Molecular determinants of the face map development in the trigeminal brainstem

Abstract

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.

Fig. 1

Elongation and arborization phases of TG axons. The whiskerpad TG brainstem whole-mount explant is illustrated on the left and TG axon elongation, collateralization, and arborization phases are depicted on the right. ATr, ascending trigeminal tract; DTr, descending trigeminal tract; TG, trigeminal ganglion; BSTC, brainstem trigeminal complex; WF, whisker follicle; WP, whiskerpad.

Fig. 2

Trigeminal system coculture setups. A: TG and its projections in the embryo. D, dorsal; R, rostral; BSTC, brainstem trigeminal complex. B: TG axon growth when a single E15 TG is cocultured with two whiskerpads. C: Age-mismatched TG with brainstem targets. E15 TG axons elongate in E15 brainstem explant but arborize in P0 brainstem explants and form synaptic contacts. D: Following TG axon growth into older brainstem explants, synaptic activity can be recorded in the trigeminal nuclei after stimulation of the TG. E: Experimental setup for whole-mount brainstem explant cultures with TG intact on both sides. Tr, trigeminal tract; R, rostral; M, medial.

Fig. 3

Left: Schematic diagram of trigeminal pathway whole-mount cultures and neurotrophin-loaded bead placement along the central trigeminal pathway. Cartoon diagrams illustrate the preparation of flattened whole-mounts and indicate the position of the neurotrophin-loaded beads with respect to the trigeminal tract and brainstem trigeminal nuclei. WP, whisker pad; BSTC, brainstem trigeminal complex; ATr, ascending trigeminal tract; TG, trigeminal ganglion; DTR, descending trigeminal tract. Right: Camera lucida drawings show the differential effects of NGF- and NT3-loaded beads on central trigeminal axons. Scale bar = 100 µm. Figure modified from Ozdinler et al. (2004).

Fig. 4

Nissl-stained sagittal sections shows the size of the trigeminal ganglion in wild-type (A), NT3 KO (B), NT3/Bax KO (C), and Bax KO (D) mice of equivalent ages. TrkA (red) and TrkC (green) immunostaining shows nonoverlapping groups of TrkA- and TrkC-positive TG cells in E13 wild-type (wt; E), Bax KO (F), Bax/NT3 double KO (G), and NT3 KO (H) mice. Scale bars = 2 mm (A–D); 100 µm (E–G). Figure provided by B. Genc.

Fig. 5

Effects of Slit2N on central trigeminal tract axons (as visualized by DiI labeling) during the elongation phase (in brainstem whole-mount cultures). A: Normal E15 rat descending trigeminal tract (Dtr) in an explant culture maintained in SFM. B: E15 rat Dtr in the presence of control HEK cells. C: E15 rat Dtr in the presence of HEK cells secreting Slit2U. D: E15 rat Dtr in the presence of HEK cells secreting Slit2N. E: E15 Dtr in the presence of conditioned medium from Slit2N-secreting cells. Branching/arborization of E15 Dtr axons medial to the tract in the BSTC in the presence of concentrated Slit2N in the medium (F) and in the presence of cells secreting Slit2N (G). In all micrographs, anterior is up and medial is to the right. Scale bar = 20 µm. For orientation and location of the Dtr, refer to Figure 1.

Fig. 6

CO staining in the PrV and SpVi of wild-type, Lmx1b, Drg11 knockout, and NR1 knockdown mice. Top row of micrographs (A–D) show the PrV of Lmx1b^+/+, Lmx1b^−/−, Drg11^+/+, Drg11^−/− mice at P0. Barrelette patterns in +/+ cases are indicated by letters a–e, corresponding to those rows of whiskers on the face. Middle row of micrographs (E–H) show whisker-related barrelette patterns in the SpVi of the same series of animals. Bottom row of micrographs show barrelette patterns in the PrV (I) and SpVi (K) of NR1^+/+ mice and the PrV of NR1KD mice (J and L, respectively) at P14. Scale bars = 200 µm (A–H); 500 µm (I–L).