Studying live cells must be the ultimate for physiologists. Imagine seeing the inner workings of a cell instead of static, stained snap shots of processes over some period of time. It’s like seeing a favorite book whose scenes you’ve played out in your head come alive on a movie screen.
The lab of Erika Holzbaur, PhD, has been studying molecular motors -- proteins that function as tiny machines within a cell -- for almost two decades. For the last few years they have been using live-cell imaging to get a better handle on what happens when the transport of cellular cargo goes off track, and how that may be the start of certain neurodegenerative diseases.
Autophagy, literally self-eating, is an essential cellular process for recycling proteins and organelles. It’s complicated and involves the formation of sacs called autophagosomes that engulf the proteins to be degraded in one part of the cell; the proteins’ transport along microtubule tracks by the molecular motor dynein to another area, and the degrading of proteins into basic amino acids in a mature sac called an autolysosomes. Mislocation of the protein tau – the cross ties of the microtubule railroad on which the trash sacs move -- in some neurodegenerative diseases may disrupt normal nourishment and waste removal in nerve cells.
In a recent Journal of Cell Biology study Holzbaur, a professor of Physiology, postdoctoral researcher Sandra Maday, Ph.D., and Karen E. Wallace, all from the Perelman School of Medicine, examined autophagosomes in neurons from transgenic mice reared with a handy florescent green biomarker. These neurons, when grown in culture, send out axon-like projections, which grew 1 mm in two days, making it easier for the team to record movies of the sacs moving along the projection.
The cargoes of the autophagosomes include SOD1, a primary disease protein of ALS, or Lou Gehrig’s disease. The team saw the sacs form and engulf cargo at the end of the projections farthest from the nucleus, and mature into the degradative autolysosomes as they moved toward the cell body. The autolysosomes also become increasingly acidic as they move along the axon, most likely to aid in more efficient degradation.
What they recorded was surprising, says Holzbaur. They thought they would see more autophagosomes being made -- a greater number of trash cans, so to speak -- since they engineered some neurons to contain mutant SOD1 proteins. But, they saw the opposite – no difference in the number of autophagosomes made and moving in control mouse neurons versus SOD1 mouse neurons.
It’s possible, they suggest, that what they observed is because autophagy may not be efficiently upregulated in their animal model of neurodegenerative disease. However, their overall observation of a live cell may provide insight into the neuron-specific cell death caused by many mutations that are ubiquitously expressed in the body.
Maday S, Wallace KE, & Holzbaur EL (2012). Autophagosomes initiate distally and mature during transport toward the cell soma in primary neurons. The Journal of cell biology, 196 (4), 407-17 PMID: 22331844