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Spatiotemporal dynamics of lesion-induced axonal sprouting and its relation to functional architecture of the cerebellum

View Article: PubMed Central - PubMed

ABSTRACT

Neurodegenerative lesions induce sprouting of new collaterals from surviving axons, but the extent to which this form of axonal remodelling alters brain functional structure remains unclear. To understand how collateral sprouting proceeds in the adult brain, we imaged post-lesion sprouting of cerebellar climbing fibres (CFs) in mice using in vivo time-lapse microscopy. Here we show that newly sprouted CF collaterals innervate multiple Purkinje cells (PCs) over several months, with most innervations emerging at 3–4 weeks post lesion. Simultaneous imaging of cerebellar functional structure reveals that surviving CFs similarly innervate functionally relevant and non-relevant PCs, but have more synaptic area on PCs near the collateral origin than on distant PCs. These results suggest that newly sprouted axon collaterals do not preferentially innervate functionally relevant postsynaptic targets. Nonetheless, the spatial gradient of collateral innervation might help to loosely maintain functional synaptic circuits if functionally relevant neurons are clustered in the lesioned area.

No MeSH data available.


Pattern of post-lesion CF collateral sprouting in vivo.(a) A representative example of in vivo time-lapse images of the same surviving CF and its traces for the time points mentioned at the top right of the CF images. Maximum projections (top-down view) of the CF in the molecular layer are shown. Solid red lines indicate the mediolateral extent of the CF at each time point while the dashed red line indicates the mediolateral boundary from the previous time point that expanded in the current time point. Magenta arrows indicate ladders categorized as outside additions while green arrows indicate ladders categorized as inside additions. A total of nine surviving CFs were imaged from the four animals and traced completely as shown in this example. Additional examples are shown in Supplementary Fig. 2. Scale bar, 10 μm. (b) Average number of ladders added (±s.e.m.) at each time point (n=4 animals). We observed 101 new ladders emerging from the 9 surviving, parent CFs out of which 48 were categorized as outside and 53 as inside. The pattern of inside addition (that is, number of ladders added to the inside) were significantly different over time (one-way repeated measures analysis of variance with Tukey post hoc analysis: F(6,18)=3.604; P=0.01; *P<0.05 time point 4 compared with time points 2, 5 and 13+). Inset shows total ladders added (±s.e.m.) at each time point.
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f2: Pattern of post-lesion CF collateral sprouting in vivo.(a) A representative example of in vivo time-lapse images of the same surviving CF and its traces for the time points mentioned at the top right of the CF images. Maximum projections (top-down view) of the CF in the molecular layer are shown. Solid red lines indicate the mediolateral extent of the CF at each time point while the dashed red line indicates the mediolateral boundary from the previous time point that expanded in the current time point. Magenta arrows indicate ladders categorized as outside additions while green arrows indicate ladders categorized as inside additions. A total of nine surviving CFs were imaged from the four animals and traced completely as shown in this example. Additional examples are shown in Supplementary Fig. 2. Scale bar, 10 μm. (b) Average number of ladders added (±s.e.m.) at each time point (n=4 animals). We observed 101 new ladders emerging from the 9 surviving, parent CFs out of which 48 were categorized as outside and 53 as inside. The pattern of inside addition (that is, number of ladders added to the inside) were significantly different over time (one-way repeated measures analysis of variance with Tukey post hoc analysis: F(6,18)=3.604; P=0.01; *P<0.05 time point 4 compared with time points 2, 5 and 13+). Inset shows total ladders added (±s.e.m.) at each time point.

Mentions: Post-lesion CF collateral sprouting in the paravermal region of lobule VI and VII was repeatedly imaged in vivo from 1 week up to 13 weeks after olivary injection of 3-AP (n=4 animals). CFs that emerged from new collaterals had a ladder-like appearance similar to their parent CF (Fig. 2a and Supplementary Fig. 2), indicating that they were innervating dendrites of nearby denervated PCs. Consistent with previous reports1123, new CF collaterals always sprouted laterally, expanding the innervation territory of the parent CF only in the mediolateral extent. Since functional zones are oriented sagittally, laterally growing CF collaterals may cross the boundary between functional zones. To quantify the temporal pattern of this mediolateral collateral sprouting, we categorized the newly added CF ladders as outside or inside based on their location of addition when compared with the previous imaging time point. Outside addition refers to the expansion of the mediolateral extent of the parent CF territory, whereas inside addition refers to further innervation of PCs within territory that the CF has already covered (Fig. 2a and Supplementary Fig. 2). Individual, surviving CFs showed variable patterns of inside/outside addition, but they showed a clear temporal profile as a population. Both outside and inside additions continued over several months (Fig. 2b). The rate of outside addition was relatively constant (Fig. 2b, magenta), suggesting that the boundary of functional zones does not limit the mediolateral expansion of the parent CF territory. Inside additions, on the other hand, were more time-dependent since significantly more CF ladders were added inside the mediolateral extents of parent CFs at 4 weeks after 3-AP injection than at any other time points (Fig. 2b, green). Amongst all the 101 ladders analysed, nearly half the ladders (49) were added in the first 4 weeks. Out of these 49 ladders, we could reliably measure the distance between the new ladders and the parent CF for 30 ladders. Out of these 30 ladders, nearly all (27) were within 45 μm of the parent CF. Since the diameter of a mature PC soma is about 15–20 μm, 45 μm is equivalent to the length of only a few PC somata contacted side by side. These results indicate that although the parent CF continues to expand its mediolateral territory over an extended period of time, the majority of new innervations resulting in this mediolateral expansion occurs in the first month and is near the parent CF.


