Should We Learn about Regeneration from Lizards?

Regenerative medicine has been a cutting edge field at present as it holds the key in revealing many unknown processes involved in tricky degenerative diseases or cancers. Many approaches have been performed to uncover the underlying mechanisms of regeneration process. Generally, the term regeneration refers to the ability of an organism to restore damaged or lost tissue with no formation of scar tissue, and the newly formed tissue is able to function the same as the original (Pirotte et al., 2016).

The capacity of regeneration in human is limited only to bone healing, repair of skin cuts and regeneration of liver, muscle, bone, blood and epithelia (Narayanan, 2015), but there are some organisms showing impressive regeneration capacity such as zebra fish, salamanders and lizards. Those animals are commonly used as models for regeneration studies as they can provide molecular insights of inherent regenerative capacity (Kurup & Ramachandran, 2011). Taxonomically, lizard which belong to the class Reptilia has the closest relation to mammals and has the ability to fully regrow its tail after breaking it off as its defense mechanism, making them more representative to be used as a model in studying tissue regeneration (Lozito & Tuan, 2015).

Researchers in the world have been fascinated by lizard’s regenerative capacity for centuries and they dedicate their time trying to unfold the mystery of this process, it can be seen from the number of international publications published by them. In Indonesia, a series of studies regarding tail regeneration of Eutropis multifasciata (member of Scincidae family) has been done by Nyoman Puniawati Soesilo, but her research is mainly focused on angiogenesis, calcification, and the roles of ependymal cells in lizard tail regeneration. Currently, there is only few Indonesian researchers focusing on studying lizard tail regeneration. Gekko gecko (Tokek) is one of the most commonly found lizards in Indonesia with regenerative capacity. 

Naturally, the tail of G. gecko consists of complex structures such as spinal cord, caudal vertebra, blood vessels, muscle bundles, connective tissue, adipose tissue and modified epidermis while the regenerated tail is a non-identical but functional replica of the original tail. The slight differences found in the regenerated tail are (1) the axial skeleton of regenerated tail is replaced by cartilage tube, (2) the spinal cord in regenerated tail is only formed by meninges a layer of ependymal cells lining the central canal, and (3) the pattern of scales in regenerated tail is simpler (Higham et al., 2013).

Studies in lizard tail regeneration open up the possibility to find out information which later can be applied in degenerative bone/cartilage diseases such as osteoarthritis, suffered by about 10% of human population aged more than 60 (Tiku & Sabaawy, 2015), or even appendage replacement in the future (Gilbert et al., 2015). Cartilage is an important connective tissue mostly found in ear, nose, and joints, and it can be damaged due to injury or disease. Human cartilage lacks of self-healing capacity which makes regenerative therapy get much attention recently (Zhang et al., 2016). Understanding how lizards are able to produce and grow cartilage tube in the newly regenerated tail following a tail loss might be helpful in engineering cartilage tissue for osteoarthritis treatment or even replicating this ability in human years to come (Lozito & Tuan, 2016).

Chondrogenesis and ossification are important processes occurred in the formation of axial skeleton in lizard regenerated tail (Bai et al., 2015), but it has been reported that only the proximal region of the cartilage tube undergoes ossification (Lozito & Tuan, 2015) even though the outer and inner edges of the cartilage tube are also calcified (Alibardi, 2015a). A transcriptomic analysis of tail regeneration in lizard Anolis carolinensis showed that the genes predicted to be required for cartilage condensation are Acan, MGP, and Col2a1; chondrocyte differentiation: Col9a1, Acan, Col2a1, and Col11a2; cartilage development: BMP3, Col9a1, Lect1, Pax7, Acan, MGP, Col2a1, and Col11a2 (Hutchins et al., 2014). 

It has been widely known that bone morphogenetic proteins (BMPs) and the BMP signaling pathway are important in mammalian articular cartilage regeneration (Zhang et al., 2015). BMP3, a member of transforming growth factor-β superfamily, is a negative regulator of osteogenesis but possesses the ability to enhance human mesenchymal proliferation and promote differentiation into nucleus-pulposus-like cells (Zhou et al., 2015). BMP3 is also known to upregulate chondrocyte proliferation by regulating the expression of BMP2 and BMP4 (Long & Ornitz, 2013) . Although BMP2/2b/4/6 has been known to positively affect the cartilage regeneration in lizards (McLean & Vickaryous, 2011; Alibardi, 2015; Lozito & Tuan, 2015; Lozito & Tuan, 2016 ), the effect of BMP3 in this process has not been studied yet. An investigation needs to be conducted to get insights regarding the role of BMP3 in lizard chondrogenesis.

Type II collagen (Col2a1) and Aggrecan (Acan) are major chondrocyte-specific extracellular matrix components, expressed throughout the chondrogenesis and regulated by CD59 protein (Wang et al., 2011; Bai et al., 2015). Their expressions are usually used as an early marker of chondrogenic cells (Mori-Akiyama et al., 2003). By observing their expressions on the regenerated tail, we can obtain information concerning the starting point of lizard chondrogenesis and how they are regulated during the process. Matrix Gla protein (MGP) is known to inhibit cartilage mineralization by binding mineral ions such as calcium and phosphate thus limiting the amount of free calcium ions available to initiate calcification (Yagami et al., 1999) or binding BMP2 and BMP4 to form an inactive complex which might also inhibit calcification (Beazley et al., 2013; Dan, Simsa-Maziel et al., 2012). This protein might not only involve in mineralization, but also in chondrocyte and cartilage development. The roles of this molecule on lizard chondrogenesis has not been studied yet.