295
Understanding Spinal Cord Regeneration
has no effect (Tseng et al. 2007). Even more interesting, tail amputation during the refractory period
results in broader apoptosis, which could be
related to the absence of regeneration (Tseng et al.
2007). Concordantly, in the transection paradigm,
R-stages showed downregulation of cell death
transcripts (Lee-Liu et al. 2014) and upregulation
of proteins that negatively regulate programmed
cell death (Lee-Liu et al. 2018) at 1 dpt, whereas
NR-stages show no down-regulation of cell death
at any analyzed time point (Table 20.2).
2. Immune cell response and infammation: The
immune response in the central nervous system has
a dual effect, defned by the different microglia/
macrophage phenotypes: the M1 or pro-inf ammatory
and the M2 or anti-infammatory (Jha et al. 2016).
The pro-infammatory response is needed to clear
cellular debris and facilitate repair, but the release
of pro-infammatory cues also could exacerbate cellular and extracellular matrix damage and increase
immune cell infltration. On the other hand, an antiinfammatory response is needed for inf ammatory
resolution, tissue remodeling, and repair (Hamilton
et al. 1999). In regenerative model animals, this
dual immune response is observed during regeneration. Zebrafsh respond to SCI with an initial
M1 phenotype, which is necessary for induction of
axon regeneration, as inhibition of TNF-α impairs
axon re-growth. This is followed by a M2 polarization within 1 dpt, which is necessary for reducing
pro-infammatory cytokines, as excessive IL-1β
also impairs axon re-growth (Tsarouchas et al.
2018). Mice show a completely different response.
Initially, the genes involved in M1 and M2 polarization are activated. This activation is predominant
for the M1-related genes, leading to M1 polarization
lasting for at least 1 month, which results in chronic
infammation (Kigerl et al. 2009). In agreement
with these results, both transcriptomic ( Lee-Liu et
al. 2014) and proteomic (Lee-Liu et al. 2018) analyses in X. laevis show that NR-stages present a positive regulation of immune response related genes
and proteins, while in R-stages, a negative regulation is mainly observed (Table 20.2).
3. Neurogenesis: After SCI, extensive cell death and
tissue damage occur, and cellular proliferation and
differentiation capacities correlate with the ability of certain animals, like urodele and anuran
amphibians and teleost fsh, to regenerate their
spinal cord ( Ferretti et al. 2003; Diaz Quiroz and
Echeverri 2013). R-stage animals respond to SCI
with an early up-regulation of transcripts related to
cell cycle (1–2 dpt) and cell division (2 dpt), whereas
NR-stage animals present a late up-regulation of
transcripts related to cell cycle (6 dpt) (Table 20.2)
(Lee-Liu et al. 2014). Following cell proliferation,
cell fate commitment is a decisive aspect for spinal
cord regeneration. Regenerative animal models are
able to generate new neurons after SCI (Benraiss
et al. 1999; Ghosh and Hui 2016; Muñoz et al.
2015), whereas mammals generate only new astrocytes and oligodendrocytes (Barnabé-Heider et al.
2010; Meletis et al. 2008). Transcript and protein
analyses in X. laevis have shown that this different capacity is in part intrinsically regulated within
the cells. R-stage animals show an up-regulation
of neurogenesis-related transcripts (Lee-Liu et al.
2014; Muñoz et al. 2015) and proteins (Muñoz et al.
2015), while they are absent or down-regulated in
NR-stage animals (Lee-Liu et al. 2014; Muñoz et al.
2015 ) ( Table 20.2 ).
4. Axon regeneration: Regenerative animal models, like
zebrafsh, regenerate axons and recover lost connections after SCI (Ghosh and Hui 2018), while mammals present a very limited capacity (Kerschensteiner
et al. 2005). In the same way, in R-stage animals, it is
possible to see axon bundles crossing the injury gap,
which are absent in the NR-stage animals (Figure
20.1) (Edwards-Faret et al. 2021; Muñoz et al. 2015).
These axons may be derived from new neurons
(Muñoz et al. 2015), regenerating axons (Gibbs and
Szaro 2006), or both. Consistent with these observations, transcripts related to axon growth, including
growth cone and axonal guidance, are differentially
regulated in R- versus NR-stages, being down-regulated only in NR-stages (Table 20.2) (Lee-Liu et al.
2014 ).
5. Metabolic regulation: More than 50% of regulated transcripts in R-stage animals are metabolic
related, being highly predominant in both up- and
down-regulated genes starting at 1 dpt and continuing until 6 dpt, while NR-stage animals present a
later regulation of metabolic related transcripts,
observed only at 6 dpt (Table 20.2). The preponderance of metabolic genes changing their expression
during the early phases of the regenerative response
can be explained by many reasons. Among others,
we propose the following:
i. The regulation of microglia/macrophage activation and M1 to M2 polarization by cellular
metabolic changes has been extensively studied
in murine cell lines. Under normal conditions,
immune cells are quiescent, and when they are
exposed to a pro-inf ammatory environment,
a metabolic switch from an oxidative to a glycolytic metabolism occurs, resulting in their
activation and polarization to a M1 phenotype,
while a new metabolic change favoring oxidative phosphorylation directs towards an M2 phenotype ( Afridi et al. 2020 ; Jha et al. 2016 ).
ii. Cellular proliferation is an energy demanding
process, which needs the production of the cellular building blocks like lipids, proteins, and
Understanding Spinal Cord Regeneration
has no effect (Tseng et al. 2007). Even more interesting, tail amputation during the refractory period
results in broader apoptosis, which could be
related to the absence of regeneration (Tseng et al.
