Noggin recruits mesoderm progenitors from the dorsal aorta to a skeletal myogenic fate

TNAP expressing adventitial pericytes contribute to myogenesis during foetal development

OBJECTIVE

During growth and differentiation of skeletal muscle, cell types other than canonical myoblasts can be recruited to a myogenic fate. Among these, TNAP+ pericytes can differentiate into skeletal or smooth muscle cells during postnatal growth and contribute to muscle regeneration. However, their role in muscle development has not been investigated. This study aims to characterise pericyte fate choices during embryonic and foetal myogenesis, occurring in the second half of gestation.

APPROACH AND RESULTS

Using Cre-loxP lineage tracing with multiple reporters including the multifluorescent Confetti, we labelled TNAP+ precursors in vivo and assessed the smooth or skeletal muscle differentiation in their lineage at a perinatal stage. We found that TNAP+ cells contribute in vivo to skeletal and smooth muscle cells, as well as other pericytes, also during pre-natal muscle development. The resulting clones showed that such fate choices are likely to depend on distinct unipotent progenitors rather than multipotent progenitors. In addition, we isolated and differentiated in vitro foetal cells derived from TNAP+ precursors, which showed that they are not spontaneously myogenic unless co-cultured with other skeletal muscle cells.

CONCLUSIONS

This work extends our understanding of the differentiative potency of these non- canonical skeletal muscle progenitors during prenatal life, with a view to a future application of this knowledge to optimise cell therapies for muscle wasting disorders.

Mesoangioblasts at 20: From the embryonic aorta to the patient bed

In 2002 we published an article describing a population of vessel-associated progenitors that we termed mesoangioblasts (MABs). During the past decade evidence had accumulated that during muscle development and regeneration things may be more complex than a simple sequence of binary choices (e.g., dorsal vs. ventral somite). LacZ expressing fibroblasts could fuse with unlabelled myoblasts but not among themselves or with other cell types. Bone marrow derived, circulating progenitors were able to participate in muscle regeneration, though in very small percentage. Searching for the embryonic origin of these progenitors, we identified them as originating at least in part from the embryonic aorta and, at later stages, from the microvasculature of skeletal muscle. While continuing to investigate origin and fate of MABs, the fact that they could be expanded in vitro (also from human muscle) and cross the vessel wall, suggested a protocol for the cell therapy of muscular dystrophies. We tested this protocol in mice and dogs before proceeding to the first clinical trial on Duchenne Muscular Dystrophy patients that showed safety but minimal efficacy. In the last years, we have worked to overcome the problem of low engraftment and tried to understand their role as auxiliary myogenic progenitors during development and regeneration.

Skeletal muscle ClC-1 chloride channels in health and diseases

In 1970, the study of the pathomechanisms underlying myotonia in muscle fibers isolated from myotonic goats highlighted the importance of chloride conductance for skeletal muscle function; 20 years later, the human ClC-1 chloride channel has been cloned; last year, the crystal structure of human protein has been solved. Over the years, the efforts of many researchers led to significant advances in acknowledging the role of ClC-1 in skeletal muscle physiology and the mechanisms through which ClC-1 dysfunctions lead to impaired muscle function. The wide spectrum of pathophysiological conditions associated with modification of ClC-1 activity, either as the primary cause, such as in myotonia congenita, or as a secondary adaptive mechanism in other neuromuscular diseases, supports the idea that ClC-1 is relevant to preserve not only for skeletal muscle excitability, but also for skeletal muscle adaptation to physiological or harmful events. Improving this understanding could open promising avenues toward the development of selective and safe drugs targeting ClC-1, with the aim to restore normal muscle function. This review summarizes the most relevant research on ClC-1 channel physiology, associated diseases, and pharmacology.

