Tbr 1 Regulates Differentiation of the Preplate

the preplate into superficial and deep components, * Nina Ireland Laboratory of thereafter termed the marginal zone and subplate, re-Developmental Neurobiology spectively (Allendoerfer and Shatz, 1994). The cortical Department of Psychiatry plate grows in an " inside-out " order, from layer 6, which University of California, San Francisco contains the earliest-born cortical plate neurons, to layer San Francisco, California 94143 2, which contains the latest-born neurons (Angevine and The preplate is thought to function primarily as a Massachusetts General Hospital and framework for further development of the cortex, or-Harvard Medical School ganizing its laminar structure and some of its connec-Boston, Massachusetts 02114 tions. In mice, preplate cells differentiate into at least ‡ Neuropsychiatric Institute two distinct types of neurons: Cajal-Retzius cells and UCLA Medical School subplate cells (though other classifications have also 760 Westwood Plaza been proposed; see Meyer et al. FUNDP Medical School have been implicated in controlling cell migrations (re-B5000 Namur viewed by Rice and Curran, 1999) and radial glia mor-Belgium phology (Supè r et al., 2000). Subplate cells are marked by expression of golli-lacZ, a transgene (Landry et al., 1998), and of calretinin (Fonseca et al., 1995). They have Summary been implicated in guiding thalamocortical afferent ax-ons into the cortex, and cortical efferent axons to their During corticogenesis, early-born neurons of the pre-However, the exact functions and origins putative transcription factor that is highly expressed of the preplate cells remain unclear, and their molecular in glutamatergic early-born cortical neurons. In Tbr1-specializations have only begun to be explored (Landry deficient mice, these early-born neurons had molecu-pressed decreased levels of Reelin, resulting in a Tbr1 is a transcription factor gene of the T box family reeler-like cortical migration disorder. Impaired sub-that is expressed soon after cortical progenitors begin plate differentiation was associated with ectopic pro-to differentiate. It is highly expressed in early-born neu-jection of thalamocortical fibers into the basal telen-rons of the preplate and layer 6 (Bulfone et al., 1995). cephalon. Layer 6 defects contributed to errors in the Neurons in layer 6 are the source of corticothalamic thalamocortical, corticothalamic, and callosal projec-axons (De Carlos and O'Leary, 1992) and have been tions. These results show that Tbr1 is a common ge-implicated as pioneer neurons in the callosal projection netic determinant for the differentiation of early-born (Koester and O'Leary, 1994). The high expression of glutamatergic neocortical neurons and provide in-Tbr1 in the preplate and cortical plate suggested that sights into …