Behavioral and electrophysiological outcomes of tissue-specific Smn knockdown in Drosophila melanogaster

Abstract

Severe reduction in Survival Motor Neuron 1 (SMN1) protein in humans causes Spinal Muscular Atrophy (SMA), a debilitating childhood disease that leads to progressive impairment of the neuro-muscular system. Although previous studies have attempted to identify the tissue(s) in which SMN1 loss most critically leads to disease, tissue-specific functions for this widely expressed protein still remain unclear. Here, we have leveraged RNA interference methods to manipulate SMN function selectively in Drosophila neurons or muscles followed by behavioral and electrophysiological analysis. High resolution measurement of motor performance shows profound alterations in locomotor patterns following pan-neuronal knockdown of SMN. Further, locomotor phenotypes can be elicited by SMN knockdown in motor neurons, supporting previous demonstrations of motor neuron-specific SMN function in mice. Electrophysiologically, SMN modulation in muscles reveals largely normal synaptic transmission, quantal release and trans-synaptic homeostatic compensation at the larval neuro-muscular junction. Neuronal SMN knockdown does not alter baseline synaptic transmission, the dynamics of synaptic depletion or acute homeostatic compensation. However, chronic glutamate receptor-dependent developmental homeostasis at the neuro-muscular junction is strongly attenuated following reduction of SMN in neurons. Together, these results support a distributed model of SMN function with distinct neuron-specific roles that are likely to be compromised following global loss of SMN in patients. While complementary to, and in broad agreement with, recent mouse studies that suggest a strong necessity for SMN in neurons, our results uncover a hitherto under-appreciated role for SMN in homeostatic regulatory mechanisms at motor synapses.

The Gemin Associates of Survival Motor Neuron Are Required for Motor Function in Drosophila

“…In this regard, whether restricted to muscle or neurons, we showed that a strong loss of function of either Gemin3 or Gemin5 has a significant impact on adult viability whereas on milder knockdown, we could detect defects in motor function. A similar tissue-selective trend has been reported for SMN in various studies [42], [59], [60]. We also show that in corroboration with a previous study [49], forced expression of normal SMN in a Gem3 ΔN mutant background does not rescue the associated motor defects, thereby indicating that SMN does not function downstream of Gemin3 but most probably in parallel to Gemin3.…”

“…However, overexpression of Gem3 ΔN or hSMN both resulted in an increase in puparial axial ratios ( FIG. 4A ), a phenotype described previously in Gemin3 null mutants [40], and on modulation of Drosophila SMN protein levels [58], [59]. Interestingly, hSMN was detected in intensely-stained nuclear and, at times, cytoplasmic puncta within muscle tissue.…”

Homeostatic Control of Presynaptic Neurotransmitter Release

“…For example, mutations in the schizophrenia-associated gene dysbindin block presynaptic homeostasis in Drosophila (27). A Drosophila model of spinal muscular atrophy includes failure of persistent presynaptic homeostasis (102). In mammals, signaling of Homer and mGluR, both of which are required for the homeostatic control of neurotransmitter receptor abundance, is implicated in mouse models of fragile X syndrome (103), as is retinoic acid (104).…”

Homeostatic plasticity at the Drosophila neuromuscular junction

“…By taking advantage of RNA interference (RNAi)-based tools and tissue specific gene knockdown, it was recently demonstrated that presynaptic Drosophila Smn is required for long-term homeostatic compensation (Timmerman and Sanyal, 2012). Yet, as with Ephexin and Gsb, Smn is dispensable for rapid homeostatic compensation (Timmerman and Sanyal, 2012).…”

“…Yet, as with Ephexin and Gsb, Smn is dispensable for rapid homeostatic compensation (Timmerman and Sanyal, 2012). These new results raise the intriguing possibility that regulation of small nuclear RNAs or mRNA metabolism could be involved in consolidating long-term homeostatic responses.…”