Introduction
Inflammatory Bowel Disease (IBD) is a group of inflammatory conditions
of the colon and small intestine. Crohn’s disease (CD) and ulcerative
colitis (UC) are the principal types of inflammatory bowel diseases. The
etiology is multifactorial, and many genetic loci have been found to
serve as susceptibility genes for different subgroups of patients with
IBD [1, 2], likewise, environmental influences are also considered
as factors involved in IBD onset [3]. In addition, to its
complicated risk factors, the pathogenesis of IBD is not entirely clear,
making it difficult to develop new drugs with therapeutic value and
acceptable side effects [2].
For more than two decades, animal models of intestinal inflammation have
provided a wealth of information about mucosal immunology and the
intestinal inflammation encountered in the inflammatory bowel diseases.
While no single model has proven to sufficiently represent the
complexity of the clinical characteristics of the human disease, the
combined data obtained from these various animal models have provided a
more detailed understanding of the underlying principles of human IBD
and delineate pathways that might help predict the successful outcome of
novel therapeutics in the clinic [4, 5].
The most widely used murine model of colitis employs dextran sodium
sulfate (DSS), a chemical colitogen with anticoagulant properties, to
induce intestinal epithelial damage [6]. The DSS colitis model lends
itself to IBD research due to its rapidity, simplicity, reproducibility
and controllability. Acute, chronic and relapsing models of intestinal
inflammation can be achieved by modifying the concentration of DSS and
the frequency of administration [6].
Numerous inflammatory mediators have been implicated in the pathogenesis
of IBD. These include cytokines, eicosanoids, reactive oxygen species,
nitric oxide, and complement activation products [7]. Similarly,
increased expression of different inflammatory mediators (TNF-α, IL-1β,
IFN-ɣ, IL-10 and IL-12) are observed as early as the first day of DSS
treatment. The production of these inflammatory mediators increases
progressively during DSS treatment [8].
Recently, we reported identification of an immunosuppressive peptide
(ENV59-GP3) from a specific human endogenous retroviral envelope
protein, ENV59, which has been adapted by the immune system and is
involved in ameliorating autoimmunity [9, 10]. The ENV59-GP3 peptide
showed strong immune modulating activity both in vitro andin vivo in SKG mouse model for rheumatoid arthritis and EAE model
for Multiple Sclerosis [9, 10].
In this study we follow up on our
previous findings and demonstrate that the ENV59-GP3 peptide treatment,
reduces inflammation in the DSS-induced mice model of colitis.
Furthermore, we show that in vitro ENV59-GP3 inhibits the
KCa3.1 potassium channel, an ion channel responsible for
hyperpolarization of the plasma membrane and regulation of calcium
influx upon lymphocyte activation.
Potassium channels might be regarded as a new group of therapeutic
targets for IBD [11, 12]. The intermediate-conductance
calcium/calmodulin-gated potassium channel KCa3.1 also
known as the IK or the Gardos channel (encoded by the KCNN4 gene)
is expressed in a variety of tissues and cell types including T cells
[13, 14], macrophages as well as DCs [14]. In T cells, sustained
Ca2+ influx via the CRAC channel is important for
sufficient cytokine production and proliferation. Influx of
Ca2+ is dependent on a negative membrane potential.
Activation of KCa3.1 by elevated intracellular
Ca2+ maintains the negative membrane potential through
efflux of K+, which helps to sustain
Ca2+ entry into the cell. Upon channel opening, the
efflux of potassium is driven by its concentration gradient and results
in hyperpolarization of the membrane providing the electrochemical
gradient necessary for influx of calcium [15].
KCa3.1 channel is known to be involved in regulating T
cell function, proliferation, and cytokine production [16], thus
suggesting that they could be involved in the pro-inflammatory responses
in IBD further up-stream of inflammatory cytokines such as IFN-ɣ and
TNF-α. Furthermore, studies have revealed a prominent role of KCNN4 in
Ca2+-dependent ion transport and intestinal
restitution [17]. In a T cell-mediated mouse model of colitis,
inhibition of KCa3.1 ameliorated the disease [11].
Likewise, the KCa3.1 channel blocker NS6180 prevents
T-cell activation and inflammation in a rat model of inflammatory bowel
disease [18]. Although loss of KCa3.1 did not
interfere with CD4 T cell differentiation, both
Ca2+ influx and cytokine production were impaired in
KCNN4−/− Th1 and Th2 CD4 T cells, whereas T-regulatory
and Th17 function were normal. In animal models for other autoimmune
diseases, the KCa3.1 inhibitor TRAM-34 or ICA-17043
(Senicapoc) have been reported to prevent experimental autoimmune
encephalomyelitis in mice [19] or to inhibit/attenuate inflammatory
responses in mice with collagen antibody induced rheumatoid arthritis.
Clinical studies with Clotrimazole (form which Tram-34 has been derived)
already provided evidence that blockers of this channel may be useful in
treatment of chronic diseases such as RA, however, non-target related
adverse effects have prevented further development in some cases
[20].