Nestin-expressing progenitor cells: function, identity and therapeutic implications

Abstract

The neuroepithelial stem cell protein, or Nestin, is a cytoskeletal intermediate filament initially characterized in neural stem cells. However, current extensive evidence obtained in in vivo models and humans shows presence of Nestin⁺ cells with progenitor and/or regulatory functions in a number of additional tissues, remarkably bone marrow. This review presents the current knowledge on the role of Nestin in essential stem cell functions, including self-renewal/proliferation, differentiation and migration, in the context of the cytoskeleton. We further discuss the available in vivo models for the study of Nestin⁺ cells and their progeny, their function and elusive nature in nervous system and bone marrow, and their potential mechanistic role and promising therapeutic value in preclinical models of disease. Future improved in vivo models and detection methods will allow to determine the true essence of Nestin⁺ cells and confirm their potential application as therapeutic target in a range of diseases.

Keywords: Central and peripheral nervous systems; Hematological malignancies; Hematopoietic stem cell niche; Mesenchymal stromal cells; Mouse models; Nestin; Neural stem and progenitor cells; Neurodegenerative diseases.

Fig. 1

Nestin structure and assembly into intermediate filament. a Molecular structure of Nestin. Nestin shares a common structure with the other intermediate filaments that consists of a central α-helical rod domain (blue) of conserved size, flanked by a globular N-terminus (‘head’, red) and C-terminus (‘tail’, green) domains. The central α-helical rod domain consists of three segments separated by two linkers, i.e., coil 1A, linker 1, coil 1B, linker 12 and coil 2, with 2A and 2B and stutter between them. b Particularities of Nestin monomer. Nestin shows a short N-terminus (≈ 8 amino acids) and a long C-terminus (> 1400 amino acids) end, compared to other intermediate filaments like vimentin (≈ 100 and 400 amino acids, respectively). c Nestin forms heterodimers. N-terminus is required for intermediate filament assembly, and free C-terminus may interact with other cytoskeleton components. The short N-terminus disables Nestin to self-assemble into higher order structures, so Nestin needs other intermediate filaments to assemble, like vimentin. The central rod domain contains hydrophobic repeats (heptad repeats) that mediate dimerization allowing two α-helices to wrap around each other and become a ‘coiled coil’. Therefore, intermediate filament dimer rod central domain is known as central rod coiled domain. d Hypothetical intermediate filament assembly. Two dimers coil in antiparallel way in head-to-tail association to make a tetramer. Assembly takes place through rapid lateral aggregation of eight tetramers to form subfilaments, which assemble axially to form the intermediate filament (diameter ~ 10 nm). Numbers in N-termini and C-termini ends indicate number of amino acid residues. N-ter N-terminus, C-ter C-terminus

Fig. 2

In vivo models for the study of Nestin⁺ cells and their progeny. a Nestin^−/− mouse model. Nestin^−/− mice are embryonically lethal (E8.5) due to neural stem cell apoptosis, which uncovered the role of Nestin in neural stem cell self-renewal and survival [36]. This model does not allow fine-tuned control of Nestin expression in space and time, so an improved strategy would be use of conditional and/or inducible knockout models, like Nestin^fl/fl intercrossed with cre lines of interest. b Nestin-gfp reporter mouse model. Pit-Oct-Unc transcription factors bind to the Nestin neural enhancer at the second intron to establish neuroepithelial cells specificity. Nestin-gfp mouse model was generated by selection only of these regulatory elements of the second intron and upstream area [2] in an attempt to direct gfp expression to neural stem cells. Later, presence of Nestin-gfp⁺ cells has been described in other tissues, particularly bone marrow. Nestin-gfp^bright and Nestin-gfp^dim cells have been reported with no robust evidence of differential and corresponding endogenous Nestin expression. In addition, gfp turnover may contribute to fluorescence intensity levels, which represents another limitation that may be overcome by fluorescent protein fusion to Nestin. c Nestin-cre^ERT2 lineage tracing in vivo. This mouse model is based on cre technology, coupled to the modified estrogen ligand-binding domain (ERT2). The system activates under administration of tamoxifen, allowing tracing of Nestin lineage in a temporal manner, both in embryonic and adult stages. However, tamoxifen has physiological effects that must be distinguished using controls lacking cre^ERT2 and treated with tamoxifen. d Nestin-cre^ERT2 iDTA mouse model for Nestin lineage depletion. This mouse model is used to reduce numbers of Nestin⁺ cells and their lineage upon tamoxifen administration. It is useful to study their contribution to disease development. However, depletion of cells occurs through diphtheria toxin expression and subsequent necrosis, which implies release of cellular content and potential sterile inflammation. Nes Nestin, Gfp green fluorescent protein, POU Pit-Oct-Unc, Tam tamoxifen, NSC neural stem cells, NSPC neural stem and progenitor cells, MSC mesenchymal stromal cells, iDTA inducible diphtheria toxin A subunit

Fig. 3

Identity of Nestin⁺ cells found in neural tissues and bone marrow. The main subsets of Nestin⁺ cells present in neural-related tissues (brain, cerebellum, retina, gut, skin) and bone marrow are shown. These subsets comprise neural stem cells, neural progenitor cells, astrocytes, radial glia-like precursors, subsets of glia cells with regenerative potential, several types of neural precursor cells outside brain, pericytes, mesenchymal stromal cells, endothelial cells, and Schwann cell precursors. They are distinct in their location, functional properties and co-expression of selective markers. Nestin⁺ cells are represented in green. In brain and bone marrow, Nestin-gfp^bright and gfp^dim cells have been described according to gfp intensity levels in Nestin-gfp mice. NSC neural stem cells, NPC neural progenitor cell, MSC mesenchymal stromal cell, Gfap glial fibrillary acidic protein, SOX2 sex determining region Y-box 2, p75 NTR p75 neurotrophin receptor, NG2 neural/glial antigen 2, Leptin R leptin receptor