Distal arthrogryposis (DA) is a collection of rare developmental disorders characterized by congenital joint contractures. Most arthrogryposis mutations are in muscle- and joint-related genes, and the anatomical defects originate cell-autonomously within the musculoskeletal tissues. However, gain-of-function (GOF) mutations in PIEZO2, a principal mechanosensor in somatosensation, cause DA subtype 5 via unknown mechanisms. We show that expression of a GOF PIEZO2 mutation in proprioceptive sensory neurons mainly innervating muscle spindles and tendons is sufficient to induce DA5-like phenotypes in mice. Overactive PIEZO2 causes anatomical defects via increased activity within the peripheral nervous system during postnatal development. Surprisingly, overactive PIEZO2 is likely to cause joint abnormalities via increased exocytosis from sensory neuron endings without involving motor circuitry. This reveals a role for somatosensory neurons: excessive mechanosensation within these neurons disrupts musculoskeletal development. We also present proof-of-concept that Botox injection or dietary treatment can counteract the effect of overactive PIEZO2 function to evade DA-like phenotypes in mice when applied during a developmental critical period. These approaches might have clinical applications. Beyond this, our findings call attention to the importance of considering sensory mechanotransduction when diagnosing and treating other musculoskeletal disorders.

Acknowledgements: Our work is supported by National Institutes of Health grant (R35 NS105067, R01 DE022358, R25 SC3GM127195, R25 GM07138, R01GM133845, intramural) and Howard Hughes Medical Institute.

Introduction Anterior column reconstruction and fusion remains the gold standard of treatment for a number of spinal pathologies. One of the challenges of interbody fusions cages is the footprint of the cage reducing the surface area of endplate available for fusion. Biodegradable polymer implants will over time present a greater area for fusion and may help to reduce problems such as stress shielding, particulate debris and retained foreign body response. Resorbable cages have been have been prepared from a number of different materials, including inorganic composites (eg hydroxyapatite / tricalcium phosphate) and polymers (Poly L-lactide-co-D,L-lactide (PDLLA)). However all of the current options for interbody fusion have reported deficiencies or complications. The synthesis, mechanical properties, and degradation behaviour of two novel biopolymers are presented and the applicability for use as materials in interbody fusion devices is discussed.

Methods Methacrylated adipic anhydride (MAA) and methacrylated sebacic anhydride (MSA) pre-polymers were synthesized by melt condensation. Conversion of the acid to the anhydride was confirmed using 1H nuclear magnetic resonance (NMR) (Bruker, Alexandria, NSW) and FT- Infrared spectroscopy (Nicolet, Waltham MA). These pre-polymers were subsequently co-polymerized with methyl methacrylate (MMA) and 0.25 wt% benzoyl peroxide at 65oC for 16hrs and post-cured at 120oC under vacuum for 2 hrs to form biodegradable networks. The co-polymerization behaviour was monitored by FT-Raman spectroscopy. The compressive mechanical properties of the polymer were determined using an Instron 5567 (Bayswater Vic.). The polymer networks were degraded in phosphate buffered saline (PBS) with various amounts of MAA and MSA.

Results The formation of the pre-polymer was confirmed with the observation of NMR peaks at 5.8 and 6.2 ppm and FT-IR peaks at 1637cm-1. Copolymerization was followed with consecutive FT-IR acquisitions with 100% conversion achieved between 10 and 30 hrs depending on the ratio of MMA to MSA or MAA. Increasing the fraction of methacrylated anhydride slowed the reaction rate.

The compressive strength of the MAA and MSA based copolymers was measured as a function of anhydride concentration. Compressive strength for MMA increased (90±9 to 140±10 Mpa) in an approximately linear manner for MAA concentrations from 10 to 40 wt.% but decreased markedly for MAA concentration of 45% (62±14 Mpa). The compressive strength of MSA decreased exponentially for concentrations ranging from 10 to 45 wt.% (140±18 to 39±1 Mpa).

Discussion The use of poly-L-lactic acid in lumbar interbody cages has been shown to be mechanically feasible with the mechanical strength of the cage material reported to be 93 Mpa (1). The material described here has controlled mechanical properties in the required range as well as a degradation behaviour that lends itself better to spinal applications than current materials