Current Stem Cell Research & Therapy - Volume 4, Issue 3, 2009

The discovery of microRNAs (miRNAs - small non-coding RNAs of ~22 nt) heralded a new and exciting era in biology. During this period miRNAs have gone from ignominy due to their origin mainly in ‘junk DNA’ to notoriety where they can be at once characterized as being all powerful (a single miRNA can target and potentially silence several hundred genes) and yet marginal (a given gene can be targeted by several miRNAs such that a given miRNA typically exerts a modest repression) [1-4]. The emerging paradox is exemplified by miRNAs that are prominently expressed in embryonic stem (ES) cells. The collective importance of miRNAs is firmly established by the fact that Dicer-/- mouse embryos die on day 7.5 due to defects in differentiation [5]. However, oppositely correlated expression that is expected of conventional repressors is increasingly being defied in multiple systems in relation to miRNA-mRNA target pairs. This is most evident in ES cells where miR-290-295 and 302 clusters the most abundant ES cell miRNAs, are both found to be driven by pluripotency genes Oct4, Nanog and Sox2 and are predicted to target Sox2 in ‘incoherent feed-forward loops’ [7]. Here the miRNAs are co-expressed and positively correlated with these targets that they repress suggesting that one of their primary roles is to fine tune gene expression rather than act as ON/OFF switches. On the other hand, let-7 family members that are notably low in ES cells and rapidly induced upon differentiation exhibit more conventional anticorrelated expression patterns with their targets [7, 8]. In an intricately designed auto-regulatory loop, LIN28, a key ‘keeper’ of the pluripotent state binds and represses the processing of let-7 (a key ‘keeper‘ of the differentiated state) [9-11]. One of the let-7 family members, let-7g targets and represses LIN28 through four 3'-UTR binding sites [12]. We propose that LIN28/let-7 pair has the potential to act as a ‘toggle switch’ that balances the decision to maintain pluripotency vs. differentiation. We also propose that the c-Myc/E2F driven miR17-92 cluster that together controls the G1 to S transition is fundamental for ES self-renewal and cell proliferation [13-18]. In that context it is no surprise that LIN28 and c-Myc (and therefore let-7 and miR-17-92 by association) and more recently Oct4/Sox2 regulated miR-302 has been shown to be among a handful of factors shown to be necessary and sufficient to convert differentiated cells to induced pluripotent stem (iPS) cells [19-29]. It is also no surprise that activation of miR-17-92 (OncomiRs) and down-regulation of let-7 (tumor suppressors) is a recurring theme in relation to cancers from multiple systems [30-48]. We speculate that the LIN28/let-7; c-MYC-E2F/miR-17-92 and Oct4/Sox2/miR-302-cyclin D1 networks are fundamental to properties of pluripotency and self-renewal associated with embryonic stem cells. We also speculate that ES cell miRNA-mRNA associations may also regulate tissue homeostasis and regeneration in the fully developed adult. Consequently, the appropriate regulation of LIN28/let-7; c-MYC-E2F/miR-17-92 and Oct4/Sox2/miR-302-cyclin D1 gene networks will be critical for the success of regenerative strategies that involve iPS cells. Perturbations in any of the key ES cell miRNA-mRNA networks maybe a hallmark of cancer stem cells (CSCs).

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by the selective loss of both spinal and upper motor neurons. One strategy in treating ALS is to use stem cells to replace lost spinal motor neurons. However, transplanted stem cell-derived motor neurons may not survive when exposed to the harsh microenvironment in the spinal cord of ALS. In particular, dysfunctional astrocytes and overactivated microglia in ALS may limit the survival of motor neurons generated from cell replacement therapy. On the other hand, stem cells may provide large quantities of motor neurons that can be used for studying glia-mediated toxic mechanisms and potential therapies in ALS. Here we will review methods and molecular factors for directed differentiation of stem cells into spinal motor neurons, the potential uses of these models for dissecting the mechanisms underlying glia-induced motor neuron degeneration and screening for new therapeutics aimed at protecting motor neurons in ALS, as well as discuss challenges facing the development of motor neuron replacement-based cell therapies for recovery in ALS.

