Role of MAP1B in axonal retrograde transport of mitochondria

Abstract

The MAPs (microtubule-associated proteins) MAP1B and tau are well known for binding to microtubules and stabilizing these structures. An additional role for MAPs has emerged recently where they appear to participate in the regulation of transport of cargos on the microtubules found in axons. In this role, tau has been associated with the regulation of anterograde axonal transport. We now report that MAP1B is associated with the regulation of retrograde axonal transport of mitochondria. This finding potentially provides precise control of axonal transport by MAPs at several levels: controlling the anterograde or retrograde direction of transport depending on the type of MAP involved, controlling the speed of transport and controlling the stability of the microtubule tracks upon which transport occurs.

Figure 1. Characteristics of wild-type, tau^−/− and MAP1B^−/− cultured neurons

(A) Western-blot analyses of brain extracts derived from control (wild-type), MAP1B^−/− and tau^−/− mice. Samples were analysed with antibodies recognizing MAP1B (125) and tau (7.51) proteins. For the loading control, anti-tubulin antibody was used. (B) Double immunofluorescence analysed by confocal microscopy with an antibody recognizing specific axonal proteins. Hippocampal neurons from wild-type (WT) and MAP1B^−/− mice were incubated with anti-tau1 antibody, and hippocampal neurons from tau^−/− mice were incubated with anti-neurofilament antibody SMI31. All of the neurons were stained with an antibody recognizing tubulin protein. Scale bar, 50 μm. (C) The range of axon calibre of hippocampal neurons (n=10) is between 0.74 and 0.94 μm for wild-type (WT) mice, between 0.56 and 0.72 μm for tau^−/− mice and between 0.72 and 0.89 μm for MAP1B^−/− mice. (D) The range of axon length of hippocampal neurons (n=10) is between 8.2 and 11.8 μm for wild-type (WT) neurons, between 7.5 and 8.1 μm for tau^−/− mice and between 3.2 and 4.8 μm for MAP1B^−/− mice.

Figure 2. Mitochondrial localization in wild-type, tau^−/− and MAP1B^−/− neurons

(A) Hippocampal neurons from wild-type (WT), tau^−/− and MAP1B^−/− mice analysed by double immunofluorescence showing tubulin and MitoTracker distribution. Scale bar, 50 μm. (B) Immunofluorescence intensity (from MitoTracker results) at distances of the cell soma up to 100 μm (wild-type) or 75 μm (tau^−/− and MAP1B^−/−) are indicated. a.u., arbitrary units.

Figure 3. Retrograde mitochondrial transport in hippocampal neurons

The movement of a single mitochondrion, at different times, is indicated for neurons from wild-type mice (A), tau^−/− mice (B) and MAP1B^−/− mice (C), in a long axon and in a short axon. Sample images from a video recording are shown. By looking at this video, it is possible to indicate that we were looking at the same (single) mitochondrion.

Figure 4. Anterograde and retrograde mitochondrial movements in neurons from wild-type, tau^−/− and MAP1B^−/− mice

(A) Anterograde and retrograde velocity distribution of mitochondrial movement in neurons from wild-type (WT), tau^−/− and MAP1B^−/− mice. (B) Relative (Rel.) frequency of anterograde and retrograde mitochondrial movements in neurons from wild-type (WT), tau^−/− and MAP1B^−/− mice. Different numbers of mitochondria (40 mitochondria for wild-type, 60 for tau^−/− and 44 for MAP1B^−/− mice) were analysed. Data are summarized in Table 1.