γ-Diketone neuropathy: axon atrophy and the role of cytoskeletal protein adduction
Introduction
Giant neurofilamentous swelling of large myelinated axons in the PNS and CNS has been historically considered the hallmark lesion induced by the industrial hexacarbon chemicals, n-hexane and methyl n-butyl ketone (MBK), and by their common active γ-diketone metabolite, 2,5-hexanedione (HD; Couri and Milks, 1982, Krasavage et al., 1980, Spencer et al., 1980a, Spencer et al., 1980b). On the basis of an assumed neurotoxicological importance, giant axonal swellings have been the focus of most mechanistic research conducted over the past 30 years (reviewed in DeCaprio, 1987, DeCaprio, 2000, Graham et al., 1991, Graham et al., 1995, LoPachin and Lehning, 1997, Sayre et al., 1985, Spencer et al., 1980a, Spencer et al., 1980b). Axon atrophy, however, has also been identified as a morphological feature of γ-diketone neuropathy (e.g., see Lehning et al., 2000, Monaco et al., 1985, Yagi, 1994). Although it was assumed by early investigators that loss of axon caliber was secondary to proximal accumulation of neurofilaments (NFs) in giant swellings Brown et al., 1978, Spencer and Schaumburg, 1977a, Spencer et al., 1980a, more recent quantitative morphometric studies have demonstrated that these axonal lesions are independent phenomena Lehning et al., 1995, Lehning et al., 2000, LoPachin et al., 2003a. Quantitative studies have also shown that axonal atrophy was a prevalent effect that occurred during the early stages of γ-diketone intoxication over a wide range of daily dose-rates (100–400 mg/kg/day). In contrast, axonal swellings were scarce in both PNS and CNS, and their expression was restricted to intoxication at lower dose-rates (i.e., 100–250 mg/kg/day; Lehning et al., 1995, Lehning et al., 2000, LoPachin et al., 2003a). Together, these observations suggest that axon atrophy is a relevant component of the pathophysiological process that leads to neurological toxicity, whereas neurofilamentous swelling is of unclear neuropathogenic significance (LoPachin and Lehning, 1997).
The molecular mechanism by which HD causes either axon atrophy or swelling is not understood. However, it is known that NF triplet proteins are an important determinant of axon caliber Friede and Samorajski, 1970, Hoffman et al., 1988, Muma and Hoffman, 1993 and that HD reacts with these cytoskeletal proteins (e.g., DeCaprio et al., 1982, Graham et al., 1982). Indeed, research conducted over the past 20 years has demonstrated that HD interacts with ε-amine groups on lysine residues of NFs and other proteins to form N-substituted 2,5-dimethylpyrrole adducts (e.g., DeCaprio et al., 1982). This pyrrole-forming reaction is now considered to be the first step in γ-diketone neuropathy. Work by Graham et al., 1991, Graham et al., 1995 has suggested that once formed, pyrrole adducts can undergo secondary oxidative reactions that yield cross-linked proteins. This research has lead to the hypothesis that protein cross-linking is an important pathophysiological step in the neurotoxic mechanism of γ-diketone action. Whereas additional molecular details are needed (e.g., extent of cross-linking vs. pyrrole formation, location of adducted lysine residues), research to date suggests that adduction of NF proteins is involved in γ-diketone-induced alterations in axon diameter (atrophy or swelling).
The purpose of this commentary is to present evidence supporting axonal atrophy as the primary morphological lesion associated with γ-diketone neuropathy. The functional consequences of atrophy and how ensuing dysfunction might be related to the development of neurological deficits are also discussed. In this commentary, we will consider the possibility that HD adduction of cytoskeletal proteins plays a mechanistic role in axon atrophy. We begin with a background discussion of the classic morphological and neurological changes induced by γ-diketone exposure. This will form a basis for developing the thesis that axon atrophy and resulting neurophysiological dysfunction are necessary events in γ-diketone neurotoxicity.
Section snippets
Classical concepts of γ-diketone neurotoxicity: neurological deficits and morphological lesions
Long-term, low dose-rate exposure or short-term, high dose-rate intoxication of laboratory animals with HD produces decreases in body weight and changes in several neurological parameters including gait abnormalities (ataxia) and reductions in hindlimb skeletal muscle strength Jortner and Ehrich, 1993, LoPachin et al., 2002a, Shell et al., 1992, Spencer and Schaumburg, 1977a. Several lines of evidence suggest that induction of neurological toxicity by HD conforms in principle to Haber's Rule
Giant neurofilamentous axonal swellings
Neurofilamentous swelling is the presumed morphological manifestation of disrupted molecular processes in affected axons of γ-diketone-intoxicated animals or humans (reviewed in DeCaprio, 1985, DeCaprio, 1987, DeCaprio, 2000, Graham et al., 1991, Graham et al., 1995). It has been assumed that the focal increase in caliber associated with the swelling promotes axonal dysfunction and eventual degeneration Spencer et al., 1980a, Spencer et al., 1980b. Consequently, neurofilamentous swellings have
NF protein turnover as a determinant of axon caliber
In the mature neuron, axon caliber is correlated with cross-sectional neurofilament (NF) protein content, which is regulated by perikaryal gene expression/synthesis and by axonal triplet protein turnover Friede and Samorajski, 1970, Hoffman et al., 1988, Muma and Hoffman, 1993, Sakaguchi et al., 1993. NFs are composed of three subunits that assemble as obligate heteropolymers: light NF (NF-L, 68–70 kDa), midsize NF (NF-M; 140–160 kDa), and heavy NF (NF-H; 190–200 kDa; reviewed in Muma and
Mechanisms of γ-diketone axon atrophy
It is not known how HD adduction of NF subunits or possibly other cytoskeletal proteins might produce axon atrophy. Previous studies of PNS and CNS from HD-intoxicated animals have revealed significant reductions in NF subunit protein contents Carden et al., 1986, Chiu et al., 2000, DeCaprio and O'Neill, 1985, Karlsson et al., 1991, Lapadula et al., 1986, Lapadula et al., 1988, LoPachin et al., 2003a, LoPachin et al., 2003b, LoPachin et al., 2003c, Watson et al., 1991. These observations, in
Conclusions
The evidence presented in this review suggests that a redefinition of γ-diketone neuropathy is necessary. Clearly, the labels “giant neurofilamentous axonopathy” or “distal axonopathy” are misleading, and therefore, future nosological schemes should take into consideration the apparent primacy of axon atrophy. Determining the relative neurotoxic risk for different chemicals that have industrial, agricultural, or household application is, in part, dependent upon assessment of classic distal
Acknowledgements
This research was supported by NIH Grants from NIEHS to R.M.L. (RO1 ES07912-07) and to A.P.D. (RO1 ES05172).
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2015, Neuroscience LettersCitation Excerpt :Both parent compounds are used in fabric manufacturing and have been associated with several human outbreaks of neuropathy following subchronic occupational exposure [1]. Early studies of HD and ACR neurotoxicity were based on the premise that distal axon regions were sites of toxicant action and that axonopathy was the pathognomonic outcome of a specific mechanism; e.g., inhibition of axolemmal Na pumps [3,4]. Because axonal swellings and degeneration were assumed to be causally related to the onset of neurotoxicity, substantial effort was devoted to deciphering the respective mechanisms [5,6].