Elsevier

Neuroscience Research

Volume 55, Issue 2, June 2006, Pages 105-115
Neuroscience Research

Review article
Towards the classification of subpopulations of layer V pyramidal projection neurons

https://doi.org/10.1016/j.neures.2006.02.008Get rights and content

Abstract

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.

Introduction

The original classification of most cerebral cortical neurons originated in the previous century. Neurons were characterised based on their size, shape and dendritic branching pattern as they appear on Golgi-stained preparations (Golgi, 1886, Ramón y Cajal, 1911, Lorente de No, 1949). The basic terminology for the identification of cortical neurons (e.g. pyramidal, stellate or granular cell, etc.) is still in use today. It was not until the advent of reliable tracing methods that connectivity became a pertinent and useful criterion. The principal neuronal types of the cerebral cortex are the excitatory pyramidal cells, which project to distant targets, and the inhibitory non-pyramidal cells, which are the cortical interneurons (Peters and Jones, 1985). These different classes of neurons originate from distinct regions. Pyramidal neurons are generated in the cortical neuroepithelium and migrate radially to reach the cortex following an inside–out gradient (Rakic, 1988), whereas most of the interneurons originate from the basal telencephalon and migrate to the cortex through tangential migration (Parnavelas, 2000, Marín and Rubenstein, 2001). Histochemical and immunohistochemical analysis revealed further details especially of the different types of interneurons (Markram et al., 2004). A list of relatively simple but reliable markers aided the identification of different interneuron classes, and these markers are still used today (Somogyi and Klausberger, 2004). Projection neurons have been further classified by the laminar position of their cell body, morphology, electrophysiology and hodology (Toyama et al., 1974, Peters and Jones, 1985), but there are relatively few neurochemical markers available for their identification. Classification of central nervous system (CNS) regions has been advanced in recent years by exploiting modern mouse molecular genetics. Markers are useful for classification, but also they are equally interesting for understanding development. The specific combination of transcription factors defines neuronal fate, and their combinatorial expression pattern closely correlates with neuronal diversity (Gray et al., 2004). Genes which regulate the production of cortical cell types have been identified (Guillemot et al., 2006, Wu et al., 2005, Hevner et al., 2006), however, the molecular profile within pyramidal cell populations in the cortex remains relatively undeveloped. The reason behind this might be the lack of integrated approaches that utilise several of the classification criteria (hodology, morphology and physiology) synchronously. Approaches employing methods where all these components are viewed in conjunction have yielded the greatest progress (Migliore and Shepherd, 2005, Nelson, 2005, Sugino et al., 2006). We wish to give an update on the efforts made in this field by reviewing recent studies on subtypes of layer V pyramidal neurons in the rodent cerebral cortex. Pyramidal cells of this class provide an exceptional model system to test ideas on cell classification and to study neuronal specification within the same cortical lamina.

Section snippets

Layer V pyramidal cells: an accessible model for the study of target selection, dendritic and physiological differentiation

Within layer V of the adult rodent cortex, pyramidal neurons with different soma size can be distinguished. Neurons with smaller sized soma tend to occupy the lower portion of layer V (Vb), whereas cells with larger soma tend to reside within the upper sector (Va). However, the location is not an absolute predictor since the two types of somas mingle with one another and there is a considerable overlap between them within layer V. The different soma sizes can be linked to distinctions in their

The general neurobiological relevance of the model

An obvious question is whether this system is general across different cerebral cortical areas in rodent, and whether it can be found in various different species including primates. While most experiments were performed in rat visual cortex, the existence of two similar types of layer V neurons were demonstrated in auditory, motor and somatosensory regions as well (Chagnac-Amitai et al., 1990, Games and Winter, 1988). In the adult rat cortex, the particular cortical area determines the

The diversity of somatodendritic morphology of layer V neurons

Although pyramidal cells share numerous common features within layer V, they are very heterogeneous in their somatodendritic morphology (Schofield et al., 1987, Hallman et al., 1990). The basic classification of type I (tufted) and type II (non-tufted) has proven to be helpful but oversimplified. At least two clear subclasses of tufted populations have been identified. One subclass possessed a simple main apical dendrite reaching layer I and terminating in a single tuft, while another has side

Electrophysiological classification

The electrophysiological distinctions between the two major types of layer V neurons have been described by Kasper et al. (1994) using sharp electrode recording. Prelabelling with fluorescent latex microspheres enabled Kasper et al. (1994) to record from neurons with identified projections and correlate the electrophysiological parameters to the somatodendritic morphology and projection target. The development of subthreshold and action potential properties of layer V neurons recently have been

Layer V marker gene expression pattern

The quest of finding neurochemical markers for layer V projection neurons started at the time when layer V neurons were being classified into type I subcortically projecting and type II callosally projecting neurons (Stanfield and Jacobowitz, 1990, Larkman and Mason, 1990). There are numerous layer V-specific protein markers. To integrate with existing classification schemes, here we describe the markers that have been demonstrated to be type-specific by combining retrograde labelling with in

Do GABAergic projection neurons contribute to subcortical or callosal connectivity?

