Int J Stem Cells.  2016 Nov;9(2):169-175. 10.15283/ijsc16020.

Unlocking the Neurogenic Potential of Mammalian Müller Glia

• Xia X
• Ahmad I

• KMID: 2362510
• DOI: http://doi.org/10.15283/ijsc16020

Abstract

Müller glia (MG) are the primary support cells in the vertebrate retina, regulating homeostasis in one of the most metabolically active tissues. In lower vertebrates such as fish, they respond to injury by proliferating and reprogramming to regenerate retinal neurons. In mammals, MG may also react to injury by proliferating, but they fail to initiate regeneration. The barriers to regeneration could be intrinsic to mammalian MG or the function of the niche that cannot support the MG reprogramming required for lineage conversion or both. Understanding these mechanisms in light of those being discovered in fish may lead to the formulation of strategies to unlock the neurogenic potential of MG and restore regeneration in the mammalian retina.

Keyword

Müller glia; Regeneration; Neurogenesis; miR-124; Lin28; Retina

MeSH Terms

Homeostasis
Mammals
Neurogenesis
Neuroglia*
Regeneration
Retina
Retinal Neurons
Vertebrates

Figure

• 

  Fig. 1 A schematic representation of mechanisms underlying the neurogenic potential of MG. A subset of MG, most likely those with dormant stem cell properties, respond to injury by proliferating presumably under Notch signaling influence. As the activated MG migrate out of the INL, the changing niche, presumably reflected in altered Notch/cytokine signaling, influence them to engage both the excitatory (Green) and inhibitory (Red) axes regulating the neurogenic potential. It is possible that the imbalance between the two axes and their inadequate niche-based recruitment prevents mammalian MG from regenerating retinal neurons. The niche could be composed of retinal neurons, immigrant astrocytes, microglia, and endothelial cells. The niche-based communication for regeneration may involve diverse signaling pathways, exemplified by Notch and cytokine signaling, acting in concert. Notch signaling besides influencing these two molecular axes, may directly influence the expression of proneural genes. ONL: outer nuclear layer, OPL: outer plexiform layer, INL: inner nuclear layer, IPL: inner plexiform layer, GCL: ganglion cell layer.

• 

  Fig. 2 The intrinsic molecular axes and their involvement in the neurogenic potential of MG. (A) A schematic diagram shows the virus-mediated ectopic expression in enriched MG to perturb Lin28a (Lin28a-let7-Hmga2/Ascl1) and miR-124 and miR-9/9* (Rest-miR-124/miR-9-9*-Sox9/Hes). (B) Lin28a overexpression caused a significant decrease and increase in transcripts characterizing MG (Glast) and neurons (beta-tubulin and Map2), respectively. Neuronal differentiation of transduced MG could also be ascertained by their neuronal morphology and expression of immunoreactivities corresponding to beta-tubulin and Map2. Neuronal differentiation of MG was accompanied by a significant increase and decrease in the expression of positive (miR-124, Hmga2, and Ascl1) and negative (REST and Hes1) regulators of neurogenesis. (C) Similarly, miR-124-9-9* overexpression led to a decrease in levels of glia-specific (Glast) and an increase in neuron-specific (beta-tubulin and Map2) transcripts. As observed in Lin28a overexpression, transduced MG displayed neuronal morphology and expressed beta-tubulin and Map2 immunoreactivities. A significant increase and decrease in the expression of positive (Lin28a and Ascl1) and negative (REST and Hes1) regulators of neurogenesis was accompanied by miR-124-9-9* mediated neuronal differentiation of MG. Levels of Hmga2 transcripts remained unchanged. That the two axes might intersect and influence each other was demonstrated by the increase of miR-124 transcript levels in Lin28 overexpression experiment and vice versa (enclosed graph, B, C). MG were enriched as described in Das et al., (6). They were transduced with Lin28a retrovirus (24) or miR124-9-9* lentivirus (27) and the effects of perturbations on gene expression and phenotype were examined by Q-PCR and immunocytochemical analyses, respectively, as described in Parameswaran et al., (22) and Xia et al., (17). Controls included MG transduced with empty retrovirus/lentivirus. Levels of transcripts and miRNA are presented after normalization with that of GAPDH and U6, respectively. Arrowheads indicate transduced MG (GFP+/RFP+) expressing neuronal markers, beta-tubulin or Map2. Scale bar, 50 μm. Data are mean±s.e.m.

Reference

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