© 2000 The Royal College of Psychiatrists
Synaptophysin gene expression in schizophrenia
Investigation of synaptic pathology in the cerebral cortex
University Departments of Psychiatry and Clinical Neurology (Neuropathology), Oxford
Department of Neuropathology, Institute of Psychiatry, London
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Declaration of interest Work funded by grants to P.J.H. from the Wellcome Trust and Stanley Foundation. N.J.C. is supported by the Medical Research Council.
Correspondence: Dr P.J. Harrison, Neurosciences Building, University Department of Psychiatry, Warneford Hospital, Oxford OX37JX
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Background Decreased expression of proteins such as synaptophysin in the hippocampus and prefrontal cortex in schizophrenia is suggestive of synaptic pathology. However, the overall profile of changes is unclear.
Aims To investigate synaptophysin gene expression in the cerebral cortex in schizophrenia.
Method The dorsolateral prefrontal (Brodmann area [BA] 9/46), anterior cingulate (BA 24), superior temporal (BA 22) and occipital (BA 17) cortex were studied in two series of brains, totalling 19 cases and 19 controls. Synaptophysin was measured by immunoautoradiography and immunoblotting. Synaptophysin messenger RNA (mRNA) was measured using in situ hybridisation.
Results Synaptophysin was unchanged in schizophrenia, except for a reduction in BA 17 of one brain series. Synaptophysin mRNA was decreased in BA17, and in BA 22 in the women with schizophrenia. No alterations were seen in BA 9/46.
Conclusions Synaptophysin expression is decreased in some cortical areas in schizophrenia. The alterations affect the mRNA more than the protein, and have an unexpected regional distribution. The characteristics of the implied synaptic pathology remain to be determined.
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Brain size is decreased and ventricular volume increased in schizophrenia (Lawrie & Abukmeil, 1998). The histological correlates of this macroscopic pathology appear to involve alterations in the cortical cytoarchitecture, with changes in the structure and organisation of neurons and their connections (Harrison, 1999a). The synaptic component of the pathology has been investigated by measuring the expression of genes whose protein product is concentrated in synaptic terminals; synaptophysin, a 38-kilodalton synaptic vesicle protein, is the most widely used marker of this kind (Masliah & Terry, 1993). In schizophrenia, decreased expression of synaptophysin has been reported in the hippocampus (Eastwood & Harrison, 1995, 1999; Eastwood et al, 1995; Vawter et al, 1999) and prefrontal cortex (Perrone-Bizzozero et al, 1996; Glantz & Lewis, 1997; Karson et al, 1999). However, other studies have shown equivocal (Browning et al, 1993; Tcherepanov & Sokolov, 1997; Honer et al, 1999) or negative (Gabriel et al, 1997; Rodriguez et al, 1998; Young et al, 1998) results. It is unclear whether the discrepancies reflect anatomical heterogeneity, lack of robustness or methodological differences (e.g. whether protein or messenger RNA (mRNA) has been measured). To address these issues we have examined synaptophysin expression in four cortical areas, using three methods, in two series of brains.
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Subjects studied
Brain tissue was collected at autopsy, with consent, in two centres, Oxford and London. Demographic details are shown in Table 1. Schizophrenia was diagnosed from the case notes according to ICD-10 (World Health Organization, 1992) and DSM-III-R (American Psychiatric Association, 1987) criteria; most cases were initially of paranoid or disorganised subtype. Nine of the subjects with schizophrenia had been living in the community, the rest were long-term in-patients. All had received antipsychotics and all but two were on medication at death. The mean duration of illness (known for the Oxford series) was 28 years (range 8-57). The control subjects had no neurological or psychiatric history. One case and one control died by suicide. Formal neuropathological assessment of all brains showed no neurodegenerative abnormalities other than minor age-related changes.
