This paper seeks to contribute to the characterization of the relation between osteogenesis
and neurogenesis by approaching it from the field of the neurobiology of language
and cognition; specifically, from an evolutionary perspective. It is difficult to
ascertain how the hominin brain changed to support modern language and cognitive abilities
because we can only rely on skull remains. But insights can be gained from fossils
because the brain and the skull exhibit a tight relationship. Skull shape and brain
shape and connectivity influence one another (Roberts et al., 2010; Lieberman, 2011).
Craniofacial anomalies and cognitive disorders frequently co-occur (see Boeckx and
Benítez-Burraco, 2014a for review). So, “osteo” considerations can shed light on “neuro”
considerations (and vice versa). Importantly, main differences between anatomically-modern
humans (AMHs) and Neanderthals pertain not to the brain size, but to the more globularized
headshape of the former (Bruner, 2004). Globularity results from an AMH-specific developmental
trajectory after birth, at a stage when the brain is the primary determinant of skull
shape (Gunz et al., 2010). Globularization is not just a morphological change of the
skull. On the contrary, factors giving rise to globularity also have important neurofunctional
consequences. The hypothesis we have explored in our recent work is that the rewiring
of the hominin brain associated to globularization brought about our most distinctive
mode of cognition (see Boeckx and Benítez-Burraco, 2014a for details).
In a series of related papers (Boeckx and Benítez-Burraco, 2014a,b; Benítez-Burraco
and Boeckx, 2015) we have examined closely some of the most critical genes that may
contribute to skull globularity and that have been selected in AMHs. These also contribute
significantly to neurogenesis, as well as to neural specification, arealization of
the neo-cortex, neuronal interconnection, and synaptic plasticity. Eventually, the
very osteogenic signals that help build our distinctive skull also contributes to
build our distinctive mode of brain organization underlying our mode of cognition
and language abilities.
Our main candidate is RUNX2. A selective sweep in this gene occurred after our split
from Neanderthals (Green et al., 2010). It is a candidate for cleidocranial dysplasia
(Yoshida et al., 2003) and controls the closure of cranial sutures (Stein et al.,
2004). Together with DLX5 and TLE1 it regulates the integration of the parietal bone
(Depew et al., 1999; Stephens, 2006), a “hotspot” for globularization (Bruner, 2004).
However, it is also involved in the development of the hippocampal GABAergic neurons
as part of the GAD67 regulatory network (Pleasure et al., 2000; Benes et al., 2007).
Moreover, it seems to be also involved in the development of thalamus (Reale et al.,
2013). Its mutations cause mental diseases in which our mode of cognition is impaired
(Talkowski et al., 2012; Ruzicka et al., 2015). Importantly, RUNX2 is deeply implicated
in the regulation of osteocalcin (Paredes et al., 2004) and osteopontin (Shen and
Christakos, 2005), which are important for both bone formation and brain organization
(e.g., osteopontin-deficient mice suffer from thalamic neurodegeneration; Schroeter
et al., 2006).
Interestingly, RUNX2 is functionally connected to many genes that are important for
brain and language development, but also to bone formation. To begin with, RUNX2 is
a regulatory target of AUTS2 (Oksenberg et al., 2014). AUTS2 is among the genes found
to be differentially expressed after RUNX2 transfection in neuroblastomic cell lines
(Kuhlwilm et al., 2013). The first half of AUTS2 displays the strongest signal of
positive selection in AMHs compared to Neanderthals (Green et al., 2010). Mutations
in AUTS2 give rise to a host of cognitive impairments (see Oksenberg and Ahituv, 2013
for review). Interestingly, these routinely co-occur with skeletal abnormalities and/or
dysmorphic features (Beunders et al., 2013). AUTS2 interacts with some other proteins
like TBR1, RELN, SATB2, GTF2I, ZMAT3, or PRC1 that play a key role at the brain level
and have been related to ASD and other developmental disorders affecting cognition
and language (Oksenberg and Ahituv, 2013). Some of them directly interact with RUNX2.
