Astrocytes and neuropsychiatric symptoms in neurodegenerative diseases: Exploring the missing links

Astrocytic Neuroligin-3 influences gene expression and social behavior, but is dispensable for synapse number

Neuroligin-3 (Nlgn3) is an autism-associated cell-adhesion molecule that interacts with neurexins and is robustly expressed in both neurons and astrocytes. Neuronal Nlgn3 is an essential regulator of synaptic transmission but the function of astrocytic Nlgn3 is largely unknown. Given the high penetrance of Nlgn3 mutations in autism and the emerging role of astrocytes in neuropsychiatric disorders, we here asked whether astrocytic Nlgn3 might shape neural circuit properties in the cerebellum similar to neuronal Nlgn3. Imaging of tagged Nlgn3 protein produced by CRISPR/Cas9-mediated genome editing showed that Nlgn3 is enriched in the cell body but not the fine processes of cerebellar astrocytes (Bergmann glia). Astrocyte-specific knockout of Nlgn3 did not detectably alter the number of synapses, synaptic transmission, or astrocyte morphology in mouse cerebellum. However, spatial transcriptomic analyses revealed a significant shift in gene expression among multiple cerebellar cell types after the deletion of astrocytic Nlgn3. Hence, in contrast to neuronal Nlgn3, astrocytic Nlgn3 in the cerebellum is not involved in shaping synapses but may modulate gene expression in specific brain areas.

Butyrate as a potential therapeutic agent for neurodegenerative disorders

Maintaining an optimum microbial community within the gastrointestinal tract is intricately linked to human metabolic, immune and brain health. Disturbance to these microbial populations perturbs the production of vital bioactive compounds synthesised by the gut microbiome, such as short-chain fatty acids (SCFAs). Of the SCFAs, butyrate is known to be a major source of energy for colonocytes and has valuable effects on the maintenance of intestinal epithelium and blood brain barrier integrity, gut motility and transit, anti-inflammatory effects, and autophagy induction. Inducing endogenous butyrate production is likely to be beneficial for gut-brain homeostasis and for optimal neuronal function. For these reasons, butyrate has gained interest as a potential therapy for not only metabolic and immunological disorders, but also conditions related to the brain, including neurodegenerative diseases. While direct and indirect sources of butyrate, including prebiotics, probiotics, butyrate pro-drugs and glucosidase inhibitors, offer a promising therapeutic avenue, their efficacy and dosage in neurodegenerative conditions remain largely unknown. Here, we review current literature on effects of butyrate relevant to neuronal function, the impact of butyrate in a range of neurodegenerative diseases and related treatments that may have potential for the treatment of neurodegenerative diseases.

High-frequency repetitive transcranial magnetic stimulation improves depressive-like behaviors in CUMS-induced rats by modulating astrocyte GLT-1 to reduce glutamate toxicity

Impaired glutamate recycling plays an important role in the pathophysiology of depression, and it has been demonstrated that glutamate transporter-1 (GLT-1) on astrocytes is involved in glutamate uptake. Studies have shown that repetitive transcranial magnetic stimulation (rTMS) is effective in treating depression, however, the exact mechanism of rTMS treatment remains unclear. Here, we used a chronic unpredictable mild stress (CUMS) protocol to induce depression-like behaviors in rats followed by rTMS treatment. Behavioral assessment was primarily through SPT, FST, OFT and body weight. Histological analysis focused on GFAP and GLT-1 expression, synaptic plasticity, apoptosis and PI3K/Akt/CREB pathway-related proteins. The results showed that rTMS treatment increased sucrose preference, improved locomotor activity, shortened immobility time as well as increased body weight. And rTMS intervention reversed the elevated glutamate concentration in the hippocampus of CUMS rats using an ELISA kit. Moreover, rTMS ameliorated the reduction in GFAP and GLT-1 expression, alleviated the decrease in BDNF, PSD95 and synapsin-1 expression, also reversed the expression levels of BAX and Bcl2 in the hippocampus of CUMS-induced rats. Moreover, rTMS also increased the protein phosphorylation level of PI3K/Akt/CREB pathway. These results suggest that rTMS treatment ameliorates depression-like behaviors in the rat model by reversing the reduction of GLT-1 on astrocytes and reducing glutamate accumulation in the synaptic cleft, which in turn ameliorates synaptic plasticity damage and neuronal apoptosis. The regulation of GLT-1 by rTMS may be through the PI3K/Akt/CREB pathway.

Astrocytes réactifs et maladies cérébrales

Les astrocytes sont des partenaires essentiels des neurones dans le système nerveux central. En réponse à de nombreuses maladies qui touchent le cerveau, les astrocytes subissent des modifications morphologiques, moléculaires et fonctionnelles : ils deviennent réactifs. Ces changements multiples sont susceptibles d’avoir un impact important sur les neurones, qui dépendent de nombreuses fonctions remplies par les astrocytes. La réponse de réactivité astrocytaire dépend du contexte pathologique. Il est donc indispensable de définir précisément les changements qui se produisent dans les astrocytes réactifs dans chaque situation pathologique, par des approches adaptées et sélectives. Cela permettra le développement de thérapies innovantes ciblant ces cellules partenaires des neurones, ainsi que l’identification de biomarqueurs spécifiques de certaines maladies cérébrales.