The video below showed up in my feed a couple of days before the article I'm sharing on how stress generates enzymes that attack the brain. Together, this information highlights how destructive stress can be on our brains and our cognitive function.
The article referenced here is open access, but it is highly technical. For those who want to read more, I am including the Discussion section below the video (at the bottom of the page).
Date: September 18, 2014
Source: Ecole Polytechnique Fédérale de Lausanne
Why is it that when people are too stressed they are often grouchy, grumpy, nasty, distracted or forgetful? Researchers have just highlighted a fundamental synaptic mechanism that explains the relationship between chronic stress and the loss of social skills and cognitive impairment. When triggered by stress, an enzyme attacks a synaptic regulatory molecule in the brain, leading to these problems.Carmen Sandi's team at EPFL discovered an important synaptic mechanism in the effects of chronic stress. It causes the massive release of glutamate which acts on NMDA receptors, essential for synaptic plasticity. These receptors activate MMP-9 enzymes which, like scissors, cut the nectin-3 cell adhesion proteins. This prevents them from playing their regulatory role, making subjects less sociable and causing cognitive impairment. Credit: EPFLWhy is it that when people are too stressed they are often grouchy, grumpy, nasty, distracted or forgetful? Researchers from the Brain Mind Institute (BMI) at EPFL have just highlighted a fundamental synaptic mechanism that explains the relationship between chronic stress and the loss of social skills and cognitive impairment. When triggered by stress, an enzyme attacks a synaptic regulatory molecule in the brain. This was revealed by a work published in Nature Communications.
Carmen Sandi's team went to look for answers in a region of the hippocampus known for its involvement in behavior and cognitive skills. In there, scientists were interested in a molecule, the nectin-3 cell adhesion protein, whose role is to ensure adherence, at the synaptic level, between two neurons. Positioned in the postsynaptic part, these proteins bind to the molecules of the presynaptic portion, thus ensuring the synaptic function. However, the researchers found that on rat models affected by chronic stress, nectin-3 molecules were significantly reduced in number.
The investigations conducted by the researchers led them to an enzyme involved in the process of protein degradation: MMP-9. It was already known that chronic stress causes a massive release of glutamate, a molecule that acts on NMDA receptors, which are essential for synaptic plasticity and thus for memory. What these researchers found now is that these receptors activated the MMP-9 enzymes which, like scissors, literally cut the nectin-3 cell adhesion proteins. "When this happens, nectin-3 becomes unable to perform its role as a modulator of synaptic plasticity" explained Carmen Sandi. In turn, these effects lead subjects to lose their sociability, avoid interactions with their peers and have impaired memory or understanding.
The researchers, in conjunction with Polish neuroscientists, were able to confirm this mechanism in rodents both in vitro and in vivo. By means of external treatments that either activated nectin-3 or inhibited MMP-9, they showed that stressed subjectscould regain their sociability and normal cognitive skills. "The identification of this mechanism is important because it suggests potential treatments for neuropsychiatric disorders related to chronic stress, particularly depression," said Carmen Sandi, member of the NCCR-Synapsy, which studies the neurobiological roots of psychiatric disorders.
Interestingly, MMP-9 expression is also involved in other pathologies, such as neurodegenerative diseases, including ALS or epilepsy. "This result opens new research avenues on the still unknown consequences of chronic stress," concluded Carmen Sandi, the BMI's director.
The above story is based on materials provided by Ecole Polytechnique Fédérale de Lausanne. Note: Materials may be edited for content and length.
Michael A. van der Kooij, Martina Fantin, Emilia Rejmak, Jocelyn Grosse, Olivia Zanoletti, Celine Fournier, Krishnendu Ganguly, Katarzyna Kalita, Leszek Kaczmarek, Carmen Sandi. (2014). Role for MMP-9 in stress-induced downregulation of nectin-3 in hippocampal CA1 and associated behavioural alterations. Nature Communications; 5: 4995 DOI: 10.1038/ncomms5995
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Published on Sep 16, 2014
A Stockholm Psychiatry Lecture given by Professor Rajita Sinha, Yale University, at Karolinska Institutet Aug 27 2014. Title of the lecture: The stressed brain: hijacking cognition, emotion, behavior and health.
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Here is the discussion section of the article summarized above.
We tested the hypothesis that MMP gelatinase activity is involved in key proteolytic processing events induced by chronic stress in a hippocampal subfield-dependent manner and in connection with behavioural changes. We show that chronic stress leads to a CA1-specific reduction in the perisynaptic expression of nectin-3 and found that this reduction is critically involved in the stress-induced deficits in social exploration, social recognition and CA1-dependent cognition. Interestingly, we found increased MMP-9-related gelatinase activity in the hippocampal CA1 in chronically stressed animals and could show that MMP-9 itself cleaves recombinant nectin-3, a process mediated via the NMDA-receptor. Consistently, intra-CA1 administration of either an MMP-9 inhibitor or an NMDA receptor antagonist during stress exposure prevented the development of stress-induced deficits in social exploration, social memory and CA1-dependent cognition. Our findings highlight a fundamental role for MMP-9 in the effects of chronic stress on brain function and behaviour.
Nectins are emerging as both targets24, 43 and mediators25 of stress actions in hippocampal-dependent memory and structural plasticity. We found molecular-, regional-, cellular compartment- and stress duration-dependent changes, with reduced nectin-3 expression after 21 days, but not 1 day, of restraint stress in the CA1 synaptoneurosomal, but not the total fraction. This was paralleled by deficits in several social behaviours and in a CA1-dependent cognitive task. Our results from cell culture experiments suggested that NMDA receptor activation during stress exposure might be implicated in the cleavage of nectin-3 in CA1 and its associated behavioural alterations. Previous work has implicated NMDA receptor activation in chronic stress-induced structural alterations in the hippocampus12, 44, 45. Our in vivo study involving the pharmacological administration of the NMDA receptor antagonist MK-801, either systemically or directly into the CA1 region, confirmed that this treatment prevented the stress-induced reduction of nectin-3 expression in the CA1 synaptoneurosomal fraction as well as the behavioural impairments induced by stress in the sociability and temporal order task.
