Anne Schaefer et al. have produced an important study about miRNAs in the mouse cerebellum. Their data suggest that miRNA function is critical for mammalian neuronal survival. The authors conclude that since dicer downregulation causes neuronal cell death, then some human neurodegenerative diseases may be caused by loss of small regulatory RNAs. (The authors wrote, “this pattern of Purkinje cell degeneration in the absence of miRNAs bears obvious similarity to processes associated with the slow progressing neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.” The “obviousness” is open to debate and surely one would draw closer analogies to the spinocerebellar ataxias because neurodegenerative diseases seem to be very cell type-specific). This is the first such study in mammals, although intriguing prior studies have previously been performed by the Bonini lab at U. Penn on flies using genes relevant to spinocerebellar ataxia.

From a technical standpoint, the paper is solid, as one would expect from the outstanding Greengard laboratory. In their...  Read more

Anne Schaefer et al. have produced an important study about miRNAs in the mouse cerebellum. Their data suggest that miRNA function is critical for mammalian neuronal survival. The authors conclude that since dicer downregulation causes neuronal cell death, then some human neurodegenerative diseases may be caused by loss of small regulatory RNAs. (The authors wrote, “this pattern of Purkinje cell degeneration in the absence of miRNAs bears obvious similarity to processes associated with the slow progressing neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.” The “obviousness” is open to debate and surely one would draw closer analogies to the spinocerebellar ataxias because neurodegenerative diseases seem to be very cell type-specific). This is the first such study in mammals, although intriguing prior studies have previously been performed by the Bonini lab at U. Penn on flies using genes relevant to spinocerebellar ataxia.

From a technical standpoint, the paper is solid, as one would expect from the outstanding Greengard laboratory. In their mice, Purkinje cell-specific Pcp2 promoter drives a Cre recombinase that causes the dicer gene (modified with loxP sites) to be knocked down. The dicer gene gets knocked down after the second week of life because that’s when the Pcp2 gene is activated. In situ hybridization shows dramatically reduced expression of some, but not all, Purkinje cell miRNAs after dicer knockdown. Most of the rest of the study is essentially neuropathological: loss of expression of both dicer and most miRNAs is followed by Purkinje cell death, dendritic withering, and, as would be expected, an ataxia phenotype develops in the mice.

Key points:

1. Some neurons need dicer (read: some miRNAs) to survive—an important point, and not one to be taken for granted!

Key additional questions:

1. Was it for want of miRNAs or some other dicer-related function that caused the cells to die?

2. If lack of miRNAs caused the cell death, was it one or a group of miRNAs that were necessary for cell viability (in coming years, we’ll no doubt see some specific miRNA knockout mice with specific phenotypes)?

3. Related but separate question—which critical biological functions in cells are regulated by those vital miRNAs?

4. Why are some miRNAs still evidently present in these cells after dicer is knocked out?

5. How do these results correlate to other types of neurodegeneration?

The field of miRNA research is in its infancy—small regulatory RNAs were the journal Science’s “Discovery of the Year” in 2002, and related research won the Nobel Prize in Medicine and Physiology in 2006. MiRNAs are short (~22 nts) RNA molecules that play remarkably powerful biological roles in plants and all known animals. MiRNAs appear to act by regulating “target” mRNAs, to which their sequences are partly complementary. Constituting ~5 percent of the human transcriptome, miRNAs in turn are predicted to regulate >30 percent of known mRNAs. MiRNAs have been shown to be expressed at high levels in brain, where hundreds of different miRNAs (most still not annotated!) are expressed in human and chimpanzee brains according to work from many labs including the Plasterk Lab in the Netherlands. A discrete role for miRNAs in human brain disease has yet to be found. However, with studies like these from the Greengard lab emerging, miRNAs may indeed play a role in human neurodegeneration with important clinical implications.

View all comments by Peter Nelson

The Olson group is a pioneer in the field of microRNA function in muscle cells. Here, the authors provide compelling evidence that miR-206, a skeletal muscle-specific “myomiR,” functions in a complex regulatory pathway to regulate ALS pathology in mice. The strength of this paper relies on the use of different mouse models, including miR-206 knockouts, to characterize the signaling pathway in vivo.

