In the adult brain, the thalamocortical tract conveys sensory information from the
external environment to the cortex. The cortex analyzes and integrates this information
and sends neural responses back to the thalamus through the corticothalamic tract. To
reach their final target both thalamocortical and corticothalamic axons have to cover
long distances during embryogenesis, changing direction several times and passing
through different brain territories. The ventral telencephalon plays a major role in the
early development of these tracts. At least three main axon guidance mechanisms act in
the ventral telencephalon. First, two different populations of pioneer neurons in the
lateral ganglionic eminence (LGE) (LGE pioneer neurons) and medial ganglionic
eminence (MGE) (MGE pioneer neurons) provide scaffolds which allow growing
corticothalamic and thalamocortical axons to cross the pallium sub pallium boundary
(PSPB) and the diencephalic telencephalic boundary (DTB), respectively. Second, the
ventral telencephalon forms a permissive corridor for thalamic axons by tangential
migration of Isl1 and Ebf1 expressing cells from the LGE into the MGE. Finally,
thalamortical and corticothalamic axons guide each other once they have met in the
ventral telencephalon (“handshake hypothesis”).
The Gli3 transcription factor has been shown to be essential for normal early embryonic
regionalization of the mammalian forebrain, although roles of Gli3 in later aspects of
forebrain development, like the formation of axonal connections, have not been investigated previously. Here, I present the analysis of axonal tract development in the
forebrain of the Gli3 hypomorphic mutant mouse Polydactyly Nagoja (Pdn). These
animals lack the major axonal commissures of the forebrain: the corpus callosum, the
hippocampal commissure, the anterior commissure and the fimbria. In addition, DiI
injections and neurofilament (NF) staining showed defects in the formation of the
corticothalamic and thalamocortical tracts. Although the Pdn/Pdn cortex forms early
coticofugal neurons and their axons, these axons do not penetrate the LGE and instead
run along the PSPB. Later in development, although a thick bundle of Pdn/Pdn cortical
axons is still observed to project along the PSPB, some Pdn/Pdn cortical axons
eventually enter the ventral telencephalon navigating along several abnormal routes until
they reach thalamic regions. In contrast, Pdn/Pdn thalamic axons penetrate into the
ventral telencephalon at early stages of thalamic tract development. However, rostrally
they deviate from their normal trajectory, leaving the internal capsule prematurely and
only few of them reach the developing cortex. Caudally, an ectopic Pdn/Pdn dorsal
thalamic axon tract projects ventrally in the ventral telencephalon not entering the
internal capsule at all. These defects are still observed in newborn Pdn/Pdn mutant mice.
Next, I investigated the developmental mechanisms causing these pathfindings defects.
No obvious defects are present in Pdn/Pdn cortical laminae formation and in the
patterning of the Pdn/Pdn dorsal thalamus. In addition, Pdn/Pdn thalamocortical axons
are able to respond to ventral telencephalic guidance cues when transplanted into wild
type brain sections. However, these axonal pathfinding defects correlate with patterning
defects of the Pdn/Pdn LGE. This region is partially ventralized and displays a reduction
in the number of postmitotic neurons in the mantle zone due to an elongated cell cycle length of LGE progenitor cells. Finally, Pdn/Pdn mutant display an upregulation of Shh
expression and Shh signalling in the ventral telencephalon. Interestingly, these
patterning defects lead to the absence of DiI back-labelled LGE pioneer neurons, which
correlates with the failure of corticothalamic axons to penetrate the ventral
telencephalon. In addition, ventral telencephalic thalamocortical guidance mistakes
happen at the same time of abnormal formation of the corridor cells.
Taken together these data reveal a novel role for Gli3 in the formation of ventral
telencephalic intermediate cues important for the development of the thalamocortical
and corticothalamic connections. Indeed, Pdn animals are the first known mutants with
defective development of the LGE pioneer neurons, and their study provides a link
between early patterning defects and axon pathfinding in the developing telencephalon
