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Dissociation of Axonal Neurofilament Content from Its Transport Rate.

Yuan A, Hassinger L, Rao MV, Julien JP, Miller CC, Nixon RA - PLoS ONE (2015)

Bottom Line: Accordingly, it may be predicted that NF content in axons can vary independently from the transport rate of NF.Moreover, knockout mice lacking NFH develop even more extreme (6-fold) proximal to distal variation in NF number, which is associated with a normal wild-type rate of NF transport.The independence of regional NF content and NF transport is consistent with previous evidence suggesting that the rate of incorporation of transported NF precursors into a metabolically stable stationary cytoskeletal network is the major determinant of axonal NF content, enabling the generation of the striking local variations in NF number seen along axons.

View Article: PubMed Central - PubMed

Affiliation: Center for Dementia Research, Nathan Kline Institute, Orangeburg, New York, United States of America; Department of Psychiatry, New York University School of Medicine, New York, New York, United States of America.

ABSTRACT
The axonal cytoskeleton of neurofilament (NF) is a long-lived network of fibrous elements believed to be a stationary structure maintained by a small pool of transported cytoskeletal precursors. Accordingly, it may be predicted that NF content in axons can vary independently from the transport rate of NF. In the present report, we confirm this prediction by showing that human NFH transgenic mice and transgenic mice expressing human NFL Ser55 (Asp) develop nearly identical abnormal patterns of NF accumulation and distribution in association with opposite changes in NF slow transport rates. We also show that the rate of NF transport in wild-type mice remains constant along a length of the optic axon where NF content varies 3-fold. Moreover, knockout mice lacking NFH develop even more extreme (6-fold) proximal to distal variation in NF number, which is associated with a normal wild-type rate of NF transport. The independence of regional NF content and NF transport is consistent with previous evidence suggesting that the rate of incorporation of transported NF precursors into a metabolically stable stationary cytoskeletal network is the major determinant of axonal NF content, enabling the generation of the striking local variations in NF number seen along axons.

No MeSH data available.


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NFM transport rate remains constant along optic axons except the first segment.Contradicting a critical assumption of the Li et al. mathematical model, NF transport rates determined at 7 (A), 14 (B), 21 (C), 42 days (D) post injection as in Fig 4, show that NFM peak advance is constant along the optic axons [for example, 0.14 ± 0.02 mm/day (Mean ± SEM, n = 19) at 7 days; 0.14 ± 0.01 mm/day (Mean ± SEM, n = 14) at 14 days] (E) until the wave exits the optic window (A-E). (F) Percentage of NFM radioactivity (in dpm) in optic nerve (segments 1–4) relative to total NFM dmp in optic pathway (segments 1–8) in long-term labeling studies from two independent analyses [7, 9]. The constant percentage in optic nerve at 75-to-192 days indicates no net movement of NF containing labeled NFM. Total number of mice analyzed in both studies equals 262–302. Gray bar, Nixon and Logvinenko, 1986; black bar, Yuan et al. 2009.
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pone.0133848.g008: NFM transport rate remains constant along optic axons except the first segment.Contradicting a critical assumption of the Li et al. mathematical model, NF transport rates determined at 7 (A), 14 (B), 21 (C), 42 days (D) post injection as in Fig 4, show that NFM peak advance is constant along the optic axons [for example, 0.14 ± 0.02 mm/day (Mean ± SEM, n = 19) at 7 days; 0.14 ± 0.01 mm/day (Mean ± SEM, n = 14) at 14 days] (E) until the wave exits the optic window (A-E). (F) Percentage of NFM radioactivity (in dpm) in optic nerve (segments 1–4) relative to total NFM dmp in optic pathway (segments 1–8) in long-term labeling studies from two independent analyses [7, 9]. The constant percentage in optic nerve at 75-to-192 days indicates no net movement of NF containing labeled NFM. Total number of mice analyzed in both studies equals 262–302. Gray bar, Nixon and Logvinenko, 1986; black bar, Yuan et al. 2009.

Mentions: Previously, investigators of axonal transport have used either the actual radiolabeled NF peak to calculate NF transport rate or the position of the “50th percentile” of NF radioactivity as an imaginary NF transport peak to calculate transport rate. The latter method is less accurate because it does not take into account the radiolabeled NF proteins moved out of the analyzed window or the population of NF that have incorporated into the stationary NF network. Moreover, the most proximal unmyelinated portion of myelinated retinal ganglion cell (RGC) axons (“initial segment”) varies in length and the largest proportion of RGC has very short initial segments close to the retinal excavation. Because the axon initial segment may contain ribosomes [33] capable of translation of proteins and a uniquely high density of specific voltage-gated channels [33–35], we measured NF transport rates specifically along the myelinated 7mm length of the optic nerve and tract in WT mice to avoid influences from this unique portion of the optic axons. Supporting the atypical apparent transport rate in the axon initial segment, we observed that the radiolabeled NFM peak appeared within the first 1mm from the eye along optic axons a few hours after intravitreal injection of radiolabeled methionine and remained there for a few days with only the front of labeled NFM moving along the axons. If we calculated NFM transport rate from the cell body and including the axon initial segment as in some previous studies [36, 37], it appeared that NFM transport slows down along the axons (Fig 7C). By contrast, once radiolabeled NF moved out of the first segment, the NFM peak moved at a constant rate (Fig 7B) of 0.14 ± 0.02 mm per day (Mean ± SEM, n = 19) along the entire length of the optic axonal window until it reached the end of this window after 7 weeks (Fig 8A–8E). We also observed an increase of NFM phosphorylation along the optic pathway in a proximal-to-distal manner using 2-dimensional gel electrophoresis (Fig 9).


