PubMed · 12420548
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Karen Lou Kennedy. 2002. Constipation in the elderly. Routine equals regularity.. https://pubmed.ncbi.nlm.nih.gov/12420548/
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The rostrocaudal position of neurons within the spinal motor pool maps systematically onto the surface of several muscles in mammals. In an effort to understand the mechanisms that generate such maps, we have been studying choices made by embryonic spinal cord neurons on muscle membrane substrates in the in vitro stripe assay. In this report we explore the effects of postnatal age of the muscle on neurite choice, and how prior denervation modifies this choice. Our results further differentiate rostral from caudal motor neurons in preferring one substrate to another. First, caudal neurites prefer to grow on P6 neonatal caudal over rostral membranes, but lose this ability to distinguish axial position of origin in older muscles. Rostral neurites prefer growth on rostral membranes, but this preference also diminishes with age. Second, when adult muscles have been denervated, both rostral and caudal neurites regain their positional growth selectivity. Third, caudal neurites are particularly sensitive to substrate choice. When growing on a preferred substrate (gluteus) caudal neurites prefer neonatal over adult membranes. These results support the concept of fundamental differences in the growth preferences of rostral and caudal spinal neurites. These differences will assist in the identification of molecular guidance cues that determine the formation of neuromuscular positional maps.
Schwann cells transiently express the transmembrane heparan sulfate proteoglycan syndecan-3 during the late embryonic and early postnatal periods of peripheral nerve development. Neonatal rat Schwann cells released soluble syndecan-3 into the culture medium by a process that was blocked by inhibition of endogenous matrix metalloproteinase activity. When Schwann cells were plated on a substratum that binds syndecan-3, the released proteoglycan bound to the substratum adjacent to the cell border. Membrane-anchored syndecan-3 was concentrated in actin-containing filopodia that projected from the lateral edges of the Schwann cell membrane. Membrane shedding was specific for syndecan-3 and was not observed for the related proteoglycan syndecan-1. Analysis of Schwann cells transfected with wild-type and chimeric syndecan-1 and syndecan-3 cDNAs revealed that membrane shedding was a property of the syndecan-3 ectodomain. Inhibition of syndecan-3 release significantly enhanced Schwann cell adhesion and process extension on dishes coated with the non-collagenous N-terminal domain of alpha4(V) collagen, which binds syndecan-3 and mediates heparan sulfate-dependent Schwann cell adhesion. Matrix metalloproteinase-dependent syndecan-3 shedding was also observed in newborn rat peripheral nerve tissue. Syndecan-3 shedding in peripheral nerve tissue was age specific, and was not observed during later stages of postnatal nerve development. These results demonstrate that Schwann cell syndecan-3 is subject to matrix metalloproteinase-dependent membrane processing, which modulates the biological function of this proteoglycan.
The basal rate of in vitro proteoglycan (PG) synthesis in explants of equine articular cartilage was subject to considerable variation in animals of the same age but was greater in younger than older animals. Synthesis of PGs in explant cultures was stimulated by a synthetic link peptide, identical in sequence to the N-terminus of the link protein (LP) of PG aggregates, in a similar manner to that demonstrated previously for human articular cartilage [Biochem. Soc. Trans. 25 (1997) 427; Arthritis Rheum. 41 (1998) 157]. Stimulation occurred in tissue from animals ranging from 1 to 30 years old but older animals required higher concentrations of peptide to produce a measurable response. Synthesis of PGs increased in a concentration-dependent manner and was paralleled by increases in the ability of aggrecan monomers to form aggregates with hyaluronan (HA). In addition to its effect on synthesis of PGs, link peptide also increased synthesis of both aggrecan and LP mRNA. Cartilage explant and chondrocyte cultures secreted small amounts of biologically active interleukin 1 (IL 1) and secretion of this cytokine was reduced considerably by the addition of link peptide. Reduction in the activity of this catabolic cytokine coupled with the increased synthesis of mRNA for aggrecan and link peptide may be the mechanism by which link peptide exerts its positive effect on the rate of PG synthesis in articular cartilage.