A study of the generation of diversity in the central nervous system

Developmental biology is concerned with understanding the mechanisms that
govern the generation of a whole organism starting from one single cell. In the central
nervous system (CNS) the development of different classes of neurons and glial cells
involves both extrinsic signals and intrinsic cues that together govern the specification
of different cell fates dependent on position within the CNS and the time of generation.
Different vertebrate species share many aspects of early development as well as
the underlying mechanisms governing the progress of development. Therefore, a
plausible assumption is that functional regions in the genome are also conserved
between species. In Paper I, we have used a comparative genomics approach to
identify Highly Conserved Non‐coding Regions (HCNRs) between the human, mouse
and pufferfish genomes. We find HCNRs to be statistically over represented in the
proximity of transcription factors associated with spatial patterning in the developing
neural tube. We show that HCNRs associated with patterning genes show an
overrepresentation of binding sites for three transcription factors (Sox, Pou and
Homedomain genes (SPHD)). By combining bioinformatics and large-scale expression
analysis, we show that SPHD enriched HCNRs are strong predictors of CNS
expression during development (83% vs. 36% of random control genes). This
suggested to us that SPHD+ HCNRs may act as CNS enhancers. Further, we isolate a
putative HCNR enhancer region and show that it acts as an enhancer both in vivo and in
vitro. Based on our findings, we propose a model where Sox and Pou proteins act as
common activators of CNS expressed genes, while homeodomain proteins, which have
been previously shown to act as repressors, act to restrict expression spatially.
While a large number of studies have provided insight into the spatial patterning
mechanisms directing the generation of distinct cell types at different positions, little is
known about the temporal mechanisms underlying the specification of different cell
types from a common pool of progenitors in the CNS. In Paper II, we have addressed
the question of how a seemingly homogenous population of progenitor cells in the
caudal hindbrain can give rise to distinct subtypes of vagal visceral motoneurons
(vMNs). We show that based on molecular marker expression we can distinguish
between at least three subtypes of vMNs at early developmental time points and that
each subtypes corresponds to a distinct projections pattern in the periphery. We show
that these subtypes are generated sequentially and that the decision to become a specific
subtype is independent of contacts with peripheral targets and cell‐cell mediated
interactions. Further, the homeodomain transcription factor Nkx6.1 and the orphan
nuclear receptor Nurr1 are required for the specification of early born subtypes and the
maturation of late born subtypes, respectively.
In Paper III we were concerned with the origins of oligodendrocytes in the
developing spinal cord and hindbrain. Oligodendrocytes have been shown to be
generated from a ventrally located domain in the spinal cord and while this ventral
origin has been widely accepted, the existence of other origins remained subject to
debate. We show, based on in vitro cultures as well as mutant analysis, that dorsal
domains in the spinal cord can give rise to oligodendrocyte precursors and that these
precursors have the capacity to develop to bona‐fide mature oligodendrocytes based on
expression of mature markers. Further we show that, at least at prenatal stages,
ventrally and dorsally generated oligodendrocytes exhibit differences in expression
profiles, suggesting potential differences between these populations. Additionally, our
data suggests that the decrease in BMP signaling, a known inhibitor of
oligodendrogenesis, in the dorsal spinal cord over time, due to the increase in the size
of the neural tube, may influence the time of induction of the dorsally generated
oligodendrocyte precursors in spinal cord. Also, our data from the spinal cord and the
hindbrain, show that ventral oligodendrogenesis at different anteroposterior levels is
governed by different genetic programs