A Micro Solution to a Macro Problem?

 

By Danielle Gerhard

Recent estimates by the National Institute for Mental Health (NIMH) have found that approximately 25% of American adults will experience a mental illness within a given year. Individuals living with a serious mental illness are more likely to develop a chronic medical condition and die earlier. In young adults, mental illness results in higher high school drop out rate. A dearth of effective medications leaves many individuals unable to hold a job, causing America a $193 billion loss in earnings per year. These saddening statistics shed light on the need for better drugs to treat mental illness.

 

Traditionally, treating a mental illness like depression, anxiety or schizophrenia involves a delicate and perpetually changing combination of drugs that target levels of neurotransmitters in the brain. Neurotransmitters are chemicals produced by the brain and used by cells to communicate with one another. Drugs used to treat mental illness either increase or decrease the release, reuptake or degradation of these chemicals from the cell. The current paradigm is that the disease solely results from neurotrasmitter imbalance. Therefore, research has predominantly focused on the specific types of cells that release them. However, neurons make up approximately 50% of all cells in the human brain. The other 50% of brain cells are glial cells and are responsible for maintaining and protecting the neurons in the brain and body.

 

One type of glial cell, microglia, are specialized macrophage-like immune cells that migrate into the brain during development and reside there throughout life. Microglia are the primary immune cells in the brain and act as first-responders, quickly mounting responses to foreign pathogens and promoting adaptive immune actions. Microglia can adapt to changes in their microenvironment by protracting or retracting their processes to maintain neuronal health, scavenging their surroundings for dead neurons and cellular debris. Moreover, it has been shown that microglia are involved in the induction and maintenance of long-term potentiation, an event that is critical for synaptic plasticity underlying learning and memory. Only in the past decade or so has this cell type begun to surface as a potential mediator in the development and continuation of mental illness. As a result of decades of neuron-focused experiments, the function of microglia have either been misunderstood or over-looked all together. Two recently published experiments contradict our conventional understanding of the etiology of mental illness.

 

A new study published in the January 29th issue of the scientific journal Nature Communciations by Dr. Jaime Grutzendler’s team at Yale University highlights a novel role for microglia in Alzheimer’s Disease (AD). Late-onset AD is thought to result from the accumulation of the protein β-amyloid (Αβ). This process is referred to as plaque aggregation and results from reduced Aβ plaque clearance. Because microglia with an activated morphology are found wrapped around areas of high Aβ accumulation, it has been hypothesized that they actually contribute to weakened neuronal projections by releasing small neurotoxic proteins, cytokines, that affect cell communication. Aβ can exist as mature and inert fibrillar Aβ but can also revert back to an intermediatary state, protofibrillar Aβ, which is toxic to neurons.

 

Dr. Grutzendler’s lab set out to to further investigate the role of microglia in Aβ plaque expansion with respect to the different forms of Aβ. Using two-photon imaging and high-resolution confocal microscopy, the team at Yale was able to show that, for the most part, microglia formed tight barriers around Aβ plaques with their processes, but in some instances microglia left plaque “hotspots” exposed. These plaque “hotspots” were associated with greater axonal and neuronal damage.

 

These findings indicate that microglia generated protective barriers around Aβ plaques that served to protect neurons from the neurotoxic effects of protofibrillar Aβ. Of note, studies using aged mice revealed that microglia were less effective at surrounding plaques leading to increased neuronal damage. Microglia regulation decreases with age thereby rendering neurons more vulnerable to environmental insults. This cell type is therefore a likely key mediator of neuronal death that leads to cognitive decline and emotional distrubances in patients suffering from AD and other neurogegenerative diseases.

 

Another recently published study highlights a novel role of microglia in addiction, a chronic disease that afflicts many individuals with mental illness, comes from Dr. Linda Watkins, of the University of Colorado, Boulder. The study, published in the February 3rd issue of the scientific journal Molecular Psychiatry, examines the role of microglia in the rewarding and reinforcing effects of cocaine.

 

It has long been understood that drugs of abuse cause activation of the dopamine (DA) system in the brain, with increased DA release from the ventral tegmental area (VTA) to the nucleus accumbens (NAc), a brain region important for their rewarding effects. Cocaine achieves this effect by blocking dopamine transporters (DATs) on the cell, resulting in increased levels of synaptic DA and sustained neuronal activity. Therefore, efforts have focused on targeting DATs to prevent the rewarding effects of cocaine and ultimately reduce addiction.

