Lecture 6 - Neural Regeneration and Neural Stem Cells Flashcards
What are the mechanisms of axonal degeneration?
Sterile alpha and TIR motif-containing protein 1 (SARM1) is the central executioner of pathological axon degeneration. The TIR motif of SARM1 has NADase activity
Nicotinamide mononucleotide adenylyl transferase (Nmnat2) is a survival factor transported down axons from the cell body that generates NAD+ from NMN
SARM1 is activated by an increase in the ratio of NMN to NAD+, both compete for binding to the auto-inhibitory N-terminal armadillo repeat (ARM) domain of SARM1
Why is regeneration impaired in adult CNS neurons?
Extrinsic inhibitory factors
Intrinsic axonal growth capacities
* Lack of robust injury signaling response
* Limited capacity for local protein synthesis
What are the regeneration problems of the adult CNS axon?
During the early phase of injury, myelin-associated inhibitors from intact oligodendrocytes and myelin debris can restrict axon regrowth. Recruitment of inflammatory cells and reactive astrocytes over time leads to the formation of a glial scar, often accompanied by a fluid-filled cyst. This scarring process is associated with the increased release of chondroitin sulphate proteoglycans, which can further limit regeneration. Together, these molecular inhibitors of the CNS glial environment present a hostile environment for axon repair
What are the guiding CNS factors regulating neurite outgrowth? (4)
Semaphorins
Ephrins
Netrins
Slits
Myelin-associated inhibitors
What are the molecular inhibitors of the adult CNS glial environment? (2)
chondroitin sulfate proteoglycans (CSPGs)
- associated with reactive astrocytes from the glial scar
myelin-associated inhibitors
- from intact oligodendrocytes and myelin debris, including myelin-associated glycoprotein (MAG, Nogo-A, oligodendrocyte myelin glycoprotein (OMgp), ephrin B3 and the transmembrane semaphorin 41) (Sema4D).
How does signalling from the Nogo-66 receptor impinge on the actin cytoskeleton?
Ultimately, though activation of RhoA - growth cone collapse
What are the differences in intrinsic axonal regrowth capacity of embryonic vs adult PNS and CNS neurons?
Adult CNS neuron
Limited intrinsic growth capacity
— Low calcium changes
— No increase in histone acetylation
— Lack of robust synthesis of RAGs
— Limited protein synthesis
— Inhibitors Of axon regrowth (PTEN. SOCS3, EFA-6)
Adult PNS neuron
High intrinsic growth capacity
— Activation of regeneration-specific program
— Recapitulation of developmental-related pathways
What is the neuronal response to axonal injury — a broad view?
(1) Injury induces back-propagating calcium wave towards the soma.
(2) result in subsequent changes in transcription.
(3) Injury induces local translation of RanBPl, Importin PI,
Vimentin, STAT 3 and CREB3.
(8) and then alter
transcription.
(12) local translation
ofNMP35 and HMGBI and
(13) tubulin deacetylation by HDAC5 promotes axon regrowth.
Intrinsic regeneration program — injury signaling and adaptation
Conditional lesioning: How was it shown that intracellular cAMP levels are an important determinant of regeneration?
*Young DRG CNS axons could regenerate, but adult ones could not
*Regeneration of the CNS branch of the DRG neuron could be enhanced by a conditioning lesion to the PNS branch
*cAMP could mimic the effects of a conditioning lesion, overcoming even the inhibitory effects of myelin-associated inhibitors
*cAMP levels drop spontaneously and precipitously with development, correlating with the lost of regenerative ability
*Neurotrophin treatment appear to elevate cAMP
*cAMP acts through PKA and the transcription factor CREB. Amongst the genes elevated by cAMP is arginase 1, an enzyme responsible for the synthesis of polyamines
Intrinsic regeneration capacity — PTEN deletion enhances axonal regeneration
What are the possible mechanisms by which PTEN-inactivation induce axon regeneration?
mTOR activation may promote the synthesis of the raw materials for axon extension, while other signaling molecules such as P13K and GSK-3 could enhance axonal transport and cytoskeleton assembly in the axon terminal.
Intrinsic regeneration capacity — the importance of local axonal protein synthesis
What does axotomy trigger?
Axotomy triggers the translation of importin 131 and vimentin mRNAs. Vimentin links PERK to the importin—dynein complex such that the injury signal is retrogradely transported to the cell body. Zipcode binding protein I (ZBPI) is required for the axonal localization of ß-actin and GAP-43 mRNAs that are translated after injury. Increased anterograde transport of mitochondria is also elicited by injury. GAP-43, growth-associated protein-43; pERK, phosphorylated extracellular
signal-regulated kinase; ZBPI, zipcode-binding protein 1.
How might stem cells be used in CNS repair?
Endogenous adult neurogenesis
Source: Adult neurogenic regions (subventricular zone (SVZ) and dentate gyrus (DG)
Manipulation: Growth factors and drugs
Hope: Trophic enhancement of regeneration or replacement
Transplantation-based therapy
Source: Allogenic or autologous
Manipulation: Undifferentiated or differentiated
Hope: Trophic enhancement of regeneration or replacement
How might endogenous neural stem cells be used in CNS repair?
New neurons for replacement of dying neurons in injury and disease can be potentially derived from stem cells in the neurogenic zones, such as the SVZ. These new neurons have to undergo directed migration toward the lesioned CNS region. Alternatively, these new neurons may be derived from resident parenchymal neural stem cells. New neurons have to survive, differentiate/mature into site-specific functional neurons, and form appropriate axonal and dendritic connections in order to contribute to functional repair of the lesioned CNS region.
What are the potential sources of stem/progenitor cells for transplantation into the injured CNS
At least in theory, stem/progenitor cells can be collected at three different stages of development: from the inner cell mass layer of the mature blastocyst; from the brain, spinal cord, olfactory system or umbilical cord of the fetus; and from the brain, spinal cord, olfactory system, bone marrow or blood of the adult.
Some of these cells have the potential to be used for autologous transplantation, including
* cells from the olfactory system
* umbilical cord blood cells (which can be frozen at birth for use in later life) haematopoietic stem cells
* bone marrow stromal cells
Why may transplantation therapy require stringent checks (even with the use of pre-differentiated cells)
The cores of the grafts may contain
slowly dividing neuroepithelial cells that
are tumorigenic
