Chronic levodopa administration followed by a washout period increased number and induced phenotypic changes in striatal dopaminergic cells in MPTP-monkeys

iPSC-based modeling of THD recapitulates disease phenotypes and reveals neuronal malformation

Tyrosine hydroxylase deficiency (THD) is a rare genetic disorder leading to dopaminergic depletion and early-onset Parkinsonism. Affected children present with either a severe form that does not respond to L-Dopa treatment (THD-B) or a milder L-Dopa responsive form (THD-A). We generated induced pluripotent stem cells (iPSCs) from THD patients that were differentiated into dopaminergic neurons (DAn) and compared with control-DAn from healthy individuals and gene-corrected isogenic controls. Consistent with patients, THD iPSC-DAn displayed lower levels of DA metabolites and reduced TH expression, when compared to controls. Moreover, THD iPSC-DAn showed abnormal morphology, including reduced total neurite length and neurite arborization defects, which were not evident in DAn differentiated from control-iPSC. Treatment of THD-iPSC-DAn with L-Dopa rescued the neuronal defects and disease phenotype only in THDA-DAn. Interestingly, L-Dopa treatment at the stage of neuronal precursors could prevent the alterations in THDB-iPSC-DAn, thus suggesting the existence of a critical developmental window in THD. Our iPSC-based model recapitulates THD disease phenotypes and response to treatment, representing a promising tool for investigating pathogenic mechanisms, drug screening, and personalized management.

Lipid-based nanodelivery approaches for dopamine-replacement therapies in Parkinson's disease: From preclinical to translational studies

The incidence of Parkinson's disease (PD), the second most common neurodegenerative disorder, has increased exponentially as the global population continues to age. Although the etiological factors contributing to PD remain uncertain, its average incidence rate is reported to be 1% of the global population older than 60 years. PD is primarily characterized by the progressive loss of dopaminergic (DAergic) neurons and/or associated neuronal networks and the subsequent depletion of dopamine (DA) levels in the brain. Thus, DA or levodopa (l-dopa), a precursor of DA, represent cardinal targets for both idiopathic and symptomatic PD therapeutics. While several therapeutic strategies have been investigated over the past decade for their abilities to curb the progression of PD, an effective cure for PD is currently unavailable. Even DA replacement therapy, an effective PD therapeutic strategy that provides an exogenous supply of DA or l-dopa, has been hindered by severe challenges, such as a poor capacity to bypass the blood-brain barrier and inadequate bioavailability. Nevertheless, with recent advances in nanotechnology, several drug delivery systems have been developed to bypass the barriers associated with central nervous system therapeutics. In here, we sought to describe the adapted lipid-based nanodrug delivery systems used in the field of PD therapeutics and their recent advances, with a particular focus placed on DA replacement therapies. This work initially explores the background of PD; offers descriptions of the most recent molecular targets; currently available clinical medications/limitations; an overview of several lipid-based PD nanotherapeutics, functionalized nanoparticles, and technical aspects in brain delivery; and, finally, presents future perspectives to enhance the use of nanotherapeutics in PD treatment.

The calretinin interneurons of the striatum: comparisons between rodents and primates under normal and pathological conditions

This paper reviews the major organizational features of calretinin interneurons in the dorsal striatum of rodents and primates, with some insights on the state of these neurons in Parkinson's disease and Huntington's chorea. The rat striatum harbors medium-sized calretinin-immunoreactive (CR⁺) interneurons, whereas the mouse striatum is pervaded by medium-sized CR⁺ interneurons together with numerous small and highly immunoreactive CR⁺ cells. The CR interneuronal network is even more elaborated in monkey and human striatum where, in addition to the small- and medium-sized CR⁺ interneurons, a set of large CR⁺ interneurons occurs. The majority of these giant CR⁺ interneurons, which are unique to the primate striatum, also display immunoreactivity for choline acetyltransferase (ChAT), a faithful marker of cholinergic neurons. The expression of CR and/or ChAT by the large striatal interneurons appears to be seriously compromised in Parkinson's disease and Huntington's chorea. The species differences noted above have to be considered to better understand the role of CR interneurons in striatal organization in both normal and pathological conditions.