For example, RUNX2 directly interacts with SATB2 (Hassan et al., 2010), a gene that
regulates stereotypic projections in the cortex (Srinivasan et al., 2012). This gene
has been related to ASD, intellectual disability, and language delays, as well as
craniofacial defects (Liedén et al., 2014) and plays a key role in osteoblast differentiation,
palate formation, and craniofacial development (Zhao et al., 2014). Crucially, the
interaction between SATB2 and RUNX2 is very relevant during osteogenesis (Hassan et
al., 2010; Gong et al., 2014). Specifically, several micro-RNAs (including miR-205
and miR-31), SATB2, RUNX2, osteopontin and osteocalcin interact complexly to modulate
the differentiation of bone mesenchymal stem cells into osteoblasts (Deng et al.,
2013; Hu et al., 2015). Interestingly, in the neural satb2 expression depends on both
Bmp and Shh (Sheehan-Rooney et al., 2013), which are genes we have highlighted in
our previous work. Moreover, SATB2 represses the expression of HOXA2 (Ye et al., 2011),
which is one of the targets of the famous “language gene” FOXP2 (Konopka et al., 2009).
HOX2A is involved in both the brain and bone formation. Accordingly, it contributes
to the hindbrain patterning (Miguez et al., 2012), acting upstream the guidance signals
Robo1, Robo2, Slit1, and Slit2 in the anteroposterior migration of pontine neurons
(Geisen et al., 2008). However, it also encodes an inhibitor of bone formation (Dobreva
et al., 2006; Ye et al., 2011), which controls the morphology of the skeleton (Tavella
and Bobola, 2010). Interestingly also, the activation of Hoxa2 in the neural crest
downregulates Bmp antagonists and leads to severe craniofacial and brain defects (Garcez
et al., 2014).
Additionally, RUNX2 interacts (via FOXO1) with DYRK1A (Huang and Tindall, 2007), a
gene located within the Down Syndrome Critical Region on chromosome 21. This gene
has been linked to microcephaly, facial dysmorphism, mental retardation, and absence
of speech (van Bon et al., 2011; Courcet et al., 2012). DYRK1A has been shown to be
involved in bone homeostasis as an inhibitor of osteoclastogenesis (Lee et al., 2009).
DYRK1A is also of interest because it phosphorylates SIRT1, which controls neural
precursor activity and differentiation (Saharan et al., 2013). SIRT1 both upregulates
RUNX2 and deacetylates RUNX2, ultimately promoting osteoblast differentiation (Shakibaei
et al., 2012; Srivastava et al., 2012), an effect which is also due to its effects
on β-catenin and FoxO in osteoblast progenitors (Iyer et al., 2014). Importantly,
resveratrol-induced SIRT1 activation promotes neuronal differentiation of human bone
marrow mesenchymal stem cells (Joe et al., 2015). Finally, RUNX2 is also functionally
related (via AUTS2) to CBL, in turn linked to Noonan syndrome-like disorder, a condition
involving facial dysmorphism, a reduced growth, and several cognitive deficits (Martinelli
et al., 2010). This gene, which encodes an inhibitor of osteoblast differentiation
and promotes the degradation of Osterix (Choi et al., 2015), is located within a region
showing signals of a strong selective sweep in AMHs compared to Altai Neanderthals
(Prüfer et al., 2014).
RUNX2 is also functionally directly linked to the FOXP2 and ROBO1 interactomes (see
Boeckx and Benítez-Burraco, 2014b for details), which are related to language disorders
and vocal learning (Graham and Fisher, 2013; Pfenning et al., 2014). To begin with,
a direct interaction between RUNX2 and FOXP2 has recently been experimentally demonstrated
(Zhao et al., 2015b). This finding was further reinforced in Gascoyne et al. (2015),
who added FOXP2 to the list of established osteoblast and chondrocyte transcription
factors such RUNX2, SP7, and SOX9. In fact, FOXP2 seems to regulate both bone formation
(it regulates endochondral ossification) (Zhao et al., 2015b), and the fate of neural
stem cells during corticogenesis (MuhChyi et al., 2013). As for the ROBO suite, some
members like HES1 and AKT1 are functionally related to RUNX2. HES1 is needed for the
correct functioning of the Slit/Robo signaling pathway during neurogenesis (Borrell
et al., 2012) and plays a role as well in the development of both GABAergic and dopaminergic
neurons. Hes1 silencing promotes bone marrow mesenchymal stem cells to differentiate
into GABAergic neuron-like cells in vitro (Long et al., 2013). Moreover, Hes1 modulates
skeletal formation and pathogenesis of osteoarthritis via calcium/calmodulin interaction
(Sugita et al., 2015). In turn AKT1 is a critical mediator of growth factor-induced
neuronal survival (Dudek et al., 1997). In mice mutations in Akt1 and Akt2 impair
bone formation (Peng et al., 2003). AKT1 has recently been shown to coordinate the
bone-forming osteoblasts and bone-resorbing osteoclasts, a process important for maintaining
skeletal integrity (Akt1 deficiency impairs osteoclast differentiation and diminishes
the rate of proliferation of osteoblast progenitors) (Mukherjee et al., 2014).