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View this table: [in a new window] | Table 1 Demographic details of subjects studied |
Tissue collection and processing
From the Oxford series, blocks were taken from the left dorsolateral
prefrontal cortex (Brodmann area (BA) 9/46), primary visual (striate) cortex
(BA 17), superior temporal cortex (BA 22), and anterior cingulate cortex (BA
24), and stored at -70°C. These regions were chosen because three of them
(BA 22, 24 and 9/46) are sites of presumed functional or structural
abnormality in schizophrenia, whereas BA 17 is not. Brains in the London
series were coronally sliced, snap-frozen and stored at -70°C. Blocks from
BA 17, 22 and 9/46 were subsequently dissected from the slices at -20°C,
randomly from either hemisphere, surrounded with embedding compound, and
stored at -70°C. All material was coded and experiments carried out blind
to diagnosis.
From both series, 18-µm cryostat sections were cut for in situ hybridisation (ISH) (to measure synaptophysin mRNA) and for immunoautoradiography (IAR) and immunoblotting (to measure synaptophysin). From BA 9/46 of the Oxford series, autoclaved formalin-fixed wax-embedded sections were also taken, because their better morphology facilitates detailed ‘per cell’ ISH analysis (Eastwood & Harrison, 1999).
Immunoautoradiography and in situ hybridisation
Methods for IAR (Eastwood &
Harrison, 1995) and ISH (Eastwood et al,
1994,
1995) have been described.
Brief details and modifications are given here.
For IAR, sections were fixed in 4% paraformaldehyde. Non-specific binding sites were blocked by incubating the slides for 30 min with 10% normal sheep serum in phosphate-buffered saline (PBS) containing 0.3% Triton X-100. Sections were incubated overnight at 4°C with mouse antisynaptophysin antibody (SY38; DAKO, High Wycombe, UK) diluted 1:100 with PBS containing 0.3% Triton X-100 and 1% normal sheep serum. After washing in PBS, sections were incubated for 1 h with 0.2 µCi/ml 35S-labelled sheep anti-mouse immunoglobulin (the secondary antibody) in PBS containing 0.3% Triton X-100 and 1% normal sheep serum. Omission of the primary antibody was used as an experimental control. Sections were apposed to film for 10 days alongside 14C microscales.
The ISH probe was a 39-base oligodeoxynucleotide, 3'-labelled with
-[35S]-deoxyadenosine triphosphate (~ 1200 Ci/mmol).
Triplicate sections were incubated overnight at 34°C in 150 µl of
hybridisation buffer containing 1 x 106 counts/min of
labelled probe and 20 mM dithiothreitol. Post-hybridisation washes were in 150
mmol sodium chloride/15 mmol sodium citrate for 3 x 20 min at 58°C
followed by 2 x 60 min washes at room temperature. Slides were apposed
to Hyperfilm betamax (Amersham) for 21 days together with 14C
microscales. The wax-embedded sections of BA 9/46 were dipped in nuclear track
emulsion, exposed for 3 weeks, developed and stained with cresyl violet. ISH
using the synaptophysin probe in the sense orientation was used as an
experimental control, in addition to previous demonstrations of the probe's
specificity under these conditions (Eastwood et al,
1994,
1995).
Immunoblotting
Immunoblotting (western blotting) was carried out on the BA 9/46 samples of
the London series, to complement the IAR measurements. Twenty cryostat
sections were collected and homogenised directly in 500 µl of suspension
buffer (100 mM sodium chloride, 10 mM Tris-HCl (pH 7.6), 1 mM ethylene diamine
tetraacetate (EDTA)) containing 1 µg/ml aprotinin and 100 µg/ml
phenylmethyl-sulphonyl fluoride and centrifuged for 5 min at 13 000
g. The resulting pellet was re-suspended in the above buffer
containing 1% w/v sodium dodecyl sulphate, boiled for 5 min, spun at 13 000
g for 5 min, and the supernatant collected. The protein concentration
of the resulting crude synaptosomal extracts was determined using the Bradford
method. Pilot studies found a linear relationship between the amount of
protein loaded (2-4.5 µg) and the signal obtained. Hence, 3 µg was
loaded for the main analyses. Protein samples were fractionated by
electrophoresis on a 10% sodium dodecyl sulphate/polyacrylamide gel for 2 h
and transferred onto a polyvinyl difluoride membrane. After blocking of
nonspecific binding sites with 2% bovine serum albumin in PBS for 30 min at
room temperature, membranes were incubated for 1 h with the SY38 antibody
(1:1000) in PBS containing 1% normal sheep serum. Membranes were given three
rinses in PBS/0.3% Triton-X100 before incubation for 1 h with
35S-labelled sheep anti-mouse immunoglobulin at 0.2 µCi/ml in
PBS. After three washes in PBS with 0.3% Triton-X100, membranes were apposed
to film for 48 h alongside 14C microscales. Samples were run in
quadruplicate.
