Motor Neuron Regeneration in Adult Zebrafish

Abstract

The mammalian spinal cord does not regenerate motor neurons that are lost as a result of injury or disease. Here we demonstrate that adult zebrafish, which show functional spinal cord regeneration, are capable of motor neuron regeneration. After a spinal lesion, the ventricular zone shows a widespread increase in proliferation, including slowly proliferating olig2-positive (olig2 ϩ ) ependymo-radial glial progenitor cells. Lineage tracing in olig2:green fluorescent protein transgenic fish indicates that these cells switch from a gliogenic phenotype to motor neuron production. Numbers of undifferentiated small HB9 ϩ and islet-1 ϩ motor neurons, which are double labeled with the proliferation marker 5-bromo-2-deoxyuridine (BrdU), are transiently strongly increased in the lesioned spinal cord. Large differentiated motor neurons, which are lost after a lesion, reappear at 6 -8 weeks after lesion, and we detected ChAT ϩ /BrdU ϩ motor neurons that were covered by contacts immunopositive for the synaptic marker SV2. These observations suggest that, after a lesion, plasticity of olig2 ϩ progenitor cells may allow them to generate motor neurons, some of which exhibit markers for terminal differentiation and integration into the existing adult spinal circuitry.

Upregulation of anti-apoptotic factors in upper motor neurons after spinal cord injury in adult zebrafish

“…Our data were in good agreement with previous findings in the adult zebrafish brain, reporting that the number of newly-generated neurons is extremely low in the region where descending neurons exist (Zupanc et al, 2005;Hinsch and Zupanc, 2007). In contrast to the upper motor neurons in the brainstem, motor neurons in the spinal cord become apoptotic after SCI, and neurogenesis from the ependymal zone around the central canal is activated via sonic hedgehog and/or Notch signaling (Reimer et al, 2008;Reimer et al, 2009;Dias et al, 2012).…”

“…BrdU incorporation assay was performed to visualize cell proliferation, as described previously (Reimer et al, 2008). In brief, fish were anesthetized and intraperitoneally injected with 50 µl of 2.5 mg/ml BrdU (Sigma-Aldrich) at 0, 2 and 4 days post-lesion (dpl).…”

“…Similar to mammals, motor function in zebrafish is controlled by motor neurons from two distinct populations, the brainstem and the spinal cord. Recently, it was reported that motor neurons in the spinal cord are newly-generated from progenitor cells located around the central canal after SCI (Reimer et al, 2008). These neurons extend axons to reinnervate their target organs.…”

Expression of the transcription factor Olig2 in proliferating cells in the adult zebrafish telencephalon

“…However, in zebrafish S100b-positive cells, as previously shown (Mä rz et al, 2010), co-express the radial glial markers BLBP and GFAP suggesting that these cells belong to the radial glial lineage.In the zebrafish adult spinal cord, Tg(olig2:EGFP)-positive cells are distributed in the gray and white matter and, in addition, radial glia-like cells at the central canal express olig2 (Park et al, 2007). Moreover, it was shown that these Tg(olig2:EGFP)-positive radial glialike cells act as progenitors of motor neurons in the regenerating spinal cord (Reimer et al, 2008), while they produce mostly glial cells in the uninjured spinal cord (Park et al, 2007). As none of the ventricular olig2-expressing cells of the telencephalon co-expresses sox10, it remains to be investigated whether these cells are indeed progenitors of oligodendrocytes or whether they give rise to neurons as in the regenerating spinal cord.…”

“…A recent study found that slowly proliferating Tg(olig2:EGFP)-positive radial glia-like cells are present in the adult zebrafish spinal cord, where they seem to act as progenitors restricted to the oligodendrocyte lineage (Park et al, 2007). Interestingly, when the spinal cord is injured, these cells not only produce oligodendrocytes but also motorneurons (Reimer et al, 2008).…”