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Nicole Whitfield
Biol 303
November 4, 2011
TAM Receptors and their Association with Multiple Sclerosis
Multiple sclerosis (MS) is a complex demyelinating disease of the central nervous system
(CNS). In other words, demyelination is when the body's immune system eats away at the
protective sheath that covers the nerves, known as myelin. Myelin helps the nerves receive and
interpret messages from the brain at a quicker speed. When nerve endings lose myelin, they can
no longer function properly. This loss of function can lead to patches of scarring that in turn
delay the transmission between the brain and the rest of the body, possibly resulting in
irreversible deterioration of the nerves.
The TAM receptor family is recognized as potentially significant in multiple sclerosis
with all members of the family regulated throughout the demyelination process. The TAM
receptors are three structurally similar protein receptor tyrosine kinases (Lai and Lemeke, 1991).
The TAM receptors are made up of three receptors termed Tyro3, Axl, and Mer (MERTK) and
are widely expressed in the vertebrate nervous system (see Figure 1) (Lai and Lemeke, 1991).
The three receptors share similarity with other tyrosine kinases in that they function as dimers.
“The TAM receptors have two N-terminal immunoglobulin (IG) domains followed by
fibronectin-III-like domains attached by a single-pass α helical transmembrane domain to an
intracellular tyrosine kinase domain” (Janssen et al., 1991). The growth arrest-specific gene 6
(Gas6) was originally detected as a gene increased with growth arrest in serum-starved NIH 3T3
cells (Schneider et al., 1988). The Gas6 protein shows structural similarity to protein S, an
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essential factor of the anticoagulation cascade (Manfioletti et al., 1993). Protein S is a ligand for
both the Mer and the Tyro3 receptors.
Figure 1: Structure of TAMs (Binder and Kilpatrick, 2009).
In addition to the nervous system, the TAM receptors (Tyro3, Axl, and Mer (MERTK))
and their ligands (protein S and Gas6) are expressed in a diversity of tissues including those of
the immune, vascular, and reproductive systems. In contrast to other tyrosine kinase receptors,
the TAM receptors are not well-expressed during development. TAM receptor expression is still
detectable, but their expression is increased post-natally extending into adulthood (Binder and
Kilpatrick, 2009). This expression pattern mirrors the significant role of the TAM receptor
signaling in regulating homeostasis in a number of diverse tissue types. TAM signaling can
regulate myelination, microglial activation, and phagocytosis. In the nervous system, TAM
receptors and their ligands are expressed by both neurons and glial cells. Glial cells are the
supporting cells that protect the neurons. Glial cells function to surround neurons to maintain
them in place, to provide nutrients and oxygen, to separate one neuron from another, and to
destroy and remove the debris of dead neurons.
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The expression of all three TAM receptors can be localized explicitly to
oligodendrocytes. The Tyro3 receptor is highly expressed in human brain tissue (Mark et al.,
1994). The expression of the Axl and Mer receptors within the CNS is limited more than that of
Tyro3 (Prieto et al., 2000). Ligands Gas6 and protein S are both highly up-regulated after birth
and the expression of both can persist at high levels in adults (Prieto et al., 1999). The
concentration of Gas6 was significantly increased in the cerebrospinal fluid (CSF) of human
patients with chronic inflammatory demyelinating polyneuropathy compared to control patients.
In addition, Gas6 promotes survival in various cell types including neurons and glial cells. Taken
together, these data support the concept that all parts of the TAM receptor-signaling pathway are
both expressed at the appropriate places and at the appropriate developmental stages to impact
CNS myelination. (Binder and Kilpatrick, 2009).
The disruption of signaling through one TAM receptor can affect expression and likely
signaling through another TAM receptor (Prasad et al., 2006). Thus, the Tyro3 receptor
expression was down regulated in mice lacking the Mer receptor. (Prasad et al., 2006).
Phagocytosis is essential to the normal performance of the retina. Also, an alteration in the Mer
gene was responsible for retinal deterioration in both rat and humans (D’Cruz et al., 2000).
A study was conducted by, Hiremath et al (1998) investigating the effects of cuprizonemediated demyelination. Cuprizone (bis-cyclohexanone oxalydihydrazone), when introduced
into the fed for mice there were reproducible and stereotyped focal demyelination. In the
cuprizone model there is a sizeable recruitment of microglia/macrophages both prior to and
throughout demyelination (Hiremath et al., 1998). In addition there is impairment to the
oligodendrocytes due to the effects of the innate immune response. In studies of recently forming
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MS lesions, initial occurrences in development show activation of microglia and oligodendrocyte
apoptosis (Binder and Kilpatrick, 2009).
