Spinal cord regeneration - the origins of progenitor cells for functional rebuilding

The endocannabinoid system regulates both ependymoglial and neuronal cell responses to a tail amputation in the axolotl

BACKGROUND

The endocannabinoid system is a neuromodulatory system implicated in cellular processes during both development and regeneration. The Mexican axolotl, one of only a few vertebrates capable of central nervous system regeneration, was used to examine the role of the endocannabinoid system in the regeneration of the tail and spinal cord following amputation.

RESULTS

The endocannabinoid receptor CB1 was upregulated in the regenerating axolotl spinal cord by 4 hours following tail amputation, and this upregulation persisted for at least 14 days. The endocannabinoid receptor CB2 was upregulated later, between 7 and 14 days after tail amputation. Both CB1 and CB2 were located in ependymoglia and neurons within the regenerating spinal cord. Treatment with inverse agonists to inhibit CB1 (AM251) or CB2 (AM630) inhibited spinal cord and tail regeneration. During the first 7 days after injury, CB1 and CB2 expression was also necessary for the proliferation of ependymoglial cells and the regeneration of axons into the newly regenerated tail tissue. However, only CB1 was necessary for the differentiation of ependymoglia into immature neurons.

CONCLUSIONS

These studies are the first to examine the role of the endocannabinoid system during spinal cord regeneration in a regeneration-competent vertebrate.

Age-dependent regenerative mechanisms in the brain

Repairing the adult mammalian brain represents one of the greatest clinical challenges in medicine. Injury to the adult brain often results in substantial loss of neural tissue and permanent functional impairment. In contrast with the adult, during development, the mammalian brain exhibits a remarkable capacity to replace lost cells. A plethora of cell-intrinsic and extrinsic factors regulate the age-dependent loss of regenerative potential in the brain. As the developmental window closes, neural stem cells undergo epigenetic changes, limiting their proliferation and differentiation capacities, whereas, changes in the brain microenvironment pose additional challenges opposing regeneration, including inflammation and gliosis. Therefore, studying the regenerative mechanisms during development and identifying what impairs them with age may provide key insights into how to stimulate regeneration in the brain. Here, we will discuss how the mammalian brain engages regenerative mechanisms upon injury or neuron loss. Moreover, we will describe the age-dependent changes that affect these processes. We will conclude by discussing potential therapeutic approaches to overcome the age-dependent regenerative decline and stimulate regeneration.

Neural Stem Cell Regulation in Zebrafish

Neural stem cells (NSCs) are progenitor cell populations generating glial cells and neurons and endowed with long-lasting self-renewal and differentiation potential. While some neural progenitors (NPs) in the embryonic nervous system are also long-lived and match this definition, the term NSC classically refers to such progenitor types in the adult. With the discovery of extensive NSC populations in the adult brain of Danio rerio (zebrafish) and of their high neurogenic activity, including for neuronal regeneration, this model organism has become a powerful tool to characterize and mechanistically dissect NSC properties. On these bases, this article will consider NSCs in the adult zebrafish brain, with a focus on its most extensively characterized domain, the telencephalon (notably its dorsal part, the pallium). Whenever necessary, we will also refer to other brain subdivisions, embryonic processes, and the mouse adult brain, whether for comparative purposes or because more information is available in these other systems.

Tail and Spinal Cord Regeneration in Urodelean Amphibians

Urodelean amphibians can regenerate the tail and the spinal cord (SC) and maintain this ability throughout their life. This clearly distinguishes these animals from mammals. The phenomenon of tail and SC regeneration is based on the capability of cells involved in regeneration to dedifferentiate, enter the cell cycle, and change their (or return to the pre-existing) phenotype during de novo organ formation. The second critical aspect of the successful tail and SC regeneration is the mutual molecular regulation by tissues, of which the SC and the apical wound epidermis are the leaders. Molecular regulatory systems include signaling pathways components, inflammatory factors, ECM molecules, ROS, hormones, neurotransmitters, HSPs, transcriptional and epigenetic factors, etc. The control, carried out by regulatory networks on the feedback principle, recruits the mechanisms used in embryogenesis and accompanies all stages of organ regeneration, from the moment of damage to the completion of morphogenesis and patterning of all its structures. The late regeneration stages and the effects of external factors on them have been poorly studied. A new model for addressing this issue is herein proposed. The data summarized in the review contribute to understanding a wide range of fundamentally important issues in the regenerative biology of tissues and organs in vertebrates including humans.

Radial glia and radial glia-like cells: Their role in neurogenesis and regeneration

Radial glia is a cell type traditionally associated with the developing nervous system, particularly with the formation of cortical layers in the mammalian brain. Nonetheless, some of these cells, or closely related types, called radial glia-like cells are found in adult central nervous system structures, functioning as neurogenic progenitors in normal homeostatic maintenance and in response to injury. The heterogeneity of radial glia-like cells is nowadays being probed with molecular tools, primarily by the expression of specific genes that define cell types. Similar markers have identified radial glia-like cells in the nervous system of non-vertebrate organisms. In this review, we focus on adult radial glia-like cells in neurogenic processes during homeostasis and in response to injury. We highlight our results using a non-vertebrate model system, the echinoderm Holothuria glaberrima where we have described a radial glia-like cell that plays a prominent role in the regeneration of the holothurian central nervous system.