Dysbindin deficiency in sandy mice causes reduction of snapin and displays behaviors related to schizophrenia

Abstract

Schizophrenia (SCZ) is a complex trait with a high heritability. The DTNBP1 gene (encoding dysbindin) is one of the leading susceptible genes of SCZ. This risk gene has been reported to be associated with clinical symptoms such as negative symptoms and cognitive deficits. Although reduction of dysbindin expression in schizophrenic brain tissue has been reported, how this contributes to its symptomatology remains uncertain. The sandy (sdy) mouse, which harbors a spontaneously occurring deletion in the Dtnbp1 gene and expresses no dysbindin protein, provides a unique tool to study the role of dysbindin in SCZ. Our recent findings reveal that the sdy mice exhibit specific defects of neurosecretion and synaptic morphology in hippocampal neurons. We here further described that sdy manifested schizophrenia-like behaviors such as social withdrawal and cognitive deficits. In sdy hippocampus, the steady-state level of snapin (a SNAP25-binding protein and a synaptic priming regulator) was reduced due to loss of dysbindin. We further characterized that a 30-residue peptide in dysbindin (90–119 amino acids) mediated the interaction with snapin. Our results suggest that the destabilization of snapin in sdy mice may lead to abnormal neurotransmission and therefore abnormal behaviors. This further defines the sdy mutant as a potential model in schizophrenia research.

Introduction

Schizophrenia is a chronic, highly inheritable (a heritability score of approximately 0.8) and debilitating psychiatric disorder with a prevalence rate estimated to be 0.5%–1% population. Genetic studies of schizophrenia (SCZ) have revealed many susceptibility genes. DTNBP1 (encoding dysbindin) is one of the leading susceptibility genes for SCZ (Straub et al., 2003, Ross et al., 2006, O'Tuathaigh et al., 2007). The association between DTNBP1 and SCZ has been replicated in several populations around the world (Schwab et al., 2003, Tang et al., 2003, Van den Bogaert et al., 2003, Funke et al., 2004, Riley and Kendler, 2006). This risk gene is associated with cognitive deficits (Donohoe et al., 2007, Burdick et al., 2007) and negative symptoms of SCZ (Fanous et al., 2005, DeRosse et al., 2006) in clinical observations. However, experimental evidence is lacking for the elucidation of the gene contribution to the expression of schizophrenic behaviors (Ross et al., 2006).

Dysbindin has a wide-spread distribution in the brain (Benson et al., 2001, Straub et al., 2003, Weickert et al., 2004, Numakawa et al., 2004). Postmortem studies have shown a specific reduction in the expression of dysbindin in the hippocampus (Talbot et al., 2004, Weickert et al., 2008) of patients with SCZ. Studies have been focused on the hippocampus formation sites to understand the cognition/memory deficits as well as other symptoms in schizophrenia patients. We recently have shown the synaptic pathology (abnormal synaptic vesicle size and number, abnormal asymmetrical synapses) in the hippocampal CA1 neurons of dysbindin-null mice (Chen et al., 2008). In vitro studies revealed that dysbindin may modulate the secretion of glutamate (Numakawa et al., 2004). This emphasizes further studies on dysbindin-deficient hippocampus to elucidate the underlying mechanism by which dysbindin may contribute to SCZ pathogenesis and symptomatology.

Dysbindin is associated with synaptic vesicles (Talbot et al., 2006), which share common features with lysosome-related organelles (Hannah et al., 1999, Sudhof, 2004). Dysbindin is known to be a subunit of biogenesis of lysosome-related organelles complex-1 (BLOC-1; Li et al., 2003), which consists of at least eight subunits (reviewed by Li et al., 2004, Li et al., 2007). Recently, studies have been focused on another BLOC-1 subunit, snapin, which binds to SNAP25 and regulates calcium-dependent exocytosis (Ilardi et al., 1999). Moreover, snapin interacts with a panel of synaptic fusion proteins including TRPV1, collectrin, EBAG9, cypin, RyR and TRPM7 to regulate synaptic fusion and release (reviewed by Li et al., 2007). Within the BLOC-1 complex, dysbindin directly interacts with snapin (Starcevic and Dell'Angelica, 2004, Nazarian et al., 2006, Talbot et al., 2006). It remains to be addressed whether the interaction between dysbindin and snapin may mediate synaptic fusion and release, thus controlling behaviors.

The sandy (sdy) mouse, which arose on the DBA/2J inbred strain and carries a spontaneously occurring deletion in the Dtnbp1 gene, expresses no dysbindin protein (Li et al., 2003). Our recent work has revealed that sdy exhibits defective synaptic structure and function by showing larger vesicle size, slower quantal release, lower release probability, and smaller size of readily releasable vesicle pool of the hippocampal CA1 neurons (Chen et al., 2008). However, whether this synaptic pathology contributes to abnormal behaviors remains unaddressed. Furthermore, the underlying mechanism of abnormal transmitter release is of interest. We here tested the schizophrenia-like behaviors in the sdy mice and further explored the underlying molecular mechanism utilized by dysbindin in regulating neurotransmission by investigating its binding partners in regulating vesicle release.