Ronald Liem, PhD

Our laboratory studies the neuronal cytoskeleton and its relationship to neurodegenerative diseases. In the mammalian nervous system a variety of neuronal intermediate filament proteins are expressed differentially during development. Overexpression and mutations of the neuronal intermediate filament proteins leads to neuronal degeneration, showing that malfunctions in neuronal intermediate filaments can be responsible for neurodegenerative diseases. We have focused on (-internexin, a neuronal intermediate filament protein, which is expressed in neurons immediately after they start to differentiate. We have found that overexpression of this protein leads to axonal swellings and degeneration in the CNS. We are interested in delineating the mechanism by which this apparent block in transport of filaments occurs and how this leads to degeneration and neuronal cell death.

The diversity of neuronal intermediate filaments in different areas of the nervous system, point to the possibility that each of these proteins interacts specifically with other components of the cytoskeleton. Interactions between neuronal cytoskeletal elements have been shown by the identification of a neuronal form of dystonin, which has both an intermediate filament binding domain and an actin binding domain. Mutations in dystonin in mice result in severe sensory neuron defects. We have shown that dystonin specifically interacts with the neuronal intermediate filament protein, peripherin, which is found predominantly in sensory neurons. We have characterized another member of this family of proteins, ACF7, which has sequence similarities to both dystonin and dystrophin. Mutations in this protein might also lead to neurodegeneration. We expect that other related proteins are likely to be present in the nervous system that may cross-link neuronal intermediate filament proteins with other cytoskeletal elements.

One of the hallmarks of Alzheimer's Disease is the presence of paired helical filaments, which are composed of the microtubule associated protein, tau. Abnormal tau phosphorylation has been shown to be occur in Alzheimer's Disease. Recent studies have identified mutations in tau in a hereditary neurodegenerative disease, frontal temporal dementia with Parkinsonisn. We are determining how these mutations might destabilize microtubules in vitro and in vivo and whether these mutations would lead to the likelihood that tau is abnormally phosphorylated. By using transgenic models, we hope to determine how these mutations might lead to neurodegeration.

Selected Publications:

• Leung, C.L. and Liem, R.K.H. Characterization of interactions between the neurofilament triplet proteins by the yeast two-hybrid system. J. Biol. Chem. 271: 14041-14044 (1996).

• Sun, D., Leung, C.L. and Liem, R.K.H. Phosphorylation of the high molecular weight neurofilament protein (NF-H) by cdk5 and p35. J. Biol. Chem. 271: 14245-14251 (1996).

• Ching, G.Y. and Liem, R.K.H. Roles of the head and tail domains in (-internexin's self-assembly and coassembly with the neurofilament triplet proteins. J. Cell Sci. 111: 321-333 (1998).

• Ho, C.L., Martys, J.L., Mikhailov, A., Gundersen, G.G. and Liem R.K.H. Novel aspects of intermediate filament dynamics revealed by green fluorescent fusion proteins. J. Cell Sci. 111: 1767-1778 (1998).

• Leung, C.L., Flores, R. L. and Liem, R.K.H. The complexity of intermediate filaments in the nervous system. Subcellular Biochemistry, 31: Intermediate Filaments pp. 497-526. Herrmann and Harris eds. (1998).

• Leung, C.L., Sun, D. and Liem, R.K.H. The intermediate filament protein peripherin is the specific interaction partner of mouse BPAG1-n (dystonin) in neurons. J. Cell Biol. 144: 435-446 (1999).

• Ching, G.Y., Chien, C.,-L., Flores, R.G. and Liem, R.K.H. Overexpression of alpha-internexin causes abnormal neurofilamentous accumulations and motor coordination deficits in transgenic mice. J. Neurosci. 19: 2974-2986 (1999).