The Role of GIMAP GTPases in Lymphocyte Homeostasis

T and B lymphocytes are classes of white blood cells which help protect us and other vertebrates from toxic or infectious agents. They make specific responses against these agents in the form of antibodies and 'killer' T lymphocytes and also develop immune 'memory' which allows us to respond more quickly and vigorously if the same agents are encountered again. The populations of lymphocytes that we possess develop through several stages in specific anatomical locations, namely the thymus (for T cells) and the bone marrow (for B cells) before emerging into the blood and lymph tissues (e.g. spleen, lymph nodes). The size and distribution of these lymphocyte populations is tightly regulated, enabling the immune system to function with high efficiency. During an immune response, lymphocytes bearing receptors specific to the invading agent divide and expand in number and become activated to fight the threat. Afterwards, these expanded populations must contract back to roughly their starting level as well as creating small numbers of memory cells. These dynamic processes are subject to complex mechanisms to ensure the accurate generation and maintenance of the repertoire of mature lymphocytes. There is currently strong interest in these mechanisms as they are very relevant to prominent health-related issues such as the effectiveness of vaccinations, the control of chronic autoimmune diseases such as rheumatoid arthritis and the frailty of immune responses often observed in the elderly.
This research will address the role played in lymphocyte development and maintenance by a family of proteins called GIMAPs which are one group within a large class of enzymes called guanosine triphosphatases (GTPases). GIMAPs are needed for lymphocytes to develop normally in mammals but the reasons for this are unknown. Some previous work suggested that they might regulate other proteins that are involved in the process of 'programmed cell death', a form of cell 'suicide' that allows unwanted cells to be removed from multicellular organisms. New evidence, however, suggests that GIMAPs may be needed for a different process, a special cellular mechanism called autophagy. Autophagy is a process by which a cell can break down its own constituent parts. This is used for the removal of defective proteins/organelles but can also be beneficial in circumstances such as starvation when a cell can extend its life by digesting non-essential components. Autophagy can also be important for the proper control of the number of mitochondria (the 'batteries' of the cell) inside some cells including lymphocytes and if this goes wrong the cells may die from oxidant stress. The experiments planned aim to uncover new evidence favouring either the 'programmed cell death' or the 'autophagy' explanation for GIMAP function: it could actually find that both are affected by the GIMAPs since new connections between the two processes are emerging in other areas of study.
Autophagy is a mechanism of strong current interest because of the potential benefits of stimulating autophagy for health across the lifespan. It is also emerging that autophagy contributes to immunity e.g. in defence against intracellular parasites (tuberculosis organisms; protozoa) and to processes during development. A lot has been learnt from detailed studies in yeast but the complexity of the system in mammals is much greater and many new discoveries can be expected in this area. The GIMAPs may be regulators of autophagy attuned to the special requirements of lymphocytes. Further knowledge of their mechanism/s of action will not only advance our understanding of how lymphocyte populations are generated and controlled but may also point to novel and specific ways of regulating lymphocyte survival and function useful in the treatment of autoimmune and inflammatory disorders, in enhancing immunity and in controlling transplant rejection mechanisms.

The mammalian GIMAP GTPases are implicated in the regulation of lymphocyte survival. In rodents, mutations in two members of this family cause severe lymphopenia in either the T lineage alone (GIMAP5) or in both T and B cells (GIMAP1). This application proposes a multidisciplinary programme of work aimed at understanding the mechanism of action of these GTPases and the extent of requirements for them in the function of mature lymphocytes. Two proposals to explain the essential pro-survival properties of GIMAP1 & 5 will be addressed (Aim 1). These are (i) their modulation of the intrinsic apoptotic pathway by interacting directly with Bcl-2 family proteins and thereby influencing mitochondrial outer membrane permeabilisation and (ii)their participation in autophagic mechanisms required for the development of mature lymphocytes from immature precursors (thymocytes; bone marrow B cells). In the latter case, particular focus will be applied to the regulated autophagic control of mitochondria, termed mitophagy, the importance of which in T cell development has recently been uncovered. The work will also ask (Aim 2) whether the requirement for GIMAPs 1 & 5 extends to long term lymphocyte survival in vivo and to the generation and maintenance of antigen-specific memory cells. It will also assess a possible role for GIMAP GTPases in T helper cell differentiation. These investigations will make use of a mouse strain bearing a conditional allele of GIMAP1 that, when combined with tissue- or inducer-dependent Cre recombinase alleles, will allow us to delete this gene selectively in lymphocyte lineages in vivo or inducibly either in vivo or in tissue culture.

The outputs from this project will be high quality basic research directly relevant to the maintenance of lymphocyte populations. The benefits of the research are likely to be seen in the medium-to-long term. Apart from those with academic interest in this area, the potential beneficiaries will include individuals whose health and well-being are compromised by loss of lymphocyte function and homeostasis. This can come about due to autoimmune disease, due to ageing or due to clinical treatments which deplete the immune system, either intentionally (in some treatments for autoimmunity or in transplantation medicine) or as a side-effect (in some cancer treatments). Therefore, patients in all these groups are potential beneficiaries of this type of research. Our work will contribute to understanding how to 'replace' or repair a lymphocyte repertoire so that it functions safely and effectively. Studies of immunological memory may contribute generally to the field of vaccines, with broad relevance to human populations.
In the areas described above, the GIMAP GTPases may turn out to have useful characteristics as targets for therapeutic intervention. In particular, the preferential expression of these genes in lymphoid cells means that, if they were targeted in order to manipulate lymphocyte populations, the side-effects on other tissues might be limited in scope. Application of this research may then benefit the commercial sector: the possibilities for this will be explored during the lifetime of this project. By becoming involved in interactions with scientists in commercial organisations, the named researcher will acquire new skills in communication and networking relevant to other employment sectors.
Our proposed gene expression analysis may identify novel gene interactions underlying the process of autophagy and its regulation. Contemporary research findings are indicating that autophagic activity plays an essential role in maintaining cellular health and that its poor functioning can cause loss of health to the individual as a whole. Thus, potential beneficiaries include persons who may suffer from neurodegenerative and other diseases ascribable to defective autophagic function. The wider proposition that autophagy mediates some of the effects of nutrient-sensing mechanisms (relevant to the positive effects of 'caloric restriction' and related treatments) lends further promise that the research will find beneficiaries, particularly in respect of maintaining good health throughout a long life.