Differences in host serotonin innervation of intrastriatal grafts are not determined by a glial scar or chondroitin sulfate proteoglycans

Differentiation of pluripotent stem cells into striatal projection neurons: a pure MSN fate may not be sufficient

Huntington's disease (HD) is an autosomal dominant inherited disorder leading to the loss inter alia of DARPP-32 positive medium spiny projection neurons ("MSNs") in the striatum. There is no known cure for HD but the relative specificity of cell loss early in the disease has made cell replacement by neural transplantation an attractive therapeutic possibility. Transplantation of human fetal striatal precursor cells has shown "proof-of-principle" in clinical trials; however, the practical and ethical difficulties associated with sourcing fetal tissues have stimulated the need to identify alternative source(s) of donor cells that are more readily available and more suitable for standardization. We now have available the first generation of protocols to generate DARPP-32 positive MSN-like neurons from pluripotent stem cells and these have been successfully grafted into animal models of HD. However, whether these grafts can provide stable functional recovery to the level that can regularly be achieved with primary fetal striatal grafts remains to be demonstrated. Of particular concern, primary fetal striatal grafts are not homogenous; they contain not only the MSN subpopulation of striatal projection neurons but also include all the different cell types that make up the mature striatum, such as the multiple populations of striatal interneurons and striatal glia, and which certainly contribute to normal striatal function. By contrast, present protocols for pluripotent stem cell differentiation are almost entirely targeted at specifying just neurons of an MSN lineage. So far, evidence for the functionality and integration of stem-cell derived grafts is correspondingly limited. Indeed, consideration of the features of full striatal reconstruction that is achieved with primary fetal striatal grafts suggests that optimal success of the next generations of stem cell-derived replacement therapy in HD will require that graft protocols be developed to allow inclusion of multiple striatal cell types, such as interneurons and/or glia. Almost certainly, therefore, more sophisticated differentiation protocols will be necessary, over and above replacement of a specific population of MSNs. A rational solution to this technical challenge requires that we re-address the underlying question-what constitutes a functional striatal graft?

Degradation of chondroitin sulfate proteoglycans potentiates transplant-mediated axonal remodeling and functional recovery after spinal cord injury in adult rats

Transplantation of growth-permissive cells or tissues was used to bridge a lesion cavity and induce axonal growth in experimental spinal cord injury (SCI). Axonal interactions between host and transplant may be affected by upregulation of inhibitory chondroitin sulfate proteoglycans (CSPGs) following various transplantation strategies. The extent of axonal growth and functional recovery after transplantation of embryonic spinal cord tissue decreases in adult compared to neonatal host. We hypothesized that CSPGs contribute to the decrease in the extent to which transplant supports axonal remodeling and functional recovery. Expression of CSPGs increased after overhemisection SCI in adult rats but not in neonates. Embryonic spinal cord transplant was surrounded by CSPGs deposited in host cord, and the interface between host and transplant seemed to contain a large amount of CSPGs. Intrathecally delivered chondroitinase ABC (C'ase) improved recovery of distal forelimb usage and skilled motor behavior after C4 overhemisection injury and transplantation in adults. This behavioral recovery was accompanied by an increased amount of raphespinal axons growing into the transplant, and raphespinal innervation to the cervical motor region was promoted by C'ase plus transplant. Moreover, C'ase increased the number of transplanted neurons that grew axons to the host cervical enlargement, suggesting that degradation of CSPGs supports remodeling not only of host axons but also axons from transplanted neurons. Our results suggest that CSPGs constitute an inhibitory barrier to prevent axonal interactions between host and transplant in adults, and degradation of the inhibitory barrier can potentiate transplant-mediated axonal remodeling and functional recovery after SCI.

Membrane-associated guidance cues direct the innervation of forebrain and midbrain by dorsal raphe-derived serotonergic axons

Unlike many neurons that extend an axon precisely to a single target, individual dorsal raphe 5-HT neurons project to multiple brain regions and their axon terminals often lack classical synaptic specializations. It is not known how 5-HT axon collaterals select between multiple target fields, or even if 5-HT axons require specific guidance cues to innervate their targets. Nor is it known how these axon collaterals are restrained within specific innervation target regions. To investigate this, we challenged explants of dorsal raphe with co-explants, or cell membrane preparations of ventral midbrain, striatum or cerebral cortex. We provide evidence for membrane-associated cues that promote 5-HT axon growth into each of these three target regions. The axon growth-promoting activity was heat-, protease- and phosphatidylinositol-phospholipase-C (PI-PLC)-sensitive. Interestingly, 5-HT axons specifically lost the ability to grow in heterotypic explants, or membrane carpets, following contact with ventral midbrain or striatal, but not cortical, explants or membranes. This inductive activity associated with striatal and ventral midbrain membranes was sensitive to both high salt extraction and PI-PLC treatment. By contrast, the activity that inhibited 5-HT axon growth onto heterotypic membranes was sensitive only to high salt extraction. These results provide evidence that a glycosylphosphatidylinositol (GPI)-linked membrane protein promotes 5-HT axon growth, and that short-range membrane-bound, as well as GPI-linked, molecules contribute to the guidance of 5-HT axon collaterals. These findings suggest that 5-HT axon collaterals acquire a target-induced growth-inhibitory response to alternative targets, increasing their selectivity for the newly innervated field.

A novel immunohistochemical method for evaluation of antibody specificity and detection of labile targets in biological tissue

Immunohistochemistry is a key tool for analyzing target molecule localization within tissues. However, accurate results require an antibody that can distinguish between similar compounds. We present a simple immunohistochemical method that can also be used to rapidly evaluate antibodies' specificities. We demonstrate this technique with serotonin, an extremely labile compound. Serotonin (5-hydroxytryptamine, 5HT) is an important neurotransmitter regulating normal cognition and several mental disorders, as well as tumor growth, cardiopathology, and embryogenesis. Immunohistochemical detection of serotonin is commonly used as a neuronal cell marker and to provide crucial information on serotonin's role as an embryonic morphogen. It is necessary to be able to distinguish serotonin from closely related molecules with significantly different biological activity. Using our method, we identify antibodies that are specific for serotonin and show that some commercial 5HT antibodies often used to identify serotonergic cells in published papers are not 5HT-specific. These data demonstrate the necessity of specifically testing antibodies (especially in areas of high clinical relevance such as 5HT). We also illustrate detection of serotonergic cells in embryonic tissue using our technique. This method offers a number of general advantages for testing specificity of antibodies to any biological molecule, and helps avoid false positives and negatives during immunohistochemistry.