The aging process: where are the drug opportunities?

Mitochondria: A Therapeutic Target for Parkinson's Disease?

Parkinson’s disease (PD) is one of the most common neurodegenerative disorders. The exact causes of neuronal damage are unknown, but mounting evidence indicates that mitochondrial-mediated pathways contribute to the underlying mechanisms of dopaminergic neuronal cell death both in PD patients and in PD animal models. Mitochondria are organized in a highly dynamic tubular network that is continuously reshaped by opposing processes of fusion and fission. Defects in either fusion or fission, leading to mitochondrial fragmentation, limit mitochondrial motility, decrease energy production and increase oxidative stress, thereby promoting cell dysfunction and death. Thus, the regulation of mitochondrial dynamics processes, such as fusion, fission and mitophagy, represents important mechanisms controlling neuronal cell fate. In this review, we summarize some of the recent evidence supporting that impairment of mitochondrial dynamics, mitophagy and mitochondrial import occurs in cellular and animal PD models and disruption of these processes is a contributing mechanism to cell death in dopaminergic neurons. We also summarize mitochondria-targeting therapeutics in models of PD, proposing that modulation of mitochondrial impairment might be beneficial for drug development toward treatment of PD.

The function and mechanisms of Nurr1 action in midbrain dopaminergic neurons, from development and maintenance to survival

Nurr1 is critical for the development and maintenance of midbrain dopaminergic (DA) neurons in mouse. Loss of Nurr1 function early during development in mice leads to the absence of midbrain DA neurons. Reduction of Nurr1 function in adulthood leads to a slowly progressive loss of striatal DA and markers for DAergic neurons, supporting its selective roles in the maintenance of DAergic neuronal survival and function. To understand the molecular mechanisms of Nurr1 action, our group has identified VIP as a potential target gene of Nurr1. Nurr1 regulates VIP mRNA and protein levels, and transactivates the VIP promoter through Nurr1-responsive cis elements. Nurr1 loss of function leads to the decrease of VIP mRNA level in developing midbrain, suggesting that Nurr1 is involved in the in vivo regulation of VIP expression in midbrain. Our group has also cloned a novel protein interactor for Nurr1. We identified a family of gene products that interact and regulate the activity of Nurr1 by screening yeast two-hybrid library and termed the longest splicing form, NuIP. In vivo NuIP protein is largely colocalized with Nurr1 in adult midbrain dopaminergic neurons. NuIP interacts and positively regulates the activity of Nurr1 protein and could also possibly mediate cross talk between Nurr1 and GTPase mediated signaling pathways. Other recently identified potential target genes and interacting proteins of Nurr1 are also summarized and discussed in this review.

Oestrogen receptors and signalling pathways: implications for neuroprotective effects of sex steroids in Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disorder with a higher incidence in the male population. In the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, 17β-oestradiol but not androgens were shown to protect dopamine (DA) neurones. We report that oestrogen receptors (ER)α and β distinctly contribute to neuroprotection against MPTP toxicity, as revealed by examining the membrane DA transporter (DAT), the vesicular monoamine transporter 2 (VMAT2) and tyrosine hyroxylase in ER wild-type (WT) and knockout (ERKO) C57Bl/6 male mice. Intact ERKOβ mice had lower levels of striatal DAT and VMAT2, whereas ERKOα mice were the most sensitive to MPTP toxicity compared to WT and ERKOβ mice and had the highest levels of plasma androgens. In both ERKO mice groups, treatment with 17β-oestradiol did not provide neuroprotection against MPTP, despite elevated plasma 17β-oestradiol levels. Next, the recently described membrane G protein-coupled oestrogen receptor (GPER1) was examined in female Macaca fascicularis monkeys and mice. GPER1 levels were increased in the caudate nucleus and the putamen of MPTP-monkeys and in the male mouse striatum lesioned with methamphetamine or MPTP. Moreover, neuroprotective mechanisms in response to oestrogens transmit via Akt/glycogen synthase kinase-3 (GSK3) signalling. The intact and lesioned striata of 17β-oestradiol treated monkeys, similar to that of mice, had increased levels of pAkt (Ser 473)/βIII-tubulin, pGSK3 (Ser 9)/βIII-tubulin and Akt/βIII-tubulin. Hence, ERα, ERβ and GPER1 activation by oestrogens is imperative in the modulation of ER signalling and serves as a basis for evaluating nigrostriatal neuroprotection.

Selective alteration at the growth-hormone- releasing-hormone nerve terminals during aging in GHRH-green fluorescent protein mice

Growth hormone (GH) secretion decreases spontaneously during lifespan, and the resulting GH deficiency participates in aging-related morbidity. This deficiency appears to involve a defect in the activity of hypothalamic GH-releasing hormone (GHRH) neurons. Here, we investigated this hypothesis, as well as the underlying mechanisms, in identified GHRH neurons from adult ( approximately 13 weeks old) and aged ( approximately 100 weeks old) transgenic GHRH-green fluorescent protein mice, using morphological, biochemical and electrophysiological methods. Surprisingly, the spontaneous action potential frequency was similar in adult and aged GHRH neurons studied in brain slices. This was explained by a lack of change in the intrinsic excitability, and simultaneous increases in both stimulatory glutamatergic- and inhibitory GABAergic-synaptic currents of aged GHRH neurons. Aging did not decrease GHRH and enhanced green fluorescent protein contents, GHRH neuronal number or GHRH-fibre distribution, but we found a striking enlargement of GHRH-positive axons, suggesting neuropeptide accumulation. Unlike in adults, autophagic vacuoles were evident in aged GHRH-axonal profiles using electron microscopy. Thus, GHRH neurons are involved in aging of the GH axis. Aging had a subtle effect at the nerve terminal level in GHRH neurons, contrasting with the view that neuronal aging is accompanied by more widespread damage.

The orphan nuclear receptors NURR1 and NGFI-B modulate aromatase gene expression in ovarian granulosa cells: a possible mechanism for repression of aromatase expression upon luteinizing hormone surge

Ovarian granulosa cells play pivotal roles in many aspects of ovary functions including folliculogenesis and steroidogenesis. In response to FSH and LH, the elevation of intracellular cAMP level in granulosa cells leads to activation of multiple ovarian genes. Here, we report findings from a genome-wide study of the cAMP-responsive gene expression profiles in a human granulosa-like tumor cell line, KGN. The study identified 140 genes that are either activated or repressed by 2-fold or greater after stimulation by the adenylyl cyclase activator forskolin. The induction patterns of some cAMP-responsive genes were further analyzed by quantitative real-time PCR. Consistent with previous observations, the LH-responsive genes, such as the nuclear receptor 4A subfamily (NURR1, NGFI-B, and NOR-1), were rapidly but transiently induced, whereas the FSH-responsive gene CYP19 encoding aromatase was induced in a delayed fashion. Interestingly, ectopic expression of NURR1 or NGFI-B severely attenuated the cAMP-responsive activation of the ovary-specific aromatase promoter. Reduction of the endogenous NURR1 or NGFI-B by small interfering RNA significantly elevated aromatase gene expression. The cis-elements responsible for NURR1/NGFI-B-mediated repression were mapped to the minimal aromatase promoter sequence that confers camp responsiveness. Furthermore, the DNA-binding domain of NURR1 was required for the repression. Taken together, these results strongly suggest a causal relationship between the rapid decline of aromatase mRNA and induction of nuclear receptor subfamily 4A expression, which concomitantly occur upon LH surge at the later stages of ovarian follicular development.