Autophagy – A Fighting Chance for Our Cells

Corresponding author Dr. Beth Levine. Picture used with permission from Dr. Beth Levine.

Study:​​ Identification of a Candidate Therapeutic Autophagy-Inducing Peptide

Authors:​ Sanae Shoji-Kawata, Rhea Sumpter Jr., MatthewLeveno, Grant R. Campbell, Zhongji Zou, Lisa Kinch, Angela D. Wilkins, Qihua Sun, Kathrin Pallauf, Donna MacDuff, Carlos Huerta, Herbert W. Virgin, J. Bernd Helms, RuudEerland, Sharon A. Tooze, Ramnik Xavier, Deborah J.Lenschow, Ai Yamamoto, David King, Olivier Lichtarge, Nick V. Grishin, Stephen A. Spector, Dora V. Kaloyanova & Beth Levine

Source:​​ Nature 2013 January, Advance Online Publication - http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11866.html

Background:

​            Though modern civilization allows us to lead relatively stable lives, our cells are continuously bombarded with hundreds of threats. These can come from bacteria, viruses, radiation, or even physiologic failures like plaque buildup (eg. Alzheimer’s) or miss-folded proteins (eg. Huntington’s or Prion’s Dx). So how do our cells cope? Like martyr’s, they can undergo apoptosis and “take one for the team” in order to ensure that we, as an organism, survive. If the damage is even greater (note: apoptosis still requires most of the mechanisms to remain functional), the cells can just burst in a process called necrosis. However, can you imagine if every insult to the cell resulted in its death? The cell loss would enormously exceed turnover capacity and the entire system would be unviable.

​            Thankfully, cells can fight off intracellular organisms or digest away protein residues, thereby recycling the contents. This process is known as Autophagy. However, like every physiological process, it has a limit. In fact, the threshold for an intracellular infection can be precisely defined as the point when the deleterious organic load exceeds the cell’s power to combat it. Some common infections – HIV, Listeria, West Nile virus, etc. – and even normal aging can reduce the cell’s capacity of autophagy, causing serious problems. Alternatively, diseases – Huntington’s, Alzheimer’s, etc. – can increase organic load and, once again, tip the scale. But what if you could increase autophagy and give each cell a fighting chance?

The Paper:

​            The role of autophagy in infection and disease, along with the proposition that increasing this process may curb symptoms, has been identified in scientific literature for a while now. However, we have yet to gain an ability to alter and take advantage of this natural cellular mechanism. In this paper, first author Dr. Shoji-Kawata and corresponding author Dr. Levine systematically present their findings about Tat-beclin1, a synthesized molecule they show to have autophagy-inducing effects.

​            Note: I am definitely notan expert on the autophagy pathway and could not even understand 90% of the charts from a google.image search. Even so, I realized there are two simple facts in common scientific knowledge that helped me when starting to read the paper: 1. Beclin1 is single-handedly responsible for activating phagolysosomes, thereby triggering autophagy, and 2. HIV alters this protein to inhibit being eaten intracellularly.

​            The first question the researchers asked was: how does HIV function to reduce autophagy? They showed that it carries a Nef factor that binds to Beclin1 at a specific location, amino acids 267-284. (By the way, if you were like me and wondering why not to just give Beclin1 to induce autophagy, it’s because the protein is about 450 amino acids long and would not cross the plasma membrane barrier.) They further noticed that the same site was required for binding and activating phagolysosomes. Proceeding to the next step, the scientists synthesized a soluble molecule that composed of two parts: 1.a part of HIV (Tat) to increase intracellular penetrationand 2.the identified sequence. They showed that this Tat-Beclin1 was similar in binding to the HIV Nef protein and was, without any pleiotropic effects, able to induce autophagy. (In fact, there was a 27-fold difference observed in number of phagolysosomes when compared to the control.)

​            As always in science, each discovery leads to more questions – this one brought to mind the possible mechanism of Tat-Beclin1. It turns out that the broader question is:if Beclin1 is always present in the cell, why is the cell not constantly undergoing autophagy? The researchers showed that a protein called GAPR1 sequesters Beclin1 in the golgi and prevents its actions. (Note: All these names definitely got confusing to me at first so I made a quick chart; I have attached it in Figure 1 for your convenience). This led them to prove that Tat-Beclin1is actually binding to GAPR1 and “inhibiting the inhibitor” – thereby freeing up Beclin1 to enable autophagy.

A Hypothesis:

​            This part was not explicitly stated in the paper, and there are possibly thousands of reasons why it may be wrong, but I just wanted to play the guessing game a bit. The researchers have showed three things: 1. Under normal conditions, GAPR1 binds Beclin1 and sequesters it in the golgi, 2. Tat-Beclin1 binds GAPR1 and makes Beclin1 free to enter cytoplasm and initiate autophagy, and 3. highly increased concentrations of GAPR1 (when its connected to the constitutively expressed myc-gene) lead to a decreased efficiency of Tat-Beclin1. Though the researchers are cautious and claim many possible mechanisms may exist, I think the most probable one is simple competitive binding.

Significance:

​            The significance of this finding is pretty much self-explanatory from the introduction – if autophagy can be induced, it can give affected cells a fighting chance to prolong their health and longevity. Dr. Levine and colleagues already point to preliminary results that hold substantial promise – “The autophagy-inducing activity of the peptide [Tat-Beclin1] was associated with clearance of small polyglutamine expansion protein aggregates, reduced titers of several positive-stranded RNA viruses, decreased intracellular survival of the bacterium, L. monocytogenes, inhibition of HIV-1 replication in human macrophages, and a reduction in the mortality of neonatal mice infected with CHIKV and WNV.”

Only time will tell if this discovery shares a Nobel Prize or turns out to be an overly hopeful experiment. Regardless, the fact that I even consider it possible to reach that stage shows how truly significant I think it is. I want to sincerely thank Dr. Levine’s lab and collaborators for extending our frontiers and wish them all the very best in their upcoming projects. Surely years of research on Tat-Beclin1 and similar peptides lies ahead of us, but we eagerly await its entrance into clinical trials and everyday medicine.

Figure 1: An oversimplified representation of the pathway in an attempt to understand the role of Beclin1. Note: this is neither verified by the authors nor any other scientific source. If you find inconsistencies, please let me know and I will immediately change the figure.