It has been shown that mature GABAergic neurons can develop long range projections intracortically (Fabri and Manzoni, 1996), and the vast majority are immunoreactive for somatostatin, neuropeptide Y and nitric oxide synthase (Tomioka et al., 2005). In developing rat neocortex, GABAergic neurons can even travel across the corpus callosum (Kimura and Baughman, 1997). At present, the general view is that roughly 1% of the callosally projecting neurons are GABAergic in the mature nervous system of

Transgenic mice expressing fluorescent proteins could provide useful tools to characterise layer V subclasses

GABA-containing interneurons are well-defined by proteins such as calbindin, calretinin, parvalbumin, neuropeptide Y, vasoactive intestinal peptide, somatostatin and cholecystokinin (Markram et al., 2004, for review). Although no specific type of interneuron can be defined by a single marker, some types express specific combinations of different markers. With the continuous isolation of molecular tags specific for layer V pyramidal neurons, we can refine the classification of projection neurons

Co-localisation studies suggest the existence of several subclasses of layer V neurons

An important aim of this study is to unify molecular classification with other aspects of layer V neuronal classification in adult and during development. It is important to correlate the combination of expressed genes with projection targets and specific somatodendritic morphology. Co-localisation studies on OTX-1 and ER81 indicate that the two markers are not expressed within the same postnatal layer V neurons (Hevner et al., 2003). Using retrograde labelling and immunohistochemistry, we have

Dual projections can be established in numerous combinations

Within any single layer of the cortex, there are numerous morphological subtypes with different cortical (intracortical or intercortical) and subcortical connectivity (Thomson and Bannister, 2003). While it is clear that there are stereotypic patterns in the connections from the earliest stages of development, it is also clear that the idea of single destination has to be abandoned. Throughout the developing CNS, promiscuous initial connections are pruned back to few targets or eliminated

Summary

Our challenge is now to understand the combinatorial effect of lineage- and area-specific gene expression profiles. These fundamental components drive neurogenesis, differentiation and regional cortical connectivity. Potential molecular markers for layer V projection neurons are continually being found. Correlation of these markers with other aspects of neuronal phenotype will offer a more comprehensive classification of layer V neurons. More importantly, markers will reveal mechanisms by which

Acknowledgements

The review is based on a talk given by ZM at the meeting held on Neuronal Differentiation in Cortical Development at Icho-kaikan in Osaka University, Suita, Osaka, in September 16–17, 2005. This meeting was held to mark the end of the Human Frontiers Science Program Grant (RG 107/2001) by Nobuhiko Yamamoto, Etienne Audinat, Daniel Lavery and Zoltán Molnár. Consortium members very much valued the interactions during the period of the grant which resulted in development of numerous collaborative

References (91)

  • M.A. Gates et al.

    Reconstruction of cortical circuitry

    Prog. Brain Res.

    (2000)
  • Y.A. Gonchar et al.

    GABA-immunopositive neurons in rat neocortex with contralateral projections to S-I

    Brain Res.

    (1995)
  • M.T. Kirkcaldie et al.

    Neurofilament triplet proteins are restricted to a subset of neurons in the rat neocortex

    J. Chem. Neuroanat.

    (2002)
  • B.G. Klein et al.

    The structural and functional characteristics of striate cortical neurons that innervate the superior colliculus and the lateral posterior nucleus in hamster

    Neuroscience

    (1986)
  • S.E. Koester et al.

    Connectional distinction between callosal and subcortically projecting cortical neurons is determined prior to axon extension

    Dev. Biol.

    (1993)
  • J.H. Lin et al.

    Functionally related motor neuron pool and muscle sensory afferent subtypes defined by coordinate ETS gene expression

    Cell

    (1998)
  • B.J. Molyneaux et al.

    Fez1 is required for the birth and specification of corticospinal motor neurons

    Neuron

    (2005)
  • D.D. O’Leary et al.

    Evidence that the early postnatal restriction of the cells of origin of the callosal projection is due to the elimination of axonal collaterals rather than to the death of neurons

    Brain Res.

