Steroid‐triggered programmed cell death of a motoneuron is autophagic and involves structural changes in mitochondria

Abstract

Neuronal death occurs during normal development and disease and can be regulated by steroid hormones. In the hawkmoth, Manduca sexta, individual accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD) at pupation that is triggered directly and cell autonomously by the steroid hormone 20-hydroxyecdysone (20E). APRs from abdominal segment six [APR(6)s] die by 48 hours after pupal ecdysis (PE; entry into the pupal stage), whereas APR(4)s survive until adulthood. Cell culture experiments showed previously that 20E acts directly on APRs to trigger PCD, with intrinsic segmental identity determining which APRs die. The APR(6) death pathway includes caspase activation and loss of mitochondrial function. We used transmission electron microscopy to investigate the ultrastructure of APR somata before and during PCD. APR(4)s showed normal ultrastructure at all stages examined, as did APR(6)s until approximately stage PE. During APR(6) death, there was massive accumulation of autophagic bodies and vacuoles, mitochondria became ultracondensed and aggregated into compact clusters, and ribosomes aggregated in large blocks. Nuclear ultrastructure remained normal, without chromatin condensation, until the nuclear envelope fragmented late in the death process. Light microscopic immunocytochemistry showed that dying APR(6)s were TUNEL-positive, which is diagnostic of fragmented DNA. These observations indicate that the steroid-induced, caspase-dependent, cell-autonomous PCD of APR(6)s is autophagic, not apoptotic, and support an early role for mitochondrial alterations during PCD. This system permits the study of neuronal death in response to its bona fide developmental signal, the rise in a steroid hormone.

Slow degeneration of zebrafish Rohon‐Beard neurons during programmed cell death

“…In addition to DNA-fragmentation and caspasedependence, apoptosis is characterized by rapid clearance of cells by phagocytosis (Savill and Fadok, 2000;Bursch et al, 2000;Henson et al, 2001;Bursch, 2001). Importantly, apoptosis can only be confirmed by electron microscope (EM) level examination to reveal changes in cellular organelles (Bursch et al, 2000;Kinch et al, 2003). Although we did not examine whether zebrafish RBs are removed by phagocytosis, our studies provide evidence that these cells often remain fairly intact for many days after DNA fragmentation, an observation inconsistent with apoptosis.…”

“…EM studies of dying RBs in Xenopus laevis (Lamborghini, 1987) are also inconsistent with the description of apoptosis and suggest that RBs may die by another mechanism. A growing body of work provides evidence that some cells, including neurons, can undergo programmed cell death by autophagy, rather than by apoptosis (Zakeri et al, 1995;Xue et al, 1999;Bursch et al, 2000;Bursch, 2001;Kinch et al, 2003). In some cases, autophagy and apoptosis are triggered by the same signals (Xue et al, 1999) and these processes are not mutually exclusive, as some cells may undergo both apoptosis and autophagy (Bursch et al, 2000;Bursch, 2001).…”

Apoptosis of neuronal cells in the brains of postembryonic silkworms Bombyx mori (Lepidoptera: Bombycidae)

“…In particular, the exposure of the second instar larval salivary glands and midgut to ecdysone results in a massive increase in dronc (a Drosophila caspase) mRNA levels, which eventually trigger cell death (Dorstyn et al, 1999). The Drosophila retinal neurons and accessory planta retractor (APR (6)) motoneurons in abdominal segment six of Manduca sexta undergo programmed death triggered by caspase or caspase-3 (Yu et al, 2002;Kinch et al, 2003). However, few studies have attempted to elucidate the process by which neuronal apoptosis is induced by caspases in insect brains during metamorphosis.…”

A Drosophila Gain-of-Function Screen for Candidate Genes Involved in Steroid-Dependent Neuroendocrine Cell Remodeling

“…Molting and metamorphosis are coordinated by two families of hormones, the juvenile hormones and the ecdysteroids (Nijhout 1994). During metamorphosis, ecdysteroids act cell autonomously to control neuronal cell fates (Robinow et al 1993;Lee et al 2000;Brown et al 2006) through evolutionarily conserved cellular mechanisms and signaling pathways (Draizen et al 1999;Kinch et al 2003;Watts et al 2003;Choi et al 2006;Hoopfer et al 2006).…”