A Link Between Circadian Rhythms in the Prefrontal Cortex and Depression

Post by Kelly Kadlec

The takeaway

The circadian clockwork in the medial prefrontal cortex was found to be altered in a mouse model of depression. The authors used pharmacological and genetic modulation of the clock molecular circuits involved to demonstrate a causal role of the circadian clock in mood regulation and show that correcting this dysregulation has an antidepressant effect. 

What's the science?

Circadian rhythms are the 24-hour physiological clocks that regulate, among other processes, our sleep. These clocks are often dysregulated in patients with mood disorders, including major depressive disorder (MDD). It is thought that the fastest-acting treatments for MDD, such as ketamine, may be effective because of their interaction with the genes that regulate circadian clocks. These treatments, unfortunately, are often only effective for short amounts of time, which motivates a need for us to better understand the biological mechanisms behind circadian rhythm dysregulation in depression and how these treatments impact them. Last week in Nature Communications, Sarrazin, Gardner, and colleagues demonstrated a causal role for the medial prefrontal cortex (mPFC) in the disruption of circadian clocks in mice that are in a stress-induced depressive state and showed that correcting the circadian clocks in mPFC produces effects similar to that of known antidepressants. 

How did they do it?

In this study, wild-type and genetically modified mice were used to study different components of the relationship between circadian rhythms and depressive states. The authors used a mouse model of depression, the chronic behavioral despair paradigm (CDM), to replicate symptoms of depression. These symptoms were evaluated using several behavioral tests, for example, the nose poke sucrose preference test, which is used to measure stress-induced anhedonia (no experience of pleasure) and motivation-oriented behavior. 

The authors measured the expression of four main circadian clock genes in brain tissue samples using quantitative reverse transcription PCR (qRT-PCR). They also manipulated the expression of two of these genes using genetic knockout lines where levels of individual circadian clock genes were reduced by half or more. Then, the authors modulated the other two genes pharmacologically. These different manipulations were studied in both the context of how they impacted the behavior of the mice and how these four circadian clock genes may interact with each other and other networks in the brain.

What did they find?

The authors find that four key circadian clock genes in mPFC are altered in mice in depressive states induced by CDM. This was reversed by administering the anti-depressant ketamine. 

In addition, the authors genetically reduced the expression of two circadian clock regulators individually and found the expected relationship between these modulations and behavior. Downregulating a positive circadian clock gene in mPFC resulted in depressive-like behaviors. In contrast, reducing the expression of a clock-suppressive gene resulted in antidepressant effects in mice subjected to CDM. Pharmacological manipulation of the other two clock genes revealed that their impacts on behavior were mediated through their action on the positive circadian clock gene.  

Finally, the authors show how these genes impact proposed mechanisms of synaptic plasticity which is thought to be critical to the pathology of depression. Increasing an antidepressant clock modulator and administering ketamine both resulted in increases in markers for synaptic plasticity. The reverse was true as well and increasing the depressive clock regulator resulted in decreases in measured plasticity. 

What's the impact?

Millions of people live with MDD, and many patients do not respond to current treatment options. The findings in this study demonstrate the potential therapeutic benefits of correcting circadian rhythms, in particular through modulating the activity and expression of clock genes in the mPFC. This provides a target for treatments of depression and furthers our understanding of the molecular mechanisms involved in the pathology of depression.   

PSEN1 Mutations Trigger Brain Degeneration Independent of Beta-Amyloid

Post by Soumilee Chaudhuri

The takeaway

Mutations in the presenilin 1 (PSEN1) gene are a major cause of familial Alzheimer’s disease, commonly known as early-onset Alzheimer’s Disease (EOAD), which affects individuals below the age of 65. This study discovered that PSEN1 mutations can lead to neurodegeneration in the brain through pathways that do not involve Amyloid-beta (Aβ) accumulation — a well-known hallmark and the most commonly studied therapeutic target of AD. 

What's the science?

PSEN1 is part of γ-secretase, an enzyme that instructs precursor proteins like amyloid precursor protein (APP) to produce amyloid-beta (Aβ) peptides, which cause AD. Over 450 mutations in the PSEN gene have been linked to AD and while PSEN1 helps protect neurons, its mutations increase the ratio of the harmful and mutated Aβ (Aβ42) to non-mutated Aβ peptide (Aβ40) levels. Therefore, it's unclear whether these PSEN1 gene mutations cause AD by increasing the harmful Aβ42/ Aβ42 ratio or impairing the gene's protective functions. This week in PNAS, Yan and colleagues investigated whether a specific PSEN1 mutation (PSEN1 L435F), a severe familial AD mutation, causes age-related neurodegeneration independently of Aβ.

How did they do it?

