Genetic dissection of the circuit for hand dexterity in primates

Abstract

It is generally accepted that the direct connection from the motor cortex to spinal motor neurons is responsible for dexterous hand movements in primates. However, the role of the 'phylogenetically older' indirect pathways from the motor cortex to motor neurons, mediated by spinal interneurons, remains elusive. Here we used a novel double-infection technique to interrupt the transmission through the propriospinal neurons (PNs), which act as a relay of the indirect pathway in macaque monkeys (Macaca fuscata and Macaca mulatta). The PNs were double infected by injection of a highly efficient retrograde gene-transfer vector into their target area and subsequent injection of adeno-associated viral vector at the location of cell somata. This method enabled reversible expression of green fluorescent protein (GFP)-tagged tetanus neurotoxin, thereby permitting the selective and temporal blockade of the motor cortex–PN–motor neuron pathway. This treatment impaired reach and grasp movements, revealing a critical role for the PN-mediated pathway in the control of hand dexterity. Anti-GFP immunohistochemistry visualized the cell bodies and axonal trajectories of the blocked PNs, which confirmed their anatomical connection to motor neurons. This pathway-selective and reversible technique for blocking neural transmission does not depend on cell-specific promoters or transgenic techniques, and is a new and powerful tool for functional dissection in system-level neuroscience studies.

Chimeric rabies SADB19-VSVg-pseudotyped lentiviral vectors mediate long-range retrograde transduction from the mouse spinal cord

“…Retrograde N2c/VSVg lentivirus was recently used to label propriospinal neuronal soma in C2-C5 by injecting the primate C6/T1 cervical spinal cord. 23 Perhaps, the most complex region to study is the cerebral cortex where apparently similar neurons send axons to diverse targets. Retrograde labeling using fluorescent dyes has uncovered some of this complexity and allowed both electrophysiological and morphological identification of these subtypes.…”

Selective Manipulation of Neural Circuits

“…Only the neurons co-infected with both viruses express the genetic tool and are manipulated. This dual viral infection system has been applied and controlled diverse circuits [41][42][43][44]. This strategy provides the most flexibility to target circuits; one needs only to know the origin and termination of a circuit.…”

“…Although these viruses can be modified to prevent transmission to the experimenter, they can be difficult to produce and can cause inflammation and injury to nervous tissue. Now, several serotypes of adeno-associated virus (AAV) such as AAV6, AAV8, and AAV9 are characterized to transduce neurons retrogradely [61][62][63][64][65], and lentivirus has been modified with its glycoprotein fused with rabies virus glycoprotein for retrograde transduction [41,42,66,67]. These viruses are easy to produce, result in less inflammation than herpes simplex and rabiesderived vectors, and have long duration and high level of transgene expression [68][69][70][71][72].…”

“…In addition, the resulting gene expression can be weak or uneven compared with the expression of transgenes. In addition, circuits that are widely dispersed, particularly in larger animals, require multiple injections of viruses [41,42]. For diffusely projecting systems (ascending monoaminergic systems of the brain stem) or for systems that originate in large areas (corticospinal system), a dual virus approach may not be practical.…”

“…The circuit is inactivated through blocking neurotransmission. This system has been used successfully to inactivate neurons and circuits selectively and reversibly [41,42,163,164]. For instance, in order to test the roles of the circuit between propriospinal neurons and spinal motor neurons in dexterous hand movement of primates, AAV encoding reverse tetracycline transactivator was injected into the origin of the circuit where the propriospinal neurons are located, and retrograde gene transfer lentivirus carrying TeNT under the control of tetracycline responsive element was injected into the termination of the circuit where the spinal motor neurons reside.…”