296
Xenopus
nucleic acids. Rapidly dividing cells, like the
ones present in tumors and embryonic tissue,
present metabolic changes favoring aerobic glycolysis, and the carbons provided by glycolysis
enter the pentose phosphate pathway for macromolecular biosynthesis ( Vander Heiden et
al. 2009 ; Krisher and Prather 2012 ; Love et al.
2014 ).
iii. NSPC maintenance and differentiation are regulated by metabolism. A glycolytic metabolism
is necessary for in vitro stem cell maintenance
and self-renewal, whereas an oxidative metabolism is associated with differentiation ( Rafalski
et al. 2012 ; Khacho et al. 2019 ). Additionally,
the expression profle and epigenetic state of the
stem cells regulate stemness and differentiation,
and many mitochondrial metabolic intermediates are used as co-factors by many histonemodifying proteins ( Khacho et al. 2019 ; Zhang
et al. 2018 ).
iv. Axon growth and the generation of collateral
projections to re-establish lost connections are
also energy-demanding processes ( Bradke et al.
2012 ). Enhancing energetic metabolism by
admini stration of creatine, which is processed
by creatine kinase for ATP generation, as well
as enhancing axonal mitochondrial transport,
enhance axonal regeneration after SCI in a
murine model ( Han et al. 2020 ).
6. Tissue remodeling: The lesion site in mammals is
characterized by the presence of a glial scar formed
by a non-neural lesion core containing f broblasts,
pericytes, and ECM components, among others,
surrounded by astrocytes that function to contain
the damage in order to prevent its spread onto adjacent viable neural tissue (O’Shea et al. 2017). In X.
laevis, R-stage animals present continuity of the
central canal and axon tracts at 20 dpt. In contrast,
NR-stage animals are not able to reconstitute the
spinal cord, and the ablation gap is flled with f broblast-like cells and ECM components (Figure 20.1;
Table 20.1) (Edwards-Faret et al. 2021; Muñoz et al.
2015). In agreement with these observations, transcriptomic analyses showed that ECM-related transcripts are up-regulated only in NR-stage animals
at 6 dpt, supporting that NR-stage, but not R-stage,
develop a glial scar (Table 20.2) (Edwards-Faret et
al. 2021; Lee-Liu et al. 2014).
20.4. FUTURE DIRECTIONS AND
IMPORTANT QUESTIONS
This chapter presented an overview of the different cellular
and genetic responses associated with SCI and regeneration,
in addition to evidence about how proper regulation of them
may explain the difference between R- and NR-capacities.
TABLE 20.2
Biological Process Regulated at the Transcriptomic
and/or Proteomic Level in R- and NR-Stages after
Spinal Cord Injury.
Biological Process
R-Stages
NR-Stages
Cell death
Early down-regulation (1 Early up-regulation (1
dpt)
dpt)
Blood coagulation Mild early up-regulation
Robust early
(1 dpt)
up-regulation (1 dpt)
Cell cycle
Early up-regulation (1, 2 dpt) Late up-regulation (6 dpt)
Development
Early down-regulation (1, Late down-regulation (6
2 dpt)
dpt)
Response to stress Early and transient
Sustained up-regulation
up-regulation (1, 2 dpt)
(1, 2, 6 dpt)
Metabolism
Sustained up-regulation
Late up-regulation (6 dpt)
(1, 2, 6 dpt)
Immune response
Mainly down-regulation
Mainly up-regulation (1,
Inf ammation
(1, 2 dpt)
2 dpt)
Neurogenesis
Early up-regulation (1, 2
Late up-regulation (6 dpt)
dpt)
Axonal growth cone No change
Late down-regulation (6
(axonal
dpt)
regeneration)
Glial scar
No change
Late up-regulation (6 dpt)
Source : Lee-Liu et al. 2014 ; Muñoz et al. 2015 ; Lee-Liu et al. 2018 ;
Edwards-Faret et al. 2021
dpt: days post transection
Even though progress has been made in understanding the
cellular and molecular mechanisms underlying these cellular responses, there is still much to elucidate.
The questions for future directions that now arise are
about the signaling pathways that are activated or repressed
in a differential manner between R- and NR-stages, allowing protection against further cell death in the R-stage and
induction of cellular proliferation. Some studies in other
model organisms have spotlighted the anti-apoptotic Bcl
family, which has been demonstrated to play a role in planarian regeneration (Pellettieri et al. 2010). Additionally, the
Wnt signaling pathway in planaria (Chera et al. 2009), and
the JNK and Wingless signaling pathways in Drosophila
(Ryoo et al. 2004), are activated during cell death after
injury, subsequently leading to proliferation and regeneration. It will be important to corroborate if these responses are
conserved across organisms and identify how they are regulated. As cell death and proliferation transcripts are already
regulated at 1 dpt in R-stage animals, a starting point would
be the study of the transcript and protein responses prior to
24 hours after SCI.
