| The complexity and intricacy of the vertebrate nervous system is due in part to the ability of neurons to form dendrites, thereby increasing their capacity to receive input. In defined culture conditions, perinatal rat sympathetic neurons obtained from the superior cervical ganglion (SCG) do not spontaneously produce dendrites unless exposed to bone morphogenetic protein-7 (BMP-7).; Initial experiments detailed herein demonstrate the utility of cationic polymers and lipids in delivering foreign genes to postmitotic neurons in vitro. This technique is then used to show that BMP-induced dendrite outgrowth in these neurons is dependent on the intracellular signal transducer Smad1. This molecule localizes to the nucleus of both sympathetic neurons and hippocampal neurons in response to treatment with either BMP7 or cerebrospinal fluid, which contains BMPs. Dendrite growth is retarded when cells are transfected with plasmids encoding a Smad1 mutant that acts in a dominant-negative manner on BMP signaling. A novel intracrine mechanism of signal transduction that utilizes fibroblast growth factor receptor-1 (FGFR1) is also shown to be involved in BMP-mediated dendrite growth. FGFR1 is present in the nucleus of sympathetic neurons treated with BMP-7, and overexpression of a dominant-negative FGFR1 construct inhibits dendritic outgrowth.; Previous studies have shown that leukemia inhibitory factor (LIF) causes retraction of dendrites and withdrawal of synapses in cultured sympathetic neurons, and is upregulated in the SCG following axotomy of postganglionic neurons.{09}However, postnatal rat pups injected with rhLIF do not show a decrease in SCG synapse density despite a marked reduction in growth rate, suggesting that neural tissue is protected against abnormally high serum levels of LIF during development.; Mutations in which there is an expansion of polyglutamine repeats in certain proteins cause degenerative diseases in central nervous system neurons. Sympathetic neurons transfected with abnormally long polyglutamine repeats decrease in size compared to control cultures and develop nuclear inclusions. Treatment with lactacystin, a specific, irreversible inhibitor of the proteasome, reduces the development of nuclear inclusions and prevents somatic atrophy.; In summary, these studies examine the intracellular mechanisms of dendrite growth in postmitotic neurons, as well as ways in which disease and injury affect dendritic outgrowth. |