Spatiotemporal dynamics of lesion-induced axonal sprouting and its relation to functional architecture of the cerebellum
Pattern of post-lesion CF collateral sprouting in vivo.(a) A representative example of in vivo time-lapse images of the same surviving CF and its traces for the time points mentioned at the top right of the CF images. Maximum projections (top-down view) of the CF in the molecular layer are shown. Solid red lines indicate the mediolateral extent of the CF at each time point while the dashed red line indicates the mediolateral boundary from the previous time point that expanded in the current time point. Magenta arrows indicate ladders categorized as outside additions while green arrows indicate ladders categorized as inside additions. A total of nine surviving CFs were imaged from the four animals and traced completely as shown in this example. Additional examples are shown in Supplementary Fig. 2. Scale bar, 10 μm. (b) Average number of ladders added (±s.e.m.) at each time point (n=4 animals). We observed 101 new ladders emerging from the 9 surviving, parent CFs out of which 48 were categorized as outside and 53 as inside. The pattern of inside addition (that is, number of ladders added to the inside) were significantly different over time (one-way repeated measures analysis of variance with Tukey post hoc analysis: F(6,18)=3.604; P=0.01; *P<0.05 time point 4 compared with time points 2, 5 and 13+). Inset shows total ladders added (±s.e.m.) at each time point.
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f2: Pattern of post-lesion CF collateral sprouting in vivo.(a) A representative example of in vivo time-lapse images of the same surviving CF and its traces for the time points mentioned at the top right of the CF images. Maximum projections (top-down view) of the CF in the molecular layer are shown. Solid red lines indicate the mediolateral extent of the CF at each time point while the dashed red line indicates the mediolateral boundary from the previous time point that expanded in the current time point. Magenta arrows indicate ladders categorized as outside additions while green arrows indicate ladders categorized as inside additions. A total of nine surviving CFs were imaged from the four animals and traced completely as shown in this example. Additional examples are shown in Supplementary Fig. 2. Scale bar, 10 μm. (b) Average number of ladders added (±s.e.m.) at each time point (n=4 animals). We observed 101 new ladders emerging from the 9 surviving, parent CFs out of which 48 were categorized as outside and 53 as inside. The pattern of inside addition (that is, number of ladders added to the inside) were significantly different over time (one-way repeated measures analysis of variance with Tukey post hoc analysis: F(6,18)=3.604; P=0.01; *P<0.05 time point 4 compared with time points 2, 5 and 13+). Inset shows total ladders added (±s.e.m.) at each time point.
Mentions: Post-lesion CF collateral sprouting in the paravermal region of lobule VI and VII was repeatedly imaged in vivo from 1 week up to 13 weeks after olivary injection of 3-AP (n=4 animals). CFs that emerged from new collaterals had a ladder-like appearance similar to their parent CF (Fig. 2a and Supplementary Fig. 2), indicating that they were innervating dendrites of nearby denervated PCs. Consistent with previous reports1123, new CF collaterals always sprouted laterally, expanding the innervation territory of the parent CF only in the mediolateral extent. Since functional zones are oriented sagittally, laterally growing CF collaterals may cross the boundary between functional zones. To quantify the temporal pattern of this mediolateral collateral sprouting, we categorized the newly added CF ladders as outside or inside based on their location of addition when compared with the previous imaging time point. Outside addition refers to the expansion of the mediolateral extent of the parent CF territory, whereas inside addition refers to further innervation of PCs within territory that the CF has already covered (Fig. 2a and Supplementary Fig. 2). Individual, surviving CFs showed variable patterns of inside/outside addition, but they showed a clear temporal profile as a population. Both outside and inside additions continued over several months (Fig. 2b). The rate of outside addition was relatively constant (Fig. 2b, magenta), suggesting that the boundary of functional zones does not limit the mediolateral expansion of the parent CF territory. Inside additions, on the other hand, were more time-dependent since significantly more CF ladders were added inside the mediolateral extents of parent CFs at 4 weeks after 3-AP injection than at any other time points (Fig. 2b, green). Amongst all the 101 ladders analysed, nearly half the ladders (49) were added in the first 4 weeks. Out of these 49 ladders, we could reliably measure the distance between the new ladders and the parent CF for 30 ladders. Out of these 30 ladders, nearly all (27) were within 45 μm of the parent CF. Since the diameter of a mature PC soma is about 15–20 μm, 45 μm is equivalent to the length of only a few PC somata contacted side by side. These results indicate that although the parent CF continues to expand its mediolateral territory over an extended period of time, the majority of new innervations resulting in this mediolateral expansion occurs in the first month and is near the parent CF.

View Article: PubMed Central - PubMed

ABSTRACT

Neurodegenerative lesions induce sprouting of new collaterals from surviving axons, but the extent to which this form of axonal remodelling alters brain functional structure remains unclear. To understand how collateral sprouting proceeds in the adult brain, we imaged post-lesion sprouting of cerebellar climbing fibres (CFs) in mice using in vivo time-lapse microscopy. Here we show that newly sprouted CF collaterals innervate multiple Purkinje cells (PCs) over several months, with most innervations emerging at 3&ndash;4 weeks post lesion. Simultaneous imaging of cerebellar functional structure reveals that surviving CFs similarly innervate functionally relevant and non-relevant PCs, but have more synaptic area on PCs near the collateral origin than on distant PCs. These results suggest that newly sprouted axon collaterals do not preferentially innervate functionally relevant postsynaptic targets. Nonetheless, the spatial gradient of collateral innervation might help to loosely maintain functional synaptic circuits if functionally relevant neurons are clustered in the lesioned area.

No MeSH data available.