G-protein coupled receptor 88 knockdown in the associative striatum reduces psychiatric symptoms in a translational male rat model of Parkinson disease

BACKGROUND

In addition to motor disability, another characteristic feature of Parkinson disease is the early appearance of psychiatric symptoms, including apathy, depression, anxiety and cognitive deficits; treatments for these symptoms are limited by the development of adverse effects such as impulse-control disorders. In this context, we investigated the orphan G protein-coupled receptor 88 (GPR88) as a novel therapeutic target.

METHODS

We used lentiviral-mediated expression of specifically designed microRNA to knock down Gpr88 in a translational male rat model of early Parkinson disease obtained by dopamine loss in the dorsolateral striatum as a result of 6-hydroxydopamine lesions. We evaluated the impact of Gpr88 knockdown on the Parkinson disease model using behavioural, immunohistochemical and in situ hybridization studies.

RESULTS

Knockdown of Gpr88 in associative territories of the dorsal striatum efficiently reduced alterations in mood, motivation and cognition through modulation of the regulator of the G-protein signalling 4 and of the truncated splice variant of the FosB transcription factor. Knockdown of Gpr88 also reduced allostatic changes in striatal activity markers that may be related to patterns observed in patients and that provide support for an "overload" hypothesis for the etiology of the psychiatric symptoms of Parkinson disease.

LIMITATIONS

Behavioural tests assessing specific cognitive and motivational parameters are needed to further characterize the effects of the lesion and of Gpr88 knockdown in early-stage and advanced Parkinson disease models, presenting more extensive dopamine loss. Additional studies focusing on the direct and indirect striatal output pathways are also required, because little is known about the signalling pathways regulated by GPR88 in different striatal cell types.

CONCLUSION

GPR88 may constitute a highly relevant target for the treatment of the psychiatric symptoms of Parkinson disease.

Knock-Down of GPR88 in the Dorsal Striatum Alters the Response of Medium Spiny Neurons to the Loss of Dopamine Input and L-3-4-Dyhydroxyphenylalanine

The effects of L-3-4-dyhydroxyphenylalanine (L-DOPA) treatment for replacing the dopamine (DA) loss in Parkinson’s disease (PD) progressively wear off and are hindered by the development of dyskinesia, prompting the search for new treatments. The orphan G protein-coupled receptor 88 (Gpr88) represents a potential new target, as it is highly and almost exclusively expressed in the projecting gamma-Aminobutyric Acid-ergic (GABAergic) medium spiny neurons of the striatum, is implicated in motor activity, and is downregulated by 6-hydroxydopamine (6-OHDA) lesions, an effect that is reversed by L-DOPA. Thus, to evaluate Gpr88 as a potential target for the management of PD and L-DOPA–induced dyskinesia (LID), we inactivated Gpr88 by lentiviral-mediated knock-down with a specifically designed microRNA (miR) (KD-Gpr88) in a 6-OHDA rat model of hemiparkinsonism. Then, we investigated the effects of the KD-Gpr88 in the DA-deprived dorsal striatum on circling behavior and LID as well as on specific markers of striatal neuron activity. The KD-Gpr88 reduced the acute amphetamine-induced and increased L-DOPA–induced turning behavior. Moreover, it normalized the upregulated expression of striatal Gad67 and proenkephalin provoked by the 6-OHDA lesion. Finally, despite promoting ΔFosB accumulation, the KD-Gpr88 was associated neither with the upregulation of prodynorphin, which is causally linked to the severity of LID, nor with the aggravation of LID following chronic L-DOPA treatment in 6-OHDA–lesioned rats. These results thus justify further evaluation of Gpr88 as a potentially novel target for the management of PD as an alternative to L-DOPA therapy.

