Local translation controls early reactive changes in perisynaptic astrocyte processes at pre-symptomatic stages of Alzheimer’s disease

INTRODUCTION

Understanding the progression of cellular dysfunction in preclinical Alzheimer’s disease (AD) is essential for developing new therapeutic strategies. Early alterations in astrocyte perisynaptic functions have been observed in AD [1], yet the molecular underpinnings remain poorly characterized. Here, we hypothesized that local protein synthesis—a critical mechanism for maintaining astrocyte polarity and subcellular compartmentalization [2, 3]—could be impaired at early AD stages.

METHODS

We studied the effect of oligomeric Aß on mRNA translation by imaging puromycylated-nascent protein chains in primary astrocytes alone or in coculture with neurons. To further characterize effects on astrocyte translation, we extracted by translating ribosome affinity purification (TRAP) [4], ribosome-bound mRNAs from hippocampal astrocytes and perisynaptic astrocyte processes (PAP) in wild type (WT) and APPswe/PS1dE9 (APP) mice at 5.5 months, a pre-symptomatic stage corresponding to the initiation of Aß plaque formation. mRNAs were analyzed by high throughput RNA sequencing and compared between genotypes and astrocyte compartments. Results were further verified by RT-qPCR in purified hippocampal synaptogliosomes as well as gliovascular units, and by fluorescent in situ hybridization (FISH). We addressed the role of the Janus Kinase (JAK)-Signal Transducer and Activator of Transcription 3 (STAT3) pathway - a master regulator of reactive astrocytes [5] - on the early mRNA expression and distribution in astrocytes by astrocyte-specific viral gene transfer of the pathway inhibitor Suppressor Of Cytokine Signaling 3 (SOCS3) in hippocampal astrocytes in WT and APP mice.

RESULTS

Aß induced global and local translational perturbations in primary astrocytes. 5.5 months APP PAPs showed prominent translational changes compared to whole APP astrocytes and WT. They were related mainly to axon development, neurotransmitter transport, inflammation and endoplasmic reticulum (ER) stress. Some upregulated mRNAs accumulated in APP PAPs from 3 months, but not in astrocyte perivascular processes (PvAP). From this stage, Serpina3n total mRNAs encoding the serine protease inhibitor A3 accumulated in astrocyte soma and PAPs, and this effect was rescued in PAPs upon JAK-STAT3 inhibition.

DISCUSSION

Our findings show that in APP mice, significant translational changes predominantly occur in PAPs at pre-symptomatic stages of AD. They are sustained, as observed for Serpina3n, by the upregulation and distribution of total mRNAs specifically in PAPs from 3 months. The JAK-STAT3 pathway contribute to the distribution of Serpina3n mRNAs in APP PAPs as early as 3 months. These early local events could be related to soluble Aβ as well as early synaptic dysregulations sensed by PAPs, which may trigger local astrocyte reactivity. Thus, targeting local translation in astrocytes may represent a promising therapeutic strategy to counteract early synaptic alterations in AD.

HIGHLIGHTS

• Soluble Aβ influences the global and local translation in primary astrocytes

• At 5.5 months, when Aβ plaques start to form, translation is severely and predominantly altered in APP PAPs.

• Local translation changes in APP PAPs impact mRNAs encoding proteins involved in astrocyte reactivity, synaptic development and functions and ER stress.

• Upregulation of Serpina3n mRNAs in APP astrocyte soma and processes occurs as early as 3 months.

• JAK-STAT3 pathway contributes to Serpina3n mRNA upregulation in PAPs as early as 3 months.