Two recent articles from the Chen lab investigate the intrinsic ability of neurons to regenerate and define the antiapoptotic protein Bcl-2 as a key player in this capability. Whether a neuron is capable of regenerating is only half of the story, though; the neuron must also be in a permissive environment. Understanding these two elements of neuronal regeneration can aid in developing therapies for the repair of central nerve damage due to injury or disease. Expression of bcl-2 is high in the embryonic central nervous system and decreases during development and acquisition of the nonregenerative phenotype. In the first article, Cho et al. show that if postnatal day 3 (P3) transgenic mice that overexpress Bcl-2 (Bcl-2tg) are subjected to optic nerve crush injury, the retinal ganglion cells (RGCs) regenerate and extend axons to the ipsilateral brain targets. (This is in contrast to the normal innervation path, which is to the contralateral side.) However, it was previously reported that if the injury occurs at P5, regeneration does not occur. Cho et al. show that RGCs from adult Bcl-2tg mice extended axons into embryonic brain slices, but this regenerative capacity diminished with the age of the target brain slice, such that essentially no neurites extended into the slices from animals older than P4. Thus, loss of regenerative capacity seems to reflect a target environment that is nonpermissive to growth, rather than an inability of the Bcl-2tg neurons to regenerate. Growth inhibition did not correlate with the appearance of myelin markers and was not prevented in mice deficient for CNS myelin; however, if 2- to 3-month-old wild-type or Bcl-2tg mice were treated with the astrocyte toxin L-a-aminoadipate immediately after optic nerve crush injury, then slight (wild-type) or robust (Bcl-2tg) nerve regeneration was observed. This and data from mice lacking glial fibrillary acidic protein (GFAP) and vimentin (two proteins associated with mature and embryonic glia, respectively) indicated that the glial response to injury in older brains is responsible for creating a nonpermissive environment for RGC regeneration.
In the second article, Jiao et al. investigate the mechanism by which Bcl-2 allows neurons to retain their regenerative capacity. Optic nerves subjected to crush injury in P3 mice overexpressing Bcl-2 or the related protein Bcl-XL (Bcl-xLtg) showed decreased apoptosis compared with RGCs from wild-type mice; however, only the Bcl-2tg showed a regenerative response. Both Bcl-2-overexpressing and Bcl-xLtg-overexpressing cells exhibited enhanced survival compared with wild-type RGCs or PC12 cells, but only RGCs from Bcl-2tg mice or Bcl-2-transfected PC12 cells showed enhanced neurite extension in culture. Localization to the endoplasmic reticulum (ER) was necessary and sufficient for Bcl-2 or Bcl-XL (when targeted to the ER) to allow neurite outgrowth in response to growth factors in transfected PC12 cells. The ER is involved in calcium signaling, and PC12 cells overexpressing Bcl-2, which is normally targeted to the ER, showed decreased ER calcium storage based on cytosolic calcium concentration measurements made after depletion of the ER with the ER Ca2+-ATPase (SERCA) inhibitor thapsigargin. Indeed, a link between ER calcium concentration and neurite outgrowth was established by showing that overexpression of SERCA2b in PC12 cells reduced neurite outgrowth in the Bcl-2-overexpressing cells and that inhibition of SERCA increased neurite outgrowth. The authors propose that by inhibiting calcium reuptake into the ER by decreasing the abundance of SERCA, Bcl-2 increases the cytosolic calcium concentration during nerve injury. This was confirmed by measuring calcium concentration in the Bcl-2-overexpressing PC12 cells, which showed increased cytosolic calcium concentration in response to various stimuli. Furthermore, in the Bcl-2-overexpressing PC12 cells, there was enhanced phosphorylation of the transcriptional regulator CREB and the mitogen-activated protein kinase ERK and enhanced activation of CREB-reporter gene expression. Enhanced signaling in the ERK and CREB pathways was observed in the Bcl-2tg RGCs after optic nerve crush injury. Thus, Bcl-2 appears to have two roles in promoting axon regeneration: an antiapoptotic role and a neurite extension role related to the effect of Bcl-2 on calcium signaling in response to injury.
K.-S. Cho, L. Yang, B. Lu, H. F. Ma, X. Huang, M. Pekny, D. F. Chen, Re-establishing the regenerative potential of central nervous system axons in postnatal mice. J. Cell Sci. 118, 863-872 (2005). [Abstract] [Full Text]
J. Jiao, X. Huang, R. A. Feit-Leithman, R. L. Neve, W. Snider, D. A. Dartt, D. F. Chen, Bcl-2 enhances Ca2+ signaling to support the intrinsic regenerative capacity of CNS axons. EMBO J. 24, 1068-1078 (2005). [Abstract] [Full Text]