Rho zero cells

Well, this post is about rho zero (ρ°) cells. TLDR: It's even more obscure than usual.

This is the basic ETC plus the ATP:ADP antiporter (ANT) and the Pi:H+ symporter (Slc25a3) added:

Most of this is very obvious but it's worth pointing out that ANT exchanges one ATP outwards with 4 negative charges for an ADP inwards which has 3 negative charges. The ADP needs an inorganic phosphate to reform ATP and this Pi carries one negative charge and enters the mitochondria via Slc25a3, facilitated by consuming one proton of the proton gradient. All is hunky dory with electrical balance, accepting some delta psi consumption.

ρ° cells are man made constructs which have no mitochondrial DNA, usually deleted by exposure to ethidium bromide. They live by glycolysis and need supplementary pyruvate and uridine to survive. They have no electron transport chain proteins because they lack core components needed to form complexes I, III, IV and the F0 (membrane) component of their F0F1 ATP synthase.

They do still form "petit" mitochondria. The F1 component of ATP synthase is present and it works. ANT and Slc25a3 are present and functional. There is CoQ, which is permanently reduced because there is nowhere for it to hand its electrons on to... A number of other cellular processes are also blocked, those which need to reduce CoQ to CoQH2 to occur. From

Restoration of electron transport without proton pumping in mammalian mitochondria

we have:

The really strange thing is that ρ° cells have a mitochondrial membrane potential and a proton gradient. This is what happens:

ATP which has been made in the cytoplasm enters the mitochondria via ANT running in reverse. The F1 component of the ATP synthase breaks down the ATP to ADP and Pi. ADP is exchanged outwards via the ANT antiporter and Pi is carried outwards in combination with a proton via the Slc25a3 symporter. This proton flux maintains the proton gradient across the inner mitochondria membrane, all of this process is being powered by glycolytic ATP synthesis.

I became interested in ρ° cells because the are so strange. But there are some practical things they tell us too. There's a venerable mini review here:

Cells depleted of mitochondrial DNA (ρ°) yield insight into physiological mechanisms

They cannot perform reverse electron transport through complex I, because there is no complex I. So no superoxide. Equally, there is none from complex III either. Clearly this has implications for what type of apoptosis they can perform and how they sense oxygen tension but more interestingly you can make ρ° versions of pancreatic beta cells.

These can't secrete insulin.

Back in the 1990s no one was thinking about RET as being essential to insulin secretion but they were pretty sure the process was based around mitochondria as well as needing glycolysis. In pancreatic beta cells glycolysis specifically inputs to the ETC at mtG3Pdh in large amounts, which will generate RET and the superoxide needed for insulin secretion. This occurs in other cells as part of insulin responsiveness, but not to the same degree as in the beta cells.

Placing some functional mitochondria in to ρ° beta cells restores insulin secretion ability.

The review suggests mtG3Pdh in beta cells acts as a sensor for cytoplasmic NADH levels. That's a nice idea. Just struck me as interesting.

Peter