Neurobiology. Inhibitory influences

Autologous cell replacement: a noninvasive AI approach to clinical release testing

The advent of human induced pluripotent stem cells (iPSCs) provided a means for avoiding ethical concerns associated with the use of cells isolated from human embryos. The number of labs now using iPSCs to generate photoreceptor, retinal pigmented epithelial (RPE), and-more recently-choroidal endothelial cells has grown exponentially. However, for autologous cell replacement to be effective, manufacturing strategies will need to change. Many tasks carried out by hand will need simplifying and automating. In this issue of the JCI, Schaub and colleagues combined quantitative bright-field microscopy and artificial intelligence (deep neural networks and traditional machine learning) to noninvasively monitor iPSC-derived graft maturation, predict donor cell identity, and evaluate graft function prior to transplantation. This approach allowed the authors to preemptively identify and remove abnormal grafts. Notably, the method is (a) transferable, (b) cost and time effective, (c) high throughput, and (d) useful for primary product validation.

The effects of cold preservation and subimmunosuppressive doses of FK506 on axonal regeneration in murine peripheral nerve isografts

BACKGROUND

FK506 is a frequently used immunosuppressant with neuroregenerative effects. The neuroregenerative and immunosuppressive mechanisms of FK506, however, are distinct, suggesting that FK506 may stimulate nerve regeneration at lower doses than are needed to induce immunosuppression. The effects of cold preservation, a technique known to improve axonal regeneration through nerve allografts, are not well studied in nerve isografts and are also reported here.

OBJECTIVES

To determine the effects of subimmunosuppressive doses of FK506 and cold preservation on nerve regeneration in isografts.

METHODS

The neuroregenerative properties of immunosuppressive and subimmunosuppressive doses of FK506 were compared in a murine model receiving either fresh or cold preserved nerve isografts. Sixty female BALB/cJ mice were randomized into six groups. Animals in groups I, III and V received fresh nerve isografts. Animals in groups II, IV and VI received cold-preserved nerve isografts. Mice in groups I and II received no medical therapy, while those in groups III and IV received subimmunosuppressive doses of FK506, and those in groups V and VI received immunosuppressive doses as confirmed by mixed lymphocyte reactivity assays. Nerve regeneration was evaluated with histomorphometry and functional recovery was evaluated with walking track analysis.

RESULTS

Pretreatment with cold preservation did not significantly affect neural regeneration. The potent neuroregenerative effect of immunosuppressive doses of FK506 was confirmed, and the ability of subimmunosuppressive doses of FK506 to stimulate axonal regeneration in murine nerve isografts is reported.

CONCLUSIONS

Less toxic subimmunosuppressive doses of FK506 retaining some neuroregenerative properties may have a clinical role in treating extensive nerve injuries.

Transplantation of olfactory ensheathing cells stimulates the collateral sprouting from axotomized adult rat facial motoneurons

Axon regrowth after CNS and PNS injury is only the first step toward complete functional recovery which depends largely on the specificity of the newly formed nerve-target projections. Since most of the studies involving the application of glial cells to the lesioned nervous system have focused primarily on the extent of neurite outgrowth, little is known regarding their effects on the accompanying processes of axonal sprouting and pathfinding. In this study, we analyzed the effects of transplanted olfactory ensheathing cells (OECs) on axonal sprouting of adult facial neurons by using triple fluorescent retrograde tracing and biometrical analysis of whisking behavior. We found that 2 months after facial nerve axotomy and immediate implantation of OECs in between both nerve stumps fixed in a silicon tube, the total number of labeled neurons was increased by about 100%, compared to animals with simple facial nerve suture or entubulation in an empty conduit. This change in the number of axon sprouts was not random. The highest increase in axon number was observed in the marginal mandibular branch, whereas no changes were detected in the zygomatic branch. This increased sprouting did not improve the whisking behavior as measured by biometric video analysis. Our results demonstrate that OECs are potent inducers of axonal sprouting in vivo. Hence OEC-filled nerve conduits may be a powerful tool to enforce regeneration of a peripheral nerve under adverse conditions, e.g., after long delay between injury and surgical repair. In mixed nerves, increased axonal sprouting will improve specificity since inappropriate nerve-target connections are pruned off during preferential motor innervation. In pure motor nerves, however, OEC-mediated axonal sprouting may result in polyneuronal innveration of target muscles.

Thrombospondin expression in nerve regeneration I. Comparison of sciatic nerve crush, transection, and long-term denervation

Patterns of expression of the extracellular matrix molecule thrombospondin (TSP) were examined during peripheral nerve regeneration following sciatic nerve crush or transection. In noninjured nerve, was present in the axoplasm, Schwann cells, endoneurium, and perineurium of the adult mouse sciatic nerve. Following nerve crush or nerve transection, levels of TSP rapidly increased distal to the trauma site. Elevated levels of TSP were present distal to regenerating axons, while expression gradually returned to normal proximal to the regenerating axons. When reinnervation was blocked, TSP levels remained high in the endoneurium in excess of 30 days, but TSP was absent by 60 days. Following reanastomosis of the proximal and distal segments after 60 days of denervation, TSP was re-expressed in the distal nerve stump. These results indicate that TSP, which is involved in neuronal migrations in the embryo and neurite outgrowth in vitro, appears to play a role in axonal regeneration in the adult peripheral nervous system.

