Truncated Forms of the Androgen Receptor are Associated with Polyglutamine Expansion in X-Linked Spinal and Bulbar Muscular Atrophy

Abstract

X-linked spinal and bulbar muscular atrophy (SBMA) is a rare form of motor neuron degeneration linked to a CAG repeat expansion in the first exon of the androgen receptor gene coding for a polyglutamine tract. In order to investigate the properties of the SBMA androgen receptor in neuronal cells, cDNAs coding for a wild-type (19 CAG repeats) and a SBMA mutant androgen receptor (52 CAG repeats) were transfected into mouse neuroblastoma NB2a/d1 cells. The full length androgen receptor proteins, of 110-112 kDa and 114-116 kDa for the wild-type and mutant protein, respectively, were detected by Western blotting in transfected cells. In addition, the presence of an expanded polyglutamine tract in the SBMA androgen receptor appears to enhance the production of C-terminally truncated fragments of the protein. A 74 kDa fragment was particularly prominent in cells expressing the SBMA androgen receptor. From its size, it can be deduced that the 74 kDa fragment lacks the hormone binding domain but retains the DNA binding domain. The 74 kDa fragment may therefore be toxic to motor neurons by initiating the transcription of specific genes in the absence of hormonal control. Immunofluorescence microscopy on transfected NB2a/d1 cells showed that, after hormone activation, the wild-type androgen receptor translocated to the nucleus whereas the SBMA androgen receptor was mainly localized in the cytoplasm in the form of dense aggregates with very little androgen receptor protein in the nucleus. This could explain the reduction in transcriptional activity of the SBMA mutant as compared with wild-type androgen receptor.

Calmodulin-Androgen Receptor (AR) Interaction: Calcium-Dependent, Calpain-Mediated Breakdown of AR in LNCaP Prostate Cancer Cells

“…AR with expanded polyglutamine repeats has been implicated in neurodegenerative disease wherein AR is cleaved between the DBD and LBD into a toxic, ''truncated'' ATD-DBD moiety (18). Caspase-3 is suggested to be the enzyme responsible for the cleavage of this pathologic AR (19)(20)(21)(22).…”

“…Other studies (11) established that proteolysis can cleave AR into two fragments: one of f50 kDa containing both the LBD and the DBD and another of approximately the same size, presumably the ATD (13). Third, the caspase family of thiol proteases is capable of cleaving AR, particularly those forms of AR with expanded polyglutamine repeats (18)(19)(20)(21)(22). To date, the cleavage of AR by the Ca 2+ -activated, thiol protease calpain has not been described.…”

Founder effect in spinal and bulbar muscular atrophy (SBMA) in Scandinavia

“…3 The gene normally has 10-36 CAG repeats, 4 but in SBMA patients the number of repeats is increased to 38-72. 5,6 Several other neurodegenerative disorders caused by trinucleotide repeat expansions have also been reported. [7][8][9] It has been shown that proteins with expanded polyglutamine stretches encoded by the CAG repeats form fibrillary aggregates in neurones, leading to apoptosis.…”

“…10 Such aggregates have also been reported in SBMA. 4,6,11 The human AR gene is located in Xq11-q12. The gene contains 8 exons.…”

Evidence for both nucleus and cytoplasm as subcellular sites of pathogenesis in Huntington'sdisease in cell culture and in transgenic mice expressing mutant huntingtin

“…As observed in vivo, mutant huntingtin and other disease proteins containing expanded polyglutamine tracts form aggregates in vitro, whereas wildtype proteins with a normal tract do not (Lunkes & Mandel 1998;Igarashi et al 1998;Skinner et al 1997;Ellerby et al 1999a;Hackam et al 1998a;Cooper et al 1998;Butler et al 1998;Merry et al 1998;. Aggregates formed in vivo and in vitro are frequently ubiquitinated (Lunkes & Mandel 1998;Cooper et al 1998;Igarashi et al 1998;Cummings et al 1998;A.…”