Spatially regulated editing of genetic information within a neuron

In eukaryotic cells, with the exception of the specialized genomes of mitochondria and plastids, all genetic information is sequestered within the nucleus. This arrangement imposes constraints on how the information can be tailored for different cellular regions, particularly in cells with complex morphologies like neurons. Although messenger RNAs (mRNAs), and the proteins that they encode, can be differentially sorted between cellular regions, the information itself does not change. RNA editing by adenosine deamination can alter the genome’s blueprint by recoding mRNAs; however, this process too is thought to be restricted to the nucleus. In this work, we show that ADAR2 (adenosine deaminase that acts on RNA), an RNA editing enzyme, is expressed outside of the nucleus in squid neurons. Furthermore, purified axoplasm exhibits adenosine-to-inosine activity and can specifically edit adenosines in a known substrate. Finally, a transcriptome-wide analysis of RNA editing reveals that tens of thousands of editing sites (>70% of all sites) are edited more extensively in the squid giant axon than in its cell bodies. These results indicate that within a neuron RNA editing can recode genetic information in a region-specific manner.

Sir Andrew Fielding Huxley OM. 22 November 1917—30 May 2012

Andrew Huxley was a physiologist, possessing a combination of practical skill, invention and mathematical ability that has few parallels, and is famous for his contributions to the understanding of how nerve and muscle work. He came from an illustrious family: his paternal grandfather was Thomas Henry Huxley, and Julian Huxley and Aldous Huxley were his half-brothers. After completing his undergraduate degree in Cambridge in 1939, he joined Alan Hodgkin in his research on the squid giant axon and they made the first intracellular recording of a nerve action potential. Together, they used the voltage clamp technique to elucidate the ionic mechanism of the action potential; for this they were awarded a share of the Nobel Prize in Physiology or Medicine in 1963. Turning to research on muscle, he elucidated the sliding filament mechanism of contraction (in parallel with Hugh Huxley), and went on to formulate a quantitative account of the steady state properties of muscle based on the interaction between myosin crossbridges and actin sites. Moving to University College London (UCL) in 1960 as Jodrell Professor of Physiology, he provided quantitative evidence for the sliding filament theory through a study of the length–tension relation. His final studies were on the transient mechanical properties of muscle, resulting in a theory of force-generation by crossbridges. During the war, Huxley was engaged in operational research on anti-aircraft artillery and naval gunnery. He was President of the Royal Society in 1980–85, and Master of Trinity College Cambridge in 1984–90.

Cellular function given parametric variation in the Hodgkin and Huxley model of excitability

How is reliable physiological function maintained in cells despite considerable variability in the values of key parameters of multiple interacting processes that govern that function? Here, we use the classic Hodgkin–Huxley formulation of the squid giant axon action potential to propose a possible approach to this problem. Although the full Hodgkin–Huxley model is very sensitive to fluctuations that independently occur in its many parameters, the outcome is in fact determined by simple combinations of these parameters along two physiological dimensions: structural and kinetic (denoted S and K, respectively). Structural parameters describe the properties of the cell, including its capacitance and the densities of its ion channels. Kinetic parameters are those that describe the opening and closing of the voltage-dependent conductances. The impacts of parametric fluctuations on the dynamics of the system—seemingly complex in the high-dimensional representation of the Hodgkin–Huxley model—are tractable when examined within the S–K plane. We demonstrate that slow inactivation, a ubiquitous activity-dependent feature of ionic channels, is a powerful local homeostatic control mechanism that stabilizes excitability amid changes in structural and kinetic parameters.

The role of gigaxonin in the degradation of the glial-specific intermediate filament protein GFAP

Alexander disease (AxD) is a primary genetic disorder of astrocytes caused by dominant mutations in the gene encoding the intermediate filament (IF) protein GFAP. This disease is characterized by excessive accumulation of GFAP, known as Rosenthal fibers, within astrocytes. Abnormal GFAP aggregation also occurs in giant axon neuropathy (GAN), which is caused by recessive mutations in the gene encoding gigaxonin. Given that one of the functions of gigaxonin is to facilitate proteasomal degradation of several IF proteins, we sought to determine whether gigaxonin is involved in the degradation of GFAP. Using a lentiviral transduction system, we demonstrated that gigaxonin levels influence the degradation of GFAP in primary astrocytes and in cell lines that express this IF protein. Gigaxonin was similarly involved in the degradation of some but not all AxD-associated GFAP mutants. In addition, gigaxonin directly bound to GFAP, and inhibition of proteasome reversed the clearance of GFAP in cells achieved by overexpressing gigaxonin. These studies identify gigaxonin as an important factor that targets GFAP for degradation through the proteasome pathway. Our findings provide a critical foundation for future studies aimed at reducing or reversing pathological accumulation of GFAP as a potential therapeutic strategy for AxD and related diseases.