Thrombospondin expression in nerve regeneration II. Comparison of optic nerve crush in the mouse and goldfish

Expression of the extracellular matrix molecule thrombospondin (TSP) was examined following retrobulbar crush injury of the goldfish and mouse optic nerve. TSP was present within the glia limitans and surrounding axon fascicles of the control normal goldfish optic nerve, but was absent from the normal mouse optic nerve. Following crush injury of the goldfish optic nerve, TSP expression increased dramatically along the path of regenerating axons and returned to near normal levels following axonal outgrowth. In contrast, during the unsuccessful attempt at regeneration following crush injury of the mouse optic nerve, TSP expression was present only in glial fibrillary acidic protein (GFAP)-negative, macrophage-rich regions distal to ganglion cell axons. These results indicate that TSP expression is increased in a temporal pattern along the path of regenerating goldfish optic nerve axons and therefore may be involved in successful central nervous system regeneration. The absence of TSP in the environment encountered by damaged mouse optic nerve axons may correlate with the lack of regeneration observed in the mouse optic nerve.

Elevation of ceramide within distal neurites inhibits neurite growth in cultured rat sympathetic neurons

Sphingolipids are abundant constituents of neuronal membranes and have been implicated in intracellular signaling. We show that two analogs of glycosphingolipid biosynthetic intermediates, fumonisin B1 (which inhibits dihydroceramide synthesis) and DL-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) (which inhibits glucosylceramide synthesis) decrease glycosphingolipid synthesis in rat sympathetic neurons. Although both fumonisin and PPMP inhibit glycosphingolipid synthesis, these inhibitors have differential effects on ceramide metabolism in axons. threo-PPMP, but not erythro-PPMP or fumonisin, induces an accumulation of [3H]palmitate-labeled ceramide and impairs axonal growth. Moreover, exogenously added, cell-permeable C6-ceramide, but not C6-dihydroceramide, mimicks the effect of PPMP. Our studies suggest that the lipid second messenger ceramide acts in distal axons, but not cell bodies, as a negative regulator of neurite growth.

Sialic acid specificity of myelin-associated glycoprotein binding

Myelin-associated glycoprotein (MAG), a nervous system cell adhesion molecule, is an I-type lectin that binds to sialylated glycoconjugates, including gangliosides bearing characteristic structural determinants (Yang, L. J.-S., Zeller, C. B., Shaper, N. L., Kiso, M., Hasegawa, A., Shapiro, R. E., and Schnaar, R. L. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 814-818). Two cell adhesion systems, COS-1 monkey kidney fibroblasts transiently transfected to express MAG and Chinese hamster ovary (CHO) cells stably transfected to express MAG, were used to probe the structural specificity of MAG-ganglioside binding. Both cell types bound to the same gangliosides: GQ1b alpha (IV3NeuAc,III6NeuAc,II3(NeuAc)2Gg4Cer) > GT1b = GD1a > GM3 > GM1, GD1b, and GQ1b (the latter do not support adhesion). Binding was enhanced by pretreatment of MAG-expressing cells with neuraminidase. MAG-expressing Chinese hamster ovary cells bound directly to gangliosides resolved on thin layer chromatograms, allowing detection of MAG binding species in a mixture. The simplest ganglioside ligand for MAG was GM3 bearing N-acetylneuraminic acid, whereas GM3 bearing N-glycolylneuraminic acid did not support adhesion. Chemical modifications of N-acetylneuraminic acid residues (on GD1a) abrogated MAG binding. Mild periodate oxidation of sialic acids to their corresponding seven-carbon (or eight-carbon) sialic acid aldehydes abolished MAG binding, as did further conversion to the corresponding primary alcohols. Eliminating the anionic charge by ethyl esterification, amidation, or reduction also abolished MAG-mediated cell adhesion. These data demonstrate that MAG-ganglioside binding is highly specific and defines key carbohydrate structural determinants for MAG-mediated cell adhesion to gangliosides.

Intrinsic changes in developing retinal neurons result in regenerative failure of their axons

The failure of mature mammalian central nervous system axons to regenerate after transection is usually attributed to influences of the extraneuronal milieu. Using explant cocultures of retina and midbrain tectum from hamsters, we have found evidence that these influences account for failure of regrowth of only a small minority of retinal axons. For most of the axons, there is a programmed loss of ability to elongate in the central nervous system. We show that there is a precipitous decline in the ability of retinal axons to reinnervate tectal targets when the retina is derived from pups on or after postnatal day 2, even when the target is embryonic. By contrast, embryonic retinal axons can regrow into tectum of any age, overcoming growth-inhibiting influences of glial factors.

Boundaries and inhibitory molecules in developing neural tissues

Numerous studies of the past decade have illuminated the importance of intercellular adhesion events for neural pattern formation. It has been documented that members of the Ig and cadherin gene superfamilies, that glycoproteins and, probably to some extent, proteoglycans of the extracellular matrix play a role in this context. Recent observations suggest that, in addition to adhesive interactions, repulsive and/or inhibitory phenoma are also of importance in regulating neural pattern formation. Several molecules are under study which are considered possible mediators of inhibitory interactions in the nervous system. The hypothesis has been advanced that some of these might be partially responsible for restrictive, boundary-like properties ascribed to glial cells in developing and regenerating tissues. The current review summarizes these studies and focusses on molecular aspects of boundary and compartmentation phenomena.