Estrogen down-regulates mRNA encoding the exocytotic protein SNAP-25 in the rat pituitary gland

SNAP-25 isoforms differentially regulate synaptic transmission and long-term synaptic plasticity at central synapses

SNAP-25 exists as two developmentally regulated alternatively spliced isoforms, SNAP-25a and SNAP-25b. We explored the function of SNAP-25a and SNAP-25b at Schaffer collateral-CA1 synapses in hippocampus using 4-week-old wild-type (WT) and SNAP-25b-deficient (MT) mice. Characterizing the protein expression of individual SNAP-25 isoforms revealed that WT females had higher levels of SNAP-25a than WT males, suggesting a sex-dependent delay of the alternative splicing switch from SNAP-25a to SNAP-25b. MT mice expressed normal levels of total SNAP-25, Syntaxin 1A and SNAP-47 in the hippocampus, but females expressed lower levels of VAMP2. Electrophysiological recordings in in vitro hippocampal slices revealed significantly reduced magnitude of LTP in MT mice. We also found reduction in paired-pulse facilitation after induction of LTP in WT males, but not in WT females, possibly related to the difference in SNAP-25a/SNAP-25b ratios, suggesting that the splicing switch may play a sex-specific role in LTP-associated increases in presynaptic release probability. Basal synaptic transmission measured in input-output relations revealed that the ability to discriminate between the intensity of presynaptic stimuli was affected in SNAP-25b-deficient mice. Learning in a behavioural paradigm of active-avoidance was impaired in MT mice, strengthening the conclusion that SNAP-25b is important for cognitive performance by altering activity-dependent synaptic plasticity.

A Multilevel Functional Study of a SNAP25 At-Risk Variant for Bipolar Disorder and Schizophrenia

The synaptosomal-associated protein SNAP25 is a key player in synaptic vesicle docking and fusion and has been associated with multiple psychiatric conditions, including schizophrenia, bipolar disorder, and attention-deficit/hyperactivity disorder. We recently identified a promoter variant in SNAP25, rs6039769, that is associated with early-onset bipolar disorder and a higher gene expression level in human prefrontal cortex. In the current study, we showed that this variant was associated both in males and females with schizophrenia in two independent cohorts. We then combined in vitro and in vivo approaches in humans to understand the functional impact of the at-risk allele. Thus, we showed in vitro that the rs6039769 C allele was sufficient to increase the SNAP25 transcription level. In a postmortem expression analysis of 33 individuals affected with schizophrenia and 30 unaffected control subjects, we showed that the SNAP25b/SNAP25a ratio was increased in schizophrenic patients carrying the rs6039769 at-risk allele. Last, using genetics imaging in a cohort of 71 subjects, we showed that male risk carriers had an increased amygdala-ventromedial prefrontal cortex functional connectivity and a larger amygdala than non-risk carriers. The latter association has been replicated in an independent cohort of 121 independent subjects. Altogether, results from these multilevel functional studies are bringing strong evidence for the functional consequences of this allelic variation of SNAP25 on modulating the development and plasticity of the prefrontal-limbic network, which therefore may increase the vulnerability to both early-onset bipolar disorder and schizophrenia.SIGNIFICANCE STATEMENT Functional characterization of disease-associated variants is a key challenge in understanding neuropsychiatric disorders and will open an avenue in the development of personalized treatments. Recent studies have accumulated evidence that the SNARE complex, and more specifically the SNAP25 protein, may be involved in psychiatric disorders. Here, our multilevel functional studies are bringing strong evidence for the functional consequences of an allelic variation of SNAP25 on modulating the development and plasticity of the prefrontal-limbic network. These results demonstrate a common genetically driven functional alteration of a synaptic mechanism both in schizophrenia and early-onset bipolar disorder and confirm the shared genetic vulnerability between these two disorders.

