Specific beta1 integrins mediate adhesion, migration, and differentiation of neural progenitors derived from the embryonic striatum

Early inductive signals within the embryonic mammalian forebrain establish two major germinal regions along the dorsal-ventral axis. The dorsal germinal zone eventually forms the cerebral cortex while the ventral ganglionic eminence primarily forms the striatum and globus pallidus. The mechanisms leading to patterning of specific forebrain structures from these distinct germinal regions are not fully understood but may involve the adhesive and migratory properties of regionally specified cells and their interactions with the extracellular environments in which they reside. In the present study, we isolated ganglionic eminence neural progenitor cells (geNPC), precursors of the adult striatum, from the ventral forebrain germinal zone and analyzed adhesion, migration, and differentiation of geNPC on various extracellular matrix (ECM) substrates in vitro. Specifically, we evaluated the role of beta1 integrins, a family of cell surface receptors important in neural development, in mediating geNPC behavior on ECM molecules expressed in embryonic brain tissue. Adhesion and migration of geNPC were significantly enhanced on laminin (LN) and fibronectin (FN) relative to other ECM substrates. Antibody perturbation experiments revealed that although geNPC express several beta1 integrins (alpha1beta1, alpha2beta1, alpha3beta1, alpha5beta1, alpha6beta1, alphavbeta1), adhesion and migration on LN and FN were primarily mediated by alpha6beta1 and alpha5beta1, respectively, and these interactions were confirmed by biochemical cross-link/extraction procedures. Finally, neuronal differentiation of geNPC was enhanced on LN, indicating a role for LN in geNPC differentiation. beta1 integrin-ECM interactions may contribute to basic mechanisms of striatal development and may explain the potent migratory capacity of geNPC transplanted into the adult brain.

Improved neural progenitor cell survival when cografted with chromaffin cells in the rat striatum

Transplantation of stem and neural progenitor cells hold great promise in the repair of neuronal tissue lost due to injury or disease. However, survival following transplantation to the adult CNS has been poor, likely due to a lack of neurotrophic factors, such as basic fibroblast growth factor (FGF-2), that are used to maintain and expand these cells in culture. Chromaffin cells produce several neurotrophic agents, including FGF-2, which may aid in both neuroprotection following injury and progenitor cell proliferation and survival. In addition, increased CNS catecholamines have been shown to improve functional recovery following insult. Thus, cotransplants of neural progenitor cells and chromaffin cells may be a useful clinical strategy. To address this, the survival of rat cortical progenitors transplanted to the adult rat striatum with and without bovine chromaffin cell cografts was assessed. Progenitors obtained from E14 embryos were prelabeled with bromodeoxyuridine (BrdU) before transplantation to enable later identification. Transplants were made both unilaterally and bilaterally, where animals received a monograft (progenitor cells alone) on one side and a cograft (progenitors + chromaffin cells) on the other. Histological results after 7, 17, and 30 days posttransplant revealed greatly improved survival of BrdU-labeled cells in the cografts and also less infiltration of presumptive immune cells. In addition, perivascular cuffing was seen in the monografts. In vitro progenitor cohorts stained positive for nestin, GFAP, and beta-tubulin III, but in vivo very few cells were found that were double labeled with BrdU and one of these markers. Thus, in contrast to in vitro findings, chromaffin cells did not enhance differentiation of progenitors in vivo during the 30 days posttransplantation. The results of these studies suggest that chromaffin cells may provide neurotrophic support to enhance survival, but not differentiation, of cortical progenitor grafts in the adult CNS.

Direct cell-cell contact required for neurotrophic effect of chromaffin cells on neural progenitor cells

Previous studies showed that neural progenitor cultures could be maintained without exogenously added FGF-2 when co-cultured with chromaffin cells. In addition, progenitor cells displayed dramatically increased neuronal differentiation in the presence of chromaffin cells. These findings suggested an approach to improved neural progenitor transplant outcomes using co-transplantation or administration of chromaffin cell-derived factors. The aim of this study was to determine whether the observed survival and differentiation effects were due to diffusible factors or required direct cell-cell contact (DC). Rat neural progenitors were cultured under six different conditions: (1) Standard N2 media with FGF-2; (2) N2 without FGF-2; (3) N2 with FGF+conditioned media (CM) from chromaffin cultures; (4) N2 without FGF-2+CM; (5) Transwells (TW), progenitor+chromaffin cells grown together but separated by a membrane allowing movement of diffusible agents but preventing direct contact; (6) direct contact co-cultures of progenitors and chromaffin cells. Cultures were evaluated for survival, proliferation, and differentiation. Cultures with FGF-2 proliferated and formed floating neurospheres while those grown in N2 without FGF-2 failed to thrive. Those grown either with CM or in transwells showed significantly improved survival. Survival was comparable to the exogenous FGF groups when progenitors were allowed direct contact with chromaffin cells. Proliferation was low in all cultures except those receiving exogenous FGF-2. Direct contact co-cultures exhibited a marked increase in beta-tubulin III+ processes compared to all other groups, indicating differentiation towards a neuronal phenotype. The results of this study suggest that diffusible agents produced by chromaffin cells can sustain viable progenitor cells in vitro even in the absence of added FGF-2 but that the effects on progenitor cell neuronal differentiation require direct cell-cell contact.

Enhanced viability and neuronal differentiation of neural progenitors by chromaffin cell co-culture

The transplantation of neural stem cells and progenitors has potential in restoring lost cellular populations following central nervous system (CNS) injury or disease, but survival and neuronal differentiation in the adult CNS may be insufficient in the absence of exogenous trophic support. Adrenal medullary chromaffin cells produce a trophic cocktail including basic fibroblast growth factor (FGF-2) and neurotrophins. The aim of this study was to evaluate whether chromaffin cells can provide a supportive microenvironment for neural progenitor cells. In order to assess this, the growth and differentiation of neural progenitor cell cultures from embryonic rat cortex were compared in standard FGF-2-supplemented neural progenitor growth media, in standard media but lacking FGF-2, or in media lacking FGF-2 but co-cultured with bovine chromaffin cells. Using bromodeoxyuridine (BrdU)-prelabeling, findings indicated poor survival of progenitor cultures in the absence of FGF-2. In contrast, the addition of chromaffin cells in co-culture appeared to 'rescue' the progenitor cultures and resulted in robust neurospheres containing numerous BrdU-labeled cells interspersed with and closely apposed to chromaffin cells. As indicated by H3 labeling, cells in co-cultures continued to proliferate, but at a substantially reduced rate compared with standard FGF-2 supplemented growth media. The co-cultures contained more beta-tubulin III-positive processes than parallel cultures maintained in FGF-2-supplemented media and these cells displayed a more mature phenotype with numerous varicosities and complex processes. These findings indicate that chromaffin cells can provide a supportive environment for the survival and neuronal differentiation of neural progenitor cells and suggest that their addition may be useful as a sustained source of trophic support to improve outcomes of neural stem cell transplantation.