Towards the classification of subpopulations of layer V pyramidal projection neurons.

The nature of cerebral cortical circuitry has been increasingly clarified by markers for the identification of precise cell types with specific morphology, connectivity and distinct physiological properties. Molecular markers are not only helpful in dissecting cortical circuitry, but also give insight into the mechanisms of cortical neuronal specification and differentiation. The two principal neuronal types of the cerebral cortex are the pyramidal and GABAergic cells. Pyramidal cells are excitatory and project to distant targets, while GABAergic neurons are mostly inhibitory non-pyramidal interneurons. Reliable markers for specific subtypes of interneurons are available and have been employed in the classification and functional analysis of cortical circuitry. Until recently, cortical pyramidal neurons have been considered a homogeneous class of cells. This concept is now changing as the powerful tools of molecular biology and genetics identify molecular tags for subtypes of pyramidal cells such as: Otx-1 [Frantz, G.D., Bohner, A.P., Akers, R.M., McConnell, S.K., 1994. Regulation of the POU domain gene SCIP during cerebral cortical development. J. Neurosci. 14, 472-485; Weimann, J.M., Zhang, Y.A., Levin, M.E., Devine, W.P., Brulet, P., McConnell, S.K., 1999. Cortical neurons require Otx1 for the refinement of exuberant axonal projections to subcortical targets. Neuron 24, 819-831]; SMI-32, N200 and FNP-7 [Voelker, C.C., Garin, N., Taylor, J.S., Gahwiler, B.H., Hornung, J.P., Molnár, Z., 2004. Selective neurofilament (SMI-32, FNP-7 and N200) expression in subpopulations of layer V pyramidal neurons in vivo and in vitro. Cereb. Cortex 14, 1276-1286]; ER81 [Hevner, R.F., Daza, R.A., Rubenstein, J.L., Stunnenberg, H., Olavarria, J.F., Englund, C., 2003. Beyond laminar fate: toward a molecular classification of cortical projection/pyramidal neurons. Dev. Neurosci. 25 (2-4), 139-151; Yoneshima, H., Yamasaki, S., Voelker, C., Molnár, Z., Christophe, E., Audinat, E., Takemoto, M., Tsuji, S., Fujita, I., Yamamoto, N., 2006. ER81 is expressed in a subpopulation of layer 5 projection neurons in rodent cerebral cortices. Neuroscience, 137, 401-412]; Lmo4 [Bulchand, S., Subramanian, L., Tole, S., 2003. Dynamic spatiotemporal expression of LIM genes and cofactors in the embryonic and postnatal cerebral cortex. Dev. Dyn. 226, 460-469; Arlotta, P., Molyneaux, B.J., Chen, J., Inoue, J., Kominami, R., Macklis, J.D., 2005. Neuronal subtype-specific genes that control corticospinal motor neuron development in vivo. Neuron 45 (2), 207-221]; CTIP2 [Arlotta, P., Molyneaux, B.J., Chen, J., Inoue, J., Kominami, R., Macklis, J.D., 2005. Neuronal subtype-specific genes that control corticospinal motor neuron development in vivo. Neuron 45 (2), 207-221]; Fez1 [Molyneaux, B.J., Arlotta, P., Hirata, T., Hibi, M., Macklis, J.D., 2005. Fez1 is required for the birth and specification of corticospinal motor neurons. Neuron 47 (6), 817-831; Chen, B., Schaevitz, L.R., McConnell, S.K., 2005. Fez1 regulates the differentiation and axon targeting of layer 5 subcortical projection neurons in cerebral cortex. Proc. Natl. Acad. Sci. U.S.A. 102 (47), 17184-17189]. These genes outline the numerous subtypes of pyramidal cells and are increasingly refining our previous classifications. They also indicate specific developmental programs operate in cell fate decisions. This review will describe the progress made on the correlation of these markers to each other within a specific subtype of layer V neurons with identified, stereotypic projections. Further work is needed to link these data with observations on somatodendritic morphology and physiological properties. The integrated molecular, anatomical and physiological characterisation of pyramidal neurons will lead to a much better appreciation of functional cortical circuits.