2007). Concordantly, in the transection paradigm,
R-stages showed downregulation of cell death
transcripts (Lee-Liu et al. 2014) and upregulation
of proteins that negatively regulate programmed
cell death (Lee-Liu et al. 2018) at 1 dpt, whereas
NR-stages show no down-regulation of cell death
at any analyzed time point (Table 20.2).
2. Immune cell response and infammation: The
immune response in the central nervous system has
a dual effect, defned by the different microglia/
macrophage phenotypes: the M1 or pro-inf ammatory
and the M2 or anti-infammatory (Jha et al. 2016).
The pro-infammatory response is needed to clear
cellular debris and facilitate repair, but the release
of pro-infammatory cues also could exacerbate cellular and extracellular matrix damage and increase
immune cell infltration. On the other hand, an antiinfammatory response is needed for inf ammatory
resolution, tissue remodeling, and repair (Hamilton
et al. 1999). In regenerative model animals, this
dual immune response is observed during regeneration. Zebrafsh respond to SCI with an initial
M1 phenotype, which is necessary for induction of
axon regeneration, as inhibition of TNF-α impairs
axon re-growth. This is followed by a M2 polarization within 1 dpt, which is necessary for reducing
pro-infammatory cytokines, as excessive IL-1β
also impairs axon re-growth (Tsarouchas et al.
2018). Mice show a completely different response.
Initially, the genes involved in M1 and M2 polarization are activated. This activation is predominant
for the M1-related genes, leading to M1 polarization
lasting for at least 1 month, which results in chronic
infammation (Kigerl et al. 2009). In agreement
with these results, both transcriptomic ( Lee-Liu et
al. 2014) and proteomic (Lee-Liu et al. 2018) analyses in X. laevis show that NR-stages present a positive regulation of immune response related genes
and proteins, while in R-stages, a negative regulation is mainly observed (Table 20.2).
3. Neurogenesis: After SCI, extensive cell death and
tissue damage occur, and cellular proliferation and
differentiation capacities correlate with the ability of certain animals, like urodele and anuran
amphibians and teleost fsh, to regenerate their
spinal cord ( Ferretti et al. 2003; Diaz Quiroz and
Echeverri 2013). R-stage animals respond to SCI
with an early up-regulation of transcripts related to
cell cycle (1–2 dpt) and cell division (2 dpt), whereas
NR-stage animals present a late up-regulation of
transcripts related to cell cycle (6 dpt) (Table 20.2)
(Lee-Liu et al. 2014). Following cell proliferation,
cell fate commitment is a decisive aspect for spinal
cord regeneration. Regenerative animal models are
able to generate new neurons after SCI (Benraiss
et al. 1999; Ghosh and Hui 2016; Muñoz et al.
2015), whereas mammals generate only new astrocytes and oligodendrocytes (Barnabé-Heider et al.
2010; Meletis et al. 2008). Transcript and protein
analyses in X. laevis have shown that this different capacity is in part intrinsically regulated within
the cells. R-stage animals show an up-regulation
of neurogenesis-related transcripts (Lee-Liu et al.
2014; Muñoz et al. 2015) and proteins (Muñoz et al.
2015), while they are absent or down-regulated in
NR-stage animals (Lee-Liu et al. 2014; Muñoz et al.
2015 ) ( Table 20.2 ).
4. Axon regeneration: Regenerative animal models, like
zebrafsh, regenerate axons and recover lost connections after SCI (Ghosh and Hui 2018), while mammals present a very limited capacity (Kerschensteiner
et al. 2005). In the same way, in R-stage animals, it is
possible to see axon bundles crossing the injury gap,
which are absent in the NR-stage animals (Figure
20.1) (Edwards-Faret et al. 2021; Muñoz et al. 2015).
These axons may be derived from new neurons
(Muñoz et al. 2015), regenerating axons (Gibbs and
Szaro 2006), or both. Consistent with these observations, transcripts related to axon growth, including
growth cone and axonal guidance, are differentially
regulated in R- versus NR-stages, being down-regulated only in NR-stages (Table 20.2) (Lee-Liu et al.
2014 ).
5. Metabolic regulation: More than 50% of regulated transcripts in R-stage animals are metabolic
related, being highly predominant in both up- and
down-regulated genes starting at 1 dpt and continuing until 6 dpt, while NR-stage animals present a
later regulation of metabolic related transcripts,
observed only at 6 dpt (Table 20.2). The preponderance of metabolic genes changing their expression
during the early phases of the regenerative response
can be explained by many reasons. Among others,
we propose the following:
i. The regulation of microglia/macrophage activation and M1 to M2 polarization by cellular
metabolic changes has been extensively studied
in murine cell lines. Under normal conditions,
immune cells are quiescent, and when they are
exposed to a pro-inf ammatory environment,
a metabolic switch from an oxidative to a glycolytic metabolism occurs, resulting in their
activation and polarization to a M1 phenotype,
while a new metabolic change favoring oxidative phosphorylation directs towards an M2 phenotype ( Afridi et al. 2020 ; Jha et al. 2016 ).
ii. Cellular proliferation is an energy demanding
process, which needs the production of the cellular building blocks like lipids, proteins, and