An Immune Pathway Drives Aging-Related Neurodegeneration and Cognitive Impairment

Post by Soumilee Chaudhuri

The takeaway

Chronic inflammation, a hallmark of underlying immune processes, is implicated in the aging process. A specific immune signaling pathway called cGAS-STING that detects the presence of DNA in cells, plays a critical role in driving chronic inflammation and functional decline during aging.

What's the science?

Natural aging is characterized by decreased organismal fitness, increased susceptibility to various diseases, and the compromise of multiple homeostatic mechanisms that interplay to balance human health. Chronic inflammation is a widely recognized factor contributing to aging and subsequent brain changes. Previous research has shown that the cyclic GMP–AMP synthase (cGAS) stimulator of interferon gene (STING) signaling pathway, or the cGAS-STING pathway, is a hallmark of aging. However, there is unclear evidence as to whether this pathway directly contributes to cellular senescence (i.e., irreversible cell cycle arrest) and age-related inflammation and dysfunction. The cGAS-STING pathway is a critical component of the innate immune system (i.e., the first immunological defense mechanism of the body) that functions to detect the presence of cytosolic DNA and, in response, trigger expression of inflammatory genes that can lead to senescence. This week in Nature, Dr. Gulen and colleagues investigate the cGAS-STING signaling pathway and show that suppression of STING reduces aging-related inflammation and improves function in multiple tissues.

How did they do it?

The researchers used a series of robust molecular biology, biochemistry, and transcriptomics approaches in preclinical models of aged mice to uncover the direct impact of the cGAS-STING pathway on aging and neurodegeneration. Aged mice used in this study were either designated into the control group (no genetic changes in mice) or the experimental group (had a deletion of the STING protein). To test the effect of STING inhibition, mice underwent the Morris Water Maze test, fear conditioning tests, grip strength tests, and others to understand the change in spatial & associative memory and physical strength conferred by STING inhibition. Immunohistochemistry of mice tissue and obtained human tissues were used to visualize the microglial cellular patterns through RNA extraction and consequent bulk and single-nuclei RNA sequencing. The authors also used primary cell culture and intracellular DNA imaging to understand the effect of damaged mitochondrial DNA in eliciting inflammation-driven pathways in aging and neurodegeneration. 

What did they find?

The authors showed that in naturally aged mice, cGAS-STING signaling contributes to a robust Type I Interferon (Type I IFN) response in brain cells and facilitates neuronal loss and cognitive impairment. They also found that beyond the cGAS and Type I IFN programs, there is a microglial gene expression program that might, synergistically or additively, contribute to similar neurodegeneration and could be a shared feature in many neurodegenerative diseases. This was important in highlighting the critical role of microglial-cGAS-STING signaling in mediating neurodegeneration. Furthermore, it was discovered that Tumor Necrosis Factor (TNF), a key neurotoxic factor, controlled the response of microglial cGAS-STING signaling and impacted other brain cells such as oligodendrocytes and astrocytes. Lastly, the authors blocked the STING protein in aged mice and were able to suppress inflammatory responses in senescent cells and tissues, leading to improvements in tissue function in the periphery as well as the brain. Animals receiving STING inhibitors displayed significant enhancements in spatial and associative memory and affected physical function with improved muscle strength and endurance.

What's the impact?

This study is the first to establish cGAS-STING as a significant driver of aging-related inflammation, neuron loss, and neurodegeneration in microglial communities in the brain. Further, this pathway facilitates a neuroimmune crosstalk mechanism in astrocytes and oligodendrocytes that could mediate senescence and neurodegeneration in the brain. This research opens avenues for further clinical research and therapeutic characterization of cognitive decline in neurodegenerative disorders like Alzheimer’s Disease (AD), Amyotrophic Lateral Sclerosis (ALS), or Frontotemporal dementia (FTD). 

Access the original scientific publication here

The Link Between Phantom Ear-Ringing and the Brain’s Emotion Centers

Post by Anastasia Sares

The takeaway

Tinnitus is a condition of hearing sound in the absence of any external stimulus, usually a ringing, hissing, or roaring. Though it was initially thought to be purely a hearing disorder related to noise exposure and other physical factors, scientists have discovered surprising connections to emotional and psychological health.