Another important aspect to study is the differentiation
of the proliferative NSPCs into postmitotic neurons. As
mentioned previously, regenerative organisms generate new
Xenopus
nucleic acids. Rapidly dividing cells, like the
ones present in tumors and embryonic tissue,
present metabolic changes favoring aerobic glycolysis, and the carbons provided by glycolysis
enter the pentose phosphate pathway for macromolecular biosynthesis ( Vander Heiden et
al. 2009 ; Krisher and Prather 2012 ; Love et al.
2014 ).
iii. NSPC maintenance and differentiation are regulated by metabolism. A glycolytic metabolism
is necessary for in vitro stem cell maintenance
and self-renewal, whereas an oxidative metabolism is associated with differentiation ( Rafalski
et al. 2012 ; Khacho et al. 2019 ). Additionally,
the expression profle and epigenetic state of the
stem cells regulate stemness and differentiation,
and many mitochondrial metabolic intermediates are used as co-factors by many histonemodifying proteins ( Khacho et al. 2019 ; Zhang
et al. 2018 ).
iv. Axon growth and the generation of collateral
projections to re-establish lost connections are
also energy-demanding processes ( Bradke et al.
2012 ). Enhancing energetic metabolism by
admini stration of creatine, which is processed
by creatine kinase for ATP generation, as well
as enhancing axonal mitochondrial transport,
enhance axonal regeneration after SCI in a
murine model ( Han et al. 2020 ).
6. Tissue remodeling: The lesion site in mammals is
characterized by the presence of a glial scar formed
by a non-neural lesion core containing f broblasts,
pericytes, and ECM components, among others,
surrounded by astrocytes that function to contain
the damage in order to prevent its spread onto adjacent viable neural tissue (O’Shea et al. 2017). In X.
laevis, R-stage animals present continuity of the
central canal and axon tracts at 20 dpt. In contrast,
NR-stage animals are not able to reconstitute the
spinal cord, and the ablation gap is flled with f broblast-like cells and ECM components (Figure 20.1;
Table 20.1) (Edwards-Faret et al. 2021; Muñoz et al.
2015). In agreement with these observations, transcriptomic analyses showed that ECM-related transcripts are up-regulated only in NR-stage animals
at 6 dpt, supporting that NR-stage, but not R-stage,
develop a glial scar (Table 20.2) (Edwards-Faret et
al. 2021; Lee-Liu et al. 2014).
20.4. FUTURE DIRECTIONS AND
IMPORTANT QUESTIONS
This chapter presented an overview of the different cellular
and genetic responses associated with SCI and regeneration,
in addition to evidence about how proper regulation of them
may explain the difference between R- and NR-capacities.
TABLE 20.2
Biological Process Regulated at the Transcriptomic
and/or Proteomic Level in R- and NR-Stages after
Spinal Cord Injury.
Biological Process
R-Stages
NR-Stages
Cell death
Early down-regulation (1 Early up-regulation (1
dpt)
dpt)
Blood coagulation Mild early up-regulation
Robust early
(1 dpt)
up-regulation (1 dpt)
Cell cycle
Early up-regulation (1, 2 dpt) Late up-regulation (6 dpt)
Development
Early down-regulation (1, Late down-regulation (6
2 dpt)
dpt)
Response to stress Early and transient
Sustained up-regulation
up-regulation (1, 2 dpt)
(1, 2, 6 dpt)
Metabolism
Sustained up-regulation
Late up-regulation (6 dpt)
(1, 2, 6 dpt)
Immune response
Mainly down-regulation
Mainly up-regulation (1,
Inf ammation
(1, 2 dpt)
2 dpt)
Neurogenesis
Early up-regulation (1, 2
Late up-regulation (6 dpt)
dpt)
Axonal growth cone No change
Late down-regulation (6
(axonal
dpt)
regeneration)
Glial scar
No change
Late up-regulation (6 dpt)
Source : Lee-Liu et al. 2014 ; Muñoz et al. 2015 ; Lee-Liu et al. 2018 ;
Edwards-Faret et al. 2021
dpt: days post transection
Even though progress has been made in understanding the
cellular and molecular mechanisms underlying these cellular responses, there is still much to elucidate.
The questions for future directions that now arise are
about the signaling pathways that are activated or repressed
in a differential manner between R- and NR-stages, allowing protection against further cell death in the R-stage and
induction of cellular proliferation. Some studies in other
model organisms have spotlighted the anti-apoptotic Bcl
family, which has been demonstrated to play a role in planarian regeneration (Pellettieri et al. 2010). Additionally, the
Wnt signaling pathway in planaria (Chera et al. 2009), and
the JNK and Wingless signaling pathways in Drosophila
(Ryoo et al. 2004), are activated during cell death after
injury, subsequently leading to proliferation and regeneration. It will be important to corroborate if these responses are
conserved across organisms and identify how they are regulated. As cell death and proliferation transcripts are already
regulated at 1 dpt in R-stage animals, a starting point would
be the study of the transcript and protein responses prior to
24 hours after SCI.
Another important aspect to study is the differentiation
of the proliferative NSPCs into postmitotic neurons. As
mentioned previously, regenerative organisms generate new