 

In addition to these established dogmas, recent studies have shown that cocaine also activates the brain’s immune system. Microglia express Toll-like receptor 4 (TLR4) and its binding protein MD-2, which are important for reconizing pathogens and activating the release of pro-inflammatory molecules such as interleukin-1β (IL-1β). Using an animal model of addiction in combination with in silico and in vitro techniques, Dr. Watkin’s team found that cocaine activates the TLR4/MD-2 complex on microglia, resulting in an upregulation of IL-1β mRNA in the VTA and increased release of DA in the NAc. Administration of the selective TLR4 antagonist (+)-naloxone blocked the cocaine-induced DA release and the rewarding effects of cocaine administration in the rodent self-administration behavioral models. Overall, the study concludes that TLR4 activation on microglial cells contributes to the rewarding and reinforcing properties of cocaine. Thus, drugs targeting this system could provesuccessful in treating addiction.

 

Through these studies and similar reports, it is becoming apparent that mental illness is more than a chemical imbalance in the brain and therefore shouldn’t be studied as such. The two studies highlighted in this article show the diverse role of microglia in the development and maintenance of mental illnesses. A more in-depth understanding of how this cell type interacts with already identified neural systems underlying mental disorders could result in the development of better-tailored drug design.

 

Signed, Sealed and Delivered… to the Brain

 

Robert Thorn

The blood brain barrier has been a problem for pharmaceutical companies for some time. The blood brain barrier separates the body’s circulatory system (the blood) from the fluid that surrounds the brain. It allows for transfer of vital nutrients and exchange of gases to make sure the brain properly functions, but not everything can pass through which poses a problem for treating brain diseases with pharmaceuticals. The classic example of the blood brain barrier hindering proper drug delivery is in the case of Parkinson’s disease. Parkinson’s disease is caused by a loss of dopamine neurons in a specific area of the brain, the substantia nigra. The first idea to treat this was to give the patient dopamine, but this was ineffective because dopamine itself could not cross the blood brain barrier. Instead it was discovered that the precursor of dopamine, L-Dopa, could cross the blood brain barrier and treat the disease. Unfortunately, other drugs that may help treat brain disease do not have such an easy method to get around the blood brain barrier.

 

A new paper in the Journal of Molecular Therapy proposes a solution to this problem. The main idea is that by attaching a drug that cannot pass through the blood brain barrier to a protein that can pass through the barrier, you may be able to get the compound you want into the brain. They set out to test this in two ways: First they wanted to prove their concept by delivering a green fluorescent protein (GFP) to the brain to test levels and then they wanted to try delivering myelin basic protein (MBP) to the brain to test for function. The interesting thing about MBP is that it has been shown to be able to reduce the brain plaques associated with Alzheimer’s disease in previous studies. Finding a way to deliver this to the brain of living beings could lead to a new way to treat Alzheimer’s patients.

 

To act as the transfer motif of the construct they chose Cholera Toxin B subunit (CTB), which is able to transcytose through the blood brain barrier. They attached CTB to both GFP and MBP with a linker that will allow enough movement so that neither the structure nor the function of CTB or the linked protein is altered. The researchers did experiments both in vivo using a wild type mouse and a mouse that has Alzheimer’s as well as doing studies ex vivo by using slices of mouse and human brains afflicted with Alzheimer’s disease in culture to test the efficacy of their drug.  They first gave oral doses of CTB-GFP to normal mice and they found that there was an increase in the amount of GFP in the brain versus just giving oral GFP. In addition, when they treated the Alzheimer’s disease mice with oral CTB-MBP they saw a decrease in the amount of plaques in the brain of these mice. These results were recapitulated in the ex vivo studies using brains from humans afflicted with Alzheimer’s Disease, showing that the drug treatment could be effective in humans as well. These results showed that the researchers had developed an interesting way to deliver drugs to the brain while being minimally invasive and bypassing the blood brain barrier.

 

While this research is still a long way from being FDA approved and in the open market, the researchers have developed a promising method to better treat neurological diseases in the future.

