Although expression of the
IGF-II has been demonstrated within the central nervous system (CNS), past studies have failed to reveal its precise roles or responses subsequent to a traumatic injury. To demonstrate that
IGF-II,
IGFBP, and IGF receptor (-R) expression alters in response to a penetrating CNS injury, we used the techniques of
ribonuclease protection assay, in situ hybridization, immunohistochemistry, Western blotting, and RIA. Under normal physiology,
IGF-II expression is restricted to the mesenchymal support structures of the brain, including the choroid plexus, where its expression is coincident with that of
IGFBP-2. Between 1-7 days post lesion (dpl), in the acute phase following a penetrant
wound to the CNS,
IGF-II and IGF-IIR
protein, but not
messenger RNA, were colocalized, with
IGF-I, IGF-IR, and
IGFBP-1, -2, -3, and -6, to neurons, macrophages, astrocytes, and microglia within the damaged tissue. Within the cerebrospinal fluid (CSF), levels of
IGF-II peptide increased to peak at 7 dpl.
IGFBP-2, -3, and -6 were also observed within the CSF, with
IGFBP-2 predominating and exhibiting an increase in binding efficiency from 7-10 dpl. In the chronic phase of injury (7-14 dpl), an increase in both
IGF-II, IGF-IIR and
IGFBP-5 messenger RNA and
protein was observed specifically and focally in the marginal astrocytes forming the limiting glial membrane of the
wound. Thus, our evidence suggests that there are two mechanisms of action for
IGF-II within the injured rat brain. During the acute phase, the secretion of
IGF-II from the choroid plexus into the CSF is up-regulated, resulting in increased transport of the
peptide to the
wound. In the CSF, transported
IGF-II is complexed to
IGFBP-2 and essentially demonstrates an endocrine mode of action with a balance of locally produced IGFBPs modulating its bioactivity in the
wound. Later in the wounding response, levels of
IGF-II decline in the CSF and the
wound neuropil, possibly with the aid of increased
IGFBP-5 levels that may help to locally sequester and down-regulate
IGF-II activity. Hence, in the chronic phase of the injury response,
IGF-II reasserts itself to a predominantly autocrine/paracrine role restricted to the mesenchymal support structures, including the glia limitans, which may help reestablish and maintain tissue homeostasis.