Identity of FEZF2-Expressing Neurons in Layer 5 of the Adult Mouse Motor Cortex

Abstract

For over a century the primary motor cortex (M1) has remained a research active area of the cerebral cortex. It is an intricately organised structure with billions of diverse neurons forming precise neuronal circuits (Anderson et al. 2010; Hooks et al. 2013). Knowing the identity of the neurons (i.e. shape, connectivity, molecular expression and electrical properties) embedded in these circuits will be a fundamental step towards understanding M1 circuit function (Brown and Hestrin 2009). A major challenge in probing neuronal identity is to understand how gene expression relates to the physiological properties of neurons. Progress is recently being made towards identifying master transcription factors that regulate the expression of genes to generate a variety of cell types with distinct identities (Molyneaux et al. 2007). One such master controller is FEZF2 (forebrain embryonic zinc finger protein 2), expression of which is both necessary and sufficient in specifying layer 5 projection neuron (PN) fate during development (Chen et al. 2005; De la Rossa et al. 2013; Molyneaux et al. 2005; Rouaux and Arlotta 2010). Importantly, Fezf2 continues to be expressed in adult PNs (Chen et al. 2005; Özdinler et al. 2011), suggesting that even after development it is required for maintenance of neuronal identity. However, the identity of neurons that express Fezf2 in the adult brain remains to be established, and particularly whether they form a distinct subpopulation in the layer 5 of M1. This research aimed to identify Fezf2-expressing PNs in layer 5 of M1. To do this, a transgenic mouse was utilized that expresses green fluorescent (GFP) protein to report Fezf2 expression (Fezf2-GFP). Fezf2-expressing PNs, and neighbouring Fezf2-negative PNs for comparison, were targeted for whole cell current clamp recording in vitro in order to extract a battery of electrophysiological parameters, and filled with biocytin for subsequent morphological reconstruction. These parameters were subjected to an unsupervised cluster analysis to test how neurons group based on electrophysiological and morphological properties without prior specification of Fezf2 expression. This investigation found a diversity of PNs in the layer 5 of M1 that fell into at least four distinct groupings, all of which contained Fezf2-expressing PNs. Evidence was found for an important role of Fezf2 in one particular group of PNs. Using retrograde labelling these Fezf2-expressing PNs were identified as corticocallosal PNs (CC PNs) that project axons to the contralateral M1. Based on Fezf2 expression, CC PNs exhibited distinct intrinsic and synaptic properties. It is proposed that the expression of Fezf2 influences the unique identity of CC PNs and supports a distinct role within the corticocallosal M1 circuitry.