The development of the vertebrate central nervous system (CNS) is orchestrated by complex genetic programs and cell-extrinsic signals that govern the differentiation of neural progenitors into the rich variety of neuronal subtypes seen in the adult. While the molecular mechanisms that regulate general aspects of early neurogenesis have begun to be elucidated, controls over lineage-specific neuronal fate determination and differentiation remain largely elusive. In particular, the molecular-genetic programs that regulate the complex neuronal heterogeneity of the mammalian neocortex are only beginning to be discerned. Callosal projection neurons (CPN) comprise a broad, diverse population of excitatory projection neurons that connect the two hemispheres of the cerebral cortex via the corpus callosum, and are the largest class of commissural neurons in placental mammals. CPN cell bodies principally reside in neocortical layers II/III, V, and, to a lesser extent, layer VI. Their absence in humans is associated with defects in abstract reasoning, problem solving, and generalization (Paul, Brown et al. 2007);and there is an emerging over-arching hypothesis that autism spectrum disorders (ASD), though heterogeneous, are likely to be in large part the result of imprecisely developed associative connectivity, especially of CPN and the corpus callosum (Egaas, et al. 1995, Vidal, et al. 2006, Herbert and Kenet 2007;Minshew and Williams 2007, Mcalonan, et al. 2009, Freitag, et al. 2009, Frazier, et al. 2009). Molecular diversity underlying evident anatomical and functional differences between CPN and other glutamatergic cortical projection neurons, as well as within the broad population of CPN, has only just begun to be studied (Alcamo, et al. 2008, Britanova, et al. 2008, Molyneaux, Arlotta, et al. 2009). In this proposal, I outline a program of research designed to investigate, with increasing precision and insight, functions of a subset of molecular controls over the development of the broad population of CPN, over subpopulations of CPN acting specifically in unique neocortical sublaminae, and over CPN with distinct connectivity.
The specific aims of this proposed research are: 1) To investigate potential function(s) of Cited2, an early, broadly expressed CPN-enriched transcriptional co-activator, in specification of CPN as a broad population;2) To investigate potential function(s) of Cav1, a gene expressed postnatally in post-mitotic differentiation and maturation of a subpopulation of dual callosal- and frontal-projecting neurons (CPN/FPN);and 3) To investigate potential function(s) of a CPN-enriched gene, Tmtc4, recently also linked to agenesis of the corpus callosum in humans, in the development of superficial layer CPN and the corpus callosum.
The proposed research has significant basic biological and clinical implications. There is an emerging over-arching hypothesis that autism spectrum disorders (ASD), though heterogeneous, are likely to be in large part the result of imprecisely developed associative connectivity, especially in cortical circuitry. Abnormalities of interhemispheric callosal projection neurons (CPN) / corpus callosum (CC) are found in substantial subsets of patients with ASD. A detailed understanding of the program of molecular-genetic controls regulating generation and maturation of this specific projection neuron population, and controls over the diversity within this population is important both for fundamental understanding of brain organization and function;for pathological understanding of and potential therapeutic approaches for subtle diseases of cortical connectivity, such as autism spectrum disorders (ASD) and agenesis of the corpus callosum;and potentially toward future development of novel cellular therapeutic approaches for neurodegenerative disorders.