Most of the seminar was about AMPA receptor (AMPAR) trafficking, so a quick refresher. AMPAR are the most common ionotropic, excitatory glutamate receptor in the brain, and it is thought that learning and memory comes from changing the number of AMPAR in synapses, viz. synaptic plasticity. Some stimulation protocols can lead to an increase in AMPARs at the synapse, called long-term potentiation (LTP), while other stimulation protocols remove AMPAR from synapses (long-term depression, LTD). Whence and whither do the AMPAR go from the synapse? From early endosomes and to recycling endosomes. All of this transport is controlled by a large web of signaling proteins, including, for today, PI(3,4,5)P3-kinase (PI3K). PI3K activity has been shown to be involved in LTP, while PIP3 phosphatases are involved in LTD.
As mentioned above, AMPAR are trafficked through various endosomes. The endosomal targeting is controlled by, in part, Rab-family GTPases. Each Rab GTPase can target a given piece of membrane towards a specific endosome. The important Rab family members today are Rabs 4, 7, and 11, outlined below. (All figures were made by me, and represent my impressions from the talk. If I got something wrong, holla at me.)
|LTD (and thus AMPAR trafficking) is regulated by various Rab GTPases. During normal LTD, you get a decrease in synaptic current of ~50% (not to scale). With either a Rab4 or Rab11 DN, LTD is increased. With a Rab7-DN, LTD is decreased.|
To look more directly at lysosomal degradation, they performed cLTD in the presence of a protease inhibitor, leupeptin, which should block lysosomal degradation (Esteban said this was the first time this simple experiment was done.). However, leupeptin had no effect on the synaptic currents measured.
To look at the role of the lysosome another way, they transfected slices with Rab7-DN, which would prevent AMPAR from being transported to lysosomes. In Rab7-DN cells, cLTD was in fact decreased, in contrast to the lack of effect of leupeptin. From this they hypothesized that once internalized, AMPAR can go two ways: to the lysosome, or back to the plasma membrane. If you block the lysosomal pathway, they then must go back to the PM. However, this means that the actual degradation itself is inessential for LTD. As a final experiment, they transfected cells with Rab11-DN, and found that this increased LTD.
PTEN and Alzheimer's
In the middle section of the talk, Esteban described two sets of experiments he published previously. Arendt et al, 2010 covered PI3K's role in AMPAR trafficking during LTP, which I covered in the very first post on this blog o' science. Jurado, et. al., 2010 showed that the PIP3 phosphatase, PTEN, is essential to LTD.
In the last third of the talk, Esteban presented new work on Alzheimer's. One facet of Alzheimer's disease is that synaptic plasticity is altered, and in simplistic terms, may favor LTD over LTP, leading to a loss of spines. Since Esteban has been investigating PI3K and PTEN, he asked whether altering PTEN function could alleviate some of the symptoms of LTD.
Mice don't normally get Alzheimer's, so to generously give it to them, Esteban's lab transfected slices with amyloid precursor protein (APP). They then performed LTP on these slices, and found that APP almost completely abolished LTP. Then to see if PTEN could effect this, they bath applied a PTEN inhibitor, and found that it could partially rescue LTP.
I slacked in note-taking during the discussion, which is probably one of the more interesting parts of a talk for strangers. Here are two points I salvaged.
Esteban mentioned that PTEN, given its influence on PIP3, can effect a wide variety of systems besides simply effecting synaptic plasticity, so more work would need to be done to understand how this works. (Indeed, given PI3K is important for LTP, it is unclear to me whether this is a downregulation of LTD, or an indirect upregulation of LTP. In either case, the balance appears to be changed.)
Second, it is not clear why PTEN had no effect when applied alone. One might expect PIP3 to accumulate, which should effect something. Esteban noted that there are ubiquitous feedback systems, which could perhaps compensate for a loss of PTEN function.