Phosphodiesterase inhibitors--are they potential neuroleptic drugs?

Benzylisoquinoline alkaloids inhibit lung fibroblast activation mainly via inhibiting TGF-β1/Smads and ERK1/2 pathway proteins

Backgrounds

Liensinine (Lien), Neferine (Nef), Isoliensinine (Iso) and Tetrandrine (Tet), benzylisoquinoline alkaloids (BIAs), have been shown inhibitory effects on pulmonary fibrosis (PF) through anti-inflammatory, anti-oxidative activities, inhibition of cytokines and NF-κB. Effects of other similar BIAs, Dauricine (Dau), Papaverine (Pap) and lotusine (Lot), on PF remain unclear. Here, we explored the effects of five bisbenzylisoquinoline (Lien, Nef, Iso, Tet and Dau) and two monobenzylisoquinoline (Pap, Lot) alkaloids on normal and PF fibroblasts.

Methods

Primary normal and PF lung fibroblasts were cultured and treated with these alkaloids. Proliferation, activation, migration and apoptosis changes were detected by MTT, wound healing assay, flow cytometry. Protein level was analyzed by Western blot.

Results

All BIAs inhibited proliferation of normal and PF lung fibroblasts induced by TGF-β. α-SMA protein level in normal and PF lung fibroblasts decreased after Lien, Nef, Iso, Tet and Dau treatment. Pap and Lot had no influence on α-SMA expression. Dau showed the strongest inhibitory effects on proliferation and activation among alkaloids. The migration rates of normal and PF lung fibroblasts were inhibited by Lien, Nef, Iso, and Dau. Lien, Nef, Iso and Dau significantly promoted apoptosis, while Tet had no effect on apoptosis. Pap and Lot had no influence on activation, migration and apoptosis. Dau significantly inhibited Smad3/4 and p-ERK1/2 protein overexpression induced by TGF-β1.

Conclusions

Bisbenzylisoquinoline alkaloids had stronger effects on inhibiting lung fibroblasts than monobenzylisoquinoline alkaloids. Dau expressed the strongest inhibitory effects, which may be related to its inhibition of TGF-β1/Smad3/4 and p-ERK1/2 pathway proteins.

Phosphodiesterase Inhibition and Regulation of Dopaminergic Frontal and Striatal Functioning: Clinical Implications

BACKGROUND

The fronto-striatal circuits are the common neurobiological basis for neuropsychiatric disorders, including schizophrenia, Parkinson's disease, Huntington's disease, attention deficit hyperactivity disorder, obsessive-compulsive disorder, and Tourette's syndrome. Fronto-striatal circuits consist of motor circuits, associative circuits, and limbic circuits. All circuits share 2 common features. First, all fronto-striatal circuits consist of hyper direct, direct, and indirect pathways. Second, all fronto-striatal circuits are modulated by dopamine. Intracellularly, the effect of dopamine is largely mediated through the cyclic adenosine monophosphate/protein kinase A signaling cascade with an additional role for the cyclic guanosine monophosphate/protein kinase G pathway, both of which can be regulated by phosphodiesterases. Phosphodiesterases are thus a potential target for pharmacological intervention in neuropsychiatric disorders related to dopaminergic regulation of fronto-striatal circuits.

METHODS

Clinical studies of the effects of different phosphodiesterase inhibitors on cognition, affect, and motor function in relation to the fronto-striatal circuits are reviewed.

RESULTS

Several selective phosphodiesterase inhibitors have positive effects on cognition, affect, and motor function in relation to the fronto-striatal circuits.

CONCLUSION

Increased understanding of the subcellular localization and unraveling of the signalosome concept of phosphodiesterases including its function and dysfunction in the fronto-striatal circuits will contribute to the design of new specific inhibitors and enhance the potential of phosphodiesterase inhibitors as therapeutics in fronto-striatal circuits.

