The macrophages in the brain, called microglia, play an important role in multiple sclerosis.  On the one hand activated microglia can generate free radicals and glutamate, which is neurotoxic, on the other hand, microglia may be involved in neural remodeling and brain repair.  It may be that microglia have inflammatory and antiinflammatory properties in a similar way that macrophages have M1 or M2 phenotypes.  Stromal vascular fraction cells contain high proportions of M2 antiinflammatory macrophages, which may be one of the reasons for the effects of autologous fat cells in treatment of multiple sclerosis.  There are some reports that neurotransmitters such as anandamine may modulate microglia production of inflammatory cytokines.  It is therefore interesting to see what type of other agents may modulate microglial activity.

In a recent paper (Russo et al. Involvement of mTOR kinase in cytokine dependent microglial activation and cell proliferation. Biochem Pharmacol. 2009 Jun 30) the effects of modulating a protein called mammalian target of rapamycin (mTOR) was examined in microglial cytokine production.

mTOR is a kinase that is associated with cell multiplication, activation, and survival.  It is downstream of several biological pathways and its activation is associated with many cancers.  Rapamycin, an immune suppressive drug that has been shown to be tolerogenic in some situations, is an inhibitor of mTOR. 

The investigators demonstrated that activation of microglial cells isolated from the cortical area of rats had activated mTOR after treatment with lipopolysaccharide (an activator of macrophages), as well as after treatment with inflammatory cytokines.  Nitric oxide release by microglial cells causes damage to neurons.  The study found out that treatment with mTOR inhibitors resulted in the inhibition of cytokine induced nitric oxide production.  Treatment of the microglial cells with mTOR inhibitors also resulted in the inhibition of proliferation and suppression of cyclooxygenase, an enzyme that causes formation of prostaglandins, which are also associated with inflammation. 

Thus the study suggests that administration of mTOR inhibitors may be a method of inhibiting microglial activation.  In fact, there is a publication combining vitamin D3 and rapamycine for the inhibition of the mouse model of multiple sclerosis (Branisteanu et al. Synergism between sirolimus and 1,25-dihydroxyvitamin D3 in vitro and in vivo. J Neuroimmunol 1997 Nov;79(2):138-47).

Microglia, the macrophages that reside in the brain, are believed to be involved in the process of neuronal degeneration in multiple sclerosis and animal models of the disease.  What are the mechanisms by which microglia may be pathological?  A recent report (Shijie et al. Blockade of glutamate release from microglia attenuates experimental autoimmune encephalomyelitis in mice. Tohuko J Exp Med 2009 Feb;217(2):87-92) suggests that microglial production of glutamate may be a cause of toxicity.

How can glutamate kill neurons?  Glutamate is used by neurons to communicate with each other.  The concentration of glutamate in the brain is tightly controlled by the blood brain barrier, which has specific glutamate transporters to only allow as much glutamate as is needed.  Additionally, when neurons communicate with each other, mechanisms exist to clear up the glutamate very rapidly after the signal is transmitted.  Too much glutamate causes what is called excitotoxicity, that is, death of the neurons from over stimulation.

In the current study the investigators demonstrated that in vitro activated microglial cells produced high concentrations of glutamate, which induced killing of neurons.  Microglial cells were demonstrated to have high concentrations of glutaminase, which generates glutamate.  When glutaminase was inhibited in vitro by addition of a small molecule inhibitor, the ability of the activated microglial cells to make glutamate, and subsequently to kill neurons was decreased. 

Furthermore, it was found that administration of the glutaminase inhibitor to mice suffering from experimental allergic encephalomyelitis (mouse multiple sclerosis) resulted in functional improvement.

These data suggest that microglial toxicity of neurons may be not only related to immunological means, but also through direct production of mediators that kill neurons.

It should be noted that Riluzole, the first drug approved for treatment of ALS works in part through suppressing interacton of glumate with its receptor, as well as upregulating activity of glutamate transporters that clear glutamate.  In fact, Riluzole has actually been demonstrated to inhibit MS-like disease in the EAE model (Gilgun-Sherki et al. Riluzole suppresses experimental autoimmune encephalomyelitis: implications for the treatment of multiple sclerosis. Brain Res 2003 Nov 7;989(2):196-204).

Macrophages, known as the “Big Eaters” are populations of immune cells that protect the body against various pathogens by engulfing them, by secreting various chemicals to let the other components of the immune system know that the body is under attack, and also by causing inflammation.  The macrophages inside the central nervous system are called “microglia”.  Glia are cells in the central nervous system that are not neurons but are involved in maintaining the function of neurons.  Specifically, glial cells are involved in providing a proper chemical environment for the neurons so that they can communicate properly.  For example, glial cells provide nutrition, physical support, and contribute to production of myelin.  Microglia comprise approximately 20% of the glial cell population.  Other glial cells include astrocytes (that amongst other things physically connect neurons to their blood supply) and oligodendrocytes (which make myelin).