Variants in SNAP25 are targets of natural selection and influence verbal performances in women

Descriptions of genes that are adaptively evolving in humans and that carry polymorphisms with an effect on cognitive performances have been virtually absent. SNAP25 encodes a presynaptic protein with a role in regulation of neurotransmitter release. We analysed the intra-specific diversity along SNAP25 and identified a region in intron 1 that shows signatures of balancing selection in humans. The estimated TMRCA (time to the most recent common ancestor) of the SNAP25 haplotype phylogeny amounted to 2.08 million years. The balancing selection signature is not secondary to demographic events or to biased gene conversion, and encompasses rs363039. This SNP has previously been associated to cognitive performances with contrasting results in different populations. We analysed this variant in two Italian cohorts in different age ranges and observed a significant genotype effect for rs363039 on verbal performances in females alone. Post hoc analysis revealed that the effect is driven by differences between heterozygotes and both homozygous genotypes. Thus, heterozygote females for rs363039 display higher verbal performances compared to both homozygotes. This finding was replicated in a cohort of Italian subjects suffering from neuromuscular diseases that do not affect cognition. Heterozygote advantage is one of the possible reasons underlying the maintenance of genetic diversity in natural populations. The observation that heterozygotes for rs363039 display higher verbal abilities compared to homozygotes perfectly fits the underlying balancing selection model. Although caution should be used in inferring selective pressures from observed signatures, SNAP25 might represent the first description of an adaptively evolving gene with a role in cognition.

The effects of acute 17beta-estradiol treatment on gene expression in the young female mouse hippocampus

Previous studies have demonstrated that treatment with 17beta-estradiol (E(2)) improves both spatial and nonspatial memory in young female mice. Still unclear, however, are the molecular mechanisms underlying the beneficial effects of E(2) on memory. We have previously demonstrated that a single post-training intraperitoneal (i.p.) injection of 0.2 mg/kg E(2) can enhance hippocampal-dependent spatial and object memory consolidation (e.g., Gresack & Frick, 2006b). Therefore, in the present study, we performed a microarray analysis on the dorsal hippocampi of 4-month-old female mice injected i.p. with vehicle or 0.2 mg/kg E(2). Genes were considered differentially expressed following E(2) treatment if they showed a greater than 2-fold change in RNA expression levels compared to controls. Overall, out of a total of approximately 25,000 genes represented on the array, 204 genes showed altered mRNA expression levels upon E(2) treatment, with 111 up-regulated and 93 down-regulated. Of these, 17 of the up-regulated and 6 of the down-regulated genes are known to be involved in learning and memory. mRNA expression changes in 5 of the genes were confirmed by real-time quantitative PCR analysis, and protein changes in these same genes were confirmed by Western blot analysis: Hsp70, a heat shock protein known to be estrogen responsive; Igfbp2, an IGF-I binding protein; Actn4, an actin binding protein involved in protein trafficking; Tubb2a, the major component of microtubules; and Snap25, a synaptosome-specific protein required for neurotransmitter release. The types of genes altered indicate that E(2) may induce changes in the structural mechanics of cells within the dorsal hippocampus that could be conducive to promoting memory consolidation.

Differential expression pattern and steroid hormone sensitivity of SNAP-25 and synaptoporin mRNA in the telencephalic song control nucleus HVC of the zebra finch

