Myelin pathogenesis and functional deficits following SCI are age-associated

Abstract

Most spinal cord injuries (SCI) occur in young adults. In the past few decades however, the average age at time of SCI and the percentage of injuries in persons over the age of 60 have increased. Studies have shown that there is an age-associated delay in the rate of remyelination following toxin-induced demyelination of the spinal cord, suggesting that there may be an age-associated difference in regenerative efficiency. Here we examine for the first time locomotor recovery, bladder recovery, and myelin pathology in young (3 months), aged (12 months), and geriatric (24 months) female rats following contusion SCI. Our assessments indicate that aged and geriatric rats have a delayed rate of locomotor recovery following contusion SCI as compared to young rats. Additionally, aged and geriatric rats have significantly slower bladder recovery as compared to young rats. Examination of myelin pathology reveals that aged and geriatric rats have significantly greater area of pathology and amount of demyelination, as well as significantly less remyelination as compared to young rats following contusion SCI. These data are the first to indicate that there is an age-associated decline in the rate and extent of both locomotor and bladder recovery following contusion SCI, and that age adversely affects the degree of general pathology, demyelination, and remyelination that accompanies contusion SCI.

Figure 1. Aging adversely affects overground locomotor recovery after SCI

(a) Young animals demonstrated significantly greater (p<0.01) locomotor capabilities post-injury as compared to aged and geriatric animals. There was no significant difference in locomotor capabilities between aged and geriatric animals (p>0.05). (b) The average change in BBB score during one week was greatest in young animals within the first week, and gradually declined thereafter. The greatest change in BBB score for aged and geriatric animals, however, was delayed until the second week, after which the rate of recovery decreased. The change in BBB score between one week and two weeks post injury was significantly greater in young animals (p<0.01). There was no significant difference in the recovery rates between groups for all other intervals post injury (p>0.05). * p< 0.01.

Figure 2. Aging adversely affects kinematic locomotor recovery following SCI

(a) Kinematic analysis of stride width indicated that aged and geriatric animals had a significantly higher stride width than young animals at multiple time points following contusion SCI (p<0.001). (b) Kinematic analysis of stride length indicated that aged and geriatric animals had significantly shorter stride lengths at multiple time points following contusion SCI as compared to young (p<0.05). (c) Kinematic analysis of digit spread indicated that aged animals had significantly greater digit spread at 4 and 7 weeks following contusion SCI (p<0.05). (d) Kinematic analysis of paw rotation indicated that aged and geriatric animals had significantly greater paw rotation at 4, 5, 6, and 7 weeks post contusion SCI (p<0.05). (* indicates significance in difference between geriatric and young animal groups; ** indicates significance in difference between aged and young animal groups; *** indicates significance in difference between aged and geriatric animal groups).

Figure 3. Aging adversely affects bladder recovery following SCI

(a) The percent of animals with bladder recovery was significantly different between young and both aged and geriatric animals (p<0.001). Young animals had 100% bladder recovery at fourteen days following SCI, whereas aged and geriatric animals reached only 57.14% and 58.33% bladder recovery, respectively, by fourteen days following SCI. (b) Linear regression analysis of bladder recovery following SCI demonstrated a greater recovery rate for young animals as compared to aged and geriatric animals. The linear equations reveal that young animals had a slope of 7.3596, whereas aged and geriatric animals had a slope of 4.5526 and 4.4819, respectively. * p<0.001.

Figure 4. Aging increases area of pathology following SCI

(a) Aged and geriatric animals had significantly greater area of pathology as compared to young animals both at the injury epicenter (p<0.001) and 2 mm cranial to the injury epicenter (p<0.05). There were no significant differences in area of pathology between aged and geriatric animals at either location examined (p>0.05), nor were there differences in area of pathology between all three groups 2 mm caudal to the injury epicenter. (b), (c), (d) Low magnification imaging of transverse spinal cord sections at the injury epicenter of young (b), aged (c), and geriatric (d) animals reveals qualitative differences in the area of pathology. * p<0.001; **p< 0.05. Scale bar= 1mm.

Figure 5. Oligodendrocyte remyelinated axons can be distinguished from normally-myelinated axons

(a), (b) Toluidine blue-stained transverse sections of contused spinal cord at the magnification used for quantification and identification of normally-myelinated axons (arrowhead), oligodendrocyte remyelinated axons (arrows), and demyelinated axons (asterisk). (c) Myelin sheath thickness against axon diameter of normally-myelinated and oligodendrocyte remyelinated axons. The G ratio was 47 ± .2 (85)^† for normally-myelinated axons and 13 ± .05 (85)^† for oligodendrocyte remyelinated axons. ^† Data are expressed as mean ± SEM; number in parentheses shows the number of axons scored. Scale bars= 5µm.

Figure 6. Aging increases demyelination and decreases remyelination following contusion SCI

(a), (b), (c) High magnification imaging of toluidine-blue stained transverse spinal cord sections at the injury epicenter of young (a), aged (b), and geriatric (c) animals reveals qualitative differences in myelin pathology. (d) Aged and geriatric animals had significantly more demyelinated axons as compared to young animals at the injury epicenter (p<0.05). Geriatric animals had significantly more demyelinated axons 2 mm cranial and caudal to the injury epicenter as compared to young animals (p<0.05). (e) Aged and geriatric animals had significantly fewer remyelinated axons as compared to young animals at the injury epicenter (p<0.05), and geriatric animals had significantly fewer remyelinated axons as compared to aged animals at the injury epicenter (p<0.05). Geriatric animals had significantly fewer remyelinated axons as compared to both young and aged animals 2 mm caudal to the injury epicenter (p<0.05). There were no significant differences cranial to the injury epicenter. * p<0.01; **p< 0.05. Scale bar= 5µm.