Region and diagnosis-specific changes in synaptic proteins in schizophrenia and bipolar I disorder

Abstract

Aberrant regulation of synaptic function is thought to play a role in the aetiology of psychiatric disorders, including schizophrenia and bipolar disorder. Normal neurotransmitter release is dependent on a complex group of presynaptic proteins that regulate synaptic vesicle docking, membrane fusion and fission, including synaptophysin, syntaxin, synaptosomal-associated protein-25 (SNAP-25), vesicle-associated membrane protein (VAMP), α-synuclein and dynamin I. In addition, structural and signalling proteins such as neural cell adhesion molecule (NCAM) maintain the integrity of the synapse. We have assessed the levels of these important synaptic proteins using Western blots, in three cortical regions (BA10, 40 and 46) obtained post-mortem from subjects with bipolar 1 disorder, schizophrenia or no history of a psychiatric disorder. In bipolar 1 disorder cortex (parietal; BA40), we found a significant increase in the expression of SNAP-25, and a significant reduction in α-synuclein compared with controls. These changes in presynaptic protein expression are proposed to inhibit synaptic function in bipolar 1 disorder. In schizophrenia, a significant reduction in the ratio of the two major membrane-bound forms of NCAM (180 and 140) was observed in BA10. The distinct functions of these two NCAM forms suggest that changes in the comparative levels of these proteins could lead to a destabilisation of synaptic signalling. Our data support the notion that there are complex and region-specific alterations in presynaptic proteins that may lead to alterations in synaptic activity in both schizophrenia and bipolar disorder.

Introduction

The finding of increased expression of synaptic proteins (SNAP-25 and synaptophysin) in Brodmann's area (BA) 9 from subjects with bipolar 1 disorder but not schizophrenia (Scarr et al., 2006) supports the hypothesis that dysregulation of neurotransmitter release and/or synaptic function has a role in the pathogenesis of a number of psychiatric disorders (Dean, 2000, Eastwood et al., 2000, Mirnics et al., 2000, Azmitia, 2001, Sobczak et al., 2002, Dean et al., 2003). Moreover, the findings on SNAP-25 and synaptophysin suggest that the cellular machinery which is rate limiting for neurotransmitter release from presynaptic nerve terminals (Benfenati et al., 1999) may be critically affected in such disorders. This supports the notion that changes in synaptic function are likely to underlie the onset of frank psychiatric illness (McGlashan and Hoffman, 2000, Eastwood, 2004).

Normal synaptic function is dependent on an intricate network of protein interactions, which mediate the transport, fusion and recycling of synaptic vesicles. Central to this network are proteins which form a vesicle binding complex, including SNAP-25 syntaxin and VAMP (otherwise known as synaptobrevin). This complex brings together the vesicular and presynaptic membranes, allowing fusion to occur (Sudhof, 1995). Beyond this complex are multiple additional proteins that mediate other steps in the process. Synaptophysin is highly expressed in central nervous system (CNS) vesicles and acts as a chaperone to VAMP (Edelmann et al., 1995). Dynamin is intrinsic to the recycling of synaptic vesicles (Robinson et al., 1993), whilst α-synuclein undergoes reversible and activity-dependent associations with vesicle membranes and has been shown to regulate synaptic activity (Kahle et al., 2002, Liu et al., 2004). In addition, NCAM, which stabilises synaptic structure, is involved in bi-directional signalling between cells at the synapse (Cotman et al., 1998, Hoffman, 1998, Wheal et al., 1998).

Whilst proteins involved in vesicle docking at the synapse and the regulation of synaptic activity have been separately investigated, at least in the CNS of subjects with schizophrenia, there have been few studies designed to determine if multiple components of synaptic function may be affected in subjects with schizophrenia or bipolar 1 disorder. The increased expression of synaptic proteins in BA9 from subjects with bipolar 1 disorder (Scarr et al., 2006) raises the question of aberrant synaptic integrity as a process underlying psychiatric disorders. Thus, we have further investigated whether the synapse-associated proteins synaptophysin, syntaxin, SNAP-25, VAMP, dynamin I, α-synuclein and NCAM are altered in expression in areas of the frontal, prefrontal and parietal cortex (BA 10, 46 and 40) from subjects with bipolar 1 disorder or schizophrenia. Several of these proteins have already been implicated in the pathology of schizophrenia (for a review see: Dean et al., 2005). The frontal cortex is of interest in both schizophrenia and bipolar disorder due to its roles in cognition, executive function and higher processes (Fuster, 2002). Many of the pathological findings observed in both schizophrenia and bipolar disorder have been particularly prominent in this region (Goldman-Rakic and Selemon, 1997, Strakowski et al., 2000, Blumberg et al., 2002). Furthermore, it is also in this area that some researchers have noted divergences in the pathology of schizophrenia and bipolar disorder (Selemon and Rajkowska, 2003). In addition, samples taken from the parietal cortex, in BA40, were included. This region is spatially and functionally distinct from the frontal cortical regions assessed, and is thus included to obtain a clearer picture of whether any changes observed are restricted to frontal cortical regions or are apparent throughout the cortex. Whilst the majority of studies have focussed on abnormalities of prefrontal cortical regions, distinct changes have been observed in the parietal cortex in both disorders (Young et al., 1994, Adler et al., 2004, Farrer et al., 2004, Monks et al., 2004, Zedkova et al., 2006, Weiss et al., 2007, Zhou et al., 2007). We decided to determine if changes in synaptic proteins might show regional specificity in either subjects with schizophrenia or bipolar disorder.