A study by Hoehn et al. (2008) investigated the role of the Axl receptor in demyelination.
They used the cuprizone model to test the hypothesis that when challenged with the copper
chelator cuprizone, remyelination in the Axl double knockout (−/−) mice would be compromised
comparative to wildtype (WT) littermates. Axl−/− and WT mice were compared at 3, 4, and 6
weeks of cuprizone treatment, and 3 and 5 week post-cuprizone treatment termination (Hoehn et
al., 2008). During the cuprizone treatment, they observed whether the removal of Axl changed
the quantity of developed oligodendrocytes or microglia or impacted phagocytosis of myelin and
cellular debris. During the recovery period, they observed whether the removal of Axl changed
the development of oligodendrocytes, axonal diameter, limited remyelination, or led to extended
axonal damage within the corpus callosum comparative to the WT littermates. There was less
remyelination detected in the Axl−/− mice three weeks after the removal of cuprizone, and
Axl−/− mice had fewer microglia after four weeks of the cuprizone treatment. At six weeks of
the cuprizone treatment, there was a delay in clearance of myelin and cellular debris in the
Axl−/− mice. There was also a delay in the clearance of apoptotic oligodendrocytes resulting in
prolonged axonal damage. In addition, the recovery period for Axl−/− mice was prolonged from
the cuprizone treatment.
On inflammatory cells, the Axl, Tyro3, and Mer TAM receptors are constitutively
expressed. Hoehn et al. (2008) determined whether there were differences between the Axl−/−
and WT mice in the microglia/macrophage population of the corpus callosum. Ionized calcium
binding adaptor molecule (Iba+) is a gene explicitly expressed due to inflammation upon the
activation of microglia. The number of microglia throughout the 3 and 6 weeks of the cuprizone
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treatment or at 3 weeks following cuprizone was not different in the WT and Axl−/− mice.
However, the number of Iba+ microglia in the corpora callosa of WT mice was increased
compared to Axl−/− mice at 4 weeks of cuprizone exposure (Figure 2).
Figure 2. During cuprizone treatment changes in microglia/macrophage number were observed
at 4 weeks of cuprizone toxicity in Axl−/− mice; p = 0.0075. Data is presented as the number of
Iba+/DAPI+ microglia, macrophages/mm2 (Hoehn et al., 2008).
Recently TAM receptors have been recognized as playing an important role in human
MS. Soluble TAM receptors can act as decoy receptors by competing for ligand binding with
the membrane-bound TAM receptors. A comparison was made of the soluble TAM receptor
levels among patients with chronic silent MS lesions, chronic active MS lesions, and healthy
controls. Chronic silent lesions had higher levels of the soluble Axl receptor, while levels of
chronic active lesions had higher levels of the Mer receptor compared with controls. These
increased levels of soluble Axl and Mer also correlated with lower levels of the Gas6 ligand
within the lesions, suggesting that the competition from the soluble receptors results in reduced
TAM receptor signaling that may prolong MS lesion activity (Weinger et al., 2009).
A genome-wide association study (GWAS) analyzed indicated that, alleles in the MERTK
gene might be correlated with MS (Australia et al., 2009). Therefore, Ma and coworkers (2011)
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led a replication study with an independent cohort of 1140 MS cases and 1140 healthy controls
to validate this finding by genotyping twenty-eight common SNPs within MERTK gene. In this
study, they found that twelve MERTK SNPs were significantly associated with MS susceptibility.
A deficiency in their ability to clear apoptotic cells was shown with macrophages in
MERTK knockout mice, leading to the progression of systemic lupus erythematosus-like
phenotypes (Scott et al., 2001; Cohen et al., 2002). In the immune system, regulation of dendritic
cell activation is shown to be associated with the MERTK gene (Sen et al., 2007). Both of these
results are consistent with the idea that MERTK represents a genuine MS susceptibility gene.
In summary, in the cuprizone model it was observed there was an increase in the number
of Iba+ microglia in the corpora callosa of WT mice but not in the Axl−/− mice at 4 weeks.
Moreover, remyelination was compromised in the Axl−/− mice, indicating that the Axl receptor
has an important role in CNS function. In the human MS lesions study, there were higher chronic
active lesions with levels of soluble Mer compared with controls indicating that loss of TAM
receptor signaling may prolong MS lesion activity. Consistent with these results, MERTK alleles
have been shown to be risk factors for the susceptibility to MS. In conclusion, there are several
processes that affect the outcome of demyelination in human MS associating TAM receptor
signaling as playing a key role.
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