PerspectiveNeuroscience

Schizophrenia: The “BLOC” May Be in the Endosomes

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Science Signaling  20 Oct 2009:
Vol. 2, Issue 93, pp. pe66
DOI: 10.1126/scisignal.293pe66

Abstract

Genome-wide association studies have identified multiple genetic polymorphisms associated with schizophrenia. These polymorphisms conform to a polygenic disease model in which multiple alleles cumulatively increase the risk of developing disease. Two genes linked to schizophrenia, DTNBP1 and MUTED, encode proteins that belong to the endosome-localized Biogenesis of Lysosome-related Organelles Complex-1 (BLOC-1). BLOC-1 plays a key role in endosomal trafficking and as such has been found to regulate cell-surface abundance of the D2 dopamine receptor, the biogenesis and fusion of synaptic vesicles, and neurite outgrowth. These functions are pertinent to both neurodevelopment and synaptic transmission, processes tightly regulated by selective cell-surface delivery of membrane proteins to and from endosomes. We propose that cellular processes, such as endosomal trafficking, act as convergence points in which multiple small effects from polygenic genetic polymorphisms accumulate to promote the development of schizophrenia.

Schizophrenia is a devastating mental illness that can lead to the inability to live independently within our society. Genetic factors and their interactions with the environment account for 80% of the susceptibility for the disease (1). The identity of these genetic factors is the subject of intense scrutiny. Recent genome-wide studies suggest that the risk of developing schizophrenia is polygenic, with multiple common allelles each contributing a small effect (2). Such studies have identified a plethora of polymorphisms associated with the illness. Moreover, the genes identified as associated with schizophrenia have markedly diverse functions; they include genes implicated in signal transduction, growth cone guidance, pre- and postsynaptic neurotransmission, and vesicular membrane protein trafficking (27).

Pathogenic hypotheses for schizophrenia have tended to emphasize individual genes of “interest” (1, 8). Broadly, these hypotheses fall into two main categories: neurochemical or neurodevelopmental (8, 9). The neurochemical hypotheses focus on alterations in neurotransmitter systems, such as dopamine, glutamate, or γ-aminobutyric acid (GABA), and are substantiated by abnormalities found in the brains of affected individuals that would affect neurotransmission as well as by the mechanisms of action of pharmacological agents used in treating or mimicking the symptoms of schizophrenia (9, 10). The neurodevelopmental hypotheses encompass structural alterations in the brain of schizophrenic patients that emerge early in life (9, 11). Here, we discuss studies by several groups that focus on the Biogenesis of Lysosome-related Organelles Complex-1 (BLOC-1) (1214). These studies implicate endosomal trafficking to and from the plasma membrane as a cellular mechanism that could link the neurochemical and neurodevelopmental hypotheses of schizophrenia.

Eight subunits make up the BLOC-1 complex: dysbindin, muted, pallidin, cappuccino, snapin, and BLOC subunits (BLOS) 1, 2, and 3 (15). This complex is present on transferrin-receptor–positive endosomes, where it regulates membrane protein targeting to synaptic vesicles, lysosomes, and lysosome-related organelles (13, 16, 17). BLOC-1 subunits are tightly bound to one another, as indicated by the observation that all of the dysbindin and pallidin protein in the brain copurify as a complex with the predicted molecular mass of the BLOC-1 complex (12). Mice with genetic defects in BLOC-1 subunits further highlight the tight structural organization of the BLOC-1 complex. Most null alleles of genes encoding BLOC-1 subunits lead to almost identical cell and organism phenotypes (1821). Among these phenotypes, the absence of any one of the BLOC-1 subunit proteins triggers the disappearance of all other BLOC-1 protein complex subunits (12, 18, 19, 22).

