A cingulate-hippocampal circuit mediates early depressive symptoms in Alzheimer’s disease

Depressive symptoms are prevalent and precedes cognitive decline in Alzheimer’s disease (AD), which worsen the clinical outcome and severely challenge the life quality of the patients. However, the neural circuitry mediating mood disturbance in AD remains elusive. Here we report that the glutamatergic projection from the anterior cingulate cortex (ACC) to the ventral hippocampal CA1 (vCA1) acts as the neural substrate of depressive symptoms in AD. We systemically mapped axonal projection in early stage of 5xFAD mice (3 months), which accompanies depressive- like behavior in these animals and identified reduced axonal projection from ACC to vCA1. This is in accordance with reduced axonal calcium signals detected with in vivo fiber photometry. Chemogenetic or optogenetic reversal of ACC-vCA1 circuitry activity efficiently ameliorated the depressive-like behaviors as well as cognitive impairment in 5xFAD mice. We further identified the synaptic molecules neuregulin-1 (Nrg1) as one vital signal regulating the synaptic glutamate transmission from the ACC to the vCA1. Collectively, these results indicated ACC-vCA1 as a critical pathway that regulates emotional states in Alzheimer’ disease. Targeting cingulate cortices with brain stimulation may treat depression in AD.

The knockdown virus (AAV2/9-DIO-shNrg1) was first injected into the ACC region, 221 then AAV2/R-Cre virus was injected into the vCA1 to specifically knockdown the 222 presynaptic Nrg1 in ACC-vCA1, AAV2/9-DIO-shRNA (scramble) served as control 223 (shNT), after one month of virus expression (3 months-old), the mice were performed 224 to behavior tests ( Figure 6D). We found that the depressive-like behaviors in 5xFAD   supplement 2D and E). We first observed dramatically reduction in sEPSC of excitatory 241 neuron within ACC in 3-month-old 5xFAD mice which may be induced by early amyloid plaque deposition. Nrg1 knockdown significantly increased both amplitude 243 and frequency of sEPSC in ACC of AD mice compared with control ( Figure 6L-N). 244 Furthermore, the paired-pulse facilitation (PPF) ratio in 5xFAD mice was significantly 245 larger than WT mice, indicating that presynaptic function was disturbed in ACC of 3 246 months-old 5xFAD mice. Nrg1 knockdown also rescued this defect which further 247 support the important pre-synaptic function of Nrg1 in AD ( Figure 6O). We did not 248 observe difference between Input (stimulation intensity) -Output (EPSC amplitude) 249 responses in the above mice, indicating postsynaptic AMPA-mediated responses were 250 not affected ( Figure 6P). Finally, the LTP impairment observed in vCA1 of AD mice 251 were restored by Nrg1 knockdown in ACC region ( Figure 6Q-R). These data suggested 252 that ectopic expression of Nrg1 specifically impaired presynaptic glutamate release and 253 LTP formation in ACC-vCA1 circuit in AD mice. 254 We then examined whether the up-regulation of Nrg1 is sufficient to trigger the old and 6-month-old AD mice. Taken together, these results describe that the ACC Nrg1 -vCA1 circuit in AD mice links depression and memory defects in AD mice.

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In the present study, a novel circuit and synaptic pathway mechanism underlying 268 depression in the early stage of AD was reported. The chemogenetic activation of the 269 ACC-vCA1 neural pathway was found to restore glutamatergic synaptic transmission

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Given the importance of both ACC and vCA1 in cognition and emotion, and in major 277 brain areas receiving projections from ACC, it can be seen that vCA1 is a limbic brain  It is common to see depression in pre-clinical AD, which is likely to show the early 297 manifestation of this disease before the appearance of cognitive impairments (Visser, 298 Verhey, Ponds, Kester, & Jolles, 2000). From that aspect, much evidence shows that 299 depression is strongly related to AD. As a result, that mental illness is considered to be 300 the risk factor for AD or the prodromic AD phase (Modrego & Ferrández, 2004). 301 Accumulating evidence proved that depression is an early feature of AD (Dreimüller et

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For whole brain quantitative mapping, a viral-based anterograde tracer, rAAV-

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CamkⅡα-EYFP was injected into the ACC of 3-month-old 5xFAD and WT mice 381 respectively. The mice were allowed to survive for four weeks.

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For retrograde transsynaptic tracing, the recombinant RV system was injected into 383 the right side of the vCA1. The RV, whose glycoprotein (G) gene is deleted from the the experiments were monitored from the side with a video tracking system (Smart 3.0).

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The immobility time in individual tests was recorded. Mice in TST were considered 420 immobile when they were passively suspended/and completely motionless, and 421 immobility also included passive swaying. The immobility in FST was defined as the 422 only motions which the animal's head above the water without any other movements.

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The mice were performed another twice independent repeated behavioral experiment 424 for one month later.

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Sucrose preference test (SPT) 427 SPT was performed to measure anhedonia behavior, which is a core symptom of  was statistically analyzed using Image J (Fuji) software.

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The vCA1 and ACC were serially micro-dissected from 400 μm thick coronal sections.

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The mouse brain samples were homogenized in cold lysis buffer (RIPA, Boster China) 521 containing a cocktail of protease inhibitors for at least 40 min, then were centrifuged at 522 12 000 rpm for 10 min at 4°C. We collected the supernatants and measured total protein