Recently in the International Journal of Molecular Sciences, Singh and colleagues reviewed the literature linking tinnitus to the limbic system: a collection of structures located deep in the brain with functions like spatial memory, threat processing, motivation, and stress. These limbic system connections may explain why tinnitus is related to other aspects of mental health.

What is tinnitus?

There are two prominent theories about the causes of tinnitus. In the first one, lack of auditory input causes the fibers that normally carry auditory signals to fire spontaneously, generating waves of activity that have no outside source. In the second theory, reduced input leads to a lack of inhibition in key auditory areas, which causes an overall increase in auditory signals despite no external sound. In short, in the absence of stabilizing auditory input, brain regions processing sound start to behave differently, going so far as to “invent” signals and interpret them as sound.

Both of these theories highlight the fact that auditory information passes through multiple structures in the brain. Because so many brain regions are involved, there is also the potential for those brain regions to influence the experience of tinnitus. In fact, the experience of tinnitus encompasses both the phantom sounds as well as a person’s reaction to those sounds (conscious or subconscious).

How is the limbic system involved?

We’ll focus on a few prominent brain regions that have been studied in relation to tinnitus.

The hippocampus: this region is tightly tied to the auditory system since auditory information helps with spatial navigation, and the hippocampus is also responsible for long-term auditory memories. Brain scans of people with tinnitus show a decrease in the size of the hippocampus, and activity in the hippocampus can help predict the unpleasantness of people’s tinnitus experience. Since noise exposure can cause noticeable changes in the hippocampus, it may be a key intermediate region in the pathway from noise exposure to tinnitus.

The amygdala: the amygdala is involved in fear and other emotional processing; it also has a strong connection to regions that regulate incoming auditory information. Activity in the amygdala may be a key part of the process that causes stress and negative emotions to intensify the experience of tinnitus.

The nucleus accumbens and anterior cingulate cortex: These are structures involved in motivation, reward, and addiction, and they work against each other. The nucleus accumbens acts to arouse us and orient our attention to both pleasant and unpleasant stimuli, and more neural activity in this area seems to aggravate tinnitus. The anterior cingulate, on the other hand, can help us control and filter out unpleasant auditory stimuli, working against tinnitus symptoms. If this pathway is weakened, it would provide less filtering and thus, stronger symptoms.

The basal ganglia: The basal ganglia are small regions deep in the brain that form a hub for much of the brain’s activity. How the basal ganglia are involved in tinnitus is somewhat unclear, but there are some dramatic examples of how either stroke or deep brain stimulation in these regions can significantly reduce tinnitus.

What's the impact?

Effective treatments for tinnitus are few and far between, and there is no gold standard. Knowing that limbic regions are involved in creating or exacerbating these symptoms, we can make better guesses at what therapies might work, like drugs that target limbic areas primarily, or deep-brain stimulation in the basal ganglia. Hopefully, more limbic-system-focused therapies are on their way.

Access the original scientific publication here.

How The Hypothalamus Regulates Hormone Release During Fasting

Post by Baldomero B. Ramirez Cantu

The takeaway

Fasting activates agouti-related peptide (AgRP)-expressing neurons in the hypothalamus which disinhibit neurons in the ​​paraventricular hypothalamus (PVH). This process leads to the release of corticosterone, a hormone that helps manage glucose levels which provides energy during fasting.

What's the science?

During fasting, the body undergoes various essential survival responses, including the activation of the hypothalamic-pituitary-adrenal (HPA) axis, which increases the levels of stress hormones (e.g. cortisol in humans or corticosterone in rodents). These hormones prevent drops in blood sugar caused by fasting and maintain glucose balance. This response is crucial for preventing low blood sugar during fasting, and although its importance is recognized, the exact mechanism behind this activation has remained a mystery. This week in Nature, Douglass, Resch, Madara et al. delve into the underlying neural mechanisms and specific neuron-type roles in the activation of the HPA axis during fasting.

How did they do it?

