Morphology, input–output relations and synaptic connectivity of Cajal–Retzius cells in layer 1 of the developing neocortex of CXCR4-EGFP mice

Max Anstötz,Enrico Mugnaini,Gianmaria Maccaferri,Joachim H R Lübke,Iris Hack,Kathleen E Cosgrove

doi:10.1007/s00429-013-0627-2

Abstract

Layer 1 (L1) neurons, in particular Cajal–Retzius (CR) cells are among the earliest generated neurons in the neocortex. However, their role and that of L1 GABAergic interneurons in the establishment of an early cortical microcircuit are still poorly understood. Thus, the morphology of whole-cell recorded and biocytin-filled CR cells was investigated in postnatal day (P) 7–11 old CXCR4-EGFP mice where CR cells can be easily identified by their fluorescent appearance. Confocal-, light- and subsequent electron microscopy was performed to investigate their developmental regulation, morphology, synaptic input–output relationships and electrophysiological properties. CR cells reached their peak in occurrence between P4 to P7 and from thereon declined to almost complete disappearance at P14 by undergoing selective cell death through apoptosis. CR cells formed a dense and long-range horizontal network in layer 1 with a remarkable high density of synaptic boutons along their axons. They received dense GABAergic and non-GABAergic synaptic input and in turn provided synaptic output preferentially with spines or shafts of terminal tuft dendrites of pyramidal neurons. Interestingly, no dye-coupling between CR cells with other cortical neurons was observed as reported for other species, however, biocytin-labeling of individual CR cells leads to co-staining of L1 end foot astrocytes. Electrophysiologically, CR cells are characterized by a high input resistance and a characteristic firing pattern. Increasing depolarizing currents lead to action potential of decreasing amplitude and increasing half width, often terminated by a depolarization block. The presence of membrane excitability, the high density of CR cells in layer 1, their long-range horizontal axonal projection together with a high density of synaptic boutons and their synaptic input–output relationship suggest that they are an integral part of an early cortical network important not only in layer 1 but also for the establishment and formation of the cortical column.

Highlights

Cajal–Retzius cells, originally described by Ramon y Cajal (1891) and Retzius (1893, 1894) are beside so-called predecessor cells of the human embryonic forebrain (BystronBrain Struct Funct (2014) 219:2119–2139 et al 2006; for review see Bystron et al 2008) and ‘subplate’ neurons (McConnell et al 1989; Friauf et al 1990; Goodman and Shatz 1993) among the earliest generated neurons in the neocortex
Selective cell death through apoptosis of neocortical Cajal– Retzius (CR) cells. It is still controversially discussed with respect to different species when CR cells in the neocortex reached their peak of postnatal expression, when they begin to disappear from layer 1 and undergo selective cell death by apoptosis or whether they are diluted in the volume of the growing neocortex
The results of our work validate the CXCR4-EGFP mouse as a tool to facilitate the identification of CR cells in developing layer 1

Summary

Introduction

Cajal–Retzius cells, originally described by Ramon y Cajal (1891) and Retzius (1893, 1894) are beside so-called predecessor cells of the human embryonic forebrain (BystronBrain Struct Funct (2014) 219:2119–2139 et al 2006; for review see Bystron et al 2008) and ‘subplate’ neurons (McConnell et al 1989; Friauf et al 1990; Goodman and Shatz 1993) among the earliest generated neurons in the neocortex. As the neocortex further develops, CR cells become, beside a heterogeneous population of GABAergic interneurons (Lavdas et al 1999; Kubota et al 2011; Wozny and Williams 2011; Jiang et al 2013), the ‘principal’ neuron of the marginal zone, which will develop later to layer 1 (L1). Their laminar position is critically driven by chemokines produced by the leptomeninges via a specific signaling mediated by the CXCR4 receptor (Paredes et al 2006). CR cells start to disappear in rat around postnatal day (P) 15 (Derer and Derer 1990; Del Rio et al 1996, 1997; Mienville and Pesold 1999) and at P22 only \3.5 % of the population found at P3–P7 were observed in EBF2-GFP mice (Chowdhury et al 2010)

Methods

Results

Conclusion