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Synapses are the functional units of the brain. They form specific contact points that drive neuronal communication and are highly plastic in their strength, density, and shape. A carefully orchestrated balance between synaptogenesis and synaptic pruning, i.e., the elimination of weak or redundant synapses, ensures adequate synaptic density. An imbalance between these two processes lies at the basis of multiple neuropathologies. Recent evidence has highlighted the importance of glia-neuron interactions in the synaptic unit, emphasized by glial phagocytosis of synapses and local excretion of inflammatory mediators. These findings warrant a closer look into the molecular basis of cell-signaling pathways in the different brain cells that are related to synaptic plasticity. In neurons, intracellular second messengers, such as cyclic guanosine or adenosine monophosphate (cGMP and cAMP, respectively), are known mediators of synaptic homeostasis and plasticity. Increased levels of these second messengers in glial cells slow down inflammation and neurodegenerative processes. These multi-faceted effects provide the opportunity to counteract excessive synapse loss by targeting cGMP and cAMP pathways in multiple cell types. Phosphodiesterases (PDEs) are specialized degraders of these second messengers, rendering them attractive targets to combat the detrimental effects of neurological disorders. Cellular and subcellular compartmentalization of the specific isoforms of PDEs leads to divergent downstream effects for these enzymes in the various central nervous system resident cell types. This review provides a detailed overview on the role of PDEs and their inhibition in the context of glia-neuron interactions in different neuropathologies characterized by synapse loss. In doing so, it provides a framework to support future research towards finding combinational therapy for specific neuropathologies. The authors sincerely thank prof. dr. David M. Wilson III for proofreading the paper. Vanmierlo, T (corresponding author), Hasselt Univ, Biomed Res Inst, Dept Neurosci, European Grad Sch Neurosci,UHasselt, Hasselt, Belgium. tim.vanmierlo@uhasselt.be

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS) characterized by heterogeneous clinical symptoms including gradual muscle weakness, fatigue, and cognitive impairment. The disease course of MS can be classified into a relapsing-remitting (RR) phase defined by periods of neurological disabilities, and a progressive phase where neurological decline is persistent. Pathologically, MS is defined by a destructive immunological and neuro-degenerative interplay. Current treatments largely target the inflammatory processes and slow disease progression at best. Therefore, there is an urgent need to develop next-generation therapeutic strategies that target both neuroinflammatory and degenerative processes. It has been shown that elevating second messengers (cAMP and cGMP) is important for controlling inflammatory damage and inducing CNS repair. Phosphodiesterases (PDEs) have been studied extensively in a wide range of disorders as they breakdown these second messengers, rendering them crucial regulators. In this review, we provide an overview of the role of PDE inhibition in limiting pathological inflammation and stimulating regenerative processes in MS. Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS) characterized by heterogeneous clinical symptoms including gradual muscle weakness, fatigue, and cognitive impairment. The disease course of MS can be classified into a relapsing-remitting (RR) phase defined by periods of neurological disabilities, and a progressive phase where neurological decline is persistent. Pathologically, MS is defined by a destructive immunological and neuro-degenerative interplay. Current treatments largely target the inflammatory processes and slow disease progression at best. Therefore, there is an urgent need to develop next-generation therapeutic strategies that target both neuroinflammatory and degenerative processes. It has been shown that elevating second messengers (cAMP and cGMP) is important for controlling inflammatory damage and inducing CNS repair. Phosphodiesterases (PDEs) have been studied extensively in a wide range of disorders as they breakdown these second messengers, rendering them crucial regulators. In this review, we provide an overview of the role of PDE inhibition in limiting pathological inflammation and stimulating regenerative processes in MS.