Image and data analysis
For ISH and IAR, film autoradiographic measurements were taken through
three representative strips of the grey matter of each cortical region; for
the immunoblots, the intensity of the 38 kDa band was measured. Values were
corrected for background (defined as the sense probe for ISH, and omission of
primary antibody for IAR) and converted to nCi/g tissue equivalents by
calibrating to the microscales. For analysis of the ISH dipped sections in BA
9/46, grain densities were measured using dark-field microscopy
(Eastwood et al, 1995)
over pyramidal neurons in laminae III, V and VI. The main synaptic targets of
these neurons are, respectively, other cortical areas, subcortical nuclei, and
mediodorsal thalamus.
Statistical analysis
Brain pH (an index of agonal state), postmortem interval, and age can
influence detection of synaptophysin and its mRNA (Eastwood et al,
1994,
1995;
Eastwood & Harrison, 1999).
Their effects were investigated using multiple regression. Analysis of
variance, with pH, post-mortem interval and age as covariates, was used to
test for effects of diagnosis and diagnosis-by-region interactions. Analyses
were carried out for the two series separately, followed by analysis of the
combined data set.
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The distribution of synaptophysin mRNA (Fig. 1A) and synaptophysin (Fig. 1C) was as anticipated (Eastwood et al, 1994). ISH histochemistry images showed enhancement of signal over laminae containing many pyramidal neurons (which express abundant synaptophysin mRNA), whereas the IAR images were more uniform, due to the presence of synaptophysin in synaptic terminals throughout the cortical ribbon. The signal in subcortical white matter was weak for both mRNA and protein, reflecting the small numbers of neurons and synapses therein. The ISH signal was absent or minimal after sense strand hybridisation, as was the IAR signal after omission of the primary antibody (not shown).
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Fig. 1 Autoradiographic images of superior temporal cortex (BA 22) showing
synaptophysin mRNA (A, B) and synaptophysin (C, D), detected by in
situ hybridisation and immunoautoradiography, respectively. A and C:
control subject; B and D: subject with schizophrenia.
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The regression analyses (Table 2) showed that increasing age, lower pH and longer post-mortem interval were associated with decreased synaptophysin mRNA. In the London series, the abundance of synaptophysin was inversely related to pH and positively related to the post-mortem interval; these correlations were opposite to what had been predicted and are probably chance findings. Nevertheless, the pH and the post-mortem interval were retained as covariates for the IAR analyses.
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View this table: [in a new window] | Table 2 Effects of age, brain pH and post-mortem interval on synaptophysin expression |
The abundance of synaptophysin mRNA
(Table 3) and synaptophysin
(
Table 5) was significantly
higher in the Oxford series than the London series, despite their comparable
demographic and storage characteristics
(Table 1, and data not shown).
Because of this, we computed Z-scores for analysis of the combined series.
(The difference may be explained by other perimortem or tissue processing
variables. Whatever the reason, it illustrates the potential pitfalls if
tissue from cases and controls come from separate sources.)
