A possible mechanism of action of the neurotoxic agent iminodipropionitrile (IDPN): a selective aggregation of the medium and heavy neurofilament polypeptides (NF-M and NF-H)

Relative susceptibility of cytoskeleton-associated and soluble neurofilament subunits to aluminum exposure in intact cells. A possible mechanism for reduction of neurofilament axonal transport during aluminum neurotoxicity

Previous studies have demonstrated the appearance of phosphorylated neurofilament (NF) subunits within perikaryal cytoskeletons following aluminum exposure. In order to examine the mechanisms leading to this altered distribution of NF subunits, we carried out biochemical analyses of NF subunits in Triton-insoluble and -soluble fractions derived from aluminum-treated NB2a/d1 cells. In addition to increases in the Triton-insoluble cytoskeleton, increases in all three NF subunits were also detected within the Triton-soluble fraction of aluminum-treated cells. To address the nature of this increase in Triton-soluble subunits, aluminum-treated and untreated cultures were harvested in the absence of Triton and fractionated by established procedures to yield fractions greatly enriched for perikarya and neurites, respectively. Each of these subcellular fractions was then subjected to further homogenization in the presence of 1% Triton and centrifugation to yield Triton-insoluble cytoskeletons and Triton-soluble material derived from perikarya and axonal neurites, respectively. Resulting Triton-soluble fractions were "clarified" by high-speed centrifugation to eliminate oligomeric assemblies or soluble neurofilaments. Immunoblot analysis demonstrated quantitative recovery of the aluminum-induced increase in Triton-soluble NF subunits in the perikaryal fraction. Additional aluminum-treated and untreated cultures were pulse-chase radiolabeled with [35S]methionine and fractionated into Triton-insoluble and soluble fractions from isolated perikarya and axonal neurites. Autoradiographic analysis of immunoprecipitated NF subunits revealed that aluminum treatment delayed the translocation of newly synthesized subunits into neurites and resulted in the accumulation of radiolabeled subunits within the Triton-soluble fraction of perikarya. These findings suggest that aluminum may exert a relatively greater effect on NF subunits that have not yet undergone axonal transport and/or incorporation into Triton-insoluble structures vs those that have already deposited into axons. This possibility was supported by the observation that a higher concentration of aluminum was required to alter the electrophoretic migration of in vitro reassembled neurofilaments vs that required for unassembled NF subunits. These findings provide possible mechanisms for the accumulation of NF subunits in perikarya during aluminum intoxication.

Experimental beta beta'-iminodipropionitrile (IDPN) neuropathy: neurofilament profile of sensory, motor and autonomic nerves as seen by immunocytochemistry on whole-mount preparations

IDPN-induced changes in a variety of sensory, motor and autonomic nerves were studied by whole-mount immunocytochemistry. A full range of proximo-distal accumulations of neurofilament-like material was found, from paranuclear round bodies in perikarya to distal and preterminal axonal dilations. Conversely, both terminal areas and nodal-paranodal regions of myelinated axons showed striking, sharply localized loss of neurofilament-immunostaining. The latter change, when transport of neurofilaments is halted by IDPN, may indicate their local processing and/or differential transport at nodal-paranodal regions.

Ubiquitin and neurofilament expression in anterior horn cells in amyotrophic lateral sclerosis: possible clues to the pathogenesis

Cytoskeletal abnormalities are a prominent pathological feature of anterior horn cells in amyotrophic lateral sclerosis (ALS), and are thought to be involved in the process of motor neuron death. Skein-like filamentous inclusions have been detected by immunocytochemical staining for ubiquitin, a stress protein involved in targeting abnormal proteins for proteolysis. So far, identification of the target protein has been elusive. We have studied the ultrastructural localization of ubiquitin and neurofilaments by post-embedding immunogold staining. In skein-like arrays, strong ubiquitin labelling was concentrated on abnormally formed 15-20 nm filaments; neurofilament labelling was localized on 10 nm filaments adjacent or in continuity with the abnormal filaments. In addition, Bunina bodies were a major site of ubiquitin accumulation. Our results suggest that ubiquitinated filaments in skein-like inclusions might originate from abnormally aggregated neurofilament proteins, which are no longer recognized by antibodies to neurofilament epitopes. Furthermore, the presence of ubiquitin in Bunina bodies suggests that, in addition to its protective role, ubiquitin might be directly implicated in the mechanism of programmed neuronal death in ALS.

Effects of selective disruption of cytoskeletal elements on steroid secretion by human adrenocortical slices

The role of the cytoskeleton in corticosteroid secretion in normal human adrenal gland was investigated in vitro, using the perifusion technique and confocal laser scanning microscopy. Vinblastine, which selectively disrupted microtubules in adrenocortical cells, did not modify the basal release of cortisol but induced a 58% inhibition of the response to adrenocorticotropic hormone (ACTH). In contrast, vinblastine did not alter dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP)-induced steroid secretion. Cytochalasin B treatment caused disappearance of microfilaments and blocked the stimulatory action of ACTH and DBcAMP on cortisol secretion. beta,beta'-Iminodipropionitrile disorganized the cytokeratin intermediate filaments but did not alter spontaneous and ACTH-evoked cortisol release. These results, which provide the first evidence for an action of cytoskeleton inhibitors on cortisol release from normal human adrenocortical cells, show that microtubules are involved in the mechanism of action of ACTH at a step preceding adenosine 3',5'-cyclic monophosphate formation, whereas microfilaments are involved in a late and common step of adrenal steroidogenesis.

Neurofilament mRNA is reduced in Parkinson's disease substantia nigra pars compacta neurons

Lewy bodies are filamentous neuronal inclusions characteristic of Parkinson's disease, and neurofilament triplet proteins are the major components of the filaments in Lewy bodies. Since the neurofilament proteins found in Lewy bodies are abnormally phosphorylated and partially degraded, the formation of Lewy bodies may be due to the defective metabolism of these proteins, and this could lead to impairments in the structure and function of neurofilament rich neuronal processes (i.e., large caliber axons). To gain further insights into the metabolism of neurofilaments in Parkinson's disease, we evaluated neurofilament mRNA levels by semi-quantitative in situ hybridization histochemistry in postmortem tissues from Parkinson's disease and control subjects. Substantia nigra pars compacta neurons were examined with digoxigenin-UTP labeled cRNA probes to the heavy and light neurofilament mRNAs. The relative abundance of these mRNAs was measured by videodensitometric image analysis of chromogenic reaction product. Using this approach, we demonstrated that the levels of both heavy and light neurofilament mRNAs were reduced in Parkinson's disease substantia nigra pars compacta neurons. Additionally, the levels of heavy neurofilament mRNA were lowest in Lewy body containing neurons in the Parkinson's disease cases. These results suggest that the formation of neurofilament-rich Lewy bodies in substantia nigra pars compacta neurons is associated with reduced levels of the heavy and light neurofilament mRNAs in Parkinson's disease. Thus, it is possible that the accumulation of abnormal neurofilament proteins in Lewy bodies and diminished neurofilament mRNAs contribute to the degeneration of substantia nigra pars compacta neurons in Parkinson's disease.