The nuclei of human adult stem cells can move within the cell and generate cellular protrusions to contact other cells

Abstract

Background The neuronal transdifferentiation of adult bone marrow cells (BMCs) is still considered an artifact based on an alternative explanation of experimental results supporting this phenomenon obtained over decades. However, recent studies have shown that following neural induction, BMCs enter an intermediate cellular state before adopting neural-like morphologies by active neurite extension and that binucleated BMCs can be formed independent of any cell fusion events. These findings provide evidence to reject the idea that BMC neural transdifferentiation is merely an experimental artifact. Therefore, understanding the intermediate states that cells pass through during transdifferentiation is crucial given their potential application in regenerative medicine and disease modelling. Methods In this study, we examined the functional significance of the variety of morphologies and positioning that cell nuclei of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can adopt during neural-like differentiation using live-cell nuclear fluorescence labelling, time-lapse microscopy, and confocal microscopy analysis. Results Here, we showed that after neural induction, hBM-MSCs enter an intermediate cellular state in which the nuclei are able to move within the cells, switching shapes and positioning and even generating cellular protrusions as they attempt to contact the cells around them. These findings suggest that changes in nuclear positioning occur because human cell nuclei somehow sense their environment. In addition, we showed the process of direct interactions between cell nuclei, which opens the possibility of a new level of intercellular interaction. Conclusions The present study advances the understanding of the intermediate stage through which hBM-MSCs pass during neural transdifferentiation, which may be crucial to understanding the mechanisms of these cell conversion processes and eventually harness them for use in regenerative medicine. Importantly, our study provides for the first time evidence that the nuclei of hBM-MSC-derived intermediate cells somehow sense their environment, generating cellular protrusions to contact other cells. In summary, human mesenchymal stromal cells could not only help to increase our understanding of the mechanisms underlying cellular plasticity but also facilitate the exact significance of nuclear positioning in cellular function and in tissue physiology.

1. Introduction

Since the first observation of a nucleus in 1700 [1], our knowledge of nuclear composition, organization, and positioning has continuously evolved [2–4]. Most textbooks depict the nucleus as a spherical or ovoid object at the center of the cell. However, different cell types have very different nuclear shapes, and the position of nuclei varies dramatically from this simple view [2, 3, 5]. Although the cell nucleus has always been considered the largest and most rigid organelle of eukaryotic cells, emerging views of the nucleus indicate a more dynamic organelle than expected [6].

There is increasing evidence that nuclei are frequently asymmetrically positioned depending on cell type, developmental stage, migratory state, and differentiation status [2, 7–10]. It has been reported that the position of the nucleus contributes to cell mechanics, such as gene regulation through relative genome segregation and the organization of cells within tissues [2, 9–11]. Furthermore, it is important to note that changes in nuclear morphology and positioning are often associated with cellular dysfunction and disease [2, 12, 13]. Therefore, the nucleus must be considered not only as the primary site for the storage of genetic material and gene transcription but also as a fundamental mechanical component of the cellular structure [6]. Despite these advances, the exact significance of nuclear positioning in cellular function and tissue physiology is still far from being clearly understood [2, 5, 7].

In our laboratory, we focus on the differentiation of human mesenchymal stem cells (hMSCs) to generate a neuronal lineage. hMSCs are considered promising candidates for regenerative medicine due to their multipotency and immunomodulatory properties. However, the mechanisms underlying their transdifferentiation into neural-like cells remain controversial. This study aims to investigate the dynamic behavior of nuclei during this process, providing new insights into cellular plasticity and intercellular communication.