Formation of Hirano Bodies Induced by Expression of an Actin Cross-Linking Protein with a Gain-of-Function Mutation

Abstract

Hirano bodies are paracrystalline actin filament-containing structures reported to be associated with a variety of neurodegenerative diseases. However, the biological function of Hirano bodies remains poorly understood, since nearly all prior studies of these structures were done with postmortem samples of tissue. In the present study, we generated a full-length form of a Dictyostelium 34-kDa actin cross-linking protein with point mutations in the first putative EF hand, termed 34-kDa ⌬EF1. The 34-kDa ⌬EF1 protein binds calcium normally but has activated actin binding that is unregulated by calcium. The expression of the 34-kDa ⌬EF1 protein in Dictyostelium induces the formation of Hirano bodies, as assessed by both fluorescence microscopy and transmission electron microscopy. Dictyostelium cells bearing Hirano bodies grow normally, indicating that Hirano bodies are not associated with cell death and are not deleterious to cell growth. Moreover, the expression of the 34-kDa ⌬EF1 protein rescues the phenotypes of cells lacking the 34-kDa protein and cells lacking both the 34-kDa protein and ␣-actinin. Finally, the expression of the 34-kDa ⌬EF1 protein also initiates the formation of Hirano bodies in cultured mouse fibroblasts. These results show that the failure to regulate the activity and/or affinity of an actin cross-linking protein can provide a signal for the formation of Hirano bodies. More generally, the formation of Hirano bodies is a cellular response to or a consequence of aberrant function of the actin cytoskeleton.

Formation of Hirano Bodies after Inducible Expression of a Modified Form of an Actin-Cross-Linking Protein

“…1, row D). We observed a more uniform presence of inclusions in populations expressing the pDXA-CT-GFP system then in populations expressing the pCT-myc system previously reported (12 CT-GFP inducible promoter expression. The p-RNR plasmid contains a promoter inducible by DNA damage, which provides a positive regulation system.…”

“…To further understand the spatial and temporal events that surround the formation of these inclusions in vivo, a live cell model that mimics the formation of these structures is necessary. The discovery that Dictyostelium discoideum cells expressing a carboxy-terminal fragment (CT) of the 34-kDa calcium-sensitive actin-binding protein (ABP34) form Hirano bodies in vivo (1,12,13) provides a tantalizing clue to a possible mechanism of protein aggregation.…”

Cofilin expression induces cofilin-actin rod formation and disrupts synaptic structure and function in Aplysia synapses

“…Given these results, the rod-like structures observed in our study seem to be similar in some aspects to the protein inclusions such as Hirano bodies, because Hirano bodies are also mainly composed of actin and actin-associated proteins, including ADF and cofilin. Furthermore, Hirano bodies are not thought to affect cell survival because surviving neurons in neurodegenerative diseases have intact cell bodies despite synapse loss (11); and the induced formation of Hirano bodies in Dictyostelium and some other cell lines is not associated with cell death and does not significantly affect cell growth (32). The cause of Hirano-body formation has not been determined, but it appears to be related to the abnormal regulation of the actin cytoskeleton (11,32).…”

“…Furthermore, Hirano bodies are not thought to affect cell survival because surviving neurons in neurodegenerative diseases have intact cell bodies despite synapse loss (11); and the induced formation of Hirano bodies in Dictyostelium and some other cell lines is not associated with cell death and does not significantly affect cell growth (32). The cause of Hirano-body formation has not been determined, but it appears to be related to the abnormal regulation of the actin cytoskeleton (11,32). Taken together, the ADF͞cofilin-actin rod has been suggested to be a precursor of the Hirano body (11).…”

Dynamics of an F-actin aggresome generated by the actin-stabilizing toxin jasplakinolide

“…It is proposed that the formation of Hirano bodies is either a cellular response of the actin cytoskeleton or a consequence of its aberrant function . This postulate arises from the Hiranobody-like formation observed in Dictyostelium and mammalian cell lines by the expression of the C-terminal fragment of the Dictyostelium 34-kDa actin cross-linking protein Maselli et al, 2003;Davis et al, 2008). Mammalian cells containing model Hirano bodies showed normal growth, morphology and motility (Davis et al, 2008).…”