Two of the BLOC-1 complex subunits, dysbindin (encoded by DTNBP1) and muted (encoded by MUTED), are associated with increased risk of schizophrenia (23, 24). Several population genetic studies have replicated the association of DTNBP1 variants with schizophrenia (3, 8). Dysbindin mRNA and protein abundance are decreased in the brain of schizophrenics (25, 26). On the basis of these data, the hypothesis that BLOC-1 is implicated in the pathogenesis of schizophrenia leads to three predictions. First, BLOC-1–deficient mice should have behavioral phenotypes that are consistent with schizophrenia. Second, genetic polymorphisms in DTNBP1 associated with schizophrenia should lead to reduced abundance of dysbindin in the brain of individuals with schizophrenia. Lastly, brain tissue from schizophrenics with decreased dysbindin abundance should also possess reduced abundance of the other BLOC-1 subunits, regardless of whether these other BLOC-1 subunit genes carry disease-associated polymorphisms. The first prediction has been documented in the dysbindin-deficient mouse sandy, which shows impaired social interactions and working memory, phenotypes consistent with animal models of schizophrenia (11, 2729). Similarly, DTNBP1 disease-associated polymorphisms decrease the abundance of DTNBP1 transcripts in human cortex (30), supporting the second prediction. However, there is neither evidence to support the last prediction nor is there information about the developmental stage at which deficiencies in BLOC-1 function may affect the brain of schizophrenic individuals.

Key observations of Ghiani et al. (12) suggest that BLOC-1 function may be required in the neonatal period, a finding that is consistent with the neurodevelopmental hypothesis of schizophrenia. They found that brain dysbindin and pallidin protein abundance was greatest perinatally and declined in adulthood. Moreover, they found that loss of BLOC-1 in pallidin-null mice led to defects in neurite outgrowth in primary cultured hippocampal neurons (12). However, the effects of BLOC-1 extend beyond developmental processes. For instance, Iizuka et al. found that BLOC-1 decreases the cell-surface abundance and activity of the D2 dopamine receptor (DRD2). In neuronal cells where BLOC-1 was decreased by small interfering RNA (siRNA), the surface abundance of DRD2 and DRD2-dependent signal transduction increased (31). Intriguingly, DRD2 (which encodes DRD2) is, like dysbindin, considered a candidate schizophrenia susceptibility gene (8). Indeed, the D2 dopamine receptor plays a central role in the well-known dopamine hypothesis of schizophrenia, which postulates that increased dopaminergic signaling is central to the disease (10). In this context, it is notable that increased surface abundance of DRD2 receptors in neurons induced by defective expression of BLOC-1 would favor enhanced dopaminergic signaling. Genetic polymorphisms in the DRD2 gene may further potentiate this enhanced dopaminergic neurotransmission. BLOC-1–dependent endosomal trafficking mechanisms in neurons may not be limited to modulation of the DRD2 receptor, but may also affect the surface abundance of diverse membrane proteins and thereby alter neuronal responsiveness to various extracellular cues.

Although the precise endosomal processes affected by BLOC-1 deficiency are unclear, two nonexclusive mechanisms may contribute to the increased abundance of these proteins at the cell surface. Evidence indicates that BLOC-1 regulates the sorting of selected membrane proteins into vesicles either by itself or in association with the adaptor complex AP-3 (13, 16, 32, 33). In fact, mice carrying null mutations in subunits of either BLOC-1 or of the ubiquitous AP-3 have increased abundance of specific synaptic vesicle proteins, most prominently vesicle-associated membrane protein 7 (VAMP7-TI), an endosomal SNARE [soluble N-ethylmaleimide-sensitive factor attachment protein receptor, a class of membrane proteins that control the selectivity of membrane fusion along the endocytic route and between endosomal-derived organelles and the cell surface, such as synaptic vesicles (34)] (13). BLOC-1 also affects membrane fusion (12, 14). Mice lacking dysbindin, and therefore the BLOC-1 complex, have slower synaptic vesicle release and decreased release probability (14). This may be related, at least in part, to the ability of BLOC-1 complexes to bind and regulate the subcellular distribution of endosomal SNAREs (12, 13, 33). Regardless, either defective biogenesis or fusion of some populations of synaptic vesicles could decrease synaptic availability of neurotransmitter, a notion that is consistent with the glutamatergic and GABAergic neurochemical hypotheses of schizophrenia, which postulate decreased activity of these neurotransmitter systems in schizophrenia (10).

