This is somewhat of a followup post.  What's really cool about this paper (to Sci, anyway), is that it brings two different areas that she's been interested in into one cool glob of SCIENCE.  And it helps to explain many of the questions that Sci got in response to two of the papers she has blogged about recently. 
They are these:
1) The Incredible Healing Mouse: Bedelbeava et al. " Lack of p21 expression links cell cycle control and appendage regeneration in mice" Proceeding of the National Academy of Sciences, 2010. 
2) The neurogenesis theory of depression and a little guy called CREB: Gur et al. " cAMP Response Element-Binding Protein Deficiency Allows for Increased Neurogenesis and a Rapid Onset of Antidepressant Response" The Journal of Neuroscience, 2007. 
And NOW, behold their MUTANT OFFSPRING:

Pechnick et al. " p21 restricts neuronal proliferation in the subgranular zone of the dentate gyrus of the hippocampus" PNAS, 2008. 
Well ok, technically it isn't a mutant offspring, because this paper was BEFORE the first paper and after the second.  So I guess it's a stepchild.  Or a sibling.  Or just the results of how Sci was searching PubMed that day. 
Anyway. 
Let's start with some background. 

We'll start with the players:
1) The subgranular zone of the hippocampus.  First, the hippocampus is a seahorse-shaped (apparently it's seahorse shaped.  Sci always thought it looked wiggly, and at one end it looks a little like a squished cinnamon roll) area of your brain which scientists currently know to be involved in things like learning and memory.  The subgranular zone of the hippocampus is an area where new neurons are born (which we call neurogenesis).  Yes, new neurons.  All the way through adulthood.  It's one of the few areas of the brain (that we know of) which can make sure neurons.  The new neurons are born, and migrate to other areas of the hippocampus, where they form part of the circuitry there. 
2) p21.  p21 is a cell cycle regulator protein.  Here's your basic cell cycle:

p21 regulates the cell cycle at the G1 phase, right before the cell doubles its DNA in preparation for eventually undergoing mitosis.  Basically, p21 is a gatekeeper. 

(p21 is the black knight of your cells, except effective)
If you have high levels of p21, the cell is prevented from going forward in the cell cycle.  If you have LOW levels of p21, the cell moves forward. 
If you read the previous stuff (is too much to tell you, Sci will sum up), neurogenesis in the hippocampus is one of the current theories for how antidepressants may work.  Basically, it is thought that some people with depression have decreased neurogenesis in the hippocampus in this subgranular zone (whether the decreased neurogenesis is a causation, or just a correlation with depressive symptoms with a different underlying cause, we don't know).  When you give animals antidepressants for a long time (like you would to humans), animals have INCREASED neurogenesis in the hippocampus, which goes along with measures for antidepressant efficacy. 
So that's neurogenesis.  What about p21?  Well, in the OTHER paper, there was this mouse (the MRL mouse) that had this incredible regenerating property.  It could regenerate damaged tissue.  This sort of thing involves increased cell proliferation.  Increased cell proliferation, in these mice, meant that p21, the cell cycle regulator that STOPs cells from going forward in the cell cycle, is LOW, so the cells go on through, and the mice heal. 
And what the scientists in this paper did was look at those mice and say "hey, if p21 can increase cell proliferation, and we need cell proliferation in the hippocampus...wait a minnite..."
And they took a mutant mouse.  This mouse is the p21-knockout mouse.  It doesn't express p21.  Which means that its cell cycle is not well regulated.  If p21 is what is regulating healing in these mice, they should heal very well.  And it turns out that they do.  So the question becomes, if these guys can grow cells so well in their ears and their limbs...what about in the BRAIN?! 
First, the authors wants to check out whether p21 is around in the brain normally.  If p21 is going to be around in the brain, it's going to be expressed in cells that are not interested in dividing, right?  So it makes sense to look for it in newborn cells, which have just gone through it all are presumably should not be interested in gettin' it on again any time soon.  Sure enough, when they looked, p21 was expressed in the hippocampus, and especially among newborn neurons.  And it makes some pretty pretty pictures. 

Up above there you can see the subgranular zone of the hippocampus, stained for p21 protein (the green) and DNA (the blue).  They wanted to look for DNA to confirm that the p21 was being expressed in the nucleus (since it regulates whether or not the cell proceeds to DNA replication, that's the place it should be). 
The authors then looked to see what happened in the hippocampus when they knocked out the gene for p21.  They used the p21 knockout mouse which I mentioned had regenerating properties.  And they found that, when you knocked out p21, they got INCREASED neurogenesis in the hippocampus. 

