Requirement of Hyaluronan Synthase-2 in Craniofacial and Palate Development

Cleft palate is a common major birth defect resulting from disruption of palatal shelf growth, elevation, or fusion during fetal palatogenesis. Whereas the molecular mechanism controlling palatal shelf elevation is not well understood, a prevailing hypothesis is that region-specific accumulation of hyaluronan, a predominant extracellular glycosaminoglycan in developing palatal mesenchyme, plays a major role in palatal shelf elevation. However, direct genetic evidence for a requirement of hyaluronan in palate development is still lacking. In this study, we show that Has2, 1 of 3 hyaluronan synthases in mammals, plays a major role in hyaluronan synthesis in the neural crest-derived craniofacial mesenchyme during palatogenesis in mice. We analyzed developmental defects caused by tissue-specific inactivation of Has2 throughout the cranial neural crest lineage or specifically in developing palatal or mandibular mesenchyme, respectively, using Wnt1-Cre, Osr2-Cre, and Hand2-Cre transgenic mice. Inactivation of Has2 either throughout the neural crest lineage or specifically in the developing palatal mesenchyme caused reduced palatal shelf size and increased palatal mesenchyme cell density prior to the time of normal palatal shelf elevation. Whereas both Has2f/f;Wnt1-Cre and Has2f/f;Osr2-Cre mutant mice exhibit cleft palate at complete penetrance, the Has2f/f; Wnt1-Cre fetuses showed dramatically reduced mandible size and complete failure of palatal shelf elevation, whereas Has2f/f;Osr2-Cre fetuses had normal mandibles and delayed palatal shelf elevation. All Has2f/f;Hand2-Cre pups showed reduced mandible size and about 50% of them had cleft palate with disruption of palatal shelf elevation. Results from explant culture assays indicate that disruption of palatal shelf elevation in Has2f/f;Hand2-Cre mutant fetuses resulted from physical obstruction by the malformed mandible and tongue. Together, these data indicate that hyaluronan plays a crucial intrinsic role in palatal shelf expansion and timely reorientation to the horizontal position above the tongue as well as an important role in mandibular morphogenesis that secondarily affects palatal shelf elevation.

Knockout of hyaluronan synthase 1, but not 3, impairs formation of the retrocalcaneal bursa

Hyaluronan (HA), a high molecular weight non-sulfated glycosaminoglycan, is an integral component of the extracellular matrix of developing and mature connective tissues including tendon. There are few published reports quantifying HA content during tendon growth and maturation, or detailing its effects on the mechanical properties of the tendon extracellular matrix. Therefore, the goal of the current study was to examine the role of HA synthesis during post-natal skeletal growth and maturation, and its influence on tendon structure and biomechanical function. For this purpose, the morphological, biochemical, and mechanical properties of Achilles tendons from wild type (WT) and hyaluronan synthase 1 and 3 deficient mouse strains (Has1-/- (Has1KO), Has3-/- (Has3KO), and Has1-/- 3-/- (Has1/3KO)) were determined at 4, 8, and 12 weeks of age. Overall, HAS-deficient mice did not show any marked differences from WT mice in Achilles tendon morphology or in the HA and chondroitin/dermatan sulfate (CS/DS) contents. However, HAS1-deficiency (in the single or Has1/3 double KO) impeded post-natal formation of the retrocalcaneal bursa, implicating HAS1 in regulating HA metabolism by cells lining the bursal cavity. Together, these data suggest that HA metabolism via HAS1 and HAS3 does not markedly influence the extracellular matrix structure or function of the tendon body, but plays a role in the formation/maintenance of peritendinous bursa. Additional studies are warranted to elucidate the relationship of HA and CS/DS metabolism to tendon healing and repair in vivo. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2622-2632, 2018.