Alpha-internexin, a novel neuronal intermediate filament protein, precedes the low molecular weight neurofilament protein (NF-L) in the developing rat brain

Abstract

alpha-Internexin is a 66 kDa protein that copurifies with intermediate filaments (IF) from rat spinal cord and optic nerve. This protein is axonally transported in rat optic nerve along with the neurofilament triplet proteins in slow component a. Polymerization in vitro and distribution in vivo confirm that alpha-internexin is a neuronal IF. We raised 2 highly specific monoclonal antibodies to alpha-internexin which were applied to frozen rat brain sections and Western blots of cytoskeletal extracts. These results indicate that alpha-internexin is primarily an axonal protein found in most, if not all, neurons of the CNS. Immunoreactive proteins of similar molecular weight were found in cytoskeletal extracts of CNS tissue from several additional species, including mouse and cow. While the distribution of alpha-internexin as given by immunocytochemical methods is similar to that of low molecular weight neurofilament protein (NF-L) in the adult, its distribution in the embryo is far more extensive. At embryonic day 16, when the expression of NF-L is still limited to a relatively small number of cells and levels of expression are low, alpha-internexin is already found at much higher levels and in cells not yet expressing NF-L in detectable quantities. Similar results are found at embryonic day 12. These data suggest that neuronal IF in the developing nervous system contain a higher proportion of alpha-internexin than their adult counterparts, and that expression of alpha-internexin precedes that of NF-L in many or most neurons of the developing brain.

?-Internexin aggregates are abundant in neuronal intermediate filament inclusion disease (NIFID) but rare in other neurodegenerative diseases

“…α-Internexin is normally differentially expressed in discrete neuronal populations: higher levels of expression have been reported in neurons with small caliber axons such as layer II neurons, cerebellar granule and basket cells [13,22]. The widespread distribution of inclusions throughout the neocortex, subcortical nuclei, brainstem and spinal cord, in NIFID does not coincide with the populations of neurons that have generally higher levels of expression of this protein.…”

“…The observation that α-internexin is a major component of the neuronal IF inclusions of NIFID is surprising because α-internexin is expressed at relatively low levels in adult brain in comparison to the NF triplet proteins [22]. α-Internexin is normally differentially expressed in discrete neuronal populations: higher levels of expression have been reported in neurons with small caliber axons such as layer II neurons, cerebellar granule and basket cells [13,22].…”

“…Neuronal loss and microvacuolation in the superficial layers, especially layer II, were seen to varying degrees in all cases of NIFID. As α-internexin is a neurodevelopmentally regulated protein and precedes the NF triplet proteins in axon growth [13,22], increased expression of α-internexin in vulnerable neurons may be a regenerative response. Increased expression or failure of axonal transport of α-internexin in vulnerable and degenerating neurons may contribute to the formation of pathological intracytoplasmic inclusions in this disease [9].…”

“…The gene for α-internexin is located on chromosome 10 and its transcript is a 499-amino acid protein with a molecular weight of 55.4 kDa and an apparent molecular mass of 66 kDa. The protein is expressed by most, if not all, neurons as they commence differentiation and its expression precedes that of the NF triplet proteins [22]. In the adult brain, α-internexin is expressed at relatively low levels in comparison to the NF proteins, and there is selective anatomical expression with greater immunoreactivity being seen in the cerebellar granule cells, the source of thin-caliber parallel fibers [13], and in the neuron cell bodies and processes of cortical layer II neurons.…”

Expression of neuron-specific tubulin defines a novel population in the proliferative layers of the developing telencephalon

“…This area of defascicu- Additional neuronal features of the TuJl-positive cells of the proliferative zones If the TuJl-positive cells of the proliferative layers are indeed neurons, then it is reasonable to expect that they express other neuron-specific molecules. However, no other neuron-specific marker tested, including antibodies to neurofilaments, has been localized to the cells of the cortical ventricular and subventricular zones (Schmechel and Marangos, 1983;Edwards et al, 1989;Kaplan et al, 1990). Nonetheless, an antibody against the highly polysialylated form of the neural cell adhesion molecule (NCAM-H), associated with migrating neurons, displayed a staining pattern in the rat embryonic cerebral cortex that partially matched the distribution of the TuJl-positive cell population at the border between the ventricular and subventricular zones (Seki and Arai,199 1).…”