The authors used a variety of techniques to investigate the role of the hypothalamus in regulating corticosterone release during fasting in mice. They primarily relied on plasma corticosterone measurements to measure HPA-axis activation, optogenetic and chemogenetic manipulations to probe the function of different cell types and to map connectivity between different hypothalamic regions, and ex-vivo preparations to assess the role of different receptor types in this pathway.

First, the authors confirmed previously reported data that fasting activates the HPA axis and increases corticosterone levels - by fasting mice for 24-hours and measuring their plasma corticosterone levels. The authors then used chemogenetics to activate or inhibit AgRP neurons and measured the effects on corticosterone levels. To further confirm the role of AgRP neurons in activating the HPA axis, the authors conducted experiments where they monitored the activity of a PVH-Crh (a specific subclass of PVH neurons crucial for initiating the release of corticosterone) by measuring their activity levels using fiber photometry, while simultaneously performing chemogenetic activation of AgRP neurons as a function of chemogenetic manipulation of AgRP neurons.

Next, they wanted to understand how AgRP neurons synaptically influence the activity of PVH-Crh neurons. Since AgRP neurons release inhibitory neurotransmitters and do not directly excite PVH-Crh neurons, they hypothesized that AgRP neurons might inhibit other neurons that in turn inhibit PVH-Crh neurons - thereby activating PVH-Crh neurons via reduced inhibition. They conducted experiments using ex-vivo electrophysiology, recording inhibitory currents onto PVH-Crh neurons while using receptor-specific agonists or antagonists (NPY and GABA). They also created genetic mutants of the NPY and GABA receptors in order to probe their role for PVH-Crh neuron inhibition in-vivo.

Finally, the authors wanted to identify the source of inhibitory GABAergic input that influences PVH-Crh neurons. They used a technique called retrograde rabies mapping to identify brain regions sending GABAergic signals to PVH-Crh neurons. Next, they employed an optogenetic-based method called channelrhodopsin assisted circuit mapping (CRACM) to confirm that neurons from a specific brain region inhibit PVH-Crh neurons ex-vivo, and fiber photometry to confirm that projections from this brain area to PVH are inhibited by AgRP neurons in-vivo.

What did they find?

The authors found that the activation of AgRP neurons increased corticosterone levels even in well-fed mice, while inhibiting these neurons suppressed the usual increase in corticosterone seen during fasting. This indicates that AgRP neurons play a crucial role in releasing corticosterone and are essential for this response during fasting. Chemogenetic activation of AgRP neurons drove rapid and sustained activation of PVH-Crh neurons while inhibition appeared to have the opposite effect. These results further support the role of AgRP neurons in this pathway, given the crucial role of PVH-Crh neurons in the release of corticosteroids and the activation of the HPA axis.

They also found that NPY and GABA can reduce inhibitory tone onto PVH-Crh neurons through receptors located on GABAergic afferents in their ex-vivo preparation. Through in-vivo experiments in genetically modified mice, they discovered that both NPY and GABA are not individually necessary for AgRP neurons to activate the HPA axis, but their combined effect is crucial. The study suggests that GABA release from AgRP neurons acting on GABA-B receptors on GABA-ergic afferents to Crh neurons in the PVH is necessary for activating the HPA axis.

Finally, the authors identify the bed nucleus of the stria terminalis (BNST) as the source of tonic inhibition to PVH-Crh neurons. Chemogenetic inhibition of inhibitory BNST neurons increased plasma corticosterone levels, indicating that inhibiting these neurons stimulates the HPA axis. Specifically inhibiting the BNST → PVH pathway also stimulated the HPA axis. Additionally, their fiber photometry results showed that stimulation of AgRP neurons suppressed the synaptic activity of BNST axon terminals in the PVH. Overall, their findings suggest that inhibitory afferents from the BNST normally suppress PVH-Crh neuron activity and that during fasting, AgRP neurons inhibit these afferents, reducing GABAergic tone onto PVH-Crh neurons and stimulating the HPA axis. 

What's the impact?

Understanding how neurons in the hypothalamus influence the body's adaptive responses to energy deficit and stress is paramount to providing insights into potential therapeutic targets for managing conditions related to metabolic and hormonal imbalances. These results help us gain a better understanding of the neural mechanisms by which AgRP neurons play a pivotal role in activating the HPA axis during fasting.