Insights into the Mode of Action of the Human Cytomegalovirus

There is form of the human cytomegalovirus that is responsible for herpes – this virus is referred to as the herpesvirus human cytomegalovirus (HCMV).  This virus can have severe repercussions for infants and those individuals who are immune-compromised such as AIDS patients.  The entire HCMV genome was completely sequenced twenty years ago.  In spite of this accomplishment, the understanding of the complete array of proteins that are produced by this virus has not been fully elucidated.  This is because of the fact that although the genome is quite small – 240 kilobases (kb) – it has been estimated that there are between 165 and 252 open reading frames (ORFs).  An ORF is the part of a reading frame in the genome that contains no stop codons – stop codons terminate transcription.

A gene is defined as that part of the genome (DNA) that contains the information for the production of a protein product.  Transcription is the first step in the process that converts the information contained within the reading frame into what is referred to as messenger RNA (m-RNA).  It is the m-RNA that migrates to the highly specialized cell organelles, the ribosomes, where proteins are ultimately produced.  This phase is called translation.  It seems that the translation products of HCMV are far more complex than previously believed.

To help unravel this apparent mystery, Dr. Noam Stern-Ginossar and colleagues at the Department of Cellular and Molecular Pharmacology at Howard Hughes Medical Institute of California at San Francisco infected human foreskin fibroblasts (HFFs) with a clinical strain of HCMV.  They subsequently harvested cells 5, 24 and 72 hours post infection and analyzed the full range of translation protein products. 

As a result of this very intensive analysis, they were able to identify 751 translated ORFs - hundreds of which had not been identified before.  The explanation that best fits the results is that transcription involves the use of alternative start sites with the net effect being the production of multiple and distinct protein products.  This result was not anticipated by the investigators and demonstrates a level of complexity that far exceeded expectations.

This kind of work is significant in that it provides important insights into the full scope of the functional and antigenic – a measure of the capacity to produce an immune response – potential of HCMV. 

Modeling the Human Brain

The human brain is a highly complex organ that is responsible for remarkably sophisticated behavior.  In order to more fully elucidate the relationship between brain structure and function, investigators involved with attempting to understand the neuroscience of brain systems especially concerning  cognition are testing large-scale simulations of brain structure and function.  This is a particularly daunting challenge.

 

For example, there is the so-called, "Blue Brain Project" that involves the simulation of about one million neurons arranged in cortical columns.  The design of this model incorporates significant biological detail that reflects known spatial relationships and neuronal connectivity data.  Another example of a human brain-modeling project involving some 100 billion neurons has also been reported. 

 

A limitation to these approaches has been that although they seek to model the complexity of brain organization they do not address the relationship between brain structure and brain function, i.e. human behavior.  Dr. Chris Eliasmith and colleagues from the Centre for Theoretical Neuroscience from the University of Waterloo, Waterloo, Ontario in Canada has attempted examine this relationship.  To do so, they have created a neuron model of 2.5 million neurons that focuses on the relationship between neuronal structure in the brain and the resulting human behavior.

 

Within this model, the inputs are represented by images of handwritten or typed characters.  All the outputs are the movement of an "arm" that possesses appropriate physical properties such as mass, length and inertia.  The designers of this model refer to it as "Spaun" – an acronym for Semantic Pointer Architecture Unified Network.  Incorporated within Spaun are typical brain-related functions such as image recognition, serial working memory and learning.  Furthermore, the eight tasks that Spaun performs are:

·         Copy drawing

·         Image recognition

·         So-called three-armed bandit task in which it is required to determine which of the three possible choices results in the statistically greatest reward

·         Successful reproduction of a list of any length

·         Counting

·         Responding to questions

·         Ability to create variables

·         Simple reasoning.

 

The model is so structured that it represents the anatomical structure of the human brain as established by current brain research.  Although a more detailed examination of the theoretical basis for this model is beyond the scope of this report, this kind of intensive work indicates the degree to which the apparent enigmas associated with high-order brain function are being methodically unraveled.

The Immunological Mechanism Behind the Battle to Contain Chronic Viral Infection

The lifelong immunity to viral infection involves a multistep process:

·         The maintenance of so-called long-lived memory lymphocytes – lymphocytes are circulating white blood cells that play a critical role in the immune response

·         Upon reinfection, these memory lymphocytes then generate large numbers of short-lived cells that actively combat infection

·         Lastly, the pool of long-lived memory cells is replenished for use in future assaults.

 

There are a number of viral infections in which the virus following active infection can become quiescent and actually be incorporated in the host DNA and subsequently be reactivated under the appropriate conditions.  An example of a low-level virus of this type is cytomegalovirus (CMV) – a herpes virus that can reside within a host for a lifetime.  To combat such an infection, a balance is achieved between the maintenance of a pool of long-lived immunological memory cells and short-lived lymphocytes designed to fight the infection as described above.  A balance of these modalities confers lifelong immunity to the individual.