The number of striatal cholinergic interneurons expressing calretinin is increased in parkinsonian monkeys

The most abundant interneurons in the primate striatum are those expressing the calcium-binding protein calretinin (CR). The present immunohistochemical study provides detailed assessments of their morphological traits, number, and topographical distribution in normal monkeys (Macaca fascicularis) and in monkeys rendered parkinsonian (PD) by MPTP intoxication. In primates, the CR+ striatal interneurons comprise small (8-12μm), medium (12-20μm) and large-sized (20-45μm) neurons, each with distinctive morphologies. The small CR+ neurons were 2-3 times more abundant than the medium-sized CR+ neurons, which were 20-40 times more numerous than the large CR+ neurons. In normal and PD monkeys, the density of small and medium-sized CR+ neurons was twice as high in the caudate nucleus than in the putamen, whereas the inverse occurred for the large CR+ neurons. Double immunostaining experiments revealed that only the large-sized CR+ neurons expressed choline acetyltransferase (ChAT). The number of large CR+ neurons was found to increase markedly (4-12 times) along the entire anteroposterior extent of both the caudate nucleus and putamen of PD monkeys compared to controls. Comparison of the number of large CR-/ChAT+ and CR+/ChAT+ neurons together with experiments involving the use of bromo-deoxyuridine (BrdU) as a marker of newly generated cells showed that it is the expression of CR by the large ChAT+ striatal interneurons, and not their absolute number, that is increased in the dopamine-depleted striatum. These findings reveal the modulatory role of dopamine in the phenotypic expression of the large cholinergic striatal neurons, which are known to play a crucial role in PD pathophysiology.

Striatal tyrosine hydroxylase-positive neurons are associated with L-DOPA-induced dyskinesia in hemiparkinsonian mice

L-3,4-Dihydroxyphenylalanine (L-DOPA) is the therapeutic gold standard in Parkinson's disease. However, long-term treatment is complicated by the induction of debilitating abnormal involuntary movements termed L-DOPA-induced dyskinesias (LIDs). Until today the underlying mechanisms of LID pathogenesis are not fully understood. The aim of this study was to reveal new factors, which may be involved in the induction of LID. We have focused on the expression of striatal tyrosine hydroxylase-positive (TH+) neurons, which are capable of producing either L-DOPA or dopamine (DA) in target areas of ventral midbrain DAergic neurons. To address this issue, a daily L-DOPA dose was administered over the course of 15 days to mice with unilateral 6-hydroxydopamine-induced lesions of the medial forebrain bundle and LIDs were evaluated. Remarkably, the number of striatal TH+ neurons strongly correlated with both induction and severity of LID as well as ΔFosB expression as an established molecular marker for LID. Furthermore, dyskinetic mice showed a marked augmentation of serotonergic fiber innervation in the striatum, enabling the decarboxylation of L-DOPA to DA. Axial, limb and orolingual dyskinesias were predominantly associated with TH+ neurons in the lateral striatum, whereas medially located TH+ neurons triggered locomotive rotations. In contrast, identified accumbal and cortical TH+ cells did not contribute to the generation of LID. Thus, striatal TH+ cells and serotonergic terminals may cooperatively synthesize DA and subsequently contribute to supraphysiological synaptic DA concentrations, an accepted cause in LID pathogenesis.