    (1981)
  • D.D. O’Leary et al.

    Occipital cortical neurons with transient pyramidal tract axons extend and maintain collaterals to subcortical but not intracortical targets

    Brain Res.

    (1985)
  • D.D. O’Leary et al.

    A transient pyramidal tract projection from the visual cortex in the hamster and its removal by selective collateral elimination

    Brain Res.

    (1986)
  • J.G. Parnavelas

    The origin and migration of cortical neurones: new vistas

    Trends Neurosci.

    (2000)
  • B. Schweitzer et al.

    Neural membrane protein 35/lifeguard is localized at postsynaptic sites and in dendrites

    Brain Res. Mol. Brain Res.

    (2002)
  • B.B. Stanfield et al.

    Antibody to a soluble protein purified from brain selectively labels layer V corticofugal projection neurons in rat neocortex

    Brain Res.

    (1990)
  • J.M. Weimann et al.

    Cortical neurons require Otx1 for the refinement of exuberant axonal projections to subcortical targets

    Neuron

    (1999)
  • B. Xu et al.

    Cortical degeneration in the absence of neurotrophin signaling: dendritic retraction and neuronal loss after removal of the receptor TrkB

    Neuron

    (2000)
  • H. Yoneshima et al.

    ER81 is expressed in a subpopulation of layer 5 projection neurons in rodent cerebral cortices

    Neuroscience

    (2006)
  • W. Akemann et al.

    Transgenic mice expressing a fluorescent in vivo label in a distinct subpopulation of neocortical layer 5 pyramidal cells

    J. Comp. Neurol.

    (2004)
  • M.C. Angulo et al.

    Distinct local circuits between neocortical pyramidal cells and fast-spiking interneurons in young adult rats

    J. Neurophysiol.

    (2003)
  • S.A. Bayer et al.

    Neocortical Development

    (1991)
  • T.N. Behar et al.

    GABA-induced chemokinesis and NGF-induced chemotaxis of embryonic spinal cord neurons

    J. Neurosci.

    (1994)
  • S. Bulchand et al.

    Dynamic spatiotemporal expression of LIM genes and cofactors in the embryonic and postnatal cerebral cortex

    Dev. Dyn.

    (2003)
  • B. Cauli et al.

    Molecular and physiological diversity of cortical nonpyramidal cells

    J. Neurosci.

    (1997)
  • Y. Chagnac-Amitai et al.

    Burst generating and regular spiking layer 5 pyramidal neurons of rat neocortex have different morphological features

    J. Comp. Neurol.

    (1990)
  • B. Chen et al.

    Fez1 regulates the differentiation and axon targeting of layer 5 subcortical projection neurons in cerebral cortex

    Proc. Natl. Acad. Sci. U.S.A.

    (2005)
  • E. Christophe et al.

    Two populations of layer V pyramidal cells of the mouse neocortex: development and sensitivity to anesthetics

    J. Neurophysiol.

    (2005)
  • I. Dori et al.

    Proportion of glutamate- and aspartate-immunoreactive neurons in the efferent pathways of the rat visual cortex varies according to the target

    J. Comp. Neurol.

    (1992)
  • G.D. Frantz et al.

    Regulation of the POU domain gene SCIP during cerebral cortical development

    J. Neurosci.

    (1994)
  • C. Golgi

    Sulla Fina Anatomia Degli Organi Centrali del Sistema Nervoso

    (1886)
  • P.A. Gray et al.

    Mouse brain organization revealed through direct genome-scale TF expression analysis

    Science

    (2004)
  • F. Guillemot et al.

    Molecular mechanisms of cortical differentiation

    Eur. J. Neurosci.

    (2006)
  • L.E. Hallman et al.

    Dendritic morphology and axon collaterals of corticotectal, cortico-pontine, and callosal neurons in layer V of the primary visual cortex of the hooded rat

    J. Comp. Neurol.

    (1990)
  • H. Hasegawa et al.

    Laminar patterning in the developing neocortex by temporally coordinated fibroblast growth factor signaling

    J. Neurosci.

    (2004)
  • R.F. Hevner et al.

    Beyond laminar fate: toward a molecular classification of cortical projection/pyramidal neurons

    Dev. Neurosci.

    (2003)
  • Hevner, R., Hodge, R.D., Daza, R.A.M., Englund, C., 2006. Transcription factors in glutamatergic neurogenesis:...
  • T. Hirata et al.

    Zinc finger gene fez-like functions in the formation of subplate neurons and thalamocortical axons

    Dev. Dyn.

    (2004)
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