 

The issue becomes more problematic in the case of persistent infections with high viral loads such as the human immunodeficiency virus (HIV) that causes AIDS.  In such infections, the ability to maintain a consistent pool of long-term memory cells can be compromised on account of the necessity to maintain a persistently high level of short-term lymphocytes.  In the case of AIDS it is believed that a constantly elevated population of CD8+ lymphocytes (CD8 is a particular protein that resides on the cell surface of these cells) that are involved in combatting the HIV infections leads to a decreased pool of memory cells and eventually results in the collapse of immunity towards HIV.

 

What is not clear is the actual mechanism that is responsible for this immunological imbalance in regard to chronic viral infections.  In attempt to elucidate this issue, Dr. Michael A. Paley and his colleagues at the Department of Microbiology and Institute for Immunology at the Perelman School of Medicine at the University of Pennsylvania worked with chronically infected mice as the animal model for their studies.  From their work, they discovered two unique subsets of CD8+ lymphocytes in response to chronic viral infection – one subset involved in the renewal of the CD8+ population and the other in differentiation.  Furthermore, they found that elimination of either of these subsets resulted in the organism's inability to control chronic infection.  These findings seem to suggest that an imbalance in differentiation and renewal may be ultimately responsible for the collapse of immunity in human patients suffering from chronic infection.

 

The clarification of this mechanism may help in the ultimate development of targeted therapies for chronic viral infections in humans.  

The Role of inflammation in Colorectal Cancer

Chronic inflammation has been implicated as playing an essential role in a number of human cancers including colorectal cancer (CRC).  Chronic inflammation is a condition in which the body's immune system's inflammatory response is turned on.  When the inflammatory response is prevalent in the intestines, powerful factors are secreted including: Interleukin-6 (IL-6), tumor necrosis factor (TNF-α), IL-23 and reactive oxygen species.  All these may act in consort to create a microenvironment that can lead to accelerated DNA damage in the intestinal epithelia and thereby increase the likelihood of leading to CRC as is demonstrated by the discovery of a colitis-associated CRC.

 

Furthermore, within the lumen of the colon, literally trillions of bacteria coexist in an environment referred to as the "microbiota."  These bacteria live in close proximity to the epithelial cells that line the colon.  From this information, the question naturally arises, "Do these bacteria, under the right conditions participate in some way with the cancer-causing process – carcinogenesis?"

 

In attempt to determine the answer to his question, Janelle C. Arthur and her colleagues at the Department of Medicine at the University of North Carolina at Chapel Hill studied the association of the microbiota, chronic inflammation and the onset of colon cancer in mice with a genetic susceptibility to colitis.  Their findings can be summarized in the following way:

 

·       Monocolonization  of the experimental animal's gut with Escherichia coli (E. coli) NC101 promoted the development of CRC in azomethane treated animals – azomethane is a potent carcinogen

·         Deletion of the particular strain of E-coli – polyketide synthase (pks+) strain known to have a deleterious impact on host genes, genotoxic, decrease the amount of invasive tumors

·         Pks+ E. coli are found in a significant percentage of human patients with inflammatory bowel disease and CRC.

 

These data demonstrate that within the scope of the mouse model, colitis, as an example of chronic inflammation in the gut, can promote CRC by altering the microbial composition of the intestinal microbiota.  In addition, these findings may have direct application to the understanding of the development of human CRC.

Iron and Good Health

There are many so-called "micronutrients" that are required in the human diet.  Many of these are metals including Sodium, Potassium, Zinc, Cobalt and Iron.  Iron plays a special role since it is a member of the transition state elements and can assume a number of different charged states; this attribute has made it quite useful as an essential cofactor in the function of many enzymes and oxygen transport systems not only in humans but also in many diverse life forms across the entire spectrum of living things.

 

On account of these properties, iron has been shown to play an important role in host-pathogen interactions during infections.  As a matter of fact, it has been clearly shown that a host's iron status can directly impact the degree and intensity of infection.  In one particular study involving Somalis who were iron-deficient, these individuals had a five-fold increase in various infections including  reactivation of preexisting malaria in relation to those individuals that were given iron supplements.  A degree of caution needs to be stressed in regards to the ingestion of iron supplements in that excessive intake can prove toxic.

 

A hormone, Hepcidin, present in humans has been shown to play a key role in the absorption of iron consumed in the diet and its subsequent distribution to many tissues throughout the body.  In addition, Hepcidin, a peptide hormone (peptides are small protein molecules), has antimicrobial properties.

 

Dr. Hal Drakesmith and his colleagues from the Molecular Immunology Group and Medical Research Council Human Immunology Unit at the Weatherall Institute of Molecular Medicine at the University of Oxford in Oxford, UK, have shown that the synthesis of Hepcidin is regulated by not only the levels of iron present in the body but also immunological status i.e. the presence of infectious agents.

 

Another aspect of iron regulatory mechanisms relates to the presence of unbound iron or heme – that is often released during infections – in the circulation.  Unbound iron can have a number of deleterious side effects:

·         It can provide an essential nutrient to an ongoing infection

·         It can generate harmful free radicals and result in tissue damage.

