Bcl-2 protection of axotomized Purkinje cells in organotypic culture is age dependent and not associated with an enhancement of axonal regeneration

TrkA regulates the regenerative capacity of bone marrow stromal stem cells in nerve grafts

We previously demonstrated that overexpression of tropomyosin receptor kinase A (TrkA) promotes the survival and Schwann cell-like differentiation of bone marrow stromal stem cells in nerve grafts, thereby enhancing the regeneration and functional recovery of the peripheral nerve. In the present study, we investigated the molecular mechanisms underlying the neuroprotective effects of TrkA in bone marrow stromal stem cells seeded into nerve grafts. Bone marrow stromal stem cells from Sprague-Dawley rats were infected with recombinant lentivirus vector expressing rat TrkA, TrkA-shRNA or the respective control. The cells were then seeded into allogeneic rat acellular nerve allografts for bridging a 1-cm right sciatic nerve defect. Then, 8 weeks after surgery, hematoxylin and eosin staining showed that compared with the control groups, the cells and fibers in the TrkA overexpressing group were more densely and uniformly arranged, whereas they were relatively sparse and arranged in a disordered manner in the TrkA-shRNA group. Western blot assay showed that compared with the control groups, the TrkA overexpressing group had higher expression of the myelin marker, myelin basic protein and the axonal marker neurofilament 200. The TrkA overexpressing group also had higher levels of various signaling molecules, including TrkA, pTrkA (Tyr490), extracellular signal-regulated kinases 1/2 (Erk1/2), pErk1/2 (Thr202/Tyr204), and the anti-apoptotic proteins Bcl-2 and Bcl-xL. In contrast, these proteins were downregulated, while the pro-apoptotic factors Bax and Bad were upregulated, in the TrkA-shRNA group. The levels of the TrkA effectors Akt and pAkt (Ser473) were not different among the groups. These results suggest that TrkA enhances the survival and regenerative capacity of bone marrow stromal stem cells through upregulation of the Erk/Bcl-2 pathway. All procedures were approved by the Animal Ethical and Welfare Committee of Shenzhen University, China in December 2014 (approval No. AEWC-2014-001219).

Ultrastructural localization of extranuclear progestin receptors in the rat hippocampal formation

Progesterone's effects on hippocampus-dependent behavior and synaptic connectivity maybe mediated through the progestin receptor (PR). Although estrogen induces PR mRNA and cytosolic PR in the hippocampus, nuclear PR immunoreactivity is undetectable by light microscopy, suggesting that PR is present at extranuclear sites. To determine whether this is the case, we used immunoelectron microscopy to examine PR distribution in the hippocampal formation of proestrus rats. Ultrastructural analysis revealed that PR labeling is present in extranuclear profiles throughout the CA1 and CA3 regions and dentate gyrus, and, in contrast to light microscopic findings, in nuclei of a few pyramidal and subgranular zone cells. Most neuronal PR labeling is extranuclear and is divided between pre- and postsynaptic compartments; approximately 30% of labeled profiles were axon terminals and 30% were dendrites and dendritic spines. In most laminae, except in CA3 stratum lucidum, about 15% of PR-immunoreactive profiles were unmyelinated axons. In stratum lucidum, where the mossy fiber axons course, more than 50% of PR-labeled profiles were axonal. The remaining 25% of PR-labeled profiles were glia, some resembling astrocytes. PR labeling is strongly dependent on estrogen priming, insofar as few PR-labeled profiles were detected in ovariectomized, oil-replaced females. Synapses formed by PR-labeled terminals were predominantly asymmetric, consistent with a role for progesterone in directly regulating excitatory transmission. These findings suggest that some of progesterone's actions in the hippocampal formation may be mediated by direct and rapid actions on extranuclear PRs and that PRs are well positioned to regulate progesterone-induced changes at synapses.

The strange case of Purkinje axon regeneration and plasticity

In the last few years Purkinje cells have become a most interesting model to investigate cellular/molecular mechanisms of axon regeneration and plasticity. Adult Purkinje cells are most peculiar for their weak cell body response to axotomy, which is accompanied by a strong resistance to injury and a virtually absolute inability to regenerate severed neurites, even in the presence of favourable environmental conditions. The same neurons show a vigorous intrinsic inclination toward axonal sprouting and structural plasticity, which can be elicited by removing extrinsic growth-inhibitory cues. These features gradually develop during early postnatal life, but the underlying mechanisms and biological significance remain unclear. This article reviews recent studies aimed at addressing these questions with respect to the general issue of brain repair. Indeed, understanding the reasons for the extremely poor regenerative capacity of Purkinje cells will be most important to elucidate basic biological mechanisms of axon regeneration and plasticity, and to promote circuit rewiring in the adult CNS.

Cell death and axon regeneration of Purkinje cells after axotomy: challenges of classical hypotheses of axon regeneration

Although adult mammalian neurons are able to regenerate their axons in the peripheral nervous system under certain conditions, they are not able to do it in the central nervous system. The environment surrounding the severed axons appears to be a key factor for axon regeneration. Many studies aiming to enhance axon regeneration in the CNS of adult mammals have successfully manipulated this environment by adding growth permissive molecules and/or neutralizing growth inhibitory molecules. In both cases, the number of axons able to regenerate was low and the different neuronal populations were not equal in their regenerative response, suggesting that manipulation of the environment is not always sufficient. This is particularly well illustrated in the cerebellar system, in which axotomized inferior olivary neurons regenerate when confronted with a permissive environment, whereas mature Purkinje cells do not. The intrinsic ability of a neuron to regenerate its axon is generally correlated with the intensity of its reaction to axotomy (expression of molecules, probability to die). Furthermore, molecules such as GAP-43 (growth-associated molecule) and c-Jun are involved in both axon regeneration and cell death suggesting that these two processes are linked. Surprisingly, Purkinje cells lose their capacity to regenerate their axon (even in the absence of myelin) during development before losing their capacity to react to an axotomy by cell death. These results emphasize the different reactions to axotomy between neuron types and underline that in Purkinje cells, the two cell decisions (axon regeneration and cell death) are differently regulated and therefore not part of the same signaling pathway.