These papers contain nice work. Since the work comes “in stereo” in a great journal, it seems all the more significant. It's rare but not unprecedented to see such similar cutting-edge research from two excellent labs.

I think these data are potentially very important. They harken back to a classical, almost a decade-old paradigm for miRNAs, namely that they are somewhat like bookmarks for a developmental stage of a particular cell lineage. miRNAs were discovered in animals in the context of the heterochronic developmental pathway in worm. Here the miRNAs regulated transcription factors, for example, the worm gene lin-14, and thus exerted a great impact on cell and organism phenotype.

In the meantime, expectations for miRNAs have broadened in terms of CNS roles, as it has been shown that miRNAs can exist as dynamic regulators of cell function in addition to assisting in the progression or maintenance of developmental states. However, both papers by Zhao et al. and Dugas et al. in Neuron suggest that the paradigm of developmental pathways needs to be kept in mind in the...  Read more

These papers contain nice work. Since the work comes “in stereo” in a great journal, it seems all the more significant. It's rare but not unprecedented to see such similar cutting-edge research from two excellent labs.

I think these data are potentially very important. They harken back to a classical, almost a decade-old paradigm for miRNAs, namely that they are somewhat like bookmarks for a developmental stage of a particular cell lineage. miRNAs were discovered in animals in the context of the heterochronic developmental pathway in worm. Here the miRNAs regulated transcription factors, for example, the worm gene lin-14, and thus exerted a great impact on cell and organism phenotype.

In the meantime, expectations for miRNAs have broadened in terms of CNS roles, as it has been shown that miRNAs can exist as dynamic regulators of cell function in addition to assisting in the progression or maintenance of developmental states. However, both papers by Zhao et al. and Dugas et al. in Neuron suggest that the paradigm of developmental pathways needs to be kept in mind in the mammalian CNS in which miRNAs regulate transcription factors for stage-specific lineage specificity.

One thing I note about these miRNAs is that they are considered in these papers to be “oligodendrocyte-specific.” This gives me pause. miR-338 was isolated from primary rat cerebral cortical cultures (Kim et al., 2004), and we have found that miR-219 is relatively enriched in hippocampal neuronal cultures relative to glial cultures (Wang et al., 2008). This puzzling question of miRNA cell type “specificity” is explicitly, but not completely, addressed in the papers being discussed. It seems to underscore the fact that a particular miRNA, much like a particular protein, can have distinct functions in different contexts.

Also, since the oligodendrocyte is a hitherto understudied potential focal point of pathogenesis (see, e.g., the recent studies by George Bartzokis concerning the potential role(s) of oligodendrocyte dysfunction in Alzheimer disease; Bartzokis, 2009), it remains to be seen if miRNAs may participate in these pathological processes.

View all comments by Peter Nelson

I read these two papers with great interest. They are elegant and provide definitive molecular explanations underlying the developmental switch from proliferating OPCs to differentiating OPCs. Cell-cycle exit is often coupled with the initiation of differentiation in different types of cells. These observations suggest a possible involvement of microRNA-dependent processes. It will be interesting to find out in future studies how microRNA biogenesis, for example, that of miR-219 in oligodendrocytes, is regulated.

View all comments by Zhigang He

These two new studies highlight once again the importance of Dicer and microRNAs in brain function. Perhaps expectedly, the authors demonstrate in a convincing way that mammalian Dicer is required for oligodendrocyte differentiation and myelination. Here, a combination of three independent mouse Cre lines was used to study the effects of Dicer loss in oligodendrocyte/Schwann cells. Interestingly, the ataxia and tremor behaviors present in the mutant mice were previously observed in CaMkII-Cre mice, in which Dicer was deleted in pyramidal neurons (Davis et al., 2008; Hebert et al., unpublished).

A few candidate microRNAs, including miR-338, miR-138, and more particularly miR-219, seem important for the loss-of-function phenotype in the Dicer cKO mice. These conclusions are based on miRNA profiling and rescue experiments on isolated cultured cells and in vivo. The partial rescue by candidate miRNAs may be related to technical issues or, more likely, to requirement of additional miRNAs in oligodendrocyte differentiation and function.