Other bone morphogenetic factors may well play a key role in the emergence of our
language-readiness and our globular brain. Among them we wish highlight the DLX suite
(particularly, DLX1, DLX2, DLX5, and DLX6) and the BMP suite (specifically, BMP2 and
BMP7): most of them also interact with RUNX2. Consider, e.g., DLX2. It is involved
in craniofacial development (Jeong et al., 2008), but it is also needed for neocortical
and thalamic growth (Jones and Rubenstein, 2004). Mutations in this gene affect craniofacial
and bone development (Kraus and Lufkin, 2006), but also cognitive development (Liu
et al., 2009). It also takes part in the regulation of neuronal proliferation within
the cortex (McKinsey et al., 2013). Concerning the BMP proteins, both BMP2 and BMP7
interact with RUNX2 and both of them play a role in bone and brain formation. BMP2
promotes the differentiation of mesenchymal cells into bone cells (Dwivedi et al.,
2012), but it is also needed for normal neurogenesis in the ganglionic eminences and
correct cortical neurogenesis (Shakèd et al., 2008). In mice Bmp2 (and also Bmp7)
upregulates Dlx1, Dlx2, Dlx5, and Runx2 (Bustos-Valenzuela et al., 2011). Much like
BMP2, BMP7 is involved in osteogenesis (Cheng et al., 2003) and skull and brain development
(Segklia et al., 2012). Mutations in this gene give rise as well to developmental
delay and learning disabilities (Wyatt et al., 2010).
We further believe that the genetic aspects highlighted here may contribute not only
to gain a better understanding of the way in which both aspects of our modernity emerged
and interact, but specifically to tune the crosstalk between the osteogenic and neurogenic
stem cell niches. Zhao et al. (2015a) have recently identified Gli1+ cells within
the suture mesenchyme as the main mesenchymal stem cell population for craniofacial
bones. Ablation of these Gli1+ cells leads to craniosynostosis, known to be associated
with cognitive deficits (Starr et al., 2007), and arrest of skull growth. Not surprisingly,
Gli1 is known to regulate Runx2 (Kim et al., 2013). In turn, Gli1 transcriptional
activity is regulated by Dyrk1a (Mao et al., 2002), whereas Hes1 directly modulates
Gli1 expression (Schreck et al., 2010). Moreover, Gli1 is the direct response gene
of Shh (Liu et al., 1998). The Shh-Gli1 pathway has been shown to regulate brain growth
(Dahmane et al., 2001; Ruiz i Altaba et al., 2002; Corrales et al., 2004), and to
control thalamic progenitor identity and nuclei specification (Vue et al., 2009),
as well as the development of the cerebellum (Lee et al., 2010). It may also be the
case that FoxP2 lies downstream of Shh, as suggested by Scharff and Haesler (2005),
who observed that the zinc finger motif of FoxP2 is highly homologous to those of
the major Shh downstream transcriptional effectors, particularly, of Gli1, Gli2, and
Gli3. Moreover, balanced Shh signaling is required for proper formation and maintenance
of dorsal telencephalic midline structure (Himmelstein et al., 2010). Dysregulation
of the neural stem cell pathway Shh-Gli1 has been observed in autoimmune encephalomyelitis
and multiple sclerosis (Wang et al., 2008). As a matter of fact, a GLI1-p53 inhibitory
loop controls neural stem cell (Stecca and Ruiz i Altaba, 2009). Most interestingly
for us, Marcucio et al. (2005) have shown that excessive Shh activity, caused by truncating
the primary cilia on cranial neural crest cells, causes hypertelorism, and frontonasal
dysplasia. This condition has been shown to be associated to mental retardation, lack
of language acquisition, and severe central nervous system deficiencies (Guion-Almeida
and Richieri-Costa, 2009). The latter example appears to lend credence to our final
claim that language and cognition are intimately related to the molecular mechanisms
associated with mesenchymal stem cell and neural stem cell populations.
Conflict of interest statement
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.