The human immune system plays a key role for human health. Dr. Gina Fiala, a scientist in Prof. Dr. Wolfgang Schamel’s team in the Department of Immunology at Freiburg University, has discovered Kidins220/ARMS in B cells. Kidins220/ARMS is part of the adaptive immune system where it plays a key role in the maturation of B cells.
B and T cells are constituents of the adaptive immune system. They are produced in the bone marrow and mature and differentiate at different sites in the human body. They carry receptors on their outer membrane that they use to recognise foreign substances and antigens. B cells (also called B lymphocytes) start their life as pre-B cells in the bone marrow, where they carry a pre-receptor on their surface. Kidins220/ARMS is involved in the activation of the B-cell receptor (BCR), which plays a crucial role in the development and maturation of B cells. Kidins220/ARMS interacts with the BCR. Kidins220/ARMS deficiency obstructs the development of B cells.
The B-cell receptor is a multiprotein complex on the surface of B cells, which sends the signals needed for the proliferation and activation of B cells at various stages of their development. “Without B-cell receptors or without their signals, there are no healthy B cells,” says Dr. Gina Fiala who is a post-doctoral researcher in Prof. Dr. Wolfgang Schamel’s team at the Institute of Biology III at the University of Freiburg.
Fiala’s research is specifically focused on B cells and B-cell receptors. B cells migrate through the blood and lymphatic vessels until they encounter an antigen. The binding of an antigen to a B-cell receptor triggers their activation. The activated B cells take up the antigen and break it down. The resulting epitopic peptide fragments are loaded onto MHC class II molecules and presented on the B-cell surface. T-helper cells, which recognise this complex, produce cytokines and induce B cells to produce antibodies.
The protein was independently discovered by two research groups working on two different research aspects and was given two different names. One group called it ARMS (ankyrin repeat-rich membrane spanning) and the other group, which originally identified it as protein kinase D substrate in neurons, called it Kidins220 (kinase-D-interacting substrate of 220 kDa). The protein is a very large, highly conserved integral membrane protein with many binding motives with which it can interact with many different proteins. Kidins220/ARMS is an excellent signalling platform as it has the ability to recruit cell-specific effectors that activate specific receptors. Kidins220/ARMS is found in all phylogenetic groups and is ubiquitously expressed on most cells of an organism. “The protein seems to play a role in general cellular processes, including immune system processes,” says Fiala.
It was initially discovered in nerve cells where it interacts with the microtubule and actin cytoskeleton, modulating neuronal differentiation, survival and synaptic plasticity. Embryos without Kidins220/ARMS die at birth due to severe neuronal and vascular defects. It helps regulate the equilibrium between cell death and cell survival by affecting the signalling pathways (e.g. ERK) that are the principal regulators of such processes. Fiala discovered Kidins220/ARMS by pure chance. “We were looking for proteins that bind to a receptor before the latter is activated. Using mass spectrometry, we found that Kidins220/ARMS interacted with the B-cell receptor, thus affecting signalling pathways from the receptor to the cell interior.” They also found that the BCR’s ability to send signals is limited without Kidins220/ARMS.
There are many known receptor interactions involving Kidins220/ARMS. The molecule interacts, for example, with B-cell receptors, which induce the development of B cells and their optimal activation when they encounter B cells with antigens (MAP kinase signalling pathway). Activated B-cell receptors induce antigen specificity by triggering a signalling cascade involving the protein ERK (extracellular-signal regulated kinase), which ultimately regulates transcription factor fine-tuning and B-cell activation. Fiala’s experiments show that without Kidins220/ARMS, the receptor’s ability to send signals into the cell interior is limited. “We were able to show that Kidins220 is vital for activating B cells,” says the immunobiologist. Without Kidins220/ARMS (for example in knock-out mouse models or in cell lines where it is deleted using shRNA (small hairpin RNA used to silence genes via RNA interference)), the production of B cells is restricted and their quality lower. BCRs have so-called light and heavy chains that are involved in antigen recognition. The absence of Kidins220/ARMS prevents the production of lambda light chains (λ LC); cells with kappa light chains (k LC) are still produced. “An absence of Kidins220/ARMS makes B cells released from the bone marrow “blind” to certain antigens because the B cells lack the lambda light chains required for doing so,” says Fiala. “And the antigens that are recognised are less sensitive and therefore bind less efficiently.” This then results in a weaker antigen binding signal and weaker immune response.
Research is still being done to find out in what way Kidins220/ARMS is involved in the transduction of signals. It was not previously known that the protein is involved in BCR-induced MAP kinase signalling. However, the Freiburg researchers have now shown that the protein is a positive regulator of BCR function and interferes relatively early on in the signalling cascade. “We assume that Kidins220 already modulates the activation of the first two kinases, i.e. Lyn and Syk,” says Fiala, going on to explain, “Kidins220/ARMS is thus a modulator of signalling strength and length.” It is assumed that Kidins220 plays a key role in cell proliferation and cell cycle progression.
As far as the immune system is concerned, a weak receptor signal can lead to an ineffective immune response and to the loss of self-tolerance because potentially autoreactive cells cannot be reliably eliminated during development. As Kidins220 plays a role in survival pathways such as MAPK or ERK signalling, it is also recognised as a potential oncogene. “Cancer cells frequently turn on the ERK pathway, which then triggers cell proliferation,” says Fiala. “Kidins220 has been shown to regulate tumour cell proliferation in response to extracellular stimuli and is therefore also able to strongly activate this pathway.”
Another research group has shown that Kidins220 is overexpressed in melanoma. It has also been proposed that the protein plays a role in the development of leukaemia. In patients that lack Kidins220/ARMS, bone marrow transplants or cell-based therapies have the potential to provide them with immune cells with complete antibodies. When Kidins220 is overexpressed, inhibitory drugs could be prescribed to achieve optimal regulation of the MAP kinase pathway.