. A MicroRNA feedback circuit in midbrain dopamine neurons. Science. 2007 Aug 31;317(5842):1220-4. PubMed.


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  1. Jongpil Kim and colleagues from Asa Abeliovich's group have produced a very important work that indicates a particular microRNA (miRNA) may play a critical role in Parkinson disease. This study is important for a number of reasons. First, it indicates a discrete role for a particular miRNA in dopaminergic function; second, previously no particular miRNA-mRNA pair had been strongly implicated in a prevalent neurodegenerative disease. In other words, these investigators have provided plausible molecular neurobiological breakthroughs for both miRNA function and dysfunction.

    The authors use various means to indicate that a particular miRNA—miR-133b—is relatively highly expressed at the tissue level in midbrain under normal conditions, but not during Parkinson disease. The main point they demonstrate conclusively is that knocking out miRNAs generally in vivo, or miR-133b by itself in culture, dramatically decreases tyrosine hydroxylase and dopamine transporter (DAT) levels in dopaminergic neurons. (The paper includes some gorgeous, albeit digitally rendered photomicrographs.) The data also support the hypothesis that miR-133b and the paired-like homeodomain transcription factor PITX3 regulate each other's expression.

    The data is very persuasive. At the same time, several points may merit reflection.

    miR-133 was previously described in a number of studies in connection with skeletal and cardiac muscle. miR-133a differs from miR-133b only at the 3' end by a single nucleotide (G for miR-133a, A for miR-133b). Theoretically this should alter the biological “activity” very little (although work from Joshua Mendell's lab at Johns Hopkins showed that the 3' end of miR-29 has an important role in trafficking the miRNA to either cytoplasm or cell nucleus). It remains to be seen whether miR-133b is indeed “specific”
    to midbrain neurons, and this brings up one of the few weaknesses of this study: I would have liked to see in-situ hybridization localizing this miRNA in human tissue. After all, substantia nigra neurons represent a small minority of “midbrain” cells. Also, while the authors do show the relative amount of miR-133a and miR-133b in the brains, there is no indication of the absolute amount expressed, which is also important. If miR-133a is still present in more abundant quantities than miR-133b, why would things alter much inside the cells when miR-133b decreased?

    Another issue with regard to this paper is that Parkinson disease is now considered a “synucleinopathy” as much as it is a “dopamine neuronopathy.” Cerebral cortical and brainstem diseases such as dementia with Lewy bodies and multiple system atrophy are in some ways more closely tied by pathoetiology to Parkinson's than, for example, MPTP toxicity, which only affects substantia nigra dopaminergic neurons. Hence, the focus of research has been more on protein degradation/ubiquitination, oxidation, folding, and so forth, rather than on the particular pathways involved with dopaminergic function. However, this paper demonstrates perhaps a new component of a “downward” spiral during the disease.

    These considerations aside, this paper represents a true landmark in tying a particular miRNA to neurodegeneration. I look forward with great anticipation to follow-up studies in this explosive field.

  2. Micro-RNAs (miRNAs) are small pieces of RNA that bind to complementary bases on specific mRNAs and downregulate protein expression from these targets. This mode of gene silencing has received a lot of attention in the few years since its discovery, culminating in last year’s Nobel Prize for Andrew Fire and Craig Mello, the two scientists who first described the phenomenon. Recently, it is becoming ever more evident that neurons rely heavily on miRNAs as a means of cell-specific gene regulation, and this point is nicely shown by Asa Abeliovich and colleagues in last week’s issue of Science. Using various approaches, the authors show that the miRNA machinery, and especially the miRNA number miR-133b, contributes to the differentiation and maintenance of dopaminergic neurons. This is the first demonstration that miRNAs are involved in the differentiation into neuronal subtypes, whereas the effects on neuronal versus non-neuronal differentiation is well documented. Most importantly, the group demonstrate that miR-133b is specifically depleted in Parkinson patients, as well as in several respective mouse models, indicating that this miRNA may well be directly involved in the pathogenesis and/or disease progression.

    The study starts out with the observation that interruption of the miRNA biogenesis pathway in a cellular model of neuronal differentiation depletes the pool of resulting dopaminergic neurons (DNs) more than other neuronal subtypes, a defect that can be significantly rescued by transfecting small RNAs, a fraction that includes mature miRNAs. The requirement of miRNA expression for DN development and maintenance was further underlined by the conditional knockout of Dicer, the crucial enzyme in the miRNA biogenesis pathway. Removing Dicer also deletes DNs from 8-week-old mice and thus causes a Parkinson-like phenotype.

    To study whether any particular miRNA correlates with DN maintenance, 224 miRNAs were screened for changes in abundance between Parkinson and control patients. One miRNA, miR-133b, was singled out for further analysis. miR-133b was specifically expressed in the midbrain of healthy controls, and severely depleted in Parkinson patients; this finding was extended to a genetic and a pharmacological mouse model of the disease. To show the importance of miR-133b for DN development and/or survival, the miRNA was overexpressed in differentiating cell cultures: TH-positive DNs were significantly depleted from the resulting pool of neurons, and the downregulation of DAT (a marker of terminally differentiated DNs), but not of Pitx3 or Nurr1 (early markers), suggests that primarily mature DNs are affected. In agreement, miR-133b depletion increases the DN population and DAT expression in vitro.

    To understand the mechanism of miR-133b action, Pitx3 was identified as a potential target. Pitx3 is a transcription factor that is important in DN differentiation, and Pitx3 mutant mice exhibit severe DN defects and are thus a model of parkinsonism. The downregulation of Pitx3 by miR-133b was verified ectopically in COS cells, as well as in primary midbrain neurons. Further, miR-133b depletion induces expression of the DN markers TH and DAT in wild-type but not Pitx3 mutant primary neurons, demonstrating that miR-133b acts downstream of Pitx3. From this, the authors make a model in which Pitx3 and miR-133b form a feedback loop that is important for DN development and/or maintenance.

    This paper will certainly stir a lot of important and exciting work in the field of neurodegeneration. Firstly, one can be almost sure that other neuronal populations similarly require miRNA-mediated gene regulation, and the absence of the respective miRNAs might well be linked to other neurodegenerative diseases, such as Alzheimer’s. Secondly, it will be important to establish how precisely miR-133b regulates DN differentiation and maintenance. To my opinion, Pitx3 downregulation is only part of the story: depletion of a Pitx3 antagonist should promote the number of DN cells—this is observed in vitro, but in vivo miR-133b depletion correlates with a loss of DNs. Most likely, miR-133b has various DN-specific roles, very much as miR-124a controls neuronal development through several parallel pathways. Finally, it will be important to see how tightly the disappearance of miR-133b is linked to parkinsonism, and whether miR-133b deficiency is cause or consequence of the disease.

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