We generated four transgenic mice with a 34 kb genomic fragment including the complete human neurofiiament heavy (NF-H) gene. This human NF-H fragment contained ail regulatory elements for tissue-specific expression, and in two transgenic lines, human NF-H proteins were produced at levels up to%-fold the levels of endogenous mouse NF-H protein. By 3-4 months of age, these NF-H transgenics progressively develop neurological defects and abnormal neurofiiamentous swellings that are highly reminiscent of those found in amyotrophic lateral sclerosis (ALS). We propose that a modest up-regulation of NF-H cross-linkers can resuit in an impairment of neurofiiament transport, causing neuronai swellings with ensuing axonopathy and muscle atrophy, a mechanism of pathogenesis pertinent to the possible etiology of ALS. introduction

Neurons contain three major classes of cytoskeletal organelies: microtubules, neurofilaments, and microfilaments. Neurofiiaments are found predominantly in axons, where they run longitudinally and parallel to each other. In mature neurons, neurofiiaments are composed of three protein subunits, light (NF-L)(66 kd), medium (NF-M)(145 kd), and heavy (NF-H)(200 kd)(Hoffman and Lasek, 1975; Liem et al., 1976). During neurogenesis, there is adifferential expression of the three subunits. The NF-L and NF-M proteins are coexpressed during early embryonic development, while the activation of NF-H expression is delayed to the postnatal period (Shaw and Weber, 1962; Julien et al., 1966; Carden et al., 1967). The three subunits share with each other and with ail intermediate filament proteins a conserved central domain of approximately 310 amino acids that is involved in the formation of coiled-coil structures (Geisier et al., 1984; Frankeet al., 1967; Steinertand Roop, 1966). in vitro reassembly experiments showed that the NF-L subunit is able to assemble into homopoiymeric 10 nm filaments (Geisier and Weber, 1961; Liem and Hutchison, 1962; Hisanaga and Hirokawa, 1990). In contrast, the NF-M and NF-H self-assemble in vitro into short filament structures and require coassembly with NF-L for formation of intermediate filaments (Gardner et al., 1964; Hisanaga and Hirokawa, 1990; Hirokawa, 1991). A role of neurofiiaments in the control of axon caliber