Chondroitin sulfate (CS), a polysaccharide moiety of proteoglycans, is one of the major components of the extracellular matrix in the central nervous system and is involved in various cellular events in the formation and maintenance of the neural network. In the developing brain, CS in the milieu of neural stem/progenitor cells (NSPCs) is believed to participate in the regulation of their functions such as proliferation and differentiation. NSPCs are expected to act as a potent cell type in cell replacement therapy for neurodegeneration in various neurological diseases. Recently, it has been shown that transplantation of NSPCs combined with removal of extracellular CS from the host nervous tissues gives a satisfactory outcome in some animal models of nervous tissue injuries including neonatal hypoxic-ischemic injury and adult spinal cord injury. The combination of cell transplantation with modification of the extracellular matrix of the host tissue could be a novel strategy for the treatment of incurable neurodegenerative diseases.

Heart failure accounts for more deaths in the United States than any other detrimental human pathology. Recently, repairing the heart after seemingly irreversible injury leading to heart failure appears to have come within reach. Cellular cardiomyoplasty, transplanting viable cell alternatives into the diseased myocardium, has emerged as a promising possible solution. Translating this approach from the laboratory to the clinic, however, has been met with several challenges, leaving many questions unanswered. This review assesses the state of investigation of several progenitor cell sources, including induced pluripotent stem cells, embryonic stem cells, bone marrow stem cells, adipose-derived adult stem cells, amniotic fluid stem cells, skeletal muscle progenitors, induced pluripotent stem cells and cardiac progenitors. Several current roadblocks to maximum success are discussed. These include understanding the need for cardiomyocyte differentiation, appreciating the role of paracrine factors, and addressing the low engraftment rates using current techniques. Tissue engineering strategies to address these obstacles and to help maximize cellular cardiomyoplasty success are reviewed.

The introduction of tyrosine kinase inhibitor treatment for CML marks one of the major success stories in the recent history of medicine. However, eradication of disease is almost never attained, because, unlike the vast majority of more differentiated cells, leukemic stem cells withstand TKI's, necessitating life-long treatment. Besides, although a relatively infrequent event under treatment with TKI's, refractory leukemic stem cells may sometimes give rise to disease transformation. In this article, we will review the definitions of CML stem cells, explain how BCR-ABL induces perturbations of critical signal transduction pathways and summarize specific characteristics that cause refractoriness of CML stem cells against TKI's. Furthermore, events that are responsible or related to transformation of the disease into blast crisis will be discussed and new research directions that should lead to successful ways to attack leukemic stem cells are proposed.

Mammalian organ regeneration is the “Holy Grail” of modern regenerative biology and medicine. The most dramatic organ replacement is known as epimorphic regeneration. To date our knowledge of epimorphic regeneration has come from studies of amphibians. Notably, these animals have the ability to reprogram phenotypically committed cells at the amputation plane toward an embryonic-like cell phenotype (dedifferentiation). The capability of mammals to initiate analogous regeneration, and whether similar mechanisms would be involved if it were to occur, remain unclear. Deer antlers are the only mammalian appendages capable of full renewal, and therefore offer a unique opportunity to explore how nature has solved the problem of mammalian epimorphic regeneration. Following casting of old hard antlers, new antlers regenerate from permanent bony protuberances, known as pedicles. Studies through morphological and histological examinations, tissue deletion and transplantation, and cellular and molecular techniques have demonstrated that antler renewal is markedly different from that of amphibian limb regeneration (dedifferentiation-based), being a stem cell-based epimorphic process. Antler stem cells reside in the pedicle periosteum. We envisage that epimorphic regeneration of mammalian appendages, other than antler, could be made possible by recreating comparable milieu to that which supports the elaboration of that structure from the pedicle periosteum.