Presynaptic Mechanisms of l-DOPA-Induced Dyskinesia: The Findings, the Debate, and the Therapeutic Implications

The dopamine (DA) precursor l-DOPA has been the most effective treatment for Parkinson’s disease (PD) for over 40 years. However, the response to this treatment changes with disease progression, and most patients develop dyskinesias (abnormal involuntary movements) and motor fluctuations within a few years of l-DOPA therapy. There is wide consensus that these motor complications depend on both pre- and post-synaptic disturbances of nigrostriatal DA transmission. Several presynaptic mechanisms converge to generate large DA swings in the brain concomitant with the peaks-and-troughs of plasma l-DOPA levels, while post-synaptic changes engender abnormal functional responses in dopaminoceptive neurons. While this general picture is well-accepted, the relative contribution of different factors remains a matter of debate. A particularly animated debate has been growing around putative players on the presynaptic side of the cascade. To what extent do presynaptic disturbances in DA transmission depend on deficiency/dysfunction of the DA transporter, aberrant release of DA from serotonin neurons, or gliovascular mechanisms? And does noradrenaline (which is synthetized from DA) play a role? This review article will summarize key findings, controversies, and pending questions regarding the presynaptic mechanisms of l-DOPA-induced dyskinesia. Intriguingly, the debate around these mechanisms has spurred research into previously unexplored facets of brain plasticity that have far-reaching implications to the treatment of neuropsychiatric disease.

Intraventricular injection of 6-hydroxydopamine results in an increased number of tyrosine hydroxylase immune-positive cells in the rat cortex

Previously we have demonstrated that intraventricular injection of 6-hydroxydopamine (6-OHDA) results in increased proliferation and de-differentiation of rat cortical astrocytes into progenitor-like cells 4 days after lesion (Wachter et al., 2010). To find out if these cells express tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine synthesis pathway, we performed immunohistochemistry in the rat cortex following intraventricular injection of 6-OHDA. Four days after injection we demonstrated a strong emergence of TH-positive (TH(+)) somata in the cortices of 6-OHDA-lesioned animals. The number of TH(+) cells in the cortex of 6-OHDA-lesioned animals was 15 times higher than in sham-operated animals, where virtually no TH(+) somata occurred. Combining TH immunohistochemistry with classical Nissl stain yielded complete congruency, and ∼45% of the TH(+) cells co-expressed calretinin, which indicates an interneuron affiliation. There was no co-staining of TH with other interneuron markers or with glial markers such as glial fibrillary acidic protein (GFAP) or the neural stem/progenitor marker Nestin, nor could we find co-localization with the proliferation marker Ki67. However, we found a co-localization of TH with glial progenitor cell markers (Sox2 and S100β) and with polysialylated-neural cell adhesion molecule (PSA-NCAM), which has been shown to be expressed in immature, but not recently generated cortical neurons. Taken together, this study seems to confirm our previous findings with respect to a 6-OHDA-induced expression of neuronal precursor markers in cells of the rat cortex, although the TH(+) cells found in this study are not identical with the potentially de-differentiated astrocytes described recently (Wachter et al., 2010). The detection of cortical cells expressing the catecholaminergic key enzyme TH might indicate a possible compensatory role of these cells in a dopamine-(DA)-depleted system. Future studies are needed to determine whether the TH(+) cells are capable of DA synthesis to confirm this hypothesis.

Methamphetamine induces low levels of neurogenesis in striatal neuron subpopulations and differential motor performance

Methamphetamine (METH) causes significant loss of some striatal projection and interneurons. Recently, our group reported on the proliferation of new cells 36 h after METH and some of the new cells survive up to 12 weeks (Tulloch et al., Neuroscience 193:162-169, 2011b). We hypothesized that some of these cells will differentiate and express striatal neuronal phenotypes. To test this hypothesis, mice were injected with METH (30 mg/kg) followed by a single BrdU injection (100 mg/kg) 36 h after METH. One week after METH, a population of BrdU-positive cells expressed the neuronal progenitor markers nestin (18 %) and β-III-tubulin (30 %). At 8 weeks, 14 % of the BrdU-positive cells were also positive for the mature neuron marker, NeuN. At 12 weeks, approximately 7 % of the BrdU-positive cells co-labeled with ChAT, PV or DARPP-32. We measured motor coordination on the rotarod and psychomotor activity in the open-field. At 12 weeks, METH-injected mice exhibited delayed motor coordination deficits. In contrast, open-field tests revealed that METH-injected mice compared to saline mice displayed psychomotor deficits at 2.5 days but not at 2 or more weeks after METH. Taken together, these data demonstrate that some of the new cells generated in the striatum differentiate and express the phenotypes of striatal neurons. However, the proportion of these new neurons is low compared to the proportion that died by apoptosis 24 h after the METH injection. More studies are needed to determine if the new neurons are functional.