To obtain insights into the mechanism(s) involved in muscle degeneration in ALS, the authors performed a microRNA array from ALS mice, which harbor the familial SOD1 G93A mutation. MiR-206 was the most significantly changed (upregulated) miRNA in this screen. It is interesting to note that other myomiRs, including miR-1, miR-133b, and miR-133a were, albeit at weaker levels, downregulated in the array; however, the authors could confirm by quantitative PCR the upregulation of miR-206 and miR-133b (which are co-expressed from the same transcript) in the diseased mice. MiR-206 upregulation coincided with denervation and ALS pathology in the mutant mice. To make a long story short, the...  Read more

The Olson group is a pioneer in the field of microRNA function in muscle cells. Here, the authors provide compelling evidence that miR-206, a skeletal muscle-specific “myomiR,” functions in a complex regulatory pathway to regulate ALS pathology in mice. The strength of this paper relies on the use of different mouse models, including miR-206 knockouts, to characterize the signaling pathway in vivo.

To obtain insights into the mechanism(s) involved in muscle degeneration in ALS, the authors performed a microRNA array from ALS mice, which harbor the familial SOD1 G93A mutation. MiR-206 was the most significantly changed (upregulated) miRNA in this screen. It is interesting to note that other myomiRs, including miR-1, miR-133b, and miR-133a were, albeit at weaker levels, downregulated in the array; however, the authors could confirm by quantitative PCR the upregulation of miR-206 and miR-133b (which are co-expressed from the same transcript) in the diseased mice. MiR-206 upregulation coincided with denervation and ALS pathology in the mutant mice. To make a long story short, the group identified both upstream (MyoD) and downstream (HDAC4, FGFBP1) effectors of the miR-206 network in vivo. Overall, an elegant study!

This work contributes significantly to our knowledge with regard to microRNA function in health and disease. Although individual microRNAs can target up to several hundred genes, it’s important to keep in mind that the observed pathological effects in vivo often result from the abnormal fine-tuning of a limited number of "key" genes. With regard to the current study, the effects of miR-206 in reinnervation could be explained, in most part, by the misregulation of HDAC4 (a miR-206 target). One could hypothesize that this mode of “targeted” pathological regulation could function in Alzheimer disease brain, where, for instance, miR-29 could contribute to plaque load by modulating BACE1 regulation.

Although well structured, this study does not, in my opinion, address the primary cause of motor neuron degeneration in ALS (mice or humans). An interesting follow-up study would be to look for changes in microRNA expression in the CNS. In this sense, this study lacks validation of microRNA (and target gene) expression in humans. Notably, the miR-206/miR-133b locus was originally described as a synapse-associated non-coding RNA (7H4)(see current study). Interestingly, it has been documented that miR-206 can become overexpressed in the brain after cellular insult (Jeyaseelan et al., 2008; Zhang and Pan, 2009). Also, one study associates miR-206 with schizophrenia (Hansen et al., 2007). In the current study, axonal regeneration seemed unaffected in the miR-206 knockout mice. Clearly, future studies are needed to elucidate the role of microRNAs, particularly miR-206, in both neuronal and muscle loss.

References:
Hansen T, Olsen L, Lindow M, Jakobsen KD, Ullum H, Jonsson E, Andreassen OA, Djurovic S, Melle I, Agartz I, Hall H, Timm S, Wang AG, Werge T (2007) Brain expressed microRNAs implicated in schizophrenia etiology. PLoS ONE 2:e873. Abstract

Jeyaseelan K, Lim KY, Armugam A (2008) MicroRNA expression in the blood and brain of rats subjected to transient focal ischemia by middle cerebral artery occlusion. Stroke 39:959-966. Abstract

Zhang B, Pan X (2009) RDX induces aberrant expression of microRNAs in mouse brain and liver. Environ Health Perspect 117:231-240. Abstract

View all comments by Sebastien S. Hebert