In the current research, first, the effect of the transgenes was examined on the locomotor activity and ethanol sensitivity. Then, the impact of these transgenes was assessed on the expression of synaptic genes, CREB, and shaggy. We expressed TauWT, Aβ42, or shaggy in Drosophila melanogaster using nSyb-Gal4 driver, which drives protein expression only in adult neurons. Therefore, the developmental effects of transgenes were avoided (http://flybase.org/reports/FBtp0041245.html).
Locomotor activity correlates with the efficiency of synaptic transmission in flies [37]. Previous studies highlighted that a number of synaptic proteins are involved in ethanol responses such as ethanol sensitivity, tolerance and preference [reviewed in 38]. In the present study, all 15-day-old transgenic flies exhibited a reduction in locomotion ability and a surge in sensitivity to ethanol. However shaggy revealed a higher impact, which may implicate that shaggy has a more adverse effect on the efficiency of neurotransmission and synaptic proteins.
As mentioned earlier, TauWT, Aβ42, and GSK3β play crucial roles in synaptic dysfunction [reviewed in 2, 4–5]. Moreover, changes in synaptic proteins, SYT1 and SNAP25, have been observed in AD patients' brains [10–12]. Therefore, in the current study, the effect of TauWT, Aβ42, or shaggy overexpression in neurons of Drosophila was investigated on syt1 and SNAP25 mRNA expression. The data obtained in our research exhibited that TauWT and Aβ42 significantly upregulated syt1 and SNAP25 transcriptions. This result was in accordance with the previous results showing that the expression of genes involves in synaptic vesicle trafficking and neurotransmitter release, including SYT1 and SNAP25, are upregulated in individuals with mild cognitive impairment (MCI) compared to age-matched controls or AD patients [10, 11] Our 15-day-old flies are also probably at the early stages of AD, as it was reported by Sofola et al. [39].
The increase in syt1 and SNAP25 expression might reflect a compensatory response to synaptic defects and loss. Synaptic dysfunction and loss occur in the very early stages of AD. In MCI, the remaining presynaptic terminals are sprouted and expanded to compensate for the absence of lost synapses [40].
The increased expression of genes involved in synaptic vesicle trafficking and release presumably increases neurotransmitters' release in the brain. The FMRI results confirm the brain activity has increased during the MCI stage [41]. Although increasing the release of neurotransmitters is expected to improve cognitive and perceptual functions, neuronal hyperactivity causes excitotoxicity leading to a number of devastating consequences such as generation of ROS and oxidative stress, and apoptosis which finally causes memory impairment [42].
It can be suggested that the neuronal cells try to compensate for the effects of these pathological lesions in TauWT and Aβ42 transgenic flies by increasing synaptic genes expression (syt1 and SNAP25) 15 days after eclosion. However, it is likely to impair synaptic plasticity and networks and ultimately lead to inability to record new information. Defect in memory impairment observed in 15 days old TauWT or Aβ42 expressing flies in our previous study [7] and deficit in locomotor ability and less sensitivity to ethanol in the current study may testify that the presumed compensatory mechanisms were inefficient.
Next, we assessed the relative expression of CREB in the brain of transgenic flies to find out if the observed changes in synaptic genes expression can reflect this transcription factor's dysregulation. CREB positively regulates the expression of genes involved in memory consolidation such as SYT1 and SNAP25 [19, 34], and has been dysregulated in AD [20–21].
According to our results, pan-neuronal expression of TauWT or Aβ42 in nerve cells dramatically increased the level of CREB mRNA, which may suggest that CREB is likely a part of compensatory mechanisms in our flies. Previous studies reported the reduction of CREB expression in cell culture treated with Aβ42 [21, 43] which is in contrast with the current results. In addition, investigation of the total and activated form of CREB protein in AD postmortem brain at late stages of the disease revealed a reduction [20–21]. However, to the best of current authors’ knowledge, no studies at the early stages or at transcription level of CREB have been yet conducted.