For these reasons, there are proteins present that can safely bind the unbound iron.  These proteins include transferrin, lactoferrin and ferritin for iron; haptoglobin for hemoglobin – the essential component for delivering oxygen to the tissues- and hemopexin for heme.  The antimicrobial properties of these iron carriers is supported by the observation that they inhibit the growth of certain bacteria in the laboratory and by the fact that lactoferrin is found in abundance in breast milk and is released from neutrophils – immunological cells that are involved in the body's first line of defense against infection.

 

These data strongly implicate iron status in the body with good health and the natural defense against infection.

Climate Change - New Concerns

There is a growing concern among climatologists that the earth may be entering an era of accelerated climate change.  This reevaluation has at its core evidence of so-called feedback loops.  Some examples of the feedback process are the following:

·         As sea ice melts it reduces the albedo effect (reflection of the sun's heat from the surface) and, therefore, leads to an increase in ocean temperature that results in a further melting of the sea ice

·         As areas of permafrost – found in Siberia - melt due to increased average temperature in the atmosphere, this releases carbon dioxide (CO₂) and the more potent greenhouse gas, methane from vast stores of biomass that exist below the permafrost.  It is estimated that the total store of this organic material represent twice the amount of carbon (CO₂) already in the atmosphere.  The release of these greenhouse gases further increases the average global temperature resulting in further melting of the permafrost.  This kind of feedback involving the permafrost has also been implicated in the warming crisis that has been reported to occur at the end of the Permian Period (~ 250 million years ago) by A. Hallam and P.B. Wignall in their book entitled, Extinctions and Their Aftermath

·         The disruption of the thermohaline circulation (THC) as discussed in detail in a previous article (Climate Change Revisited).  It is important to understand that the THC influences global climate by transporting heat and freshwater between the oceans globally.

 

The prospect of these feedback mechanisms accelerating climate change has so concerned James E Hansen, Director of the NASA Goddard Institute for Space Studies that he has warned that, "Unless the world slashes CO₂ levels back to 350 ppm (the current level is 391), we will have started a process that is out of humanity's control."  He cited the possibility of a sea level rise of five meters during this century - if this should become reality it would be catastrophic.  Other scientists in his field believe the rise will not be that extreme but do envision a sea level rise as high as two meters.  This projection is also very troubling.

 

These data indicate that not only are we currently feeling the impact of climate change but also that these changes may very well be accelerating.  This is a global crisis that necessitates a global solution. 

Climate Change Revisited

The recent weather catastrophe that struck an area of the United States where some fifty million individuals reside brings to the forefront once again the issue of climate change.  The storm surge precipitated by the hurricane referred to as, "Sandy," was unprecedented in its severity for this region of the country.  The damage that it wrought in both its physical manifestations and its emotional impact on the lives of those directly affected together with the mounting cumulative evidence including extreme hot weather, droughts, the frequency and severity of forest fires, etc. are representative of the expected consequences of the ever-increasing concentration of greenhouse gases in the atmosphere as predicted by sophisticated computer-modeling developed by climate scientists.   

 

I have eluded to some of these factors in previous reports, that show a connection between increased surface temperature of the oceans and the expected increased severity and possible frequency of hurricanes and typhoons that derive their energy from ocean temperature. I would like to take this opportunity, however, to address a very particular aspect of climate change that may directly relate to extreme weather conditions more likely to impact global weather patterns in the more distant future..

 

Since the upper layers of the ocean have direct interaction with the atmosphere, changes in this upper level have a direct bearing on climate over shorter periods of time (decades).  However, deep ocean circulation has a direct influence over future climate conditions.  For water to sink from the ocean's surface to the deeper ocean, it must be more dense – the two criteria that meet this requirement are temperature and salt content i.e. the water must be cold and salty.  The water in the Atlantic Ocean and the Greenland Sea – between Greenland and the Scandinavian countries –  and the ocean around Antarctica are large bodies of water that meet these requirements.  This deep dense water contributes to the deep water circulation that is global in extent.  This process is knows at the thermohaline circulation (THC).  It is important to understand that the THC influences global climate by transporting heat and freshwater between the oceans globally.

 

As ice melts as a direct consequence of the level of greenhouse gases in the atmosphere, the water in its immediate vicinity becomes less dense as a result of simple dilution.  As a consequence, such water will be less likely to sink to the deep ocean.  Consequently, this weakening of the THC, lessens the heat flow from the tropics to the North Atlantic.  It is anticipated that if the level of greenhouse gases is allowed to increase unabated over the long term, the THC would be expected to breakdown entirely.  If this should happen, the winters in the North Atlantic and Western Europe could be become extremely severe.

 

This kind of scientific analysis demonstrates that if humanity fails to address the problem of greenhouse gas emission in a meaningful way, both the near term and long term effects could prove disastrous to life as we know it.