Steric-blocking antisense, such as Morpholino oligos, can interfere with miRNA activity.

The first report that Morpholinos can block miRNAs is:
Kloosterman WP, Wienholds E, Ketting RF, Plasterk RH. Substrate requirements for let-7 function in the developing zebrafish embryo. Nucleic Acids Res. 2004 Dec 07;32(21):6284-91.
While the paper stated the knockdowns worked, no techniques or data were presented regarding the Morpholino knockdowns.

A recent paper describing the experimental techniques and results of a Morpholino-miRNA knockdown is:
Flynt AS, Li N, Thatcher EJ, Solnica-Krezel L, Patton JG. Zebrafish miR-214 modulates Hedgehog signaling to specify muscle cell fate. Nature Genetics 2007; 39: 259 - 263.

I have posted notes on the Flynt et al. paper on my blog,
http://blog.myspace.com/127542408

I'll post updates here as new papers are published using Morpholinos for miRNA knockdowns.

- Jon

People usually use LNA to knockdown miRNA, right? What are the advantages and disavantages of this morpholino oligo method?

The per-base affinity of LNA is higher than for Morpholinos. This means that Morpholinos need a longer complementary sequence to bind to the target RNA, increasing specificity. However, Morpholinos are also more tolerant of single-nucleotide mismatches than LNA, so if single-nucleotide discrimination is needed then in that case LNA might be the better choice.

LNAs are nuclease resistant, but can be nucleolytically degraded (albeit much more slowly than RNA). Morpholinos are not degraded by nucleases.

If you decide to compare prices, note that Morpholinos are delivered in measured quantities (typically 300 nanomoles) while LNAs are often prepared as scale syntheses, with fairly low guaranteed minimum delivered quantities. Where measured amounts of LNAs are available, the price per nanomole of LNA greatly exceeds the price per nanomole of Morpholinos (on the site I just checked, the price per nanomole of an unlabeled LNA was sixty times the price per nanomole for an unlabeled Morpholino).

There is little published so far for Morpholino-miRNA knockdowns. Watch for an upcoming paper by Wigard Kloosterman in PLoS Genetics (resubmitted after addressing reviewer's comments).

Here's a publication update for miRNA blocking by Morpholinos.

A collaboration with Wigard Kloosterman, first of Ronald Plasterk's lab then Rene Ketting's lab at the Hubrecht Laboratory in the Netherlands, has shown that Morpholino oligos can strongly inhibit the activity of miRNAs in zebrafish and block their detection by in-situ hybridization and Northern blots. Knockdowns of miRNA activity have generally used oligos targeting the miRNA guide strand. A Morpholino oligo targeting an miRNA guide strand can interfere with the activity of the miRNA. It is difficult to control for the specificity of the knockdown when using this technique alone. However, Morpholinos targeting the nucleolytic processing sites of an immature miRNA can prevent maturation of the miRNA. This allows sets of nonoverlapping Morpholino oligos targeting a primary miRNA to be used as specificity controls; if two non-overlapping oligos targeting the same miRNA produce the same phenotype, this supports the hypothesis that the phenotype is due to knocking down the activity of the targeted miRNA and not due to an off-target effect. These techniques are explored in the following paper:
Kloosterman WP, Lagendijk AK, Ketting RF, Moulton JD, Plasterk RHA. Targeted inhibition of miRNA maturation with morpholinos reveals a role for miR-375 in pancreatic islet development. PLoS Biol. 2007;5(8): e203.

Lin YC, Hsieh LC, Kuo MW, Yu J, Kuo HH, Lo WL, Lin RJ, Yu AL, Li WH. Human TRIM71 and Its Nematode Homologue are Targets of let-7 MicroRNA and Its Zebrafish Orthologue is Essential for Development. Mol Biol Evol. 2007 Sep 21; [Epub ahead of print]

An miRNA's target site on an mRNA can be protected by a complementary Morpholino oligo.
Choi WY, Giraldez AJ, Schier AF. Target Protectors Reveal Dampening and Balancing of Nodal Agonist and Antagonist by miR-430. Science. 2007 Aug 30; [Epub ahead of print]

Martello G, Zacchigna L, Inui M, Montagner M, Adorno M, Mamidi A, Morsut L, Soligo S, Tran U, Dupont S, Cordenonsi M, Wessely O & Piccolo S. MicroRNA control of Nodal signalling. Nature Advance Online Publication, August 2007. doi:10.1038/nature06100

Does anyone know whether morpholinos will also effectively knockdown miRNAs in cultured mammalian cells?

Hi Baoyu,

Morpholinos are routinely used for translation or splicing inhibition in cultured mammalian cells. I haven't seen a publication describing the use of Morpholinos for blocking miRNA in cultures, but I expect they will work well.

In embryos, microinjection delivers the oligos to the single-celled or few-celled zygote and the daughter cells all contain the Morpholinos. The additional challenge posed by cultures compared to embryos is that in cultures, delivery of the oligos into the cytosol of the cells requires a technique like electroporation or use of an endosomal release agent (e.g. Endo-Porter). Morpholino delivery using cationic lipids like lipofectamine is not efficient because the cationic lipids form electrostatic complexes with anionic nucleic acids, but the uncharged Morpholino oligo cannot form a similar complex with the cationic lipids. If your cells tolerate electroporation and you have access to the equipment, I'd start with that. If not, try Endo-Porter.

Here's another for the reference list:

Eberhart JK, He X, Swartz ME, Yan YL, Song H, Boling TC, Kunerth AK, Walker MB, Kimmel CB, Postlethwait JH. MicroRNA Mirn140 modulates Pdgf signaling during palatogenesis. Nat Genet. 2008 Feb 10; [Epub ahead of print]

I wrote some notes on the Morpholino work in the Eberhart et al. paper in my blog under the heading "Cleft palate, pgdf and miRNA", posted 20 Feb 2008:
http://blog.myspace.com/127542408