You can see there labeling for BrdU, which labels baby cells.  You can see that the knockout mice (p21 -/-) had higher levels of labeling for BrdU, which indicates that they have more baby cells around. 
So now we know that p21 is linked to neurogenesis in the hippocampus.  What about antidepressants? 
So they took normal mice and treated them with an antidepressant (Imipramine, it's very commonly used in the lab, it's a tricyclic antidepressant, though, and isn't used too much in the clinic anymore because there are a lot of side effects) for 21 days, to simulate the chronic treatment that humans go through before they feel any effects.  They they looked for p21 and for neurogenesis.  They found that Imipramine increases neurogenesis (as Sci would have expected, all classes of antidepressants increase neurogenesis in rodents), and it ALSO decreased p21! 

Up there you can see panel B and panel F. Panel B is p21 normally, and panel F is after Imipramine treatment.  You can see that p21 glows a lot less. 
Not only that, the authors provided a behavioral correlation, too.  Sci has mentioned several times before the Forced Swim Test, which is a test used in rodents to look at antidepressant efficacy.  The decreased p21 following imipramine treatment correlated with changes in the forced swim test in these mice. 
So what do we take away from this? 
1) antidepressants can decrease p21
2) decreased p21 can increase neurogenesis in the hippocampus. 
3) Increased neurogenesis in the hippocampus can have antidepressant effects. 
4) So MAYBE decreasing p21 itself can increase neurogenesis in the hippocampus and provide a mechanism for antidepressant effects of the drugs you see around you every day. 
Or maybe not.  Sci's isn't calling it until she sees the paper showing that decreasing p21 has antidepressant effects.  But it's a cool paper, and brings together the other two papers Sci mentioned before.  It's a great big glob of Science!Pechnick, R., Zonis, S., Wawrowsky, K., Pourmorady, J., & Chesnokova, V. (2008).  p21Cip1 restricts neuronal proliferation in the subgranular zone of the dentate gyrus of the hippocampus Proceedings of the National Academy of Sciences, 105 (4), 1358-1363 DOI: 10.1073/pnas.0711030105

Wow...that is an old X-men clip; it has The Beast before he went blue! 
Great post as always, can't really add much to the content as its now been almost two years since I covered any animal biology - been spending too much time with the plants and bacteria. 

I sincerely hope this is a goer!  Now I'm on a monster dose of anti-depressant and I feel better than I've felt for more than 6 years, I can be further heartened by the fact that my memory ought to improve, since my hippocampus is growing new bits!  Nice connections, Sci!  Your hippocampus must be working well most of the time! 

Dr Becca: actually, p21 KO mice are a false positive in the FST, like MRL mice, they just never stop struggling.  So I mean, that looks great, but it's also where the FST breaks down in terms of predictability, are they "anti-depressed" (if we can ever say that) or are they just wicked hyperactive? 

Ah, OK.  Yeah, the FST is a tricky test in terms of what you're actually measuring besides the effectiveness of anti-depressants.  I'm teaching a 1st-year grad seminar this spring (I heart my 1st years!),  and we were discussing the theory that stopping struggling in the FST could indicate higher adaptability, energy conservation, etc, rather than "depression" (and likewise, longer struggling could mean the animal is inflexible, perseverating, etc).  I would guess this is a discussion you've had at some point as well--any thoughts? 

Dr. Becca: Yeah, Sci's opinion on this may be long winded.  Here we go.  My problem with the FST and TST is that a lot of people tend to over interpret them.  The ONLY thing an FST or TST can really tell you is whether a drug is going to make a decent antidepressant right off the bat.  It MAY be able to provide some indication of which neurotransmitter systems are activated (swimming vs climbing may indicate serotonin vs norepinephrine), but other than that, we really can't say anything.  We certainly can't say that the mouse is "depressed", "less depressed", "tired", or anything else really, because a mouse can't tell us that.  The FST and TST have great predictive validity, but I personally really dislike having to interpret the tests any further than that. 

Most simple but convincing explanation of the p21 gene as far as I have seen.  I only have but one question.  What will the (complete) removal of the p21 gene in the human body accomplish in medical progress?