Dissociation of Axonal Neurofilament Content from Its Transport Rate.

Yuan A, Hassinger L, Rao MV, Julien JP, Miller CC, Nixon RA - PLoS ONE (2015)

NFM transport rate remains constant along optic axons except the first segment.Contradicting a critical assumption of the Li et al. mathematical model, NF transport rates determined at 7 (A), 14 (B), 21 (C), 42 days (D) post injection as in Fig 4, show that NFM peak advance is constant along the optic axons [for example, 0.14 ± 0.02 mm/day (Mean ± SEM, n = 19) at 7 days; 0.14 ± 0.01 mm/day (Mean ± SEM, n = 14) at 14 days] (E) until the wave exits the optic window (A-E). (F) Percentage of NFM radioactivity (in dpm) in optic nerve (segments 1–4) relative to total NFM dmp in optic pathway (segments 1–8) in long-term labeling studies from two independent analyses [7, 9]. The constant percentage in optic nerve at 75-to-192 days indicates no net movement of NF containing labeled NFM. Total number of mice analyzed in both studies equals 262–302. Gray bar, Nixon and Logvinenko, 1986; black bar, Yuan et al. 2009.
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pone.0133848.g008: NFM transport rate remains constant along optic axons except the first segment.Contradicting a critical assumption of the Li et al. mathematical model, NF transport rates determined at 7 (A), 14 (B), 21 (C), 42 days (D) post injection as in Fig 4, show that NFM peak advance is constant along the optic axons [for example, 0.14 ± 0.02 mm/day (Mean ± SEM, n = 19) at 7 days; 0.14 ± 0.01 mm/day (Mean ± SEM, n = 14) at 14 days] (E) until the wave exits the optic window (A-E). (F) Percentage of NFM radioactivity (in dpm) in optic nerve (segments 1–4) relative to total NFM dmp in optic pathway (segments 1–8) in long-term labeling studies from two independent analyses [7, 9]. The constant percentage in optic nerve at 75-to-192 days indicates no net movement of NF containing labeled NFM. Total number of mice analyzed in both studies equals 262–302. Gray bar, Nixon and Logvinenko, 1986; black bar, Yuan et al. 2009.
Mentions: Previously, investigators of axonal transport have used either the actual radiolabeled NF peak to calculate NF transport rate or the position of the “50th percentile” of NF radioactivity as an imaginary NF transport peak to calculate transport rate. The latter method is less accurate because it does not take into account the radiolabeled NF proteins moved out of the analyzed window or the population of NF that have incorporated into the stationary NF network. Moreover, the most proximal unmyelinated portion of myelinated retinal ganglion cell (RGC) axons (“initial segment”) varies in length and the largest proportion of RGC has very short initial segments close to the retinal excavation. Because the axon initial segment may contain ribosomes [33] capable of translation of proteins and a uniquely high density of specific voltage-gated channels [33–35], we measured NF transport rates specifically along the myelinated 7mm length of the optic nerve and tract in WT mice to avoid influences from this unique portion of the optic axons. Supporting the atypical apparent transport rate in the axon initial segment, we observed that the radiolabeled NFM peak appeared within the first 1mm from the eye along optic axons a few hours after intravitreal injection of radiolabeled methionine and remained there for a few days with only the front of labeled NFM moving along the axons. If we calculated NFM transport rate from the cell body and including the axon initial segment as in some previous studies [36, 37], it appeared that NFM transport slows down along the axons (Fig 7C). By contrast, once radiolabeled NF moved out of the first segment, the NFM peak moved at a constant rate (Fig 7B) of 0.14 ± 0.02 mm per day (Mean ± SEM, n = 19) along the entire length of the optic axonal window until it reached the end of this window after 7 weeks (Fig 8A–8E). We also observed an increase of NFM phosphorylation along the optic pathway in a proximal-to-distal manner using 2-dimensional gel electrophoresis (Fig 9).

Bottom Line: Accordingly, it may be predicted that NF content in axons can vary independently from the transport rate of NF.Moreover, knockout mice lacking NFH develop even more extreme (6-fold) proximal to distal variation in NF number, which is associated with a normal wild-type rate of NF transport.The independence of regional NF content and NF transport is consistent with previous evidence suggesting that the rate of incorporation of transported NF precursors into a metabolically stable stationary cytoskeletal network is the major determinant of axonal NF content, enabling the generation of the striking local variations in NF number seen along axons.

View Article: PubMed Central - PubMed

Affiliation: Center for Dementia Research, Nathan Kline Institute, Orangeburg, New York, United States of America; Department of Psychiatry, New York University School of Medicine, New York, New York, United States of America.

ABSTRACT
The axonal cytoskeleton of neurofilament (NF) is a long-lived network of fibrous elements believed to be a stationary structure maintained by a small pool of transported cytoskeletal precursors. Accordingly, it may be predicted that NF content in axons can vary independently from the transport rate of NF. In the present report, we confirm this prediction by showing that human NFH transgenic mice and transgenic mice expressing human NFL Ser55 (Asp) develop nearly identical abnormal patterns of NF accumulation and distribution in association with opposite changes in NF slow transport rates. We also show that the rate of NF transport in wild-type mice remains constant along a length of the optic axon where NF content varies 3-fold. Moreover, knockout mice lacking NFH develop even more extreme (6-fold) proximal to distal variation in NF number, which is associated with a normal wild-type rate of NF transport. The independence of regional NF content and NF transport is consistent with previous evidence suggesting that the rate of incorporation of transported NF precursors into a metabolically stable stationary cytoskeletal network is the major determinant of axonal NF content, enabling the generation of the striking local variations in NF number seen along axons.

No MeSH data available.


Related in: MedlinePlus