Targeting midkine and pleiotrophin signalling pathways in addiction and neurodegenerative disorders: recent progress and perspectives

Midkine (MK) and pleiotrophin (PTN) are two neurotrophic factors that are highly up-regulated in different brain regions after the administration of various drugs of abuse and in degenerative areas of the brain. A deficiency in both MK and PTN has been suggested to be an important genetic factor, which confers vulnerability to the development of the neurodegenerative disorders associated with drugs of abuse in humans. In this review, evidence demonstrating that MK and PTN limit the rewarding effects of drugs of abuse and, potentially, prevent drug relapse is compiled. There is also convincing evidence that MK and PTN have neuroprotective effects against the neurotoxicity and development of neurodegenerative disorders induced by drugs of abuse. Exogenous administration of MK and/or PTN into the CNS by means of non-invasive methods is proposed as a novel therapeutic strategy for addictive and neurodegenerative diseases. Identification of new molecular targets downstream of the MK and PTN signalling pathways or pharmacological modulation of those already known may also provide a more traditional, but probably effective, therapeutic strategy for treating addictive and neurodegenerative disorders.

LINKED ARTICLES

This article is part of a themed section on Midkine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-4.

Levodopa induces long-lasting modification in the functional activity of the nigrostriatal pathway

Much controversy exists concerning the effect of levodopa on striatal dopaminergic markers in Parkinson's disease (PD) and its influence on functional neuroimaging. To deal with this issue we studied the impact of neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic levodopa administration on striatal (18)F-DOPA uptake (Ki) in an animal model of PD. The levels of several striatal dopaminergic markers and the number of surviving dopaminergic neurons in the substantia nigra (SN) were also assessed. Eleven Macaca fascicularis were included in the study. Eight animals received weekly intravenous injections of MPTP for 7weeks and 3 intact animals served as controls. MPTP-monkeys were divided in two groups. Group I was treated with placebo while Group II received levodopa. Both treatments were maintained for 11months and then followed by a washout period of 6months. (18)F-DOPA positron emission tomography (PET) scans were performed at baseline, after MPTP intoxication, following 11months of treatment, and after a washout period of 1, 3 and 6months. Monkeys were sacrificed 6months after concluding either placebo or levodopa treatment and immediately after the last (18)F-DOPA PET study. Striatal dopamine transporter (DAT) content, tyrosine hydroxylase (TH) content and aromatic l-amino acid decarboxylase (AADC) content were assessed. In Group II (18)F-DOPA PET studies performed at 3 and 6months after interrupting levodopa showed a significantly increased Ki in the anterior putamen as compared to Group I. Levodopa and placebo treated animals exhibited a similar number of surviving dopaminergic cells in the SN. Striatal DAT content was equally reduced in both groups of animals. Animals in Group I exhibited a significant decrease in TH protein content in all the striatal regions assessed. However, in Group II, TH levels were significantly reduced only in the anterior and posterior putamen. Surprisingly, in the levodopa-treated animals the TH levels in the posterior putamen were significantly lower than those in the placebo group. AADC levels in MPTP groups were similar to those of control animals in all striatal areas analyzed. This study shows that chronic levodopa administration to monkeys with partial nigrostriatal degeneration followed by a washout period induces modifications in the functional activity of the dopaminergic nigrostriatal pathway.