New Insight into CLN5 Protein May Help Develop New Therapeutic Strategies, Study Reports

Researchers have described how the CLN5 protein — the mutation of which underlies a type of Batten disease — turns into a mature soluble protein, information they hope can be used to identify new therapeutic strategies for the disease.

The study, “CLN5 is cleaved by members of the SPP/SPPL family to produce a mature soluble protein,” was published in Experimental Cell Research.

Neuronal ceroid lipofuscinoses (NCLs), also known as Batten disease, comprise a group of five childhood genetic disorders with a wide range of symptoms, including vision loss, lack of motor coordination, and impaired cognition. These conditions can be caused by mutations in 14 different genes (CLN1 to CLN14),  but investigators still do not know the precise functions of the proteins for which most of them provide instructions.

One such gene is CLN5. At least 35 different CLN5 mutations have been found to cause the late-infantile Finnish variant of NCL, also known as neuronal ceroid lipofuscinosis 5 (CLN5-NCL).

The CLN5 protein is composed of 407 amino acids and contains a transmembrane domain — a region that enables the protein to enter membranes of different cell compartments — which is broken down so that the inactive CLN5 preprotein becomes a mature soluble one.

Once active, CLN5 is transported to lysosomes — special compartments within cells that digest and recycle different types of molecules — where it is thought to regulate the transit of recycled materials between different cell compartments.

Although researchers already had some clues on the function of CLN5, so far the mechanisms involved in its activation remained unknown. Now Canadian researchers may have finally solved the mystery.

Using a combination of biochemistry and molecular biology techniques, the team demonstrated that CLN5 is first translated as a type 2 transmembrane protein, after which it is broken down by an enzyme called SPPL3 into a mature soluble protein.

Proteins are composed of different sequences of amino acids and are translated, or produced, from a “template” of messenger RNA, which is the intermediate molecule between DNA and protein, in a specific direction: N-terminus to C-terminus, commonly referred to as left to right. The N-terminal is the start of a protein sequence that contains a free amine group. The C-terminal tail is the end of the amino acid chain, terminated by a free carboxyl group.

The N-terminal often contains sequences, just like postal codes, that direct the delivery of a protein to the proper site within a cell.

Researchers found that the N-terminal fragment of CLN5 was not required for the transport of the protein to lysosomes. Moreover, they discovered this fragment was subsequently cleaved by SPPL3 and another enzyme called SPPL2b, and then degraded by a protein complex called the proteasome.

Researchers hope these molecular findings may contribute to the development of new therapeutic strategies to treat children affected by the disease.