Interestingly, previous reports have...  Read more

These two new studies highlight once again the importance of Dicer and microRNAs in brain function. Perhaps expectedly, the authors demonstrate in a convincing way that mammalian Dicer is required for oligodendrocyte differentiation and myelination. Here, a combination of three independent mouse Cre lines was used to study the effects of Dicer loss in oligodendrocyte/Schwann cells. Interestingly, the ataxia and tremor behaviors present in the mutant mice were previously observed in CaMkII-Cre mice, in which Dicer was deleted in pyramidal neurons (Davis et al., 2008; Hebert et al., unpublished).

A few candidate microRNAs, including miR-338, miR-138, and more particularly miR-219, seem important for the loss-of-function phenotype in the Dicer cKO mice. These conclusions are based on miRNA profiling and rescue experiments on isolated cultured cells and in vivo. The partial rescue by candidate miRNAs may be related to technical issues or, more likely, to requirement of additional miRNAs in oligodendrocyte differentiation and function.

Interestingly, previous reports have shown that miR-219 and miR-138 are functionally expressed in neurons (Kocerha et al., 2009; Siegel et al., 2009). Indeed, miR-219 seems important for NMDA receptor signaling, whereas miR-138 controls dendritic spine morphology. Although one must be careful in the interpretation of cell “enriched” or “specific” (i.e., 10 and 100 times, respectively, more abundant when compared to other tissues), these miRNAs are clearly highly expressed in the brain. It remains possible that these miRNAs share different subcellular localization, depending on cell type. For instance, miR-138 is enriched in neuronal dendrites (Siegel et al., 2009).

Is miR-219 physiologically more important in oligodendrocytes compared to neurons? Not necessarily. In addition to cell-type specificity, the organism has developed many ways to control miRNA function, including developmental timing, subcellular localization, relative expression levels, and post-transcriptional modifications. Complex organs such as the brain have likely developed an additional level of miRNA regulation based on cell-specific gene targets, perhaps using a combination of unique co-factors. In this way, the same miRNA could target different genes depending on cellular context. Of course, this line of thinking could be extrapolated to ubiquitously expressed miRNAs. In accordance with this hypothesis, it has been proposed that the “brain-specific” miR-29 could play an important role in various cardiovascular diseases (Hebert, 2009). More related to these studies, microarray studies have shown that ubiquitously expressed miR-20 family members (miR-20a, miR-106a, and miR-R17-5p) are downregulated in Dicer-deficient oligodendrocytes.

Whether miR-219 and/or other candidate miRNAs are specifically involved in myelination diseases in humans remains an attractive possibility. Interestingly, changes in miRNA expression levels have been associated with multiple sclerosis, including downregulation of miR-219 and miR-338 (Junker et al., 2009).

References:
Davis TH, Cuellar TL, Koch SM, Barker AJ, Harfe BD, McManus MT, Ullian EM (2008) Conditional loss of Dicer disrupts cellular and tissue morphogenesis in the cortex and hippocampus. J Neurosci 28:4322-4330. Abstract

Hebert SS (2009) Putative Role of MicroRNA-Regulated Pathways in Comorbid Neurological and Cardiovascular Disorders. Cardiovasc Psychiatry Neurol 2009:849519. Abstract

Junker A, Krumbholz M, Eisele S, Mohan H, Augstein F, Bittner R, Lassmann H, Wekerle H, Hohlfeld R, Meinl E (2009) MicroRNA profiling of multiple sclerosis lesions identifies modulators of the regulatory protein CD47. Brain 132:3342-3352. Abstract

Kocerha J, Faghihi MA, Lopez-Toledano MA, Huang J, Ramsey AJ, Caron MG, Sales N, Willoughby D, Elmen J, Hansen HF, Orum H, Kauppinen S, Kenny PJ, Wahlestedt C (2009) MicroRNA-219 modulates NMDA receptor-mediated neurobehavioral dysfunction. Proc Natl Acad Sci U S A 106:3507-3512. Abstract

Siegel G et al. (2009) A functional screen implicates microRNA-138-dependent regulation of the depalmitoylation enzyme APT1 in dendritic spine morphogenesis. Nat Cell Biol 11:705-716. Abstract

View all comments by Sebastien S. Hebert

The manuscript by Rademakers and colleagues provides evidence that increased binding of miR-659 to the 3’UTR of the GRN gene could underlie an important risk for TDP-43-positive frontotemporal dementia (FTLD-U). These data bring strong clinical support for the role of microRNAs in neurodegenerative disorders in humans. These results are consistent with a loss of function of the GRN gene in the disease, further linking gene dosage effects in neurodegenerative disorders (as seen, e.g., with APP in Alzheimer disease and SNCA in Parkinson disease).