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View this table: [in a new window] | Table 3 Synaptophysin mRNA in the cerebral cortex in schizophrenia |
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View this table: [in a new window] | Table 4 Synaptophysin mRNA in BA 9/46 pyramidal neurons |
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View this table: [in a new window] | Table 5 Synaptophysin in the cerebral cortex in schizophrenia |
Synaptophysin mRNA
In the Oxford series there was an overall effect of diagnosis (P
<0.001), and a diagnosis-by-region interaction (P=0.047).
Synaptophysin mRNA was decreased in BA 17 (P=0.002) and BA 22
(P=0.043), with a trend in BA 24 (P=0.06)
(Table 3; compare Figs
1A and
1B). No change was seen in BA
9/46 overall (Table 3), nor in
any of the constituent pyramidal neuron populations measured therein using the
dipped sections (Table 4). In
the London series there was an effect of diagnosis (P=0.019) but no
interaction with region (P=0.2), reflecting the smaller sample size
and absence of BA 24. The decrease of synaptophysin mRNA from BA 17, though of
similar magnitude to that in the Oxford series, was not significant
(P=0.056). In the combined sample there was an effect of diagnosis
(P <0.001), with less synaptophysin mRNA in schizophrenia in BA 17
(P <0.001) and in BA 22 (P=0.012).
In a post hoc analysis there was a diagnosis-by-gender interaction, significant in the Oxford series (P=0.029) and in the combined sample (P=0.006). In BA 22, synaptophysin mRNA was decreased in the brains of women with schizophrenia (P <0.01) but not in the brains of the men, with a similar trend in BA 9/46 (P=0.058; Table 3). In contrast, the effects of schizophrenia on synaptophysin mRNA in BA 17 and BA 24 were similar in both genders.
There were no significant (P <0.01) correlations of synaptophysin mRNA with duration of illness (after partialling out the effect of age), age at onset, or extent of antipsychotic exposure (rated on a threepoint scale).
Synaptophysin
Synaptophysin measured by IAR did not differ between diagnostic groups in
the Oxford (P=0.15) or London (P=0.09) series
(Table 5; compare Figs
1C and
1D). Neither were there
diagnosis-by-region interactions (Oxford: P=0.3; London:
P=0.9). Nevertheless, we examined the data for BA 17 and BA 22
separately because of the loss of the mRNA in those areas. A modest reduction
was seen in BA 17 in the Oxford series (P <0.05) but not in the
London series nor in the combined sample. There were no changes in BA 22.
Immunoblotting corroborated the IAR results, showing synaptophysin to be
unchanged in schizophrenia in BA 9/46 (cases: 686 (178) nCi/g; controls: 680
(95) nCi/g).
No diagnosis-by-gender interactions were seen for synaptophysin. Nor did its abundance correlate with duration of illness, age at onset or medication history.
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Synaptic abnormalities are prominent among the contemporary neuropathological findings in schizophrenia. They provide a plausible anatomical basis for the suggestion that aberrant functional connectivity underlies this disorder (Harrison, 1999a). Nevertheless, the existence of synaptic pathology is by no means firmly established, and its features remain to be characterised.
The main positive finding of this study is that synaptophysin mRNA is decreased in some regions of the cerebral cortex (Table 3). The reduction is statistically robust and seen in two series of brains. Antipsychotic treatment is unlikely to have confounded the results, since chronic administration of the drugs to rats leaves the expression of synaptophysin either unaffected or modestly increased (see Harrison, 1999b), and exposure to medication did not correlate with synaptophysin expression here or in previous studies. As such, the basic hypothesis that there is a synaptic abnormality in the cerebral cortex in schizophrenia is supported. Timing and aetiology cannot be determined from a post-mortem study, but the presence of synaptic pathology is consistent with an anomaly in synaptogenesis or synaptic pruning during neurodevelopment (Keshavan et al, 1994; Eastwood et al, 1995). Similarly, despite the relatively large sample size, this present study is underpowered (and limited by the retrospective nature of assessments) to investigate clinicopathological correlations of potential interest, such as between the expression of synaptophysin and the severity of illness or cognitive functioning. The reduced expression of synaptophysin in schizophrenia may well reflect decreased synaptic density, just as in neurodegenerative disorders (Masliah & Terry, 1993; Heffernan et al, 1998), but alternative explanations are possible (e.g. the synapses are smaller, or contain fewer vesicles). Either way, the changes seem likely to be associated with impaired synaptic functioning.
