Molecular Components of the Synaptic Basal Lamina That Direct Differentiation of Regenerating Neuromuscular Junctions

Abstract

Results of experiments outlined here provide evidence that components of the myofiber basal lamina sheath direct the formation of active zones in regenerating motor nerve terminals and the development of infoldings and the aggregation of AChRs in the plasma membrane of regenerating myofibers. As a step toward identifying the basal lamina molecules that aggregate AChRs, we are now studying an ECM fraction from the Torpedo electric organ that causes AChRs to aggregate on cultured myotubes. We have solubilized and purified the electric organ AChR-aggregating molecules over 1000-fold. Only nanogram amounts of the most purified extracts are required to cause detectable AChR aggregation. We have also shown that similar activity can be extracted in relatively small amounts from muscle. Antiserum raised against the partially purified electric organ material completely blocked and immunoprecipitated the AChR-aggregating activity in extracts of the electric organ and muscle and bound to components of the basal lamina of frog muscle fibers. Although several polypeptides are present in our most purified extracts, an antiserum against polypeptides in the range of 80 kD completely blocked AChR aggregation by soluble extracts of the electric organ. These findings demonstrate the feasibility of isolating molecules from the synapse-rich electric organ that cause AChR aggregation and comparing them by immunological techniques with those in basal lamina at the neuromuscular junction.

Comparison of Six Different Substrata on the Plating Efficiency, Differentiation and Survival of Human Dorsal Root Ganglion Neurons in Culture

“…Nerve fi bers are guided to their targets by specific di rectional cues which are thought to involve the extracellular matrix [7,8,10,26,28]. Ex periments by Nitkin et al [22] have shown that the extracellular matrix of the nervous system controls synapse formation at myoneural junctions. Similarly, myelination of axons by Schwann cells has been reported to be enhanced by coculturing Schwann cells and neurons with a reconstituted basement membrane whose major components include collagen and laminin [6], In vitro, the importance of various compo nents of the extracellular matrix in promoting neurite outgrowth of neurons derived from nonhuman sources has been known for some time.…”

Clustering of Voltage-Sensitive Sodium Channels on Axons Is Independent of Direct Schwann Cell Contact in the Dystrophic Mouse

“…However, in the absence of direct contact with Schwann cells, low levels of a diffusible signal released from Schwann cells at more remote locations still might induce more slowly the formation of clusters on the bare axons. For acetylcholine receptors on muscle, a diffusible clustering factor has been identified as agrin, which is released by the innervating presynaptic terminal during synapse formation (Nitkin et al, 1983). More recently, agrin has been reported to induce sodium channel clustering on cultured muscle fibers (Sharp and Caldwell, 1996).…”

Extracellular Matrix Molecules And Their Receptors: Functions In Neural Development

“…Studies on regeneration of amphibian neuromuscular junctions have demonstrated a critical role for the basal lamina in inducing formation of both pre-and postsynaptic specializations (see Nitkin et al 1983, Sanes & Chiu 1983). The basal lamina at the neuromuscular synapse has been shown to be molecularly distinguishable from non-synaptic basal lamina.…”

“…The glycoprotein agrin was identified initially as a factor that induces clustering of acetylcholine receptors on skeletal myotubes (cf. Nitkin et al 1983). Purified agrin has been shown to induce concentrations of both ECM-associated and membrane-associated synaptic molecules on skeletal myotubes.…”