An attractive aspect of an endosomal hypothesis is its ability to accommodate the polygenic nature of the genetic association data. For example, one-third of the 29 genes considered top scores for increased schizophrenia susceptibility in genetic association meta-analyses are either regulated by or participate in endosomal mechanisms (Table 1) (3). Furthermore, the products of several genes identified as contributing to the risk of developing schizophrenia in recent genome-wide studies are either regulated by or participate in endosomal trafficking (Table 1). Consider two gene products from Table 1: Vesicular monoamine transporter 1 (VMAT1, encoded by SLC18A) (35), which is involved in presynaptic storage of dopamine in vesicles, and DRD2 (encoded by DRD2), a postsynaptic dopamine receptor. The possible pertinence of polymorphisms in these two genes to the dopamine hypothesis is clear. However, under the umbrella of an endosomal hypothesis, we can consider how the products of several of the genes in Table 1 might regulate dopaminergic neurotransmission. The subcellular distribution of VMAT1 and DRD2 could be under control of proteins involved in endosomal sorting and vesiculation encoded by genes such as DTNBP1 (encoding dysbindin), CLTCL1 (encoding clathrin isoform-1), and CENTG2 [encoding ARF1 guanosine triphosphatase–activating protein-1 (AGAP1)]. The clathrin isoform encoded by the CLTCL1 gene is deleted from chromosome 22 of a subset of schizophrenics (4). Clathrin and clathrin adaptor complexes orchestrate the formation of vesicles from multiple intracellular compartments, including endosomes (36). AGAP1, the protein encoded by CENTG2, regulates the recruitment of AP-3 to endosomal membranes, a necessary step in the formation of vesicles from endosomes (37). AGAP1 directly interacts with AP-3, which in turn associates with both clathrin and BLOC-1 to form vesicles from endosomes (37, 38). Thus, under a polygenic model of schizophrenia, disease-associated alleles in SLC18A, for example, could have small and clinically silent individual effects in either the abundance or the function of VMAT1. Minor VMAT1 phenotypes could be further potentiated and rendered clinically meaningful by the compounded effect of multiple other disease-associated alleles affecting, for example, mechanisms regulating dopamine receptor surface abundance. Such genes will include those genes encoding BLOC-1 subunits and other molecules involved in endosomal sorting and vesiculation, such as clathrin, AP-3, and AGAP1.

Table 1 Schizophrenia susceptibility genes related to endocytic trafficking.

Selected supporting references are listed with capital letters in the right-hand column. The following studies are included: the SzGene study (3), the studies that identified microdeletions in chromosomes 22 (Chr22) and 15 (Chr15) (4, 7), genome-wide association studies (GWAS) conducted on the Molecular Basis of Schizophrenia Study (MGS GWAS) (5), the Stefansson’s genome-wide association study (6) and the International Schizophrenia Consortium genome-wide association studies (ISC GWAS) (2).

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Endosomal trafficking controls cell-surface receptor numbers in most cell types. Trafficking qualitatively and quantitatively affects signal transduction and cell-surface composition in developing and adult organisms (39). In neurons, endosomal trafficking mechanisms define pre- and postsynaptic composition and function by controlling the biogenesis of synaptic vesicles as well as the subcellular distribution of neurotransmitter transporters and receptors (Fig. 1) (40, 41). Focusing on endosomal trafficking builds a conceptual bridge among traditional models of schizophrenia pathogenesis. Although we have focused on the BLOC-1 complex and its role in endosomal trafficking, analysis of present and future genetic data may define other fundamental cellular machineries or processes that could contribute to disease pathogenesis. We believe that focusing on cellular mechanisms rather than isolated genes of “interest” will facilitate the formulation of hypotheses with predictions amenable to exploration in the genomes and brain tissue of affected individuals.

Fig. 1

Putative events regulated by BLOC-1–dependent mechanisms in neurons. Diagram depicts the BLOC-1 possible sites of action. BLOC-1 modulates sorting of membrane proteins and SNAREs either alone or in conjunction with AP-3 at endosomes in the cell body affecting the composition of presynaptic secretory organelles. Additionally, BLOC-1 binds and may regulate SNARE-dependent membrane fusion at the nerve terminal.

Acknowledgments

This work was supported by grants from the National Institutes of Health to V.F. (NS42599 and GM077569) and T32 GM008367, NIH, Training Program in Biochemistry, Cell, and Molecular Biology to P.V.R.

References and Notes

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