Delayed short-term tamoxifen treatment does not promote remyelination or neuron sparing after spinal cord injury

Abstract

The tamoxifen-dependent Cre/lox system in transgenic mice has become an important research tool across all scientific disciplines for manipulating gene expression in specific cell types. In these mouse models, Cre-recombination is not induced until tamoxifen is administered, which allows researchers to have temporal control of genetic modifications. Interestingly, tamoxifen has been identified as a potential therapy for spinal cord injury (SCI) and traumatic brain injury patients due to its neuroprotective properties. It is also reparative in that it stimulates oligodendrocyte differentiation and remyelination after toxin-induced demyelination. However, it is unknown whether tamoxifen is neuroprotective and neuroreparative when administration is delayed after SCI. To properly interpret data from transgenic mice in which tamoxifen treatment is delayed after SCI, it is necessary to identify the effects of tamoxifen alone on anatomical and functional recovery. In this study, female and male mice received a moderate mid-thoracic spinal cord contusion. Mice were then gavaged with corn oil or a high dose of tamoxifen from 19-22 days post-injury, and sacrificed 42 days postinjury. All mice underwent behavioral testing for the duration of the study, which revealed that tamoxifen treatment did not impact hindlimb motor recovery. Similarly, histological analyses revealed that tamoxifen had no effect on white matter sparing, total axon number, axon sprouting, glial reactivity, cell proliferation, oligodendrocyte number, or myelination, but tamoxifen did decrease the number of neurons in the dorsal and ventral horn. Semi-thin sections confirmed that axon demyelination and remyelination were unaffected by tamoxifen. Sex-specific responses to tamoxifen were also assessed, and there were no significant differences between female and male mice. These data suggest that delayed tamoxifen administration after SCI does not change functional recovery or improve tissue sparing in female or male mice.

The neuroprotective effects of estrogen and estrogenic compounds in spinal cord injury

“…Only one study showed that estrogen did not affect the lesion size using hematoxylin and eosin staining, however they noted that their protocol was unlikely to be sensitive enough to detect changes in lesion size in their moderate SCI lesion model (Letaif et al, 2015). One study also found that tamoxifen had no effect on lesion size (Pukos & McTigue, 2020). This study, however, reportedly used a very high (300mg/kg) dose of tamoxifen in mice.…”

“…Two studies, however, demonstrated no effect. One study showed that estrogen (Letaif et al, 2015) and another, using a dose of tamoxifen well outside of the clinical range, showed that tamoxifen (Pukos & McTigue, 2020) had no effect on axon integrity and sparing.…”

“…Several studies showed that estrogen, tamoxifen, and bazedoxifene did increase the amount of white matter spared after SCI (Chaovipoch et al, 2005;Colón et al, 2018;Colón et al, 2016;Kachadroka et al, 2010;Kim et al, 2021;Osuna-Carrasco et al, 2016;Siriphorn et al, 2012). Other studies, however, found no effect (Hubscher et al, 2010;Kachadroka et al, 2010;Pukos & McTigue, 2020;Siriphorn et al, 2012). These differences were independent of the SCI model, as the studies included a mix of crush injuries, hemicontusion, and moderate contusion.…”

Chronic demyelination and myelin repair after spinal cord injury in mice: A potential link for glutamatergic axon activity

“…PDGFRα-CreER T2 : Tau-mGFP mice were given tamoxifen by oral gavage to induce recombination prior to SCI or at different times post-injury, then 5-ethynyl-2 0 -deoxyuridine (EdU) was dissolved in their drinking water to label proliferating cells. Although tamoxifen may be neuroprotective and can accelerate OL differentiation and remyelination in models of primary demyelination (Gonzalez et al, 2016), the tamoxifen regimen used in this report and prior studies does not change OL function or remyelination after SCI (Pukos & McTigue, 2020). One cohort of mice did not receive EdU and underwent in vivo electrophysiology recordings and behavioral testing.…”

Paclitaxel Chemotherapy Elicits Widespread Brain Anisotropy Changes in a Comprehensive Mouse Model of Breast Cancer Survivorship: Evidence From In Vivo Diffusion Weighted Imaging

“…Brains were frozen, cut at 10µm on a cryostat, and mounted. Sections were immunolabeled for Iba1 or GFAP as previously described (Invitrogen) ( 57 ). Next, three images from hypothalamus (paraventricular nucleus and lateral hypothalamic nucleus) and hippocampus (CA3 region) were collected for each brain, and the immunoreactive area of GFAP and Iba1 of each section was quantified using image analysis (Image J).…”