published proteome-wide interactomes datasets described in the following references (Camargo et al., 2007, Huttlin, 2015 #921; Wang et al., 2011; Havugimana et al., 2012; Corominas et al., 2014; Wan et al., 2015). These six datasets were analyzed and curated using the Genemania platform (Warde-Farley et al., 2010). Binary interactions obtained in Genemania were plotted using Cytoscape 3.5.1 (Shannon et al., 2003).

Figure 2. Dysbindin/BLOC1S8 Coprecipitates with Subunits of the BORC Complex

A. Immunoprecipitation of the BLOC-1 complex assembled in Drosophila neurons with anti-GFP antibodies. Extracts were prepared from Drosophila heads expressing recombinant UAS-Dysbindin-Venus driven by the pan-neuronal GAL4 driver C155 as described (Mullin et al., 2015). Controls were performed from heads of animals expressing GAL4 driver C155 alone. Note that the Drosophila orthologues of BORCS6 and KXD1 coprecipitate with Dysbindin-Venus. B. Depicts a heat map of the number of peptide identified by mass spectrometry in experiments performed by Pu at al. (Pu et al., 2015). Note that precipitations with BORCS6 as a bait precipitate dysbindin/BLOC1S8 (1-2 peptides).

Figure 3. Deleterious Mutations in Genes from BLOC-1, BORC, and Primary Cilia Interactomes in Schizophrenia Patients

Graphs show the log10 p value of the frequency of predicted deleterious mutations comparing schizophrenia affected individuals and non-affected controls. P-values are empirical, uncorrected one-sided burden tests. A. presents components of the BLOC-1/BORC interactome. B. presents genes from the primary cilia interactome. In A, KYNU, UFL1and SHKBP1 are presented as comparisons since they were identified as genes with increased burden in schizophrenia patients (Fromer et al., 2014; Purcell et al., 2014).Data were obtained with the GeneBook engine http://bit.ly/2fkvgLq that summarizes multiple exome studies (Fromer et al., 2014; Purcell et al., 2014). C presents genes that affect cilia found within the 108 high risk genomic loci identified by the Schizophrenia Working Group of the Psychiatric Genomics, Consortium (Schizophrenia Working Group of the Psychiatric Genomics, 2014)

Figure 4. Primary Cilium Interactome and the Primary Cilium Hypothesis of Neurodevelopmental Disorders and Schizophrenia

A. Presents an interactome of the BLOC-1 interactome, primary cilia complexes involved in protein targeting and barrier formation. Interactome was generated as indicated in figure 1A. The barrier components CEP290, SEPT5, and SEPT6 associate with schizophrenia risk (see figure 3) and connect with multiple components of the primary cilium interactome. B. Presents a diagram of the primary cilium hypothesis of neurodevelopmental disorders and schizophrenia. Primary cilium is depicted as a red rod emanating from the cell body of a neuron. Red indicates the presence of dopamine receptors targeted and concentrated into the primary cilium. Mutations that cause neurodevelopmental disorders or schizophrenia affect primary cilium morphology and redistribute dopamine and/or other receptors away from the primary cilium and into the plasma membrane, which are represented as a thin red line delimiting the cell body and dendrites. Small triangles represent GABAergic terminals whose number are reduced in BLOC-1 and cilia deficiencies. An unbalance between primary cilium and plasma membrane signal transduction is postulated to be pathogenic for neurodevelopmental disorders and schizophrenia endophenotypes.