Gonadal steroid hormones play an important role in the process of sexual differentiation of brain areas and behavior such as singing and song learning in songbirds. These hormones affect behavior controlling circuits on both the gross morphological and ultrastructural levels. Here we study whether the expression of genes coding for synaptic proteins is sensitive to gonadal steroid hormones and whether such altered expression coincides with changes in brain area size. We treated adult male zebra finches with the aromatase inhibitor fadrozole, to reduce estrogen synthesis and analyzed the mRNA expression of the synaptic proteins synaptoporin (SPO) and synaptosomal-associated protein 25 kDa (SNAP-25) in song control areas and surrounding tissues of adult male zebra finches. SPO and SNAP-25 are differently expressed throughout the song system. Generally, the telencephalic song nuclei expressed SNAP-25 at high intensity whereas SPO expression was area-specific. Elevated levels of SNAP-25 mRNA were present in the nucleus hyperstriatalis ventrale pars caudale (HVC) and in the robust nucleus of the archistriatum (RA). SPO mRNA was found in moderate levels in the HVC, in low levels in the lateral nucleus magnocellularis (lMAN) and Area X, and was absent in the RA. The treatment significantly increased the mRNA level of SPO in the HVC, whereas SNAP-25 expression level was not affected. These expression patterns are not explained by the decrease of HVC volume after treatment. The decreased HVC size is not area-specific but correlates with an overall reduction in size and an overall increase in cell density of the forebrain.

Effect of hypothyroidism on synaptosomal-associated protein of 25 kDa and syntaxin-1 expression in adenohypophyses of rat

The aim of this study was to analyze the expression of SNAP-25 and syntaxin-1 in the adenohypophyses of hypothyroid rats. Rats were divided into: 1) controls; 2) thyroidectomized 40 days; and 3) thyroidectomized 40 days with replacement of T4 20 days after surgery. Adenohypophyses were studied by immunohistochemistry and immunoblot analysis using antibodies against SNAP-25, syntaxin-1 and TSH. By immunostaining, SNAP-25 and syntaxin-1 were conspicuous and were localized around cytoplasmic vacuoles of thyroidectomy cells. Immunoblot analysis shows that thyroidectomy increases adenohypophysial SNAP-25 expression and decreases syntaxin-1 levels. T4 administration for 20 days produces a recovery similar to control values. In conclusion, thyroidectomy produces changes in both expression and immunoreactivity of SNAP-25 and syntaxin-1 in adenohypophyses of rats and these effects can be reversed by T4 administration.

NGF enhances depolarization effects on SNAP-25 expression: induction of SNAP-25b isoform

The 25 kDa synaptosomal associated protein (SNAP-25), which is implicated in neuronal plasticity and neurosecretion, exists as two isoforms generated by alternative splicing of exons 5a and 5b. The aim of the present study was to characterize factors influencing isoform expression. We report that chronic depolarization of PC12 cells alone or in the presence of NGF induces the expression of isoform-b, in addition to a 1.8- to 3-fold increase in SNAP-25 mRNA and protein as determined by immunoblotting and combined RT-PCR and Southern blot analysis. When cerebellar granule neurons were cultured in elevated K+, the predominant isoform switched from SNAP-25a to SNAP-25b. Taken together these results suggested that chronic depolarization regulates the transcription and processing of SNAP-25 mRNA.

Accumulation of synaptosomal-associated protein of 25 kDa (SNAP-25) and other proteins associated with the secretory pathway in GH4C1 cells upon treatment with estradiol, insulin, and epidermal growth factor

Treatment of rat pituitary GH4C1 cells with estradiol, insulin, and epidermal growth factor induces secretory granule accumulation, PRL storage, and stabilization of ICA512, a membrane protein associated with secretory granules. In these investigations we found that the same treatment induced accumulation over 2-fold of other proteins in the secretory pathway, including synaptosomal-associated protein of 25 kDa (SNAP-25), synaptotagmin III, synaptobrevin, synaptophysin, and cyclophilin B, and did not affect accumulation of others, including synaptotagmin I, calnexin, and glucose-regulated protein 94. The induction of proteins was not a coordinate event, because epidermal growth factor alone maximally stimulated SNAP-25 accumulation, but not that of synaptotagmin III. Induction of SNAP-25 accumulation occurred without an increase in its synthesis, and induction of cyclophilin B occurred without an increase in its messenger RNA accumulation, suggesting that accumulation may be caused by stabilization of the proteins. SNAP-25 immunofluorescence was located in the cytoplasm and on the plasma membrane and sometimes was heavily concentrated in protrusions from the cell surface, especially in hormone-treated cells. Frequenin immunofluorescence was also sometimes concentrated in intense patches, but did not colocalize with SNAP-25. Growth hormone and prolactin immunofluorescence was not found in the protrusions and sometimes did not colocalize with each other when they were present in the same cell. Hormone treatment of GH4C1 cells therefore induces accumulation of specific proteins in all parts of the secretory pathway and causes morphological changes in addition to accumulating secretory granules.

