Our work will allow a better understanding of the genetic and molecular basis of congenital visceral smooth muscle malformations.

FOR BEGINNERS

Smooth muscles, unlike skeletal muscle, contract slowly and involuntarily (spontaneous contractile activity). These smooth muscles are found in vascular walls, digestive, respiratory and urinary tract. Their function is to assist in the transport of various media (i.e. blood for blood vessels, air in the bronchi, urine in ureters).

During embryonic development, differentiation of smooth muscle cells is a crucial process for the development of blood vessels and hollow organs (eg intestine, ureter, lungs). Alterations of this mechanism contribute to the development of various diseases such as atherosclerosis, hypertension or asthma.

We showed in mice that the transcription factor TSHZ3 played a key role in the differentiation of ureteral mesenchyme into smooth muscle. Work in progress will allow a better understanding of the genetic and molecular basis of congenital visceral smooth muscle malformations.

We also highlighted the importance of TSHZ3 in the establishment of the neural circuit that controls breathing. Tshz3 is a candidate gene for childhood respiratory defects in humans.

FOR SPECIALISTS

Tshz3 gene codes for a zinc finger transcription factor (TSHZ3) (Caubit et al., 2000) involved in several developmental processes in the ureter, the central nervous system and skeletal muscle (Caubit et al., 2008 , Caubit et al., 2010; Faralli et al., 2011).

1. Morphogenesis of the ureter involves reciprocal interactions between the epithelial and the mesenchymal compartment. Mesenchymal cells contribute to the formation of smooth muscles wich are essential to carry dynamically the urine produced by the kidney to the bladder. Tshz3^LacZ/LacZ mutant mice die at birth and exhibit a phenotype of hydroureter (Figure 1).

Analysis of Tshz3^LacZ/LacZ embryos showed that the ureter mesenchyme does not differentiate into smooth muscle. We have shown that the factor TSHZ3 is required downstream of SHH and BMP4 signaling pathways and upstream the “master” transcription factor Myocardin for differentiation of smooth muscle cells (Caubit et al., 2008, Lye et al., 2010 ) (Figure 2 and 3).

Defects observed in the Tshz3^LacZ/LacZ mutant mice are comparable to those observed in patients with obstruction of the ureteropelvic junction. Smooth muscle cells present the particularity to move along a continuum between two extremes: a proliferative state during development to a highly differentiated and contractile state. The transcriptional mechanisms that confer this phenotypic plasticity remain largely misunderstood. Our work is underway to better describe the transcriptional mechanisms involved in the different stages of the myogenic program.

2. In collaboration with researchers from the University of Aix-Marseille and Paris-Sud 11, we have shown that Tshz3 is a gene required for breathing and therefore, for survival at birth (Caubit et al. 2010) (Figure 4).

In mammals, fetus develops in a liquid environment where the umbilical cord provides oxygen and pulmonary functions are virtually absent. At birth, the baby transitions from aquatic intrauterine life to autonomy in the air. How does the body prepare to such a brutal transition? We already know that several neuronal circuits are involved in neonatal respiration in mammals. More specifically, two regions in the hindbrain have been identified (préBötzinger complex and para-facial respiratory group). These neurons initiate a pacemaker activity, that is to say, a rhythm in the brain stem initiating automatic respiratory movements and preparing newborns to birth. Our work shows that TSHZ3 is expressed in the para-facial respiratory group and plays a major role in the neuronal activity of this region. Tshz3^LacZ/LacZ mutant newborn mice do not breathe at birth and die after a few minutes. In these newborn (Tshz3^LacZ/LacZ mutant), preBötzinger complex and para-facial respiratory group seems to be correctly formed but neurons from the para-facial respiratory group do not exhibit their characteristic  rhythmic activity (Figure 5).

Figure 5

Thus, a single gene, Tshz3 is able to control at the level of neurons, the development of several elements and cellular events that are critical for the acquisition of breathing at birth. In the future, collaboration with medical research teams could bring a better understanding of the implication of Tshz3 in human respiratory disorders, from sleep apnea syndrome to sudden infant death syndrome, the leading cause mortality of newborns in Western countries. Through this work, we have also shown that Tshz3 is required for the survival of motor neurons of the nucleus ambiguus.

Work in progress aims to better characterize the role of TSHZ3 factor in the differentiation and/or survival process of different neuronal populations in the central nervous system.