Page 33 - Read Online

Kaya et al. Neuroimmunol Neuroinflammation 2019;6:5 I http://dx.doi.org/10.20517/2347-8659.2018.70 Page 7 of 15

site. In order to achieve this goal, the first challenge is to thoroughly understand the exact pro-apoptotic
2+
mechanisms driven by Ca influx and the key factors involved in these mechanisms. From this point of
view, superior neuronal protection in SCI involves a two-pronged approach: (1) reversal of the apoptotic
effect on injured neurons caused by the apoptotic deregulation of the ERK/MAPK and PI3K/AKT pathways;
and (2) inhibition of pathological calpain protease activation.

2+
The goal of our laboratory is to understand the pro-apoptotic mechanism of Ca deregulation and to
2+
prevent apoptosis by inhibiting the downstream effects of lethal Ca influx in neurons. To this effect, we
are studying the novel cell cycle regulatory protein Speedy/RINGO due to its p53-dependent anti-apoptotic
[12]
function which has previously been observed in U2OS osteosarcoma cells , calpain-induced degenerating
[13]
[14]
[15]
primary neurons , ALS motor neurons and astrocytes and microglia of spinal cord lesions .
A NOVEL CELL CYCLE REGULATOR, SPEEDY/RINGO, AS A STRONG CANDIDATE PROTEIN
fOR NEUROPROTECTION IN SCI
The main function of Speedy/RINGO is the regulation of the cell cycle in mitotic cells. However, recent
studies show that Speedy/RINGO also has an anti-apoptotic effect in DNA-damaged mitotic cells, allowing
[12]
for their survival . Speedy/RINGO has been shown to have a strong protective effect for mitotic cells
exposed to extrinsic or intrinsic apoptotic factors such as UV irradiation. This anti-apoptotic function of
[13]
Speedy/RINGO has also been utilized and confirmed in post-mitotic degenerating primary neurons and
[14]
in ALS motor neurons . Speedy/RINGO performs this function by inhibiting caspase-3 activation and
apoptosis in the presence of the gene regulatory protein p53 [12,13] . Since p53-mediated apoptosis is inevitable
for SCI patients, utilizing Speedy/RINGO’s anti-apoptotic feature may turn the tide in the battle against SCI.

Speedy/RINGO protein structure and function
In eukaryotic cells, cell cycle progress is strictly controlled by cyclin-dependent kinases (CDKs) which are
regulated by cyclins. Cyclins regulate CDK activity during different phases of the cell cycle by binding and
phosphorylating them. Although cyclins are the key regulators of CDK activity, Speedy/RINGO, a novel cell
[71]
cycle regulator, is shown to bind and regulate CDK activity in many eukaryotic cell types .
Speedy/RINGO was first identified in Xenopus oocytes as a meiotic cell cycle regulator accelerating G2/M
[72]
progression during oocyte maturation .

Unlike cyclins, Speedy/RINGO binds and activates CDKs by a yet unelucidated phosphorylation-free
[71]
mechanism .
There are at least three major branches in the Speedy/RINGO family (A, B and C), with a fourth branch
(D) suspected. Speedy/RINGO A, the human homologue Spy1, is the most conserved and the most slowly
evolving branch of Speedy/RINGO family. This is the branch used in our laboratory. Branch A is found in
nearly all types of cells in fish, chickens, sea urchins and mammals. Expression levels are higher in testis
[71]
tissue than in tissues such as brain, heart, lung, placenta, prostate, small intestine, etc. . Since Speedy/
RINGO is primarily a mitosis regulatory protein, it is not widely expressed in post-mitotic neurons.

Although the main function of Speedy/RINGO is cell cycle regulation, studies have attributed a p53-
dependent anti-apoptotic function to Speedy/RINGO in DNA-damaged mitotic cells, resulting in those cells
[12]
evading apoptosis and, thus, surviving .

Speedy/RINGO protein inhibits apoptosis and leads mitotic cells to become cancerous
When apoptotic insult occurs during a cell cycle, the resulting DNA damage triggers cell cycle arrest.
[73]
This arrest, in turn, activates checkpoint responses to allow cells to repair the DNA damage . Increased