Other aspects of the results were unexpected: first, the distribution of changes, which were most striking in BA 17 (visual cortex) and did not occur in BA 9/46; and second, the mRNA decrements were not accompanied by equivalent reductions by synaptophysin (Table 5). These related anatomical and molecular issues are explored below, since they determine the overall interpretation of the findings.
Synaptophysin expression in the cortex in schizophrenia
Decreased synaptophysin has been reported in BA 9/46 and adjacent
prefrontal regions in schizophrenia in three studies
(Perrone-Bizzozero et al,
1996; Glantz & Lewis,
1997; Karson et al,
1999), but results of a fourth study were negative
(Gabriel et al,
1997), and Honer et al
(1999) could only find a
reduction when subjects dying by suicide were omitted. We failed to find a
change in synaptophysin in BA 9/46 in either brain series, using IAR or
immunoblotting, so that the matter remains unresolved. There are no obvious
characteristics which distinguish the studies which found reductions from
those which did not. The discrepancy is reminiscent of, and might indeed be
related to, the continuing controversy regarding hypofrontality in
schizophrenia (Weinberger & Berman,
1996). There is more consistency regarding synaptophysin mRNA,
since two other studies of prefrontal cortex also found no change
(Rodriguez et al,
1998; Karson et al,
1999). These measurements did not, however, rule out the
possibility of a localised reduction from a sub-population of constituent
neurons. The cellular analysis (Table
4) is informative in this respect, showing that synaptophysin mRNA
expression is maintained in BA 9/46 in schizophrenia in the three major
pyramidal neuron groups.
Synaptophysin and its mRNA were also unchanged in BA 24, in agreement with the findings of Honer et al (1997), but not with the finding of a 25% elevation in an elderly sample (Gabriel et al, 1997). The latter may reflect ongoing age-associated changes in this region in schizophrenia.
Visual symptoms are not a characteristic feature of schizophrenia, and BA 17 was initially included in our studies as an ‘internal control’ region. In the event, the reduction of synaptophysin expression was greatest and most robust in BA 17 (Tables 3 and 5). The implication that this region is not entirely normal, at least histologically, in schizophrenia is supported by neuronal morphometric data (see Selemon & Goldman-Rakic, 1999) and one previous synaptophysin study (Perrone-Bizzozero et al, 1996). Moreover, we have found other mRNAs to be reduced in BA 17 (Burnet et al, 1996; Eastwood & Harrison, 1998), with a trend for the total mRNA content to be decreased (Harrison et al, 1997), implying that gene expression in this region is non-specifically impaired in schizophrenia. The origin and consequences of this are obscure.
The reduction in synaptophysin mRNA in BA 22 in brains of schizophrenia cases was limited to women. As this was not predicted, the observation must be replicated before suggesting that there are gender differences in synaptic pathology in schizophrenia. However, we note that diagnosis-by-gender interactions have been reported in other recent morphometric studies of schizophrenia (Highley et al, 1998), and this should be investigated further.
Interpreting differential changes in synaptophysin mRNA and
protein
Interpreting the significance of a loss of mRNA when the level of encoded
protein is preserved is not straightforward. It might be due to an altered
balance of transcriptional v. translational gene regulation (see
Burnet et al, 1996).
For example, only a small fraction of an mRNA is translationally active and
being used for protein synthesis. Perhaps the proportion of
‘active’ synaptophysin mRNA is increased in schizophrenia,
compensating for the decreased total amount. Or it may be that the methods
used are more sensitive in detecting changes in abundance of an mRNA than of a
protein. A further possibility — that the mRNA levels are simply more
labile and susceptible to confounding variables (e.g. pH or post-mortem
interval) — is not likely, given that these factors were matched between
groups and covaried for in the analyses.