Synaptosome-associated protein of 25 kilodaltons in oocytes and steroid-producing cells of rat and human ovary: molecular analysis and regulation by gonadotropins

The synaptosome-associated protein of 25 kDa (SNAP-25) is crucially involved in exocytosis in neurons. The aim of this study was to investigate whether it is present in the ovary. We found SNAP-25 to be expressed in nonneuronal cells of the rat and human ovary, namely in all oocytes and in steroidogenic cells, including granulosa cells (GC) of large antral follicles and luteal cells. Both isoforms, SNAP-25a and b, were found in the ovary. Oocytes obtained by laser capture microdissection were shown to express SNAP-25b, whereas SNAP-25a was found in rat GC and human luteinized GC. Immunohistochemical observations of strong SNAP-25 staining in GC of large growing antral follicles compared with absent or weak staining in small follicles suggested a role in folliculogenesis. To study a presumed regulation of SNAP-25, we used a rat GC line (GFSHR-17), which expresses FSH receptors, and luteinizing human GC, which express LH receptors. FSH elevated SNAP-25 mRNA and protein levels about fivefold within 24 h in GFSHR-17 cells. The cAMP analogue dibutyryl-cAMP (db-cAMP) mimicked this action of FSH. The effects of both db-cAMP and FSH were inhibited by the protein kinase A (PKA) inhibitor H89. In contrast, SNAP-25 protein and mRNA-levels were not altered by LH/hCG in luteinized human GC. Our results for the first time identify SNAP-25b in oocytes and SNAP-25a in steroidogenic cells of the mammalian ovary. SNAP-25a and b may be involved in different exocytotic processes in these cell types.

Differential expression of SNAP-25a and SNAP-25b RNA transcripts in cranial nerve nuclei

Synaptosomal-associated protein of 25 kDa (SNAP-25) is involved in the molecular regulation of neurotransmitter release. SNAP-25 exists in two isoforms, SNAP-25a and SNAP-25b, which arise from alternate splicing and which are differentially expressed throughout the nervous system. In situ hybridization was used to examine the presence and subcellular localization of SNAP-25a and SNAP-25b RNA transcript expression in motor and parasympathetic nuclei associated with cranial nerves. SNAP-25a RNA transcripts were strongly expressed in the parasympathetic Edinger-Westphal nucleus and dorsal motor nucleus of the vagus nerve but weakly expressed in motor nuclei such as the oculomotor, trochlear, trigeminal, facial, ambiguus, hypoglossal and accessory nuclei and in motoneurons of mouse lumbar spinal cord. In contrast, SNAP-25b RNA transcripts were not detectable in the Edinger-Westphal nucleus and dorsal motor nucleus of the vagus nerve but were strongly expressed in the oculomotor, trochlear, trigeminal, facial, ambiguus, hypoglossal, and accessory nuclei and in the motoneurons of mouse lumbar spinal cord. In the parasympathetic cranial nerve nuclei, displaying high levels of SNAP-25a RNA transcripts, the labeling was cytoplasmic, whereas the labeling was nuclear in the cranial nerve motor nuclei, displaying lower levels of transcripts. In contrast, labeling of SNAP-25b RNA transcripts was cytoplasmic in cranial nerve motor nuclei and not detectable in parasympathetic cranial nerve nuclei. Possible explanations for the region-specific and differential subcellular localization of SNAP-25a and SNAP-25b RNA transcripts are discussed.