Thursday, July 11, 2013

Throw another adipocyte on the fire

Humans are able to live in so many different climates, in a wide range of temperatures and yet our inner core body temperature remains nearly constant.  This ability to thermoregulate has something to do, of course, with clothing and the ability to cool and heat our living spaces, but our bodies also offer many adaptations to regulate body temperature.  If it’s too hot, we sweat, releasing excess heat through evaporative cooling.  If it’s too cold, we shiver, producing heat in our working muscles.  The production of heat through physiological mechanisms is called thermogenesis and also includes a non-shivering version.  Today’s paper is about non-shivering thermogenesis, which is when our fat cells produce heat.

Non-shivering thermogenesis
To understand how non-shivering thermogenesis works, we need to take a step back and discuss cellular respiration.  The cells of our body store energy from food in the chemical bonds of a molecule called ATP.  During cellular respiration, a cell will convert glucose or fat into carbon dioxide, while slowly tapping into the energy in those food molecules in order to make ATP.  The final step of cellular respiration is that the energy from the electrons in glucose are passed from protein to protein, releasing energy that is used to pump protons into a membrane-bound cellular space.  You can think of these protons as a form of potential energy, like stuffing a closet full of balls.  When you open up the closet door, all the balls come tumbling out, releasing their potential energy in the process.  During cellular respiration, this potential energy is used by an enzyme to make ATP.  During non-shivering thermogenesis, though, the potential energy stored in all those protons stuffed into a small space is released by the cell as heat.  Thus, the energy from food is used to heat the body rather than being stored in ATP.

The main type of cell that does non-shivering thermogenesis is brown adipocytes, or fat cells.  Brown fat is very common in infants, but is also found in adult humans in the upper chest and neck.  The purpose of brown fat is to provide heat for the body.  Thus, non-shivering thermogenesis is activated by a drop in body temperature.  The cold temperature is sensed by the brain, which activates the sympathetic nervous system (the “fight or flight” response), which signals to the brown fat cells to express the genes necessary to bypass ATP production and release heat instead.  In a recent paper published in PNAS, Ye et al. describe how a different type of fat cell is able to skip all the nervous system steps and sense the cold directly (red arrow in diagram).  It is pretty cool that the fat cells are able to sense temperature, as if they were neurons, and can act autonomously to remedy the situation.  No need for a brain here!

Independent thermogenesis
Through a series of experiments, the authors demonstrate that a particular type of fat cell will express genes necessary for non-shivering thermogenesis when exposed to cold, independent of sympathetic nervous system activation.

In one experiment, they grew fat cells at different temperatures and measured gene expression using a technique called quantitative PCR (qPCR).  The idea behind this technique is that if a gene is highly expressed, there will be a lot of mRNA in the cell (remember the “central dogma” of molecular biology) and qPCR is a method for measuring the concentration of mRNA for a particular gene.  They focused their measurements on thermogenic genes that are known to be part of the non-shivering thermogenesis mechanism, such as Ucp1, which is the enzyme that actually allows the protons to fall back across the membrane, thereby releasing their energy as heat.  They found that these fat cells that were exposed to the cold expressed more Ucp1 mRNA, even in the absence of any nervous system.  These are just cells in a dish, so this must be an intrinsic property of fat cells.

It wasn’t just any fat cell that had this response.  In fact, brown adipocytes did not express more Ucp1 in the cold.  It was a different type of fat cell called a white adipocyte.  What is white fat?  The majority of fat in our body is white fat and its purpose is to store fat for energy (for cellular respiration) and to act as a thermal insulator, so we don’t lose as much heat through our skin.  There is one subtype of white fat that has been shown to do non-shivering thermogenesis and it was this type that could express thermogenic genes, like Ucp1, in the cold, independent of the nervous system.

Okay, so these white fat cells don’t need input from the nervous system, but do they still use the same intracellular pathway to turn on expression of these genes?  Normally, when a fat cell is activated by the sympathetic nervous system, it sets off a molecular cascade of events inside the cell, which involves activation of molecules in a pathway called the cAMP pathway (as shown in the diagram).  The authors inhibited this pathway in various ways and found that the cells could still respond to the cold as before, so this effect must use a different pathway.

There are still a number of open questions, such as: how do fat cells sense temperature?  Do they use the same types of receptors as temperature-sensitive neurons?  Why are some white fat cells independent, but brown fat cells need the nervous system to activate thermogenesis?  One thing that is clear, however, is that white fat cells are clearly important for temperature regulation as well as fat storage.  The authors suggest that tapping into thermogenesis might be a good way to help obese patients get rid of excess energy storage by releasing it as heat.  This pathway that is independent of the sympathetic nervous system could allow medications to target only the fat cells without involving the sympathetic nervous system which controls so many other functions in the body.

Something to think about as the cold Bay Area summer sets in.