Inhibition of PTEN activity promotes IB4-positive sensory neuronal axon growth.

Traumatic nerve injuries have become a common clinical problem, and axon regeneration is a critical process in the successful functional recovery of the injured nervous system. In this study, we found that peripheral axotomy reduces PTEN expression in adult sensory neurons; however, it did not alter the expression level of PTEN in IB4-positive sensory neurons. Additionally, our results indicate that the artificial inhibition of PTEN markedly promotes adult sensory axon regeneration, including IB4-positive neuronal axon growth. Thus, our results provide strong evidence that PTEN is a prominent repressor of adult sensory axon regeneration, especially in IB4-positive neurons.

c-Myc controls the fate of neural progenitor cells during cerebral cortex development.

The anatomical structure of the mammalian cerebral cortex is the essential foundation for its complex neural activity. This structure is developed by proliferation, differentiation, and migration of neural progenitor cells (NPCs), the fate of which is spatially and temporally regulated by the proper gene. This study was used in utero electroporation and found that the well-known oncogene c-Myc mainly promoted NPCs' proliferation and their transformation into intermediate precursor cells. Furthermore, the obtained results also showed that c-Myc blocked the differentiation of NPCs to postmitotic neurons, and the expression of telomere reverse transcriptase was controlled by c-Myc in the neocortex. These findings indicated c-Myc as a key regulator of the fate of NPCs during the development of the cerebral cortex.

Calcium/calmodulin-dependent protein kinase II regulates mammalian axon growth by affecting F-actin length in growth cone.

While axon regeneration is a key determinant of functional recovery of the nervous system after injury, it is often poor in the mature nervous system. Influx of extracellular calcium (Ca2+ ) is one of the first phenomena that occur following axonal injury, and calcium/calmodulin-dependent protein kinase II (CaMKII), a target substrate for calcium ions, regulates the status of cytoskeletal proteins such as F-actin. Herein, we found that peripheral axotomy activates CaMKII in dorsal root ganglion (DRG) sensory neurons, and inhibition of CaMKII impairs axon outgrowth in both the peripheral and central nervous systems (PNS and CNS, respectively). Most importantly, we also found that the activation of CaMKII promotes PNS and CNS axon growth, and regulatory effects of CaMKII on axon growth occur via affecting the length of the F-actin. Thus, we believe our findings provide clear evidence that CaMKII is a critical modulator of mammalian axon regeneration.

Regulation of adult mammalian intrinsic axonal regeneration by NF-κB/STAT3 signaling cascade.

The inflammatory response is a critical regulator for the regeneration of axon following nervous system injury. Nuclear factor-kappa B (NF-κB) is characteristically known for its ubiquitous role in the inflammatory response. However, its functional role in adult mammalian axon growth remains elusive. Here, we found that the NF-κB signaling pathway is activated in adult sensory neurons through peripheral axotomy. Furthermore, inhibition of NF-κB in peripheral sensory neurons attenuated their axon growth in vitro and in vivo. Our results also showed that NF-κB modulated axon growth by repressing the phosphorylation of STAT3. Furthermore, activation of STAT3 significantly promoted adult optic nerve regeneration. Taken together, the findings of our study indicated that NF-κB/STAT3 cascade is a critical regulator of intrinsic axon growth capability in the adult nervous system.

Deletion of Krüppel-like factor-4 promotes axonal regeneration in mammals.

Axonal regeneration plays an important role in functional recovery after nervous system damage. However, after axonal injury in mammals, regeneration is often poor. The deletion of Krüppel-like factor-4 (Klf4) has been shown to promote axonal regeneration in retinal ganglion cells. However, the effects of Klf4 deletion on the corticospinal tract and peripheral nervous system are unknown. In this study, using a mouse model of sciatic nerve injury, we show that the expression of Klf4 in dorsal root ganglion sensory neurons was significantly reduced after peripheral axotomy, suggesting that the regeneration of the sciatic nerve is associated with Klf4. In vitro, dorsal root ganglion sensory neurons with Klf4 knockout exhibited significantly enhanced axonal regeneration. Furthermore, the regeneration of the sciatic nerve was enhanced in vivo following Klf4 knockout. Finally, AAV-Cre virus was used to knockout the Klf4 gene in the cortex. The deletion of Klf4 enhanced regeneration of the corticospinal tract in mice with spinal cord injury. Together, our findings suggest that regulating KLF4 activity in neurons is a potential strategy for promoting axonal regeneration and functional recovery after nervous system injury. This study was approved by the Animal Ethics Committee at Soochow University, China (approval No. SUDA20200316A01).