Severe arterial injury heals with a complex clonal structure involving a large fraction of surviving smooth muscle cells.

BACKGROUND AND AIMS: Smooth muscle cell (SMC) lineage cells in atherosclerosis and flow cessation-induced neointima are oligoclonal, being recruited from a tiny fraction of medial SMCs that modulate and proliferate. The present study aimed to investigate the clonal structure of SMC lineage cells healing more severe arterial injury. METHODS: Arterial injury (wire, stretch, and partial ligation) was inflicted on the right carotid artery in mice with homozygous, SMC-restricted, stochastically recombining reporter transgenes that produced mosaic expression of 10 distinguishable fluorescent phenotypes for clonal tracking. Healed arteries and contra-lateral controls were analyzed after 3 weeks. Additional analysis of cell death and proliferation after injury was performed in wildtype mice. RESULTS: The total number of SMC lineage cells in healed arteries was comparable to normal arteries but comprised significantly fewer fluorescent phenotypes. The population had a complex, intermixed, clonal structure. By statistical analysis of expected versus observed fractions of fluorescent phenotypes and visual inspection of coherent groups of same-colored cells, we concluded that >98% of SMC lineage cells in healed arteries belonged to a detectable clone, indicating that nearly all surviving SMCs after severe injury at some point undergo proliferation. This was consistent with serial observations in the first week after injury, which showed severe loss of medial cells followed by widespread proliferation. CONCLUSIONS: After severe arterial injury, many surviving SMCs proliferate to repair the media and form a neointima. This indicates that the fraction of medial SMCs that are mobilized to repair arteries increases with the level of injury.

Generation and characterization of a novel knockin minipig model of Hutchinson-Gilford progeria syndrome.

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare genetic disorder for which no cure exists. The disease is characterized by premature aging and inevitable death in adolescence due to cardiovascular complications. Most HGPS patients carry a heterozygous de novo LMNA c.1824C > T mutation, which provokes the expression of a dominant-negative mutant protein called progerin. Therapies proven effective in HGPS-like mouse models have yielded only modest benefit in HGPS clinical trials. To overcome the gap between HGPS mouse models and patients, we have generated by CRISPR-Cas9 gene editing the first large animal model for HGPS, a knockin heterozygous LMNA c.1824C > T Yucatan minipig. Like HGPS patients, HGPS minipigs endogenously co-express progerin and normal lamin A/C, and exhibit severe growth retardation, lipodystrophy, skin and bone alterations, cardiovascular disease, and die around puberty. Remarkably, the HGPS minipigs recapitulate critical cardiovascular alterations seen in patients, such as left ventricular diastolic dysfunction, altered cardiac electrical activity, and loss of vascular smooth muscle cells. Our analysis also revealed reduced myocardial perfusion due to microvascular damage and myocardial interstitial fibrosis, previously undescribed readouts potentially useful for monitoring disease progression in patients. The HGPS minipigs provide an appropriate preclinical model in which to test human-size interventional devices and optimize candidate therapies before advancing to clinical trials, thus accelerating the development of effective applications for HGPS patients.

Coupling of Bone Resorption and Formation in Real Time: New Knowledge Gained From Human Haversian BMUs.

It is well known that bone remodeling starts with a resorption event and ends with bone formation. However, what happens in between and how resorption and formation are coupled remains mostly unknown. Remodeling is achieved by so-called basic multicellular units (BMUs), which are local teams of osteoclasts, osteoblasts, and reversal cells recently proven identical with osteoprogenitors. Their organization within a BMU cannot be appropriately analyzed in common histology. The originality of the present study is to capture the events ranging from initiation of resorption to onset of formation as a functional continuum. It was based on the position of specific cell markers in longitudinal sections of Haversian BMUs generating new canals through human long bones. It showed that initial resorption at the tip of the canal is followed by a period where newly recruited reversal/osteoprogenitor cells and osteoclasts alternate, thus revealing the existence of a mixed "reversal-resorption" phase. Three-dimensional reconstructions obtained from serial sections indicated that initial resorption is mainly involved in elongating the canal and the additional resorption events in widening it. Canal diameter measurements show that the latter contribute the most to overall resorption. Of note, the density of osteoprogenitors continuously grew along the "reversal/resorption" surface, reaching at least 39 cells/mm on initiation of bone formation. This value was independent of the length of the reversal/resorption surface. These observations strongly suggest that bone formation is initiated only above a threshold cell density, that the length of the reversal/resorption period depends on how fast osteoprogenitor recruitment reaches this threshold, and thus that the slower the rate of osteoprogenitor recruitment, the more bone is degraded. They lead to a model where the newly recognized reversal/resorption phase plays a central role in the mechanism linking osteoprogenitor recruitment and the resorption-formation switch. © 2017 American Society for Bone and Mineral Research.