 

 

Cleaning Your Body When You Sleep

Celine Cammarata

Sleep is a great mystery for scientists.  Nearly all living things do it, and sleep deprivation quickly leads to cognitive deficits, health problems, and death, so we can safely assume that sleep is important.  But for what exactly, no one is sure.  This week, a new paper in Science has made a splash by showing compelling evidence that sleep plays a key role in washing waste products from the brain, leaving it clean and refreshed for a new day of use.

 

Waste products are a natural part of life; all cellular processes produce waste, and being particularly busy cells, neurons tend to churn out a lot.  But unlike most parts of the body, where the lymphatic system takes care of clearing metabolic waste, in the brain proteins are washed out from the space surrounding cells via the exchange of clean cerebrospinal fluid (CSF) from the ventricular system in and around the brain with interstitial fluid containing waste products.

 

In the current study, the authors examined how readily labelled CSF traveled around the brains of mice in various states, and found that CSF influx to the brain was about 95% lower when animals were awake than during sleep.  Similar comparatively high CSF flow was seen when mice were anesthetized.

 

The investigators hypothesized that the observed difference in CSF flow may be due to differences in the interstitial space when animals were asleep or awake; reduced space between cells in awake mice could impede the movement of CSF.  When they tested this, they found that indeed, interstitial space was significantly greater during sleep or anesthesia, making an easier route for CSF.

 

Better flow of CSF means solutes are more easily flushed out of the area surrounding cells.  The authors demonstrated that β-amyloid, a major waste product in the brain, was cleared much more efficiently in sleeping and anesthetized mice than in awake animals, as was an inert test tracer, C-inulin.

 

The finding that anesthesia acts similarly to natural sleep suggests that it is the animal’s state, rather than circadian rhythms, that dictates the solute clearing properties of sleep, possibly via changes in cell volume that would in turn effect interstitial space.  Because they’re know to be important in arousal, adrenergic neurotransmitters are a good candidate to signal such changes.  The authors found that, consistent with this idea, inhibiting adrenergic neurotransmitters in awake animals improved CSF flow.

 

These findings suggest that a key role of sleep, and the reason sleep is so critical to brain function, may have to do with clearing waste products and restoring the brain the a healthy, clean state for the next days’ use.

Leafing through the Literature

Thalyana Smith-Vikos

Highlighting recently published articles in molecular biology, genetics, and other hot topics

Can I get some of your gut bacteria?

While there have been many reports popping up in the literature that demonstrate a connection between gut microbiome and diet, Ridaura et al. have elegantly showed how the mammalian microbiome affects diet in a specific yet alterable manner that can be transmitted across individuals. The researchers transplanted fecal microbiota from adult murine female twins (one obsess, one lean) into mice fed diets of varying levels of saturated fats, fruits and vegetables. Body and fat mass did depend on fecal bacterial composition. Strikingly, mice that had been given the obese twin’s microbiota did not develop an increase in body mass or obesity-related phenotypes when situated next to mice that had been given the lean twin’s microbiota. The researchers saw that, for certain diets, there was a transmission of specific bacteria from the lean mouse to the obese mouse’s microbiota.

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In vivo reprogramming

Abad et al. have performed reprogramming of adult cells into induced pluripotent stem cells (iPSCs) in vivo. By activating the transcription factor cocktail of Oct4, Sox2, Klf4 and c-Myc in mice, the researchers observed teratomas forming in multiple organs, and the pluripotency marker NANOG was expressed in the stomach, intestine, pancreas and kidney. Hematopoietic cells were also de-differentiated via bone marrow transplantation. Additionally, the iPSCs generated in vivo were more similar to embryonic stem cells than in vitro iPSCs by comparing transcriptomes. The authors also report that in vivo iPSCs display totipotency features.

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Connection between pluripotency and embryonic development

Lee and colleagues have discovered that some of the same pluripotency factors (Nanog, Oct4/Pou5f1 and SoxB1) are also required for the transition from maternal to zygotic gene activation in early development. Using zebrafish as a model, the authors identified several hundred genes that are activated during this transition period, which is required for gastrulation and removal of maternal mRNAs in the zebrafish embryo. In fact, nanogsox19b and pou5f1 were the top translated transcription factors prior to this transition, and a triple knockdown prevented embryonic development, as well as the activation of many zygotic genes. One of the genes that failed to activate was miR-430, which the authors have previously shown is required for the maternal to zygotic transition. Thus, Nanog, Oct4 and SoxB1 induce the maternal to zygotic transition by activating miR-430.