Pericytes in Muscular Dystrophies

The muscular dystrophies are an heterogeneous group of inherited myopathies characterised by the progressive wasting of skeletal muscle tissue. Pericytes have been shown to make muscle in vitro and to contribute to skeletal muscle regeneration in several animal models, although recent data has shown this to be controversial. In fact, some pericyte subpopulations have been shown to contribute to fibrosis and adipose deposition in muscle. In this chapter, we explore the identity and the multifaceted role of pericytes in dystrophic muscle, potential therapeutic applications and the current need to overcome the hurdles of characterisation (both to identify pericyte subpopulations and track cell fate), to prevent deleterious differentiation towards myogenic-inhibiting subpopulations, and to improve cell proliferation and engraftment efficacy.

Smad1/5/8 are myogenic regulators of murine and human mesoangioblasts

Mesoangioblasts (MABs) are vessel-associated stem cells that express pericyte marker genes and participate in skeletal muscle regeneration. Molecular circuits that regulate the myogenic commitment of MABs are still poorly characterized. The critical role of bone morphogenetic protein (BMP) signalling during proliferation and differentiation of adult myogenic precursors, such as satellite cells, has recently been established. We evaluated whether BMP signalling impacts on the myogenic potential of embryonic and adult MABs both in vitro and in vivo. Addition of BMP inhibited MAB myogenic differentiation, whereas interference with the interactions between BMPs and receptor complexes induced differentiation. Similarly, siRNA-mediated knockdown of Smad8 in Smad1/5-null MABs or inhibition of SMAD1/5/8 phosphorylation with Dorsomorphin (DM) also improved myogenic differentiation, demonstrating a novel role of SMAD8. Moreover, using a transgenic mouse model of Smad8 deletion, we demonstrated that the absence of SMAD8 protein improved MAB myogenic differentiation. Furthermore, once injected into α-Sarcoglycan (Sgca)-null muscles, DM-treated MABs were more efficacious to restore α-sarcoglycan (αSG) protein levels and re-establish functional muscle properties. Similarly, in acute muscle damage, DM-treated MABs displayed a better myogenic potential compared with BMP-treated and untreated cells. Finally, SMADs also control the myogenic commitment of human MABs (hMABs). BMP signalling antagonists are therefore novel candidates to improve the therapeutic effects of hMABs.

Theranostic impact of NG2/CSPG4 proteoglycan in cancer

NG2/CSPG4 is an unusual cell-membrane integral proteoglycan widely recognized to be a prognostic factor, a valuable tool for ex vivo and non-invasive molecular diagnostics and, by virtue of its tight association with malignancy, a tantalizing therapeutic target in several tumour types. Although the biology behind its involvement in cancer progression needs to be better understood, implementation of NG2/CSPG4 in the routine clinical practice is attainable and has the potential to contribute to an improved individualized management of cancer patients. In this context, its polymorphic nature seems to be particularly valuable in the effort to standardize informative diagnostic procedures and consolidate forcible immunotherapeutic treatment strategies. We discuss here the underpinnings for this potential and highlight the benefits of taking advantage of the intra-tumour and inter-patient variability in the regulation of NG2/CSPG4 expression. We envision that NG2/CSPG4 may effectively be exploited in therapeutic interventions aimed at averting resistance to target therapy agents and at interfering with secondary lesion formation and/or tumour recurrence.

Type-2 pericytes participate in normal and tumoral angiogenesis

Tissue growth and function depend on vascularization, and vascular insufficiency or excess exacerbates many human diseases. Identification of the biological processes involved in angiogenesis will dictate strategies to modulate reduced or excessive vessel formation. We examine the essential role of pericytes. Their heterogeneous morphology, distribution, origins, and physiology have been described. Using double-transgenic Nestin-GFP/NG2-DsRed mice, we identified two pericyte subsets. We found that Nestin-GFP(-)/NG2-DsRed(+) (type-1) and Nestin-GFP(+)/NG2-DsRed(+) (type-2) pericytes attach to the walls of small and large blood vessels in vivo; in vitro, type-2, but not type-1, pericytes spark endothelial cells to form new vessels. Matrigel assay showed that only type-2 pericytes participate in normal angiogenesis. Moreover, when cancer cells were transplanted into Nestin-GFP/NG2-DsRed mice, type-1 pericytes did not penetrate the tumor, while type-2 pericytes were recruited during its angiogenesis. As inhibition of angiogenesis is a promising strategy in cancer therapy, type-2 pericytes may provide a cellular target susceptible to signaling and pharmacological manipulation in treating malignancy. This work also reports the potential of type-2 pericytes to improve blood perfusion in ischemic hindlimbs, indicating their potential for treating ischemic illnesses.

