Processing bodies were first described in the scientific literature by Bashkirov et. al. in 1997, in which they describe "small granules… discrete, prominent foci" as the cytoplasmic location of the mouse exoribonuclease mXrn1p. It wasn’t until 2002 that a glimpse into the nature and importance of these cytoplasmic foci was published. In 2002, researchers demonstrated that multiple proteins involved with mRNA degradation localize to the foci. During this time, many descriptive names were used to identify the processing bodies, including "GW-bodies" and "decapping-bodies"; however "P-bodies" was the term chosen and is now widely used and accepted in the scientific literature. Recently evidence has been presented suggesting that GW-bodies and P-bodies may in fact be different cellular components. The evidence being that GW182 and Ago2, both associated with miRNA gene silencing, are found exclusively in multivesicular bodies or GW-bodies and are not localized to P-bodies. Also of note, P-bodies are not equivalent to stress granules, the two structures support overlapping cellular functions but generally occur under different stimuli. Hoyle et al. suggests a novel site termed EGP bodies, or stress granules, may be responsible for mRNA storage as these sites lack the decapping enzyme.

P-bodies are distinct foci within the cytoplasm of the eukaryotic cell

Eukaryotic Cell

Eukaryotic Cell is an academic journal published by the American Society for Microbiology. The title is commonly abbreviated EC and the ISSN is 1535-9778 for the print version, and 1535-9786 for the electronic version....

 consisting of many enzymes involved in mRNA turnover. P-bodies have been observed in somatic cells originating from vertebrates and invertebrates, including plants and yeast. To date, P-bodies have been demonstrated to play fundamental roles in general mRNA decay, nonsense-mediated mRNA decay, AU-rich element mediated mRNA decay, and microRNA induced mRNA silencing. It is important to note that not all mRNAs which enter P-bodies are degraded, as it has been demonstrated that some mRNAs can exit P-bodies and re-initiate translation.

The following activities have been demonstrated to occur in or to be associated with P-bodies:

• decapping
  Messenger RNA decapping
  The process of messenger RNA decapping consists of hydrolysis of the 5' cap structure on the RNA exposing a 5' monophosphate. This 5' monophosphate is a substrate for the exonuclease Xrn1 and the message is quickly destroyed...
  
   and degradation of unwanted mRNAs
• storing mRNA until needed for translation
• aiding in translational repression by miRNA
  Mirna
  Mirna may refer to:geographical entities* Mirna , a river in Istria, Croatia* Mirna , a river in Slovenia, tributary of the river Sava* Mirna , a settlement in the municipality of Mirna in Southeastern Sloveniapeople...
  
  s (related to siRNAs
  Small interfering RNA
  Small interfering RNA , sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA molecules, 20-25 nucleotides in length, that play a variety of roles in biology. The most notable role of siRNA is its involvement in the RNA interference pathway, where it...
  
  )

In neurons, P-bodies move by motor proteins in response to stimulation. This is likely tied to local translation in dendrite

Dendrite

Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

s.

P-bodies and microRNA

microRNA mediated repression occurs in two ways, either by translational repression or stimulating mRNA decay. miRNA recruit the RISC

RNA-induced silencing complex

RNA-Induced Silencing Complex, or RISC, is a multiprotein complex that incorporates one strand of a small interfering RNA or micro RNA . RISC uses the siRNA or miRNA as a template for recognizing complementary mRNA. When it finds a complementary strand, it activates RNase and cleaves the RNA...

complex to the mRNA to which they are bound. The link to P-bodies comes by the fact that many, if not most, of the proteins necessary for miRNA gene silencing are localized to P-bodies as covered by the recent review by Kulkarni et al (2010). These proteins include, but are not limited to, the scaffold protein GW182, Argonaute (Ago), decapping enzymes and RNA helicases.
The current evidence points toward P-bodies as being scaffolding centers of miRNA function, especially due to the evidence that a knock down of GW182 disrupts P-body formation. However, there remain many unanswered questions about P-bodies and their relationship to miRNA activity. Specifically, it is unknown whether there is a context dependent (stress state versus normal) specificity to the P-bodies mechanism of action. Based on the evidence that P-bodies sometimes are the site of mRNA decay and sometimes the mRNA can exit the P-bodies and re-initiated translation, the question remains of what controls this switch. Another ambiguous point to be addressed is whether the proteins that localize to P-bodies are actively functioning in the miRNA gene silencing process or whether they are merely holding at standby.

Further reading

• Kulkarni et. al. provide a review of P-bodies and a table of all proteins detected in the P-bodies as of 2010.