The mechanism underlying gene expression dysregulation in flies expressing shaggy seemed different. CREB expression and also syt1 and SNAP25 showed downregulation in shaggy transgenic flies. Intriguingly, it has been demonstrated that neuronal overexpression of shaggy leads to a decrease in the number and size of synapses and presynaptic terminals and a decline in neurotransmitter release [44–46]. Our finding is in agreement with earlier research indicated that there is a decrease in transcription of genes involved in exocytosis of neurotransmitters like SYT1 and SNAP25 in AD patients compared to age-matched control or MCI [10, 11]. Furthermore, there is a decrease in the protein level of CREB in the postmortem brain of individuals suffering from AD at later stages [20–21]. Moreover, it has been demonstrated that overexpression of GSK3β in neurons of mice resulted in downregulation of Syt1 transcription in the hippocampus [6].
Phosphorylation of CREB on serine 129 by GSK3 leads to its inactivation [36]. As CREB is a self-regulator, its inactivation can lead to a decrease in its mRNA level [19]. Therefore, it can be suggested that shaggy, likely through the inactivation and downregulation of CREB in flies' brains, leads to a decrease in the target genes of this transcription factor.
At last, to examine whether the change in CREB transcript level is through GSK3/CREB pathway, we spotted the expression of shaggy in TauWT or Aβ42 transgenic flies. The current study showed that ectopic expression of TauWT in the drosophila nerve system upregulated transcription of shaggy, which is consistent with the finding of a preceding study demonstrating that mRNA level of GSK3β has been elevated in the hippocampus at the early stages of Alzheimer’s disease [33]. However, we observed that Aβ42 did not affect shaggy expression. Our data proposed that the upregulation of CREB as a part of the compensatory mechanism in TauWT or Aβ42 transgenic flies is probably independent of GSK3/CREB pathway.
It has been exhibited that GSK3β can trigger abnormal hyperphosphorylation and aggregation of TauWT [2], but there is no study on the effect of TauWT on shaggy levels or activation. So we are the first to report that the expression of TauWT may affect the expression of shaggy, GSK3β orthologue. It would be more interesting to investigate the impact of TauWT on protein level and activity of GSK3β, as well. In spite of the upregulation of shaggy in TauWT expressing flies, there was an increase in synaptic genes expression, which may put forward that in the early stages of the illness, the compensatory mechanism is more powerful than the effect of GSK3.
Increased production of Aβ42 and higher activation of GSK3β are also in a vicious circle [2]. 15-day-old flies that expressed Arctic Aβ42 only in adult neurons revealed a reduction in the levels of inhibitory ser9 phosphorylation of shaggy, leading to an increase in its activity [39]. As we did not observe any changes in the mRNA levels of shaggy in Aβ42 expressing flies, it seems that Aβ42 probably dysregulates GSK3β activity without affecting its transcriptional levels. Further investigation is required to verify the mechanisms involved in GSK3 dysregulation in AD.
To sum up, here we have shown, for the first time, that TauWT, Aβ42, and shaggy differentially and independently alter the expression of synaptic genes (syt1 and SNAP25) and transcription factor of CREB in transgenic flies at an early time point of their life cycle. While both TauWT and Aβ42 had similar effects on synaptic gene expression, shaggy had a different impact on the expression of those genes. It seems that dysregulation of synaptic genes occurs as a consequence of changes in CREB expression and is likely independent of GSK3/CREB pathway. While TauWT increased the levels of shaggy, no change was observed in Aβ42 expressing flies.
In addition, shaggy has a more significant impact on ethanol sensitivity and motor dysfunction in flies. This could be due to more dramatic effect of shaggy on synaptic dysfunction compared to TauWT and Aβ42, as we observed in the current research.
With regard to the result of this study, it appears that GSK3 has more adverse effect on Alzheimer’s disease’s symptoms such as locomotor and synaptic dysfunction. It seems that using GSK3 inhibitors in treating Alzheimer disease or improving patient’s quality of life, accompanied by drugs that target tau and amyloid beta, would be essential.