Proprioceptive sensory feedback is an integral element of motor control. By providing the CNS with a real time representation of limb and body position, muscle feedback is critical in the planning and adjustment of motor output. This is clearly illustrated in individuals affected by acute or chronic sensory neuropathy, for whom simple activities such as walking down a flight of stairs, signing their name, or buttoning up a shirt, can become challenging tasks.
Research in our laboratory is aimed at understanding the molecular and cellular basis of the proprioceptive sensory system. Specifically, we are interested in delineating the molecular correlates of the main proprioceptor subtypes and how each of these subclasses contributes to generating coordinated motor output. Using the mouse as a model system, we employ molecular (including single cell RNAseq), genetic, and viral strategies, in order to i) define the molecules that drive proprioceptor subtype specification, ii) map the proprioceptive connectivity patterns in spinal cord, and iii) assess the behavioral consequences of the loss of proprioceptors through genetic (proprioceptor) inactivation studies.
Complementing our in vivo work, we are developing protocols to derive somatic sensory neurons – including proprioceptors – from human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). The goal of these efforts is two-fold: To facilitate more in-depth molecular and biochemical studies of proprioceptor development, and to develop in vitro model systems for sensory neuropathies such as Friedreich Ataxia (FA) or chemotherapy-induced peripheral neuropathy (CIPN) using patient-derived sensory neurons.
By flexibly exploiting both in vitro ESC/iPSC-derived sensory neurons and our in vivo mouse models, we hope to advance our understanding of proprioceptor development and function in health and disease.
Specific projects that are ongoing in the lab:
1. Molecular basis of proprioceptor subtype identity
Proprioceptive sensory information derives from three main proprioceptor subclasses: group Ia and group II muscle spindle (MS) afferents, which both register changes in muscle length, and group Ib Golgi tendon organ (GTO) afferents, which provide updates on muscle load/tension. Despite considerable efforts, the molecular basis of proprioceptor subtype diversity remains largely unresolved.
We recently identified several new proprioceptor-enriched transcripts and are currently working to assign these new markers to the correct proprioceptor subclass using various in vivo labeling techniques. In addition, we are using single cell RNAseq approaches to enable a temporal analysis of the proprioceptor transcriptome while they differentiate into distinct proprioceptor subtypes in order to understand the molecular mechanisms through which the distinct MS/GTO subtypes emerge during development
2. Role of proprioceptive muscle afferents in motor control
Sensory feedback is critical for normal motor control, yet many important questions remain unanswered. What is the relative contribution of proprioceptive muscle feedback versus feedback from other sensory receptors? Is proprioceptive muscle feedback required for the formation of spinal motor circuits? We developed an intersectional genetic strategy that permits the acute inactivation of all (or select subsets of) proprioceptive muscle afferents while preserving cutaneous mechanoreceptors. Using this new mouse model we are performing behavioral studies to assess the consequences of the loss of proprioceptive muscle feedback at various stages during development. In addition, using a combination of anatomical and physiological studies we are assessing if and how proprioceptive feedback is required for the development of spinal motor circuits, and, how these circuits may adapt when confronted with a sudden loss in proprioceptive feedback.
3. Derivation of sensory neurons from hESC/iPSCs
Building on the pioneering work of others, we developed a new protocol for the efficient in vitro derivation of somatic sensory neurons (SNs) from human embryonic or induced pluripotent stem cells (hESCs/iPSCs). A large proportion of these in vitro derived SNs exhibit molecular and physiological features that are consistent with a nociceptive SN identity, yet a subset of neurons express markers that are indicative of mechanoreceptive touch-, or proprioceptive SNs. Using Crispr/Cas9 gene-editing strategies, we are generating fluorescent reporters for each of these individual SN subtypes. These reporters will enable us to directly assess the efficiency of differentiation for individual SN subclasses, and moreover, will permit molecular and biochemical studies to further our understanding of the developmental mechanisms that drive these sensory fates.
4. Modeling peripheral neuropathy
Sensory neuropathy can have many different causes – it can be hereditary, it can be caused by autoimmune or metabolic disease, or it can be due to toxic agents such as chemotherapy drugs. Effective treatment options are limited, in part due to our limited understanding of the molecular events that lead to disease onset, as well as due to a lack of adequate model systems based on human sensory neurons to evaluate putative neuroprotective molecules. Our goal is to utilize patient iPSC-derived sensory neurons that are engineered to express our fluorescent sensory neuron reporters to model peripheral neuropathy, including chemotherapy-induced peripheral neuropathy (CIPN) and Friedreich Ataxia (FA). Through a systematic analysis of cellular (e.g. microtubule and mitochondrial dynamics) and molecular changes within the affected SNs, we hope to deduce the underlying cause of the neuropathy and to identify the genetic variables that influence disease severity.