Interaction between Axons and Specific Populations of Surrounding Cells Is Indispensable for Collateral Formation in the Mammillary System

Abstract

BackgroundAn essential phenomenon during brain development is the extension of long collateral branches by axons. How the local cellular environment contributes to the initial sprouting of these branches in specific points of an axonal shaft remains unclear.Methodology/Principal FindingsThe principal mammillary tract (pm) is a landmark axonal bundle connecting ventral diencephalon to brainstem (through the mammillotegmental tract, mtg). Late in development, the axons of the principal mammillary tract sprout collateral branches at a very specific point forming a large bundle whose target is the thalamus. Inspection of this model showed a number of distinct, identified cell populations originated in the dorsal and the ventral diencephalon and migrating during development to arrange themselves into several discrete groups around the branching point. Further analysis of this system in several mouse lines carrying mutant alleles of genes expressed in defined subpopulations (including Pax6, Foxb1, Lrp6 and Gbx2) together with the use of an unambiguous genetic marker of mammillary axons revealed: 1) a specific group of Pax6-expressing cells in close apposition with the prospective branching point is indispensable to elicit axonal branching in this system; and 2) cooperation of transcription factors Foxb1 and Pax6 to differentially regulate navigation and fasciculation of distinct branches of the principal mammillary tract.Conclusions/SignificanceOur results define for the first time a model system where interaction of the axonal shaft with a specific group of surrounding cells is essential to promote branching. Additionally, we provide insight on the cooperative transcriptional regulation necessary to promote and organize an intricate axonal tree.

Specification of posterior hypothalamic neurons requires coordinated activities of Fezf2, Otp, Sim1a and Foxb1.2

“…In mouse mutants for the winged helix-loop-helix protein FOXB1 the MB does not develop properly, suggesting a crucial role for this transcription factor (Alvarez-Bolado et al, 2000;Labosky et al, 1997;Wehr et al, 1997). More recent studies revealed additional transcription factors, including SIM1, PAX6 and PITX2, that are involved in late aspects of MB differentiation, such as the development of the mammillothalamic tract (Marion et al, 2005;Skidmore et al, 2012;Szabó et al, 2011). However, only these few regulators of MA neuron development are currently known.…”

A novel TaulacZ allele reveals a requirement for Pitx2 in formation of the mammillothalamic tract

“…The principal mammillary tract (pmt) first appears at E10.0 and is one the earliest visible neuronal projections in the developing mouse brain (Easter et al ,1993). The pmt gives rise to mammillotegmental (mtg) and mammillotectal (mtc) tracts which are visible by E14.0 (Szabo et al ,2011). Between E17 and E18 in mouse, collaterals of the pmt branch off and project rostrally toward the anterior thalamus, giving rise to the mammillothalamic tracts (mtt) (Valverde,1998; Valverde et al .,2000).…”

“…Analysis of mouse mutants has revealed a small number of transcription factors that regulate formation of principal mammillary, mammillothalamic, and mtg projections: FOXB1 (Alvarez‐Bolado et al ,2000), PAX6 (Szabo et al ,2011; Valverde et al ,2000), and SIM1/SIM2 (Marion et al ,2005). Collaterals from the mtg projections form in the mouse around E17.5 and give rise to the mtt, through a process that depends on PAX6 and FOXB1‐mediated signaling events at the border between dorsal and ventral thalamus (Alvarez‐Bolado et al ,2000; Szabo et al ,2011; Valverde et al ,2000). Pax6 and Foxb1 mutants both have abnormalities in the mtt but have intact mtg projections, whereas Sim1/Sim2 double mutants lack both the mtt and mtg tracts (Alvarez‐Bolado et al ,2000; Marion et al ,2005; Valverde et al ,2000).…”

“…Pax6 is normally highly expressed near the bifurcation of the pmt and mtt. Pax6 mutant embryos selectively lack mtt (Tsuchiya et al ,2009; Valverde et al ,2000) and a specific group of Pax6 ‐expressing cells derived from the prethalamus is essential to elicit the pmt‐mtt axonal branching (Szabo et al ,2011). We tested whether PAX6‐positive cells are altered in Pitx2 mutants, using immunofluorescence on paraffin embedded sagittal sections of control versus conditional mutant E18.5 embryos.…”

Collateral branching of long‐distance cortical projections in monkey

“…To some extent, this is another version of the random versus specificity debate, as more usually formulated in the context of postsynaptic targets. In one model system, the mammillothalamic tract, branching at a specific point, is elicited by an interaction with a group of PAX6‐expressing cells, and the cooperation of transcription factors Foxb1 and Pax6 differentially regulates navigation and fasciculation (Szabo et al, ). This applies, however, to a well‐defined axon bundle, and it seems unlikely that similar mechanisms operate in the case of cortical axons, where the trajectories are typically more widely dispersed.…”