Increased expression of insulin-like growth factor-I (IGF-I) during embryonic development produces neocortical overgrowth with differentially greater effects on specific cytoarchitectonic areas and cortical layers

Abstract

The in vivo actions of insulin-like growth factor-I (IGF-I) on the growth and development of the cerebral cortex were investigated in transgenic (Tg) mice that overexpress IGF-I in the brain, beginning as early as embryonic day (E) 13. Compared to non-Tg littermate controls, Tg mice at postnatal day (P) 12 exhibited significant increases in total cortical volume (31%) and in total neuron number (27%). The numerical density of neurons did not differ significantly between Tg and control mice, except in layer I. Comparing cytoarchitectonic areas in Tg mice, significantly greater increases in cortical volume were found for the motor cortex (42%), compared to somatosensory cortex (35%). Similarly, greater increases in total neuron number were found for motor cortex (44%) compared to somatosensory cortex (28%). Comparing individual cortical layers in Tg mice, the greatest increase in neuron number was found in layer I for both motor (93%) and somatosensory (76%) regions, followed by layer V (36–53%)>II/III (26–47%)>VI (26–37%)>IV (22–34%). Our results demonstrate that increased expression of IGF-I in vivo during embryonic and early postnatal development produces substantial overgrowth of the neocortex. IGF-I-mediated growth and development exhibits differential effects in some cytoarchitectonic areas and in lamina-specific neuron populations, most notably the neurons of layer I.

Introduction

Insulin-like growth factor I (IGF-I), a 70-amino acid peptide, is a member of the insulin superfamily. IGF-I triggers its actions by binding to the type 1 IGF receptor (IGF1R), initiating a cascade of intracellular events ultimately leading to the transcription of genes whose products are responsible for regulating processes such as cell cycle progression and cellular differentiation (for reviews, see Refs. [26], [36]). Evidence from recent in vitro studies indicates that IGF-I has a role in embryonic central nervous system (CNS) development. IGF-I augments neural stem cell proliferation [4], [17], [24] and promotes neuron survival [7], [23], [58], [60]. IGF-I also influences oligodendrocyte development in vitro by promoting oligodendrocyte precursor proliferation and differentiation [35], [42], [47] and oligodendrocyte survival [8], [62].

Mutant mouse models of IGF-I function strongly support a role for IGF-I in brain growth and development (for review, see Ref. [20]). The brain weights of mice carrying a null mutation of the IGF-I gene (IGF-I knockout mice) are significantly reduced, as are the volume and number of neurons in many brain regions [11], [38]. Conversely, Tg mice that overexpress IGF-I in the brain exhibit increased brain weight and volume, as well as regional increases in neuron number [12], [19], [31], [41], [49], [52], [61]. These studies clearly indicate that IGF-I contributes to the regulation of brain development.

Relatively few studies, either in vitro or in vivo, have specifically addressed the effects of IGF-I on cerebral cortex development. Both IGF-I and IGF1R are expressed in the primitive cerebral cortex of rodents during embryonic development [6], [15]. IGF1R is expressed by postmitotic neurons and neural stem cells in the ventricular zone of the developing cerebral cortex [15], while in situ hybridization studies indicate that IGF-I mRNA is expressed throughout the embryonic cortex [6]. High levels of IGF-I and IGF1R expression also are apparent in the rodent cerebral cortex during the first 3 weeks of postnatal life [9], [14], [15].

IGF-I has been shown to promote the survival of cultured cerebral cortical neurons [60]. In a line of Tg mice in which IGF-I was overexpressed postnatally in the brain, Gutierrez-Ospina and collaborators [31] documented increases in cortical surface area, as well as total barrel area and neuron number within individual barrels in the posterior medial barrel subfield of the somatosensory cortex. In IGF-I knockout mice, the thickness of the cerebral cortex is reduced and cell density is increased in all regions of the cortex compared to normal mice. The apparent number of projection neurons in layers III and V in the parietal and temporal regions of the cerebral cortex of IGF-I knockout mice, however, does not differ from normal mice [11]. These studies indicate that IGF-I influences the development of the cerebral cortex and suggest a differential effect of IGF-I on specific neuronal populations and/or functional areas of the cerebral cortex.

In the present study, the in vivo effects of IGF-I on the prenatal and early postnatal development of the cerebral cortex were investigated in a new line of Tg mice, termed nestin/IGF-I Tg mice, in which IGF-I is overexpressed in the brain during embryonic development. The transgene was composed of (1) regulatory elements that include the second intron of the human nestin gene and the minimal promoter of herpes simplex virus intermediate early gene ICP4, (2) the human IGF-IA cDNA fused to a signal sequence from rat somatostatin, and (3) a sequence containing polyadenylation signals and sites derived from the human growth hormone gene [52]. In nestin/IGF-I Tg mice, IGF-I overexpression begins as early as embryonic day (E) 13 and continues into postnatal life, with regional expression being the highest in the cerebral cortex [52]. Stereological analyses were conducted to determine the effect of IGF-I on the total volume and mean thickness of the neocortex, and on the numerical density and total number of neurons in the cortex. Separate analyses were conducted in individual cortical laminae for two different cytoarchitectonic regions, the primary motor and primary somatosensory cortices.