For pre-synaptic proteins there is an additional issue, first raised by a similar finding of a greater decrease of mRNA than protein in the hippocampus (Eastwood & Harrison, 1995; Eastwood et al, 1995). An mRNA is found only in the neuronal cell body, whereas the protein is in the axon terminals (Fig. 2). In the case of cortical pyramidal neurons, for example, these are located elsewhere, in other cortical regions, the thalamus, striatum and other subcortical nuclei (Peters & Jones, 1984), where there may be losses of synaptophysin to accompany the decreases in its mRNA identified here. Equally, neurons projecting into one of the measured areas might be overexpressing synaptophysin, compensating for decreased expression by intrinsic neurons. In other words, our mRNA data suggest that neurons in BA 17 and BA 22 are forming fewer, or otherwise aberrant, synaptic connections, but we have not located the affected synapses. Though important, this can only be a partial explanation, since many cortical synapses do arise from interneurons and from axon collaterals of adjacent pyramidal neurons. Further studies are needed to distinguish molecular from anatomical explanations for the greater loss of synaptophysin mRNA than synaptophysin.
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Fig. 2 Cartoon illustrating anatomical aspects of synaptophysin expression. The
large rectangle represents a cortical area (e.g. BA 9/46). Synaptophysin mRNA
is localised to the cell body of neurons (shown in grey), whereas
synaptophysin itself is concentrated in their synaptic terminals (black
diamonds). The synapses of interneurons (circles) and some of those arising
from pyramidal neurons (triangles) are located in the same area. However the
main projection of pyramidal neurons is to other brain areas (represented by
the smaller rectangle, below, outlined with broken lines), and this fraction
of their synaptophysin will not be detected. Equally, some of the
synaptophysin (black circle) in the area being studied is in the terminals of
neurons whose cell body (open triangle) lies elsewhere, and whose mRNA will
therefore not have been measured.
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Further characterisation of synaptic pathology in schizophrenia
Because of the complexities and discrepancies discussed here, it would be
premature to conclude that synaptic abnormalities are a feature of
schizophrenia, or that the characteristics of any synaptic pathology are
known. There is, however, enough evidence to warrant further studies.
Since synaptophysin is present in virtually all synapses, its expression cannot reveal whether subtypes defined by phenotype (e.g. glutamatergic v. GABA-ergic synapses) or by subcellular location (e.g. axo-spinous v. axo-somatic synapses) are differentially affected in a disease. Neither does the expression of unchanged synaptophysin preclude the existence of discrete alterations in a sub-population of synapses. Indeed, there are already suggestions that in schizophrenia the synaptic pathology in the hippocampus (Harrison & Eastwood, 1998; Young et al, 1998) differs from that in the prefrontal cortex (Aganova & Uranova, 1992; Honer et al, 1997; Thompson et al, 1998; Woo et al, 1998). Molecular dissection as well as regional delineation will, therefore, be needed if the existence and nature of synaptic pathology in the disorder are to be revealed.
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CLINICAL IMPLICATIONS
- Decreased synaptophysin mRNA implies that some neurons in occipital and
superior temporal cortices in schizophrenia form fewer or abnormal
synapses.
- The inferred synaptic pathology is mild and varies from one region to
another.
- The alterations in schizophrenia may be more extensive in women than in
men.
LIMITATIONS
- Synaptic pathology could exist without an alteration in synaptophysin,
and vice versa.
- The diagnostic specificity of the findings is uncertain.
- The cause and timing of the changes are unknown.
We thank David Cotter, Margaret Esiri, Ian Everall, Josie Heffernan, Rob Kerwin, Bianca Watts and Padraig Wright for their contributions. The London series was collected courtesy of the MRC Brain Bank at the Institute of Psychiatry.
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