Cadherins mediate sequential roles through a hierarchy of mechanisms in the developing mammillary body

Abstract

Expression of intricate combinations of cadherins (a family of adhesive membrane proteins) is common in the developing central nervous system. On this basis, a combinatorial cadherin code has long been proposed to underlie neuronal sorting and to be ultimately responsible for the layers, columns and nuclei of the brain. However, experimental proof of this particular function of cadherins has proven difficult to obtain and the question is still not clear. Alternatively, non-specific, non-combinatorial, purely quantitative adhesive differentials have been proposed to explain neuronal sorting in the brain. Do cadherin combinations underlie brain cytoarchitecture? We approached this question using as model a well-defined forebrain nucleus, the mammillary body (MBO), which shows strong, homogeneous expression of one single cadherin (Cdh11) and patterned, combinatorial expression of Cdh6, −8 and −10. We found that, besides the known combinatorial Cdh pattern, MBO cells are organized into a second, non-overlapping pattern grouping neurons with the same date of neurogenesis. We report that, in the Foxb1 mouse mutant, Cdh11 expression fails to be maintained during MBO development. This disrupted the combination-based as well as the birthdate-based sorting in the mutant MBO. In utero RNA interference (RNAi) experiments knocking down Cdh11 in MBO-fated migrating neurons at one specific age showed that Cdh11 expression is required for chronological entrance in the MBO. Our results suggest that neuronal sorting in the developing MBO is caused by adhesion-based, non-combinatorial mechanisms that keep neurons sorted according to birthdate information (possibly matching them to target neurons chronologically sorted in the same manner). Non-specific adhesion mechanisms would also prevent cadherin combinations from altering the birthdate-based sorting. Cadherin combinations would presumably act later to support specific synaptogenesis through specific axonal fasciculation and final target recognition.

Birthdate of parvalbumin-neurons in the Parvafox-nucleus of the lateral hypothalamus

“…Of the total number of neurons of the Parvafox-nucleus, 10% manifest double staining for both Foxb1 and Parv- (Bilella et al, 2014), thereby implying that these cells stem from the caudal periventricular zone (Alvarez-Bolado et al, 2000). Our study indicates that in rats the Parv-positive cells are born first around E10, their number peaks at E12 and decreases at E13, confirming what was already observed for the Foxb1-neurons in mice (AlvarezBolado et al, 2000;Szabó et al, 2015;Workman et al, 2013). This is a time-interval in which the majority of the lateral hypothalamic nerve cells are born (Croizier et al, 2014).…”

“…Foxb1 is a winged helix transcription factor involved in the development of the diencephalon during the embryogenesis and to a lesser extent during the postnatal development (Altman and Bayer, 1978;Alvarez-Bolado et al, 2000;Croizier et al, 2014;Iball et al, 1966;Markakis and Swanson, 1997;Szabo et al, 2015). Foxb1 is expressed in a "patch" of the caudal hypothalamic neuroepithelium that gives rise to neurons later forming defined hypothalamic nuclei.…”

Time to put the mammillothalamic pathway into context

“…The rostral boundary of the MBs is defined by the poorly differentiated pre-mammillary nucleus, while dorsally, the MBs are limited by horizontally oriented axonal fibres coursing beneath the supramammillary nucleus. The MBs’ morphogenesis relies on a tightly orchestrated cascade of genetic events ( Alvarez-Bolado et al, 2000 ; Marion et al, 2005 ; Szabo et al, 2015 ; Tsuchiya et al, 2009 ; Valverde et al, 2000 ; Zappala et al, 2007 ), disruption of which can result in deficits akin to those following targeted ablation of the MBs or the MTT ( Radyushkin et al, 2005 ).…”

Editorial: Development of the hypothalamus

“…Finally, the hypothalamus and its heterogeneity can be used as a model to investigate key brain development problems. Szabó et al ( 2015 ) approached a neglected developmental problem, the specific aggregation of neurons to form neuronal nuclei, using the mammillary body as a model. The authors show that two information systems, based on birthdates and adhesion mechanisms, work together in order to guarantee appropriate neuronal aggregation as well as specific axonal fasciculation.…”