I think Amber Dance did a fantastic job reviewing the highlights of this paper. I would like to discuss additional issues with regard to certain technical and mechanistic aspects of these findings, which could be taken into account when interpreting the data.

First, miR-659, located on chromosome 22 in humans, seems to be relatively very weakly expressed in adult brain (with cycle threshold [Ct] values of approximately 32 as measured by qRT-PCR). Therefore, whether endogenous miR-659 levels are sufficient to regulate GRN levels...  Read more

The manuscript by Rademakers and colleagues provides evidence that increased binding of miR-659 to the 3’UTR of the GRN gene could underlie an important risk for TDP-43-positive frontotemporal dementia (FTLD-U). These data bring strong clinical support for the role of microRNAs in neurodegenerative disorders in humans. These results are consistent with a loss of function of the GRN gene in the disease, further linking gene dosage effects in neurodegenerative disorders (as seen, e.g., with APP in Alzheimer disease and SNCA in Parkinson disease).

I think Amber Dance did a fantastic job reviewing the highlights of this paper. I would like to discuss additional issues with regard to certain technical and mechanistic aspects of these findings, which could be taken into account when interpreting the data.

First, miR-659, located on chromosome 22 in humans, seems to be relatively very weakly expressed in adult brain (with cycle threshold [Ct] values of approximately 32 as measured by qRT-PCR). Therefore, whether endogenous miR-659 levels are sufficient to regulate GRN levels in vivo remains speculative. Mechanistically, one must envisage that regulation of GRN mRNA by miR-659 occurs in a cell-autonomous fashion. One possibility, not shown here, is that miR-659 is expressed in specific cell types, such as the granular cell layer of the cerebellum where GRN protein is decreased (it should be noted that the qRT-PCR for miR-659 was performed on whole tissues). In my opinion, this would strongly strengthen the biological significance of the proposed mode of regulation.

Here, the authors use basic, but widely accepted in vitro systems to validate their hypothesis. First, artificial overexpression of miR-659 (at a concentration of 12 nM) in human M17 neuroblastoma cells leads to decreased expression of endogenous GRN protein levels (note that inverse experiments using antisense oligonucleotides to block endogenous miR-659 was not performed, possibly due to the extremely low levels of this microRNA in these cells). Whether GRN mRNA levels are affected in these conditions is not shown. Then, additional studies were conducted in mouse Neuro2A cells using luciferase-based constructs containing the GRN 3’UTR. In these latter experiments, functional effects on GRN expression are seen with the mutant TT construct at concentrations starting at 5 pM of exogenous miR-659. Again from a mechanistic point of view, it would be interesting to see whether the “increased” binding (i.e., increased sequence complementarity) of miR-659 to the mutant TT allele causes an siRNA effect (thus degradation of mRNA). It should be noted, however, that, in affected patients, GRN mRNA (from total tissue sections) is not affected.

Interestingly, the predicted target site (more particularly the “seed” sequence) for miR-659 in the GRN 3’UTR is only conserved in humans, and is not found in other mammals including mouse and dog (e.g., see www.targetscan.org). Similarly, miR-659 is, at least for now, only found in humans. Interestingly, the GRN 3’UTR is quite short (approximately 300 bp in length). In comparison, the BACE1 and APP 3’UTRs, which equally have functional microRNA target sites, are approximately 4,000 bp and 2,000 bp in length, respectively.

Overall, these findings provide novel and important clues into the development of FTLD-U. In addition, this study contributes to the potential role of microRNA pathways in the development of neurodegenerative disorders in human. I agree that relatively few patients were analyzed here to make definitive conclusions with regard to the biological relevance of these findings.

View all comments by Sebastien S. Hebert

Thank you for this fascinating article.

In future, when describing those with Rett syndrome, would you please be aware that some girls/women are not necessarily mentally retarded. Some are severely dyspraxic, i.e., mostly unable to show their understanding by being unable to make appropriate actions.

My own Rett daughter is 45 years old. She has R255X, said to be one of the most severe mutations. She has completed her school Leaving Certificate in the four subjects of English, History of Revolutions, Australian History, and Psychology. She also completed year 11 general math.

She is now studying her second unit at university—very slowly, as you can imagine. She types using a small keyboard/laptop/word-prediction program.

View all comments by Jill Johnson

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