Receptor for CNTF and the biological role of CNTF
Binding of CNTF to the high affinity receptor complexes CNTFRα/gp130/LIFRβ and to the low affinity receptor complex IL-6R/gp130/LIFRβ (modified from
[1] [2])
CNTF exerts its biological function by binding into a tripartite
receptor complex consisting of a specific CNTF receptor subunit α (CNTFRα) linked to
the cell membrane with a glycosyl-phosphatidylinositol linkage, and two
signal-transducing transmembrane subunits LIF receptor beta (LIFRβ) and gp130.
Binding of a hCNTF dimer to CNTFRα is the absolute first requirement to initiate a
signal transduction cascade, which ultimately results in the enhanced transcription
of genes encoding e.g. acute phase plasma proteins, and suppressors of cytokine signaling.
CNTFRα has been observed to exist also as a soluble receptor (sCNTFRα) [3], attributed
to a phospholipase C-mediated cleavage [4].
Additionally, hCNTF has been observed to be
able to bind both to membrane-bound and soluble human Interleukin-6 receptor (IL-6R) and use as a substitute it to form the receptor complex
necessary for signal transduction [5] [6]. These observations are considered to
offer explanation as to why cells expressing LIFRβ and gp130
but not CNTFRα, are nonetheless responsive to CNTF.
CNTF's known biological functions involve affecting the growth, differentiation, survival and repair of various types of neurons and glial cells in both the central and peripheral nervous systems [7] [6]; evidence also suggests CNTF to be expressed in other tissues, such as adipocytes and hepatocytes among others [7] [6].
As the transcription of CNTF mRNA and CNTF distribution have been noticed to be altered upon neural injury,
it has been suggested, that CNTF is not an absolute requirement for neural
development, but in fact mainly acts in response to neural injuries and stresses .
Furthermore, mice homozygous for an inactived CNTF gene have been observed to develop normally, and to display losses
in motor neurons only later during adulthood. Although humans homozygous for an inactivated CNTF gene
caused by a mutation have not been observed to suffer from major neurological abnormalities [1],
this has been linked with early onset of amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) [6].
However, mice that have a CNTFRα knock-out die already during the perinatal stage and exhibit severe motor
neuron deficits [7], suggesting that CNTFRα might have a second ligand.
Disease and Clinical applications
A frameshift mutation in exon 2 of the CNTF gene affecting some 2-3 % of the population [8] causes earlier onset of the disease and quicker declination of motor neuron function in MS patients [9].
MS patients with functional CNTF show 1,7-fold increase of CNTF mRNA expression in the cortex, suggesting CNTF secretion as an innate response to progressing neuronal damage [10].
While not directly linked to disease, CNTF has been shown to inhibit the secretion of vascular endothelial growth factor (VEGF). VEGF, in turn, is upregulated in many diseases of the retina, causing increased angiogenesis. Thus CNTF-induced downregulation alleviates the symptoms of many ocular diseases [2]. It has been proposed that treatment with CNTF could counteract vision loss caused by age-related macular degeneration, retinitis pigmentosa and retinitis pigmentosa linked to Usher syndrome. In these cases the alleviating effect is produced by CNTF inducing regeneration of outer segments of cone cells in the retina [11]. Using encapsulated cells transfected with the human CNTF gene has been studied as a delivery method in clinical studies[12], [13].
As CNTF has also been shown to affect energy balance, it also has potential clinical applications in the prevention and treatment of obesity and type II diabetes [7] [14].
Structure
Like with many other cytokines, the tertiary structure of CNTF consists of four anti-parallel α-helices (Arg13–His41), (Glu69–Val96), (Phe105–Leu129) and (Phe152–Ser180), where helices A-B and C-D are connected
by two cross-over loops and helices B-C by one short loop. A partial crystal structure of a truncated form of hCNTF (2-187) is displayed here [15].
The CNTFRα-binding surface epitope of CNTF was studied with random mutagenesis and identified to consist of residues Arg25, Arg28, Gln63, Trp64, Gln74, Asp175 and Arg177. These residues are located in helix A, the loop between helices A-B, helix B and helix D, and are [16].
In addition, the LIFR-binding epitope of CNTF was identified to consist of Glu36-Met56, Leu91-Ile109 and Gly147-Leu162 [17]. The ability of hCNTF to bind both to CNTFRα and IL-6R has been mapped to
residue Gln63 [5].
The dimeric structure of CNTF is attributed to the properties of hydrophobic (Leu91, Leu113, Leu114, Ala117, Tyr121, Ile128), polar (Gln95) and charged residues (Arg81, His84, Glu92, His106, His110, Glu124, Glu125) in the B and C helices of monomeric CNTF. These interface sites are highly conserved among species, except for Gln95. In the dimeric structure, , forming hydrogen bonds to His84 and Tyr121 of each monomer[15].
