Tissue-specific stem cells: lessons from the skeletal muscle satellite cell

Abstract

In 1961, the satellite cell was first identified when electron microscopic examination of skeletal muscle demonstrated a cell wedged between the plasma membrane of the muscle fiber and the basement membrane. In recent years it has been conclusively demonstrated that the satellite cell is the primary cellular source for muscle regeneration and is equipped with the potential to self renew, thus functioning as a bona fide skeletal muscle stem cell (MuSC). As we move past the 50(th) anniversary of the satellite cell, we take this opportunity to discuss the current state of the art and dissect the unknowns in the MuSC field.

Figure 1. Satellite cell functions

(A) Adult single muscle fiber showing Pax7+ satellite cells (pink) located on individual multinucleated adult muscle fibers, myonuclei stained with DAPI (blue). (B) An example of satellite cell contribution to muscle repair using Pax7CreER^™.R26Rβ-gal is shown by X-gal reaction on an adult lower limb muscle prior to and after injury (reproduced from Shea et al, 2010). Note the many X-gal⁺ muscle fibers running throughout the regenerating muscle. (C) Pax7⁺ cells located in satellite cell position in uninjured muscle and their contribution to both myofiber repair and repopulation in the satellite cell position after regeneration (reproduced from Shea et al., 2010). (D) A schematic showing the participating roles of satellite cells. Pax7 expressing satellite cells (red) reside between the plasmalemma (brown line) of the muscle fiber and the basal lamina (black line). A mature adult muscle contains thousands of differentiated nuclei (green) and ~10 SCs (this number is highly dependent on age, muscle length and metabolic character). In response to injury, Pax7⁺ cells proliferate, most differentiate and fuse to regenerate myofibers (blue), while a minority self renew, return back to quiescence to replenish the satellite cell pool (left). After genetic ablation of satellite cells, muscle regeneration is prevented. During rapid hypertrophic stimuli, satellite cells fuse to add differentiated myonuclei. After genetic ablation of satellite cells, muscle hypertrophy occurs without addition of myonuclei, suggesting satellite cells are dispensable for protein accumulation and hypertrophy (center). During normal tissue homeostasis the contribution of satellite cells to maintenance of tissue or the satellite cell pool remains unknown (right).

Figure 2. MuSC heterogeneity

Origins of the MuSC pool. SC precursors (Pax7CreER⁺) located in the myotome begin expressing Pax7 (red) at ~E10.5, most express Myf5⁺ and Myod⁺ before occupancy into the niche. During early postnatal growth (P0–P7), a subset of Alkaline Phosphatase (AP⁺) derived pericytes as detected with APCre (grey) express Pax7 (red) and occupy the MuSC niche (dark red). A subset of AP⁺ pericytes is capable of contributing to myofiber differentiation (dark red myonuclei). Whether perictyes that commit to myogenic fusion express Pax7 during their lineage progression is unknown (thick red arrows).

Figure 3. Non-random segregation of chromosomes and asymmetric fate determination

The satellite cell pool is composed of primitive (Pax7hi, green) and lineage primed (Pax7low, grey) subsets. Pax7hi cells are in a dormant (less metabolically active) state. Pax7low satellite cells are lineage-primed. Following cytokinesis, satellite cell daughters can each inherit some chromosomes bearing newer (purple) or older template strand (dark green). Pax7hi subsets undergo non-random segregation of chromosomes, whereby one daughter contains exclusively chromosomes bearing older template strands, while its sister contains only chromosomes bearing newer template strands. In contrast, Pax7low cells undergo random segregation of chromosomes ensuring each daughter cell inherits an older and newer template strand. Subsets of Pax7hi and Pax7low are capable of multiple rounds of self-renewal (curved arrows) and differentiation (blue cells), however Pax7hi cells are endowed with enhanced self-renewal capability. Daughter cells containing newer template strands express high levels of Numb, Myod and Dek1 and are biased to differentiate (blue cell). Pax7hi cells can give rise to Pax7low, but not vice versa, suggesting a hierarchical relationship in the satellite cell pool based on Pax7 expression. In addition to cell fate determination, non-random segregation of chromosomes may act to protect genome integrity during cell proliferation. Due to low metabolic output of primitive satellite cells they may be protected from oxidative damage. Lineage-primed satellite cells are capable of self-renewal but may be acutely sensitive to proliferative demands and other forms of genotoxic insults.

Figure 4. Regulation of MuSC functions in satellite cells

(A) The MuSC niche. The satellite cell (red) resides in a quiescent state in contact with the plasmalemma (brown) of the myofiber and basal lamina (black) in a milieu of inhibitory (white arrows) and stimulatory factors (red arrows) that retain satellite cells in a stem cell state and inhibit their differentiation. Other cells in close proximity to satellite cells in vivo such as TCF4⁺ fibroblasts, FAPs, PICs and pericytes may constitute components of the MuSC niche and signal to the satellite cell and vice versa. (B) Schematic of MuSC regulation. Satellite cells transition from quiescence into a transient amplification stage most progress along a myogenic lineage to differentiate while a subset self renew and return back to quiescence (thick blue arrows). Markers denoting each stage of lineage progression are shown in black, molecules required for distinct satellite cell functions are shown in red. Quiescent satellite cells express Spry1, Notch-3 and miR-489. RBP-J and Myf5 is expressed in quiescent and cycling satellite cells. RBP-J and miR-489 (red) are required to retain satellite cells in quiescence. Spry1 and Notch-3 are required in cycling satellite cells for their return to quiescence and homeostasis of satellite cell pool after injury. Myf5 is required for normal transient amplification of progenitors. Myod, Myogenin and Myosin Heavy Chain (MyHC) are required for differentiation. Normal step-wise lineage progression of satellite cells depends on RBP-J (thin blue arrow), possibly through modulation of Notch-1 signaling.