The researchers engineered a mouse model carrying the PSEN1 L435F mutation, associated with EOAD, and bred it with mice lacking the amyloid precursor protein (APP) to reduce Aβ production in the offspring mice. Mice of both sexes, aged 2, 12, and 18 months old were used in this experiment, and detailed analyses of their brain tissue post-mortem allowed for the measurement of cortical atrophy, neuron loss, and other signs of neurodegeneration. Additionally, they also worked with several other mouse models that specifically deleted PSEN1 genes in brain cells. This setup allowed the researchers to investigate how these genetic changes impact neurodegeneration across different ages and different strains of mice all containing the PSEN1 mutation but lacking APP.

What did they find?

The researchers found that even with the APP deletion and Aβ absent, mice with the PSEN1 L435F mutation still developed significant cortical neurodegeneration, similar to that seen in AD in humans. The researchers also noted increased cell death and brain inflammation. Even with no Aβ production, these mice still experienced significant cortical volume reduction, neuron loss, increased cell death, and brain inflammation at all ages (2, 12, and 18 months). This suggests that the neurodegenerative processes in these mice are driven by alternative, non-amyloid pathways. 

What's the impact?

Mice with PSEN1 mutations exhibited severe neurodegeneration, synaptic impairment, and memory deficits, independent of amyloid beta levels. These findings challenge the amyloid-centric model of Alzheimer’s disease, particularly in cases involving PSEN1 mutations. Understanding these non-amyloid pathways could open up new avenues for treating Alzheimer’s by targeting alternative mechanisms of neurodegeneration. Despite extensive anti-Aβ therapeutic efforts, such as the approval of lecanemab, clinical outcomes have been modest. These findings emphasize the need for a broader understanding of AD, including the role of PSEN1 function, to develop more effective therapies.

Access the original scientific publication here

Altering the Stress Response To Reduce Anxiety

Post by Annie Phan

The takeaway

The most common mental health disorders, anxiety and depression, are linked to chronic stress. To design better pharmacological treatments for stress-related disorders, it is important to identify and undo the disruption of chemical interactions in the brain that occur with repeated exposure to stress.

What's the science?

Current medications for anxiety and depressive disorders target related neurotransmitter systems, like serotonin, to yield therapeutic effects. However, they also have side effects that may reflect biological processes unrelated to these disorders. This week in PNAS, Pandey and colleagues used mouse models of chronic stress to investigate signaling pathways that were changed in response to stress manipulation. Then, they attempted to reverse the anxiety behavior by pharmacologically repairing this pathway.

How did they do it?

The researchers used a mouse model of chronic stress where they physically restrained young adult mice for 3 hours daily for 14 consecutive days. They also validated their results with a second mouse model of stress, maternal separation, where young pups from postnatal day 5 to 21 were isolated from their mother for 3 hours daily. To assess the mice for anxiety- and depressive-like behaviours, they used common rodent assays like the elevated plus maze, marble burying test, and forced swim test. To study the molecular events in the brain specifically related to the synapse, they performed biochemical tests to evaluate the amount of relevant proteins and electrophysiological tests to evaluate excitatory and inhibitory transmission. To further investigate the reduction of protein expression, they performed a proteomic screen by co-immunoprecipitation and then mass spectrometry analysis, providing insight into how these proteins interact as part of a signaling pathway.  

What did they find?

First, the researchers found that following the chronic stress protocol, the mice in both chronic stress models showed anxiety and depressive-like behavior, and this was in parallel with the decrease in several inhibitory proteins at the synapse including Neuroligin2 (NL2). When they investigated the decrease in NL2 expression further, they found that upregulation of proteins including Src kinase and calmodulin (CaM) resulted in reduced interaction between MyosinVa (MyoVa) and NL2, indicating this pathway was altered in response to chronic stress. The authors manipulated this pathway by suppressing Src kinase activity via an injection of a Src kinase inhibitor, PP2 for 7 consecutive days after the chronic stress protocol. An inactive analog PP3 was used as a control. They observed a reduction in anxiety-like behavior in mice treated with PP2. Overall, inhibiting the elevated activity of Src kinase in chronically stressed mice re-established the expression of synaptic protein NL2 — which was previously reduced by stress — thereby restoring inhibitory synaptic transmission. 

What's the impact?

The public health burden of anxiety and depression calls for the need to create more effective and well-tolerated medication for these disorders. This study found a novel therapeutic target sensitive to the stress response, the Src-CaM-MyoVA-NL2 pathway, which is related to the GABA system responsible for inhibitory transmission. Targeting this pathway could help reduce anxiety behavior and improve outcomes for those affected. Further research in humans is needed to validate these findings and explore whether therapies targeting this pathway might be effective. 

ccess the original scientific publication here.