From villain to hero

‘The dose makes the poison’ goes the adage, but could it be so for carbon monoxide and hydrogen sulphide? Could these notorious toxins be used for the treatment of complex neurological disease? Don’t rule it out say one team of neuroscientists…

Carbon monoxide (CO) and hydrogen sulphide (H₂S) are best known as toxic gases. CO is an odourless gas produced during the incomplete combustion of carbon, for example in faulty heating and gas appliances. Although carbon monoxide is rightly regarded as a highly toxic gas, it has been known since the 1890’s that organisms generate CO endogenously. CO can interfere with oxygen transport in blood due to its ability to bind to haemoglobin; however its toxicity is believed to be due to disruption of mitochondrial function.

H₂S on the other hand is known for its pungent rotten egg-like smell properties. Most sources of H₂S are natural, however, there are some industrial sources (e.g. pulp and paper manufacturing).

What is now becoming apparent through scientific research is that these gases could be potential therapeutic treatments for a host of cardiovascular and neurological diseases. In light of this research we have been looking at the effects of CO in the brain with the hope that CO could be a potential therapeutic remedy for neurodegenerative diseases such as Alzheimer’s disease (AD). Recently, we undertook undergraduate summer projects sponsored by The Alzheimer's Society and the Physiological Society in Dr Dallas’s lab to look at the physiology of microglial cells in response to amyloid beta and CO.

The human brain is made up of a collection of various cells types. Nerve cells get most of the attention when it comes to neuroscience research however the so called support cells are increasingly being viewed as pivotal players in brain physiology. These support cells or glia include three specific cell types; astrocytes, microglia and oligodendrocytes. All these cells contribute to brain physiology and there is a growing interest in them in the pathogenesis of neurological disease. In particular microglia cells have been implicated in AD through both genetic and in vivo studies. Under normal conditions, microglia are constantly patrolling the brain; this is to enable detection of potential threats. However in response to pathogens these cells can suddenly become “aggressive”. To tackle a threat they usually change their shape, appearance, protein production and gene expression in a matter of minutes. This process can become damaging to the surrounding cells, including nerve cells.

Our research revealed that amyloid beta (A?), the toxic peptide responsible for the nerve cell death in AD, was not toxic to microglia. This is in contrast to previous work highlighting the vulnerability of both nerve cells and astrocytes to A?. While no apparent changes were observed in cell health, we observed distinct changes in calcium homeostasis. Calcium homeostasis is an essential process within the cells in the central nervous system. Disruption of Ca²⁺ homeostasis may incur neuronal death, which is seen in neurodegenerative diseases such as Alzheimer’s disease. Indeed, current calcium channel blockers (preventing calcium influx) are proposed as drug for the treatment of nervous system disorders. Our experiments revealed a significant increase in the intracellular Ca²⁺ concentration after A? exposure. This increase was suppressed when cells were exposed to CO in addition to A?. Further experiments are planned to examine the mechanism by which CO modulates A? effects on Ca²⁺ handling in microglia. Microglia express an array of ion channels, which are now considered a substrate for some of the physiological and indeed pathological effects of CO. In addition, it is unclear as to the role of the A? induced changes in microglia calcium handling. Changes in calcium have been implicated in regulating microglia motility and phagocytosis. This would be pertinent in AD as microglia attempt to clear the brain of accumulating amyloid beta. This further research will clarify a role for CO in modulating microglia physiology and if CO therapy would prove beneficial in treating neurological disease.

Our research adds to the growing body of evidence supporting the role for carbon monoxide as an endogenous signalling molecule in the brain. Most research into the benefits of carbon monoxide production revolves around the regulation of the precursor enzymes, the heme oxygenases (HO-1 and HO-2). Indeed, organisms that lack HO-1 (gene name HMOX-1) have a compromised life span.

The knock out mouse is >95% embryonically lethal (those that survive have a shortened life span, organ abnormalities and enhanced sensitivity to a multitude of stresses) and a clinical case report identified an individual deficient in HO-1 that died prematurely. A common feature among many age-related neurodegenerative conditions is the increased expression of HO-1 in the brain. Thus, while HO-2 is constitutively expressed in both neurons and astrocytes, HO-1 is inducible in both cell types following oxidative stress or ischemic insult and is also observed in neurodegenerative diseases.

Both oxygenases degrade heme to liberate biliverdin, ferrous iron (Fe²⁺) and CO. This reaction breaks down the pro-oxidant heme and generates a highly effective antioxidant in bilirubin (produced from biliverdin via bilirubin reductase). In addition, CO is an important signalling molecule, interacting with diverse intracellular signalling pathways. Much evidence indicates that heme oxygenases protect the brain against acute glutamate excitotoxicity and in various in vitro and in vivo models of stroke. It is not yet established how protection is afforded by any specific heme product. However, with the development of carbon monoxide releasing molecules (CORMs) the case for CO is becoming stronger. Recent studies suggest that CO is neuroprotective against experimental stroke, oxidative stress and intracerebral haemorrhage. Furthermore research has pointed the finger at glial cells in their contribution to CO signalling in that CO derived from astrocytes in response to brain hypoxia protects neighbouring neurons from apoptosis.

While this research area is progressing at a rapid pace further work is required. The next steps required will be to conduct further experiments to test the functional effects of exposure to these gases and come up with detailed mechanisms of action of these gases. Indeed it is also necessary to examine the effects of these gases on the diverse cell types within the body. Recent evidence highlights this in that CO can be beneficial to the brain but in stark contrast it is dangerous for the heart. As previously stated, a lot more research is still required in order for a potential therapy to be produced, we can still speculate about how the gases could provide a promising future to patients with neurodegenerative diseases. For a start, carbon-monoxide releasing drugs (CORMs) are widely used in current research projects and pharmaceutical companies are working on the next generation of compounds. H₂S is lagging behind in terms of available donor molecules (NaHS is routinely used); however research groups at the University of Exeter are tackling this problem. Most recently a medical device (Covox DS), capable of delivering controlled amounts of CO has been investigated in controlled medical trials.

This device aims at respiratory disorders such as asthma; however it would be logical to speculate that similar devices could be adapted as therapeutic options to slow down neurodegenerative diseases such as Alzheimer’s disease in a future. 

The Authors: Hafeeza Ayuoob, Vytautas Kontrimas and Mark Dallas of the School of Pharmacy at the University of Reading