A single lentivirus vector mediates doxycycline-regulated expression of transgenes in the brain

A single lentivirus vector allowing doxycycline-regulated expression of transgenes in the brain was generated by incorporating the tetracycline (Tet)-dependent regulatory system into the backbone of the vector. Two distinct expression cassettes were inserted upstream and downstream from the central Flap sequence that provides for enhanced transduction of nondividing cells. The first cassette was used to express the transgene under the control of the Tet-dependent minimal cytomegalovirus promoter. The second cassette was employed to express constitutively the Tet-dependent transactivator rtTA2-M2, which activates the Tet-dependent promoter after binding of doxycycline (Tet-on system). Vectors carrying luciferase and tyrosine hydroxylase as the transgene were constructed, tested in astroglia-rich primary cultures, and injected into the striata of rats. The constructs allowed in vitro and in vivo robust expression of the transgene that could be regulated over two orders of magnitude by the addition and withdrawal of doxycycline. The vector may thus be useful for many applications in gene therapy research, including the development of a therapeutic protocol for the treatment of Parkinson's disease based on the restoration of regulated dopamine production.

Neuronal signals are required for estrogen-mediated induction of progesterone receptor in cultured rat Schwann cells

Rat glial cells from the central nervous system (CNS) and the peripheral nervous system (PNS) express steroid hormone receptors. Whereas progestin receptors (PR) in cultured glial cells of the CNS are estrogen-inducible, similar increase of PR in cultured Schwann cells, the glial cells of the PNS, prepared from newborn rat sciatic nerves, could not be demonstrated. In the present work we have used fetal dorsal root ganglion cultures to study the effect of estrogen and its antagonist ICI 164,384 on the expression of PR in rat Schwann cells. The PR levels were measured by hormone binding in whole cell assays or in cell cytosol, 18 h after excision of the ganglion from the cultures. Treatment of DRG-Schwann cell cultures with estradiol (E2) increased PR levels from about 60 to 160 fmol per mg cytosol protein, in untreated and estrogen-treated cells, respectively. This increase was dose-dependent; maximal induction was obtained at 50 nM E2-concentration. Treatment of the cultures with the antagonist ICI 164,384 completely inhibited the estrogen-induction of PR, whereas ICI alone did not influence receptor levels in Schwann cells. The estrogen-induction of PR was dependent on the presence of dorsal root ganglion during the period of estrogen treatment. Excision of the neuronal mass from the cultures caused a rapid decrease and disappearance of estrogen-inducible progestin receptors, whereas the concentration of non-inducible PR-binding sites remained unchanged. Estradiol had no influence on DRG-Schwann cell proliferation, only replated secondary Schwann cells showed a slightly higher level of proliferation in presence of 100 nM E2 and 5 microM forskolin. Receptors for estrogen (ER) were also demonstrated in DRG-Schwann cells by ligand binding experiments. Specific ER-binding was 36 +/- 8 fmol bound estradiol per mg cytosol protein. Finally, both PR and ER were visualized in Schwann cells by indirect immunofluorescence staining using specific anti-receptor antibodies. These findings suggest that the expression of estrogen-inducible progestin receptors in cultured glial cells of the PNS is mediated via intracellular estrogen receptors and that it requires the presence of neuronal signal(s).

Proliferation and differentiation properties of permanent Schwann cell lines immortalized with a temperature-sensitive oncogene

Permanent Schwann cells lines have been established in culture after stable transfection of newborn rat Schwann cells with the pJC-SVLTtsA vector, expressing a thermosensitive oncogene driven by the early promoter-enhancer region of the gliotropic GS/B variant of the papovavirus JC. The proliferation and differentiation of two clonal cell lines have been studied. The cells of these lines display the morphology of primary Schwann cells and express Schwann cell differentiation markers such as the S-100 protein, laminin, the low-affinity receptor to nerve growth factor and the glial fibrillary acidic protein. One of the lines is able to differentiate further. Indeed, in the presence of dorsal root ganglion neurones, the cells synthesize the myelin Po protein and are capable of some myelination, although to a lesser extent than secondary Schwann cells.