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) characterized by focal inflammatory lesions and prominent demyelination. Even though the currently available therapies are effective in treating the initial stages of disease, they are unable to halt or reverse disease progression into the chronic progressive stage. Thus far, no repair-inducing treatments are available for progressive MS patients. Hence, there is an urgent need for the development of new therapeutic strategies either targeting the destructive immunological demyelination or boosting endogenous repair mechanisms. Using in vitro, ex vivo, and in vivo models, we demonstrate that selective inhibition of phosphodiesterase 4 (PDE4), a family of enzymes that hydrolyzes and inactivates cyclic adenosine monophosphate (cAMP), reduces inflammation and promotes myelin repair. More specifically, we segregated the myelination-promoting and anti-inflammatory effects into a PDE4Dand PDE4B-dependent process respectively. We show that inhibition of PDE4D boosts oligodendrocyte progenitor cells (OPC) differentiation and enhances (re)myelination of both murine OPCs and human iPSC-derived OPCs. In addition, PDE4D inhibition promotes in vivo remyelination in the cuprizone model, which is accompanied by improved spatial memory and reduced visual evoked potential latency times. We further identified that PDE4B-specific inhibition exerts anti-inflammatory effects since it lowers in vitro monocytic nitric oxide (NO) production and improves in vivo neurological scores during the early phase of experimental autoimmune encephalomyelitis (EAE). In contrast to the pan PDE4 inhibitor roflumilast, the therapeutic dose of both the PDE4B-specific inhibitor A33 and the PDE4D-specific inhibitor Gebr32a did not trigger emesis-like side effects in rodents. Finally, we report distinct PDE4D isoform expression patterns in human area postrema neurons and human oligodendroglia lineage cells. Using the CRISPR-Cas9 system, we confirmed that pde4d1/2 and pde4d6 are the key targets to induce OPC differentiation. Collectively, these data demonstrate that gene specific PDE4 inhibitors have potential as novel therapeutic agents for targeting the distinct disease processes of MS. This work has been supported by FWO (12G0817N, 1S57521N, G041421N, and 12G0817N), Fondation Charctot Stichting (ID2020- 0019), Nationale Belgische Multiple Sclerose Liga (Charco18VT), MS Liga Vlaanderen and Stichting MS Research (18-1016 MS). MS, EP, JP and TV have a proprietary interest in selective PDE4D inhibitors for the treatment of demyelinating disorders and neurodegenerative disorders. JP has a proprietary interest in the PDE4 inhibitor roflumilast for the treatment of cognitive impairment as well as PDE4D inhibitors for the treatment of Alzheimer’s disease. We thank Prof. Dr. O.N. Viacheslav (University Medical Center Hamburg-Eppendorf, German Center for Cardiovascular Research) and Prof. Dr. M. Conti (University of California), for providing the PDE4B KO animals. Furthermore, we thank Rewind Therapeutics for providing the visual evoked potential equipment.

Charcot-Marie-Tooth disease type 1A (CMT1A) is an inherited neuropathy of the peripheral nervous system. Symptoms include distal muscle weakness and atrophy, as well as sensory loss. Currently, no therapies for CMT1A are available. CMT1A is characterized by Schwann cell (SC) dedifferentiation and demyelination. The differentiation of SCs to the myelinating phenotype and the subsequent myelination process are positively regulated by cAMP. Therefore, we hypothesized that elevating intracellular cAMP levels by inhibiting PDE4D, a main cAMP-hydrolyzing enzyme in SCs, can stimulate SC differentiation, myelination and subsequent functional repair in the C3-PMP22 mouse model. Primary SCs were treated with 3 different inhibitors and their differentiation was investigated by assessing gene expression of several SC differentiation markers by qPCR, as well as the protein levels of several markers using immunocytochemistry. These experiments were inconclusive. Secondly, the therapeutic potential of PDE4D inhibitor GEBR-32a was evaluated in vivo. Mice were injected with 0.3 mg/kg GEBR-32a twice per day for 7 weeks, and motor function was assessed using several behavioral assays. We found that GEBR-32a treatment significantly improves motor function in C3-PMP22 mice. Furthermore, electrophysiological recordings showed a significant increase in nerve conduction speed. In conclusion, these findings indicate a promising therapeutic potential of PDE4D inhibition in CMT1A, although the exact molecular mechanisms need to be evaluated in vitro.

Dopamine is a neurotransmitter of the brain and tight modulation of its release is essential for a proper brain function. Dysfunction of dopamine signaling is associated with various diseases, such as Parkinson's disease and psychosis. The activity of dopamine releasing neurons is modulated in the midbrain and elucidating these modulatory mechanisms is essential for unveiling the etiology of these diseases and development of new treatment strategies. The neurotransmitter glycine plays a major role in the modulation of dopamine neuron activity, yet it is unclear which subunits are involved. To address this gap, we first confirmed the inhibitory effect of glycine on basal dopamine neuron firing in the substantia nigra pars compacta (SNc) within the midbrain. Next, since preliminary data from our lab indicate a significant role for the alpha 2 subunit of the glycine receptor (GlyR'2) in dopaminergic signaling, we checked the presence of the subunit on dopamine neurons and repeated the firing experiments in GlyR'2 knock-out littermates to determine its functional role. Our findings clearly demonstrate the involvement of glycine receptors in modulation of dopamine neuron activity, but modulation was independent of GlyR'2s at baseline activity and activity in presence of synaptic glycine concentrations. However, it is conceivable that high-affinity GlyR'2s are involved in the modulation at lower, tonic glycine concentrations and/or in phasic activity.