Heat-Induced Antigen Retrieval: An Effective Method to Detect and Identify Progenitor Cell Types during Adult Hippocampal Neurogenesis
Summary
During adult hippocampal neurogenesis, a distinct set of genetic markers are expressed when a quiescent neural stem cell sequentially progresses and develops into a functionally integrated neuron in the circuit. Using heat-induced antigen retrieval, progenitor cell types that are otherwise difficult to detect are identified with improved effectiveness.
Abstract
Traditional methods of immunohistochemistry (IHC) following tissue fixation allow visualization of various cell types. These typically proceed with the application of antibodies to bind antigens and identify cells with characteristics that are a function of the inherent biology and development. Adult hippocampal neurogenesis is a sequential process wherein a quiescent neural stem cell can become activated and proceed through stages of proliferation, differentiation, maturation and functional integration. Each phase is distinct with a characteristic morphology and upregulation of genes. Identification of these phases is important to understand the regulatory mechanisms at play and any alterations in this process that underlie the pathophysiology of debilitating disorders. Our heat-induced antigen retrieval approach improves the intensity of the signal that is detected and allows correct identification of the progenitor cell type. As discussed in this paper, it especially allows us to circumvent current problems in detection of certain progenitor cell types.
Introduction
Neurogenesis, the generation of new neurons from neural stem cells, is now known to constantly occur into adulthood in two specialized regions in the brain. These include the subventricular zone (SVZ) of the olfactory bulb and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus. Over the past decade, the field of adult hippocampal neurogenesis has seen significant work. The region has attracted much interest since newborn neurons in the SGZ contribute to enhanced neural plasticity that can sustain specific brain functions1-5. Adult hippocampal neurogenesis has been implicated to play important roles in mood regulation, regeneration, and learning and memory. Thus, there has been a concerted effort to understand the development and regulation of neurons in this rich neurogenic niche.
An unavoidable and important aspect of studying adult hippocampal neurogenesis is the identification of separate stages that an activated neural stem cell passes through on its way to becoming a fully functional neuron. In this process, the quiescent neural stem cell is activated and proceeds through a series of early active and late active progenitor cell types (Figure 1). These distinct populations of neural progenitors can be identified by morphology and their expression of molecular markers such as MCM2, nestin, Tbr2 and Doublecortin (DCX) and NeuN that guide the conversion of RGLs into their respective progenitor cell types. Nestin is an intermediate filament protein that is expressed across the radial glia-like cells and some early progenitor cell types. Another marker is Tbr2 that is specifically expressed in the amplifying progenitor cells. The Tbr2 transgene expression is initially switched off in the Type-1 cells (radial-glia like), turned on across the Type-2a, Type-2ab and Type-2b progenitors and switched off among the more differentiated Type-3 and immature neurons (Figure 1). DCX is a neuronal migration marker which is expressed in the Type-2ab, Type-2b and Type-3 neural progenitors. Using this combination of three markers, we can label distinct subtypes of neural progenitors. These include the Type-1 (MCM2+nestin+Tbr2-), Type-2a (MCM2+nestin+Tbr2+), Type-2ab (partial MCM2+nestin-Tbr2+ and partial MCM2+Tbr2+DCX-), Type-2b (MCM2+Tbr2+DCX+) and Type-3 (MCM2+Tbr2-DCX+). Post-mitotic cells such as the immature and functionally integrated neurons can be identified by utilizing DCX and the mature neuronal marker, NeuN.
Traditional methods of IHC utilize antibodies to identify antigens for specific cell types based on their gene expression. Subsequent visualization through high resolution imaging techniques such as confocal microscopy can be utilized for their identification. However, currently there are problems that exist with these methods that don’t allow efficient identification of the radial glia-like cells and early progenitor cell types. It is difficult to identify these particular cell types because the applied antibody is not able to penetrate and efficiently bind the nestin protein. Nestin is the intermediate filament protein that comprises the radial processes produced by the radial glia-like cell. The inefficient binding of an antibody to the antigen can be a result of many factors including fixation time, temperature and technique utilized6-7. To help solve such issues, we have developed an antigen-retrieval or “boiling” method. Besides nestin, this method of antigen-retrieval also improves staining for other markers such as Ki67, BrdU, glial fibrillary acidic protein (GFAP), Tbr2 and MCM2. Our method combines together chemical and physical approaches. It involves chemical fixation of the tissue in paraformaldehyde and subsequent high temperature boiling of thin coronal hippocampal sections in a buffer of a denaturant and chaotropic treatment. This enhances accessibility of the antigen to the antibody, improves antibody specificity and allows improved identification of progenitor cell types. Our approach is simple to use requiring mostly already available tools and reagents in the laboratory. We have utilized it extensively to study development of neural stem cells and their regulation via intrinsic genetic or extrinsic environmental factors8.
Protocol
Representative Results
Discussion
The most critical steps for successful antigen-retrieval and staining of progenitor cell types are: 1) utilizing well perfused and fixed tissue of the optimal coronal thickness; 2) allowing sufficient time for boiling and subsequent cooling during antigen retrieval; 3) manual dexterity in mounting fixed coronal sections and preventing their damage when doing so.
It is critical in IHC to use perfused tissue that has undergone sufficient fixation but not over-fixation. A primary difficulty can b…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The methodology was originally developed in Dr. Hongjun Song’s laboratory at the Institute for Cell Engineering, Department of Neurology, Johns Hopkins School of Medicine. This work was funded by NIMH (R00MH090115), NARSAD, the Fraternal Order of Eagles’ Mayo Cancer Research Fund and a start-up package from Mayo foundation awarded to M.H.J. and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (A3014385) awarded to W.R.K.
Materials
| Name of the reagent | Company | Catalogue number | Comments (optional) |
| SuperFrost Plus Slide | Fisherbrand | 12-550-15 | Provides strong adherence to hippocampal tissue during boiling |
| PVA/DABCO Mounting Media | Sigma Aldrich | 10981-100 ml | |
| Nestin (chicken) | Aves Lab | NES | Neural stem cell marker |
| GFAP (rabbit) | Dako North America | Z033401-2 | Astrocyte and neural stem cell marker |
| MCM2 (mouse) | BD Transduction Laboratory | 610701 | Proliferation marker |
| DCX (goat) | Santa Cruz Biotechnology | SC8066 | Immature neuron marker |
| Tbr2 (rabbit) | Abcam | Ab23345 | Amplifying progenitor marker |
| NeuN (mouse) | Millipore | MAB377 | Mature neuron marker |
| Cy2 (anti-rabbit) | Jackson Immunoresearch | 111-226-047 | |
| Cy2 (anti-chicken) | Jackson Immunoresearch | 303-165-006 | |
| Cy5 (anti-mouse) | Jackson Immunoresearch | 315-175-047 | |
| Cy5 (anti-goat) | Jackson Immunoresearch | 305-165-047 | |
| DAPI | Life Technologies Corporation | D1306 | |
| Dako Pen | S2002 | Dako | Water-repellant pen |
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