 

A microRNA promotes sugar stability

Pederson and colleagues report that a C. elegans microRNA, miR-79, targets two factors critical for proteoglycan biosynthesis, namely a chondroitin synthesis and a uridine 5′-diphosphate-sugar transporter. Loss-of-function mir-79 mutants display neurodevelopmental abnormalities due to altered expression of these biosynthesis factors. The researchers discovered that this dysregulation of the two miR-79 targets leads to a disruption of neuronal migration through the glypican pathway, identifying the crucial impact of this conserved microRNA on proteoglycan homeostasis.

Struggling to keep up with all the mIRs? Create your feed for miR-430 or miR-79.

 

Establishing heterochromatin in Drosophila

It is known that RNAi and heterochromatin factor HP1 are required for organizing heterochromatin structures and silencing transposons in S. pombe. Gu and Elgin built on this information by studying loss of function mutants and shRNA lines of genes of interest in an animal model, Drosophila, during early and late development. The Piwi protein (involved in piRNA function) appeared to only be required in early embryonic stages for silencing chromatin in somatic cells.  Loss of Piwi leads to decreased HP1a, and the authors concluded that Piwi targets HP1a when heterochromatin structures are first established, but this targeting does not continue in later cell divisions. However, HP1a was required for primary assembly of heterochromatin structures and maintenance during subsequent cell divisions.

 

The glutamate receptor has a role in Alzheimer’s

Um and colleagues conducted a screen of transmembrane postsynaptic density proteins that might be able to couple amyloid-β oligomers (Aβo) bound by cellular prion protein (PrPC) with Fyn kinase, which disrupts synapses and triggers Alzheimer’s when activated by Aβo-PrPC . The researchers found that only the metabotropic glutamate receptor, mGluR5, allowed Aβo-PrPC  to activate intracellular Fyn. They further showed a physical interaction between PrPC and mGluR5, and that Fyn is found in complex with mGluR5. In Xenopus oocytes and neurons, Aβo-PrPC caused an increase in intracellular calcium dependent on mGluR5. Further, the Aβo-PrPC-mGluR5 complex resulted in dendritic spine loss. As a possible therapeutic, an mGluR5 antagonist given to a mouse model of inherited Alzheimer’s reversed the loss in synapse density and recovered learning and memory loss.

 

Keep playing those video games!

Anguera et al. investigated whether multitasking abilities can be improved in aging individuals, as these skills have become increasingly necessary in today’s world. The scientists developed a video game called NeuroRacer to test multitasking performance on individuals aged 20 to 79, and they observed that there is an initial decline in this ability with age. However, by playing a version of NeuroRacer in a multitasking training mode, individuals aged 60-85 achieved levels higher than that of 20-year-olds who had not used the training mode, and these successes persisted over the course of 6 months. This training in older adults improved cognitive control, attention and memory, and the enhancement in multitasking was still apparent 6 months later. The results from playing this video game indicate that the cognitive control system in the brains of aging individuals can be improved with simple training.

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Can Protein Synthesis Protect Against Alzheimer’s?

Celine Cammarata

For many, the name Alzheimer’s Disease brings to mind plaques, tangles, and a vague knowledge that these somehow sicken neurons to cause a crippling dementia.  This week, Ma et al. demonstrated that part of that “somehow” lies in dysregulation of protein synthesis.  It makes sense: long term memory requires synthesis of new proteins, while Alzheimer’s Disease (AD)  involves deficits in memory.  Earlier work supports the idea.  The protein eukaryotic initiation factor 2 α (eIF2α) is a general regulator of translation, and when eIF2α is phosphorylated most translation is inhibited.  AD in human patients has been associated with increased eIF2α phosphorylation.  Thus, the researchers reasoned, inhibiting some of the proteins that phosphorylate eIF2α might help alleviate some of the diseases effects.

The investigators chose the protein PERK, one of several able to phosphorylate eIF2α, and bred Continue reading “Can Protein Synthesis Protect Against Alzheimer’s?”