ADF/Cofilin-actin rods in neurodegenerative diseases

Abstract

Dephosphorylation (activation) of cofilin, an actin binding protein, is stimulated by initiators of neuronal dysfunction and degeneration including oxidative stress, excitotoxic glutamate, ischemia, and soluble forms of beta-amyloid peptide (Abeta). Hyperactive cofilin forms rod-shaped cofilin-saturated actin filament bundles (rods). Other proteins are recruited to rods but are not necessary for rod formation. Neuronal cytoplasmic rods accumulate within neurites where they disrupt synaptic function and are a likely cause of synaptic loss without neuronal loss, as occurs early in dementias. Different rod-inducing stimuli target distinct neuronal populations within the hippocampus. Rods form rapidly, often in tandem arrays, in response to stress. They accumulate phosphorylated tau that immunostains for epitopes present in "striated neuropil threads," characteristic of tau pathology in Alzheimer disease (AD) brain. Thus, rods might aid in further tau modifications or assembly into paired helical filaments, the major component of neurofibrillary tangles (NFTs). Rods can occlude neurites and block vesicle transport. Some rod-inducing treatments cause an increase in secreted Abeta. Thus rods may mediate the loss of synapses, production of excess Abeta, and formation of NFTs, all of the pathological hallmarks of AD. Cofilin-actin rods also form within the nucleus of heat-shocked neurons and are cleared from cells expressing wild type huntingtin protein but not in cells expressing mutant or silenced huntingtin, suggesting a role for nuclear rods in Huntington disease (HD). As an early event in the neurodegenerative cascade, rod formation is an ideal target for therapeutic intervention that might be useful in treatment of many different neurological diseases.

Figure 1

Summary of signaling pathways discussed in text for stress-induced cofilin-activation leading to rod formation.

Figure 2

Accumulation of (B) phosphorylated tau (12E8 antibody staining) at (A) cofilin-actin rods in antimycin A-treated hippocampal neurons. (C) Overlay of images. E18 rat hippocampal neurons were cultured 5 days and then treated with antimycin A for 30 min. Cells were fixed with 4% formaldehyde for 45 min and permeabilized for 90 s in 0.05% Triton X-100. Higher levels of Triton X-100 or treatment for longer periods reduces immunostaining of cofilin in rods. Methanol permeabilization, which is ideal for cofilin-actin rod immunostaining, destroys the 12E8 epitope. Bar=10 μm.

Figure 3

Nuclear ADF/cofilin-actin rods induced in neurons by 10% DMSO. (A–D) Time-course of rod formation after addition of 10% DMSO. A=0 time; B=15 min; C=45 min; D=90 min. Cells were fixed in 4% paraformaldehyde for 30 min, permeabilized with −20° methanol for 3 min and stained for ADF/cofilin with affinity purified rabbit 1439 antibody [99]. Bar for A–D= 10 μm. Transmission electron micrographs of sections through the cell body of hippocampal neurons that were untreated (E) or treated 90 min with 10% DMSO (F). Rods in longitudinal section and cross-section are present around the nucleus but are also within white box. Bar for E and F= 1 μm. (G) Nuclear rod from another cell that was stained with immunogold for ADF/cofilin. Virtually all of the ADF/cofilin immunogold staining lies over rods. Bar=0.1 μm.

Figure 4

Ribbon structures of human cofilin with space filling models of the peptides of the non F-actin-binding face used for studies on disruption of rod formation. (A) View of molecule from the normal actin binding surface. (B) View of molecule showing exposed peptides on non-actin binding interface. Peptides shown are: S8-V20 (red), R21-K30 (yellow), C39-N46 (green), N46-D59 (cyan) and K73-R81 (magenta).