While microglia are normally involved in protecting the brain from infections, they also are believed to play a pathological role in multiple sclerosis.  For example, microglial production of inflammatory cytokines, free radicals, and matrix metalloproteases is believed to be one of the reasons why multiple sclerosis damages the central nervous system (Benveniste et al. Role of macrophages/microglia in multiple sclerosis and experimental allergic encephalomyelitis. J Mol Med 1997 Mar;75(3):165-73).  This makes sense in some ways since T cells producing the cytokine interleukin-17 (IL-17), called Th17 cells, have been found both in mice and humans with multiple sclerosis.  Interestingly, microglia express receptors for IL-17, and the level of receptor expression increases during disease induction (Das Sarma et al. Functional interleukin-17 receptor A is expressed in central nervous system glia and upregulated in experimental autoimmune encephalomyelitis. J Neuroinflammation 2009 Apr 28;6:14).  On the flip side of the coin, microglial cells have been postulated to also be involved in repair of the brain during multiple sclerosis (Napoli and Neumann.  Protective Effects of Microglia in Multiple Sclerosis. Exp Neurol. 2009 May 3).

Macrophages are interesting cells.  As we discussed previously in the same why that T cells may be effector T cells and T regulatory cells, broadly speaking, so too macrophages may be proinflammatory (called M1) and antiinflammatory (called M2).  In fact, one of the reasons why using your own fat stem cells may be therapeutically beneficial in multiple sclerosis is because there are high concentrations of antiinflammatory M2 macrophages found in the fat.  The classical distinction between these two types of macrophages has been that M1 macrophages produce nitric oxide when they are activated and that M2 macrophages produce products of the enzyme arginase.  Mesenchymal stem cells, which have demonstrated activity against the mouse model of multiple sclerosis, have been demonstrated to be able to induce an M2-like phenotype in macrophages by stimulating them to produce the antiinflammatory cytokine interleukin-10.  Before we continue, we should state that macrophages are derived from monocytes, and there is some work suggesting that monocytes in special situations may act as stem cells.

Now if mesenchymal stem cells have the ability to induce production of interleukin-10 in macrophages, what if someone discovered a chemical that could induce production of this antiinflammatory cytokine (which theoretically would be beneficial for multiple sclerosis) simply by administration of the chemical?  While this would not induce the regenerative or growth factor effects of mesenchymal stem cells, it may be useful for inhibiting the destruction cause by activated macrophages. 

A recent paper (Correa et al. Anandamide enhances IL-10 production in activated microglia by targeting CB(2) receptors: Roles of ERK1/2, JNK, and NF-kappaB. Glia 2009 Jun 29) demonstrated that the endocannabinoid anandamide may actually have this property.

Anandamide a neurotransmitter of the cannabinoid family that occurs naturally in the human central nervous system.  It activates the CB1 receptor in the brain and the CB2 receptor in the periphery.  It is known that activation of cannabinoid receptors is associated with decreased symptoms in animal models of multiple sclerosis (Cabranes et al. Decreased endocannabinoid levels in the brain and beneficial effects of agents activating cannabinoid and/or vanilloid receptors in a rat model of multiple sclerosis. Neurobiol Dis 2005 Nov;20(2):207-17).  It is also known that deletion of the enzyme Fatty Acid Amide Hydrolase (FAAH), whose role is to metabolize anandamide, causes regression of symptoms of multiple sclerosis in animals, presumably by increasing the amount of endogenous anandamide (Webb et al. Genetic deletion of Fatty Acid Amide Hydrolase results in improved long-term outcome in chronic autoimmune encephalitis. Neurosci Lett. 2008 Jul 4;439(1):106-10).  Therefore in the paper that we are discussing, the investigators wanted to see if anandamide may alter production of cytokines and behavious of microglial cells.

The authors demonstrated that administration of anandamide results in upregulation of interleukin-10 production from microglial cells that have been activated.  The induction of interleukin-10 seemed to be associated with suppression of the transcription factor NF-kB, which is involved in numerous inflammatory conditions.  Furthermore, the investigators demonstrated that microglial cells treated with anandamide had suppressed ability to produce the inflammatory cytokines IL-12 and IL-23, which was correlated to inhibition of Th1 and Th17 generation.

These data suggest a very important point, which is that numerous neurotransmitters may have effects on immunological cells.  Theoretically, this should not be that surprising.  The immune system and the nervous system are the only systems of the body that share the features of: a) Distinguishing between what is self and what is non-self; b) Ability for specificity; c) Memory; and d) Innate reactivity.  At a practical level, the major immunological organs such as they thymus and spleen are highly innervated.  Additionally, most immunological cells, including dendritic, T cells, B cells, and NK cells all have expression of receptors for neurotransmitters. 

Therefore we at Stemnow.com believe that this area of “neuroimmunology” will be an exciting one to watch in the upcoming years.