Deciphering cAMP-dependent mechanisms for T cell regulation in the context of multiple sclerosis

Abstract

Multiple sclerosis (MS) is an inflammatory, demyelinating disease with unknown etiology, leading toneurodegeneration of the central nervous system (CNS). It is the most common chronic neuroinflammatory disease, affecting around 2.5 million people worldwide. So far, there is no cure or preventive treatment for MS with treatment options only delaying the disease progression. MS is orchestrated by a misfunctioning immune system and thus commonly thought to be anautoimmune disease. Contribution of the immune system to MS onset and progression is diverse but high impact is thought to come from autoreactive T cells which orchestrate the migration of immune cells to the CNS, guide the attack of the myelin sheath and lead to an activation of CNS-resident cells resulting in demyelination and blood brain barrier breakdown. Targeting these autoreactive, active T cells to ameliorate the disease course or even inhibit disease onset is a major goal in MS research. Hence,pathways controlling T cells physiology are important targets to decipher potential treatment options. One such pathway is the cAMP-mediated pathway, as intracellular level of cAMP can affect T cell physiology. Within this study, different modulators of intracellular cAMP levels were studied in the context of T cell function and autoimmune diseases. First target was the Gs-coupled G-protein coupled receptor (GPCR) GPR52. The orphan GPR52 contributes to intracellular cAMP levels via its constitutive activity which can be seen in neurons and transfected HEK cells, but also in T cells. However, its effect on T cell function has so far not been untangled. Within this thesis it was shown that neither Gpr52-deficiecency nor pharmacological modification of GPR52 resulted in altered T cell physiology. Moreover, Gpr52-deficiency had no effect on the disease course of the experimental autoimmune encephalomyelitis (EAE), the animal model of MS. As second target, the phosphodiesterase (PDE) PDE2A was chosen. PDEs are the only known enzymes which can hydrolyze cAMP, thereby decreasing intracellular cAMP levels. Other PDEs have already been shown to affect T cell function and MS mouse models. However, PDE2A has not been studied in this context. Here, it was shown that inhibition of PDE2A with the selective inhibitor BAY 60-7550 decreased T cell proliferation while not affecting T cell activation or differentiation. Moreover, Pde2a expression was increased in T cells isolated from mice undergoing the EAE. Together with the finding that PDE2A is upregulated in activated T cells, an involvement in autoimmune diseases gets possible which should be determined in further in vivo experiments. Third, the natriuretic peptides (NP), namely atrial NP (ANP), brain NP (BNP), and C-type NP (CNP), where tested upon their potential to alter T cell function. These peptides, which have been studied extensively in cardiovascular research, can alter PDE2A activity via modulating the cGMP/cAMP crosstalk. While BNP had no significant effect on T cell function, ANP and CNP severely modified T cell physiology. Most dominant effect was observed upon CNP treatment, hence further focus was set on this peptide. It was shown that CNP affects T cell function via its cognate receptor NPR2. Moreover, CNP increased T cell activation also in human T cells, indicating that CNP is an interesting target in translational research. In summary, this study deciphered the involvement of three cAMP modulators on T cell function. It was shown, that GPR52 is dispensable for T cell function. PDE2A and the NP, especially CNP, on the other hand can modulate T cell function and therefore should be considered as targets for promising therapeutical approaches to counteract autoimmune diseases like MS.