Proper formation of whisker barrelettes requires periphery-derived Smad4-dependent TGF-β signaling

Abstract

Mammalian somatosensory topographic maps contain specialized neuronal structures that precisely recapitulate the spatial pattern of peripheral sensory organs. In the mouse, whiskers are orderly mapped onto several brainstem nuclei as a set of modular structures termed barrelettes. Using a dual-color iontophoretic labeling strategy, we found that the precise topography of barrelettes is not a result of ordered positions of sensory neurons within the ganglion. We next explored another possibility that formation of the whisker map is influenced by periphery-derived mechanisms. During the period of peripheral sensory innervation, several TGF-β ligands are exclusively expressed in whisker follicles in a dynamic spatiotemporal pattern. Disrupting TGF-β signaling, specifically in sensory neurons by conditional deletion of Smad4 at the late embryonic stage, results in the formation of abnormal barrelettes in the principalis and interpolaris brainstem nuclei and a complete absence of barrelettes in the caudalis nucleus. We further show that this phenotype is not derived from defective peripheral innervation or central axon outgrowth but is attributable to the misprojection and deficient segregation of trigeminal axonal collaterals into proper barrelettes. Furthermore, Smad4-deficient neurons develop simpler terminal arbors and form fewer synapses. Together, our findings substantiate the involvement of whisker-derived TGF-β/Smad4 signaling in the formation of the whisker somatotopic maps.O ne prominent characteristic of the rodent whisker-somatosensory system is its precisely organized topographic sensory maps (1-3). Each whisker is innervated by peripheral axons of a subset of trigeminal sensory neurons whose cell bodies reside in the trigeminal ganglia (TG) and central axons project to the brainstem (4). Sensory afferents carrying information from individual whiskers segregate and converge to form modular structures termed barrelettes, whose spatial organization exactly mirrors that of the whiskers in the periphery (5). The barrelette map in the brainstem emerges during development and serves as a template for the subsequent generation of homologous upstream structures in the thalamus and cortex, termed barreloids and barrels, respectively (5, 6). Interestingly, induction of an extra whisker by exogenous expression of Shh during early development leads to the formation of an extra barrelette with a topographic position corresponding to that of the ectopic whisker (7). Together with other studies (8-10), these data suggest that the formation of the whisker map is under the strong instructive influence of the periphery. The whisker-derived signals regulating barrelette formation remain mostly unknown, however.Previous work showed that BMP4 signaling induces differential expression of genes in trigeminal sensory neurons innervating different areas of the face along the dorsoventral axis (11,12). At later developmental stages, multiple TGF-β superfamily ligands are expressed in whisker follicles during the period of sensory...

Facial whisker pattern is not sufficient to instruct a whisker-related topographic map in the mouse somatosensory brainstem

“…Our analysis of Edn1 −/− mutants further demonstrate that ectopic whisker arrays are selectively targeted by the mandibular branch, according to their position on the lower jaw, and that this innervation in turn induces significant maxillary-like molecular changes in mandibular TG neurons, supporting whisker-specific retrograde signalling (da Silva et al, 2011;Hodge et al, 2007). However, such molecular changes are not sufficient to instruct a maxillary-like, row-specific, pre-target axon sorting of the mandibular tract, nor to instruct the establishment of row-and/or whisker-related patterns of collateral targeting in the dorsal 'mandibular' area of PrV.…”

“…Whisker-related patterns of axon targeting are established prenatally in the hindbrain trigeminal nuclei Distinct rows in the whisker pad are innervated by neuronal populations with segregated latero-medial distribution within the TG maxillary portion (da Silva et al, 2011;Erzurumlu and Jhaveri, 1992;Erzurumlu and Killackey, 1983;Hodge et al, 2007), whereas barrelette topography is not prefigured by a pre-ordered position of neurons within row-specific TG sub-populations (da Silva et al, 2011). We therefore investigated the establishment of whisker-specific afferent patterns in the developing brainstem and carried out row-and whiskerspecific neuronal tracings with multicolour lipophylic NeuroVue fluorescent dyes, which allow controlled diffusion by progressive dye release from a filter membrane (Jensen-Smith et al, 2007).…”

“…Further refinement by whisker-related activity and/or retrograde molecular signalling from periphery contributes to achieve final one-to-one topography at postnatal stages. Recent studies addressed the importance of facial signals to refine central projections (da Silva et al, 2011;Hodge et al, 2007) whereas others highlighted the importance of brainstem patterning to receive appropriate and spatially restricted peripheral projections and allow topographic representation (Erzurumlu et al, 2010; Oury et al, 2006). However, the relative importance of facial signals versus TG pre-target axon sorting versus central prepatterning in the building of topographic equivalence between the face and the brainstem is still unclear.…”

“…Such peripheral changes were reflected on the somatotopy of the barrel map (Ohsaki et al, 2002;Van der Loos et al, 1984). Moreover, retrograde signalling from the developing face was shown to be involved in establishing spatial patterns of gene expression in the TG and transferring somatotopic information to the brainstem (da Silva et al, 2011;Hodge et al, 2007). Such experiments seemed to indicate an instructive role of signalling from facial receptors to establish ordered connectivity at the peripheral and central level for somatotopic map generation.…”

“…−/− mutant mandibular primary neurons upon ectopic whisker pad innervation Bmp4 and TGF-β retrograde signalling from the whisker pad and/or follicles has been suggested to be involved in organising topographic patterns of TG neuron central connectivity (da Silva et al, 2011;Hodge et al, 2007). In particular, Bmp4-dependent signalling regulates early positional differences of Tbx3 and Hmx1 transcription factor expression in distinct divisions of the TG (Hodge et al, 2007).…”