Cellular Patterns of Transcription
Factor Expression in Developing
Inma Cobos, Jason E. Long, Myo T. Thwin and
John L. Rubenstein
Nina Ireland Laboratory of Developmental Neurobiology,
Department of Psychiatry, University of California at San
Francisco, San Francisco, CA 94158, USA
Most g-aminobutyric acidergic interneurons in the neocortex and
hippocampus are derived from subpallial progenitors in the medial
ganglionic eminence and migrate tangentially to the pallium, where
they differentiate into a diverse set of neuronal subtypes. Toward
elucidating the mechanisms underlying the generation of interneuron
diversity, we have studied in mice the expression patterns in dif-
ferentiating and mature neocortical interneurons of 8 transcription
factors, including 6 homeobox (Dlx1, Dlx2, Dlx5, Arx, Lhx6, Cux2), 1
basic helix-loop-helix, (NPAS1), and 1 bZIP (MafB). Their patterns
of expression change during interneuron differentiation and show
distinct distributions within interneuron subpopulations in adult
neocortex. This study is a first step to define the combinatorial
codes of transcription factors that participate in regulating the
specification and function of cortical interneuron subtypes.
Keywords: calretinin, GAD, inhibition, parvalbumin, somatostatin
c-Aminobutyric acidergic (GABAergic) interneurons are essen-
tial components of neocortical and hippocampal microcircuit
organization and function. For example, they are implicated in
the generation of synchronous population discharge patterns
that are thought to be involved in various cognitive processes
(Freund 2003; Whittington and Traub 2003). Defects in GABAer-
gic transmission have been implicated in diverse neurological
and psychiatric diseases including epilepsy, depression, anxiety,
schizophrenia, autistic disorders, and addiction (Freund 2003;
Noebels 2003; Horike and others 2005; Lewis and others 2005).
These complex functions involve an extremely diverse set
of morphologically and physiologically distinct GABAergic in-
terneurons (Ramon y Cajal 1911; Jones 1975; DeFelipe 1993;
Kubota and others 1994; Gonchar and Burkhalter 1997;
Kawaguchi and Kubota 1997; Gupta and others 2000; McBain
and Fisahn 2001; Markram and others 2004; Sugino and others
2006). Classification of cortical interneurons has been challeng-
ing and is still under debate, in part because of the considerable
overlap in expression of markers in different anatomical and
physiological interneuron classes. These markers include Ca2+-
binding proteins, such as calbindin, calretinin (CR), and parval-
bumin (PV), and neuropeptides, such as somatostatin (SOM),
cholecystokinin, neuropeptide Y (NPY), and vasoactive intestinal
peptide. Recent work combining anatomical, physiological, and
molecular tools suggests that although the expression of single
gene cannot isolate any one anatomical class, profiles of expres-
sion of Ca2+-binding proteins and neuropeptides can reliably
predict anatomical types (Toledo-Rodriguez and others 2005).
An interesting question is how this interneuron diversity is
generated in the developing telencephalon. Accumulating evi-
dence suggests that different subtypes of interneurons are
specified by the action of distinct combinations of transcrip-
tion factors that begin acting in interneuron progenitors in
different compartments of the telencephalic neuroepithelium.
This appears to be a mechanism underlying cell fate specifica-
tion for different regions and cell types within the subpallium
(Campbell 2003), spinal cord (Briscoe and others 2000), and
retina (Livesey and Cepko 2001). Recent data provide evidence
that some morphologically and physiologically defined cortical
interneuron subtypes are derived from distinct subpallial sub-
divisions (Nery and others 2002; Xu and others 2004; Butt and
others 2005; Flames and Marin 2005). For example, PV+and
SOM+interneurons are derived from progenitors that express
the transcription factors Nkx2.1 and Dlx within the medial
ganglionic eminence (MGE), whereas CR+interneurons appear
to derive from Nkx2.1–;Dlx+-expressing progenitors in the
caudal ganglionic eminence (Xu and others 2004). In addition
to intrinsic molecular cues, interneuron diversity could be
regulated by ‘‘extrinsic’’ factors produced in the environment of
the differentiating interneurons. Extrinsic factors could include
neurotrophins or neural activity (Marty and others 1997); they
could act in distinct cortical layers or regions and during
distinct time windows.
Postmitotic subpallial-derived interneurons remain relatively
immature during an extended developmental period. They do
not exhibit fully distinct morphological or physiological prop-
erties until they have reached their final positions in the cortex
and integrated into the cortex circuitry. Differentiating inter-
neurons may change their intrinsic properties and competency
to respond to local cues over time; these changes could con-
tribute to subtype specification. Therefore, it is important to
identify the transcription factors that are expressed at various
times during the prolonged period of cortical interneuron dif-
ferentiation. Transcription factors expressed during perinatal
and early postnatal stages could be important for regulating
their laminar position, neurite outgrowth, patterns of dendrite
and axon morphogenesis, formation of synapses onto different
target cells and different target-cell domains (soma, dendrites,
or axons), and their electrophysiological properties. Here,
we approach this subject by studying the patterns of expression
of 8 transcription factors (Dlx1, Dlx2, Dlx5, Arx, Lhx6, Cux2,
NPAS1, MafB) in postmitotic immature and mature cortical
interneurons in mice.
The mouse mutant strain with double null alleles of Dlx1 and Dlx2
(Qiu and others 1997) and the transgenic lines Dlx1 bacterial artificial
chromosome (BAC) and Lhx6 BAC (GENSAT project, http://www.
gensat.org) were used. Mouse colonies were maintained at the
Cerebral Cortex 2006;16:i82--i88
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Transcription Factors in Cortical Interneurons
Cobos and others
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