Stimulation of mTOR-independent autophagy and mitophagy by rilmenidine exacerbates the phenotype of transgenic TDP-43 mice.

Autophagy, which mediates the delivery of cytoplasmic substrates to the lysosome for degradation, is essential for maintaining proper cell homeostasis in physiology, ageing, and disease. There is increasing evidence that autophagy is defective in neurodegenerative disorders, including motor neurons affected in amyotrophic lateral sclerosis (ALS). Restoring impaired autophagy in motor neurons may therefore represent a rational approach for ALS. Here, we demonstrate autophagy impairment in spinal cords of mice expressing mutant TDP-43Q331K or co-expressing TDP-43WTxQ331K transgenes. The clinically approved anti-hypertensive drug rilmenidine was used to stimulate mTOR-independent autophagy in double transgenic TDP-43WTxQ331K mice to alleviate impaired autophagy. Although rilmenidine treatment induced robust autophagy in spinal cords, this exacerbated the phenotype of TDP-43WTxQ331K mice, shown by truncated lifespan, accelerated motor neuron loss, and pronounced nuclear TDP-43 clearance. Importantly, rilmenidine significantly promoted mitophagy in spinal cords TDP-43WTxQ331K mice, evidenced by reduced mitochondrial markers and load in spinal motor neurons. These results suggest that autophagy induction accelerates the phenotype of this TDP-43 mouse model of ALS, most likely through excessive mitochondrial clearance in motor neurons. These findings also emphasise the importance of balancing autophagy stimulation with the potential negative consequences of hyperactive mitophagy in ALS and other neurodegenerative diseases.

Axonal Growth of Midbrain Dopamine Neurons is Modulated by the Cell Adhesion Molecule ALCAM Through Trans-Heterophilic Interactions with L1cam, Chl1, and Semaphorins.

The growth of axons corresponding to different neuronal subtypes is governed by unique expression profiles of molecules on the growth cone. These molecules respond to extracellular cues either locally though cell adhesion interactions or over long distances through diffusible gradients. Here, we report that that the cell adhesion molecule ALCAM (CD166) can act as an extracellular substrate to selectively promote the growth of murine midbrain dopamine (mDA) neuron axons through a trans-heterophilic interaction with mDA-bound adhesion molecules. In mixed-sex primary midbrain cultures, the growth-promoting effect of ALCAM was abolished by neutralizing antibodies for components of the Semaphorin receptor complex Nrp1, Chl1, or L1cam. The ALCAM substrate was also found to modulate the response of mDA neurites to soluble semaphorins in a context-specific manner by abolishing the growth-promoting effect of Sema3A but inducing a branching response in the presence of Sema3C. These findings identify a previously unrecognized guidance mechanism whereby cell adhesion molecules act in trans to modulate the response of axonal growth cones to soluble gradients to selectively orchestrate the growth and guidance of mDA neurons.SIGNIFICANCE STATEMENT The mechanisms governing the axonal connectivity of midbrain dopamine (mDA) neurons during neural development have remained rather poorly understood relative to other model systems for axonal growth and guidance. Here, we report a series of novel interactions between proteins previously not identified in the context of mDA neuronal growth. Significantly, the results suggest a previously unrecognized mechanism involving the convergence in signaling between local, adhesion and long-distance, soluble cues. A better understanding of the molecules and mechanisms involved in establishment of the mDA system is important as a part of ongoing efforts to understand the consequence of conditions that may result from aberrant connectivity and also for cell replacement strategies for Parkinson's disease.

Chronic activation of the relaxin-3 receptor on GABA neurons in rat ventral hippocampus promotes anxiety and social avoidance.

Anxiety disorders are highly prevalent in modern society and better treatments are required. Key brain areas and signaling systems underlying anxiety include prefrontal cortex, hippocampus, and amygdala, and monoaminergic and peptidergic systems, respectively. Hindbrain GABAergic projection neurons that express the peptide, relaxin-3, broadly innervate the forebrain, particularly the septum and hippocampus, and relaxin-3 acts via a Gi/o -protein-coupled receptor known as the relaxin-family peptide 3 receptor (RXFP3). Thus, relaxin-3/RXFP3 signaling is implicated in modulation of arousal, motivation, mood, memory, and anxiety. Ventral hippocampus (vHip) is central to affective and cognitive processing and displays a high density of relaxin-3-positive nerve fibers and RXFP3 binding sites, but the identity of target neurons and associated effects on behavior are unknown. Therefore, in adult, male rats, we assessed the neurochemical nature of hippocampal RXFP3 mRNA-expressing neurons and anxiety-like and social behavior following chronic RXFP3 activation in vHip by viral vector expression of an RXFP3-selective agonist peptide, R3/I5. RXFP3 mRNA detected by fluorescent in situ hybridization was topographically distributed across the hippocampus in somatostatin- and parvalbumin-mRNA expressing GABA neurons. Chronic RXFP3 activation in vHip increased anxiety-like behavior in the light-dark box and elevated-plus maze, but not the large open-field test, and reduced social interaction with a conspecific stranger. Our data reveal disruptive effects of persistent RXFP3 signaling on hippocampal GABA networks important in anxiety; and identify a potential therapeutic target for anxiety disorders that warrants further investigation in relevant preclinical models.

Modulation of forebrain function by nucleus incertus and relaxin-3/RXFP3 signaling.

The nucleus incertus (NI) in the pontine tegmentum sends ascending projections to the midbrain, hypothalamus, amygdala, basal forebrain, hippocampus, and prefrontal cortex, and has a postulated role in modulating several forebrain functions. A substantial population of GABAergic NI neurons expresses the neuropeptide, relaxin-3, which acts via the Gi/o -protein-coupled receptor, RXFP3, present throughout the forebrain target regions. Broad and specific manipulations of these systems by activation or inhibition of the NI or modulating RXFP3 signaling have revealed key insights into the likely influence of the NI/relaxin-3/RXFP3 system on modalities including arousal, feeding, stress responses, anxiety and addiction, and attention and memory. This range of actions corresponds to a likely impact of NI/(relaxin-3) projections on multiple integrated circuits, but makes it difficult to draw conclusions about a generalized function for this network. This review will focus on the key physiological process of oscillatory theta rhythm and the neural circuits that promote it during behavioral activation, highlighting the ability of NI and relaxin-3/RXFP3 signaling systems to modulate these circuits. A better understanding of these mechanisms may provide a way to therapeutically adjust malfunction of forebrain activity present in several pathological conditions.