The role of phosphodiesterases in schizophrenia : therapeutic implications

Recent studies have suggested that currently available antipsychotic medications, while useful in treating some aspects of schizophrenia, still possess considerable limitations. Improving the treatment of negative symptoms and cognitive dysfunction, and decreasing adverse effects remain significant challenges. Many new drug strategies have been proposed in recent years and increasing evidence suggests that members of the phosphodiesterase (PDE) gene family may play a role in the aetiology or treatment of schizophrenia. PDEs are key enzymes responsible for the degradation of the second messengers cAMP (3',5'-cyclic adenosine monophosphate) and cGMP (3',5'-cyclic guanosine monophosphate). Mammalian PDEs are composed of 21 genes and are categorized into 11 families based on sequence homology, enzymatic properties and sensitivity to pharmacological inhibitors. Representatives from most families have been identified in the brain by the presence of protein or RNA, and numerous studies suggest that PDEs play an important role in the regulation of intracellular signalling downstream of receptor activation in neurons. Insights into the multiple brain processes to which PDEs contribute are emerging from the phenotype of genetically engineered mice that lack activity of specific PDEs (knockout mice), as well as from in vitro and in vivo studies with PDE inhibitors.This article provides a brief overview of recent studies implicating PDE inhibition, focusing on PDE4 and PDE10, as targets for treating the positive, negative or cognitive symptoms associated with schizophrenia.

Phosphodiesterases in the central nervous system

Phosphodiesterases (PDEs) represent important cornerstones of cGMP signaling in various tissues. Since the discovery of PDE activity in 1962, it has become clear that the functional characteristics of PDEs and their role in cyclic nucleotide signaling are fairly complex. On the one hand, members of the PDE family responsible for the hydrolysis of cGMP affect cellular responses by shaping cGMP signals derived from the activation of soluble cytosolic and/or membrane bound particulate guanylyl cyclases. Conversely, PDEs may function as downstream effectors in the cGMP signaling cascade. To make things even more sophisticated, cGMP modulates the activity of several PDEs either directly, by binding to a regulatory domain, or indirectly, through phosphorylation, and the result can be either inhibition or stimulation of the enzyme, depending on the subtype. Furthermore, cross-talk between cGMP and cAMP signaling is achieved by cGMP-dependent modulation of PDEs hydrolyzing cAMP and vice versa. Mammals possess at least 21 PDE genes and often express a set of PDEs in a tissue- and differentiation-dependent manner. Given these premises, it is still a challenging task to elucidate the physiological function(s) of individual PDE genes. The present chapter focuses on the role of PDEs as regulators of neuronal functions. Useful information regarding this topic has been gained by studying (1) the expression pattern of PDEs in the CNS, (2) the association of PDEs with specific macromolecular signaling complexes and (3) the phenotypes associated with mutations or ablation of PDE genes in man, mice and fruit flies, respectively. PDEs degrading cGMP and/or being regulated by cGMP have been implicated in cognition and learning, Parkinson's disease, attention deficit hyperactivity disorder, psychosis and depression. Correspondingly, modulators of PDEs have become attractive tools for treatment of these disorders of CNS function.

Distinct roles of PDE4 and PDE10A in the regulation of cAMP/PKA signaling in the striatum

Phosphodiesterase (PDE) is a critical regulator of cAMP/protein kinase A (PKA) signaling in cells. Multiple PDEs with different substrate specificities and subcellular localization are expressed in neurons. Dopamine plays a central role in the regulation of motor and cognitive functions. The effect of dopamine is largely mediated through the cAMP/PKA signaling cascade, and therefore controlled by PDE activity. We used in vitro and in vivo biochemical techniques to dissect the roles of PDE4 and PDE10A in dopaminergic neurotransmission in mouse striatum by monitoring the ability of selective PDE inhibitors to regulate phosphorylation of presynaptic [e.g., tyrosine hydroxylase (TH)] and postsynaptic [e.g., dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32)] PKA substrates. The PDE4 inhibitor, rolipram, induced a large increase in TH Ser40 phosphorylation at dopaminergic terminals that was associated with a commensurate increase in dopamine synthesis and turnover in striatum in vivo. Rolipram induced a small increase in DARPP-32 Thr34 phosphorylation preferentially in striatopallidal neurons by activating adenosine A(2A) receptor signaling in striatum. In contrast, the PDE10A inhibitor, papaverine, had no effect on TH phosphorylation or dopamine turnover, but instead robustly increased DARPP-32 Thr34 and GluR1 Ser845 phosphorylation in striatal neurons. Inhibition of PDE10A by papaverine activated cAMP/PKA signaling in both striatonigral and striatopallidal neurons, resulting in potentiation of dopamine D(1) receptor signaling and inhibition of dopamine D(2) receptor signaling. These biochemical results are supported by immunohistochemical data demonstrating differential localization of PDE10A and PDE4 in striatum. These data underscore the importance of individual brain-enriched cyclic-nucleotide PDE isoforms as therapeutic targets for neuropsychiatric and neurodegenerative disorders affecting dopamine neurotransmission.