A novel short splice variant of the tumour suppressor LKB1 is required for spermiogenesis

Abstract

LKB1 was discovered as a tumour suppressor mutated in Peutz-Jeghers syndrome, and is a gene involved in cell polarity as well as an upstream protein kinase for members of the AMP-activated protein kinase family. We report that mammals express two splice variants caused by alternate usage of 3'-exons. LKB1(L) is the previously described form, while LKB1(S) is a novel form in which the last 63 residues are replaced by a unique 39-residue sequence lacking known phosphorylation (Ser(431)) and farnesylation (Cys(433)) sites. Both isoforms are widely expressed in rodent and human tissues, although LKB1(S) is particularly abundant in haploid spermatids in the testis. Male mice in which expression of Lkb1(S) is knocked out are sterile, with the number of mature spermatozoa in the epididymis being dramatically reduced, and those spermatozoa that are produced have heads with an abnormal morphology and are non-motile. These results identify a previously undetected variant of LKB1, and suggest that it has a crucial role in spermiogenesis and male fertility.

Phosphorylation of Serine 399 in LKB1 Protein Short Form by Protein Kinase Cζ Is Required for Its Nucleocytoplasmic Transport and Consequent AMP-activated Protein Kinase (AMPK) Activation

“…In the alternatively spliced LKB1 short form, 34 amino acids replace the last 63 amino acids present in the C terminus of human LKB1 L (25). This substitution leads to loss of the farnesylation site and the Ser-431 (human Ser-428) phosphorylation site in the LKB1 C-terminal domain (22,24). Ser-431 has been the focus of various studies seeking to understand the role of C-terminal phosphorylation in regulation of LKB1 activity.…”

“…There are two splice variants of LKB1 that differ in their C terminus, the LKB1 long form (LKB1 L ) and the LKB1 short form (LKB1 s ) (22)(23)(24). In the alternatively spliced LKB1 short form, 34 amino acids replace the last 63 amino acids present in the C terminus of human LKB1 L (25).…”

L’AMPK, régulateur de l’énergie et des fonctions des spermatozoïdes

“…Les spermatides de rongeurs possèdent une isoforme spécifique de LKB1, l'AMPK kinase impliquée dans l'activation de l'AMPK, la LKB1S, illustrant l'importance de cette voie dans la fertilité masculine. Ainsi, chez les souris dont le gène LKB1 a été délété (LKB1 KO [knock-out]), l'absence de la forme courte de la kinase 5 entraîne une diminution de la spermatogenèse avec pour conséquence une réduction du nombre de spermatozoïdes matures dans l'épididyme et une augmentation des spermatozoïdes morphologiquement anormaux et non-mobiles [29]. Dans les cellules de Sertoli, qui sont les cellules nourricières de la lignée germinale, l'activation de la voie de l'AMPK conduit, chez le rat et la souris, à une production accrue de lactate, en réponse à une augmentation de l'absorption du glucose et, en consé-quence, des niveaux de GLUT1 (transporteur 1 de glucose) et MCT4 (transporteur 4 monocarboxylate) [19,30] …”

Molecular chaperone complexes with antagonizing activities regulate stability and activity of the tumor suppressor LKB1

“…WB, western blotting. replaced by a unique 39-residue sequence (Towler et al, 2008;Denison et al, 2009). LKB1-DN is generated by the alternative use of a downstream start codon and lacks the N-terminal 124 residues of the full-length protein.…”