Using AAV-induced OE of nectin-3 either in the whole hippocampus or specifically in the CA1 area, we obtained evidence for a causal role of nectin-3 reduction in chronic stress-induced behavioural alterations, with the exception of the aggressive phenotype. We confirmed that the effects of nectin-3 OE were not due to altered physiological responses to the stress procedure (for example, body weight changes or corticosterone responses) or to changes in anxiety or locomotion. We found increased nectin-1 expression associated with AAV-nectin-3 OE throughout the hippocampus, consistent with evidence in knockout mice indicating that downregulation of either nectin-1 or nectin-3 induces a parallel decrease in the levels of the other nectins in the hippocampus46. Synaptophysin levels were not changed by nectin-3 OE and/or chronic stress, which is line with findings described for nectin-3 knockout mice46. In addition, using the same chronic restraint stress protocol as described here, changes in the size of postsynaptic densities were observed but not in synaptic density in the CA1 (ref. 5). Interestingly, consistent with evidence that nectins recruit cadherins to cooperatively promote cell adhesion47, we found a reduction in the CA1 perisynaptic N-cadherin levels. The specificity of these molecular changes in CA1 was supported by a lack of significant changes in the stressed animals’ synaptoneurosomal compartment of SynCAM-1 in the same brain region. To verify that the molecular changes specifically observed in CA1 were associated with well-established CA1-dependent behaviours, we tested animals in the temporal order task that is sensitive to CA1, but not to CA3, lesions40. With regard to region-specificity, our findings for CA1 are in contrast with recent evidence in mice showing reduced nectin-3 expression in CA3 (refs 24, 25). This disparity may be attributed to differences in the animal species or stress procedures.
MMPs are a family of proteolytic enzymes that degrade components of the extracellular matrix and cleave specific cell-surface proteins48, making them particularly suitable to sustain neural remodelling processes15. The degradation of cell adhesion molecules is one of the main mechanisms whereby MMPs affect neural plasticity9, 22 and the synapse-associated nectin-3 decrease suggested the potential involvement of proteolytic processing. Nectin-1 has been shown to undergo ectodomain shedding by alpha-secretase32; however, the molecular players involved in nectin-3 shedding remained unknown.
We found that decreased nectin-3 expression in the hippocampal CA1, but not in the CA3, synaptoneurosomes of the stressed animals is accompanied by increased gelatinase activity. This suggested an increase in MMP-2 and/or MMP-9 activity, as these two MMPs are the most prominent gelatinases expressed in the brain. Our cell culture experiments also indicated that NMDA receptor stimulation leads to increased nectin-3 proteolytic cleavage that is MMP-9 dependent. The involvement of MMP-9 and not MMP-2 is consistent with a previous study showing that MMP-2 does not interact with nectin-3 (ref. 49). Furthermore, we provide direct evidence that MMP-9 cleaves recombinant nectin-3. Interestingly, MMP-9 cleaves several postsynaptic proteins involved in trans-synaptic adhesion via their interaction with presynaptic proteins. The list of such MMP-9 targets includes β-dystroglycan that binds to neurexins42 as well as neuroligin-1 also binding neurexins50. Our findings are in line with previous reports implicating hippocampal MMP-9 in changes in dendritic spine morphology51 as well as in the cellular processes that contribute to a stressful learning task20. Importantly, we show that intra-CA1 treatment with a specific MMP-9 inhibitor prevented the emergence of chronic stress-induced effects in social exploration and CA1-dependent cognition. Therefore, our results are consistent other findings that indicate a crucial role for extracellular proteolysis in the stress-induced behavioural alterations, with former studies highlighting the role of serine proteases, including the tissue-plasminogen activator12 and neuropsin10.
Although deregulated social behaviour is a hallmark of many psychiatric disorders52, studies focusing on the link between chronic stress and psychopathology has mainly concentrated on studies in mood and cognition2, 7, whereas the effects of stress on social behaviours are much less known. In agreement with our previous study8, we confirm here that chronic restraint stress for 21 days leads to clear alterations in the social domain, including reduced sociability, impaired social memory and increased aggressive behaviours. The hippocampus has been implicated in social behaviours both in rodents53 and in humans54. Consistent with our findings, social recognition in rats was disrupted by CA1 damage55. However, although the effects of stress on sociability and social memory were rescued with nectin-3 OE, increased aggressive behaviours were not modified by this treatment. We have recently found that targeting the cell adhesion molecule neuroligin-2 expression or function in the hippocampus alters aggressive behaviour8, 56, suggesting the involvement of the hippocampus in the regulation of aggression. However, it should be noted that those treatments were not confined to the CA1 area, which, on its own, might not modulate aggressive behaviours.
In summary, our findings identify a key role for MMP-9 proteolytic processing of nectin-3 in the hippocampal CA1, through a mechanism that engages NMDA receptors, among the processes leading to chronic stress-induced changes in social and cognitive behaviours. In addition to nectin-3, recently identified as potential mediators in stress-related disorders25, our study highlights MMP-9 activity as a novel target for the treatment of stress-related neuropsychiatric disorders, in particular depression, which is typically characterized by deficits in the social and cognitive domains.