Pericytes in development and pathology of skeletal muscle

Increasing attention is currently devoted to the multiple roles that pericytes (also defined as mural, Rouget, or perivascular cells) may play during angiogenesis, vascular homeostasis, and pathology. Many recent excellent reviews thoroughly address these topics (see below); hence, we will not discuss them in detail here. However, not much is known about origin, heterogeneity, gene expression, and developmental potential of pericytes during fetal and postnatal development. This is likely because of the paucity of markers expressed by pericytes and the absence of truly unique ones. Thus, in vivo identification and ex perspective isolation are challenging and explain the relative little data available in comparison with neighbor but far more characterized cells such as the endothelium. Despite this preliminary knowledge, we will propose that contribution to growing mesoderm tissues may be an important role for pericytes. Thus, their ability to contribute to tissue regeneration may be a consequence of their role in tissue growth. However, in a severely damaged or diseased tissue, acute or chronic inflammation likely results in the production of signaling molecules that are different from those present in developing tissues, thus explaining why pericytes are easily diverted from a regenerative to a fibrotic fate.

Dll4 and PDGF-BB convert committed skeletal myoblasts to pericytes without erasing their myogenic memory

Pericytes are endothelial-associated cells that contribute to vessel wall. Here, we report that pericytes may derive from direct conversion of committed skeletal myoblasts. When exposed to Dll4 and PDGF-BB, but not Dll1, skeletal myoblasts downregulate myogenic genes, except Myf5, and upregulate pericyte markers, whereas inhibition of Notch signaling restores myogenesis. Moreover, when cocultured with endothelial cells, skeletal myoblasts, previously treated with Dll4 and PDGF-BB, adopt a perithelial position stabilizing newly formed vessel-like networks in vitro and in vivo. In a transgenic mouse model in which cells expressing MyoD activate Notch, skeletal myogenesis is abolished and pericyte genes are activated. Even if overexpressed, Myf5 does not trigger myogenesis because Notch induces Id3, partially sequestering Myf5 and inhibiting MEF2 expression. Myf5-expressing cells adopt a perithelial position, as occasionally also observed in wild-type (WT) embryos. These data indicate that endothelium, via Dll4 and PDGF-BB, induces a fate switch in adjacent skeletal myoblasts.

Skeletal muscle neural progenitor cells exhibit properties of NG2-glia

Reversing brain degeneration and trauma lesions will depend on cell therapy. Our previous work identified neural precursor cells derived from the skeletal muscle of Nestin-GFP transgenic mice, but their identity, origin, and potential survival in the brain are only vaguely understood. In this work, we show that Nestin-GFP+ progenitor cells share morphological and molecular markers with NG2-glia, including NG2, PDGFRα, O4, NGF receptor (p75), glutamate receptor-1(AMPA), and A2B5 expression. Although these cells exhibit NG2, they do not express other pericyte markers, such as α-SMA or connexin-43, and do not differentiate into the muscle lineage. Patch-clamp studies displayed outward potassium currents, probably carried through Kir6.1 channels. Given their potential therapeutic application, we compared their abundance in tissues and concluded that skeletal muscle is the richest source of predifferentiated neural precursor cells. We found that these cells migrate toward the neurogenic subventricular zone displaying their typical morphology and nestin-GFP expression two weeks after brain injection. For translational purposes, we sought to identify these neural progenitor cells in wild-type species by developing a DsRed expression vector under Nestin-Intron II control. This approach revealed them in nonhuman primates and aging rodents throughout the lifespan.